| 1. |
- Cho, Choong-Rae, et al.
(författare)
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Preparation of Na0.5K0.5NbO3/La0.6Sr0.2Mn1.2O3/LaAlO3 Thin Film Structures by Pulsed Laser Deposition
- 1999
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Ingår i: Multicomponent oxide films for electronics. - Warrendale, Pa : Materials Research Society. - 1558994815 - 9781558994812 ; , s. 249-254
-
Konferensbidrag (refereegranskat)abstract
- We report on ferroelectric/giant magnetoresistive Nao.sKo.sNbCVLao.oSrojMniO} (NKN/LSMO) heterostructures gro\vn onto LaAlOj (001) single crystal using KrF pulsed laser ablation of stoichiometric ceramic target. Main processing parameters have been optimized to obtain smooth LSMO template layer, avoid NKN-LSMO interdiffusion, preserve NKN stoichiometry against the lost of volatile potassium and sodium and achieve reasonable reliability of NKN film performance. X-ray diffraction 0- 20 scans and rocking curves evidence for single-phase content and high c-axis orientation both in template LSMO and top NKN layers. Ferroelectric measurements yield remnant polarization P, of 1.5 (.iC/cm2 and spontaneous polarization Ps of 7 jiC/cm2 at electric field strength of 130 kV/cm. At room temperature, dielectric permittivity e' and dissipation factor tancJhave been found to vary from 595 to 555 and 0.046 to 0.029 respectively in the frequency range of 0.4 to 20 kHz. At 10 kHz dielectric permittivity linearly increases from 410 to 650 in the temperature range 77 K to 415 K while the dissipation factor below 320 K does not exceed 3%.
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| 2. |
- Adolfsson, Erik, et al.
(författare)
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Matrix crack induced stiffness reductions in [(0(m)/90(n)/+theta(p)/-theta(q))(s)](m) composite laminates
- 1995
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Ingår i: Composites Engineering. - : Elsevier BV. - 0961-9526. ; 5:1, s. 107-123
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Tidskriftsartikel (refereegranskat)abstract
- Two- and three-dimensional linearly elastic glass/epoxy and carbon/epoxy laminates of the type [(0m/90n/ + theta(p)/ - theta(q))s]M containing periodically distributed matrix cracks have been analysed by aid of the finite element method. The presented finite element model enables modelling of several important thick and thin ply stacking sequences like cross-plies, angle plies and quasi-isotropic laminates. Due to periodicity it suffices to model a representative volume element of the laminate. The boundaries of this unit cell represent prospective crack surfaces. In this way varying crack configurations and crack densities could be simulated. By application of periodic boundary conditions the stiffness tensors for laminates containing different crack configurations were calculated. The results are presented in the form of reduced engineering stiffness parameters as functions of matrix crack densities for a thick quasi-isotropic [(0-degree/90-degrees/ + 45-degrees/ -45-degrees)s]M glass/epoxy laminate, a thick [(0-degree/90-degrees + 55-degrees/ - 55-degrees)s]M carbon/epoxy laminate and a thin (0-degree/ +45-degrees/ -45-degrees)s glass/epoxy laminate. Comparisons are made to an approximate analytic model developed previously. An excellent agreement between the analytic predictions and the finite element results was found for all cases under consideration.
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| 3. |
- Kornmann, Xavier, et al.
(författare)
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Nanocomposites based on montmorillonite and unsaturated polyester
- 1998
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Ingår i: Polymer Engineering and Science. - : Wiley. - 0032-3888 .- 1548-2634. ; 38:8, s. 1351-1358
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Tidskriftsartikel (refereegranskat)abstract
- The concept of nanoscale reinforcement provides opportunity for synthesis of new polymer materials with unique properties. Montmorilonite (MMT) was derived from bentonite, purified, activated by sodium ions and mixed with reacting unsaturated polyester (UP). X-ray and transmission electron microscopy data were in support of the formation of a partially delaminated nanocomposite material. At an MMT content of only 1.5 vol%, the fracture energy, GQ of the nanocomposite was doubled, 138 J/m2 as compared with 70 J/m2 for the pure UP.
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| 4. |
- Asp, Leif E., et al.
(författare)
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Effects of a composite-like stress state on the fracture of epoxies
- 1995
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Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 53:1, s. 27-37
-
Tidskriftsartikel (refereegranskat)abstract
- The strain to failure of a transversely loaded composite is much lower than for the pure matrix in uniaxial tension. Several studies of composites suggest the triaxial matrix stress state as one of the explanations. In order to investigate this experimentally, a triaxial tensile test previously used for rubbers (the poker-chip test) was successfully applied to four epoxies in the glassy state. The chosen specimen geometry mimicked the most severe stress state in the matrix as determined by finite element analysis of a transversely loaded glass-fiber/epoxy composite. The poker-chip strains to failure in the primary loading direction were 0.5-0.8%, whereas uniaxial strains to failure were 1.8-7%. The triaxial stress state in composite matrices may therefore by itself be a sufficient explanation for low values of transverse composite strains to failure.
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| 5. |
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| 6. |
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| 7. |
- Kifetew, G, et al.
(författare)
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The effect of drying on wood fracture surfaces from specimens loaded in wet condition
- 1998
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Ingår i: Wood Science and Technology. - 0043-7719 .- 1432-5225. ; 32:2, s. 83-94
-
Tidskriftsartikel (refereegranskat)abstract
- The study describes the effect of drying on fracture surfaces of Scots pinePinus silvestris L. Microtomed specimens of isolated and combined early-and latewood, in green and oven-dried/resoaked state were loaded to failure in uniaxial tension parallel to the grain. The fracture surfaces were studied using scanning electron microscopy (SEM). Both green early- and latewood samples showed rough fracture surfaces, which in latewood was dominated by intrawall failure. In the resoaked state, transwall failure dominated and fracture surfaces were more flat, indicating a more brittle fracture process. Although variation in the data was large, the strength of the resoaked samples were generally lower than those of paired green samples. The observations support irreversible cell wall damage formed during drying which severely affects the failure mechanism.
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| 8. |
- Varna, Janis, et al.
(författare)
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Effect of voids on failure mechanisms in RTM laminates
- 1995
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Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 53:2, s. 241-249
-
Tidskriftsartikel (refereegranskat)abstract
- Most studies on the effects of voids aim for determination of the relationship between void content and the fracture property of interest. The effect of voids on the mechanisms of transverse failure was therefore investigated for predominantly unidirectional resin transfer moulded laminates of glass-fibre-fabric/vinylester. Laminates with the highest average content of voids had a transverse strain to failure as high as 2% whereas low void content laminates failed at 0.3%. Only a few large and well-defined transverse cracks formed in low void content laminates before final failure. Multiple transverse cracks with irregular shape as well as numerous smaller cracks formed in the high void content laminates. The irregularity of these cracks resulted in lower stress concentration and stress level in the small amount of weft bundles orientated in the loading direction. A simple model was developed in order to demonstrate this and explain the high strain to failure of high void content laminates.
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| 9. |
- Varna, Janis, et al.
(författare)
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Effects of voids on damage mechanism in RTM laminates under tensile loading
- 1994
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Ingår i: Conference papers. - : Institute of Materials. ; , s. 8.1-8.5
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Konferensbidrag (refereegranskat)abstract
- The effect of voids on the transverse mechanical behavior was investigated for unidirectional resin transfer molded laminates of glass fiber fabric/vinylester. Laminates with the highest average content of voids had a transverse strain to failure as high as 2 percent whereas low void content laminates failed at 0.3 percent. Only a few large and well-defined transverse cracks formed in low void content laminates before final failure. Multiple transverse cracks with irregular shape as well as numerous smaller cracks formed in the high void content laminates. A simple model was developed in order to explain the high strain to failure of high void content laminates
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| 10. |
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| 11. |
- Andersons, J., et al.
(författare)
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Statistical model of coating fragmentation under equibiaxial load
- 1998
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Ingår i: Materials and Manufacturing Processes. - : Informa UK Limited. - 1042-6914 .- 1532-2475. ; 13:4, s. 597-602
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Tidskriftsartikel (refereegranskat)abstract
- A statistical model of coating cracking under equibiaxial tension is proposed based on a Weibull strength distribution for the coating. Crack length and spacing distributions are derived assuming that cracks initiate in random locations and propagate straight till stopping upon encountering a geometiical obstacle (another crack). The theoretical distributions are verified by comparing with simulated cracking patterns obtained by the Monte-Carlo method. An analysis of crack patterns of SiO2 coatings on a PET film under biaxial tension is performed. Qualitative agreement with the theoretical crack spacing distribution is observed.
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| 12. |
- Asp, Leif, et al.
(författare)
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Criterion for crack initiation in glassy polymers subjected to a composite-like stress state
- 1996
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 11, s. 1291-1301
-
Tidskriftsartikel (refereegranskat)abstract
- Three epoxy systems of interest as composite matrix materials are examined for their yielding and failure behavior under uniaxial, biaxial and triaxial stress states. Yield criteria applicable to glassy polymers, i.e. accounting for the hydrostatic stress effect on the deviatoric stress to yielding, are assessed. It is found that under stress states resembling those in matrix constrained between fibers, e.g. equibiaxial and equitriaxial tension, yielding is suppressed while brittle failure, presumably caused by crack growth from cavitation, occurs. A criterion for this mode of failure is proposed as the critical dilatational strain energy density. Experimental data are found to support this criterion.
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| 13. |
- Asp, Leif E., et al.
(författare)
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A biaxial thermomechanical disk test for glassy polymers
- 1997
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 37:1, s. 96-101
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Tidskriftsartikel (refereegranskat)abstract
- Failure criteria for polymers need to include effects from the stress state. For this reason, biaxial test results are of interest. However, biaxial test methods usually require expensive equipment. In the test method presented here, a disk of epoxy is bonded between a steel ring and a steel disk. The temperature is then lowered until fracture is observed. Experiments were performed on three different glassy epoxy polymers. The biaxial stress state was analyzed by finite element analysis and by an approximate analytical model. Experimental observations support the ability of the method to provide material property data. An approximate analytical model was found sufficiently accurate for stress analysis and determination of the stress state at failure.
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| 14. |
- Asp, Leif, et al.
(författare)
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Effects of fiber and interphase on matrix-initiated transverse failure in polymer composites
- 1996
-
Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 45:6, s. 657-665
-
Tidskriftsartikel (refereegranskat)abstract
- Failure initiation in polymer-matrix composites loaded transverse to the fibers is investigated by a numerical parametric study where the effects of constituent properties, interphase properties and thickness are examined. Failure initiation in the matrix only is studied, interfacial debonding not being considered. Two modes of failure - yielding and cavitation-induced brittle failure - are examined. A criterion for the cavitation-induced brittle failure has been proposed previously and failure prediction based on this criterion was found to agree with experimental data for a glass-fiber-reinforced epoxy. The present study shows that the elastic modulus of fibers has a large effect on the stress and strain to failure initiation. A rubbery interphase material is found in most cases to have a beneficial effect. The site at which failure initiates and the governing mode of failure initiation are also affected by the fiber modulus and the interphase properties
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| 15. |
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| 16. |
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| 17. |
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| 18. |
- Asp, Leif, et al.
(författare)
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Prediction of failure initiation in polypropylene with glass beads
- 1997
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 18:1, s. 9-15
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of glass bead content and residual stresses on failure initiation in isotactic polypropylene composites has been investigated by finite element analysis for the cases of interfacial debonding, plastic yielding, and cavitation. Residual thermal stresses are demonstrated to have a large effect on global failure initiation stress. Yielding and cavitation occur at higher global stresses than debonding. Modeling results, as well as previous experimental data, support debonding as the initial failure mechanism
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| 19. |
- Asp, Leif, et al.
(författare)
-
Prediction of matrix-initiated transverse failure in polymer composites
- 1996
-
Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 56:9, s. 1089-1097
-
Tidskriftsartikel (refereegranskat)abstract
- A study has been conducted of failure in unidirectionally-reinforced fiber composites loaded in tension normal to the fibers. The case considered is when this failure is governed by failure of the matrix rather than fiber/matrix debonding. Both yielding and cavitation-induced brittle failure of the matrix are considered. The latter mode of failure was suggested previously as the likely mode to occur in epoxies under stress states that are purely or nearly hydrostatic tension. Three fiber packing arrangements (square, hexagonal and square-diagonal) with different fiber volume fractions were studied numerically by a finite element method to determine the local stress states. It is found that cavitation-induced brittle failure occurs much before yielding in all cases. Experimental data taken from the literature support this finding.
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| 20. |
- Asp, Leif
(författare)
-
Transverse failure initiation in polymer composites
- 1995
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Transverse failure is one of the most important failure modes in polymer composites. The phenomenon often causes the first deviations from nonlinear laminate behavior. Also, in pressure vessels and pipes, fluid leakage through a path of transverse cracks is often the limiting design criterion. In the present work, experimental and theoretical studies focused on the micromechanical level have been carried out. The objective was to investigate transverse failure initiation in the matrix. The other major mechanism of failure initiation, fiber/matrix debonding, was not considered. The triaxial nature of the matrix stress state in glass fiber/epoxy was confirmed by finite element analysis. Experimental results for glassy epoxies subjected to composite-like stress states demonstrated large reductions in strain to failure as compared with uniaxial loading. The triaxial stress state is therefore by itself a sufficient explanation for the low transverse strain to failure in polymer composites. Plastic yielding in the matrix was demonstrated not to be the cause of failure initiation. Instead cavity induced cracking was suggested as a failure mechanism. A criterion was proposed based on a critical value for the dilatational energy density. Comparison with experimental results for epoxies subjected to a variety of multiaxial load-cases supported the criterion. Additional support was obtained from comparison with experimental results in the literature for transverse failure of glass fiber/epoxy at different fiber contents. Although the epoxy matrix was different from those in the present study, general trends in data were supported by predictions based on the criterion and finite element analysis. Thermal residual stresses were found to be important for high fiber contents. Based on the criterion, a conservative estimate of composite strain to failure was obtained. This is reasonable since the criterion predicts initiation, not final failure. Based on the model, effects from changes in constituent properties were examined in a parametric finite element analysis. Fiber modulus was found to strongly influence transverse failure. Introduction of a third phase interphase between fiber and matrix was also investigated. Beneficial results on transverse failure strain caused by matrix initiation was observed for thin rubbery interphases.
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| 21. |
- Berglund, Lars A., et al.
(författare)
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Effect of intralaminar toughness on the transverse cracking strain in cross-ply laminates
- 1991
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Ingår i: Advanced Composite Materials. - 0924-3046 .- 1568-5519. ; 1:3, s. 225-234
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Tidskriftsartikel (refereegranskat)abstract
- The effect of toughness on the onset of transverse cracking was studied in [0m/90n]s cross-ply laminates. Experimental observations indicated that carbon fiber composites based on tougher matrices show weaker constraint effect although their mechanical cracking strain improves in the low constraint region. From LEFM theory it was predicted that the total transverse cracking strain in a thick 90-deg layer of a cross-ply laminate is 1 . 58 times higher than the transverse failure strain of the unidirectional composite. This was verified by experimental data. The LEFM-based model by Isida was applied to the transverse cracking problem in order to predict the total transverse cracking strain for different cross-ply laminates. Model predictions showed good agreement with experimental data for the brittle matrix T300/934 carbon fiber composite. However, for carbon fiber composites the predictions did not describe the experimentally observed weakening of the constraint effect with increasing toughness.
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| 22. |
- Berglund, Lars A., et al.
(författare)
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Fracture of epoxies subjected to uniaxial and triaxial stress fields
- 1994
-
Ingår i: Deformation, yield and fracture of polymers. - : Institute of Materials.
-
Konferensbidrag (refereegranskat)abstract
- The uniaxial fracture properties of dogbone specimens are often used to estimate the performance of polymers in other loading situations, e.g. composite matrices. Although triaxial stress states are likely to affect the fracture behavior, this effect is difficult to quantify. In the present study, a method previously used for rubbers (the poker-chip method) was used to subject epoxies to a triaxial tension stress field. The method was successfully used and its validity supported by stress analysis and fractography. Results showed the fracture behavior of the poker-chip specimen to be dramatically different compared to the uniaxial case. A general decrease in strain to failure was observed for the triaxial test data. Yield criteria, shown to predict yielding in epoxies for biaxial stress states, suggest some other mechanism to control failure of the poker-chip specimen
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| 23. |
- Berglund, Lars A., et al.
(författare)
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Transverse Cracking and Local Delamination in [04/90n]s and [90n/04]s Carbon Fiber/Toughened Epoxy Laminates
- 1992
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Ingår i: Journal of reinforced plastics and composites (Print). - : SAGE Publications. - 0731-6844 .- 1530-7964. ; 11:6, s. 643-660
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Tidskriftsartikel (refereegranskat)abstract
- [04/90n]s and [90n/04 ]s laminates were used to study 90°-layer failure mechanisms in HTA/6376 carbon fiber/toughened epoxy. The different stacking sequences were chosen to vary the stress states, since experimental results show differences in failure strains as well as in the local delamination behavior at the crack tip. Differences in the onset of transverse cracking and local delamination behavior between [04/90m]s and [90 n/04]s laminates was discussed using a model based on linear elastic fracture mechanics (LEFM). Although quantitative LEFM-predictions do not agree with data for the carbon fiber/toughened epoxy investigated, the general predictions for differences in and causes of local delamination behavior were confirmed by experimental data. The fact that the main features were correctly predicted, encourages the development of some modified LEFM-approach to describe toughened matrix composites.
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| 24. |
- Berglund, Lars A., et al.
(författare)
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Transverse cracking in laminated composites
- 1995
-
Ingår i: Advanced technology for design and fabrication of composite materials and structures. - : Kluwer Academic Publishers. - 0792333039 ; , s. 191-201
-
Konferensbidrag (refereegranskat)abstract
- Models based on variational analysis are compared with respect to the elastic properties of cross-ply laminates with transverse cracks. All models use the principle of minimum complementary energy for calculation of the stress state in a laminate element between two cracks, as was the case with the original model by Hashin (1985). The models are based on assumptions of different degree of accuracy, where the most refined model was developed by the present authors. Experimental data for the longitudinal stiffness are compared with predictions for glass fiber/epoxy cross-ply laminates. The agreement between model predictions and data is good provided the initial assumptions of the stress state are sufficiently accurate
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| 25. |
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| 26. |
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| 27. |
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| 28. |
- Ericson, M.L., et al.
(författare)
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A method of measuring energy dissipation during crack propagation in polymers with an instrumented ultramicrotome
- 1996
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Ingår i: Journal of Materials Science. - 0022-2461 .- 1573-4803. ; 31:3, s. 655-662
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Tidskriftsartikel (refereegranskat)abstract
- In order to characterize very local energy dissipation during crack propagation in polymers, an ultramicrotome was instrumented to measure the energy dissipated during sectioning. The work to section per unit area, W s, was measured for five different amorphous polymers [polymethyl methacrylate (PMMA), polystyerene (PS), polycarbonate (PC) and two epoxy resins] in the glassy state. When the section thickness was varied between 60 and 250 nm, W s varied between 15 and 100 Jm-2, depending on the material and section thickness. The method and the results are compared with other methods used for determining the energy dissipation at a local level as well as at a macroscopic level in polymers. The differences between different polymers were found to be contradictory to macroscopic fracture toughness, G lc, measurements. The material that showed the highest W s had the lowest G lc values reported. Possible mechanisms for energy dissipation during sectioning are also discussed.
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| 29. |
- Ericson, Mats L., et al.
(författare)
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Design and potential of instrumented ultramicrotomy
- 1997
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Ingår i: Polymer. - 0032-3861 .- 1873-2291. ; 38:17, s. 4485-4489
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Tidskriftsartikel (refereegranskat)abstract
- Ultramicrotomes are generally used for preparation of very thin sections for transmission electron microscopy. Recently it has been shown that when the sample holder of the ultramicrotome is instrumented with a force transducer, it is possible to measure the very small sectioning force during sectioning, and calculate the energy dissipated. In the present work, the instrumentation is further improved. The new sample holder, which uses two piezo-electric force transducers can measure two force components simultaneously. It is not only robust and stiff, but it also shows high sensitivity and reproducibility. It is possible to detect sectioning forces lower than 0.1 mN. The method is demonstrated on two amorphous polymers, poly(methyl methacrylate) and epoxy. Fracture energies in the same order of magnitude as theoretical predictions from chemical bond fracture only are recorded. It is therefore suggested that the method of instrumented ultramicrotomy is a useful tool when information on covalent bond density is needed. Potential future applications are identified including research on nano-scale fracture, characterization of molecular anisotropy and developments of the ultramicrotome
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| 30. |
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| 31. |
- Ericson, Mats L., et al.
(författare)
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Processing and mechanical properties of orientated preformed glass-mat-reinforced thermoplastics
- 1993
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 49:2, s. 121-130
-
Tidskriftsartikel (refereegranskat)abstract
- The stiffness and strength of moulded, glass-mat-reinforced thermoplastics (GMT) components would be increased by the presence of highly orientated fibres at critical locations. A previously described method to produce preformed GMT materials was therefore further developed to make orientation of the fibres in the preform possible. Two ‘orientation plates' were used to orientate the fibres during spray-up of a glass-fibre/polyethylene preform. The preform was then heated by hot gas and compression moulded. The ratio of the highest and lowest stiffnesses of a given plate was in the range of 27 to 38. Micromechanics equations were used with classical lamination theory to design a model laminate with stiffness properties in close agreement with experimental data. Reasons for the relatively wide fibre orientation distribution and low fibre length efficiency factor obtained for the model laminate are discussed and improvements suggested.
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| 32. |
- Gamstedt, E.K., et al.
(författare)
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Fatigue damage mechanisms in unidirectional carbon-fibre-reinforced plastics
- 1999
-
Ingår i: Journal of Materials Science. - 0022-2461 .- 1573-4803. ; 34:11, s. 2535-2546
-
Tidskriftsartikel (refereegranskat)abstract
- The fatigue life behaviour and the underlying micromechanisms have been studied in two different Types of unidirectional carbon-fibre-reinforced plastics loaded in tension-tension along the fibre direction. The carbon fibres (AS4) were the same in the two composite systems. One thermoplastic matrix (polyetheretherketone, PEEK) and one thermosetting matrix (epoxy toughened with a thermoplastic additive) were used. The macroscopic fatigue behaviour was characterised by fatigue life diagrams. Surface replicas were taken intermittently during the course of the fatigue tests to monitor the active fatigue damage micromechanisms. The thermoset based composite showed a higher fatigue resistance with few microcracks initiated at distributed fibre breaks growing at a decelerating rate. The thermoplastic composite had a more pronounced fatigue degradation with a steeper fatigue life curve, which was caused by widespread propagating debonds and matrix cracks. The use of a tougher and more ductile matrix results in an inferior fatigue life performance, due to a more widely distributed accumulation of damage that propagates at a higher rate.
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| 33. |
- Gamstedt, E.Kristofer, et al.
(författare)
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Fatigue mechanisms in unidirectional glass-fibre-reinforced polypropylene
- 1999
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 59:5, s. 759-768
-
Tidskriftsartikel (refereegranskat)abstract
- Polypropylene (PP) and polypropylene modified with maleic anhydride (MA-PP) reinforced by continuous longitudinal glass fibres have been investigated. The most prominent effect of the modification with maleic anhydride in the composite is a stronger fibre/matrix interface. The effects of interfacial strength on fatigue performance and on the underlying micromechanisms have been studied for these composite systems. Tension-tension fatigue tests (R=0.1) were carried out on 0 glass-fibre/PP and glass-fibre/MA-PP coupons. The macroscopic fatigue behaviour was characterized in terms of stiffness reduction and fatigue-life curves. The results showed that the longitudinal Young's modulus degraded more rapidly for glass-fibre/PP, which was caused by a higher degree of damage growth and accumulation. The improvement in monotonic strength was negligible, but the fatigue life was prolonged by about one decade for the composite with the stronger interface by use of the maleic-anhydride grafted polypropylene matrix. During the fatigue testing, the microscopic mechanisms were monitored intermittently by a surface replication technique. From microscopic observations, it could be concluded that the better fatigue resistance of glass-fibre/MA-PP can be attributed to the greater interfacial strength and the resistance to debond propagation
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| 34. |
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| 35. |
- Gamstedt, E.K., et al.
(författare)
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Micromechanisms in tension-compression fatigue of composite laminates containing transverse plies
- 1999
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 49:2, s. 167-178
-
Tidskriftsartikel (refereegranskat)abstract
- For both monotonic and fatigue loading conditions, debonding is the subcritical micromechanism which leads to transverse cracking and ultimately influences final failure of the composite structure. Previous studies show that tension-compression fatigue is more detrimental than tension-tension fatigue to transverse and multidirectional laminates. By analysing the debonding mechanisms and modelling thereof, the macroscopic fatigue behaviour can be better understood. Also, the dominant crack-propagation mode can be identified which may be of use in selection of constituent material properties to optimise fatigue resistance. In this study, glass-fibre-reinforced vinyl-ester was used. The adverse effect of compressive load excursions was verified by counting the transverse cracks in cross-ply laminates. The mechanisms were studied in low-cycle fatigue of a specimen containing a single transverse fibre. Compressive load cycles led to significantly increased debond growth. In tension, contact zones developed at the crack tips for sufficiently large debonds. Because of the mismatch in elastic properties, an opening zone appeared at the tips of the interfacial crack when the same debond was subjected to a compressive load. Since debond propagation is more susceptible to mode I loading, the sensitivity to tension-compression fatigue is explained by the effective crack-tip opening in compressive loading for sufficiently large debond cracks. This has also been verified by finite-element analysis.
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| 36. |
- Gamstedt, Kristofer
(författare)
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Fatigue damage mechanisms in polymer matrix composites
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymer matrix composites are finding increased use in structural applications, in particular for aerospace and automotive purposes. Mechanical fatigue is the most common type of failure of structures in service. The relative importance of fatigue has yet to be reflected in design where static conditions still prevail. The fatigue behavior of composite materials is conventionally characterized by a Wöhler or S-N curve. For every new material with a new lay-up, altered constituents or different processing procedure, a whole new set of fatigue life tests has to be repeated for such a characterization. If the active fatigue damage micromechanisms and the influence of the constituent properties and interface were known, it would be possible, at least qualitatively, to predict the macroscopic fatigue behavior. A study of the fatigue damage mechanisms would also give indications of the weakest microstructural element, which is useful information in materials selection for improvement in service properties. In tensile fatigue of a multidirectional laminate, the critical elements are the longitudinal plies which are the last to fail. Although failure of neighboring off-axis plies as well as delamination will influence the fatigue process, an understanding of the behavior of the longitudinal plies forms an important foundation. Effects of plies of other directions may then be interpreted based on this foundation. Fatigue of longitudinal plies is therefore focused on in the present study. The underlying fatigue damage mechanisms were investigated for unidirectional O' carbon fiber reinforced plastics (CFRP) and glass fiber reinforced polypropylene (GF/PP) in tension-tension fatigue. By use of a surface replication technique the evolution of fatigue damage could intermittently be monitored during the course of fatigue testing. In the CFRPS, the matrix was an epoxy resin or polyetheretherketone (PEEK). In the GF/PP system, the matrix was modified with maleic anhydride (MA) to achieve a stronger fibermatrix interface. The macroscopic fatigue behavior was characterized by fatigue life diagrams. A statistical method has been devised to systematically characterize fatigue life data in terms of fatigue life diagrams. On the microscopic level, the CF/epoxy and GF/MA-PP composites have relatively strong interfaces and showed localized and scarce fiber breaks from which matrix cracks propagated perpendicular to the fiber direction. In CF/epoxy, fiber bridged cracks with squeezed fiber tips appeared. Conversely, CF/PEEK and GF/PP have weaker interfaces, and the principal mechanisms were extensive and distributed debonding or longitudinal matrix cracking followed by further fiber breakage. Macroscopically, the weak interface composites showed shorter fatigue lives and more rapid fatigue degradation. This suggests that higher interfacial strengths lead to improved fatigue performance. Modeling studies were undertaken for the two observed mechanisms; debonding from a fiber break, and fiber bridged cracking. The stochastic breakage of fibers next to a growing debond was parametrically investigated with a shear lag model. The stress profile in the surviving fibers becomes attenuated and more distributed as the debonds grow. This results in longer axial distances between fiber breaks, and hence a more jagged and uneven crack propagation. A larger variability in strength along the fibers has basically the same break distributing effect. With a more homogeneous stress distribution caused by long debonds, the variability in fiber stress at failure of the intact fibers decreases. This can explain the experimentally observed lower scatter in fatigue life of composites exhibiting a more homogeneous distribution of damage caused by debonding. Furthermore, the experimental results of fiber bridged cracking was modeled with a fracture mechanics approach. The crack growth curve can be plotted in terms of the effective stress intensity factor where the contribution of the cohesive crack surface forces from the bridging fibers are taken into account. This curve falls somewhat closer to that of the neat matrix material compared to the unbridged crack, but the difference is still considerable. Besides the fiber bridging, there should therefore be other active toughening mechanisms that slows the crack propagation down to account for the fatigue resistant behavior of the tested material. In fatigue of multidirectional laminates, tension-compression loading has shown to be more detrimental than tension-tension loading. The reason for this behavior has not been entirely clarified. The adverse effect of the compressive load excursions is partly caused by the formation of transverse cracks. This was verified by counting transverse cracks in cross-ply laminates. Since debonding is the subcritical mechanism which leads to transverse cracking and eventually influences ultimate failure, the debonding was studied in low cycle fatigue of a single transverse fiber. In tension, contact zones developed at the crack tips for sufficiently large debonds. Due to the inherent geometry and the mismatch in elastic properties of the constituents, an opening zone appeared at the crack tips of the debond in compression. This was also verified by finite element analysis. Since debond propagation is more suseptible to mode I loading, the sensitivity to tension-compression loading is explained by the effective opening zone in compression.
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| 37. |
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| 38. |
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| 39. |
- Holmberg, J.A., et al.
(författare)
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Manufacturing and performance of RTM U-beams
- 1997
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 28:6, s. 513-521
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Tidskriftsartikel (refereegranskat)abstract
- Geometrical limitations and robustness of resin transfer moulding were investigated for single curved laminates. The design of experiments approach was used to determine how the out of plane tensile strength is affected by variations in performing method, radius, fibre content and vacuum assistance. U-beams with an inner radius of 0.8 mm were manufactured and demonstrated good mechanical performance. The strength of these beams was in fact less sensitive to defects than the strength of beams with a 5 mm radius. Different preforming methods can have a strong influence on beam strength due to effects on the void content in the finished laminates. The particular preforming problems encountered are described and possible solutions are suggested and tested. For example utilisation of vacuum assistance during mould filling reduces void content, which in turn tends to reduce the sensitivity of the beam strength to other defects such as wrinkling.
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| 40. |
- Holmberg, J.A., et al.
(författare)
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Micromechanisms of delamination failure in RTM U-beams
- 1997
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 28:8, s. 709-717
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Tidskriftsartikel (refereegranskat)abstract
- In a previous study, resin transfer moulded U-beams were subjected to opening tensile loads applied to the flanges. The failure behaviour was sensitive to the void content of the material. In the present study, we use optical microscopy of polished edges to observe mechanisms of failure in-situ as delaminations develop in the beam radius. Descriptions of these failure mechanisms are presented. Voids were found to facilitate the growth phase of the delamination process in the beam radius. The mechanisms described will be used in a forthcoming stress analysis to explain the experimentally observed failure behaviour of beams with different radii and void contents.
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| 41. |
- Holmberg, J. Anders
(författare)
-
Resin transfer moulded composite materials : processing, structure, property relationships
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis contains analysis of the performance of polymer composites manufactured by resin transfer moulding (RTM). The thesis consists of six papers, where the first two deals with the tensile strength of composites reinforced with randomly oriented glass fibres, which still is the most common reinforcement in RTM manufacturing. The following four papers covers aspects of manufacturing induced defects, out-of-plane strength and dimensional tolerances of carbon fibre reinforced high temperature epoxy, which is a relatively new application for RTM. The apparent tensile strength of random fibre composites is usually significantly higher if measured in bending than in tension. In Paper A, it is shown that the deduction of strength from flexural tests is complicated by several factors, leading to large errors if not accounted for. Approximate closed form expressions are derived to account for friction at the support rollers, nonlinear material behaviour and large deflections and suggested for future use. It is then shown that properly deduced flexural strengths can be predicted from tensile test data by Weibull theory, if applied on effective fibre stress. In Paper B, the Weibull theory is extended to biaxial load cases for random fibre composites. Failure strain predictions based on tensile test data are in good agreement with experimental results from three- and four-point bending and biaxial bending. In Paper C, the robustness and geometrical limitations of resin transfer moulding were investigated for single curved laminates. The design of experiments approach was used to determine how the out of plane tensile strength is affected by variations in preforming method, radius, fibre content and vacuum assistance. Carbon fibre /epoxy U-beams with an inner radius of 0.8 mm were manufactured and demonstrated good mechanical performance. The strength of these beams were in fact less sensitive to defects than the strength of beams with a 5 mm radius. Beams with a 5 mm radius were very sensitive to the presence of voids. A good correlation was obtained between local void content and beam strength, indicating that void content is the only important defect. In-situ optical microscopy was used in Paper D to observe the micromechanisms of failure as delaminations develop in the beam radius. Beams with and without voids were studied to determine how the voids affect the failure initiation. The failure mechanism proposed, based on the observations, involves the position of the voids and the microstructure of the reinforcement. The material system investigated was a plain carbon fibre weave impregnated with epoxy by resin transfer moulding. The voids were primarily located in the matrix rich regions created by the crimp in the reinforcement. Voids were found to have no effect on the initial formation of a debond crack at the interface between adjacent fibre bundles. Instead, the voids increase the stress concentration at the crack tip and therefore crack growth starts at a lower load. In Paper E this issue is brought further by numerical modelling of an initial crack adjacent to a void. The calculated increase in energy release rate, due to the presence of a void, suggests a strength reduction in qualitative agreement with the experimental results presented in Paper C. The reasons for why the smaller radius exhibits a lower defect sensitivity is also discussed in Paper E. Finally, Paper F is a study of the springback phenomenon. The mechanisms responsible for springback during a typical RTM cure schedule are discussed. An approximate model for prediction of springback is developed, incorporating the effects of chemical shrinkage and the phase change at the glass transition temperature. The effect from chemical shrinkage is estimated from fibre and matrix properties and it is demonstrated both theoretically and experimentally that the effect from chemical shrinkage is significant. Model predictions of springback angles are in good agreement with experimental data from the beam series presented in Paper C.
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| 42. |
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| 43. |
- Hui, C.Y., et al.
(författare)
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Estimation of interfacial shear strength : an application of a new statistical theory for single fiber composite test
- 1999
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 59:13, s. 2037-2046
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Tidskriftsartikel (refereegranskat)abstract
- A simple procedure is proposed for estimating the effect of different sizings on the strength of fiber/matrix interface. This procedure is based on a newly developed statistical theory for single-fiber composite tests. This procedure is used to evaluate the interfacial shear strength of HTA carbon fiber in an epoxy matrix with two different sizings. It is shown that if the experiment data are presented in the form of the normalized variables, and σ/σc, the two sets of data corresponding to the two different sizings can be collapsed into a single curve which can be obtained theoretically. Excellent agreement between experiments and theory is found. An argument is presented for the use of the interfacial shear strength as an engineering parameter to characterize interface adhesion. We also investigate the effect of residual stresses due to thermal-expansion coefficient mismatch between the fiber and the matrix on the interpretation of single-fiber composite tests. We found that the experimentally determined Weibull parameters are very sensitive to the effect of these residual stresses and must be taken into consideration when interpreting single-filament composite tests.
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| 44. |
- Jacobs, Edgard, et al.
(författare)
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On the use and limitations of concentric cylinder models for polymer matrix composites
- 1999
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Ingår i: ICCM-12 Europe. - Tours : ICCM. - 2951452624
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Konferensbidrag (refereegranskat)abstract
- Models to calculate the stiffness matrix of a unidirectional composite involve often the use of a concentric cylinder as unit cell. In this paper, an existing concentric cylinder model of Pagano&Tandon has been extended, by applying the Mori-Tanaka and generalised self-consistent scheme theories to this basic model. The aim is to improve the stiffness prediction for three phase composite systems. It will also be pointed out that, although improvements in stiffness prediction can be achieved, the use of these concentric cylinder models for local stress state analysis in polymer matrix composites is rather questionable.
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| 45. |
- Jekabsons, Normunds
(författare)
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Micromechanics approach to stiffness characterization of woven composites
- 1999
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Thesis consider the stiffness of woven composites, methods of its calculation and stiffness reduction due to damage accumulation during the service. First, reliable methods for stiffness calculation of undamaged woven composites are considered. There are a large amount of simple approximate methods available in literature. They are very convenient to use but the error introduced by used assumptions may be considerable. In the first part of thesis a detailed investigation of this question is performed comparing approximate results with reiterated homogenization method. The second part of thesis deals with carbon fiber reinforced ceramic matrix composites and with the stiffness reduction due to damage accumulation. The morphology of the considered porous plain weave composite is complicated by stitches (fiber bundles) in the out-of-plane direction. Experimental data are presented showing dramatic changes in all elastic properties due to loading. FEM modeling is performed in order to analyze the initial stiffness of this material and its reduction versus the determined number of microcracks. An analytical model convenient for use is derived that gives results rather close to those obtained by finite element method. The Fracture Mechanics approach is used to estimate the critical energy release rate G_IC for several damage modes.
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| 46. |
- Joffe, Roberts, et al.
(författare)
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Acoustic emission from composite laminates with simple damage characteristics
- 1995
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Ingår i: Fifth International Symposium on Acoustic Emission from Composite Materials, July 10-14, 1995, Sundsvall's KonferensCenter, Hotel Sundsvall, Sundsvall Sweden. - Columbus, Ohio : American Society for Nondestructive Testing. - 157117012X ; , s. 179-186
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Konferensbidrag (refereegranskat)abstract
- Interpretation of acoustic emission (AE) signals in terms of damage mechanisms in composite materials and structures is often difficult. This is because of the multitude of damage mechanisms. In the present study, glass fiber/epoxy (GF/EP) (0m/90n)s cross-ply laminates were studied in unixial tensile loading. Since the damage mechanisms in such laminates are primarily matrix cracking, acoustic emission interpretation was facilitated. Real-time optical microscopy and AE studies were conducted in order to identify details of the damage development. It was found that a multitude of secondary cracking events occurred at higher strains. These events may explain why stress analysis models based on straight and regularly spaced transverse cracks are not successful. In addition, it was clearly demonstrated that in order to interpret damage development with some confidence from AE-data, direct observations of damage events must also be performed.
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| 47. |
- Joffe, Roberts, et al.
(författare)
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Analysis of single fiber fragmentation data
- 1995
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Ingår i: Deformation and fracture of composites : 3rd International conference. - London : Institute of metals. ; , s. 126-131
-
Konferensbidrag (refereegranskat)
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| 48. |
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| 49. |
- Joffe, Roberts, et al.
(författare)
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Analytical modeling of stiffness reduction in symmetric and balanced laminates due to cracks in 90° layers
- 1999
-
Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 59:11, s. 1641-1652
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Tidskriftsartikel (refereegranskat)abstract
- Stiffness reduction in [S,90n]s symmetric laminates, containing orthotropic sub-laminates (S) and cracked 90° layer, is analyzed. Closed-form expressions relating stiffness changes to the transverse crack density are derived. They contain only material properties, laminate geometry and a stress-perturbation function that is proportional to the normalized average crack-opening displacement. Stress-distribution models [shear lag, based on variational approach, and finite-element analysis (FEA)] are adopted for the [S,90n]s configurations and used to calculate the stress-perturbation function. Predictions are compared with experimental data for [θ, 904]s θ=0, 15, 30, 40 glass-fiber/epoxy-resin laminates. Generally the FEA model slightly underestimates stiffness reduction whereas both of the variational models used lead to similar results, slightly lower than experimental. Even the shear-lag model may be successfully used if the shear-lag parameter is first obtained from fitting test results for cross-ply laminate of the same material.
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| 50. |
- Joffe, Roberts
(författare)
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Damage accumulation and stiffness degradation in composite laminates
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the presented thesis analytical modeling of elastic properties degradation and modeling of damage evolution in composite laminates is performed. Transverse cracking initiation on a fiber/matrix level is briefly studied by testing model composites and cross-ply laminates. Analysis of obtained results show limitation in application of model composites. Models originally obtained for cross-ply laminates (shear lag, variational) are adapted for more general lay-up of laminates [S,90n]s, where S can be any balanced sub-laminate. Closed form expressions to calculate elastic properties changes as a function of crack density are derived. These models are verified by comparing prediction with experimental results. In this work damage evolution modeling is performed by Monte-Carlo simulation. Two approaches that use different failure criteria are executed. One method employs strength failure criteria and second uses fracture mechanics approach. Experimental data for [+Q/-Q/90/90/90/90]s laminates are used to verify correctness of the applied models. Influence of the thickness of 90-layer on damage evolution is also analyzed. The investigation of laminates containing off-axis layers is performed. The results show that degradation of the shear modulus occurs in those layers during the loading. Degradation of the shear modulus influences longitudinal modulus of the whole laminate as well as stress distribution in 90-layer.
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