| 1. |
- Allen, D.H., et al.
(författare)
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Modeling the viscoelastic response of GMT structural components
- 2001
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 61:4, s. 503-515
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Tidskriftsartikel (refereegranskat)abstract
- A glass-mat-reinforced thermoplastic (GMT) composite material has been fabricated from randomly oriented continuous glass fibers embedded in a polypropylene matrix. The mechanical constitution of this composite has been characterized by using a linear viscoelastic micromechanically based material model. This material model has subsequently been implemented to several finite-element computer codes for analysis of structural components fabricated from polypropylene GMT. In this paper several example problems have been studied in order to determine the applicability of this modeling approach to predicting time-dependent deformations due to creep in GMT components. These example problems have been solved by utilizing two commercially available codes: ABAQUS, and ANSYS. Furthermore, results obtained with the codes have been compared to both analytic and experimental results, with varying degrees of success. The paper details these results for each of the example problems considered herein. © 2001 Elsevier Science Ltd. All rights reserved.
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| 2. |
- Andersons, J., et al.
(författare)
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Glass fibre strength distribution determined by common experimental methods
- 2002
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 62:1, s. 131-145
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Tidskriftsartikel (refereegranskat)abstract
- The tensile strength of brittle fibres is routinely described by the Weibull distribution. The parameters of the distribution can be obtained from tests on single fibres and fibre bundles or from model composite tests. However, there is growing evidence that the distribution parameters obtained by different experimental techniques differ systematically. In order to investigate the possible causes of such discrepancies, single-fibre tension, fibre bundle, and single-fibre fragmentation tests are employed in this study to obtain strength distribution of commercial E-glass fibres. The results reveal parameter dependence on the approach used to extract the distribution parameters from experimental data. Particularly, in the case of single-fibre tension tests, the shape parameter obtained from average fibre strength vs. length data is larger than that obtained at a fixed gauge length. It is assumed that the apparent fibre strength scatter is caused by both the inherent flaw structure along a fibre and by the fibre-to-fibre strength variability within a batch, due to slightly differing processing and handling history of the fibres. Fibre fragmentation test results are used to derive the Weibull distribution parameters applicable to the fibre batch. The strength distribution obtained is compared with strength data for the single fibres, and reasonably good agreement is observed.
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| 3. |
- Andersson, B., et al.
(författare)
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Micro- and meso-level residual stresses in glass-fiber/vinyl-ester composites
- 2000
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Ingår i: Composites Science And Technology. - : Elsevier Science Ltd, Exeter, United Kingdom. - 0266-3538 .- 1879-1050. ; 60:10, s. 2011-2028
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Tidskriftsartikel (refereegranskat)abstract
- Residual stresses in glass-fiber composites were studied on the micro and meso scales by computational and experimental methods. Transmitted polarized light images of thin sections were compared with 3D finite-element solutions of a sample containing 1410 fibers. Calculated point-wise stresses were derived from a linear thermoelastic model with negligibly small numerical errors. Regions with calculated maximum compressive stresses showed good agreement with experimentally observed optical bands. A material with poor interfacial adhesion showed weaker optical effects indicating fiber/matrix debonding. On the basis of these results it stems likely that irreversible matrix deformation and debonding can take place in the curing phase.
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| 4. |
- 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
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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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| 5. |
- 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
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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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| 6. |
- Asp, Leif, et al.
(författare)
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Effects of fiber and interphase on matrix-initiated transverse failure in polymer composites
- 1996
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Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 45:6, s. 657-665
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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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| 7. |
- Asp, Leif, et al.
(författare)
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Prediction of matrix-initiated transverse failure in polymer composites
- 1996
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Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 56:9, s. 1089-1097
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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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| 8. |
- Asp, Leif
(författare)
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The effects of moisture and temperature on the interlaminar delamination toughness of a carbon/epoxy composite
- 1998
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 58:6, s. 967-977
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Tidskriftsartikel (refereegranskat)abstract
- HTA/6376C composite has been investigated for influence of temperature and moisture content on the interlaminar delamination toughness in mode I, mode II and mixed mode conditions. Dry and moisture-saturated specimens were tested over the temperature range - 50 °C to 100 °C. Evaluation methods based on load/displacement and load measurements were employed. In pure mode II the critical strain-energy release rate drops with moisture content and increase in temperature. In mixed mode the critical strainenergy release rate also decreases with moisture content, but no general trends in the dependence on temperature is observed. The critical strain-energy release rate in pure mode I is unaffected by changes in moisture content and was found to increase slightly at elevated temperatures. During crack propagation, enhanced fiber bridging due to increases in temperature and moisture content promotes R-curve behavior in the pure mode I tests. The resulting mode mixity of the mixed-mode bending (MMB) tests is found to be severely affected by the evaluation methods. Methods based on load measurements only are considered to give unreliable strain-energy release rates as the measured compliance/displacement relationships were found to be non-linear even prior to crack growth. Further studies are needed to assess the mixed-mode ratio in the MMB test.
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| 9. |
- Borg, Rikard, et al.
(författare)
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Modeling of delamination using a discretized cohesive zone and damage formulation
- 2002
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 62:10-11, s. 1299-1314
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Tidskriftsartikel (refereegranskat)abstract
- Delamination initiation and growth are analyzed by using a discrete cohesive crack model. The delamination is constrained to grow along a tied interface. The model is derived by postulating the existence of a maximum load surface which limits the adhesive forces in the process zone of the crack. The size of this maximum load surface is made dependent on the amount of dissipated crack opening work, such that the maximum load surface shrinks to zero as a predefined amount of work is consumed. A damage formulation is used to reduce the adhesive forces. Mode I, II and III loading or any combined loading is possible. An analytical solution is obtained for a single mode opening and the implications of this result on the governing equations is discussed. The delamination model is implemented in the finite element code LS-DYNA and simulation results are shown to be in agreement with experimental results. © 2002 Elsevier Science Ltd. All rights reserved.
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| 10. |
- Borg, Rikard, 1962-, et al.
(författare)
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Simulating DCB, ENF and MMB experiments using shell elements and a cohesive zone model
- 2004
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 64:2, s. 269-278
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Tidskriftsartikel (refereegranskat)abstract
- A delamination model for shell elements is presented. It consists of an adhesive penalty contact formulation for initially tying shells together and a cohesive zone model for degrading the adhesive forces. An adhesive contact used between shell elements has to account for the thickness offset, such that the rotational degrees of freedom in the shell elements are included in the algorithm. This is considered in the present contact model and the complete delamination model is implemented in the explicit Finite Element code LS-DYNA. By preventing delamination growth the delamination model can be turned into a tied contact. As such it is used in two FE-models, where plates are bonded together and subjected to various loads. The adhesive penalty contact performs well. The complete delamination is validated by simulating the Double Cantilever Beam, End-Notch Flexural and Mixed Mode Bending setups, and the results are shown to be in agreement with experimental data.
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