| 1. |
- Alfredsson, K. Svante, et al.
(författare)
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Stability of beam-like fracture mechanics specimens
- 2012
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Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 89, s. 98-113
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Tidskriftsartikel (refereegranskat)abstract
- Test specimens used to determine the interlaminar strength of composites as well as the strength of adhesive layers are known to occasionally suffer from instability. Thus, even though the experiments are performed under controlled load-point displacement, the experiments are terminated prematurely by unstable crack propagation. Often there exists a critical crack length which must be exceeded in order to obtain stable crack propagation. In this paper, a general method to assess the stability of beam-like fracture mechanics specimens is developed. Both systems subjected to a single load and the more general situation with several independent loads are treated. A simple formula is derived for the critical crack length for one-parameter loading. The only parameter necessary as input is the compliance of the un-cracked specimen. For the case of non-proportional loading, stability is determined by studying the eigenvalues of a symmetric matrix. Other findings quantified in the paper are the effect of orthotropy, the influence of a flexible interphase layer and the influence of the compliance of the loading device.
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| 2. |
- Biel, Anders, et al.
(författare)
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A Critical Study of an Alternative Method to Measure Cohesive Properties of Adhesive Layers
- 2012
-
Ingår i: Proceedings of the 19th European Conference on Fracture. - : Kazan Scientific Centre of the Russian Academy of Sciences. - 9785905576188
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Konferensbidrag (refereegranskat)abstract
- A perfect experiment is only sensitive to the properties to be analysed. However, evaluation of experimental results is always based on assumptions. Depending on the assumptions, the derived results are more or less correct. In this paper a method based on linear elastic fracture mechanics is compared to a method based on the path independence of the J-integral and the assumptions of the existence of a cohesive zone. Contrary to the other methods, the J-integral method only rests on the assumption that the material of the specimen has a strain energy density that not explicitly depends on the position in the direction of crack propagation. That is, the conditions for J to be path independent. Evaluation of simulated experiments gives the exact value of the fracture energy. The alternative method is based on linear elastic fracture mechanics. Contrary to the conventional methods we use an expression where the crack length is eliminated in favour of the flexibility of the specimen.Influences of assumptions are studied both experimentally and numerically. Differences in stiffness are achieved by changing the type of adhesive and the layer thickness. Two different adhesives are studied. One is a modern crash resistant epoxy adhesive, SikaPower-498. This is a relatively stiff and tough adhesive. The other adhesive is a soft and extremely tough polyurethane based adhesive, Sikaflex-UHM. Two layer thicknesses are tested; 1.0 mm for the epoxy and 3.0 mm for the polyurethane based adhesive. The results show that the two methods give similar results for the thinner and stiffer epoxy adhesive but differences are recorded for the soft polyurethane based adhesive. This analysis gives a better understanding of the evaluation methods and their limitations and possibilities to extract cohesive laws.
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| 3. |
- Biel, Anders, et al.
(författare)
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Damage and plasticity in adhesive layer : an experimental study
- 2010
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Ingår i: International Journal of Fracture. - : Springer Science+Business Media B.V.. - 0376-9429 .- 1573-2673. ; 165:1, s. 93-103
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Tidskriftsartikel (refereegranskat)abstract
- An experimental method is developed to identify a suitable model of in-elastic behaviour of an adhesive layer. Two prototype models are considered: an elastic-plastic model where the in-elasticity is considered due to permanent straining of the adhesive and an elastic-damage model where the in-elasticity is due to a reduction in elastic stiffness. Simulations show that the evaluated property is sensitive to the choice of model. In the experimental study of an engineering epoxy adhesive, the elastic-damage model fits the experiments. The study also reveals that plasticity and damage accumulated at the crack tip influences the evaluated fracture properties.
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| 4. |
- Biel, Anders, et al.
(författare)
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Damage and plasticity of adhesive layers : an experimental study
- 2011
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Ingår i: Procedia Engineering. - : Elsevier. - 1877-7058. ; 10, s. 2280-2285
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Tidskriftsartikel (refereegranskat)abstract
- Time independent inelasticity is often modelled as due to plasticity and/or damage. The difference is manifested at reversed loading; plasticity reveals itself by a remaining strain in the unloaded state while damage is revealed by a decrease in the elastic stiffness during unloading. With thin adhesive layers, the deformation is inhomogeneously distributed along the layer. Large deformations occur at the ends of the layer. In the more central parts, the layer is virtually undeformed. This makes a direct measurement of the unloading properties virtually impossible. In the present paper, novel experiments are performed in order to evaluate the inelastic properties of epoxy adhesives. The load is first increased to a level corresponding to 50, 60, 70 or 80% of the fracture energy. The load is then reversed. The first step creates a zone of inelastically deformed adhesive at the start of the layer. During a final loading phase, the properties of this zone are analysed. Major differences due to the loading direction are observed. Some comparisons with simulation models are performed.
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| 5. |
- Biel, Anders, et al.
(författare)
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Influence of Edge-boundaries on the Cohesive Behaviour of an Adhesive Layer
- 2012
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Ingår i: ASME 2012 International Mechanical Engineering Congress and Exposition, November 9–15, 2012, Houston, Texas, USA. - : ASME Press. - 9780791845240 ; , s. 507-511
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Konferensbidrag (refereegranskat)abstract
- In comparison with other adhesives e.g. epoxies, polyurethane adhesives (PUR) are soft. In automotive applications, the thickness of PUR-adhesive layers is between about 2 to 5 mm. Since these adhesives cure by moisture, the width of the joints is limited. Often, the width is only marginally larger than the thickness of the layer. In numerical FE-simulations it is now common to represent epoxy adhesive layers by cohesive elements. With this model, both stress distribution and fracture can be modelled using mesh sizes that allows for large-scale analyses. Material properties are usually the result from experiments with coupon type specimens, e.g. the double cantilever beam specimen (DCB). With PUR-adhesives this approach is problematic. The adhesive is very flexible and effects from the edge-boundaries cannot be ignored. In order to study the influence of the edge-boundaries in peel loading, experiments with the DCB-specimens are performed. Specimens with a layer thickness of 3 mm and three different widths between 10.6 mm to 40.6 mm are studied. The PUR-adhesive SikaFlex-UHM is used. All the experiments are performed at a constant loading rate. The cohesive law is measured. The experimental results show that the maximum peel stress is increasing with an increasing width of the specimen, i.e. when the influences from the edges decrease. When the width increases from 10.6 mm to 40.6 mm, the maximum evaluated peak stress increases from about 5 MPa to about 7 MPa. From visual inspections during the experiments it is conjectured that crack growth starts with voids initiating inside the adhesive. At a critical point, the voids rapidly reach the surface and crack growth starts.
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| 6. |
- Carlberger, Thomas, et al.
(författare)
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Dynamic testing and simulation of hybrid joined bi-material beam
- 2010
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Ingår i: Thin-walled structures. - : Elsevier. - 0263-8231 .- 1879-3223. ; 48:8, s. 609-619
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Tidskriftsartikel (refereegranskat)abstract
- A specimen is developed for real-like low velocity impact testing of bi-material joint configurations. Six different joint configurations are analysed. Two engineering adhesives are evaluated with and without discrete mechanical fasteners, i.e. adhesive and hybrid joints. Experiments and simulations are performed. The simulations are performed using adhesive cohesive finite elements. Simulations show good agreement with experiments in impact energy and overall deformation mode. The histories of applied load vs. load-point deflection show reasonably good correlation. The results show that the impact energy consumption depends on the joint integrity. A threshold value for the fracture energy of the adhesive seems to exist. Beneath this value, adhesive and discrete fastener work together increasing the impact energy capacity. Above this value the discrete fastener has a negative effect, and may be regarded as a stress concentration.
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| 7. |
- Carlberger, Thomas, et al.
(författare)
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Influence of Layer Thickness on Cohesive Properties of an Epoxy-Based Adhesive : An Experimental Study
- 2010
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Ingår i: The journal of adhesion. - : Taylor & Francis Group. - 0021-8464 .- 1563-518X .- 1545-5823. ; 86:8, s. 814-833
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Tidskriftsartikel (refereegranskat)abstract
- Cohesive laws are determined for different layer thicknesses of an engineering adhesive. The shape of the cohesive law depends on the adhesive layer thickness. Of the two parameters of the cohesive law—the fracture energy and thestrength—the fracture energy is more sensitive to thickness variation than the strength. The fracture energy in peel mode (Mode I) increases monotonically as the thickness is increased from 0.1 to about 1.0 mm. At an adhesive thickness of 1.5 mm, the fracture energy is slightly lower than for a 1.0 mm adhesive thickness, indicating a maximum between 1.0 and 1.5 mm. In shear mode (Mode II), the thickness dependence is not as strong, but an increasing trend in fracture energy with increasing adhesive thickness is evident. A slight decrease in strength with increasing adhesive thickness is found in both loading modes.
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| 8. |
- Carlberger, Thomas, 1962-, et al.
(författare)
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Vilka hållfasthetsegenskaper har limfogen? : Simulering på Högskolan i Skövde ger svaren
- 2014
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Ingår i: Fordonskomponenten. - Göteborg : Fordonskomponentgruppen (FKG). - 2000-7299. ; :3, s. 46-47
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Forskningsöversikt (populärvet., debatt m.m.)abstract
- Att beräkna limfogars hållfasthet är inte helt enkelt, vilket beror på spänningskoncentrationens inflytande vid limfogens kanter.Materialmekanik vid Högskolan i Skövde har specialiserat sig på hållfasthetssimulering av limfogar genom kohesiv modellering.
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| 9. |
- Eklind, Alexander, et al.
(författare)
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High cycle fatigue crack growth in Mode I of adhesive layers : modelling, simulation and experiments
- 2014
-
Ingår i: International Journal of Fracture. - : Springer Netherlands. - 0376-9429 .- 1573-2673. ; 190:1-2, s. 125-146
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Tidskriftsartikel (refereegranskat)abstract
- The capability to predict high cycle fatigue properties of adhesive joints is important for cost-efficient and rapid product development in the modern automotive industry. Here, the adaptability of adhesives facilitates green technology through the widening of options of choosing and joining optimal materials. In the present paper a continuum damage mechanics model is developed based on the adhesive layer theory. In this theory, through-thickness averaged variables for the adhesive layer are used to characterise the deformation, damage and local loading on the adhesive layer. In FE-simulations, cohesive elements can thereby be used to model the adhesive layer. This simplifies simulations of large scale complex built-up structures. The model is adapted to experimental results for two very different adhesive systems; one relatively stiff rubber based adhesive and one soft polyurethane based adhesive. The model is able to reproduce the experimental results with good accuracy except for the early stage of crack propagation when the loads are relatively large. The model also predicts a threshold value for fatigue crack growth below which no crack growth occurs. The properties of the model are also compared with the properties of Paris’ law. The relations between the parameters of the continuum damage mechanics law and the parameters of Paris’ law are used to adapt the new law. It also shows that the properties of a joined structure influence the Paris’ law properties of the adhesive layer. Thus, the Paris’ law properties of an adhesive layer are not expected to be transferable to joints with adherends having different mechanical properties.
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| 10. |
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| 11. |
- Marzi, Stephan, et al.
(författare)
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On experimental methods to investigate the effect of layer thickness on the fracture behavior of adhesively bonded joints
- 2011
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Ingår i: International Journal of Adhesion and Adhesives. - : Elsevier. - 0143-7496 .- 1879-0127. ; 31:8, s. 840-850
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Tidskriftsartikel (refereegranskat)abstract
- A number of different experimental methods are used at two independent laboratories to evaluate the influence of layer thickness on the fracture properties atone batch of a crash resistant epoxy adhesive. Both mode I and II are considered. Novel, as well as state of the art methods are used. In mode I, the Double Cantilever Beam (DCB) and two versions of the Tapered Double Cantilever Beam (TDCB) specimens are utilized; in mode II, two versions of the End Notch Flexure (ENF) and the End-Loaded Shear Joint (ELSJ) specimens are used. Good agreement between the test results is achieved for thin layers in both fracture modes. For thicker layers the variation is larger. (C) 2011 Elsevier Ltd. All rights reserved.
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| 12. |
- Stigh, Ulf
(författare)
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Cohesive Laws for Delamination of CFRP : Experiments and Models
- 2010
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Ingår i: ASME 2010 International Mechanical Engineering Congress and Exposition. - : ASME Press. - 9780791838914 - 9780791844465 ; , s. 45-50
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Konferensbidrag (refereegranskat)abstract
- In the present paper, we study delamination of a carbon fibre reinforced composite at a small length scale, i.e. without consideration of crack bridging. The study is performed within the framework of cohesive modelling. We propose methods based on the applications of the path independent J-integral to measure the cohesive law for delamination. With a DCB-specimen, the cohesive law corresponding to mode I loading is measured and with an ENF-specimen, the law corresponding to mode II loading is measured. These laws are used to calibrate a mixed-mode cohesive law. It is argued that the most important parameters of a cohesive law are the ability to provide the correct fracture energy and strength. The cohesive law is developed using a minimum of adjustable parameters to provide a transparent calibration process.
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| 13. |
- Stigh, Ulf, et al.
(författare)
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Fracture of Adhesive Layers in Mode II
- 2012
-
Ingår i: Proceedings of the 19th European Conference on Fracture. - : Kazan Scientific Centre of the Russian Academy of Sciences. - 9785905576188
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Konferensbidrag (refereegranskat)abstract
- Measuring fracture properties of adhesives in Mode II is often problematic. Indeed, no method can today be regarded as established by the community. In this paper a number of methods are presented. Experiments show that the evaluated properties of the same adhesive sometime vary considerably with the choice of specimen. Even just modest variations in loading conditions using the same specimen can yield considerable variation in the evaluated properties. Sources for these deficiencies are identified.It has long been understood that Mode II testing using the end-notched flexure specimen (ENF) is conditionally stable. That is, the length of the crack has to be large enough to achieve a stable experiment. This is also the case for other Mode II specimens. A condition for stability is derived leading to an easily evaluated equation. Moreover, careful studies of the crack tip area during Mode II experiments often reveal an expansion of the adhesive during the final phase of loading. That is, negative Mode I loading. Due to the stiffness of the adherends, this leads to a compressive transversal loading of the process zone. Experiments and simulations show that the evaluated fracture energy depends on this constraint. A more detailed analysis of Mode II loading considering large-scale process zones gives some insight into the problem. It is also clear that Mode II has to be more carefully defined than is necessary for Mode I. Due to the transversal expansion of the process zone associated with shear, we may choose to define Mode II as a state of pure shear deformation or a state of pure shear stress. In experiments, none of these states are easily achieved. Moreover, transversally loaded short specimens can result in a process zone extending under the loading point. The result is compression of the process zone and exaggerated evaluated fracture energy. This problem is especially important to consider when evaluating soft and tough adhesives. If better understood and modelled, these effects might also be used in design so that an adhesive joint is loaded in a more favourable way.
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| 14. |
- Stigh, Ulf, et al.
(författare)
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On cohesive laws for delamination of composites
- 2010
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Ingår i: 14th European conference on Composite materials. - Budapest : University of Technology and Economics. - 9789633130087 - 9633130085 ; , s. 1-10
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Konferensbidrag (refereegranskat)abstract
- Analysis of delamination of carbon fibre reinforced composite using cohesive models is studied. A method to measure the cohesive law associated with delamination is presented. The method allows for identification of a cohesive law fit to model the fracture process at the crack tip, i.e. not considering fibre bridging. Due to the small size of the cohesive zone, an elaborated method involving simulations of the fracture process is developed. The results show larger scatter in the parameters of the cohesive law than in the fracture energy.
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| 15. |
- Stigh, Ulf, et al.
(författare)
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Shear properties of an adhesive layer exposed to a compressive load
- 2014
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Ingår i: Procedia Materials Science. - : Elsevier. - 2211-8128. ; 3, s. 1626-1631
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Tidskriftsartikel (refereegranskat)abstract
- Adhesive joints are designed to transfer load in shear since both the fracture energy and the fracture stress are larger in shear than in peel. Shear deformation is isochoric, however, the fracture process involves nucleation and growth of a multitude of slanted microcracks. In order to grow, these microcracks open up. Thus, adhesive layers show a tendency to deform in peel during shear fracture. This opening is localized to the fracture process zone and the adherends have to separate locally over the process zone to allow for the adhesive to swell. Depending on the stiffness of the adherends, the opening mode is more or less prohibited. With stiffer adherends, the opening is obstructed more efficiently than with softer adherends. Micromechanical studies indicate that the constraints of the peel deformation during shear loading have a profound influence on the strength of the joint. In the present study, we compress the process zone during experiments. Repeated experiments with ENF-specimens are performed. A compressive force is applied on the first part of the adhesive layer by use of a pneumatic cylinder. The experiments are evaluated using the path independent J-integral. Together with measurements of the shear and peel deformation of the adhesive layer at the start of the layer, the complete shear stress vs shear deformation relations are evaluated. It is shown that the inhibited peel deformation gives a substantial increase of the fracture energy
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| 16. |
- Stigh, Ulf, et al.
(författare)
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Shear strength of adhesive layers – models and experiments
- 2014
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Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 129, s. 67-76
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Tidskriftsartikel (refereegranskat)abstract
- The mode II properties of adhesives joints are of special interest since these joints are strongest if loaded in shear. Today no standardized method is available to measure shear properties. After a brief discussion of different models used to analyse adhesive joints, we identify some of the reasons for problems that arise in some of the more frequently used experimental methods. It is shown that transversally loaded specimens with elastically deforming adherends can lead to unstable crack growth provided the un-cracked specimen is flexible. With tough adhesives, a substantial process zone develops at the crack tip. That is, most specimens are in a state of large scale yielding. If not properly taken into account, the evaluated properties will be in error. Moreover, the process zone may grow in under the loading point which hinders its evolution and yield errors in the evaluated properties. Modest variations in loading conditions using the same specimen can yield considerable variation in the evaluated properties. However, properly designed and used, both the thick-adherend lap-shear joint and the end-notched flexure specimen provide useful results.
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| 17. |
- Stigh, Ulf, et al.
(författare)
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Some aspects of cohesive models and modelling with special application to strength of adhesive layers
- 2010
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Ingår i: International Journal of Fracture. - : Springer Netherlands. - 0376-9429 .- 1573-2673. ; 165:2, s. 149-162
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Tidskriftsartikel (refereegranskat)abstract
- An overview of recent development of cohesive modelling is given. Cohesive models are discussed in general and specifically for the modelling of adhesive layers. It is argued that most cohesive models model a material volume and not a surface. Detailed microscopic and mesomechanical studies of the fracture process of an engineering epoxy are discussed. These studies show how plasticity on the mesomechanical length scale contributes to the fracture energy in shear dominated load cases. Methods to measure cohesive laws are presented in a general setting. Conclusions and conjectures based on experimental and mesomechanical studies are presented. The influence of temperature and strain rate on the peak stress and fracture energy of cohesive laws indicates fundamentally different mechanisms responsible for these properties. Experiments and mesomechanical studies show that in-plane straining of an adhesive layer can give large contributions to the registered fracture energy. Finite element formulations including a method to incorporate this influence are discussed.
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| 18. |
- Svensson, Daniel, et al.
(författare)
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An experimental method to determine the critical energy release rate associated with longitudinal compressive failure in CFRP
- 2013
-
Ingår i: Proceeding of the 19th international conference on composite materials (ICCM 19). - 9780969679714
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Konferensbidrag (refereegranskat)abstract
- A test specimen for measurement of the critical energy release rate associated with longitudinal compressive failure is proposed. High strains are localized by decreasing the out-of-plane thickness towards the anticipated damage region which consists of a unidirectional (UD) laminate. Thus, the compressive fibre failure mode is isolated. Microscopic studies show that the UD-material fails in a kinking mode. A method based on a generalized form of the J-integral and full-field measurements of the strain field is developed to extract the fracture energy. The fracture energy in four experiments is measured to be 20-40 kN/m. Finite element simulations are performed to validate the experimental results. The essential features of the response are captured by modelling the damage region with cohesive elements.
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| 19. |
- Svensson, Daniel
(författare)
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Experimental methods to determine model parameters for failure modes of CFRP
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The focus of this thesis is to develop methods to predict the damage response of Carbon Fibre Reinforced Polymers (CFRP). In the pursuit of reducing the manufacturing cost and weight of CFRP components, it is crucial to enable modelling of the non-linear response associated with various failure modes. Two failure modes are considered in this thesis: fibre compressive failure and interlaminar delamination. Multidirectional laminated composites are commonly used when a low weight is desired due to their high specific strength and stiffness. In a carbon/epoxy composite, almost exclusively the fibres carry the load. However, along the fibre direction, the compressive strength is considerably lower than the tensile strength. With the same reasoning, the transverse strength is considerably lower than the in-plane strength. This makes delamination and fibre compressive failure two of the major concerns in structural design. Moreover, the presence of delaminations severely reduces the compressive strength of a laminate. This can cause catastrophic failure of the structure. In Paper A, we suggest a test method for determining fracture properties associated with fibre compressive failure. A modified compact compression specimen is designed for this purpose and compressive failure takes place in a region consisting exclusively of fibres oriented parallel to the loading direction. The evaluation method is based on a generalized J-integral and full field measurements of the strain field on the surface of the specimen. Thus, the method is not restricted to small damage zones. Paper B focuses on measuring cohesive laws for delamination in pure mode loading. The cohesive laws in mode I and mode II are measured with the DCB- and ENF-specimen, respectively. With a method based on the J-integral, the energy release rate associated with the crack tip separation is measured directly. From this, the cohesive laws are derived. It is concluded that the nonlinear response at the crack tip is crucial in the evaluation of the mode II fracture energy.
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| 20. |
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| 21. |
- Walander, Tomas, et al.
(författare)
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An evaluation of the temperature dependence of cohesive properties for two structural epoxy adhesives
- 2012
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Ingår i: 19th European Conference on Fracture. - : Kazan Scientific Centre of the Russian Academy of Sciences. - 9785905576188
-
Konferensbidrag (refereegranskat)abstract
- Cohesive modelling provides a more detailed understanding of the fracture properties of adhesivejoints than provided by linear elastic fracture mechanics. A cohesive model is characterized by astress-deformation relation of the adhesive layer. This relation can be measured experimentally.Two parameters of the stress-deformation relation are of special importance; the area under thecurve, which equals the fracture energy, and the peak stress. The influence of temperature of theseparameters is analyses experimentally and evaluated statistically for two structural epoxy adhesivesin the span from of -40°C to +80°C. The adhesives are used by the automotive industry and atemperature span below the glass transition temperature is considered. The results show that thattemperature has a modest influence on the adhesives Mode I fracture energy. For one of theadhesives, the fracture energy is independent of the temperature in the evaluated temperature span.In mode II, the influence of temperature is larger. The peak stresses decreases almost linearly withan increasing temperature in both loading cases and for both adhesives.
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| 22. |
- Walander, Tomas
(författare)
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Cohesive modelling of the temperature dependence of epoxy based adhesives in Mode I and Mode II loading
- 2013
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this work, the influence of the temperature on the cohesive laws for two epoxy adhesives is studied at temperatures below the glass transition temperature for both Mode I and Mode II loading. Cohesive laws are measured experimentally under quasi-static loading conditions in the temperature range -30≤T≤80"C" . Three parameters of the cohesive laws are studied in detail: the elastic stiffness, the peak stress and the fracture energy. Methods for determining the elastic stiffness in Mode I and Mode II are derived and evaluated. With these methods, the results in this work show that it is possible to measure all three parameters for each pure mode loading case by the use of only the DCB- and the ENF-test specimens. Even though the measures tend to spread in values, this can significantly reduce the cost for performing experiments. It is shown that most of the cohesive parameters are decreasing with an increasing temperature in both loading modes and for both adhesives. An exception is the Mode I fracture energy for one of the adhesives. This is shown to be independent of the temperature in the studied temperature range. For the same adhesive, the Mode II fracture energy is shown to be continuously decreasing with an increasing temperature. The experimental results are verified by finite element analyses. The simulations only consider uncoupled cohesive behaviours. By use of the experimental results, simplified bi-linear cohesive laws to be used at any temperature within the studied temperature range are derived for one adhesive in both loading modes. This is desired in order to simulate adhesively bonded structures that suffer a wide range in temperature.
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| 23. |
- Walander, Tomas, et al.
(författare)
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Fatigue damage of adhesive layers : experiments and models
- 2014
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Ingår i: Procedia Materials Science. - : Elsevier. - 2211-8128. ; 3, s. 829-834
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Tidskriftsartikel (refereegranskat)abstract
- Mode I fatigue crack growth at load levels close to the threshold is studied with the aim of improving the understanding of the fatigue properties. We also aim at identifying a suitable damage evolution law for large-scale simulation of built-up structures. A fatigue test rig is designed where up to six specimens are tested simultaneously. Each specimen is evaluated separately indicating the specimen-to-specimen variation in fatigue properties. A rubber-based and a PUR-based adhesive are tested. The two adhesives represent adhesives with very different material properties; the rubber adhesive is a stiff structural adhesive and the PUR adhesive is a soft modular adhesive. The experiments are first evaluated using a traditional Paris’ law approach. Inspired by an existing damage evolution law, a modified damage evolution law is developed based on only three parameters. The law is implemented as a user material in Abaqus and the parameters are identified. The results from simulations show a very good ability to reproduce the experimental data. With this model of fatigue damage, a zone of damage evolves at the crack tip. The extension of this zone depends on the stiffness of the adherends; stiffer adherends leads to a larger damage zone. This means that the rate of crack growth depends on the stiffness of the adherends. Thus, not only the state at the crack tip governs the rate of crack growth. This is in contrast to the results of a model based on Paris’ law where only the state at the crack tip, through the energy release rate, governs the rate of crack growth. This indicates that the threshold value of the energy release rate may depend on the stiffness of the adherends.
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| 24. |
- Walander, Tomas, et al.
(författare)
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Temperature dependence of cohesive laws for an epoxy adhesive in Mode I and Mode II loading
- 2013
-
Ingår i: International Journal of Fracture. - : Springer Netherlands. - 0376-9429 .- 1573-2673. ; 183:2, s. 203-221
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the temperature on the cohesive laws for an epoxy adhesive is studied in the glassy region, i.e. below the glass transition temperature. Cohesive laws are derived in both Mode I and Mode II under quasi-static loading conditions in the temperature range C. Three parameters of the cohesive laws are studied in detail: the elastic stiffness, the peak stress and the fracture energy. Methods for determining the elastic stiffness in Mode I and Mode II are derived and evaluated. Simplified bi-linear cohesive laws to be used at any temperature within the studied temperature range are derived for each loading mode. All parameters of the cohesive laws are measured experimentally using only two types of specimens. The adhesive has a nominal layer thickness of 0.3 mm and the crack tip opening displacement is measured over the adhesive thickness. The derived cohesive laws thus represent the entire adhesive layer as having the present layer thickness. It is shown that all parameters, except the Mode I fracture energy, decrease with an increasing temperature in both loading modes. The Mode I fracture energy is shown to be independent of the temperature within the evaluated temperature span. At C the Mode II fracture energy is decreased to about 2/3 of the fracture energy at C. The experimental results are verified by finite element analyses.
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