| 1. |
- Biel, Anders, et al.
(författare)
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An analysis of the evaluation of the fracture energy using the DCB-specimen
- 2007
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Ingår i: Archives of Mechanics. - : Polish Academy of Science. - 0373-2029. ; 59:4-5, s. 311-327
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Tidskriftsartikel (refereegranskat)abstract
- The methods to estimate the fracture energy using DCB-specimens as advocated in common standards. For instance, ASTM D 3433 and BS 7991:2001 are based on a compliance method, i.e. on linear elastic fracture mechanics (LEFM). Since the mechanical properties of almost all adhesives are non-linear, errors are generated. In some of the standards, the non-linear behaviour is compensated for by the use of correction terms generated from the experiments. An analysis of the methods of evaluation the fracture energy from experiments is performed. This analysis is performed first by simulating an experiment using realistic data for an engineering adhesive and then, by analysing the results with different methods. In this way, the correct fracture energy is known beforehand and the error in the evaluated fracture energy can be determined. In the present work it is shown that the magnitude of this error depends on the length of the crack. The results show that some commonly used methods generate substantial errors when a large region of non-linear deformation precedes the crack tip. It is also shown that methods based on nonlinear fracture methods do not produce this kind of error.
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| 2. |
- Biel, Anders, et al.
(författare)
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Effects of constitutive parameters on the accuracy of measured fracture energy using the DCB-specimen
- 2008
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Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 75:10, s. 2968-2983
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Tidskriftsartikel (refereegranskat)abstract
- Several methods exist to estimate the fracture energy for adhesive joints using the double cantilever beam specimen and linear elastic fracture mechanics. Since the mechanical properties of all adhesives are non-linear, errors are generated. By use of an exact solution experiments are simulated. These are evaluated with eight different methods. The influence of the constitutive parameters is systematically studied. This influence is small for most methods. The error due to the choice of evaluation method is considerably larger. One of the commonly used methods gives accurate results; the error is less than 3%. However, most methods yield substantial errors.
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| 3. |
- Carlberger, Thomas, et al.
(författare)
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An explicit FE-model of impact fracture in an adhesive joint
- 2007
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Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 74:14, s. 2247-2262
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Tidskriftsartikel (refereegranskat)abstract
- Dynamic fracture of an adhesive layer in a structure is analysed. The structure represents some specific properties of an automotive structure and is simple enough to allow for closed form solutions obtained by the method of characteristics. These solutions are compared to results of explicit FE-analyses. The FE-solutions agree with the closed form solutions. Damage is included in the FE-model. Three constitutive models of the adhesive layer are used. It is shown that an amplification of the strain rate is achieved in the adhesive layer. It is also shown that an artificially increased flexibility of the adhesive in an aluminium structure gives only minor influences of the general behaviour. In some load cases, the adhesive layer will experience repeated loading/unloading. It is shown that in these cases an explicit FE-analysis with a “large” time step is more prone to give immediate rupture. Thus, the method is conservative.
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| 4. |
- Carlberger, Thomas, et al.
(författare)
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Explicit FE-formulation of interphase elements for adhesive joints
- 2008
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Ingår i: International Journal for Computational Methods in Engineering Science and Mechanics. - : Taylor & Francis. - 1550-2287 .- 1550-2295. ; 9:5, s. 288-299
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Tidskriftsartikel (refereegranskat)abstract
- The potential of adhesive bonding to improve the crashworthiness of cars is attracting the automotive industry. Large-scale simulations are time consuming when using the very small finite elements needed to model adhesive joints using conventional techniques. In the present work, a 2D-interphase element formulation is developed and implemented in an explicit FE-code. A simplified joint serves as a test example to compare the interphase element with a straightforward continuum approach. A comparison shows the time-saving potential of the present formulation as compared to the conventional approach. Moreover, the interphase element formulation shows fast convergence and computer efficiency.
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| 5. |
- Carlberger, Thomas, et al.
(författare)
-
Influence of temperature and strain rate on cohesive properties of a structural epoxy adhesive
- 2009
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Ingår i: International Journal of Fracture. - : Springer. - 0376-9429 .- 1573-2673. ; 155:2, s. 155-166
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Tidskriftsartikel (refereegranskat)abstract
- Effects of temperature and strain rate on the cohesive relation for an engineering epoxy adhesive are studied experimentally. Two parameters of the cohesive laws are given special attention: the fracture energy and the peak stress. Temperature experiments are performed in peel mode using the double cantilever beam specimen. The temperature varies from −40 to + 80°C. The temperature experiments show monotonically decreasing peak stress with increasing temperature from about 50 MPa at −40°C to about 10 MPa at + 80°C. The fracture energy is shown to be relatively insensitive to the variation in temperature. Strain rate experiments are performed in peel mode using the double cantilever beam specimen and in shear mode, using the end notch flexure specimen. The strain rates vary; for peel loading from about 10−4 to 10 s−1 and for shear loading from 10−3 to 1 s−1. In the peel mode, the fracture energy increases slightly with increasing strain rate; in shear mode, the fracture energy decreases. The peak stresses in the peel and shear mode both increase with increasing strain rate. In peel mode, only minor effects of plasticity are expected while in shear mode, the adhesive experiences large dissipation through plasticity. Rate dependent plasticity, may explain the differences in influence of strain rate on fracture energy between the peel mode and the shear mode.
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| 6. |
- Högberg, J. Li, et al.
(författare)
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Constitutive behaviour of mixed mode loaded adhesive layer
- 2007
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Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 44:25-26, s. 8335-8354
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Tidskriftsartikel (refereegranskat)abstract
- Mixed mode testing of adhesive layer is performed with the Mixed mode double Cantilever Beam specimen. During the experiments, the specimens are loaded by transversal and/or shear forces; seven different mode mixities are tested. The J-integral is used to evaluate the energy dissipation in the failure process zone. The constitutive behaviour of the adhesive layer is obtained by a so called inverse method and fitting an existing mixed mode cohesive model, which uses a coupled formulation to describe a mode dependent constitutive behaviour. The cohesive parameters are determined by optimizing the parameters of the cohesive model to the experimental data. A comparison is made with the results of two fitting procedures. It is concluded that the constitutive properties are coupled, i.e. the peel and shear stress depend on both the peel and shear deformations. Moreover, the experiments show that the critical deformation in the peel direction is virtually independent of the mode mixity.
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| 7. |
- Högberg, J. Li, et al.
(författare)
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Specimen proposals for mixed mode testing of adhesive layer
- 2006
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Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 73:16, s. 2541-2556
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Tidskriftsartikel (refereegranskat)abstract
- The experimental methods to determine the fracture properties for adhesives under mixed mode loading is not as well established as compared to such methods for adhesives under pure mode loading. Some controversies exist regarding the decomposition of the mode mixity. For a flexible linear elastic adhesive, the mode mixity of a single-layer adhesive joint is directly related to the deformation of the adhesive layer at the crack tip. The governing equations for linear elastic single-layer adhesive joints show that the mode mixity depends on the external loads, the properties of the adherends and often on the flexibility of the adhesive layer. This implies some fundamental problems that have to be addressed before an experimental method can be established. The purpose of this paper is to investigate different specimen configurations for mixed mode loading. Requirements for the design of a specimen configuration are given. A new specimen configuration is proposed based on the geometry of a semi-infinite symmetric DCB-specimen. According to this study, the proposed test specimen offers exceptional flexibility, variety and stability.
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| 8. |
- Leffler, Karin, et al.
(författare)
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On the existence of a unique stress-deformation relation for an adhesive layer loaded in shear
- 2005
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Ingår i: 11th International Conference on Fracture (ICF11), Turin, Italy 20-25 March 2005. - : Curran Associates, Inc.. - 9781617820632 ; , s. 3763-3768
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Konferensbidrag (refereegranskat)abstract
- An experimental method to determine the complete stress vs. deformation relation for a thin adhesive layer loaded in shear is presented. The experiments are performed by use of a classical specimen geometry, i.e. the end-notch flexure specimen, though the experiments are evaluated based on a novel inverse technique. With this technique, the instantaneous energy release rate is first evaluated by use of a theory for the specimen based on the Euler-Bernoulli beam theory. Effects of a flexible adhesive layer are considered in an approximate way. From the energy release rate, the stress-deformation relation is evaluated using an inverse method. In order for the theory to be valid, the adherends of the specimen are only allowed to deform elastically. Quasi-static experiments are performed using a servo-hydraulic testing machine. In the experiments, the displacement of the loading point is gradually increased to obtain a constant velocity of the shear deformation at the crack tip. Formation of micro-cracks and the propagation of a macro-crack are monitored during the experiments by use of a CCD-camera attached to a microscope. By varying the heights of the adherends, the size of the process zone in front of the crack tip changes from about 200 to 400 times the thickness of the adhesive layer. The results of the experiments give a fracture toughness of 2.5 kJ/m 2 , a critical shear deformation of 0.13 mm, and a maximal strength of 30 MPa independent of the specimen geometry. The experiments show consistent results. The results show that if the process zone is large as compared to the thickness of the adhesive layer, the shear stress – shear deformation relation can be considered as a constitutive property of the adhesive layer.
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| 9. |
- Leffler, Karin, et al.
(författare)
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Shear behaviour of adhesive layers
- 2007
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Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 44:2, s. 530-545
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Tidskriftsartikel (refereegranskat)abstract
- An experimental method to determine the complete stress versus deformation relation for a thin adhesive layer loaded in shear is presented. The method is based on a classic specimen geometry; the end-notch flexure specimen. The experiments are evaluated using an inverse method. First, the variation of the energy release rate with respect to the shear deformation at the crack tip is measured during an experiment. Then the traction–deformation relation is derived using an inverse method. The theory is based on the path-independence of the J-integral and considers the effects of a flexible adhesive layer.Quasi-static experiments on three different specimen geometries are performed using a servo-hydraulic testing machine. The experiments give consistent results. This shows that the traction–deformation relation can be taken as independent of the dimensions of the adherends. Thus, the constitutive relation can be considered as a property of the adhesive layer. The deformation process at the crack tip is also monitored during the experiments by the use of a digital camera attached to a microscope.
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| 10. |
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| 11. |
- Olasz, Lorant, 1973-
(författare)
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Residual Stresses and Strains in Cross-linked Polyethylene Power Cable Insulation
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The subject of this thesis is modeling of the manufacturing process of high voltage power cables with the aim of predicting residual stresses and strains in the cable insulation. The studied material is cross-linked polyethylene (XLPE), which at room temperature is a semi-crystalline viscoelastic solid. Above the crystallization temperature the material exhibits a rubber type behavior due to the crosslinks.An extensive set of uniaxial tensile relaxation tests were used for the mechanical characterization of XLPE. These experiments were complemented by pressure volume temperature experiments as well as density and crystallinity measurements. Based on the experiments, initially a linear and later a non-linear viscoelastic power law model was formulated, incorporating temperature and crystallinity dependence. The non-linear model is based on the Schapery formulation. Evaluations of the model were performed with additional uniaxial experiments. These included comparisons between predicted stress responses and measured values during relaxation tests with transient temperature histories, during two step relaxation experiments and during uniaxial tests with constant strain rate loading.The initial modeling work focused on the prediction of residual stresses which develop during the cooling stage of the manufacturing process. As the constitutive model incorporates temperature and crystallinity dependence, the mechanical problem is coupled to the heat transfer and crystallization problems. Calculations were performed for a vertical manufacturing line. The effects of a viscoelastic material model are illustrated by a comparison to a stress state predicted by a thermo-elastic material model.A final study concerns the modeling of the residual strains in the insulation. It was found that strains originating at cross-linking of the molecules play a signifi- cant role for buildup of residual strains. Calculations are performed for the same vertical process line as before. Good agreement was found between predicted and experimentally obtained residual strains. Based on the residual strain state an estimate is made for the upper limit of shrink-back of the cable insulation.
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| 12. |
- Salomonsson, Kent, et al.
(författare)
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An adhesive interphase element for structural analyses
- 2008
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Ingår i: International Journal for Numerical Methods in Engineering. - : Wiley. - 0029-5981 .- 1097-0207. ; 76:4, s. 482-500
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Tidskriftsartikel (refereegranskat)abstract
- A special purpose finite element is developed for structural simulations of complex adhesively bonded structures. In the interphase element, the adhesive is explicitly regarded as a material phase between two substrates. The element considers large rotations. Furthermore. it considers in-plane straining of the adhesive due to large curvatures of the bonded shells. This feature appears especially important when considering bonding of thin plastically deforming metallic shell structures. Simulations are made on specimens where the adherends deform both elastically and plastically. The results are in good agreement with previously performed experiments. Copyright (0 2008 John Wiley & Sons, Ltd.
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| 13. |
- Salomonsson, Kent, et al.
(författare)
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Influence of root curvature on the fracture energy of adhesive layers
- 2009
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Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 76:13, s. 2025-2038
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Tidskriftsartikel (refereegranskat)abstract
- Previously performed experiments to study the mode I behavior of an adhesive layer revealed an apparent increase in the fracture toughness when the adherends deformed plastically. Attempts to simulate the experiments are made; both with elastically and plastically deforming adherends. Thus, effects of the size of the process zone and the deformation of the adherends are revealed. The adhesive layer is modeled using finite elements with different approaches; cohesive elements and representative volume elements. The adherends are modeled with solid elements. With a long process zone, all models give good results as compared to the experiments. However, only the model with representative volume elements gives good agreement for large root curvatures and correspondingly short process zones. The results are interpreted by analyzing the deformation and mechanisms of crack propagation in the representative volume elements. It is shown that with large root curvature of the adherends, the in-plane stretching of the adhesive layer gives a substantial contribution to the fracture energy. A simple formula is derived and shown to give an accurate prediction of the effects of the root curvature. This result indicates the limits of conventional cohesive zone modeling of an adhesive layer of finite thickness.
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| 14. |
- Salomonsson, Kent
(författare)
-
Meso-Mechanical Modeling and Analysis of Adhesive Layers
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is concerned with the modeling, simulation and analysis of adhesive layers. By use of an in situ scanning electron microscopy study it is found that the adhesive studied in the present thesis has a very complex structure with two different compounds, a mineral and an epoxy/thermoplastic blend. A representative volume element (RVE) model is developed to study the behavior of the adhesive layer at the meso-level. It is a continuum model where interface finite elements are implemented at the boundaries of the continuum elements in order to enable crack initiation and propagation of micro cracks. On a structural level, two deformation modes, modes I and II, dominate the behavior of thin adhesive layers. With the RVE it is possible reproduce experimental stress-deformation relations from both modes. However, in a real structure, mixed mode loading usually occur. A range of mode mixes is studied, using the RVE, from an un-loaded state until fracture of the layer. The results indicate that the behavior of the interface elements dominate for mode mixes close to mode I and plasticity in the continuum elements dominates for mode II dominated mode mixes. Furthermore, effects of large root curvatures of the adherends is analyzed numerically by simulating plastically deforming double cantilever beam specimens using the finite element model. The developed RVE is implemented in the models to simulate the behavior of the adhesive layer. By this methodology, virtual experiments can be analyzed with extreme detail. It is shown that in-plane straining of the adhesive layer significantly influences the strength of adhesive joints at large plastic strain of the adherends. There is a never ending need in industries to minimize computational time. To this end, an interphase finite element for structural analyses is developed. The element considers in-plane straining of the adhesive layer due to large curvatures of surrounding substrates.
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| 15. |
- Salomonsson, Kent, et al.
(författare)
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On the apparent influence of the adherends on the fracture toughness of adhesive layers
- 2007
-
Ingår i: Interface design of polymer matrix composites. - 9788755036260
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Konferensbidrag (refereegranskat)abstract
- A detailed model of experiments with the double cantilever beam specimen is set up. Analysis of the model shows that an experimentally deduced apparent increase of fracture energy with severely deforming adherends is due to contributions of in-plane straining of the adhesive layer to the fracture energy. An analysis with the J-integral confirms the result.
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| 16. |
- Schmidt, Peter, 1965-
(författare)
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Computational Models of Adhesively Bonded Joints
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Simulations using the Finite Element Method (FEM) play an increasingly important role in the design process of joints and fasteners in the aerospace industry. In order to utilize the potential of such adhesive bonding, there is an increasing need for effective and accurate computational methods. The geometry and the nature of an adhesive joint are, however, not so simple to describe effectively using standard FEM-codes. To overcome this difficulty, special FEM-elements can be developed that provide a material surface treatment of the adhesive and the joined parts.In order to create a model that reflects the above features, one may introduce proper scalings on the geometry and on the material properties in terms of a perturbation parameter. Within the framework of three-dimensional elasticity, together with an asymptotic expansion method, a material surface model is obtained through a systematic procedure. In such a derivation, no a priori assumptions for the displacements or stress fields are needed. The final result is a variational equation posed over a single reference surface which forms the basis of a structural element for the compound joint.Through the usage of continuum damage mechanics and the framework of a generalized standard material, the linear elastic model is extended to include an elastic-plastic material model with damage for the adhesive. The model is FE-discretized and an important implication is that the (quasi-static) propagation of the local failure zone in the adhesive layer can be simulated. The failure load is obtained as a computational result and consequently no postulated failure criterion is needed. The derived FE-method opens up the possibility to efficiently model and analyze the mechanical behavior of large bonded structures.
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| 17. |
- Stigh, Ulf
(författare)
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Continuum Damage Mechanics and the Life-Fraction rule
- 2006
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Ingår i: Journal of applied mechanics. - : American Society of Mechanical Engineers. - 0021-8936 .- 1528-9036. ; 73:4, s. 702-704
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Tidskriftsartikel (refereegranskat)abstract
- This paper gives a short review of two different methods for life prediction at high temperature; namely continuum damage mechanics (CDM) and the linear life-fraction rule (LFR). It is well known that the class of CDM theories with a separable evolution law gives a life prediction in accordance with the LFR. However, it appears to be an open question if this is a necessary condition. It is here shown that in order for a CDM theory to comply with the LFR it must have a separable evolution law. That is, if we can assume that a material follows the LFR, it is necessary to chose a separable evolution law for this material. The reverse is also true, to get a life-fraction different from unity, we must chose a nonseparable evolution law.
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| 18. |
- Stigh, Ulf
(författare)
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Limning för hög hållfasthet
- 2009
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Ingår i: Svetsen. - : Svetskommissionen. - 0039-7091. ; 68:3, s. 8-9
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 19. |
- Stigh, Ulf, et al.
(författare)
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Measurement of cohesive laws and related problems
- 2009
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Ingår i: Proceedings of the ASME International Mechanical Engineering Congress and Exposition. - : ASME Press. - 9780791838631 - 9780791843840 ; , s. 293-298
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Konferensbidrag (refereegranskat)abstract
- Cohesive modelling provides a simple method to introduce a process region in models of fracture. It is computationally attractive since it blends into the structure of finite element programmes for stress analysis. The development of computational methods and applications of cohesive modelling has accelerated during recent years. Methods to measure cohesive laws have also been developed. One class of such methods is based on the path-independence of the J-integral. By choosing a path encircling the cohesive zone, J can be shown to be given by the area under the traction-separation relation for the cohesive zone. Using an alternative path, J can in some cases be directly related to the applied load and deformation with relatively modest or no assumptions on the material behaviour. Thus, the cohesive law can be measured.Methods to measure cohesive laws for different specimen geometries are presented. The method are used to measure the cohesive law in peel, shear and mixed-mode for an adhesive layer. A new method to measure cohesive laws in shear is presented. The method is shown to give accurate data with a much smaller test pecimen than earlier methods.
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