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  • 1.
    Biel, Anders
    et al.
    University of Skövde, The Virtual Systems Research Centre. University of Skövde, School of Technology and Society.
    Stigh, Ulf
    University of Skövde, The Virtual Systems Research Centre. University of Skövde, School of Technology and Society.
    Walander, Tomas
    University of Skövde, The Virtual Systems Research Centre. University of Skövde, School of Life Sciences.
    A Critical Study of an Alternative Method to Measure Cohesive Properties of Adhesive Layers2012In: Proceedings of the 19th European Conference on Fracture, Kazan Scientific Centre of the Russian Academy of Sciences , 2012Conference paper (Refereed)
    Abstract [en]

    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.

  • 2.
    Biel, Anders
    et al.
    University of Skövde, School of Technology and Society.
    Walander, Tomas
    University of Skövde, School of Technology and Society.
    Stigh, Ulf
    University of Skövde, School of Technology and Society.
    Influence of Edge-boundaries on the Cohesive Behaviour of an Adhesive Layer2012In: ASME 2012 International Mechanical Engineering Congress and Exposition, November 9–15, 2012, Houston, Texas, USA: Volume 8: Mechanics of Solids, Structures and Fluids, ASME Press, 2012, p. 507-511Conference paper (Refereed)
    Abstract [en]

    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.

  • 3.
    Eklind, Alexander
    et al.
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Walander, Tomas
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Carlberger, Thomas
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Stigh, Ulf
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    High cycle fatigue crack growth in Mode I of adhesive layers: modelling, simulation and experiments2014In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 190, no 1-2, p. 125-146Article in journal (Refereed)
    Abstract [en]

    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.

  • 4.
    Marzi, Stephan
    et al.
    Fraunhofer IFAM, Bremen, Germany.
    Hesebeck, Olaf
    Fraunhofer IFAM, Bremen, Germany.
    Brede, Markus
    Fraunhofer IFAM, Bremen, Germany.
    Nagel, Christof
    Fraunhofer IFAM, Bremen, Germany.
    Biel, Anders
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Walander, Tomas
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Stigh, Ulf
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Effects of the bond line thickness on the fracture mechanical behaviour of structural adhesive joints2014In: Proceedings of the Annual Meeting of the Adhesion Society 2014, Adhesion Society , 2014, p. 189-192Conference paper (Refereed)
  • 5.
    Marzi, Stephan
    et al.
    Fraunhofer IFAM, Germany.
    Walander, Tomas
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Hesebeck, Olaf
    Fraunhofer IFAM, Germany.
    Brede, Marcus
    Fraunhofer IFAM, Germany.
    A Controlled Mixed-Mode Bending (CMMB) test to investigate the fracture of structural adhesive joints2014Conference paper (Refereed)
    Abstract [en]

    A so called Controlled Mixed-Mode Bending (CMMB) test is presented, which has been developed toinvestigate the fracture of crash-optimized, elastic-plastic adhesives loaded in mixed-mode. While mostcommon mixed-mode tests, like e.g. the MMB test, work fine with brittle adhesives, the controlled MMBtakes into account the large crack opening displacements at the crack tip and ensures a constant mode mixat the crack tip by the regulation of two actuators. Consequently, the definition of mode mixity is asignificant difference to state-of-the-art experiments, which define the mode mixity in terms of the ratio ofdissipated energy in the single modes and which are therefore based on analytical models or assumptionsconcerning the energy dissipation during the test. A further target of the presented CMMB test is to obtain information on the complete shape of the socalled traction separation law, which describes the relation between stress and displacement inside theadhesive layer. Such traction separation law is often used to define the failure behaviour of an adhesivejoint within a numerical analysis, using cohesive elements in a finite element code or similar approaches. Beside the theory and the idea of the CMMB test, experimental results for the adhesive SikaPower 498 arepresented and deeply discussed with respect to difficulties and limitations of the proposed method and therealized experimental setup.

  • 6.
    Stigh, Ulf
    et al.
    University of Skövde, The Virtual Systems Research Centre. University of Skövde, School of Technology and Society.
    Biel, Anders
    University of Skövde, The Virtual Systems Research Centre. University of Skövde, School of Technology and Society.
    Walander, Tomas
    University of Skövde, The Virtual Systems Research Centre. University of Skövde, School of Technology and Society.
    Fracture of Adhesive Layers in Mode II2012In: Proceedings of the 19th European Conference on Fracture, Kazan Scientific Centre of the Russian Academy of Sciences , 2012Conference paper (Refereed)
    Abstract [en]

    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.

  • 7.
    Stigh, Ulf
    et al.
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Biel, Anders
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Walander, Tomas
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Shear strength of adhesive layers – models and experiments2014In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 129, p. 67-76Article in journal (Refereed)
    Abstract [en]

    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.

  • 8.
    Walander, Tomas
    University of Skövde, School of Technology and Society. University of Skövde, The Virtual Systems Research Centre.
    Cohesive modelling of the temperature dependence of epoxy based adhesives in Mode I and Mode II loading2013Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    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.

  • 9.
    Walander, Tomas
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre. Chalmers University of Technology.
    Influences of temperature, fatigue and mixed mode loading on the cohesive properties of adhesive layers2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis concerns some aspects that have influence on the strength of adhesive layers. The strength is determined by the stress deformation-relation of the layer. This relation is also referred to as cohesive law. The aspects having influence on the cohesive laws that are studied in this work are temperature, fatigue, multi-axial fatigue and mixed mode loading.

    For each aspect, a model is developed that can be used to describe the influence of the aspects on the cohesive laws numerically, e.g. by using the finite element method. These models are shown to give good agreement with the experimental results when performing simulations that aims at reproducing the experiments. For the aspect of temperature, a FE-model is suggested that can be used to simulate the mechanical behaviour in pure mode loadings at any temperature within the evaluated temperature span. Also, a damage law for modelling high cycle fatigue in a bonded structure in multi-axial loading is presented. Lastly, a new experimental set-up is presented for evaluating strength of adhesives during mixed mode loading. The set-up enables loading with a constant mode-mix ratio and by the experimental results, a potential model for describing the mechanical behaviour of the evaluated adhesive is presented.

  • 10.
    Walander, Tomas
    et al.
    University of Skövde, School of Technology and Society.
    Biel, Anders
    University of Skövde, School of Technology and Society.
    Stigh, Ulf
    University of Skövde, School of Technology and Society.
    An evaluation of the temperature dependence of cohesive properties for two structural epoxy adhesives2012In: 19th European Conference on Fracture, Kazan Scientific Centre of the Russian Academy of Sciences , 2012Conference paper (Refereed)
    Abstract [en]

    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.

  • 11.
    Walander, Tomas
    et al.
    University of Skövde, School of Technology and Society. University of Skövde, The Virtual Systems Research Centre.
    Biel, Anders
    University of Skövde, School of Technology and Society. University of Skövde, The Virtual Systems Research Centre.
    Stigh, Ulf
    University of Skövde, School of Technology and Society. University of Skövde, The Virtual Systems Research Centre.
    Temperature dependence of cohesive laws for an epoxy adhesive in Mode I and Mode II loading2013In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 183, no 2, p. 203-221Article in journal (Refereed)
    Abstract [en]

    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.

  • 12.
    Walander, Tomas
    et al.
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Eklind, Alexander
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Carlberger, Thomas
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Stigh, Ulf
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Fatigue damage of adhesive layers: experiments and models2014In: Procedia Materials Science, ISSN 2211-8128, Vol. 3, p. 829-834Article in journal (Refereed)
    Abstract [en]

    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.

  • 13.
    Walander, Tomas
    et al.
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Eklind, Alexander
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Carlberger, Thomas
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Stigh, Ulf
    University of Skövde, School of Engineering Science. University of Skövde, The Virtual Systems Research Centre.
    Rietz, Andreas
    Scania STC, Södertälje, Sweden.
    Prediction of mixed-mode cohesive fatigue strength of adhesively bonded structure using Mode I data2016In: International Journal of Adhesion and Adhesives, ISSN 0143-7496, E-ISSN 1879-0127, Vol. 66, p. 15-25Article in journal (Refereed)
    Abstract [en]

    A cohesive zone model is presented for analyzing the fatigue life of an adhesive joint in the range of 104–106 load cycles. The parameters of the model are derived from Mode I double cantilever beam experiments. Fatigue experiments with adhesively joined components from the automotive industry are performed, and the results from the experiments are compared to the results of simulations. The error in the predicted fatigue strength is of the same order as the statistical deviation of the fatigue experiments, indicating that the simulation method produces acceptable predictions of the fatigue strength for applications in e.g. early product development.

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