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Mechanical behaviour of adhesive layers: experimental methods, cohesive laws, and fracture mechanics
University of Skövde, School of Technology and Society.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Adhesive joining is today viewed as one of the key technologies to achieve decreased emissions in the automobile industry. To decrease weight, optimal material selection often results in different materials for different parts. This leads to the necessity to join mixed material. Here, the use of adhesives is the most promising joining technology. For a rational design process, good models for strength analysis of adhesively joined structures are essential. With cohesive modelling, fracture of the adhesive layer is modelled with a stress-deformation law. This law - often denoted a cohesive law - gives the traction exerted on the adherends due to the deformation of the adhesive layer. This thesis is concerned with experimental methods to measure cohesive properties of engineering adhesives and standardized methods to measure the fracture energy of adhesives. A new method to measure cohesive laws is developed. With this method, the cohesive law of an epoxy adhesive is measured in shear. In peel loading, with elastically deforming adherends, the cohesive law is shown to be independent of the geometry of the specimen. If the adherends deform plastically the fracture energy increases. Experiments are performed in order to determine the temperature dependence of the cohesive layer for an epoxy adhesive. It is shown that the peak stress is strongly dependent on the temperature while the fracture energy shows only small temperature dependence. Experiments are also performed to study the influence of strain rate in peel and shear loading. The experiments show that the peak stress increases with an increasing strain rate and that the fracture energy increase in peel loading and decreases in shear with increasing strain rate. A new method to experimentally determine the relation between damage and plasticity in the adhesive during the fracture process is developed. For the present adhesive, it is shown that only minor plasticity occurs during the fracture process in peel loading. For peel, several commonly used methods to evaluate the fracture energy using the double cantilever beam specimen are critically studied. For some methods the error in evaluated fracture energy is larger than 40 %. It is shown that the evaluated fracture energy is more dependent on the choice of method than on the cohesive properties of the adhesive layer.

Place, publisher, year, edition, pages
Chalmers tekniska högskola , 2008. , 16 p.
Series
Doktorsavhandlingar vid Chalmers tekniska högskola, ISSN 0346-718X ; 2816
Keyword [en]
experimental method, adhesive layer, cohesive law, fracture, fracture energy, energy release rate, peel, shear
National Category
Materials Engineering
Research subject
Technology
Identifiers
URN: urn:nbn:se:his:diva-2941ISBN: 978-91-7385-135-0 OAI: oai:DiVA.org:his-2941DiVA: diva2:210687
Public defence
2008-10-06, 13:00 (English)
Opponent
Available from: 2009-04-03 Created: 2009-04-03 Last updated: 2013-04-16Bibliographically approved
List of papers
1. An experimental method to determine the complete stress-deformation relatioin for a structural adhesive layer loaded in shear
Open this publication in new window or tab >>An experimental method to determine the complete stress-deformation relatioin for a structural adhesive layer loaded in shear
2003 (English)In: The 9th International Conference on The Mechanical Behaviour of Materials, 2003Conference paper, (Other academic)
Identifiers
urn:nbn:se:his:diva-1925 (URN)
Available from: 2007-06-11 Created: 2007-06-11 Last updated: 2010-04-08
2. On the effective constitutive properties of a thin adhesive layer loaded in peel
Open this publication in new window or tab >>On the effective constitutive properties of a thin adhesive layer loaded in peel
2006 (English)In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 141, no 1-2, 227-246 p.Article in journal (Refereed) Published
Abstract [en]

An experimental method to determine the complete stress-elongation relation for a structural adhesive loaded in peel is presented. Experiments are performed on the double cantilever beam specimen, which facilitates a more stable experimental set-up as compared with conventional methods like the butt-joint test. The method is based on the concept of equilibrium of the energetic forces acting on the specimen. Two sources of energetic forces are identified: the start of the adhesive layer and the positions of the two acting loads. By use of the concept of equilibrium of energetic forces, it is possible to measure the energy release rate of the adhesive layer instantaneously during an experiment. The complete stress-elongation relation is found to be the derivative of the energy release rate with respect to the elongation of the adhesive layer at its start. By this procedure, an effective property of the adhesive layer is measured. That is, the fields are assumed to be constant through the thickness of the layer and only vary along the layer. To investigate the validity of this approach, experiments are performed on five different groups of specimens with different dimensions. This leads to large variations in the length of the damage zone at the start of the adhesive layer. Four of the experimental groups are used to determine the stress-elongation relation. This is found to be independent of the geometry. For the remaining experimental group, the adherends deform plastically and simulations are performed with the stress-elongation relation determined from the four elastic groups. It is found that the relation cannot be used to accurately predict the behaviour of the experiments where the adherends deform plastically. This indicates that the stress-elongation relation has limited applicability.

Place, publisher, year, edition, pages
Springer Netherlands, 2006
Keyword
Stress-elongation relation, Adhesive layer, Experimental method, Energetic force, J-integral, Damage zone, Anticlastic deformation
National Category
Engineering and Technology
Research subject
Technology
Identifiers
urn:nbn:se:his:diva-1970 (URN)10.1007/s10704-006-0075-6 (DOI)000243017400017 ()2-s2.0-33845344381 (Scopus ID)
Available from: 2008-04-16 Created: 2008-04-16 Last updated: 2013-04-10Bibliographically approved
3. Cohesive laws for adhesives at repeated loading: an experimental method
Open this publication in new window or tab >>Cohesive laws for adhesives at repeated loading: an experimental method
(English)Manuscript (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:his:diva-2940 (URN)
Available from: 2009-04-03 Created: 2009-04-03 Last updated: 2010-01-14
4. Effects of constitutive parameters on the accuracy of measured fracture energy using the DCB-specimen
Open this publication in new window or tab >>Effects of constitutive parameters on the accuracy of measured fracture energy using the DCB-specimen
2008 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 75, no 10, 2968-2983 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Elsevier, 2008
Keyword
Test standards, DCB-specimen, Toughness testing, Fracture energy, Adhesive
National Category
Materials Engineering
Research subject
Technology
Identifiers
urn:nbn:se:his:diva-2806 (URN)10.1016/j.engfracmech.2008.01.002 (DOI)000256014600009 ()2-s2.0-40849083441 (Scopus ID)
Available from: 2009-03-02 Created: 2009-03-02 Last updated: 2013-04-15Bibliographically approved
5. An analysis of the evaluation of the fracture energy using the DCB-specimen
Open this publication in new window or tab >>An analysis of the evaluation of the fracture energy using the DCB-specimen
2007 (English)In: Archives of Mechanics, ISSN 0373-2029, Vol. 59, no 4-5, 311-327 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Polish Academy of Science, 2007
National Category
Materials Engineering
Research subject
Technology
Identifiers
urn:nbn:se:his:diva-2807 (URN)000250482000002 ()2-s2.0-35649007956 (Scopus ID)
Available from: 2009-03-02 Created: 2009-03-02 Last updated: 2013-04-11Bibliographically approved
6. Influence of temperature and strain rate on cohesive properties of a structural epoxy adhesive
Open this publication in new window or tab >>Influence of temperature and strain rate on cohesive properties of a structural epoxy adhesive
(English)Manuscript (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:his:diva-2939 (URN)
Available from: 2009-04-03 Created: 2009-04-03 Last updated: 2010-01-14Bibliographically approved

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Citation style
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