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Ductile Adhesively Bonded Timber Joints
1. Ductile Adhesively Bonded Timber Joints Simon Hehl, Research Assistant Thomas Tannert, Research FellowTill Vallée, ProfessorTimber & Composites LaboratoryBern University of Applied Sciences
2. Outline Ductility in the context of bonded timber joints Experimental investigations Numerical modeling Discussion Conclusions
3. Ductility in the context of bonded timber joints Ductility holds a central status in civil engineering Allows for load redistribution, announces failure Ductility is usually achieved through plastic behavior Yielding of steel Plastic behavior of concrete in compression Timber has to be considered as being brittle Especially when it comes to tensile and shear
4. Ductility in timber structures Timber is not ductile If not considering some plastic behavior in compression Ductility in timber structures can be found in the connections In the context of adhesives, it boils down to consider the use of plastic adhesives Preliminary work done for Glulam Similar research performed of G/C-FRP
5. Experimental investigationsSeries 1 Tests on adhesively bonded double lap joints (DLJ) Timber: Spruce Consideration of three different adhesives A commonly used stiff epoxy Two softer plastic adhesives Three overlap lengths
7. Experimental investigationsSeries 2 Tests performed on DLJ involving graded adhesive layers Soft adhesive in the middle portion vs. stiff outside Variation of the grading level, i.e. proportion of soft vs. stiff
8. For all adhesives No significant differences in the load-displ’ curves Brittle failure in the timber No objective signsof ductility! No influence on strength Experimental investigations
11. Experimental investigations Neither did the use of ductile adhesives enhance ductility nor increase the ultimate strength of adhesively bonded joints Since the use of softer, especially plastic, adhesives leads to a reduction of stresses, this results sounds contradictory
12. Numerical modeling All configurations were numerically modeled Using Ansys v11 Material properties gathered for this purpose including the orthotropic properties of timber Including the plastic properties of the adhesives Determination of all stress components that trigger failure σx,σy, τxy
16. Numerical modeling FEA confirms that ductile adhesives indeed lead to a reduction of stress magnitudes Correspondingly an increase of joint strength would be expected This leads to the conclusion that joint strength is not exclusively driven by a stress based approach Actually, the problem is best described using a probabilistic method
17. Numerical modeling Each element has a different volume, Vi, compared to the specimen on which strength was determined, V0 Its strength is thus different… Each of the elements i has a probability of survival …determined using Weibull …Ps of joint = ΣPs,i Size effects Weibull probabilistic
20. Discussion The use of ductile adhesives leads to a reduction in stress magnitudes, but also to increasing the volumes over which these stresses act There is a lack of correlation between stress magnitude and joint strength Probabilistic methods can be used to explain the apparent contradiction raised by this study
21. Numerical modeling CHAMFERED DLJ Stresses reduced Till Vallée, Juan Murcia, Thomas Tannert and David Quinn, Influence of stress reduction methods on the strength of adhesively bonded composed of brittle adherends, subm. To Int. JourAdh&dh.
22. Beyond adhesives and/or timber T. Vallée, J. R. Correia, and T. Keller, „Probabilistic strength prediction for double lap joints composed of pultruded GFRP profiles, part II: Strength prediction“, Composites Science and Technology, vol. 66, no. 13, pp. 1915-1930, 2006.
23. Conclusions The lack of ductility of timber, and structures made thereof is a serious issue To overcome this issue, several researchers point towards the use of ductile adhesives …their claim is based on a correlation between structural response and adhesive ductility, i.e. stress reductions This study, amongst others, gives indications that this approach is highly hypothetical …and offers an explanation why the lack of the expected pay-off using ductile adhesives was predictable, and never clearly demonstrated experimentally
24. Thank you for your attention I’m happy to answer your questions till.vallee@fibreworks.org
