Högskolan i Skövde

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.