Numerical and experimental analyses of a fracture mechanics test for adhesively bonded joints

The use of a fracture mechanics test to evaluate the joint strength through the determination of the strain energy release rate G is nowadays well est ablished. The joint strength for fluorinated polymer (PVDF) sheets bonded w ith an epoxy adhesive was studied using a double cantilever beam (DCB). In order to obtain small-scale yielding, the adhesive joint of the polymer spe cimens was strengthened by steel sheets. Pre-cracks were initiated at the c enter of the bond thickness separating the two PVDF surfaces, with nominal lengths ranging from 5 to 27.5 mm. We did not measure the evolution of the crack length, which is generally very difficult to obtain with good precisi on. The measurement of the load-point displacement was used instead. The op ening load versus this load-point displacement was recorded. The slope of t he first part of this curve gives the value of the initial stiffness of the joint specimen. The stiffness of the various specimens enables us to acces s the real experimental initial crack length, which was smaller than the no minal value, by comparison of the experimental values with the numerical on es. From the second part of the curve, the strain energy release rate value s for the crack propagation in the initial step (G(1)) and in the steady st ep (G,) are deduced. They were calculated from a least-squares linear fit o btained from the load-point displacement versus the inverse square of the l oad curve. The experimental results are discussed in light of an analytical analysis using the thin beams approach, improved with an elastic foundatio n model developed by Maugis describing the deformation of materials behind the crack tip, and of a numerical approach based on a finite element analys is. In this numerical model, an elastic-plastic behavior of the materials h as been assumed. Analytical and numerical approaches are compared and their validity and limitations are discussed.