Sheared blocks, bond failures

Fracture energies have been determined for bond failure at either end of the bond line for a sheared rubber block, but the results are inconclusive—it is not clear where bond failure will occur, or at what load, even when the fracture properties of the rubber are known. Thus, the initiation of cracks, especially at interfaces and comers, needs further study. 

Another compressive shear test used for wood joints is ASTM D905-49. Test joints are prepared by bonding two blocks of wood, free from defects, with the grain of each block parallel to the length direction. Test specimens are cut from this bonded joint and tested in a special shearing apparatus (Figure 13). Loads are applied using a crosshead movement of 0.015 inch/min. The shear stress is calculated with the failure load and bonded shear area. 

As shown in O Fig. 15.1, uncross-linked adhesives can exhibit two maxima in peel with shear rate. At slow peel rates they will fail cohesively. When pulled sufficiently fast their failure becomes adhesive, the peel force drops, but then rises with peel rate - as with cross-Unked adhesives - until falling again when slip-stick failure occurs. Commercial styrenic block copolymer-based adhesives can exhibit high peel force cohesive failure at standard test speeds when bonded to high-energy substrates like polished steel. 

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