Measuring fracture energy

Figure 2. Measured fracture energy (Glc) vs. molecular weight (distribution peak) for narrow molecular weight fractions of...

Yet, for systems A and C, the measured fracture energies remain low compared with the critical fracture energy of the bulk aluminum 10 J Moreover, we do not observe islands of passivation material on the A1 fracture surface and, inversely, we do not observe A1 on debonded surfaces of the passivation films. This suggests that the loss of interfacial adhesion is close to a brittle fracture process despite the influence of plasticity of the A1 substrate and crack blunting at the interface. This sort of brittle mode of interfacial failure, including plastic flow in a ductile material (the substrate), has been observed or discussed for a sapphire/Au interface. ... 

Fig.5. Measured fracture energy for thin metal films debonded from (un)treated polyimide...

The situation is more delicate when the two materials have different moduli. In this case, if the beams are of identical thickness the failure will no longer be purely mode I. In these circumstances the crack will deviate from the interface into the material with the lower deformation resistance, leading to additional energy dissipation. In these circumstances the measured values of the interfacial fracture energy will be larger than Gic- This problem can be overcome by using an asymmetrical test, in which the thicknesses of the two beams are unequal. At a particular ratio of thicknesses the measured fracture energy will be a minimum and this may be taken as the true value of G c. 

References to the effects of temperature and test rate have been cited in Chapter 1. The work has been done by Mostovoy and Ripling and others.Their findings suggest that adhesive bonds may behave as viscoelastic solids. This implies that test rate and temperature have opposite effects, i.e., increasing temperature or decreasing test rate will increase the measured fracture energy. 

To judge the intrinsic adhesion at an interface by the measured fracture energy may be misleading. For example, if an adhesive is modified by adding fillers... 

Figure 7.24 Effect of crosshead displacement rate on the measured fracture energy, Gic, and type of crack growth [108]. (a) DGEBA-TEPA adhesive (b) DGEBA-TETA adhesive. O stable brittle crack growth (Type C) A initiation and A arrest of unstable crack growth (Type B). (Test temperature = 22 °C TDCB joints aluminium alloy substrates.)...

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