Mixed-mode failure

It is seen in Table 1 that SIMS Is applicable to several areas of investigation in adhesive bonding. SIMS may be used in a variety of ways including species imaging of the surface (SUMS) which may be especially useful for clarifying mixed mode failure surfaces. 

Ion beams provide useful information either as a diagnostic tool or as a precision etching method in adhesion research. The combination ISS/SIMS method used along with other techniques such as SEM provides a powerful tool for elemental analysis of surface composition. These results, as well as earlier work in this laboratory, indicate that the surface composition can be significantly different from the bulk due to contamination, selective chemical etching and segregation. These same techniques also provide an analysis of the mode of failure in adhesive joints. Many failures classified as "adhesive" on the basis of visual inspection are frequently mixed mode failures or failures at a new interface containing elements of both adhesives and adherend. 

Ductile-brittle transitions are more accurately located by variable temperature tests than by altering impact speed in an experiment at a fixed temperature. This is because a linear fall in temperature is equivalent to a logarithmic increase in straining rate. The ductile-brittle transition concept can be clarified by sketches such as that in Fig. 11-25. In the brittle region, the impact resistance of a material is related to its LEFM properties, as described above. In the mixed mode failure... 

Ducept F, Gamby D, Davies P, A mixed mode failure criterion from tests on symmetric and asymmetric specimens Comp. Sci Technology, 59, 1999, p609-619. 

Ducept F, Davies P, Gamby D Mixed mode failure criteria for a glass/epoxy composite and an adhesively bonded composite/composite joint, Int. Journal of Adhesion Adhesives, 20, 3, 2000 p233-244. 

Ducept et al., [13] studied mixed mode failure criteria for a glass/epoxy composite and an adhesively bonded composite joint. In their study, the initiation failure point detected by acoustic signal and by the non-linearity point on the load/displacement curve and found good correspondence. Magalhaes et al., [14] studied the application of acoustic emission to investigate the creep behavior of composite bonded lap shear joints. 

In addition to pure modes I and II testing, there is considerable interest in determining the fracture resistance of joints subject to mixed-mode (I/n) loading as adhesive joints in engineering structures will frequently experience such mixed-mode conditions in service. There has also been a significant effort in the literature directed toward developing mixed-mode failure criteria for adhesively bonded joints, such that fracture resistance envelopes might be predicted if the resistance to the pure modes is known. 

Mixed mode failure surface of a specimen tested in creep with un-reinforced epoxy adhesive (Khalili et al. 2009)... 

Average and one standard deviation, [2] Double failure, [5] Mixed-mode failure. 

Figure 6 shows typical appearance of failure of the SiC/Mo-foil/SlC and SlC/Tl-foll/SlC sandwiches after the DNS test. Three kinds of the failures were observed In this study (a) joint failure, (b) mixed-mode failure, and (c) base failure. Notch to notch fractures were observed In all failure-modes. In the joint failure, the cracks propagated along the joint layer. In the base failure, the cracks ran through the SiC base. The mixed-mode failure is defined as a failure mode not characterized with either joint or base failure, and the crack appears to propagate in both the joint and base. 

Figure 8. DNS strength of the Mo foil-joined SiC exhibiting the mixed-mode failure as a function of mean crack spacing in the bonded zone.

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