Understanding fracture behavior at the interfaces of brittle composite materials requires appropriate measurement techniques for fracture toughness. Due to their simplicity and convenience, indentation techniques are attractive solutions. One such technique is the interface indentation fracture (IIF) test, which measures the relative toughness of interfaces between brittle materials by introducing a series of indents at various angles of incidence (0–90°) to the interface, from which crack growth will either be by penetration through the interface or by deflection (debonding) along it. Larger angles of incidence promote penetration and smaller angles promote deflection, so by noting the critical angle at which propagation changes from penetration to deflection, the IFF test can make inferences about relative fracture toughness of different interfaces tested under similar conditions. However, as previous work by Parmigiani and Thouless has shown, the penetration vs. deflection behavior of a crack incident to an interface is a function not only of interface fracture toughness but also of interface strength. Interface cohesive zone elements in a finite element model incorporating both fracture toughness and strength criteria were used to study the propagation behavior of cracks normally incident to brittle composite interfaces. In the follow up work presented here, the cohesive zone method (CZM) has been extended to study cracks that occur at varying angles of incidence to these interfaces. Results show that IIF testing does not always result in unique values for relative fracture toughness; when interface strength is varied, it is possible for identical IIF-test critical angles to correspond to differing interface toughness values and, conversely, for differing critical angles to correspond to identical fracture toughness values. To properly employ the IFF test method, this phenomenon must be taken into account.

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