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Mode I
Mode II
Mode III
A major consideration in the design of adhesively bonded structures is the possibility of crack growth, either within the adhesive or at the adhesive-adherend interface. Crack propagation can be catastrophic when the fracture toughness G of the adhesive-adherend system has been exceeded. Debonds or delaminations are probably the most life-limiting defects that occur in layered or laminated structures, and may arise during processing or subsequent service. Common structural features, such as thickness changes, holes or bonded joints, generate through-thickness stress concentrations, which may initiate debonding under static or cyclic loading conditions. The situation is exacerbated at elevated temperatures and in the presence of aggressive environments (e.g. water vapour or solvents).
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Mode I |
Mode II |
Mode III |
Crack initiation and growth usually occurs under mixed-mode conditions, a combination of Mode I (crack-opening), Mode II (forward-shear) and Mode III (scissor-shear or tear). As a result, fracture toughness tests have been developed to quantify resistance to crack initiation and growth under tensile and shear loading conditions. These test methods are used to assess the influence of adherend surface condition, adhesive, adhesive-adherend interactions, primers, etc., on the fracture strength. Efforts in recent years have been directed towards generating reliable test methods for determining fracture toughness under mixed-mode loading conditions.
There are a number of reservations as to the relevance and reliability of fracture toughness data, particularly in applying test results to actual engineering structures. Although serious doubts have been raised, it is recognised that a fracture mechanics approach to studying the failure behaviour of adhesive joints has proved invaluable in understanding the fundamental mechanisms of adhesion. Fracture toughness data obtained from mode I fatigue tests has been used to predict the fatigue life of adhesively bonded joints with good agreement obtained between the theoretical predictions and experimental results. This section considers Mode I, Mode II and Mixed-Mode I/II test methods. For more details see fracture toughness testing.
Next: Mode I
