Environmental failure fracture mechanics

Proof of durability and safe performance are, rightly, onerous requirements for any innovations in the construction industry. The parameters affecting environmental durability have been summarised, and water has been identified as the most hostile environment for bonded joints that is commonly encountered. Identification of the general failure mechanisms is useful because it highlights the procedures necessary for the satisfactory fabrication of reliable and durable bonded joints. It also enables the development and adoption of appropriate test methods, since real joint configurations are of limited use in assessing experimentally environmental effects (e.g. bonded areas must be minimised in order to allow environmental access within a reasonable time-scale). Fracture mechanics methods. 

In the case of structural joints, cohesive failure of the adhesive is often observed. Indeed, in many instances, the requirement will be to engineer failure within the bulk of the adhesive layer. Modelling of the adhesive response as a function of water uptake with temperature is carried out to represent the influence of environmental exposure. With fatigued structural joints, by combining the use of fracture mechanics and appropriate modelling tools, the resultant crack propagation rates and consequently durability levels can be predicted as a function of various environmental and mechanical test parameters, such as frequency. Fatigue threshold values can be determined, which are used to predict durability performance. " ... 

The various methods used to study the effect of environmental exposure on structural bonds have been described, pointing out that such methods provide results of a comparative nature only. There are at present few test methods that provide information useful for making predictions of durability of actual bonded structures. Most of the test methods involve the simultaneous effects of an applied load and environmental exposure some involve exposure to cyclic conditions. As explained, these conditions are much more detrimental to joint performance than is exposure to a single test condition. The tests based on fracture mechanics principles have provided some especially useful information and insights into the mechanisms of environmental failure. 

Future work must address two areas to provide the foundation for statistically based analyses of high-cycle CF (as well as environmental LCF and FCP). For simple laboratory conditions, the Weibull analysis of mechaniccil HCF failure probability [82] must be extended to include CF. Second, variable load, temperature, and environment chemistry histories are likely to be complex in applications and significantly affect CF Hfe. Such history effects have not been studied. The scaling of Basquin relationship data to predict the Ufe of a structure is qualitative and uncertain. Either the local strain approach to CF crack formation/eeurly growth life or the fracture mechanics analysis of CF propagation provide a better foundation for life prediction and failure analysis. 

Introduction. Electrochemical corrosion of the substrate may lead to a loss of joint strength and greatly reduce the service-life. However, one has to be wary of misinterpreting any initial observations of the failed joints often signs of gross corrosion are seen on the fracture surfaces of joints which have suffered environmental attack, but such corrosion may have frequently resulted after interfacial failure had occurred due to one of the mechanisms given in Section 2.2. Thus, in such cases, corrosion of the substrates is a post-failure event, rather than a primary cause of environmental failure. 

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