Failure and Damage
Failure of a structure or a system, in general, refers to the condition that the structure or system stops functioning satisfactorily. The criteria to decide the satisfactory functioning can be subjective or quantitative. In general, in engineering applications the failure is quantified using various criteria. The following are some of the criteria used to quantify a failure:
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Strength -
Form failure
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Stiffness -
Yielding
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Fatigue life -
Bending
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Corrosion resistance -
Impact resistance
Resistance to lightening Resistance to hazardous environmental agents
However, the list is in-exhaustive with many such criteria. In general, failure is understood as complete de-functioning of the structure.
In case of composites, the failure of a lamina or laminate needs special attention. In case of laminates there are a number of local failures before it completely breaks into two or more pieces. The local level failure is called as “damage”. In case of fibrous composites the term “local” refers to the individual constituent phases – fibre and matrix. Thus, damage in case of fibrous composites is a micro level event.
It is important to note that the ultimate failure (rupture/breaking) of the laminate takes place by gradual accumulation of damage. In turn, this is manifested at the lamina or laminate level by some form of failure. Thus, the “first failure” in laminates does not mean the “final failure”. The development of additional local failures with increasing loads or time is termed as “damage accumulation”. The terms “damage growth” and “damage propagation” are equivalently used for damage accumulation. The branch of mechanics which deals with the study of initiation and accumulation of damage until and including complete rupture is called as “damage mechanics”.
In this lecture we are going to see the fibre-matrix level failure mechanisms in detail. The failure at lamina/laminate or macro-level is the ultimate result of the local failures. Thus, the understanding of these mechanisms is a key point in the development of a reliable and accurate failure theory for laminated composites. Further, this understanding also helps in developing new materials with higher strength.
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