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Computational Mechanics of Composite Materials part 9

Tham khảo tài liệu 'computational mechanics of composite materials part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fracture and Fatigue Analysis of Composites 225 Let us note that direct determination of fatigue cycle number makes it possible to derive without any further computational simulations the life of the structure till the failure while the stiffness reduction approach is frequently used together with the FEM or BEM structural analyses. The crack length growth and damage function approach are used together with the structural analysis FEM programs usually to compute the stress intensity factors. However final direct or symbolic integration of crack length or damage function is necessary to complete the entire fatigue life computations. Considering the mathematical nature of the fatigue life cycle estimation the deterministic approach can be applied where all input parameters are defined uniquely by their mean values. Otherwise the whole variety of probabilistic approaches can be introduced where fatigue structural life is described as a simple random variable with structural parameters defined deterministically and random external loads. The cumulative fatigue damage can be treated as a random process where all design parameters are modelled as stochastic parameters. However in all probabilistic approaches sufficient statistical information about all input parameters is necessary which is especially complicated in the last approach where random processes are considered due to the statistical input in some constant periods of time using the same technology to assure the same randomness level . The analysis of fatigue life cycle number begins with direct estimation of this parameter by a simple power function A5.1 consisting of stress amplitude as well as some material constant s . Alternatively an exponential-logarithmic equation can be proposed A5.2 where temperature strength and residual stresses are inserted. Both of them have a deterministic form and can be randomised using any of the methods described below. The weak point is the homogeneous character of the .