The loading requirements in a fracture mechanics analysis may vary considerably depending upon the goal of the analysis. Some examples include:
- Parametric study of crack sizes to determine stress intensity factors under a number of load conditions
- Evaluation of j-integrals at multiple load increments1 in a non-linear plastic collapse analysis
- Thermo-mechanical interaction during a transient loading condition
- Fatigue crack growth analysis with spectrum loading
- Sustained load crack growth analysis with load as a function of time
- Combined fatigue and sustained load crack growth in which da/dn and da/dt effects contribute significantly to the overall crack growth
An example of complexity of load types and time histories
The variety of possible requirements is tackled in Zencrack by the combination of two processes:
- extraction of the results of one or more load increments from the finite finite element analysis
- a load system methodology for defining the load sequence during crack growth calculations
The processing of multiple increments from the finite element analysis is sufficient for cases without crack growth. It allows, for example, creation of K vs time plots through a transient loading sequence.
Any local mixed mode effects arising at the crack front as a result of the geometry and loading are embodied in the stress and displacement solution of the finite element analysis. A crack growth direction can therefore be calculated for each processed load increment and this direction may change through the loading sequence.
For crack growth, the load system approach allows a framework to be constructed that defines the loading history applied to the structure. The load system framework is then cross-referenced to the results from the finite element analysis. This load system approach can be used to define simple cases such as constant amplitude fatigue loading through to complex general cycles which have combined fatigue and time dependent crack growth effects.
1 The term "load increment" is used here to describe the solution of a single load state in the finite element analysis. In Abaqus terminology this is the solution of an increment within a step. In Ansys terminology it is the solution of a sub-step within a step.
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