Finite element analysis (FEA) can play a major part in carrying out an engineering critical assessment (ECA); TWI often uses this method when conducting work for members.
Finite element analysis for ECA typically requires the creation of a model of an engineering structure, including postulated or measured defects, to calculate stress intensity factors and collapse loads, which are then used to prepare failure assessment diagrams.
These models usually include sharp cracks and are non-linear, so high-quality material property data is required. A clear understanding of the loading conditions is also essential to ensure that appropriate boundary conditions are used. For design of structures subjected to cyclic loading it is necessary to create FEA models to calculate hot spot stresses at specific locations, including welds, for fatigue life calculations.
TWI uses commercial FEA software to create complex models of engineering structures. The models must be created both with and without defects, which may include sharp cracks or locally thinned areas due to corrosion.
Where it is necessary to assess several scenarios the creation of a set of models may be automated. The models, which usually contain high levels of plasticity and/or contact between different components, are non-linear, and are run on multi-processor servers at TWI to ensure that results are produced in a realistic timescale.
Validation and interpretation of the results is an important consideration. An understanding of the effect of constraint on the behaviour of materials allows data from small-scale tests to be used in the assessment of large structures.
Applications and benefits
The use of FEA in assessments, where permitted by the relevant codes and standards, allows fabricators and operators to avoid the excess conservatism that is built into the analytical approaches used by the codes. This will typically allow materials, weight and cost savings, while maintaining the required level of safety.
An example is the treatment of residual stresses in welds. Without evidence to the contrary, standards require the assumption of residual stresses of yield stress magnitude at welds. Suitable FEA models can be used to demonstrate that residual stresses are lower than this, allowing a reduction in the conservatism of the ECA.
TWI has made extensive use of FEA for ECA in consultancy projects for our Members. This includes experience of the assessment of pipelines (including during and after high strain due to reeling), pipe-in-pipe systems, pressure vessels, chains, and a wide range of other engineering structures.
The expertise behind this work was developed via internal research, including contributions to the development of standards (eg BS 7910, 'Guide to methods for assessing the acceptability of flaws in metallic structures'; and R6, 'Assessment of the integrity of structures containing defects').
We have also developed automated software to prepare and run high-quality models, and extract the required information, for a wide range of defects in pipeline girth welds. This allows cost-effective and rapid investigation of the effect of parameters such as defect shape, weld geometry, cladding, weld-offset, HAZ size etc.
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