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Defect tolerant fatigue design


Acceptance limits of defects for cost effective manufacturing of fatigue critical components

Material and manufacturing defects such as porosity, inclusions, and crack like defects have a significant effect on the fatigue strength of load bearing components. However, depending on the size, shape, location and distribution of the defects their effect on the component fatigue strength can be accepted in many cases. In practice all industrial components contain material and manufacturing defects and the challenge is how to define the defect acceptance limits. Even large defects can be accepted at low stress locations of components by careful design and controlled work flows.

Load bearing components can be designed so that the effect of defects on the fatigue strength is minimized and the products are cost effective. This is possible by using modern manufacturing process simulation tools, such as casting simulation and SLM simulation, as part of the design work flow. The manufacturing process simulation tools provide estimates of residual stresses, distortions and defects of components in early phase of the design process.

Assessment of fatigue strength vs. manufacturing quality for castings, forgings and additive manufacturing (AM) components, welded details and materials

The Defect tolerant fatigue design -tools developed by VTT provide means for analyzing the material and manufacturing defects. Welds, castings, forgings and additive manufacturing components and materials can be studied by the tools.

The analysis procedure starts with microscopy analysis of macro specimens of material or component samples. The defects are then detected and classified from the microscopy images by image analysis. A fast estimate of the material fatigue strength is obtained based on the proven and widely used Murakami-Endo approach. This approach is well suited for defect sizes up to 1 mm in diameter. The defects can also be studied by computational stress analysis (FEA). The image based FEA is best suited for clustered and large defects.

A large number of microscopy images can be processed fast by the automatic analysis procedures, to cover the variation of the defects for example at several location of the component. Together with stress analysis of the component the effect of the sizes and locations of the defects on the fatigue strength of components can be studied. The acceptance limits for defects can be determined for use in design, purchasing or in trouble shooting and failure analysis cases as basis for assessment if the product has been fit for purpose.

See also infographic about defect tolerant fatigue design

Allowable porosity size for load bearing SLM components
Analysis chain to cover the fatigue related factors for SLM
Define the allowable defect size vs stress level by fast methods
Apply surface finishing for fatigue strength only where needed
Shorten build time by allowing more porosity at low stress areas