Residual stresses are generated in a part during manufacturing which typically involves sequential thermo-mechanical processing including forming and heat-treatment steps. These stresses can then be influential during subsequent machining operations where the removal of material results in the redistribution of bulk stresses within the part leading to potentially significant distortions and problems with dimensional tolerances. Contour method is a residual stress measurement technique that can provide information about the magnitude and distribution of these stresses. It is a destructive technique whereby the part is sectioned to release the internal stresses, manifested as waviness of the cut face. The surface profile is then scanned, using either coordinate measuring systems or non-contact optical means, to measure out of plane distortions that will then be applied as boundary condition in an inverse simulation, to obtain a full cross-sectional map residual stresses. The technique offers evaluation of the bulk stresses with a good spatial resolution, reasonable accuracy, and is not limited by the shape or size of a part.
State of the Art/Maturity
Knowledge of the internal stress state of the material enables engineers and machinists to understand the evolution of distortion during machining and design a strategy to acquire a right-first-time component, by optimising the cutting operations and therefore accounting for non-conformances. Thus, costly trial-and-error procedures can be substituted by a systematic examination of clamping configurations, material removal sequences, tool path optimisation, and compensation procedures. More than that, Contour Method offers the opportunity to link prior manufacturing processes to the end state of the material allowing for the optimisation of the entire manufacturing route, the development of a complete digital-twin and re-think how we are doing things currently. Though, the technique is not suited for in-situ, in-line, non-destructive inspection of parts; it rather accelerates the development and application of such techniques in production, such as Ultrasonic, compensating for their lack of resolution by providing the means for their validation and calibration.
Practical applications for Machining
- Mapping of internal stresses for design of optimum machining strategies
- Missing link for the integration of machining in the (re-)design of manufacturing route
- Development and calibration of in-situ, in-line, Non-Destructive Residual Stress techniques using Ultrasonic, to assess variations in parts/batches.