RESIdual STress ANalysis for Critical Elements – RESISTANCE

The development and shaping of structural components as well as their assembly generate significant residual stress fields in the structures. These stresses are superimposed on the in-service loads and have an influence on structures fatigue durability and buckling. When these residual stresses are not known, the implementation of a conservative design approach will lead to an oversizing which can be both costly and limiting in terms of performance, with a particular increase in the weight of the structures. Beyond the optimization of existing designs, the use of new materials and innovative manufacturing processes (additive manufacturing in particular) also makes it necessary to characterize the residual constraints generated. Finally, the improvement of the knowledge of the internal stresses of components already in service may allow their in-service life extension.
Residual stress fields are by nature heterogeneous and most often multiaxial. Therefore, their knowledge requires the characterization at different points and in different directions in space. For this purpose, there are different techniques that differ in the possible directions of measurement, the volume of measurement, and the depth of measurement.
The standardized and controlled methods are currently reserved for measurements close to the surface. Few methods can be used to characterize the stresses in the core of thick and large structures such as submarine hulls or nuclear reactor containments. If the contour method is now relatively widespread in the research community, including in France, it is limited to the measurement of small parts (< 1m). Only the deep hole drilling method can be applied on real parts but this method is currently mastered by only a few foreign organizations. The development of this method in France is therefore of strategic interest, particularly in the field of defense and nuclear energy (civil and military).
The objective of the ASTRID RESISTANCE (RESIdual STress ANalysis for Critical Elements) project is to combine the results of in-depth stress measurements in order to allow the reconstruction of stress fields in the core of thick components.
Several technical and scientific obstacles will have to be overcome in the RESISTANCE project. Experimentally and during an 18-month post-doctoral fellowship, it will first be necessary to develop an expertise on the deep hole drilling method and to validate its application. For this, at some reference structures will be designed and produced with different levels of complexity ranging from a 2D flat structure allowing to study the repeatability of the measurements towards more complex structures with strong stress gradients in the 3 directions representative of the targeted structures.
Numerically, it will be necessary to adapt the principle of the reconstruction method by deformation, inherent to an incomplete mechanics problem in the sense that it will not be possible to have experimentally all the information at every point. The available information may be contradictory due to the different precision of each measurement and the biases associated with each technique (in particular the appearance of plastic deformation during trepanning). An innovative reconstruction method will be developed through a PhD thesis, and will integrate the consideration of measurement uncertainties in order to reconstruct a compatible averaged field. Finally, an original approach will aim at enriching the method with the measurement of the strain hardening profile which will allow improving the spatial distribution of the introduced deformation fields.