All the stresses that exist in materials, also without the application of any external loads, are termed residual stresses. Residual stresses can originally exist in a component and naturally add to stresses induced by applied loads. As a result, residual stresses influence the behaviour of mechanical components and can impair the structural and dimensional stability, as well as the fatigue and fracture resistance of components. A residual tensile stress actually facilitates crack propagation and therefore reduces the fatigue life of a mechanical component. Residual stresses limit the loading capacity and safety of mechanical components during operation and in certain circumstances it becomes necessary to quantify those stresses.
Residual stresses can be caused by the following main factors:
Non-uniform heating or cooling of a component during manufacturing and fabricating processes (e.g., casting, welding, moulding, heat treatment)
Machining processes to remove shavings or plastic deformation (turning and forging)
Through or surface heat treatments (e.g., tempering, welding or grinding)
Sand-blasting or shot-peening.
The typical fields of application of residual stress are:
Airplane and aerospace applications
Automotive Industry: series production, sport races and competitions
Energy production: steam, wind and nuclear power plants
Oil and Gas: compressor and turbine parts
Railway production: wheels and railway samples
Production control: quality control of the surface and heat treatment
Overview of the measuring methods
Measurement of residual stresses can be carried out using different techniques. Residual stress techniques are based on indirect type of measurements. It means that thanks to the observation of the variation of different mechanical quantities (strain, diffraction) it is possible to evaluate the original stress that has caused this effect.
The available methods can generally be divided into 3 different groups, depending on the damage level that they cause in the specimens under investigation:
non-destructive techniques (X-ray Diffraction)
semi destructive techniques (Hole Drilling, Ring Coring)
destructive techniques (Sectioning, Slitting, Layering and Contour)
Non-destructive methods are aimed to measure residual stresses that do not cause surface damages to the component under test. These methods have the advantage not to damage the mechanical specimen; anyway, many of them have limitations regarding the maximum depth of investigation and the material constituting the component.
The most popular non-destructive method is X-ray diffraction, based on Bragg law, which offers the best results on the surface or in the first microns of depth. This method works on semi-crystalline materials, particularly on aluminium alloy and steels. It could be the preferable option for surface analysis or quality control of a specimen, also because it has a low measurement cost but it requires an expensive hardware to realize the tests.
The semi-destructive methods are based on the principle of removing a very small quantity of material from the specimens to release strains: generally, they consist in drilling a small hole or a core with limited dimension. These methods are considered semi-destructive because in most cases the presence of the hole or the core does not affect the functionality of the specimens under test.
After this operation the stress equilibrium is modified and a dedicated strain gage rosette, with a minimum of 3 grids, placed on the measurement area, can be used to acquire the strains released to obtain the new equilibrium of the workpiece. The strain data acquired by the strain gage sensor are used for the back calculation of residual stresses using special influence functions. Such methods find a very wide diffusion as they allow to perform measures easily and at a low cost.
The most used semi-destructive methods are the hole-drilling and the ring-coring.
Hole-drilling method can be applied on several testing materials including standard metals (Aluminium alloys, steels, cast irons), engineering advanced materials (Titanium alloy and Nickel Alloy), polymeric and composites material. Ring-coring is generally used on metals and particularly on large metal components (forged or melted). These methods provide results starting from the first microns to a total depth of 5mm depending on the test configuration.
The destructive methods for the measurement of the residual stresses require the complete destruction of the specimens under test after the removal of a part of the workpiece (by cutting, turning, sectioning). They can be based on the measurement of the strains (generally using a strain gauge, a strain gauge rosette or a specially oriented strain gauge chain) or on the analysis of the displacements generated after the cutting.
In the first family of these methods can be found the sectioning method, slitting or Layering (Layer Removal for flat plates or the Sach’s boring out method for cylindrical parts). Cuttings of different type and orientation and removing materials in different layers can be realized on the workpiece until a small area of the specimens is completely or partially cut. Depending on the test condition, these methods can provide results about the residual stress in the depth, or information about surface residual stresses.
In the second family of techniques can be found the Contour Method: this method provides full-field residual stress measurements with a 2D map of stresses. It requires a special cutting of the specimens using a wire EDM; after this process, the surface displacements are measured using a coordinate measuring machine.
A complete table of the different methods, including the related depth of analysis is reported below: