Residual Stress Measurement
Residual stress laboratory and stress analysis center
Residual stress laboratory and stress analysis center
A list of the main residual stress measurement services with details, advancements and field of applications is reported here below:
The hole drilling strain gage method for measuring the residual stresses consists in drilling a small hole on the surface of the workpiece under testing. The drilling process allows to relieve the residual stresses and the associated strains around the drilled hole. The relieved strains can be measured with a strain gage rosette with 3 independent grids. The original residual stress can be evaluated starting from the measured strains and using the calibration coefficients. Calculation can be done according different methods including the ASTM E837 standard.
Depending on the strain gage rosette used and the related diameter of the drilled hole, residual stress can be evaluated in the depth range from 0.01 mm to 2.00 mm.
Metallic materials (Aluminium Alloy, Ferritic Steel, Austenitic Steel, Titanium Alloy, Nickel Alloy, Magnesium Alloy) – Polymeric materials – Composite material
Semi-destructive method. The typical dimension of the drilled hole is approximately 2.0 mm
Presence of international standard (ASTM E837) and accredited laboratory (ISO/IEC 17025)
Moderate cost for unit measurement
Possibility to perform measurements both in laboratory and directly on field
The ring coring strain gage method for measuring the residual stresses consists in drilling annular groove on the surface of the workpiece under testing. The drilling process allows to relieve the residual stresses and the associated strains around the drilled groove (external diameter 18mm / internal diameter 14mm). The relieved strains can be measured with a strain gage rosette with 3 overlapped independent grids. The original residual stress can be evaluated starting from the measured strains and using the calibration coefficients. Calculation can be done according different methods including the KWU method. The ring coring method is generally used for industrial aims to determine residual stresses on huge forged parts and large casted items.
Up to 5.0 mm in depth. Near the surface stress resolution is lower than the hole drilling method
Metallic materials (Aluminium Alloy, Ferritic Steel, Austenitic Steel, Titanium Alloy, Nickel Alloy, Magnesium Alloy) – Polymeric materials – Composite material
Semi-destructive / destructive method. The external diameter of the annular groove is approximately 18.0 mm
Presence of industrial procedures for carrying out the measurements
Intermediate cost for unit measurement
Possibility to perform measurements both in laboratory and directly on field
The strain gage mechanical methods are destructive techniques for the measurement of residual stresses using strain gages and strain gage rosettes installed directly on the surface of the workpiece under test. The mechanical methods consist in disturbing the equilibrium of the material by removing material from the component itself: depending on the chosen technique, the machining can be done by sectioning, turning or milling. The released strains are measured by the strain gage or strain gage rosette and then these values are used for the evaluation of residual stress. The most typical destructive mechanical methods are the Sectioning method, the Slitting method and the Layering methods (including the Sach’s boring out for cylindrical parts the or Layer removal for flat workpieces).
Depending on the chosen method, the total depth of analysis can reach about the entire thickness of the workpiece under testing.
Metallic materials (Aluminium Alloy, Ferritic Steel, Austenitic Steel)
Destructive method
Higher cost for unit measurement comparing with semi-destructive techniques
Higher depth of investigation
The X-ray diffraction method (XRD) is a mostly non-destructive technique for the measurement of the surface residual stresses. The X-rays are diffracted by atoms that composed the grain structure of the crystalline metallic materials. According to the Braggs law, the x-ray diffraction angle is related with the atomic lattice spacing. When the material under test is loaded, the atomic lattice spacing changes if compared with the unstressed conditions. These values increase in case of tensile stresses or decrease in case of compressive stresses. Therefore residual stress is then calculated by applying the standard relation defined by the elasticity theory. For the complete determination of the stress field, measurements need to be carried out in three independent directions for any test location.
Limited to the surface of the workpiece. In some case it is possible to increase the depth of analysis by removing material (electropolishing) and repeating the step-by-step measurement process.
Metallic materials (Aluminium Alloy, Ferritic Steel, Austenitic Steel)
Non destructive method for surface measurements / Semi destructive or destructive if XRD analysis are repeated step-by-step using elettropolishing for the evaluation of residual stress profile in the depth
Low cost for unit measurement in case of surface measurement / Moderate cost of the activity in case of stress profiling (depending on the depth of investigation).
For surface measurements: high spatial resolution and accuracy.
The contour method is a destructive technique for the measurement of residual stresses. This method is able to produce a 2D map of residual stress in a workpiece section. The Contour method is based on a variation of Bueckner’s elastic principle of superposition. The complete measurement process starts with the cutting of the specimens by EDM. Then the displacements along the cutting surface are accurately measured with a small spatial resolution by using a coordinate measurement machine with a touch probe. Finally, using finite element analysis (FEM), the acquired surface displacements are processed and a map of 2D residual stress is calculated.
Map of residual stress for the entire cutting surface
Metallic materials (Aluminium Alloy, Ferritic Steel, Austenitic Steel)
Destructive
High cost for unit measurement: otherwise the price is competitive considering the amount of data that can be provided
Full map of 2D stress of the cutting surface