Residual Stress Analysis (X-Ray Diffraction)
What is residual stress?
Residual stress is mechanical stress that is present in materials independent of external load. It is caused by plastic deformation and exists in practically every component to a varying degree. All manufacturing techniques and processing procedures introduce residual stresses that may significantly influence the strength and service life of the respective component. If residual and operating stresses overlap in an unfavorable way, failure can occur at external loads below the strength limits of the material. In turn, the focused introduction of residual stress can improve components in their operational strength. Quantitative information on the magnitude of residual stresses and the stress field orientation are therefore important quality characteristics and can be failure-relevant variables. In this respect, X-ray diffraction (XRD) represents a well-established technique for precise residual stress analysis on poly-crystalline materials.
Non-destructive residual stress analysis on metal parts using X-ray diffraction (XRD)
X-ray radiation is diffracted by crystallites of the component material (via Bragg reflection). Using the 2-theta method, information on the atomic distances in the crystal lattice (d-spacings) can be derived. In an elastically stressed material, the crystal lattice is deformed compared to the stress-free state. This is employed in X-ray residual stress analysis using the sin2ψ method. X-ray diffraction is performed on the sample at different tilt angles, enabling precise detection of length changes in the atomic distances in the tilt direction. Using elastic constants, these length changes can be converted into compressive or tensile stress values (given in MPa). Residual stress can thus be analyzed in a specific component direction. The stress tensor is calculated by tilting in three different component directions.
The information depth of the X-rays is small and measures only a few microns. Residual stress is therefore determined at the component surface. Through repeated, local material removal by means of electrolytic polishing, it is possible to derive a residual stress depth profile without significantly changing the stress pattern of the component. This can be done at high spatial resolution.
Instrumentation: StressX Diffractometer
Our compact diffractometer is mounted on a 6-axis robot. This setup enables the investigation of components of variable shapes and dimensions. We carry out precise residual stress measurements on a wide range of metals and alloys (e.g. steel, Ni, Cu, Ti, Al, etc.). Different X-ray tubes are used for specific materials for achieving optimal measurement results.
Artefact-free removal of the component surface by electrolytic polishing enables measurements of the residual stresses at different component depths.
Determination of retained austenite content
Quantification of residual austenite in steel is possible by means of X-ray diffraction. Austenite (γ-iron) has a characteristic diffraction pattern that distinguishes it from ferrite (α-iron), allowing precise phase quantification down to low concentrations (<1%). We use a diffractometer optimized for this application, that is equipped with a 65 kV molybdenum X-ray tube. The analysis complies with norm ASTM E975.
- Material fatigue: Residual stress in form of compressive or tensile stresses can be beneficial or detrimental to material strength.
- Surface residual stress directly affects the service life of hardened parts.
- Surface residual stress induced by machining or cold working (e.g. shot peening) change the mechanical properties of metals. Introduction of a specific residual stress patterns, such as in form of compressive residual stress, can improve fatigue strength.
- Welding residual stresses are caused by differences in thermal expansion and contraction between weld metal and base material. Residual stress and external load can add up and cause structural failure.
- Additive manufacturing: Process-induced residual stresses (e.g. through selective laser or electron beam melting) play a decisive role in the service life of an additively manufactured functional component.
Norms and Standards
The residual stress analysis complies with norms ASTM E915, ASTM E2860 and EN 15305. In addition, we carry out measurements according to OEM regulations, e.g. VW (PV 1005), Daimler and other manufacturers.
Residual austenite quantification corresponds to ASTM E975 norm.
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