Fraunhofer Institute for Mechanics of Materials IWMResidual stress analyses
Our aim is to create residual stress optimized components for our clients.
To do so, we determine manufacturing and use-caused residual stresses, assess them with regard to the reliability, safety and lifetime of parts and develop recommendations and concepts for component design, raw material use and manufacturing processes.
In order to prepare requirements to use components safely and reliably, we investigate and analyze residual stresses in connection with the underlying material microstructure, environmentally-caused degradation (such as hydrogen or corrosion) and manufacturing influences (e.g. by welding, heat treatment or surface treatment).
We determine residual stresses in our laboratories or at the customer’s premises, with both X-ray and mechanical procedures being used. Each procedure is chosen in relation to the specific problem in order to achieve an economical and reliable determination. Our measurement procedures are continually developed and supplemented by new advances. The investigated components range in size from a millimeter to meters. In addition, we carry out texture and phase analyses. An economic measurement strategy results from the efficient selection of analysis procedures adapted to the client’s problem. Furthermore, we simulate and calculate residual stress and the resulting structural changes using numerical methods which occur following certain manufacturing steps.
Based on our analyses and assessment, we develop optimization concepts and strategies for handling a wide array of failure related mechanisms:
- Structure optimization in heat treatment
- Process-caused strain hardening
- Abrasion and corrosion protection from the perspective of residual stress optimization
- Crack formation due to hydrogen brittleness
- Cold crack formation after welding
- Hydrogen-forced fatigue
- Change in strength properties/deformability
- Stress corrosion cracking
- Shot peening of brittle raw materials
- Undesired or misunderstood changes in properties as well as the function of the components produced or used (such as warping, crack formation, tiring, malfunction, corrosion)
- Residual stresses occurring or changing in operations due to thermal, mechanical, chemical or combined influences
- The improvement of manufacturing procedures such as shaping, processing, post-treatment, peening, and joining
- The targeted generation of favorable residual stresses in manufacturing
We prefer to carry out close-to-surface residual stress analyses using X-ray diffraction. This procedure is based on the determination of distortions of the crystal lattice and is therefore only suitable for (partially) crystalline raw materials. The X-ray radiation normally used has a low penetration depth from a few µm up to 10 µm which is why a high depth resolution is achieved. Larger depth ranges can be achieved by gradual electrochemical material removal with subsequent measurements. The procedure is then no longer non-destructive.
Residual stress mapping on large components
Increase of welding residual stress on site
Microdefractometer for efficient stress determination with the highest local resolution
For measurement of residual stress by means of the classical sin2ψ procedure, recording of the corresponding lattice deformation with various tilts in ψ is often necessary to determine an individual normal strain component.
In the absence of shearing stresses, a linear relationship between the measured network level spacing d and sin2ψ applies. The strain component (amount and sign) in question can then be calculated from the gradient of the line.
Accessibility of the measurement position is required to apply the sin2ψ procedure. The Fraunhofer IWM has developed a CAD routine for this to compare the marginal conditions of the sin2ψ procedure with the shape and size of each component. A strategy for determining residual stress can thus be developed cost effectively without complex consultations and sharing of samples.
From a depth of approximately 20 µm, residual stress in surface layers can also be determined using borehole or circular core procedures. To do so, the current state of residual stress is partially broken down by gradual high-speed milling of a blind hole or a ring groove and the resulting expansions recorded using strain gauges. The originally presented residual stress in the depth of a component can be recalculated from the strain depth profiles. Residual stress depth distributions up to around 1.2 mm (borehole) or 5 mm (ring groove) in depth can therefore be determined.
Residual stress determination using a borehole procedure
The Fraunhofer IWM has a range of different X-ray diffraction facilities. In addition to diffractometers with the customary equipment for powder diffractometry, we have a micro-diffractometer for lateral high-resolution analyses (for example in notches), an automated 4-point bending equipment to determine the special X-ray elastic constants, and several mobile diffractometers for residual stress investigations on large components. We work almost exclusively with place-sensitive X-ray detectors so many measurements can be taken in a short time frame, which allows a mapping of component areas.