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Methods and areas of applications
Residual stress in pulled tungsten wires (micro-diffractometry)
Residual tension in high-performance ceramics (X-ray diffraction)
Economic determination of the residual stress of friction stir welded space travel components
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 use the determined residual stress values for component assessment, as an input for strength calculations and to verify simulation results. We assist in the development of manufacturing processes as well as in the optimization of components and measurement technologies.
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:
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.
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.
Tungsten wires for the lighting industry are pulled to a thickness of less than 100 µm in a multi-step process. Although the process has been established for decades, longitudinal cracks, known as splits, frequently occur. These splits can expand across long distances along the wire axis in the cooled wire. Residual stress is the driving force for crack expansion. Developing the pulled texture plays an important role in this damaging procedure as it is jointly responsible for direction-dependent properties in the wire.
Determining the texture and residual stress in thin wires by X-rays is a difficult experimental challenge. Defocusing effects can only be controlled with strongly focused primary optical paths resulting in low intensities that lead to acceptable measurement times through the use of the micro-diffractometer specialized for this purpose. Gradual electrochemical erosion is required to determine the residual stress depth distributions, which leads to redistributions of residual stress and needs to be corrected mathematically. Together with Finite Element Modelling of the pulling process, the experiments allow for the optimization of pull parameters to avoid unfavorable high tension residual stress in the edge layer of the wires.
Process and abrasion-caused damage and residual stresses influence the edge layer hardness of ceramics. X-ray diffraction analyses and the hardness investigations adapted to the expected operational demands can provide meaningful results in these cases.
Both macroscopic residual stress, which overlays the operational stress and dislocation densities, which often are a measure for the mechanical load in processing or operation can be determined via X-ray diffraction. Surprisingly often for brittle raw material such as ceramic, because of the influence of processing procedures on the edge layer hardness, an increase in hardness due to rough processing procedures are established. X-ray determination of edge layer changes, as well as assessment using fracture mechanics show that in some cases the influence of strengthened hardness from residual stress processing exceeds the influence of process-caused damage. Similar connections are also found when considering the tribological behavior of ceramic components in the case of a preferred mechanical edge layer strain. Due to the reduced contact ratio in the run-in phase, roughly lapped services initially offer lower lubrication effects than finely lapped surfaces. The high and deep pressure residual stress induced by processing can however reduce raw material erosion over long wear paths.
Friction stir welding is becoming increasingly important in airplane construction and space travel as a joining method for high-strength aluminum alloys. Significant advantages in comparison to molten pool welding are less warping and less influence on strength due to heat. This procedure involves a rotating pin being driven under high pressure into the components to be connected. The material is therefore heated by the friction and viscoplastically deformed. With a progressive, lateral feed motion, the material of the components to be joined can merge and form a weld comparative to an extrusion-like process. Although the temperatures that develop in the process are considerably lower than in molten pool welding, residual stress caused by heat and deformation develops in the material. This can have an unfavorable effect on fatigue behavior, crack growth and resistance against stress crack corrosion of the weld joints and must be considered when designing the components.
Understandably, no complete components are used for residual stress and hardness analyses, and instead samples removed from larger components are preferred. In order to optimize the manufacturing processes, sample welding is primarily performed on geometrically simpler, straight components. The question is therefore raised as to whether the state of residual stress samples comes close to the state of residual stress in the component or at least allows for a conservative assessment of the residual stress influence.
Fraunhofer IWM uses various, destruction-free, partially destructive, and fully destructive procedures for residual stress analyses. Residual stress in edge layer areas is relevant for the damage process in friction stir welding. Residual stress close to the surface determined using X-ray diffraction analyses which do not cause any damage is preferable. Deep distributions in the edge layer can however be economically determined with the incremental borehole procedure. In order to uncover lateral and deep-lying stress maxima, the surface residual stress distributions were therefore determined by X-ray and also measured using the borehole procedure at maximum residual stress areas of deep distributions.
Removing samples from high-volume components can lead to the redistribution of wide-reaching residual stress fields. In order to be able to specify the relevant state of residual stress for the component, residual stress redistributions that appeared when moving components and samples were determined using strain gauges. As only elastic expansion appeared, the residual stress distributions determined in these samples could be corrected.
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 (up to 50 µm) analyses (for example in notches), a high temperature chamber for in situ diffraction analyses up to 2000 °C (for example for temperature and time-resolved tracking of heat treatment processes), 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, including on-site at the customer’s premises. We work almost exclusively with place-sensitive X-ray detectors so many measurements can be taken in a short time frame, including on multi-phase raw materials.
Fraunhofer Institute for Mechanics of Materials IWM
