Residual stress measurement methods such as x-ray diffraction (XRD) are often employed for root-cause failure investigations and life prediction efforts in safety-critical components and structures. For certain components, their design performance often relies on “engineered” residual stresses imparted during manufacturing (e.g., peening, rolling, etc.), so it is important to confirm that these beneficial residual stresses have been applied correctly. Conversely, where undesirable residual stresses have been imparted during manufacturing as a “side effect” of processing, these should be identified, evaluated and mitigated if required. For example, if a part has been welded, it could be susceptible to stress corrosion cracking (SCC) near the heat-affected zone (HAZ) adjacent to the weld; this cracking may occur if there is a high level of tensile residual stress remaining in the HAZ after welding processes.1 Such a condition could result in catastrophic consequences, especially if in either an aerospace or an automotive application, thus the importance of residual stress characterization in these instances.
The distribution of residual stress within a part is not always obvious just from its appearance so it can be challenging to locate the specific problem area within the component. During processes such as grinding, different regions of the material yield at different rates. With some areas getting constrained more than others, the resulting stresses are inconsistent across the part. While the net sum of all residual stresses across any cross-section of a component is always zero, balancing tensile and compressive stresses exist, either internally or across the surface of the part. For instance, if a component undergoes grinding, the action of the grinding wheel hitting the material causes plasticisation, which could impart tensile and/or compressive residual stress into the surface of the part. Because of the potential for tensile residual stress to be imparted, it is important to understand the magnitude and location of the stress in order to prevent issues such as SCC. In any instance in which stress is introduced into a component, both the surface stress and the bulk stress can be altered, potentially causing problems with the part.
The difficulty in characterizing a complete residual stress profile in a component is twofold: firstly, the distribution of the stress must be determined in order to properly locate the areas of high tensile stress, as steep stress gradients could be present (e.g., in the HAZ of a welded part); and secondly, the setup required to measure residual stress at several different locations, especially within a complex part, can be cumbersome and time-consuming. Residual stress mapping is a useful tool in solving both of these problems, as it can be used to select specific points for measurements and also greatly reduce instrument setup requirements. Mapping is especially important in detecting the changes in residual stress that are often present when moving away from a feature such as a hole or weld. Mapping removes the guesswork of selecting single-point measurement locations where high-magnitude stresses are presumed to be located and ensures that areas of interest are not missed, especially in regions where steep surface stress gradients exist.
In selecting specific areas to be mapped, the geometry of the component also plays a significant role. In most cases, if there are no known features (such as welds or holes) within the part and if the geometry is not complex, mapping is typically performed in two dimensions to identify any stress gradients that may be present. For example, in investigating SCC, the area of potential crack propagation can be identified using a square or rectangular map that will cover an area in both X and Y directions. A square map is also appropriate for instances in which the specific areas of stress are unknown. However, if there is symmetry or a specific feature along one axis of the part, then only one dimension may be necessary for examination. Tracing across the component with a line map would be a useful strategy for welds or roots of gears if they have a consistent stress along the axis of symmetry (perpendicular to the line map profile). Finally, for investigations in which the problem area is known in advance or measurements are being performed for quality control purposes, a few single-point measurements can be performed on the part. Because of the accuracy of modern mapping technology, measuring via manual alignment can be bypassed, and once the part is set up, different locations can easily be measured on the part as needed.
As the originators of residual stress mapping, Proto has long been utilizing advanced mapping technology to make residual stress measurements more efficient. In previous mapping systems, a mechanical pointer on the head of the instrument travelled along the part, and the XY stage could be moved into several different positions until a map was generated. Measurements could be performed at approximately every millimeter along the part’s surface. While this method helped reduce setup time, mapping was still a fairly lengthy process due to the nature of the pointer and the movement of the instrument stage. To streamline this process, Proto designed a single device to make mapping much simpler. The addition of the LP200 Profilometer to their instrument line-up has made complicated residual stress maps much quicker and easier to generate.
The LP200 is a laser profilometer system capable of scanning a part and producing a 3D model on which various points can be selected for subsequent residual stress measurements. Comprised of a sensor head, control box, motorized XY stages, and a manual Z stage, the LP200 can produce a surface map that is then sent to the XRDWIN software program, where automated mapping setup takes place. When the part is transferred to the residual stress system, in this case Proto’s iXRD instrument, the goniometer head automatically adjusts to stay perpendicular to the measurement area, providing accurate results. In XRD residual stress experiments, the surface of the part must remain perpendicular to the goniometer head. Proto’s residual stress systems work with the profilometer’s map data to ensure that the part remains in the correct position for each measurement. Measuring the residual stress of parts that are curved in two directions is an even greater challenge; as the user moves across the part, the goniometer head must be continuously repositioned to follow the curve. Thus, for optimal results, the maps should be coupled with a dual-axis goniometer such as the Proto LXRD Chi or the roboXRD system, which can move across the part in both X and Y directions while always keeping the goniometer head at the correct angle. This allows maps to be generated on parts that previously could not have been measured without tedious manual setup.
Specific locations can be selected from the 3D map for residual stress characterization, which enables users to quickly and precisely select the locations that are most susceptible to fatigue or failure on their part. For example, in a part that has been welded, the user can select certain distances away from the weld using the on-screen ruler to determine the residual stress at specific distances within the HAZ. Distinct zones can be selected from the map to reduce measurement setup time resulting from having to move the part into various orientations.
The map areas created within the profilometer’s software have various selection options, including specific shapes, lines, squares, circles, and more. While surface measurements can provide similar information about a part, the LP200 is far more accurate in the data it generates, and it reduces the errors that can result from aligning an instrument manually. Scanning can be performed as fast as 50 millimeters per second, and the maximum achievable accuracy is under 0.5 microns, providing a distinct advantage over alternative mapping technologies.
The LP200 is well-suited for mapping parts with curved areas or otherwise complex geometries. For example, Proto recently generated residual stress maps of a cylindrical weldment starting from the original metal material, through the HAZ, and into the weld metal. Surface residual stress was analyzed because sustained tensile residual stress is the main factor that causes stress corrosion cracking (the other two factors being susceptible materials and corrosive environments, which are difficult to correct). The 3D surface profile map generated by the LP200 was completed in only eight minutes, and the square mapping tool was then utilized in order to select residual stress measurement locations in the areas of interest. The component was transferred to an iXRD for residual stress characterization, where it was discovered that mainly uniform residual stress was present in the HAZ. The post-weld processes that were applied to this part resulted in residual stress mostly in the range of -200 to -300 MPa (Figure 5). However, a few regions were found to have only mildly compressive stresses (just under 0 MPa); since pipes are susceptible to SCC, those areas are concerning because they could become tensile when combined with in-service loads. Depending on grain orientation, cracks can develop even with very low tensile stress, so more compressive stress may need to be introduced in this part to mitigate SCC.
Depending on the specific issue being investigated with residual stress measurements, it may also be necessary to measure through the depth of a part to locate other regions of high stress that could affect its performance. If depth measurements need to be performed on a curved component such as the welded part discussed above, it can be difficult to tell where the original reference surface began just by looking at the part after it has been electropolished. With the LP200’s mapping technology, the depth of the hole can easily and accurately be measured via interpolation of the 3D map data, and the information can then be used to generate residual-stress-versus-depth curves. This is another useful function of the profilometer that eliminates the need for depth measurements with tools such as depth gauges, as these methods can be more time-consuming.
The aforementioned advances in residual stress mapping technology allow users of the XRD method to quickly and easily create detailed residual stress maps of parts with steep stress gradients, even in complex geometries. Moreover, the same advances enable at-depth measurements at specific points, as well as the ability to characterize the depth of electropolished spots. The quick and accurate 3D profiles that are generated can easily be integrated into a residual stress system, thus revolutionizing root-cause failure investigations and life prediction efforts in components with detrimental residual stresses either on the surface or at depth.
1. J.A. Pineault, M.E. Brauss, J.S. Eckersley, “Residual Stress Characterization of Welds Using X-ray Diffraction Techniques,” Proceedings from AWS Convention, Welding Mechanics and Design, Chicago, 1996.
For Information: Proto Mfg. Inc.
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