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Residual-stress,Find, Reduce, Prevent.SOLUTIONS with Effective, Practical Advice
Residual-stress presence is the unavoidable consequence of any welding process. In general these are stresses that can exist in a body not subjected to external forces. The accurate detection of such internal stresses and the determination of their distribution and amount may be important in certain critical applications. Such an investigation may be needed for controlling the quality of the products and for assuring the design service lifetime. This subject was briefly addressed in a short note published (11) in Issue 71 of our Practical Welding Letter for July 2009. Welding Residual-stress formation was described in our page on In a welded structure subjected to service loads, internal stresses, produced as a reaction to those loads, are summed algebraically to Residual-stress. The sum may either alleviate or dangerously increase the local composite stress present, to the point of endangering the stability or the resistance to fatigue of the same. The physical distortion generated in a structure by a welding process can be evaluated qualitatively by visual examination. An experienced observer can perform this examination, knowing which stresses can be produced and where by the specific welding sequence employed. Two conclusions can be suggested by the results. One is the application of well considered local heating application for straightening the structure. This can be done to counteract the deformation incurred, essentially introducing additional Residual-stress to oppose the distortion. See PWL#020 in section 3. The other is the study and the suggestion of a different welding sequence, capable of limiting the effects of Residual-stress, by a thoughtful redistribution of the shrinkage stresses generated by welding. Residual stress can be beneficial or detrimental to the performance of a material or the life of a component. In the case of fatigue crack growth, tensile residual stress adds to the mean stress, and this in turn affects strongly the crack growth rate especially in the near threshold regime. Therefore, the knowledge of the residual stress distribution is very important for determining safety and durability of a structure. The theoretical study of Residual-stress can be performed by numerical finite elements analysis through simulation. This is done by describing local application of heat and consequent metallurgical transformations (including volume changes) in a schematic virtual representation of a real body. By following with each iteration the deformations incurred and the stresses generated due to hindered expansion one can obtain a global picture of the situation. The experimental Residual-stress analysis is performed mainly by two different techniques.
Non Destructive Techniques. Presently, the accurate way to perform non destructive measure of Residual-stress is by X-ray Diffraction (XRD). X-ray use the interatomic spacing of certain lattice planes in absence of stresses, as the gage length for measuring strain. Under residual stresses the concurrent strains (deformations) change the above spacing. The accurate measure of the changes indicates the existence and the amount of stresses. X-ray methods measure only surface strains, useful for researching fatigue problems. In addition, XRD permits non-contact measurements with exceptional spatial resolution, and the ability to measure hardened materials because cutting is not required. Similar to the above but different in the probing radiation is the Neutron Diffraction method, which has much higher penetration and high spatial resolution, but at higher cost and limited availability. Also the Electron Diffraction technique can be used, but with serious limitations in applicability. Destructive Techniques. If a part or specimen containing a given Residual-stress configuration can be sectioned, previously existing residual stresses can be calculated. That is done from the measurements of the strain (deformation) obtained as the locked-in stresses are released by cutting. One popular method involves drilling a hole in a selected region, monitored by a set of three strain gages arranged at 120 degrees from each other, or by other techniques involving interferometry or holography. Upon machining, the surface of a sample containing residual stresses will relax into a different shape: the measured strains provide data for calculation of the previous Residual-stress. Other methods involve controlled removal of material and measurement of intervening strains, by suitable techniques. Special Applications. A special case involves the welding deposition of a layer on a substrate. This can induce stresses causing the base metal to curve. From the measurement of the curvature, in simple cases, one can deduce the amount of residual stresses involved. Strictly related to the study of Residual-stress is the subject of stress relieving that has to be designed so that the peak stresses are effectively decreased without inducing unwanted deformations. A website page on Stress Relieving is available to interested readers. An Article on Stress Relieving was published (7) in Practical Welding Letter for April 2004. Click on PWL#008 to see it. An Introduction to ASM Handbook Volume 4C on
For Resources on Residual Stress Measurements see Bulletin 53 attached to Practical Welding Letter #085.
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