Residual stress

It works in the laboratory, but not yet for industrial applications

Expert, how is it measured in the laboratory?

It seems there’s no way to measure it yet!

There is currently no way to measure residual stress. If anyone knows how to do it, please share the method so that we can all learn from it.

I came across calculations related to residual stress when studying engineering mechanics back then, but I’ve forgotten it all now; it’s really embarrassing

It can be determined through mechanical property tests.

The blind hole method: Some people in our laboratory are conducting experiments on using ultrasonic shock to eliminate residual welding stresses. To compare the stresses before and after the shock treatment, this blind hole method is employed. I’m not entirely sure of the details; from what I saw, they select several locations along the weld and in the heat-affected zone to attach strain gauges, then drill small holes at the center of these strain gauges. During the drilling process, the strains that occur are measured, and the residual stresses are calculated based on those measurements. That’s roughly how it works; it might not be entirely accurate, as I’m not an expert in this field. I hope those who are more knowledgeable can correct me

Residual stresses in large welded components and countermeasures: Whether large welded components, especially those used in building steel structures, require or are capable of undergoing aging treatment is a matter of concern； While aging treatment processes have been widely applied in the manufacturing of machine tools, lifting equipment, transportation systems, ports, general-purpose machinery, energy equipment, and chemical processing equipment, it is rare to see their use in large welded structures, especially in building steel frameworks. There are three reasons for this: 1) Lack of knowledge – some designers are unaware of the concepts of residual stress and aging treatment, as well as China’s standards regarding such processes ； There is a fundamental difference between the \"non-use\" resulting from ignorance and the \"non-use\" determined by technical standards ； 2) Too risky: Due to the large size of the building’s steel structure, the overall thermal aging process cannot be used, and local thermal aging on-site is generally not considered either due to construction difficulties and associated risks. When aging treatment is necessary, only technically mature thermal aging processes are used; for example, the load-transferring columns of the Jinmao Building were constructed by first subjecting the entire components to thermal aging and then welding them on site. Vibration aging and vibration (modulated) welding processes are not adopted due to the lack of testing and technical standards. 3) Not possible: The construction party is constrained by the design drawings, and aging treatment cannot be applied to components for which such treatment is required. 1. Residual stresses in large welded components: The welding methods used for large welded components and building steel structures are not significantly different from those used for ordinary welded components; therefore, residual welding stresses also exist in them. Taking the Shanghai Anting Yunzhaobang Bridge as an example, the steel grade is Q345B, with σs=345MPa. First, perform segmented welding at the factory, and then carry out circumferential weld joining on-site ； However, the segment to be placed on the bridge piers was too large to be transported, so it had to be divided into two half-segments for the left and right bridge girders. These two half-segments were first welded together on site (as shown in Figure 1), and then the circumferential welds were used to join them together. The blind-hole method was used to measure the residual stresses in the longitudinal weld seam, and the results are shown in Table 1: Table 1: Residual stress conditions after welding of the Yunzaobang Bridge. Location, Stress (Mpa): Maximum principal stress, Minimum principal stress, Shear stress, Longitudinal stress (in the direction of the weld seam), Transverse stress. Upper surface, Longitudinal weld seam: Extreme values – 315, –95; Mean values – 157, 2, 78, 64, 94. Lower surface, Longitudinal weld seam: Extreme values – 81, –74; Mean values – 62, –46, 54, 31, –15. Access hatch, Seal weld seam: Extreme values – 261, 94, 79, 232, 133; Mean values – 184, 103, 41, 173, 114. The data in Tables 2, 3, and 4 also confirm this situation. The weld seams on the lower surface were welded first, resulting in relatively low residual stress levels, whereas the stress levels on the upper surface are quite high; some individual values are close to the stress level of the base material σs, while the average values are close to or exceed σs/2. Welded components, due to the high tensile residual stresses present, as well as the heat-affected zones, weld toe defects, and stress concentrations at the weld areas, develop weak points in terms of both structure and mechanics. This can lead to deformation of the component during operation, as well as early cracking, stress corrosion, fatigue fracture, and brittle fracture. Therefore, where possible, adopting appropriate aging processes to improve microstructural properties and eliminate residual stresses can effectively enhance the stability and safety of components. 2. Elimination of residual stresses in large welded components: In fact, aging processes are already being used in some high-demand large welded steel structures, including heat aging, vibration aging, and TIG remelting processes that are supported by technical standards: 2.1 Heat aging process: For large welded steel structures, an on-site welding process is often employed after overall heat aging of the key joint components; the framework of the Shanghai Jinmao Tower is a fully welded structure, with individual components manufactured in the factory, and the load-bearing components – namely the transfer columns – undergoing overall heat aging before being welded together on site. Jiangyin transmission towers also use a similar process. The results of evaluating the effectiveness of the thermal aging process using the blind hole method are shown in Table 2: Measurements of residual stresses before and after thermal aging were taken to assess its ability to eliminate welding-induced residual stresses. The data are as follows: Table 2: Data on the use of thermal aging to eliminate welding residual stresses in the Jinmao Tower and Jiangyin Tower projects. Project, Residual Stress (MPa): σmax, σmin, σX, σY. Shanghai Jinmao Tower, Transfer Columns (Q235): Average value before heat treatment: 135, 51, 58, 128; Average value after heat treatment: 79, 16, 30, 64; Difference before and after heat treatment: 56, 35, 28, 62; Percentage change: 41%, 68%, 48%, 50%. Jiangyin Tower, Butt Joint Test Plates (Q345): Average value before heat treatment: 185, 63, 146, 101; Average value after heat treatment: 87, 21, 71, 37; Difference before and after heat treatment: 95, 42, 75, 64; Percentage change: 53%, 67%, 52%, 64%. It is generally believed that the stress-relief effect of thermal aging is between 40% and 80%, and the results in Table 2 are consistent with this range ； However, the residual stresses resulting from welding will still remain in the steel structure ； 2.2 TIG remelting process Generally speaking, TIG welding can improve the transverse residual stresses in the weld zone, as well as reduce stress concentration caused by weld toe defects. Studies show that trimming the weld toe can extend fatigue life. Apart from this minor existing defect, either blunt its sharp opening. The crack was trimmed using the TIG method to restore its condition prior to cracking, thereby extending the fatigue life. The favorable cross-sectional shape of the weld reduces stress concentration in the geometric shape, thereby also enhancing the fatigue crack resistance of the entire joint. When these existing toe defects are perpendicular to the applied stress, methods to extend fatigue life are most effective. Shanghai Baoye Engineering Technology Company carried out repairs on the girders of heavy gantry cranes. Two measurements were taken of the residual stresses in the weld seams of the simulation welding test plates before and after TIG remelting, and the stress changes were determined using X-ray methods, as shown in the following table: Table 3: Comparison of average residual stresses before and after remelting (Material: Q345) Number Longitudinal residual stress (MPa) Reduction percentage Transverse residual stress (MPa) Reduction percentage Average before remelting Average after remelting Average before remelting Average after remelting 1 209 1989 5.0 56 57 -2.3 2 206 240 -16.4 59 64 -8.0 3 236 2130 9.6 -57 29 -150.9 4 265 245 7.7 259 84 67.5 5 189 201 -6.4 206 114 44.6 6 221 219 0.7 105 70 33.4 It can be seen that TIG remelting does not significantly improve the longitudinal residual stresses at the weld joints; the absolute value of these stresses does not decrease much. However, it does have a certain effect on improving the uniform distribution of longitudinal residual stresses ； TIG remelting has a significant effect on reducing the transverse residual stresses in the welds of the test plates; the absolute value of these residual stresses decreases markedly, and their distribution becomes more uniform ； The residual stress tends to reduce uniformly the stress concentration in the welded parts during use, thereby increasing their service life. 2.3 Vibration Stress Relaxation (VSR) Vibrational stress relaxation involves applying alternating stresses to a component; these stresses combine with the residual stresses present in the component to reach the material’s yield stress, resulting in localized plastic deformation ； This plastic deformation usually occurs first at the locations where the residual stresses are highest, thereby releasing the constrained deformation there and achieving the effect of reducing and homogenizing the residual stresses. VSR was used to eliminate welding residual stresses in the drum bodies produced by Shanghai Zhenhua Port Machinery Co., Ltd. (with a diameter of 1400, a length of 13800, and a wall thickness of 50). The comparison of residual stresses before and after VSR application is shown in Table 4: Table 4: Stress changes before and after vibration aging of the drum bodies. Location Residual stress (MPa) σmax σmin σX σY Drum Girth weld Average before heat treatment 272 –1 176 95 Average after heat treatment 119 19 112 26 Difference before and after heat treatment 153 –20 64 69 Percentage change (%) 56 –2061 36 72 Drum Web fillet weld Average before heat treatment 127 12 106 33 Average after heat treatment 89 33 83 39 Difference before and after heat treatment 38 –21 23 –6 Percentage change (%) 30 –175 21 –18 It is generally believed that the stress-relief effect of vibration aging is between 20–50%, and the results in Table 4 are consistent with this range ； However, unlike thermal aging, its minimum principal stress shows an upward trend as the maximum principal stress decreases ； But its stress-relieving effect is precisely the overall effect. All three of these commonly used methods for eliminating residual stresses in large welded components exhibit relatively significant stress-reduction effects. However, different stress-relief measures yield varying results depending on the welding structure, welding process, and materials involved, as well as differing in terms of economic cost. The thermal aging method provides good stress-relief effects, but the size of the components is limited by the space available in the heat treatment furnace ； TIG remelting can significantly reduce transverse stress and improve stress concentration in the joint, but it has little effect on reducing longitudinal stress, and its work efficiency is low ； The VSR method is simple and fast, with very significant results, and it has been applied in advanced fields such as the nuclear industry (nuclear reactor components, fusion devices), maglev transportation, and aerospace. However, it does not possess the capability to remove hydrogen through thermal aging or to restore plasticity. Furthermore, the hammering process has also been successfully applied to stress relief treatment of the final welding seams of large water turbines at Baosteel’s converters and Dongfang Electric Corporation. Due to the great variations in base material, welding materials, welding processes, and operating conditions, it is necessary to correctly select the stress-relief process based on the requirements of the materials and components, as well as in accordance with technical standards. 2.4 Vibration Welding Process Vibration welding, also known as vibration-modulated welding, can improve the quality of welds; it is a new technology that is currently under development both domestically and internationally. Vibration welding refers to the application of mechanical vibrations with different parameters to various components during the welding process, that is, welding carried out under vibrating conditions. During the welding process, slight vibrations of the welded parts within a certain frequency range inevitably have an impact on the weld pool and the heat-affected zone: 1) The vibrations influence the flow and spread of the liquid metal in the weld, as well as driving the upward movement of gases and oxidation impurities within it, which results in fewer macroscopic defects in the weld ； 2) The interference of vibration on catalytic nucleation and grain growth during the crystallization process leads to the refinement of the weld structure and the coarse-grained region in the heat-affected zone of the joint ； 3) At the beginning of cooling and solidification, the driving effect of vibration on shrinkage slip reduces welding deformation ； 4) The vibration-induced drive of slippage, along with the accompanying vibration aging effect, promotes the relaxation of potential energy within the joint, resulting in a decrease in the peak value of welding residual stresses ； The measurement results of the residual stress from vibration welding on 90 mm thick plates are shown in Figure 2 ； Vibration welding plays a role in refining the grain structure from the onset of the welding process. Subsequently, by inducing thermoplastic deformation in the hot state, strain is adjusted to reduce residual stresses; this effectively prevents the formation of welding cracks and distortion of the workpiece, improves the fatigue life of the components, and enhances the mechanical properties of the welds. Preliminary tests on Q235 steel show that vibration welding can achieve the following effects: 1. Reducing the width of the distribution of longitudinal high tensile residual stresses (width at σ>100 Mpa: from none to approximately >60 mm) ； Use – about 80 Mpa ； Impact is applied to the weld toe and weld surface using a frequency of approximately –16 KHz: 1. Ultrasonic impact has the effect of relieving stress in the surface layer to a certain depth; when impact is applied to the entire weld area, it can reduce stress in layers with a depth of 2–4 mm by 34–55%, meeting the requirements of the JB/T5926-91 standard ； The impact effects of electroslag welding and 2. submerged arc welding are shown in Figures 3 and 4. 3. The weld toe impact method can be used to repair defects in the weld toe and reduce stress concentration. Together with the effect of its compressive stress zone, it can reduce the residual stress in the unimpacted weld to a certain extent, by up to 19%, which has a significant impact on improving the fatigue life of the joint. 4. Impact can cause a slight increase in surface hardness, but the average value does not exceed HRC10, remaining at a low hardness level. 5. The impact process is a stress-relief technique of the \"surface effect\" type that features point impact contact and compressive stress yielding, along with certain \"volume effect\" ultrasonic vibration aging effects; it is particularly suitable for the local treatment of short welds under high constraint conditions. In cases such as local welding repairs, assembly welding of large components, and welding of small components onto thick-walled structures, the weld areas are subjected to high constraint stresses, and delayed cold cracking is likely to occur after welding. 6. Due to the characteristics of the impact processing technique, its working efficiency is relatively low, at approximately 1200 mm2/min. It is estimated that 10–11 hours are required to process a pillar that is 4M long ； Therefore, it is not suitable for stress relief treatment of large components with multiple welds ； Figure 3: Impact stress-relief effect of electroslag welding. Figure 4: Impact stress-relief effect of submerged arc welding. 2.5 Explosion method: Special adhesives are applied along the weld direction near the weld seam. **After detonation, successive shock waves are generated, forcing the areas of peak stress in the structure to undergo plastic deformation, thereby achieving stress relief. It is reported that the thickness can be reduced by up to 70 mm, with an effectiveness of 60%, and the process is completed instantly, making it suitable for large and very large structures. 3. Conclusions  Large welded steel structures have high residual stresses after welding; aging treatment can significantly reduce these stress levels, which is beneficial for safety and service life ； However, in building steel structures, the aging process has not yet been widely applied ；  Thermal aging, TIG remelting, vibration aging, and explosion processing can be applied to large welded steel structures, yielding significant results ； Hamming and ultrasonic impact can be applied to large welded steel structures and are very effective in improving fatigue performance, but their work efficiency is low. Experiments have shown that vibration welding can improve weld quality, and it is recommended that such tests be carried out on a wider scale. (End of text) (References: omitted) ； The main data in this paper come from the author’s field measurements.)

I really haven’t heard of any testing methods in this area; if such methods do exist, they are likely to be destructive in nature.

It’s really remarkable that stress can be detected with such accuracy. I never came into contact with it before.