This paper uses a 3D thermo-mechanical finite element analysis to evaluate welding residual stresses in austenitic stainless steel plates of AISI 304L. The finite element model has been verified by the hole drilling method. The validated finite element (FE) model is then compared with the ultrasonic stress measurement based on acoustoelasticity. This technique uses longitudinal critically refracted (LCR) waves that travel parallel to the surface within an effective depth. The residual stresses through the thickness of plates are evaluated by four different series (1 MHz, 2 MHz, 4 MHz, and 5 MHz) of transducers. By combining the FE and LCR method (known as FELCR method) a 3D distribution of residual stress for the entire welded plate is presented. To find the acoustoelastic constant of the heat-affected zone (HAZ), a metallographic investigation is done to reproduce HAZ microstructure in a tensile test sample. It has been shown that the residual stresses through the thickness of stainless steel plates can be evaluated by the FELCR method.
Finite Element Welding Simulation, Ultrasonic Stress Measurement, Welding Residual Stress, Austenitic Stainless Steel, Through Thickness Stress, Acoustoelasticity
Residual stresses of welded structures are produced as a result of nonuniform thermal expansions and contractions during the welding processes. Nondestructive measurement of residual stress is important to optimize the structures’ design and control their mechanical strength. Unfortunately, there is no reliable method that nondestructively gives complete satisfaction in the in-situ stress monitoring of the welded structures. Material, geometry, surface quality, cost, and accuracy of the measurement are some of the parameters that must be taken into account in choosing a proper method. Therefore, the development of several methods like X-ray diffraction, incremental hole drilling, and the ultrasonic waves are inevitable .
Stress measurement by ultrasonic waves is a non-destructive, easy to use, and reasonably inexpensive method recently used in some industrial applications. However, it is slightly sensitive to the microstructure effects like grain size , carbon rate , texture , and structure  and also to the operating conditions (temperature , coupling , etc.). The ultrasonic evaluation of the residual stresses requires the division between the microstructure and the acoustoelastic effects which are simultaneously reasons for changing in ultrasonic properties of the structure materials.
Welding simulation by Finite Element (FE) has become a popular method for the prediction of welding residual stresses and deformations. Earlier studies of welding simulation accounted for the nonlinearities caused by temperature-dependent material properties and plastic deformations . The majority of those studies, due to weakness in the computational capabilities of the previous computers, were limited to two-dimensions on the plane perpendicular to the welding direction. Good agreements have been observed between the numerical predictions and experimental results (-) which encourage using FE welding simulation in residual stress evaluation.
The main goal of this study is evaluating residual stress through the thickness of stainless steel plates with 10 mm thickness. Finite element welding simulation, hole-drilling method and LCR ultrasonic waves are employed to reach this goal. According to the achieved results, it can be concluded that:
1) The LCR ultrasonic method measures the average of stresses in determined penetration depth of transducers. Therefore, if measuring the stress at an exact distance from the surface is needed, the ultrasonic method is not recommended.
2) Since the penetration depth of the LCR wave is limited, both side measurement of thick plates is recommended.
3) The finite element results are in good agreement with welding logic which says the maximum of tensile residual stress is produced in the weld centerline; it will be transformed to compressive stress near the HAZ and finally free stress zone in the parent material.
4) The average results of FE residual stress in 2mm from the surface are in good agreement with those of the hole-drilling method.
5) The finite element result for residual stress distribution of the weld centerline shows a rapid change in the weld start and endpoint and a mild increase in the final third of the weld line.
6) Ultrasonic 3D distribution of residual stresses is compared with finite element analysis and shows an acceptable agreement in the main and back weld.
7) The deviation of ultrasonic and FE results is increased by using high-frequency transducers.
8) Less agreement between FE and ultrasonic results is observed in HAZ because of its small width and also dimensional changes through the thickness.
9) Improper effect of welding endpoint on the residual stress is a surface effect and cannot penetrate more than 1 mm.
10) FE and ultrasonic results show that the maximum of tensile residual stress in the back weld is less than the main weld.
11) It is possible to investigate the top, bottom, and root of the main and back weld by LCR waves.
According to the results of this study, the FELCR method (which is the combination of finite element welding simulation and ultrasonic stress measurement by LCR waves) can evaluate welding residual stress through the thickness of investigated stainless steel plate. FELCR method can be introduced as the most cost-effective nondestructive method to produce full 3D maps of welding residual stresses. Its requirements are a computer for simulation and ultrasonic equipment (which can be provided at a lower price than the other non-destructive equipment for stress measurement). However, it can non-destructively predict the residual stress through the thickness with verification.
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FULL Paper PDF file:Using Finite Element and Ultrasonic Method to Evaluate Welding Longitudinal Residual Stress through the Thickness in Austenitic Stainless Steel Plates
Using Finite Element and Ultrasonic Method to Evaluate Welding Longitudinal Residual Stress through the Thickness in Austenitic Stainless Steel Plates
Elsevier, Materials & Design
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Professor Siavosh Kaviani was born in 1961 in Tehran. He had a professorship. He holds a Ph.D. in Software Engineering from the QL University of Software Development Methodology and an honorary Ph.D. from the University of Chelsea.