Residual Stress Analysis
Residual Stress Analysis: Theoretical Foundations
Professor, what is residual stress?
Stress that exists within a structure in the absence of external forces. Generated by welding, heat treatment, forming, and surface treatment. Significantly affects fatigue life, buckling load, and stress corrosion cracking.
Mechanism of Residual Stress Generation
Professor, what is residual stress?
Stress that exists within a structure in the absence of external forces. Generated by welding, heat treatment, forming, and surface treatment. Significantly affects fatigue life, buckling load, and stress corrosion cracking.
Summary
What is Residual Stress: Generation Mechanism
Residual stress is a self-equilibrating stress that remains within a material after external forces are removed. The three main causes are non-uniformity of plastic deformation, temperature gradients, and phase transformations. In welding, tensile residual stress is generated by rapid cooling and contraction around the molten pool. Measurements confirm that residual stress near the weld line in butt welding of mild steel can reach near the yield point (355 MPa). On the other hand, shot peening (particle impact processing) intentionally introduces compressive residual stress into the surface layer, extending fatigue life by 2 to 5 times. The wing span of a Boeing 737 is about 30m, and compressive residual stress is crucial for fatigue management of the lower wing surface.
Computational Methods for Residual Stress Analysis
Residual Stress FEM
Welding residual stress:
1. Welding Heat Source Model (Goldak, etc.) — Calculate temperature field with moving heat source
2. Thermo-Elasto-Plastic Analysis — Temperature → thermal expansion → plastic deformation → cooling →
Inherent Strain Method: Input pre-determined inherent strain into the weld zone. Obtain residual stress with a single-step elastic analysis. Computation time reduced to 1/100 ~ 1/1000.
Summary
Comparison of X-ray Diffraction and Neutron Diffraction Methods
Comparison of the two main residual stress measurement methods: X-ray diffraction (sin²ψ method) is limited to a depth of a few μm from the surface but the equipment is compact and suitable for on-site measurement, accuracy ±20 MPa. Neutron diffraction can non-destructively measure up to several cm inside a structure, and measurements using the neutron beam at JRR-3 (Tokai Village, Ibaraki, Japan Atomic Energy Agency) achieved accuracy of ±5 MPa. In 2020, the dedicated residual stress beamline ENGINEERING at J-PARC (Ibaraki) enabled 3D residual stress mapping of welded joints (resolution 0.5mm³).
Residual Stress Analysis in Practice
Practical Checklist
Reduction by Post-Weld Heat Treatment (PWHT)
Post-Weld Heat Treatment (PWHT) is the most reliable method to relieve residual stress. JIS B 8285 (Post-Weld Heat Treatment Standard) specifies holding at 600~650°C for 1~4 hours for carbon steel. For BWR (Boiling Water Reactor) piping fittings (SUS304), residual tensile stress causes stress corrosion cracking (IGSCC), so since the 1980s, alternatives to PWHT omission such as hydrostatic testing (Mechanical Stress Improvement Process, MSIP) and Low Plasticity Burnishing (LPB) have been adopted. In the Westinghouse AP1000, PWHT is incorporated as a design standard for all welded locations.
Residual Stress Analysis: Software & Solver Comparison
Tools
Comparison of Dedicated Solvers for Welding Residual Stress Analysis
For welding residual stress analysis, there are three major players: SYSWELD (ESI Group), Simufact Welding (MSC), and ABAQUS with Goldak model. SYSWELD comes with a measured database of arc efficiency η=0.85~0.95 and is an automotive industry standard, while Simufact Welding is strong in shipbuilding (for MHI) due to integration with process simulation. Verification accuracy against ASME Sec.IX welding qualification tests shows SYSWELD has the highest performance with error within 7%.
Advanced Technology
Advanced
Non-Destructive Measurement of Residual Stress by Neutron Diffraction
Neutron diffraction boasts the highest accuracy for non-destructive residual stress measurement. The BL19 beamline at J-PARC (Tokai Village, Ibaraki) can measure residual stress inside steel with ±5 MPa accuracy. It is reported that non-destructive evaluation of welded axle parts of JR Central N700S Shinkansen up to 30mm depth achieved 95% agreement with SYSWELD analysis values.
Residual Stress Analysis: Common Issues & Debugging
Troubles
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