Symposium 16: Residual Stress in Fatigue and Fracture

In recent years a number of studies have been performed in which the impact of residual stress on structural performance, especially fatigue performance, has been evaluated both experimentally and analytically. These efforts are leading to an emerging paradigm in which residual stresses are represented explicitly in structural design, analysis, manufacturing and sustainment calculations. In order for this new approach to be become minimally viable, it is necessary to be able to quantify both the residual stresses in the structural component in question, and the impact that those residual stresses have on component strength and life. However, to achieve general acceptance, especially for critical applications in which the presence of the residual stress directly impacts whether or not the component will meet its design requirements, deterministic quantification alone may not be sufficient; it may be necessary to quantify the uncertainties in both the residual stresses and the resulting fatigue lives.
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Taper-Lok fasteners provide great benefit to fatigue performance but create a complex scenario for analysis to fully account for its effects. The interference due to the oversized tapered fastener introduces tensile hoop stress around the hole and compressive radial stress that combine with applied loading to effectively reduce stress amplitude and improve fatigue performance. The combination of the stress due to interference, applied stress, and the likelihood of plastic deformation near the hole results in a complicated scenario for damage tolerance analysis. The objective of this work was to develop an analytical approach to support explicit incorporation of the physics of a Taper-Lok fastener installation for B-1 critical locations. The work included experimental measurements and finite element predictions of residual stress utilizing manufactured coupons and aircraft excised structure. A comprehensive fatigue crack growth test program was conducted to obtain validation data using coupons representative of wing rear spar and wing carry-through lower cover control points. The analytical approach and validation data developed in this work are discussed in detail.
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The cold expansion process is widely used in industry in order to introduce compressive residual stresses around fastener holes, up to 2 mm beneath the surface. These compressive residual stresses are beneficial since they will prevent crack initiation from the surface and decrease subsequent crack growth rates. However, residual stress relaxation may occur due to the thermomechanical loading of the area. This study aims to investigate residual stress relaxation under thermo-mecanical cyclic loads.
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The effect of compressive residual stress on the fatigue limit was investigated using fatigue tests on specimens with and without compressive residual stress. The results demonstrated that the fatigue limit of AISI4140 steel with compressive residual stress can be predicted using the fatigue limit diagram, considering the local yielding of the compressive residual stress layer.
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Weld residual stresses (WRS) are an important driver of primary-water stress corrosion cracking (PWSCC) in nuclear reactor piping, and thus can have an large influence on crack growth predictions. Consequently, it is important to be able to accurately predict WRS using finite element (FE) modeling. This study describes a proposed procedure for the validation of WRS predictions in nuclear primary piping systems using 2D axi-symmetric FE models.
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As the UK fleet of power plants changes technology from Advanced Gas Cooled Reactors to Light Water Reactors (LWR), the fatigue life of structural components in the primary coolant loop becomes of high interest. This is because of cyclic loading of LWRs caused by solid-liquid interactions which are less prominent in gas cooled reactors. Concurrently, modern welding techniques such as electron-beam (EB) welding are of great interest in LWR designs thanks to their benefits such as the ability to be automated, smaller heat affected zones and less material complexity as they can be deployed with no filler material (Horne et al., 2019). A common focus in studying weld fracture is the weld toe; this is because it has been observed that cracks often initiate in this region typically due to higher expected carbide deposition within the heat affected zone acting as stress concentrators. As EB welds have very narrow heat affected zone, the expected region in which cracks may initiate, is less obvious. This work compares three crack initiation sites taken from a modern reactor material (stainless steel 316L) pipe containing a circumferential EB butt weld and evaluates the fatigue crack growth rate (FCGR) within the linear region of the
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Tensile residual stresses caused by welding potentially lend to detrimental consequences on the structural integrity and durability of tubular joints in the engineering field. This paper presents the experimental and numerical investigations to determine the welding residual stresses in X-tubular joints. This paper describes a new modeling approach to establish the finite element model of the tubular joint with a multi-pass weld to simulate the welding process with multiple welding passes and analyses the welding residual stresses. Assisted by the non-destructive residual stress measurement by the X-ray diffraction approach, the numerical results realized by the proposed modeling approach and the thermal-mechanical simulation method agree well with the X-ray diffraction measurement.
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Quenching is a heat treatment process for the rapid cooling of a metallic workpiece in water, oil or air to obtain certain desired material properties. The accurate determination of resulting residual stress and distortion of a large aerospace aluminum part is challenging due to the nature of fast transient thermal process that includes the coupling of thermal, metallurgical, and mechanical interactions. The use of heat transfer coefficients (HTCs) in empirical tools requires an extensive testing matrix to calibrate these HCTs based on measured temperature data at selected locations of the workpiece. The use of a thermal multi-phase FSI tool is essential for the rational design of the flow rate quenchant with agitation to reduce the quenching residual stress by decreasing the thermal gradient from the center of the work piece to the surface. Given the temperature and phase profiles predicted from the Fluid Structure Interaction (FSI) based heat transfer module, a residual stress and distortion prediction module is developed by including fields mapping, temperature and phase dependent property evolution, and a user-defined material model for Abaqus. The fatigue performance of a quenched T-stiffener is evaluated in the presence of quenching induced residual stress under different dipping orientations.
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Subcritical crack growth of nuclear components is a current concern in operating light water nuclear reactors. Weld residual stresses (WRS) can drive stress corrosion crack growth, affect fatigue crack growth, lead to reheat cracking issues if the components are operated in the creep regime, and can affect the fracture response of components. This paper provides several examples where crack growth, driven by weld residual stress fields, has led to safety concerns in several nuclear components. This is especially true for the dissimilar metal welds that are present in most PWR reactors. Mechanical mitigation examples are also discussed which are used to reduce the WRS fields or alter them to compression which can mitigate stress corrosion cracking.
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