J. K. Hong

Coupled thermomechanical analysis of friction stir welding process using simplified models

Abstract It is widely recognised that the fundamental mechanisms associated with the weld formation process and their relationships with welding parameters are complex and remain to be fully understood. The present paper reports a series of general findings based on a set of simplified numerical models that were designed to elucidate various aspects of the complex thermomechanical phenomena associated with friction stir welding. The following phenomena were investigated in separate numerical models: (i) coupled friction heat generation; (ii) plastic flow slip zone development; and (iii) three-dimensional heat and material flow. The friction induced heat generation model was used to quantify the contributions of coupled thermomechanical friction heating, including non-linear interfacial phenomena between the tooling (e.g. stir pin) and material being welded. The plastic work induced heating effects were also examined. The plastic slip formation mechanisms were then investigated by considering contributions from various heating mechanisms. Finally, a simplified three-dimensional heat and material flow model, based on the observations from the coupled friction heat generation model, was used to establish some initial insight regarding the heat and material flow. The results from the three subproblem areas were then generalised in the form of a simple parametric relationship between welding variables (i.e. travel and rotating speeds) and weld formation conditions. A series of assumptions were made in constructing these individual models since there exists little information on actual material behaviour under friction stir welding conditions. However, the findings from the present study not only illuminate some of the important weld formation mechanisms in friction stir welding, but also provide an effective framework for more focused investigations into some of the fundamental phenomena identified in the three subproblem areas: such investigations will be reported separately in a future publication.

Stresses and stress intensities at notches: 'Anomalous crack growth' revisited

Abstract In this paper, a new notch stress estimation scheme is presented within the context of conventional finite element solutions. The estimation method is based on a separation of an actual notch stress state into two parts. One part is a far-field stress in the form of membrane and bending components that satisfy far-field equilibrium conditions, and the other is a self-equilibrating part that provides an effective measure of the notch stress state. The self-equilibrating part can be directly related to notch geometry and captured using a rather coarse finite element model. The singular stress behavior at an arbitrary notch is then described by a closed form solution formulated using the self-equilibrating part of the stress state for a small crack emanating from a notch. The corresponding stress intensity factor solutions are then presented by considering both the far-field stress (also called structural stress) and self-equilibrating notch stress. The stress intensity solutions are formulated using existing solutions for typical simple crack geometry. One important feature of the notch stress and stress intensity solutions is that the current solutions not only capture the singular characteristics as the notch tip is asymptotically approached, but also recover the far-field (or nominal) stress state. Finally, the effectiveness of the present notch stress estimation scheme is demonstrated by using a series of well-known short crack growth data exhibiting ‘anomalous growth’. It has been found that, for instance, the anomalous growth discussed in, for example, Fat Eng Mat Struct 6 (1983) 315; Eng Fract Mech 29 (1988) 301; as well as Surface Crack Growth: Models, Experiments, and Structures (1990) 333, can be unified with long crack data as straight lines, without resorting crack closure considerations. As a result, a two-stage crack growth model is proposed within the context of K-dominant crack growth. The first stage is dominated by the notch-induced self-equilibrating part of the stress state and the second stage is dominated by the equivalent far-field stress state or structural stresses that satisfy equilibrium conditions and can be effectively computed in a mesh-insensitive manner. The implications of these findings on fatigue growth rate data generation and fatigue life predictions will also be discussed.

The Master S-N Curve Approach to Fatigue Evaluation of Offshore and Marine Structures

As reported in the last OMAE conference (Dong, 2003), a robust structural stress method has been developed and validated for fatigue evaluation of ship structures through a major joint industry project. The structural stress method not only provides a consistent method for characterizing stress concentration effects on fatigue in different joint types and loading modes, but also offers a rapid estimation procedure for stress intensity factors for arbitrary joint geometries and loading modes in fracture mechanics context. As a result, a master S-N curve approach has been recently developed by using the mesh-insensitive structural stress parameter and its direct linkage to fracture mechanics principles. The master S-N curve is described by an equivalent structural stress range parameter which provides a single parameter description of stress concentration effects, thickness effects, and loading mode effects on fatigue in welded joints. A massive amount of S-N data since 1947, encompassing drastically different joint types, plate thickness, and loading modes have been used to validate the effectiveness of the master S-N curve approach. With the master S-N curve method, plate joints in ship structures, tubular joints in offshore structures, as well as pipe joints for riser applications can be collapsed into a singe curve, referred to as the master S-N curve. This paper provides the detailed theoretical development, application examples, and validation results. The applications for the master S-N curve approach will be illustrated by using various offshore/marine examples.Copyright

Fatigue of Tubular Joints: Hot Spot Stress Method Revisited

A series of well-known tubular joints tested in UKSORP II have been re-evaluated using the mesh-insensitive structural stress method as a part of the on-going Battelle Structural Stress JIP efforts. In this report, the structural stress based analysis procedure is first presented for applications in tubular joints varying from simple T joints, double T Joints, YT joints with overlap, and K joints with various internal stiffening configurations. The structural stress based SCFs are then compared with those obtained using traditional surface extrapolation based hot spot stress methods. Their abilities in effectively correlating the fatigue data collected from these tubular joints are demonstrated. These tests are also compared with the T curve typically used for fatigue design of tubular joints as well as the structural stress based master S-N curve adopted by ASME Section VIII Div 2. Finally, some of the implications on fracture mechanics based remaining life assessment for tubular joints are discussed in light of the results obtained in this investigation.

On the Residual Stress Profiles in New API 579/ASME FFS-1 Appendix E

In this paper, the mechanics basis underlying the parametric through-thickness residual stress profiles proposed for the new joint fitness for service document referred to as API 579/ASME FFS-1 Appendix E is presented. The proposed residual stress profiles are described to a large extent by a unified parametric function form valid for a broad spectrum of pipe and vessel welds. The functional relationship is established based on the comprehensive knowledge base developed within a recent major international joint industry project (JIP) under the auspice of Pressure Vessel Research Council (PVRC) and a large amount of residual stress measurement data from recent literature. One of the most important features associated with the proposed revision is that residual stress profile is uniquely determined by two important sets of governing parameters: (1) parameters relevant to pipe geometry, i.e., r/t and t; (2) a parameter related to welding linear heat input Q (J/mm), referred to as the characteristic heat input

Assessment of Asme’s Fsrf Rules for Vessel and Piping Welds using a New Structural Stress Method

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Fatigue of piping and vessel welds: ASME'S FSRF rules revisited

which has a dimension of J/mm3. As a result, the corresponding through-wall residual stress distribution exhibits a continuous change as a function of r/t, t, and

Fatigue Evaluation Procedures for Multiaxial Loading in Welded Structures Using Battelle Structural Stress Approach

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Low-Cycle Fatigue Evaluation Using the Weld Master S-N Curve

, instead of falling into a few discrete and unrelated profiles, as seen in the current Codes and Standards.

Study on Weld Fatigue Evaluation Under Sour Service Environment Using Battelle Structural Stress Method

Fatigue design rules for welds in the ASME Boiler and Pressure Vessel Code are based on the use of Fatigue Strength Reduction Factors (FSRF) against a Code-specified fatigue design curve generated from smooth base metal specimens without the presence of welds. Similarly, Stress Intensification Factors (SIF) that are used in the ASME B31 Piping Codes are based on component S-N curves with a reference fatigue strength based on straight pipe girth welds. Typically, the determination of either the FSRF or SIF requires extensive fatigue testing to take into account the stress concentration effects associated with various types of component geometry, weld configuration, and loading conditions. As the fatigue behaviour of welded joints is being better understood, it has been generally accepted that the difference in fatigue lives from one type of weld to another is dominated by the difference in stress concentration. However, general finite element procedures are currently not available for effective determination of such stress concentration effects. This is mainly due to the fact that the stress solutions at a notch (e.g., at weld toe) are strongly influenced by mesh size at and near a weld, resulting from notch stress singularity. In this paper, a mesh-insensitive structural stress method is used to re-evaluate the S-N test data. Its applications in consistently representing the stress concentration effects on fatigue S-N data for pipe girth welds are demonstrated. A single master S-N approach is presented by means of a mesh-insensitive structural stress parameter formulated within the context of fracture mechanics. The major findings are as follows: (a) The mesh-insensitive structural stress method provides a simple and effective mean for characterising stress concentrations at vessel and pipe welds (b) The structural stress based parameter provides an effective measure of stress intensity at welds, which can be related to fatigue lives. (c) Once the mesh-insensitive structural stress is used, the S-N data processed thus far can be reasonably consolidated into one narrow band. Therefore, single master S-N curve for vessel and piping welds can now be established, regardless of piping weld types or geometries (straight pipe girth welds, different types of flange welds, elbow welds, mitre bends, etc.), and can be used to general a master fatigue design curve.