Mesh-insensitive Structural Stress Research Articles

A Robust Structural Stress Method for Fatigue Analysis of Offshore/Marine Structures

Recent rapid advances in developing mesh-insensitive structural stress methods are summarized in this paper. The new structural stress methods have been demonstrated to be effective in reliably calculating structural stresses that can be correlated with fatigue behavior from simple weld details to complex structures. As a result, a master S–N curve approach has been developed and validated by a large amount of weld S–N data in the literature. The applications of the present structural stress methods in a number of joint types in offshore/marine structures will be illustrated in this paper. The implications on future applications in drastically simplifying fatigue design and evaluation for offshore/marine structures will also be discussed, particularly for using very coarse finite element mesh designs in ship structures.

A Robust K Estimation Scheme using Mesh-Insensitive Structural Stresses

In this paper, the applications of a new structural stress method, referred as a mesh-insensitive structural stress method developed recently, in estimating stress intensity factors for welded joints will be discussed. The new structural stress method is based on the considerations of separating an actual stress state into two parts: (1) equilibrium-equivalent part and (2) self-equilibrating part. Both parts are consistent with the far-field stress state with respect to a hypothetical crack location within fracture mechanics context. Therefore, the stress intensity factors (K) can be estimated once the mesh-insensitive structural stresses become available. The method is particularly suited for applications in welded joints in complex structures. In this paper, a brief description of the mesh-insensitive structural stress procedure is reviewed. A robust K estimation procedure is then presented. The new K solution procedures were then validated against a series of well-known K solutions for welded joints in the literature. Finally, discussions are given in terms of the implications of the present solutions in applications in complex structures.

The Master S-N Curve Approach to Fatigue of Piping and Vessel Welds

In support of the new revision of ASME Div 2 Codes, a unified master S-N curve approach has been developed using a recently developed mesh-insensitive structural stress procedure. The effectiveness of the new master S-N curve approach has been documented in a number of earlier publications for many joint types and loading conditions, including pipe and vessel welds. In this paper, additional S-N data for pipe and vessel girth welds are analysed using the mesh-insensitive structural stress procedure and the master S-N curve approach. The new data include a large series of full-scale pipe tests for applications in deepwater riser recently completed. A series of full-scale vessel and pipe tests under pressure pulsation have also been analysed. It is particularly worth noting that some of the vessel tests were used to support ASME Sec. III related applications, in which some tests incorporated welds with artificially machined flaws and one test was a stainless steel pipe removed from an actual reactor. Regardless of the drastically different joint geometries and loading modes, an excellent correlation among all the S-N data analysed have been achieved by using the master S-N curve approach. Furthermore, it seems that all these pipe and vessel test results can also be correlated with all plate joint data collected thus far.

용접구조물의 피로설계를 위한 유한요소 해석 및 통합 피로선도 초안 개발

Fatigue design rules for welds in the ASME Boiler and Pressure Vessels 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 that are used in the ASME B31.1 Piping Code. are based on component S-N curves with a reference fatigue strength based on straight pipe girth welds. But the determination of either the FSRF or stress intensification factor 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 behavior 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. In this paper, a mesh-insensitive structural stress method is used to re-evaluate the S-N test data, and then more effective method is proposed for pressure vessel and piping fatigue design.

Assessment of Asme’s Fsrf Rules for Vessel and Piping Welds using a New 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.

Temperature and humidity effects on fatigue life distribution of carbon steel: Sakai, T., Ramulu, M. and Suzuki, M. Int. J. Fatigue Mar. 1991 13 (2), 117–125

This paper describes the application procedures of a nodal force based mesh-insensitive structural stress parameter for analysis of a comprehensive set of spot weld fatigue test data collected from a series of advanced high strength sheet steels. The structural stress parameter is calculated in an equilibrium sense in terms of bending and membrane components from nodal forces and moments at each grid point along the periphery of a weld nugget. Based on fracture mechanics considerations, an equivalent structural stress parameter is then used to take into account the effects of loading mode and sheet thickness on the fatigue of spot welded joints. The equivalent structural stress is proven effective in consolidating the large amount of fatigue data of spot welds for transformation induced plasticity (TRIP), dual-phase (DP), and high strength low alloy (HSLA) steels subjected to both tensile shear and coach peel loadings. As a result, a single master S–N can be established for fatigue design and life prediction of spot-welded structures.

Fractographic observations and predictions on fatigue crack growth in an aluminium alloy under miniTWIST flight-simulation loading: Seigl, J., Schijve, J. and Padmadinata, U.H. Int. J. Fatigue Mar. 1991 13, (2), 139–147

This paper describes the application procedures of a nodal force based mesh-insensitive structural stress parameter for analysis of a comprehensive set of spot weld fatigue test data collected from a series of advanced high strength sheet steels. The structural stress parameter is calculated in an equilibrium sense in terms of bending and membrane components from nodal forces and moments at each grid point along the periphery of a weld nugget. Based on fracture mechanics considerations, an equivalent structural stress parameter is then used to take into account the effects of loading mode and sheet thickness on the fatigue of spot welded joints. The equivalent structural stress is proven effective in consolidating the large amount of fatigue data of spot welds for transformation induced plasticity (TRIP), dual-phase (DP), and high strength low alloy (HSLA) steels subjected to both tensile shear and coach peel loadings. As a result, a single master S–N can be established for fatigue design and life prediction of spot-welded structures.

Fatigue crack initiation and stage-I propagation in polycrystalline materials. II. Modelling: Provan, J.W. and Zhai, Z.H. Int. J. Fatigue Mar. 1991 13, (2), 110–116

This paper describes the application procedures of a nodal force based mesh-insensitive structural stress parameter for analysis of a comprehensive set of spot weld fatigue test data collected from a series of advanced high strength sheet steels. The structural stress parameter is calculated in an equilibrium sense in terms of bending and membrane components from nodal forces and moments at each grid point along the periphery of a weld nugget. Based on fracture mechanics considerations, an equivalent structural stress parameter is then used to take into account the effects of loading mode and sheet thickness on the fatigue of spot welded joints. The equivalent structural stress is proven effective in consolidating the large amount of fatigue data of spot welds for transformation induced plasticity (TRIP), dual-phase (DP), and high strength low alloy (HSLA) steels subjected to both tensile shear and coach peel loadings. As a result, a single master S–N can be established for fatigue design and life prediction of spot-welded structures.

Accuracy by using newer criteria of strength for the fatigue behaviour of metals under multiaxial alternating loads: Troost, A., Akin, O. and Klubberg, F. Materialwissen. Werkstofftech. Jan. 1991 22, (1), 15–22 (in German)

This paper describes the application procedures of a nodal force based mesh-insensitive structural stress parameter for analysis of a comprehensive set of spot weld fatigue test data collected from a series of advanced high strength sheet steels. The structural stress parameter is calculated in an equilibrium sense in terms of bending and membrane components from nodal forces and moments at each grid point along the periphery of a weld nugget. Based on fracture mechanics considerations, an equivalent structural stress parameter is then used to take into account the effects of loading mode and sheet thickness on the fatigue of spot welded joints. The equivalent structural stress is proven effective in consolidating the large amount of fatigue data of spot welds for transformation induced plasticity (TRIP), dual-phase (DP), and high strength low alloy (HSLA) steels subjected to both tensile shear and coach peel loadings. As a result, a single master S–N can be established for fatigue design and life prediction of spot-welded structures.

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The 3rd Edition of API 579-1/ASME FFS-1 2016 Fitness-For-Service includes a new Part 14 dedicated to fatigue assessment. An important section in this part covers the fatigue assessment of welded joints. In this paper, an overview of the fatigue methods for welded joints is provided and extensions are recommended. First, an overview is given of the classical fatigue method used in the ASME B&PV Code based on smooth bar fatigue curves in conjunction with a fatigue strength reduction factor. In addition, the mesh insensitive structural stress method is outlined using an equivalent stress parameter based on fracture mechanics considerations in conjunction with a master S-N curve based on the analysis of over 2000 high and low cycle S-N test data. The resulting master S-N curve approach is applicable to high cycle fatigue and low cycle fatigue if a Neuber correction is introduced. In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to the low cycle fatigue regime in which plastic deformations can be significant while an elastic core is present. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure can be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in the high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke’s law. Thus, both low cycle and high cycle fatigue behavior can now be treated in a unified manner. In the low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy a linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. A PVRC Joint Industry Project is currently sponsoring work on the structural strain method that will lead to its incorporation in the next edition of API 579-1/ASME FFS-1.