Thomas P. Forte

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

The Battelle structural stress method is examined for the evaluation of multiaxial loading fatigue behavior in welded structures. Even though the structural stress and its master S-N curve approach have been mainly focused on normal loading dominant (Mode I) failure cases, the evaluations on multiaxial loading weld fatigue using structural stress parameters have been relatively recently performed such as using the modified Gough ellipse [1] and the path-dependent maximum range (PDMR) cycle counting procedure [2].In this article, in order to evaluate the multiaxial fatigue behavior, an effective equivalent structural stress range (EESS) parameter is defined as a von Mises form of combined normal and in-plane shear equivalent structural stress ranges. The newly developed in-plane shear equivalent structural stress range for in-plane shear dominant loading (Mode III) is introduced. This in-plane shear equivalent structural stress range parameter has been formulated based on the evaluation of fatigue behavior under in-plane shear loading. Also, the EESS parameter is a function of damage parameter based on the PDMR procedure.In this article, the procedure employing the EESS parameter is evaluated and validated using published weld fatigue data. The multiaxial fatigue date is consolidated within a small scatter band regardless of in-phase, out-of-phase, and non-proportional loading as well as torsional loading. Finally, the design master S-N curve is proposed for multiaxial loading weld fatigue.It is found that the dimensionless bend ratio parameter, Iτ (rτ)1/mτ for in-plane shear loading is a much more significant correction than that for normal loading when the ratio of bending structural stress to the total structural stress, rτ increases. This procedure will be beneficial for fatigue design with preventing over-conservatism.Copyright

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

Risers, pipelines and flowlines for deep water applications are subject to corrosive environments. Especially, in the presence of hydrogen sulfide which makes the field sour, their fatigue performance becomes significantly degraded. In order to quantify the sour degradation effect, a knock-down factor has been introduced. This factor is defined as the fatigue life reduction relative to the in-air fatigue life.Several sets of fatigue test results in sour service environments have been published. These include strip specimens of different sizes, e.g., diameters, wall thicknesses, and arc lengths. Naturally, the knock-down factor must be based upon a statistically valid number of fatigue test results obtained from the same specimen geometry and the same loading conditions tested in air and in sour conditions. Currently, the database available in the open literature is too limited to properly define a knock-down factor. Moreover, there is a great deal of scatter within the database and each test in a sour environment is costly and time consuming. Thus, it is difficult to establish a statistically valid database upon which to base the knock-down factor.A mesh-insensitive structural stress method has been developed by Battelle researchers and has been proven to be highly effective in correlating the fatigue behavior of welded joints. In 2007, the Battelle structural stress based weld fatigue master S-N curve was included in ASME Section VIII Div. 2 because it successfully consolidated more than 800 fatigue test results for weld toe failures onto a single master S-N curve with very little scatter, regardless of specimen shape, size, loading type, and steel alloy [1–2].A knock-down factor is derived by applying the Battelle structural stress method to the existing database for sour environment tests and by using the current in-air database as the reference condition. This approach will reduce the uncertainty in the knock-down factor because it allows a wider range of sour environment data from specimens of different sizes, types, and loading conditions to be combined, while simultaneously reducing scatter. As such, a unified knock-down factor can be determined with greater statistical validity and wider applicability for design recommendations in sour conditions.Copyright

Probabilistic Approach for Fatigue Life Assessment Based on S-N Curve

Components in pressure vessels and pipes are usually subjected to mechanical and thermal cyclic loadings, which cause fatigue failure. The statistic and probabilistic assessment of these components based on S – N or e–N curves is of great importance for fatigue design. Recently, the standard practice, as adopted by ASTM, BS, DNV and many other standards, for statistical analysis of linear or linearized stresslife (S – N) and strain-life (e–N) fatigue data has been critically reviewed. The shortcomings of the standard procedure based only on the variation of cycles have been clearly demonstrated by examining the general trend of a large amount of S-N data. A new deterministic statistical method based on the equivalency between the changes of stress range and cycles to failure has been subsequently proposed and validated. In this paper a probabilistic approach based on the equivalency method is developed to quantify the uncertainty of engineering structures subjected to inherent randomness in material properties, and its effectiveness is also demonstrated.Copyright

Multi-Axial Fatigue Life Assessment of Wind Turbine Structural Components

Modern wind turbines, which are usually made of composite materials, are fatigue critical structures that are subjected to variable multi-axial fatigue loading. Therefore, they should be designed as safely as necessary to withstand the fatigue loads over the designed life time. Path-Dependent Maximum Range (PDMR) is a multi-axial fatigue life assessment tool developed by Battelle researchers. PDMR has been successfully applied to fatigue analysis of isotropic structures under general variable amplitude, multi-axial fatigue loading histories. The effectiveness of the PDMR method has been validated by its ability to correlate a large amount of fatigue data available in the literature. For uniaxial loading data, PDMR gives exactly the same results as ASTM standard Rainflow cycle counting method. In this paper, the PDMR method is extended to composite materials, such as glass fiber reinforced plastics (GFRP) and carbon fiber reinforced plastics (CFRP). The proposed multi-axial fatigue damage model effectively correlates fatigue lives of unidirectional composites for various off-axis ply angles under cyclic tensile loading. With this extended capability, the PDMR can now be used to assess the multi-axial fatigue life of composite structures used in wind energy industry.Copyright

Integrity Analysis for Dents in Pipelines

Incident analyses over the last several decades consistently show that outside force is the largest single cause of reportable incidents for the hazardous-liquid- and gas-transmission pipeline systems throughout the world. This paper addresses outside-force incidents due to “acts of man” leading to mechanical damage in the form of dents. As such incidents are significant worldwide, there are worldwide efforts to prevent future incidents, and develop approaches to manage this threat. This paper evaluates the criticality of dents in a more general framework than acceptable dent depth used in codes. This analysis is done with reference to typical line pipe mechanical properties, as well as less know parameters like true -fracture ductility and fracture-initiation toughness. Analyses results characterize severity and rank criticality as a function of dent type in reference to the general PRCI model for mechanical damage. Results are presented that show current code-acceptance criteria for dents are over conservative in general, sometimes significantly, particularly for plain dents. Full-scale test data are introduced to support the analytical results. Results are also presented to assist in evaluating the utility and accuracy of ILI deformation tools, and their calibration with reference to measured field dent size.Copyright

Fatigue Evaluation Procedures for Multi-Axial Stress State in Welded Joints

The Battelle structural stress method (BSSM) for fatigue life evaluation is examined for multi-axial stress states that develop due to uniaxial loading in welded structures. The resultant multi-axial stress state due to simple uniaxial loading is easily observed in common joint types such as a plate with a welded tube or a plate with an angled attachment. In these joint types, under simple loading, the stress distribution at the location of failure along the weld line shows significant in-plane shear stress (parallel to the weld line) as well as normal stress (normal to weld line). Although the fatigue data, as exemplified by the inverse slope of the S-N curve for the subject joints under uniaxial loading, are observed to be similar to that for normal-loading-mode dominant (Mode I) failures in welded joints, when only the normal structural stress is considered for these joints the predictions of both the fatigue failure location and the fatigue life using the master S-N curve approach are inaccurate because the in-plane shear stress plays a significant role in the development of the crack.The slope of fatigue data exhibited in S-N curves taken from weld fatigue data for resultant multi-axial stress state generated by uniaxial loading is different from multi-axial fatigue loading conditions for tubular joints as discussed in the recent work [OMAE2014-23459].In this article, the fatigue behavior of welded joints with multi-axial stress states is evaluated using an effective equivalent structural stress range parameter that is formulated as a von Mises form of the combined normal and in-plane shear equivalent structural stress ranges. When the effective equivalent structural stress range parameter is employed, the fatigue failure location can be predicted correctly. It is also found that the cycles-to-failure data from the subject joint types are comparable with the master S-N curve for Mode I loading dominant behavior (inverse slope of 3.125). Therefore, the master S-N curve that was developed for Mode I failures can be equally applicable for fatigue life prediction for these joints by replacing the equivalent structural stress range with effective equivalent structural stress range on the ordinate axis.Copyright

A Rapid Convex Hull Algorithm for Implementing Path-Dependent Multi-Axial Fatigue

The Path-Dependent Maximum Range (PDMR) is a general-purpose multi-axial fatigue life assessment tool recently developed by Battelle researchers. The PDMR has been successfully applied to fatigue analysis of engineering components under variable amplitude, non-proportional, multiaxial fatigue loading histories. PDMR begins by seeking the maximum possible distance (or range) between any two points in the equivalent stress/strain space over a given fatigue loading history, while also identifying the associated loading path-length. The process continues recursively until each loading path has been counted. PDMR then collects the cycles calculated and the associated path-lengths for subsequent calculations of the fatigue damage. The effectiveness of the PDMR method has been validated by its ability to correlate a large amount of fatigue data. In a computerized PDMR calculation, most of the central processor unit’s (CPU) time is spent searching for the maximum range. While a brute-force search is the simplest to implement, and will always find a solution if it exists, its cost, in many practical problems, tends to grow very quickly as the size of the loading spectrum increases with O(n2 ) time complexity, where n is the number of spectrum data points. In this paper, Andrew’s monotone chain algorithm, a sophisticated and reliable convex hull algorithm is implemented into the PDMR to reduce the solution time. Like the widely used angular Graham Scan sort, Andrew’s monotone chain runs in O(n log n) time due to the merge-sort approach. The Rotating Caliper algorithm, which is another computational algorithm for quickly determining all antipodal pairs of vertices on a predetermined convex hull, is also introduced. Several examples have clearly demonstrated that these algorithms can be used in combination to significantly decrease the execution time for the PDMR in engineering fatigue analysis and design.Copyright

Study of Crack Extension Effects on a Weld-Induced Residual Stress Field

For many applications, welding is the best manufacturing process for joining two separate components. However, the welding process with its highly localized heating and cooling results in the residual stresses both along the surface and through the thickness of the joint. These residual stresses will affect the rate of growth of a fatigue crack as it propagates through the joint and therefore will affect the service life. Therefore, service life predictions must consider the residual stresses as well as their redistribution that occurs as the crack grows.Analytical approaches to assess fitness-for-service such as API 579 are available to evaluate the effect of weld-induced residual stresses. However, current industry practices calculate stress intensity factors (SIFs) for fracture and fatigue crack growth life estimates without considering the redistribution of the residual stresses as the crack propagates, and thus tend to be very conservative.In this paper, Battelle’s weld residual stress modeling methods are combined with a procedure for calculating the SIF using local crack tip displacements that accounts for the redistribution of the residual stresses due to crack extension. In addition, the finite element model used to determine the weld residual stresses is also used to determine the SIF. Therefore, the complete mechanical response to the welding, including residual stress, deformed geometry, elastic and plastic strains, etc. are available for the crack growth analysis. This new unified procedure is demonstrated for simple joints and is compared to a “simplified” crack growth analysis that uses stress mapping. The unified procedure clearly characterizes mixed mode crack behavior at the crack tip. In this paper the procedure is presented in terms of how the crack affects the weld-induced residual stress field; it is equally well suited for practical design use to assess structural integrity in the presence of a weld-induced residual stress field with or without external service loading.Copyright