Offshore pipelines and risers transfer oil and gas across long distances, from seabed to production facility to the surface. The long pipelines are formed by welding together pipe segments. The welded joints formed are a source of stress concentration and defects from which fatigue cracks can grow. Hence, it is imperative that the effect of the weld geometry on the stress concentration be understood so that appropriate measures can be taken to assess the potential remaining service life of the welded structure. The effects can be understood by the linear elastic fracture mechanics approach, where the stress intensity factors quantify the stress concentration. While the classical equations of Newman and Raju have been long available for semi-elliptical surface cracks in plates, no similarly elegant stress intensity factor solutions are available for pipes. There have been solutions in tabular form which can be cumbersome in practice. Moreover, solutions of welded pipe geometries have not been developed. The objectives of the current work are to develop closed-form solutions for stress intensity factors for external semi-elliptical surface cracks in plates. The welded pipe geometry will also be studied to develop solutions for the weld toe magnification factors of welded pipe geometries. The stress intensity factors can be used to determine the propagation rate of cracks in pipe or welded pipe geometries. The stress intensity factors are obtained by the J-integral output of the three-dimensional finite element method. First, a plate with a circular crack is modelled. The initial step transforms the model to a plate with a semi-elliptical crack with the appropriate crack aspect ratio and width. A second transformation follows to transform the geometry to pipe form. The main parameters studied are the relative crack depth to thickness, crack aspect ratio, radius and thickness. The developed stress intensity factor solutions can be reduced to the classical equations. The new solutions show good agreement compared to previous work. A similar approach is developed to study the welded pipe geometry to develop weld toe magnification factor solutions. The weld toe magnification factor solutions for certain geometries are presented as a function of the relative crack depth. The stress intensity factor solutions are then applied to predict the crack growth rates of cracks in pipe geometries. The prediction was conducted by a program written to assess the fatigue life of single and multiple cracks in pipes and welded pipes. The fatigue life assessment of welded pipes using the weld toe magnification factor solutions shows how significantly the weld geometry affects fatigue life.

Fatigue crack growth at welded joints often propagates from as many as tens to hundreds of small weld toe cracks along the weld toe line in offshore welded structures. This paper will present a fatigue algorithm for modeling many small weld toe cracks propagating from a welded joint. Cracks usually initiate at the weld toe region of the structures and propagate as surface cracks at the stress concentration regions of the weld-toe line. The presence of such weld defects or crack-like flaws can have a severe detrimental effect on their fatigue life and fracture resistance. Currently, there is a lack of studies that considers the effects of multiple cracks and their distribution density in welded joints. This work focuses on the fatigue analysis and modeling of multiple weld toe cracks, specifically in T-butt joints. Fatigue crack growth prediction is usually determined by the stress intensity factor range and crack propagation rate through Paris law. To predict the Stress Intensity Factor (SIF) of a weld toe crack, the magnification ( M k ) factor was used. The M k factor is influenced by the size of the welded attachment, as well as the size and depth of the weld toe crack. Simplified solutions for practical prediction of M k factors were determined from 3D extended finite element method (XFEM) by modelling a semi-elliptical weld toe crack in a T-butt weld for cracks of different dimensions. The accuracy of the M k factor solutions was verified by comparison to HSE fatigue data on 16 mm thick tubular joints. The M k factor solutions were used to predict the growth of fatigue cracks using a model based on Paris Law and SIF solutions by Newman and Raju with plastic zone size corrections. Fatigue life was predicted for plates with and without attachments. It could be seen that the predicted life of a weld toe crack was severely reduced with the addition of a welded attachment. The model was extended to the multiple surface cracks commonly observed at the weld toe, where each crack is treated as independent, following established code procedures. The multiple cracks will coalesce as they propagate, until a single dominant crack emerges and fracture occurs. In this paper, the relationship between the fatigue life and the number and density distribution of the initial cracks was investigated. Fatigue life was predicted for plates with attachments with 1, 2, 10 and 100 cracks initially. The results show that as the number of cracks increases, the predicted fatigue life decreases.

In this paper, the strain-rate dependent mechanical properties and stress-strain curve behavior of Sn3.8Ag0.7Cu (SAC387) solder is presented for a range of strain-rates at room temperature. The apparent elastic modulus, yield stress properties and stress-strain curve equation of the solder material is needed to facilitate finite element modeling work. Tensile tests on dog-bone shaped bulk solder specimens were conducted using a non-contact video extensometer system. Constant strain-rate uni-axial tensile tests were conducted over the strain-rates of 0.001, 0.01, 0.1 and 1 (s −1 ) at 25°C. The effects of strain-rate on the stress-strain behavior for lead-free Sn3.8Ag0.7Cu solder are presented. The tensile yield stress results were compared to equivalent yield stress values derived from nano-indentation hardness test results. Constitutive models based on the Ramberg-Osgood model and the Cowper-Symond model were fitted for the tensile test results to describe the elastic-plastic behavior of solder deformation behavior.

The combined sequential reliability test of thermal cycling aging followed by board level drop test for lead-free SnAgCu soldered assemblies were investigated. Interfacial IMCs, Kirkendall voids formation and interconnect failure mode are studied subject to TC aging. Kirkendall voids were observed with Ar+ sputtering etching. The failure sites and mechanism were examined and correlated with IMC and void formation. Significant decrease of drop life was observed for both SAC/ENIG and SAC/Cu-OSP assemblies after thermal cycling aging. Growth of Kirkendall voids and IMC significantly weakened the solder joint interface during TC aging. Drop impact crack path changed from the IMC to the IMC/Cu interface.