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Heavy vehicle structures are made from welded carbon steel frames. During operation these frames are subjected to random dynamic loads, which induce fatigue at the welded joints. A Finite Element based process for calculating fatigue life of welded joint under single excitation random loading is proposed in this study. The proposed method com-bines Equivalent Equilibrium Structural Stress (E2S2) method for weld fatigue and PSD based vibration fatigue technique for handling random loads. Fatigue life of a welded T-joint is analysed using the proposed method in frequency domain and validated against a transient dynamic analysis. The main advantage of the proposed method is the analysis run time is reduced almost 12 times compared to transient analysis. Effect of geometric changes on weld fatigue life is studied. It is found the tube thickness increase at lower thickness ranges significantly increases the fatigue life compared to higher thickness ranges. / May 2016

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At cyclic operation of gas turbines, components are subjected to thermo-mechanical fatigue (TMF) due to the high temperature gradients that arises. In this thesis life assessment of welds in combustion chambers are at focus. Siemens Industrial Turbomachinery AB uses a method they call the HQ-method to estimate the TMF life of components, but how well this estimation work with welds have not been investigated before and is therefore unclear. Because of this unclearness, an additional reduction factor is used take this into account. The goal of this thesis is to conduct a study of welds in the combustion chambers to get an overview of welds that are present, and to identify eventual problems with those. Furthermore, an analysis of a selected weld is performed and the results are evaluated by using the HQ-method. For this analysis, a TIG weld on a test specimen of Hastelloy X is selected as both the weld method and the material are commonly used in combustion chambers. The specimen is chosen due to the possibility to verify the results using a test rig designed for TMF-tests. The results show that the HQ-method, with the assumptions made today, yields a life of welds considered very low compared with the base metal. Also the location of the most severely loaded point is questionable. Further investigations also show that residual stresses in the weld relax quickly and the creep rate of the weld does not make any particular difference to expected life. To verify the results and clarify some questions it is therefore suggested that the commenced investigation continues with real tests on the weld. As the results show that the creep rate is unimportant to the fatigue life, the parameter should be ruled out from the investigation and the objective should instead be to find the accurate yield strength of the weld, and study if failure occurs in or outside the weld. / Vid cyklisk drift av gasturbiner utsätts komponenterna för termomekanisk utmattning (TMF) på grund av de höga temperaturgradienter som uppkommer. I detta arbete är livslängdsbedömningen av svetsar i brännkammare i fokus. Siemens Industrial Turbomachinery AB använder en metod som de kallar för HQ-metoden för att bedöma livslängden av komponenter utsatta för TMF men hur väl denna bedömning stämmer på svetsar är oklart. På grund av denna oklarhet används en extra reduceringsfaktor för att ta hänsyn till detta. Målet med detta arbete är att göra en undersökning av svetsar i brännkammare för att få en överblick av vilka svetsar som finns samt att identifiera eventuella problem med dessa. Vidare ska en analys på en utvald svets utföras och resultaten ska utvärderas med HQ-metoden. Till denna analys väljs en TIG-svets på en provstav av Hastelloy X då både svetsmetoden och materialet är vanligt förekommande i brännkammare. Provstaven valdes eftersom den ger en möjlighet att verifiera resultaten i en provrigg avsedd för TMF-prov. Resultaten visar att HQ-metoden, med de antaganden som görs idag, ger en livslängd på svetsar som är väldigt låg i förhållande till grundmaterialet. Också placeringen av den mest påkända punkten kan ifrågasättas. Vidare visar även undersökningen att restspänningar i svetsen relaxerar snabbt och att kryphastigheten i svetsen inte gör någon speciell skillnad på den förväntade livslängden. För att verifiera resultaten samt klargöra vissa frågetecken föreslås därför att den påbörjade undersökningen fortsätter med riktiga tester på svetsar. Eftersom resultaten visar att kryphastigheten endast är av ringa betydelse ska den parametern uteslutas ur undersökningen och målet ska istället vara att finna den korrekta sträckgränsen i svetsen samt studera om brott uppkommer i eller utanför svetsen.

Tailor welded blanks (TWBs) are used in the automotive industries as a method to meet economic, environmental and governmental demands. Conventionally, TWBs incorporated mild and low strength steels such as interstitial free and draw quality steels because of their excellent formability traits. However, due to their low strength they are unsuitable for energy absorption applications; thus, the interest of incorporating advanced high strength steels (AHSS) into the TWBs. Dual phase (DP) steel is a type of AHSS that is of interest because of its combination of high strength and good formability that is comparable to high strength low alloy (HSLA) steels. However, welding DP steel causes softening in the heat affected zone (HAZ), which leads to premature failure and reduces formability. The aim of this thesis was to study the effect of weld design on the formability of TWBs with DP steels and with HSLA steel. This thesis is divided into three parts; the first part examines TWBs with different weld line positions, weld line orientations and strain paths. The second part investigates bead-on plate curvilinear blanks and its effect on formability of the blanks. The last part examines the effects of multiple welds on the formability of TWBs.

Existing steel bridges are subjected to both increasing traffic loads and natural aging, both are capable of causing severe durability problems. Dependable rehabilitation methods are attracting attention as the promising methods to enhance structural durability and/or structural performance. One possible rehabilitation method, for improving fatigue performance, is the use of residual stress-based post-weld treatments such as peening. A number of studies has been performed and it has been proven that residual stress-based treatments are an effective way of increasing the fatigue lives of newly built steel bridges, and even enhancing the fatigue performance of existing structures. Provisions have been developed to ensure the proper execution of peening and several codes have considered its beneficial effect in the fatigue design of welded structures. Various analytical approaches are used to predict the fatigue performance of welded structures and the beneficial effects of residual stress-based post-weld treatments. In most codes and recommendations, variations of the “S-N curve” approach are employed. Linear elastic fracture mechanics (LEFM) and strain-based fracture mechanics (SBFM) are widely accepted approaches for making more precise predictions of the treatment benefit. Cohesive zone fatigue models are also recently introduced for predicting fatigue crack growth in as-received and peened welds. Despite all the research conducted, there are still two main unanswered questions related to the application of peening treatments. First, it is claimed that peening can be more effective for civil structures where a considerable portion of the total applied stress is due to permanent loads and thus peening is applied under load. However, most of research done so far has studied effects peening prior to the introduction of the structural self weight. Secondly, considering the nature of these treatments, some concerns have been raised regarding their effectiveness under actual in-service loading conditions, as most of the reported test-based studies only demonstrated the fatigue performance improvement under constant amplitude tension-only loading conditions. The current study was undertaken to examine the fatigue performance of welds peened-under load and to determine the effectiveness of peening for improving the fatigue performance of welds subjected to realistic in-service loading conditions. Moreover, a previously developed strain-based fracture mechanics (SBFM) model for predicting fatigue performance of welded details under different loading and treatment conditions, and a previously developed damage-based cohesive zone model for steel specimens were evaluated and calibrated. Fatigue tests were conducted on welded steel specimens, simulating different loading and peening conditions. Dye penetrant was used to stain cracked specimens upon detection of cracks and a crack front marking loading scheme was used to study the crack front shape. The alternating current potential drop (ACPD) method was used for continuous crack growth monitoring for both as-welded and peened specimens under different loading schemes. It was observed that cracks propagated at different rates in specimens treated under load than in the normally peened and as-welded specimens. Material tests were also conducted to determine the mechanical properties of the steel base metal. Secondary effects of peening were investigated by microhardness measurements and weld toe measurements. A number of typical weld toe defects was also detected. Residual stress measurements showed a uniformly distributed tensile residual stress near the surface of the untreated specimen. Needle peening the specimen resulted in a significant change in the residual stress distribution through the specimen thickness. In all cases, peening resulted in a significant increase in the fatigue life. However, greater fatigue life improvements were observed in lower stress ranges. Of the specimens tested under constant amplitude loading, those peened under load experienced the largest fatigue lives. For the variable amplitude loading tests, the untreated specimens had mean fatigue lives slightly less than observed in the constant amplitude tests. A previously developed strain-based fracture mechanics (SBFM) model was used to estimate analytically the effectiveness of peening applied to welded details. The model was able to predict the fatigue lives for both the as-welded and peened specimens for all loading conditions. It correctly estimated the additional benefit of peening when applied under a relatively small prestress level. The model predictions were used to estimate the additional benefit of peening under load. A previously developed cohesive zone model was introduced and applied to predict fatigue crack growth in a weld detail under cyclic loading. Fatigue tests were simulated using the finite element program ABAQUS. The material parameters α and β were chosen by iteration. Other fatigue tests were simulated and the model correctly predicted the effects of varying the applied stress range, R ratio, and residual stress level on the fatigue behaviour.

A novel technique by employing a capacitance displacement measurement system to measure the post-weld-shift (PWS) caused by laser welding in the butterfly-type laser diode module packaging process is proposed. Reduction of the PWS is an important issue in developing low-cost and high-performance semiconductor laser module. Prior to the reduction and compensation of the PWS, a measurement system of PWS must be constructed. In comparison to the high-magnification camera with image capturing system (HMCICS) limited in resolution of 0.07£gm due to its pixels, a measurement system with a higher resolution of 0.0254£gm is used. During the measurement procedure, the PWS of the ferrule probed by the sensors is converted into the fiber misalignment shifts. The coupling efficiency can be improved over 70% after compensation. The result indicates that the PWS can be qualitatively measured and quantitatively computed.

The energy absorbing behavior of stainless steel 304L during the pulsed Nd:YAG laser welding is investigated in this thesis. The equivalent absorptivity is estimated from the comparison of measured and finite element method (FEM) results simulated melting pool shape parameters, e.g. pool width, pool depth, cross-section area and total volume of the pool. To simulate the actual pulsed laser beam, the energy density of heating source is performed as a Guassian distribution in the transection of a circular laser beam. For evaluating the feasibility and the accuracy of the estimated equivalent absorptivity, the multi-pulsed Nd:YAG laser welding is simulated by using the estimated absorptivities. A good agreement between this simulated and measured melting pool shapes are found in the multi-pulsed laser welding. The equivalent absorptivity can be interpolated from different parameters of the molten pool. However, absorptivity curve fitted from the cross-section area and total volume of the melting pool provide a more stable value. Results also indicate that the absorptivity and the pulse energy are in inverse proportion. The thermal-elastic-plastic FEM model is employed to simulate the fusion and solidification process of the pulsed laser welding. A complicate residual stress distribution introduced from the shrinkage in the solidification process is also calculated and presented. The distribution of post-weld-deformation near the melting pool has also been studied in this thesis. This post-weld-deformation may be a key factor in high precision laser welding, e.g. laser packaging for the optoelectronic components. The absorptivity estimated in this thesis may be helpful to simulate the laser welding process accurately.