Fracture path transition, from interfacial fracture to a pulled button, in peel tests of spot welds in SAE 1039 steel was evaluated to determine the controlling material properties. Welds were tested in the as welded condition and after tempering at various temperatures to develop a range of hardness and strength in the weld metal. Two transitions were found, from complete interfacial fracture to partial interfacial and from partial interfacial fracture to a pulled button. Samples tempered at less than 350 °C exhibited complete interfacial fracture while those tempered at 500 °C or higher pulled full buttons. Each transition was accompanied by a large increase in the energy absorbed during fracture. Both partial and complete interfacial fracture occurred by intergranular fracture along prior austenite grain boundaries. Optical microscopy utilizing a tint etch indicated that austenite existed as films on prior austenite grain boundaries of samples tempered at less than 500 °C and TEM confirmed that these films were austenite. Weld metal toughness was found to control the fracture path. If fracture initiation was delayed until the applied load caused plastic deformation of the coupons pulled button fracture occurred. Fracture initiation prior to plastic deformation of the coupons resulted in interfacial or partial interfacial fractures. A small secondary hardening peak was observed in samples tempered at 450 °C. Secondary hardening, which normally results from alloy carbide precipitation, was due to AlN precipitation in this aluminum killed plain carbon steel. / Ph. D.

This study investigates the reliability of SnPb and SnAg solder joints in semiconductor packages subjected to thermal cycling. More specifically, solder joint crack growth and life are experimentally measured, and FEM models are run to explain the test results. Ultimately a life-prediction model is proposed for both SnPb and SnAg solder joint packages. Motorola 357-plastic ball grid array packages on printed wiring boards were thermal cycled with the following test parameters: SnPb and SnAg solders, three post-process conditions (aged, aircooled and quenched), four package layouts on the printed circuit boards (singledense, single-sparse, double-alternating, and double-dense), three accelerated thermal cycling protocols (0°C to 100°C, -40°C to 125°C, and -55°C to 125°C), and tests run at Motorola and the University of Texas. At predetermined thermal cycles, packages were removed from the environmental chambers, dyepenetrated, packages removed to expose the solder joints, and optical images taken. Images were processed to measure crack area, shape, orientation and length to show crack growth. Selected joints were sectioned and polished to investigate microstructure and failure modes. Selected boards were connected to an ANATECH event detector to monitor life from joint failures. FEM crack initiation and propagation models were developed to better understand failure mechanisms. Major experimental results are: 1) SnPb joints have about 50% faster crack growth rates than SnAg joints, subsequently SnPb joints have half the life of SnAg joints, 2) air-cooled and quenched packages had similar failure characteristics, but aged SnPb joints had lower life and aged SnAg joints had significantly longer life than the comparable nonaged joints, 3) double-dense package layout significantly decreased life (by 75%) over the other package layouts, which were similar to each other, 4) the test results at the two locations (UT and Motorola) were similar for SnPb solder joints, but significantly different for SnAg solder joints, and 5) the largest cracks occurred at the corners of joints just under the die edge. Major FEM simulation results are: 1) the crack initiation life of SnAg joints is approximately 100% longer than SnPb joints, 2) shear load is a major cause of crack growth, but the contribution of tensile load increases as the cracks grow, 3) primary cracks at the board interface appear to reduce the propagation rate of the primary crack on the package interface, 4) secondary cracks are suppressed when compressive stresses prevent voids from nucleating, 5) the double-dense configuration shows no PWB warping due to symmetry, and its stresses are larger than for the other package layouts, and (6) the stresses and strains for single-dense, single-sparse, and double-alternating package layouts are similar because the stresses/strains are dominated by local effects due to the CTE mismatch between the die and board. Based upon the experimental results and FEM simulations, a lifeprediction model based upon a severity metric was proposed. The metric estimates damage to the solder joints and links material properties and parameters associated with package layout and thermal test conditions to the time-dependent creep, time-independent plastic deformation, and a time-dependent and geometric effective stress of the solder. The severity metric predicted life very well for most of the data tested and was more accurate than the industry-standard life-prediction models for SnPb solder joints.

In a bus chassis, welded connections are often preferred to other fastening techniques due to low cost and broad competence among suppliers but are usually the weakest parts due to fatigue life. Evaluation of welds is often costly in time and competence. Long iteration times are often leaving welded designs unoptimized and poorly understood by designers and engineers. The goal of this thesis is to gather knowledge about a plug-welded bracket commonly found welded to beams in bus chassis and body, aiding the design and dimensioning of such brackets. A factorial design study was performed using FEM analysis with the “Effective notch method,” revealing the effects of seven different design parameters on the stresses in the weld. A theory to analytically calculate the profile of the bracket is presented. The theory is taking the beam’s second moment of area and the bending moment into consideration, essentially tuning the bracket’s stiffness to the beam and the load situation. An improved bracket is presented, analyzed, and compared to the design study. The results show how stresses in the weld are affected by the stiffness of the bend in the bracket, the location of bolts (or other fastening technique) and the plug weld dimensions. When the weld is made to run longer along the beam, the importance of adjusting the stiffness in the bracket increases in order to balance the load throughout the whole weld. / Svetsade fästelement är ofta att föredra i ett busschassi över andra tekniker av kostnadsskäl och att en bred kompetens finns inom svets hos många underleverantörer. Men på grund av utmattning är ofta svetsade infästningar den svagaste punkten i ett chassi. Utvärdering av svetsar är kostsamt i tid och kompetens. Långa iterationstider i svetsutvärderingar lämnar ofta svetsen ooptimerad och undermåligt förstådd av designers och ingenjörer. Målet med detta arbete är att samla kunskap runt ett pluggsvetsat fästelement som ofta förekommer i busschassier och busskarosser, för att underlätta design och dimensionering av dessa fästelement. En faktoriell designstudie var utförd med FEM analys enligt ”Effective Notch” metoden för att undersöka effekten från sju olika designparametrar på spänningskoncentrationerna i svetsen. En teori för att analytiskt beräkna profilen på fästelementet presenteras. Teorin tar balkens böjtröghetsmoment och böjmoment med i beräkningen för att optimera fästelementets styvhet till balken och lastsituationen. Ett förbättrat fästelement är presenterat, analyserat och jämförd med designstudien. Resultaten visar hur spänningarna i svetsen påverkas av styvheten i den böjda delen av fästelementet, placeringen av bultar (eller annan infästningsteknik) och dimensioner på ”svetspluggen.” När svetsen är går längre längs med balken blir fästelementets styvhet allt viktigare för att kunna balansera lasten igenom det svetsade området.

No description available.

The behaviour of spot welded joints is to be studied in shear mode by using the end notched flexure (ENF) specimen. The specimen counts on several spot welds spaced with the same gap along the entire length except for the notch. Different configurations of the test specimen spacing gap and spot diameter are to be preliminary designed in software PTC-CREO 2.0 in order to obtain accurate results. The results obtained from the software are to be compared with the experimental analysis performed by means of a servo-hydraulic testing machine. Both the virtual and the experimental results will be used to extract the stress-shear displacement relation around the first spot weld which is in contact with the notch, that is, the corresponding cohesive law. In addition, a comparison between adhesive and spot weld behaviours will be carried out by means of analytical equations in order to prove an existing equalisation between each other. Despite assuming several sources of error and after facing some problems related to the experimental work, accurate convergences between experimental and theoretical results were not accomplished in any of the three tests performed in lab. Due to the plastic deformation of the specimens in lab, the cohesive law was possible to be extracted only from the theoretical analysis, but not from the experimental one. In its place, the load-shear displacement was extracted. Some alternatives to solve this issue and to improve the performance of the tests are given at the end of this paper.

The objectives of this thesis comprise of overcoming the challenges in designing lightweight welded structures such as material selection, choice of fatigue design methods, and increased performance by using improvement techniques. Material selection of welded joints is dependent on the filler and base material strengths. Partially and fully penetrated cruciform and butt welded joints were designed in under-matching, matching, and over-matching filler materials. Base material steel grades were S600MC, S700MC, and S960. Current design rules are developed for welds in steel up to yield strength of 700MPa. Therefore, design rules in Eurocode3, AWS d1.1, and BSK 07 were verified and recommendations for developing design rules for designing welded joints in S960 were concluded. Numerical methodology for estimating static strength of welded joints by simulating heat affected zone was also developed. Another objective of the thesis work was to overcome the challenges in selection of fatigue design methods. The available design curves in standards are developed for uniaxial stress states, however, in real life the welds in mechanical structures are subjected to complex multiaxial stress states. Furthermore; weld toe failures are frequently investigated, weld root failures are seldom investigated. Therefore, in this work the multiaxial fatigue strength of welded joints failing at the weld root was assessed using experiments and various nominal and local stress based approaches. Butt welded joints with different weld seam inclinations with respect to applied uniaxial loading were designed to assess the root fatigue strength in higher multiaxial stress ratio regime. The fatigue strength of multi-pass tube-to-plate welded joints subjected to internal pressure only and combined internal pressure and torsion in and 90° out of phase loading was also investigated. Test data generated in this thesis was evaluated together with the test data collected from literature. Last objective of the thesis included investigation of the increased performance in fatigue strength by post weld treatment methods such as HFMI. The behavior of residual stresses induced due to HFMI treatment during fatigue loading is studied. Numerical residual stress estimations and residual stress relaxation models are developed and the effect of various HFMI treatment process parameters and steel grade on the induced residual stress state is investigated. Specimens studied were non load carrying longitudinal attachments and simple plates. Residual stresses in both test specimens were measured using X-ray diffraction technique. / <p>QC 20170206</p>

Thesis. 1976. M.S.--Massachusetts Institute of Technology. Dept. of Ocean Engineering. / Microfiche copy available in Archives and Engineering. / Includes bibliographical references. / M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Chang-Shung Chai. / Ph.D.

<p>U disertaciji je predstavljen postupak spajanja topljenjem nezavarljive legure aluminijuma EW AN 7049A u stanju T 652 postupkom zavarivanja trenjem -<br />miješanjem. Korišćenjem pet različitih tipova alata za zavarivanje, varijacijom brzine zavarivanja i broja obrtaja, zavareni spoj dostiže efikasnost od 70% do 80 % zatezne čvrstoće osnovnog materijala. Karakterizacija zavarenog spoja izvedena je standardnim metodama za eksperimentalno određivanje energije udara i udarne žilavosti na Charpy epruvetama, a ispitivanja mehanike loma zavarenih spojeva savijanjem u tri tačke SENB epruveta (ASTM E 1820), poprečno na pravac zavarivanja u zoni najviše pogođenom termomehaničkim uticajem alata.<br />Kriterijumima elasto-plastične mehanike loma ( CTOD-otvaranje vrha prsline i J-<br />integrala) utvrđena je veća otpornost prema širenju prsline u različitim mjestima zavarenog spoja (centru, strani napredovanja i na povratnoj strani metala šava) nego u osnovnom materijalu.</p> / <p>The thesis presents the procedure of joining by melting of unweldable alloy of Al EW AN 7049A of status T 652 by the procedure of friction-stir welding. By using five various types of tools for welding, variation of welding velocity and number of revolutions, the welded joint shall reach the efficiency of 70% -80% of tensile strenght of basic material. Characterization of welded joint was made by standard methods for experimental determination of impact energy and impact toughness on Charpy tubes, and breakage mechanical properties by bending in three points of SENB tubes (ASTM E 1820), transversely to direction of welding in the zone of highly affected thermo mechanical impact of tool. Criteria of elastoplastic mechanics of the fracture (CTOD crack opening displacement and J-integral) determined bigger resistivity to cracking at various locations of welded joint (center, side of advancing and on retreating side of the seam metal) than on the basic material.</p>

Laser ultrasonics can be used to nondestructively evaluate structures to determine the existence and location of surface and interior flaws. The goal of this research was to determine if laser ultrasonic techniques can be applied to the inspection of aluminum plate. titanium tubes, and large welded plate structures. The research was carried out with a Q-switched pulsed ruby laser emitting light of 694 nm wavelength. Ultrasonic waves were experimentally generated and recorded in the aluminum plate and the titanium tube. A comprehensive literature study was completed to determine if the technique can be applied to welded structures. For the two experimental cases, the ultrasonic waves were received by a piezoelectric pinducer which was located on the opposite side of the plate. and on the outside of the tube. A digital oscilloscope captured the signals from the pinducer. The signals were then analyzed to determine echo spacing and frequency content. The physical characteristics of the laser pulse such as the energy and full-width-half-height and amplitudes were measured via a photodiode system and a calorimeter. The aluminum plate confirmed that the system was functioning properly, as the ultrasonic echoes that were generated matched the expected results from previous experimentation. The titanium tube data turned out to be difficult to interpret due to the complex geometry and mode conversion. The welding research showed that ultrasound can be used to identify many types of flaws in a welded joint. Currently, few researchers have applied the laser based ultrasound to flaw detection in finished welds, although several have looked at using the laser ultrasound as an input to a control system for a weld in progress. The literature research uncovered the need for further studies on the application of laser based ultrasound to flaw detection in completed welds. / Graduation date: 1997