Advanced Materials Research Vols. 891-892

Abstract: The development and application of friction stir welding (FSW) technology in steel structures in the shipbuilding industry provide an effective tool of achieving superior joint integrity especially where reliability and damage tolerance are of major concerns. Since the shipbuilding components are inevitably subjected to dynamic or cyclic stresses in services, the fatigue properties of the friction stir welded joints must be properly evaluated to ensure the safety and longevity. This research intends to fulfill a clear knowledge gap that exists nowadays and, as such, it is dedicated to the study of welded steel shipbuilding joints in GL-A36 steel, with 4 mm thick. The fatigue resistance of base material and four plates in as-welded condition (using several different parameters, tools and pre-welding conditions) were investigated. The joints culminate globally with defect-free welds, from which tensile, microhardness, and fatigue analyses were performed. The fatigue tests were carried out with a constant amplitude loading, a stress ratio of R=0.1 and frequency between 100 and 120 Hz. The experimental results show the quality of the welding process applied to steel GL-A36 which is reflected in the mechanical properties of joints tested.

Abstract: Friction StirChannelling (FSC) is an innovative solid-state manufacturing technology able toproduce, in a single step, continuous integral channels in monolithic platesthat can be used to produce conformal cooling systems as well as heatexchanging devices. FSC is based on the same principles of Friction StirWelding (FSW) process and the features of the channels produced by FSC can becontrolled by the processing parameters and tool geometry. Bending fatiguetests have been performed using friction stir channelling specimens of 5083‑H111aluminium alloy with different channel geometries. Fatigue tests have beencarried out at 120°C and 200°C in a servo-hydraulic testing machine coupledwith a furnace, in order to understand the role of channel geometry on thefatigue behaviour at elevated temperature. Results were compared with thoseobtained at room temperature. The specimens were tested until fracture or up to 3x10⁶ cycles. For the channel geometries tested, the fatiguestrength of friction stir channelling specimens is dependent on the testingtemperature, decreasing with the temperature increased. At all temperaturesstudied, for both channel geometries, fatigue-crack always initiated into theinterior of the specimen, namely on the boundary between the channel nugget andthe thermo‑mechanically affected zone. It was observed that crack initiationhas been occurred earlier at elevated temperature than at room temperature.

Abstract: Microstructure and pre-existing surface flaws in smooth notch geometries significantly affect the fatigue life of welded joints. Traditionally, a welded joint is assumed to incorporate crack-like defects and the crack propagation dominates the total fatigue life. For a smooth weld notch geometry, the macro crack initiation period becomes more significant, and this difference cannot be modelled with the existing stress or fracture mechanics ‑based approaches. In this paper, a microstructure and strain ‑based fatigue life approach is presented. In the approach, the fatigue damage process is modelled as a repeated crack initiation process within a material volume related to the microstructure. The novelty of the developed approach is that the size of the damage zone is defined from the grain size statistics without using fracture mechanics. The approach is able to consider the changes in the stress gradient, stress triaxiality and plasticity during the fatigue crack initiation and growth. The developed approach has been validated with experiments on submerged-arc and laser-hybrid welded joints. The predicted fatigue life, crack growth path and rate showed good agreement with the experiments. For a welded joint with smooth and favourable notch shape, the short crack growth, i.e. macro crack initiation period is dominant and it has a significant influence on the fatigue life.

Abstract: The AISI/SAE 4340 steel and 300M have been used in severe conditions where high strength and toughness are necessary. Since there is a particular interest in the aerospace industry, they are being used in aircraft landing gear, rocket engines vessels and satellite launch vehicles. Therefore, the understanding of the weldability of these steels has been the subject of special interest. This study aims to evaluate and compare the microstructure and mechanical strength by tensile and fatigue in steel sheets of 4340 and 300M steels after laser welding and tempering heat treatments. The results showed that the welds had high hardness and small heat affected zones. Nevertheless, there was not a decrease in the uniaxial tensile properties for the 300M and 4340 steels. The decrease in fatigue life due to the welding process, compared to the unwelded condition, was small, indicating that the parameters welding were adequate and that this process is viable.

Abstract: This paper examines the link between the design codes, good design and the prevention of field failures. There are many design codes for the fatigue design of welds. They contain many differences influenced by the history of particular industries. Engineering may not be well served by these codes since on one hand they are normally conservative, thus creating excessively heavy structures. On the other hand the design codes often do not include the most important factors in the life expectancy of a weld namely the weld procedure, the quality of the weld and environmental effects. The paper concludes that a thorough review and unification of the industry design codes is overdue. The issue of manufacture of welds has to be put at the centre of the consideration of fatigue design.

Abstract: Additive Manufacturing (AM) technologies are considered revolutionary because they could fundamentally change the way products are designed. Selective Laser Melting (SLM) is a metal based AM process with significant and growing potential for the manufacture of aerospace components. Traditionally a material needs to be listed in the Metallic Materials Properties Development and Standardization (MMPDS) handbook if it is to be considered certified. However, this requires a considerable amount of test data to be generated on the materials mechanical properties. Therefore, the MMPDS certification process does not lend itself easily to the certification of AM components as the final component can have similar mechanical properties to wrought alloys combined with the defects associated with traditional casting and welding technologies. These defects can substantially decrease the fatigue life of a fabricated component. The primary purpose of this investigation was to study the fatigue behaviour of as-built Ti-6Al-4V (Ti64) samples. Fatigue tests were performed on the Ti-6Al-4V specimens built using SLM with a variety of layer thicknesses and build (vertical or horizontal) directions. Fractography revealed the presence of a range of manufacturing defects located at or near the surface of the specimens. The experimental results indicated that Lack-of-Fusion (LOF) defects were primarily responsible for fatigue crack initiation. The reduction in fatigue life appeared to be affected by the location, size and shape of the LOF defect.

Abstract: The results of linear elastic finite element analyses of stress concentration factor (SCF) and through-thickness stress distribution for 80 weld toe T-butt plate geometries are presented in parametric form for tension (membrane) loading. The closed-form solutions, which describe the stress state of the two-dimensional plane stress models studied in terms of weldment angle, weld toe radius, weld attachment width and plate thickness, are accurate and wide ranging. The SCF is presented in full parametric form and also as a simple reduced expression, quantifying the degree of error from raw data in each case. An expression for the stress distribution through the potential Mode I crack plane of the uncracked geometries is also presented in full parametric form. It is anticipated that these HBC equations will be particularly useful in the calculation of weight functions for stress intensity factors (SIFs) of fatigue cracks emanating from weld toes in T-butt welded joints, especially in the presence of known residual stresses, or those resulting from peening.

Abstract: Most of the distresses in flexible pavement are due to cracking and rutting. Geogrids distribute the street and loads to adequate area and cause augmentation bearing capacity. This project surveys the function of carbon and Glass fiber Geogrids on Delay of Crack propagation in flexible pavement and placement configuration in pavement structure. Four point bending loading applied to beams with dimension 50.8*63.5*381 mm and 6.5 mm tolerance (AASHTO TP8-96). According to the received specimen from cutting slabs of case study toward chart fatigue life, for each condition reinforced and unreinforced beams 3 sample in 4-strain level were experimented. Simulatedrepeated loading with a frequency 10 Hz simulate high-speed traffic was applied to beam fatigue specimen. Based on definition (AASHTO T321-07) assumed achieving to 50% first bending stiffness for end of the fatigue life indicate a significant reduction in the rate of crack propagation in reinforced samples with carbon geogrids compared to unreinforced ones, augmentation service life is up to 3.5fold and placing the geogrid at a one-third depth of overlay thickness from the bottom provide the maximum service life.

Abstract: The purpose of this study was to evaluate the influence of heat treatment on the microstructural change and low cycle fatigue (LCF) resistance of an electron beam welded (EBWed) dissimilar joint between Ti-6Al-4V and Ti17 alloys. The aging with solution (STA) had a more significant effect on the microstructure and hardness than aging, compared to the as-welded joint. The post-welded joints in both aging and STA conditions were basically cyclic stable at low strain amplitudes up to 0.6%, while cyclic softening occurred at higher strain amplitudes. The fatigue life in the aging condition was slightly longer than that in the STA condition at the lower strain amplitudes. Fatigue crack initiation occurred from the specimen surface or near-surface defect, and fatigue crack propagation was characterized mainly by the fatigue striations coupled with secondary cracks in both aging and STA conditions.

Abstract: Rolling contact fatigue and wear of rails are inevitable problems for railway system due to wheel and rail contact. Increased rail wear and increased fatigue damage such as shelling, head check, etc. require more frequent rail exchanges and more maintenance cost. The fatigue crack growth and wear forming on the contact surface are affected by a variety of parameters, such as vertical and traction load, friction coefficient on the surface. Also, wear and crack growth are not independent, but interact on each other. Surface cracks are removed by wear, which can be beneficial for rail, however too much wear shortens the life of rail. Therfore, it is important to understand contact fatigue and wear mechanism in rail steels according to a variety of parameters. In this study, we have investigated fatigue and wear characteriscs of rail steel using twin disc testing. Also the comparative wear behavior of KS60 and UIC 60 rail steel under dry rolling-sliding contact was performed.