V. Infante

Fatigue performance of tungsten inert gas (TIG) and plasma welds in thin sections

Abstract The greatest potential for the use of TIG and plasma welding is in the joining of thin sections, less than 10 mm thickness. This may introduce an additional benefit from the fatigue viewpoint, since fatigue strength is expected to increase with a decrease in plate thickness. Superior fatigue performance was confirmed for TIG and plasma transverse butt, cruciform and non-load carrying fillet welds of a carbon-manganese steel, all failing from the weld toe, justifying a one class increase in Eurocode 3. Weld details, which failed by fatigue cracking in the weld throat, showed no influence of welding process. Extensive measurements of the weld toe geometries confirmed that TIG and plasma welds had more favourable profiles than MMA welds, with lower weld toe angles and larger weld toe radii. Fracture mechanics modelling confirmed that the differences in weld toe geometry were consistent with the differences in fatigue life actually observed. Weld toe radius appeared to be more significant than weld toe angle.

Optimization of FS Welding Parameters for Improving Mechanical Behavior of AA2024-T351 Joints Based on Taguchi Method

In the present study, the design of an experiment technique, the Taguchi method, has been used to optimize the friction stir welding (FSW) parameters for improving mechanical behavior of AA2024-T351 joints. The parameters considered were vertical downward forging force, tool travel speed, and probe length. An orthogonal array of L9 (34) was used; ANOVA analyses were carried out to identify the significant factors affecting tensile strength (Global Efficiency to Tensile Strength—GETS), bending strength (Global Efficiency to Bending—GEB), and hardness field. The percentage contribution of each parameter was also determined. As a result of the Taguchi analysis in this study, the probe length is the most significant parameter on GETS, and the tool travel speed is the most important parameter affecting both the GEB and the hardness field. An algebraic model for predicting the best mechanical performance, namely fatigue resistance, was developed and the optimal FSW combination was determined using this model. The results obtained were validated by conducting confirmation tests, the results of which verify the adequacy and effectiveness of this approach.

Aluminium Friction-stir Weld-bonded Joints

A push towards more energy-efficient transport solutions has led to an increasing lightweight trend in structural design, requiring new materials, manufacturing, and assembly processes. The development of solid-state welding techniques, such as friction-stir welding (FSW), and the continuous improvement of adhesive technology, has created opportunities for new structural design concepts. Although FSW is capable of producing sound defect-free welds with high tensile strength efficiency in butt joint configuration, in the case of lap joints, the formation of a “hook”-like defect results in worse properties than base material. The combination of adhesive bonding (AB) with FSW aims to overcome this issue and create a hybrid joining technique. This work aims to develop a hybrid technique combining FSW and AB aggregating static strength testing and numerical modelling efforts. AB joints showed a 60% higher strength than FSW lap joints, but when combining FSW with adhesive, the hybrid joint managed to match the adhesive joints strength. Finite elements method (FEM) models developed for both AB and FSW lap joint showed some level of agreement, but when attempting to combine both models to discretize the hybrid joints the developed model failed to mimic the more complex failure mode.

A failure analysis study of cast steel railway couplings used for coal transportation

Abstract This paper presents the results obtained in a failure analysis study carried out in cast steel railway couplings of the trains of one coal transportation line. A significant number of failures have occurred in these components which led to great losses and disruption of services. The type of failure was mainly fatigue cracking. This paper presents the results from detailed microstructural and fractographic analysis. Strain gauge data were analyzed and from this data fatigue cycles were derived. Cumulative frequency diagrams of stress range and mean stress were obtained and fatigue damage values were calculated. Damage factors were obtained in several zones of the coupling and for the different stages of the journey and loading conditions. Results of a 3D stress analysis obtained with a FE program are presented. The values of the principal and equivalent stresses were obtained in the critical areas of the coupling. Stress concentration factors were calculated from the computed stress distribution and for the critical areas of the coupling.

Fatigue Behaviour of Friction Stir Welded Steel Joints

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.

A failure analysis study of wet liners in maritime diesel engines

Abstract This paper presents the results of a study regarding the failure mechanisms detected in alloyed cast iron liners of marine engines. The liners have shown cracks and extensive corrosion after about 1200 service hours, located in the adjustment flange between the liners in the cylinder block. This study includes the material characterisation using chemical, microstructural, hardness and fractographic analysis. The metallurgical results have shown that the material was acceptable concerning the composition and microstructure. The paper also presents the results obtained for the distribution of stresses and temperatures in the critical areas of the cylinder liner. A 2D finite element code was used for this study, using axisymmetric elements with reduced integration. The results have shown that the thermal stresses are significantly more important than the mechanical stresses, the latter being due exclusively to the effect of the combustion gases in the cylinder. For the two geometries which were analysed, the peak stresses were slightly above 100 MPa in tension. This value of stress is high for the cast iron of the liners, and leads to a great potential risk of initiation and propagation of the tiny surface corrosion defects found in these zones. The detailed geometric changes introduced in the second design of the liner gave a little improvement in the stress distribution in the critical areas, although this may not be enough to avoid cracking conditions in the critical areas.

Mechanical Characterization of Friction Stir Channels under Internal Pressure and In-Plane Bending

Friction Stir Channelling (FSC) is a simple and innovative technique of manufacturing integral and continuous channels (also referred as conformal channels) in monolithic plates in a single step. This paper is focused on the mechanical behaviour of integral and continuous FS channels produced in a monolithic plate of the aluminium alloy AA7178‑T6 with 13mm of thickness, typically used in structural aircraft applications. Internal pressure tests were conducted on specimens with a longitudinal friction stir channel. In-plane bending tests were carried out in specimens with longitudinal and transversal channels; different conditions were analysed for each FSC parameters set. Results were analysed and compared concerning the FS channels microstructure and base material mechanical properties. Bending tests results show that tool rotation speed has more influence in the FSC specimens bending strength than tool travel speed and internal pressure tests show that increase tool rotation speed increases the minimum pressure that leaking points arise.

Optimization of fibers orientation in a composite specimen

ABSTRACT This article is devoted to the study of the optimal design of fibers orientation in a composite specimen with the objective to minimize the displacement. The composite specimen considered is within the scope of aerospace and mechanical applications. The objective function associated with the composite design is computed based on a static analysis of a finite element solid model, which allows one to define (or control) the fibers orientation. The recent global and local optimization using direct search methods (GLODS) is used for the optimization process. To validate and compare the numerical and optimized results, the specimens were manufactured and tested experimentally. The orientation of the layers that minimize the maximum displacement is achieved through the computational interaction of the optimization program, GLODS, in loop with the finite element program, ANSYS. It is shown that the optimized lamination schemes found by GLODS minimized about 60% of the displacement compared to the nonoptimized specimens.

Using a standard specimen for crack propagation under plain strain conditions

Purpose – Stress state has a major influence on different phenomena, namely those involving diffusion and plastic deformation (like crack closure and high‐temperature fatigue crack growth, void formation or ductile fracture). The isolation of plane stress and plane strain states is crucial in fundamental studies of material behavior. The isolation of plane stress state is achieved with thin specimens, whilst the isolation of plane strain state is usually done increasing the thickness or introducing lateral grooves. The purpose of this paper is to propose a specimen geometry able to isolate the plane strain state, based on the standard M(T) geometry.Design/methodology/approach – A numerical study was carried out aiming at obtaining a stress triaxiality parameter, h, as a function of different geometrical features of the specimen, such as the notch radius, notch depth and specimen thickness.Findings – Results show that a pure plane strain state is achievable (i.e. 97 percent of specimen thickness has h>0.97...

Role of Friction Stir Channel Geometry on the Fatigue Behaviour of AA5083-H111 at 120°C and 200°C

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.