Papers by Keyword: Fatigue

Abstract: This paper presents the results of microstructural and mechanical characterization of Friction Stir Welding joints of two aluminum-based particles reinforced composites. The composites were FSW in the extruded and T6 heat treated condition. No post-weld heat treatment was carried out on the FSW joints. Hardness, tensile, low-cycle fatigue and impact tests were carried out. Microstructural and fractographic characterization were performed both on the base and FSW material, in order to investigate the effect of the solid-state welding process on the reinforcement particles and aluminum matrix. The FSW produced high quality joints with good microstructural characteristics: the welded zone displayed a refinement of the Al matrix grain size and reinforcement particles, and a better particle distribution. The FSW specimens showed high efficiency, both in the tensile, impact and fatigue tests.

Abstract: Chemicals play an important role in the enhancement of oil and gas production and processing. They control corrosion, prevent organic and inorganic deposits, aid in phase separation and control microbial problems. Several factors can have significant impact on the safety, maintenance, operation, and service life of the chemical injection point. Failures encountered in the high pressure retrievable chemical injection points are presented. This paper also provides guidelines for materials selection and design of chemical injection points in upstream facilities.

Abstract: Grain boundaries hinder twin boundary motion in magnetic shape-memory alloys and suppress magnetic-field-induced deformation in randomly textured polycrystalline material. The quest for high-quality single crystals and the associated costs are a major barrier for the commercialization of magnetic shape-memory alloys. Adding porosity to polycrystalline magnetic-shape memory alloys presents solutions for (i) the elimination of grain boundaries via the separation of neighboring grains by pores, and (ii) the reduction of production cost via replacing the directional solidification crystal growth process by conventional casting. Ni-Mn-Ga foams were produced with varying pore architecture and pore fractions. Thermo-magnetic training procedures were applied to improve magnetic-field-induced strain. The cyclic strain was measured in-situ while the sample was heated and cooled through the martensitic transformation. The magnetic field-induced strain amounts to several percent in the martensite phase, decreases continuously during the transformation upon heating, and vanishes in the austenite phase. Upon cooling, cyclic strain appears below the martensite start temperature and reaches a value larger than the initial strain in the martensite phase, thereby confirming a training effect. For Ni-Mn-Ga single crystals, external constraints imposed by gripping the crystal limit lifetime and/or magnetic-field-induced deformation. These constraints are relaxed for foams.

Abstract: The martensitic transformation in austenitic stainless steels can be induced by plastic deformation at room temperature. The benefit of this transformation is commonly used to strengthen stainless steels grades, i.e. their yield and tensile resistance can be adjusted according to the requirement by cold rolling. In this paper, the martensitic transformation was induced by means of torsion deformation. Several torsion angles were selected to achieve different percentages of martensite at the surface of the specimens and then the effect on the fatigue life of the steel was studied. Fatigue testing results showed dissimilar behavior depending on the stress ratio (R) applied during the test. As a conclusion, the presence of martensite in the surface increases the fatigue life for high stress ratios (R=0.8), while at low R values martensitic transformation has no positive effect.

Abstract: The 3D finite element simulations are conducted for the cold working of a fastener hole in a low carbon steel plate. The simulation models the actual cold working process where the hole edge is chamfered with die-press. The agreement of finite element method and experimental results is good enough. The residual stresses are analyzed under the different die-press parameters and contact conditions i.e. die-pressing depth, friction factor, the die taper. The main results in this work are as follows: With increasing in chamfer depth the maximum compressive residual stress shows an increasing within the chamfer range of C1.5; the compressive residual stress is decrease with increasing friction; with increase in die taper the maximum compressive residual stress shows the slow increase. The efficiency for enhancing the load-carrying capacity of structural components with cylindrical holes subjected to bending load has been proved by means of FE simulation. The study shows that the simulations of cold working are necessary for if predicted residual stresses are to be used to assess fatigue life and for design die-pressing tool, improvement of parameters of the process of DP working.

Abstract: It is well known that shot peening has a marked benefit on fatigue life for the majority of applications. This effect is attributed mainly due to the compressive residual stress state at the component’s surface due to shot peening. The present paper evaluates the ability of several fatigue life prediction models, commonly used for general analyses, to predict the behaviour of components with compressive residual stress due to shot peening. Advanced elastic-plastic finite element analyses were carried out in order to obtain stress, strain, strain energy and fracture mechanics parameters for cracks within a compressive residual stress field. With these results several total fatigue life prediction models (including critical distance methods) and fracture mechanics based models were applied in order to predict fatigue life. Fatigue life predictions were compared with several experimental fatigue tests carried out on specimens, representative of a critical region of a compressor disc in a gas turbine aero engine. The results obtained showed that total fatigue life methods, even if combined with critical distance methods, give conservative results when shot peening is considered. Fatigue life was successfully predicted using the method proposed by Cameron and Smith, by adding initiation life to crack propagation life. This last method was also successfully applied for the prediction of non-propagating cracks that were observed during the experimental tests.

Abstract: Strength mismatch effect across weld interfaces, generated by welding weak and strong steels, influences fatigue and fracture properties of a welded bimetallic composite. Advancing fatigue crack tip in weak parent steel is shielded from the remote load when it reaches near the interface of ultra strong weld steel. Entry of crack tip plasticity into weld steel induces load transfer towards weld which dips crack growth rates thereby enhancing the fatigue life of the composite. A computational model for fatigue life prediction of strength mismatched welded composite under K dominant conditions is validated by experimental work in this paper. Notched bimetallic compact tension specimens, prepared by electron beam welding of weak alloy and strong maraging steels, are subjected to fatigue testing in high cycle regime.

Abstract: In this paper a review of the state of the art on the study of the fatigue and the contact/rolling contact fatigue (RCF) resistance of thin hard-coated components is provided. Physical and chemical vapor deposition (PVD and CVD) methods are used to deposit such films. A fair number of references reports experimental data highlighting the improvements achieved with coating deposition on both steels and light alloys. Numerical modelling has also been devoted to shedding light on the behaviour of coated components and reliable previsional procedures have been arranged to foresee the number of cycles until fatigue damage initiation and failure.

Abstract: In this paper, the influence of deep cryogenic treatment (DCT) on mechanical properties of two commercial steels is analysed. Hardened AISI 302 stainless steel and 18NiCrMo5 carburized steel specimens were subjected to DCT after standard treatments. For both materials, the fatigue behaviour is a key property considering their usual applications requirements. Surface hardness, tensile properties and axial fatigue resistance of both materials were measured and compared with and without DCT. From the analysis of the experimental results and from their interpretation in the light of the previous literature, some useful indications are obtained about the DCT potential fallout on design and construction of structural components.

Abstract: The fatigue crack initiation and growth in a high loaded bolted bar connection made of high strength steel S1100Q is presented. The material parameters for the fatigue crack initiation f’, f’, b and c are determined using low cycle fatigue test according to ASTM E 606 standard. The fracture mechanics parameters C and m are determined according to ASTM E 647 standard. Based on low cycle fatigue parameters the computational analysis is performed to determine the number of stress cycles required for the fatigue crack initiation. The remain service life up to the final failure is than determined using the known parameters C and m and calculated stress intensity factor, where 3D numerical analysis is performed. The bolted bars are also experimentally tested. Comparison of computational and experimental results shows a reasonable agreement.