Showing papers on "Fatigue limit published in 2016"

Mechanical behavior of additive manufactured, powder-bed laser-fused materials

Abstract: Mechanical behavior of four metallic alloys fabricated with layered, laser-heated methods of additive manufacturing (AM) was compared to that of similar alloys produced with conventional methods (wrought and machined). AM materials were produced by a leading commercial service provider, as opposed to incorporating material specimens produced by unique or specially-adapted equipment. The elastic moduli were measured in flexure, stress–strain characteristics were measured in tensile deformation, and fatigue strengths were measured in fully reversed bending. The effects of fabrication orientation, surface polishing, and hot isostatic pressing upon mechanical behavior were studied. The fatigue strengths exhibited by SLM AlSi10Mg and DMLS Ti6Al4V in the as-fabricated condition proved to be significantly inferior to that of conventional material. These lower fatigue strengths are a consequence of multiple fatigue cracks initiating at surface defects, internal voids and microcracks, and growing simultaneously during cyclic loading. Measured fatigue strengths of DMLS 316L and 17-4PH approached those of corresponding wrought materials when subjected to principal stresses aligned with the build planes. When cyclic stresses were applied across the build planes of the DMLS stainless steels, fatigue fractures often developed prematurely by separation of material. Post-processing the DMLS Ti6Al4V and SS316L with hot isostatic pressure elevated the fatigue strength significantly. Measurements of surface roughness with an optical profilometer, examinations of the material microstructures, and fractography contribute to an understanding of the mechanical behavior of the additive materials.

The influence of cell morphology on the compressive fatigue behavior of Ti-6Al-4V meshes fabricated by electron beam melting.

Abstract: Additive manufacturing technique is a promising approach for fabricating cellular bone substitutes such as trabecular and cortical bones because of the ability to adjust process parameters to fabricate different shapes and inner structures. Considering the long term safe application in human body, the metallic cellular implants are expected to exhibit superior fatigue property. The objective of the study was to study the influence of cell shape on the compressive fatigue behavior of Ti-6Al-4V mesh arrays fabricated by electron beam melting. The results indicated that the underlying fatigue mechanism for the three kinds of meshes (cubic, G7 and rhombic dodecahedron) is the interaction of cyclic ratcheting and fatigue crack growth on the struts, which is closely related to cumulative effect of buckling and bending deformation of the strut. By increasing the buckling deformation on the struts through cell shape design, the cyclic ratcheting rate of the meshes during cyclic deformation was decreased and accordingly, the compressive fatigue strength was increased. With increasing bending deformation of struts, fatigue crack growth in struts contributed more to the fatigue damage of meshes. Rough surface and pores contained in the struts significantly deteriorated the compressive fatigue strength of the struts. By optimizing the buckling and bending deformation through cell shape design, Ti-6Al-4V alloy cellular solids with high fatigue strength and low modulus can be fabricated by the EBM technique.

Fatigue strength of Co-Cr-Mo alloy clasps prepared by selective laser melting.

Abstract: We aimed to investigate the fatigue strength of Co-Cr-Mo clasps for removable partial dentures prepared by selective laser melting (SLM). The Co-Cr-Mo alloy specimens for tensile tests (dumbbell specimens) and fatigue tests (clasp specimens) were prepared by SLM with varying angles between the building and longitudinal directions (i.e., 0° (TL0, FL0), 45° (TL45, FL45), and 90° (TL90, FL90)). The clasp specimens were subjected to cyclic deformations of 0.25mm and 0.50mm for 10(6) cycles. The SLM specimens showed no obvious mechanical anisotropy in tensile tests and exhibited significantly higher yield strength and ultimate tensile strength than the cast specimens under all conditions. In contrast, a high degree of anisotropy in fatigue performance associated with the build orientation was found. For specimens under the 0.50mm deflection, FL90 exhibited significantly longer fatigue life (205,418 cycles) than the cast specimens (112,770 cycles). In contrast, the fatigue lives of FL0 (28,484 cycles) and FL45 (43,465 cycles) were significantly shorter. The surface roughnesses of FL0 and FL45 were considerably higher than those of the cast specimens, whereas there were no significant differences between FL90 and the cast specimens. Electron backscatter diffraction (EBSD) analysis indicated the grains of FL0 showed preferential close to orientation of the γ phase along the normal direction to the fracture surface. In contrast, the FL45 and FL90 grains showed no significant preferential orientation. Fatigue strength may therefore be affected by a number of factors, including surface roughness and crystal orientation. The SLM process is a promising candidate for preparing tough removable partial denture frameworks, as long as the appropriate build direction is adopted.

Static and fatigue mechanical behavior of three dental CAD/CAM ceramics.

Abstract: Purpose The aim of this study was to measure the mechanical properties and fatigue behavior of three contemporary used dental ceramics, zirconia Cercon® (ZC), lithium disilicate e.max® CAD (LD), and polymer-infiltrated ceramic Enamic® (PIC). Methods Flexural strength of each CAD/CAM ceramic was measured by three point bending (n=15) followed by Weibull analysis. Elastic modulus was calculated from the load–displacement curve. For cyclic fatigue loading, sinusoidal loading with a frequency of 8 Hz with minimum load 3 N were applied to these ceramics (n=24) using three point bending from 103 to 106 cycles. Fatigue limits of these ceramics were predicted with S–N fatigue diagram. Fracture toughness and Vickers hardness of the ceramics were measured respectively by single edge V-notch beam (SEVNB) and microindentation (Hv 0.2) methods. Chemical compositions of the materials׳ surfaces were analyzed by EDS, and microstructural analysis was conducted on the fracture surfaces by SEM. One-way ANOVA was performed and the level of significance was set at 0.05 to analyze the numerical results. Results The mean flexural strength of ZC, LD, and PIC was respectively 886.9, 356.7, and 135.8 MPa. However, the highest Weibull modulus belonged to PIC with 19.7 and the lowest was found in LD with 7.0. The fatigue limit of maximum load for one million cycles of ZC, LD, and PIC was estimated to be 500.1, 168.4, and 73.8 GPa. The mean fracture toughness of ZC, LD, and PIC was found to be respectively 6.6, 2.8, and 1.4 MPa m1/2, while the mean Vickers hardness was 1641.7, 676.7, and 261.7 Hv. Fracture surfaces followed fatigue loading appeared to be smoother than that after monotonic loading. Conclusions Mechanical properties of ZC were substantially superior to the two other tested ceramics, but the scattering of data was the least in PIC. The fatigue limit was found to be approximately half of the mean flexural strength for all tested ceramics.

Experimental and finite element studies of bolted, bonded and hybrid step lap joints of thick carbon fibre/epoxy panels used in aircraft structures

Abstract: A prior study conducted by the authors had investigated the behaviour of thin composite double lap joint repairs. This investigation was an extension to the previous study and focused on analysing the behaviour of thick step lap joint repairs which are more complicated in nature. Finite element analysis (FEA) was performed to verify the static and fatigue strength of bolted, bonded and hybrid joint configurations. Thick carbon fibre/epoxy laminates and aerospace grade film adhesive and fasteners were selected. Several configurations were considered in this case. These include varying the number of fasteners in the joint region as well as inclusion of bondline defects. Adhesive non-linear material properties, fastener contacts and frictional forces were all included in the three dimensional (3D) finite element (FE) models. The progressive failure process was simulated and the Multicontinuum Theory (MCT) was used to determine the stress states in all specimens considered. The strain energy release rate (SERR) as a function of crack length was also utilised to gain further understanding into the fatigue behaviour. Overall the FE models are able to accurately predict the bonded, bolted and hybrid joint strengths. SERR results suggest it is vital to place fasteners closer to the ends of the overlap to suppress the peak peeling stresses and to delay the effects of early crack initiation. This overall improves the longevity of a conventional bonded joint design.

Influence of moisture uptake on the static, cyclic and dynamic behaviour of unidirectional flax fibre-reinforced epoxy laminates

Abstract: This papers aims to characterize the influence of moisture uptake on the mechanical behaviour of unidirectional flax fibre-reinforced epoxy laminates. Monotonic and cyclic tensile tests and free vibration characterization are carried out. Results show that UD flax-epoxy composites, when exposed to hygrothermal conditioning at 70 °C and 85% RH, exhibit a diffusion kinetic which follows a one dimensional Fickian behaviour. The mass uptake at equilibrium is approximately 3.3% and the diffusion coefficient 6.5 × 10 −6 m 2 s −1 . Water vapour sorption is shown to induce a significant change in the shape of the tensile stress-strain curve, a decrease in the dynamic elastic modulus of about 20% and a 50% increase in the damping ratio. Contrary to all expectations, water saturation does not degrade the monotonic tensile strength of such a flax-epoxy composites and leads to an increase in the fatigue strength for a high number of cycles.

Microstructure and mechanical properties of Ti–15Zr alloy used as dental implant material

Abstract: Ti-Zr alloys have recently started to receive a considerable amount of attention as promising materials for dental applications. This work compares mechanical properties of a new Ti-15Zr alloy to those of commercially pure titanium Grade4 in two surface conditions - machined and modified by sand-blasting and etching (SLA). As a result of significantly smaller grain size in the initial condition (1-2µm), the strength of Ti-15Zr alloy was found to be 10-15% higher than that of Grade4 titanium without reduction in the tensile elongation or compromising the fracture toughness. The fatigue endurance limit of the alloy was increased by around 30% (560MPa vs. 435MPa and 500MPa vs. 380MPa for machined and SLA-treated surfaces, respectively). Additional implant fatigue tests showed enhanced fatigue performance of Ti-15Zr over Ti-Grade4.

Effect of mean stress and ratcheting strain on the low cycle fatigue behavior of a wrought 316LN stainless steel

Abstract: This work reports the low cycle fatigue behavior of a wrought 316LN stainless steel under different control modes at room temperature. Under symmetrical strain and stress cycling, the steel exhibits consistent loading-amplitude-dependent cyclic hardening/softening and fatigue life characteristics. Under asymmetrical stress cycling, the steel is significantly hardened due to mean stress, and the fatigue life at the same strain amplitude is significantly reduced due to ratcheting strain. With the increase of mean stress, though the ratcheting strain level is increased, the fatigue life is prolonged. The effect of mean-stress hardening and ratcheting strain on fatigue life is discussed in terms of strain amplitude and micro-crack initiation and propagation. The Smith-Walker-Topper (SWT) model and a newly proposed fatigue life model based on the Coffin-Manson equation were used to predict the fatigue life under mean stress, and the proposed model yields more robust predictions.

Microstructure and directional fatigue behavior of Inconel 718 produced by selective laser melting

Abstract: Recent research efforts in additive manufacturing have focused on developing parts made of Inconel 718 (IN 718), a nickel-based superalloy, which is an attractive material for aerospace and energy high-temperature applications. Here the selective laser melting (SLM) process is used to transform alloy powder into a solid IN 718 parts followed by optimal stress-relief and subsequent precipitation hardening treatment. Two main aspects were investigated. The IN 718 microstructure generated by the SLM process was characterized using metallographic techniques and found to be distinctly directional because it is a result of a layer-by-layer material build-up typical of the SLM process. The high cycle fatigue behavior of SLM IN 718 was determined using a novel test method designed to determine and quantify the directional material behavior, which is important information for part design and process optimization. The fatigue S-N data show that the direction parallel to the build direction is associated with the lowest fatigue strength. The role of the as-produced surface characteristics on fatigue crack initiation is discussed.

Experimental investigation of performance reliability of macro fiber composite for piezoelectric energy harvesting applications

Abstract: Macro fiber composite (MFC) has been extensively used in actuator/sensor/harvester applications. Fatigue due to cyclic high electric fields in actuator applications has been studied extensively. However, fatigue failure of MFC due to high stress or strains in energy harvesting applications has attracted little attention. The aim of the study is to obtain the upper limit of dynamic strain on MFC which can be used as failure limit in the design process of piezoelectric energy harvesters (PEHs). The examined PEH is comprised of a cantilever beam made of aluminum and a patch of MFC bonded at its root for power generation. Energy harvesting tests are conducted at various base accelerations around 30 Hz (near resonant frequency) and the voltage output and maximum strain on MFC are measured. Severe loss in the performance of the harvester is observed within half million cycles of testing at high strain amplitude. Hence several reliability tests for extended periods of time are carried out at various strain amplitudes. The harvesters are tested at resonant frequencies around 30 Hz and 135 Hz for over 20 million and 60 million cycles, respectively. Degradation in voltage output, change in natural frequency and formation of cracks are considered as failures. Based on the experimental results, an upper limit of 600 μϵ is proposed as the safe amplitude of strain for reliable performance of MFC. Tensile tests are also carried out on MFC patches to understand the formation of cracks and shift in resonant frequency at low strains. It is observed that cracks are formed in MFC at strains as low as 1000 μϵ. The observations from this work are also applicable to MFC bending actuators undergoing cyclic strains.

Influence of the build orientation on the fatigue strength of EOS maraging steel produced by additive metal machine

Abstract: This paper presents a research dealing with the dependence of the fatigue strength of maraging steel parts, manufactured by direct selective laser sintering, on the production build orientation. Three sets of specimens have been manufactured according to the ISO 1143 Standard (2010) by EOSINT M280 additive manufacturing machine, with the following heat and mechanical treatments, in agreement with the recommendations by the material manufacturer and current literature. The expected outcomes are the Fatigue Limit values of the material and the maximum number of cycles observed at different stress levels for three different build orientations (three different angles, 0°, 45° and 90°, between the build direction and the longitudinal axis of the samples). The results have been processed and compared by statistical methods in order to determine the fatigue curves in the finite life domain and the fatigue limits, along with their confidence bands and intervals, and to investigate the significance of the build orientation factor.

Experimental determination of the mode I delamination fracture and fatigue properties of thin 3D woven composites

Abstract: The mode I delamination fracture toughness and fatigue strength of thin-section three-dimensional (3D) woven composite materials is experimentally determined. The non-crimp 3D orthogonally woven carbon–epoxy composites were thin (2 mm) and consequently their through-thickness z-binder yarns were inclined at a very steep angle (about 70°) from the orthogonal direction. The steep z-binder angle has a marked effect on the delamination toughening and fatigue strengthening mechanisms. Experimental testing revealed that the fracture toughness and fatigue resistance increased progressively with the volume content of z-binders. However, the steep angle caused the z-binder yarns bridging the delamination crack to deform and fail in shear and through-thickness tension, rather than in-plane tension which usually occurs in thick 3D woven composites. Mode I pull-off tests on a single woven z-binder yarn embedded within the composite revealed that the crack bridging traction load, strain energy absorption and failure mechanism were strongly affected by the steep angle.

A new rapid thermographic method to assess the fatigue limit in GFRP composites

Abstract: Conventional procedures and methods used for obtaining the fatigue performance of materials represent a critical aspect of mechanical characterization because of time consuming tests with a high number of specimens. In the last few years, great efforts have been made to develop a number of methods aimed at reducing testing time and, subsequently, the cost of the experimental campaign. In the process, thermographic methods have shown to be a useful tool for the rapid evaluation of fatigue damage and fatigue limit. This work deals with a new procedure for the evaluation of fatigue limit and the monitoring of damage in GFRP material by means of thermography. Although damage mechanisms in composite materials are difficult to understand, the proposed procedure allows us to obtain a number of parameters providing information relating to the onset of failure phenomena. It is worth noting that the reported procedure provides results in good agreement with those attained by the standard test methods.

About the thermomechanical behaviour of a carbon fibre reinforced high-temperature thermoplastic composite

Abstract: Novel high-temperature thermoplastic polymers offer potential advantages over thermoset ones and represent a promising alternative in advanced composite applications. This work proposes to determine the thermal properties and resistance of unidirectional carbon fibre reinforced thermoplastic polyimide composite and to characterize the influence of temperature on its mechanical behaviour and properties, including tensile properties, interlaminar shear strength and failure mechanisms. Characterization is performed on composite tapes and on ring-shaped specimens manufactured using a heated-head thermoplastic filament winding process. Results show that the thermal degradation of such composite material occurs at temperature higher than 400 °C. The glass transition temperature is approximately 250 °C. The tensile strength is higher than 1200 MPa in the fibre direction on a temperature range varying from −50 to 250 °C. The material has also an outstanding fatigue strength under tension in this material direction. At 200 °C, the fatigue strength for a high number of cycles (2·10 6 ) is still approximately 50% of the static strength. One of the weak point of this composite laminate is the relatively low interlaminar shear strength at high temperature.