Average Fatigue Strength Research Articles

Fatigue Response of Additive-Manufactured 316L Stainless Steel

This study investigated the fatigue performance of 316L stainless steel fabricated via laser powder bed fusion (LPBF). Stress-controlled fatigue tests were performed at different stress amplitudes on vertically built samples using a frequency of 15 Hz and a stress ratio of 0.1. The stress amplitudes were varied to provide the cyclic response of the materials under a range of loading conditions. The average fatigue strength was determined to be 92.94 MPa, corresponding to a maximum stress of 185.87 MPa. The microstructures were observed through scanning electron microscopy (SEM) with the aid of electron backscattered diffraction (EBSD), and the average grain size of the as-built samples was determined to be 15.6 µm, with most grains having a <110> preferred crystallographic orientation. A higher kernel average misorientation value was measured on the deformed surfaces, revealing the increased misorientation of the grains. Defects were observed on the fractured surfaces acting as crack initiators while deflecting the crack propagation paths. The fatigue failure mode for the LPBF 316L samples was ductile, as illustrated by the numerous dimples on fracture surfaces and fatigue striations.

Fatigue Properties of Thixoformed Al-6wt%Si-2.5wt%Cu

One of the main functions of semisolid metal alloy forming processes, notably rheocasting and thixoforming, is the manufacture of parts, casings and frames for mechanical assemblies and systems. These parts not only must have the required minimum mechanical properties in terms of yield strength and elongation, but also must be able to withstand cyclic tensile and compressive forces. However, there is little fatigue strength data for the materials used for these parts. The present work seeks to fill this gap by determining the fatigue limit at 107 cycles of Al-6.0wt%Si-2.5wt%Cu alloy, or simply Al6Si2.5Cu, thixoformed in a pneumatic press at 585 °C (the temperature corresponding to 40 % solid fraction) with isothermal treatment times of 30 and 60 seconds. The parts were also subjected to T6 heat treatment, for which they were solution heat treated at a temperature of 520 °C for 4 hours followed by aging at 180 °C for 10 hours. For all the conditions tested, the microstructures were characterized to determine the grain size, appearance and shape of the silicon particles, in addition to the residual porosity. For the best conditions observed, 30 s holding time and T6 heat treatment, the grain size varied between 100 µm and 130 µm; the shape factor was around 0.60, indicating an excellent degree of roundness; and there was low residual porosity of around 0.3 %, resulting in a yield strength of up to 240 MPa with 4.5 % elongation. The average fatigue strength was estimated by the staircase method and was between 95 MPa and 98 MPa for 107 cycles.

Analisa Analisa Kelelahan Dan Kekerasan Pada Logam Axle shaft Dengan Pengelasan Gesek

The axle shaft is a component of the power transfer system which often experiences mechanical failure. The purpose of this study was to determine the degree of fatigue and hardness of metal shafts with variations in frictional load as information on friction welding recommendations on metal shafts. The research method used is the experimental method, which is a way to compare specimens with variations in friction welding loads of 2 kg, 4 kg, and 6 kg using the Rockwell method of hardness testing and fatigue testing using a rotary bending machine. From the research results it is recommended that friction welding on metal axle shafts can be carried out with a friction load of 6 kg because from the results of the fatigue test SPBG6 has the highest average fatigue strength compared to SPBG2 and SPBG4. This was reinforced by the results of the SPBG6 hardness test which showed the lowest hardness level for the parent metal at 80.44 HRA.

Fatigue behaviour of titanium scaffolds with hierarchical porosity produced by material extrusion additive manufacturing

Metallic porous structures (scaffolds) produced by additive manufacturing represent an important class of personalised implants used in load-bearing orthopaedic applications. As such, their fatigue performance must be excellent to prevent the need for revision surgery. This paper provides insight into the high-cycle fatigue behaviour of novel titanium scaffolds with hierarchical porosity and properties comparable to those of human bone that were produced by direct ink writing (DIW) and tested under cyclic loading typical for bone implants. Opposite to traditional expectations in the field of metal fatigue, scaffolds produced with an open intrastrand pore network (14.3%) endured nearly an order of magnitude more cycles than those with relatively compact strands (5.9%) and their normalized fatigue strength (62% of their yield strength in 106 cycles) was competitive with many current titanium scaffolds produced by other additive manufacturing technologies. Improved fatigue performance was related to fatigue crack growth shielding effects that reduced the crack growth rate, prolonged the crack path, and increased energy absorption. Consequently, these novel hierarchically porous titanium structures prepared by DIW can serve safely under cyclic loading conditions, and at the same time can provide multiple open porosity-related functionalities in advanced biomedicine and other industrial sectors.

Improvement of traction-traction fatigue properties of A100 steel plate-hole-structure by double shot peening

To reduce the stress concentration and improve fatigue resistance of A100 steel plate-hole-structure, double shot peening (DSP) of the primary shot peening with strong intensity iron shots and the secondary shot peening with weak intensity ceramic shots is carried on material surface and hole and the effect of surface integrity on fatigue properties is investigated. The results show that the surface of hole presents severe surface plastic deformation (SSPD) with increasing DSP intensity. A high intensity compressive residual stress (CRS) distributed in deep layer is generated. Meanwhile, the compressive residual stress is much more stable during the fatigue process, which is beneficial to improve fatigue properties by inhibiting fatigue crack initiation and early propagation. In addition, due to the reduced stress concentration at the edge of hole and higher compressive residual stress, the fatigue resistance of A100 steel plate-hole-structure is remarkably improved. The average fatigue strength of DSP-treated specimens with medium intensity primary shot peening was at least improved by 59.6%, and the fatigue notch sensitivity was reduced by 61%. However, it is also discovered the secondary shot peening cannot repair the rough surface and surface damage caused by excessive primary shot peening, which compromise the improvement of fatigue resistance.

Metastable high entropy alloys: An excellent defect tolerant material for additive manufacturing

Fatigue failure is ubiquitous in structural components. In additively manufactured (AM) components, the processing induced defects limit the fatigue performance. Further, the stochastic nature of defects in laser-powder bed fusion (L-PBF) make it difficult to predict the fatigue life in these components. In this work, we explored exceptional work hardening (WH) of a metastable Fe40Mn20Co20Cr15Si5 high entropy alloy (CS-HEA) to obtain high fatigue-resistance with L-PBF. Further, a fatigue life estimation tool based on the statistical size distribution of microstructural entities such as grains, pores and solid-state inclusions and, their mutual interaction was used to estimate the fatigue life of as-printed material. Upon deformation, CS-HEA exhibited γ (f.c.c.) → ε (h.c.p.) martensitic transformation and subsequent twinning in ε (h.c.p.) phase. Such deformation behavior resulted in sustained WH and is specifically beneficial in the vicinity of critical pores. A high normalized fatigue strength of 0.65 with respect to the yield strength was thus obtained in as-printed condition. Further, the model accurately predicted extended crack initiation life for CS-HEA. The current work therefore provides guidance towards developing defect-tolerant alloys for L-PBF and presents a tool for estimation of fatigue life of AM alloys with unconventional WH behavior.

A new evaluation of the fatigue design criteria of roller compacted concrete (RCC) pavements

Although mechanical fatigue loading can generally cause critical stresses in pavement systems, there have been few studies that specifically investigate fatigue behavior of roller-compacted concrete (RCC). Possible reasons for this lack include the complexity and time-consuming nature of fatigue tests as well as difficulties in producing RCC prismatic specimens because of stiff consistency in their fresh state. In this experimental study, to simulate actual field-compaction procedures, three RCC mixes of different performance categories were compacted in a plate mold by first applying a vibratory plate compactor, followed by a small-scale vibratory hand roller. Fatigue behavior of RCC mixtures was investigated by applying fatigue loads at five different stress ratios on beam specimens taken from the compacted plates. The average fatigue strength of RCC corresponding to 2 million loading cycles was found to be about 62.5% of the ultimate static strength, showing that RCC performs better than conventional concrete under fatigue. Using a Weibull distribution, the failure probabilities of these mixtures were also determined and expressed as an S-N-P curve. The relationships between RCC fatigue life and mixture design parameters showed that the degree of compaction of a specimen significantly affects the fatigue performance of RCC. A fatigue-life model considering the mixture parameters was also developed, providing a more realistic and novel solution than other RCC fatigue life-estimation models found in the literature that consider only stress ratios. This model is less conservative than those currently used in pavement design.

Processing, structure, and properties of additively manufactured titanium scaffolds with gyroid-sheet architecture

While the relationships between processing, structure, and properties of solid titanium alloys produced by additive manufacturing have been established, these relationships are less understood for porous materials, particularly those with rough surfaces inherent to L -PBF. For orthopedics applications, porous architecture and surface roughness are desirable for bone growth, and thus optimization of fatigue life despite these inherent fatigue drivers is critical. The present results establishes relationships between post-processing, microstructure, and resulting fatigue properties for gyroid-sheet scaffolds with as-fabricated surfaces. By comparison of known factors driving fatigue behavior, the relative effect of each on normalized fatigue strength was quantified. Normalized compressive fatigue strength of the gyroid-sheet scaffolds which underwent no surface treatments was observed to be > 50%. The result is higher than that seen for tension fatigue of analogous gyroid-sheet scaffolds, or compared to previously reported normalized compressive fatigue strength of strut based scaffolds. The high strength and fatigue resistant behavior of gyroid-sheet scaffolds despite the inherent surface roughness of L -PBF is desirable for biomedical applications.

Probabilistic modeling of the size effect and scatter in High Cycle Fatigue using a Monte-Carlo approach: Role of the defect population in cast aluminum alloys

In this study, a probabilistic model based on Monte-Carlo theory is applied to predict the fatigue behavior of cast aluminum alloys. The objective of the proposed approach is to investigate the effect of porosity (i.e. the defect size distribution and spatial density) on the fatigue strength and its associated scatter for uniaxial fatigue loads with the load ratio R = 0.1. The proposed model is applied to two cast aluminum alloys with very different defect characteristics. The results for these two alloys confirm that the model can be used to predict the average fatigue strength with a relative error <5%. It also accurately reproduces the experimentally observed trends concerning the scatter in fatigue strength. The scatter is underestimated but is of the same order of magnitude as the experimental values. It is believed that this is because the proposed model considers that the porosity is the only source of scatter. It is demonstrated that the model is well adapted for the prediction of the volume or scale effect in fatigue. This approach can also be used to estimate the Representative Volume Element in Fatigue for cast aluminum alloys.

Surface roughness effects on the fatigue behaviour of gyroid cellular structures obtained by additive manufacturing

Surface roughness effects on the fatigue behaviour of gyroid cellular Ti-6Al-4 V structures obtained by additive manufacturing were investigated. The normalized fatigue strength values for 10^6 cycles, obtained from the S-N curves, were in the range of 0.21–0.23. Finite element analyses showed that stress peak points were located on the stress concentrators due to surface defects, and scanning electron microscopy images showed that crack initiation was located at the surface. It is concluded that the expected good fatigue behaviour due to the curvature in the junctions of the struts, is diminished because of the superficial roughness contributing to the notch effect.

Fatigue behaviour of bolted joints for rack structures

Racking systems applied in logistics warehouses are built with thin steel sections that are joined by bolted connections. In fully automatic systems, these structures are loaded 7/7 days and 24/24 hours raising fatigue issues due to the cyclic nature of the resulting stresses. According to the current industrial practice, those bolted joints are tight with no torque control, leading to the concept of snug tight bolts, which behaves similarly to non-preloaded bolted joints. Eurocode 3 has no reference about fatigue rules of bolted connections between thin-walled cold formed steels. This paper aims at analyzing the fatigue behaviour of bolted (snug tight vs. preloaded) joints in order to provide fatigue design rules for possible update of Eurocode 3 existing rules.A significant fatigue testing program was performed to investigate the influence of different bolts arrangements, stress R-Ratios and bolt preloads, including snug tight, on fatigue life. The obtained results were analyzed under ASTM E739 procedure in order to obtain S-N curves.This work revealed an average fatigue strength of 114 MPa at 2E6 cycles for the snug tight bolted joints. Preloaded bolt joints presented an average fatigue strength of 108 MPa at 2E6 cycles. Discrepancies between S-N curves suggested by Eurocode 3 and fatigue data obtained experimentally were verified.

Mechanical strength of a new plate compared to six previously tested opening wedge high tibial osteotomy implants

BackgroundThis study aimed to assess the mechanical static and fatigue strength provided by the FlexitSystem plate in medial opening wedge high tibial osteotomies (MOWHTO), and to compare it to six previously tested implants: the TomoFix small stature, the TomoFix standard, the ContourLock, the iBalance, the second generation PEEKPower and the size 2 Activmotion. Thus, this will provide surgeons with data that will help in the choice of the most appropriate implant for MOWHTO.MethodsSix fourth-generation tibial bone composites underwent a MOWHTO and each was fixed using six FlexitSystem plates, according to standard techniques. The same testing procedure that has already been previously defined, used and published, was used to investigate the static and dynamic strength of the prepared bone-implant constructs. The test consisted of static loading and cyclical loading for fatigue testing.ResultsDuring static testing, the group constituted by the FlexitSystem showed a fracture load higher than the physiological loading of slow walking (3.7 kN > 2.4 kN). Although this fracture load was relatively small compared to the average values for the other Implants from our previous studies, except for the TomoFix small stature and the Contour Lock. During fatigue testing, FlexitSystem group showed the smallest stiffness and higher lifespan than the TomoFix and the PEEKPower groups.ConclusionsThe FlexitSystem plate showed sufficient strength for static loading, and average fatigue strength compared to the previously tested implants. Full body dynamic loading of the tibia after MOWHTO with the investigated implants should be avoided for at least 3 weeks. Implants with a wider T-shaped proximal end, positioned onto the antero-medial side of the tibia head, or inserted in the osteotomy opening in a closed-wedge construction, provided higher mechanical strength than implants with small a T-shaped proximal end, centred onto the medial side of the tibia head.

Non-Auxetic Mechanical Metamaterials.

The concept of “mechanical metamaterials” has become increasingly popular, since their macro-scale characteristics can be designed to exhibit unusual combinations of mechanical properties on the micro-scale. The advances in additive manufacturing (AM, three-dimensional printing) techniques have boosted the fabrication of these mechanical metamaterials by facilitating a precise control over their micro-architecture. Although mechanical metamaterials with negative Poisson’s ratios (i.e., auxetic metamaterials) have received much attention before and have been reviewed multiple times, no comparable review exists for architected materials with positive Poisson’s ratios. Therefore, this review will focus on the topology-property relationships of non-auxetic mechanical metamaterials in general and five topological designs in particular. These include the designs based on the diamond, cube, truncated cube, rhombic dodecahedron, and the truncated cuboctahedron unit cells. We reviewed the mechanical properties and fatigue behavior of these architected materials, while considering the effects of other factors such as those of the AM process. In addition, we systematically analyzed the experimental, computational, and analytical data and solutions available in the literature for the titanium alloy Ti-6Al-4V. Compression dominated lattices, such as the (truncated) cube, showed the highest mechanical properties. All of the proposed unit cells showed a normalized fatigue strength below that of solid titanium (i.e., 40% of the yield stress), in the range of 12–36% of their yield stress. The unit cells discussed in this review could potentially be applied in bone-mimicking porous structures.

Fatigue Life Prediction for Transverse Crack Initiation of CFRP Cross-Ply and Quasi-Isotropic Laminates.

Carbon fiber reinforced plastic (CFRP) laminates are used as main structural members in many applications. Transverse cracks that form in 90° layers of CFRP laminates are mostly initial damage in the case where tensile loading is vertically applied to the 90° layers of CFRP laminates, and they are the origin of more serious damage of delamination and fiber breakage. It is thus important to predict quantitatively the transverse crack initiation of CFRP laminates subjected to cyclic loading to ensure the long-term reliability of the laminates. The initiation and multiplication behaviors of transverse cracks strongly depend on the laminate configuration, thickness, and thermal residual stress. Therefore, a model based on the Walker model was proposed to predict transverse crack initiation in CFRP cross-ply and quasi-isotropic laminates under cyclic loading in the present study. The usefulness of the proposed model was verified with 10 different CFRP laminates formed from four different prepregs with epoxy resin matrices. The analysis results were in good agreement with experimental results. The fatigue life was expressed with three constants, which related to the fatigue strength reduction, the normalized fatigue strength at N = 1 cycle, and the contribution of stress amplitude to the fatigue life, and they are independent of the laminate configuration.

Effect of porosity on the fatigue strength of cast aluminium alloys: from the specimen to the structure

The fatigue strength of cast aluminium alloys is known to be greatly affected by different defect types related to the manufacturing process, particularly microshrinkage pores created during the solidification phase of the casting process. Even if certain classification procedures are given in the standard ASTM E155-15 [1], the presence of defects is not readily related to the capacity of a component or a structure to meet the requirements of the mechanical technical specifications.The present study aims at establishing a clear link between certain microstructural features and the average fatigue strength. This is possible by looking for the average size of critical defects and using a relevant statistical analysis. More exactly, the Murakami approach based on the statistics of extremes is employed. The main originality of this work lies in the application of this approach to the case of a real structure submitted to high cycle fatigue damage: engine cylinder heads, used in the automotive industry. Indeed, both fatigue tests and microstructural characterizations are carried out on cylindrical specimens and real structures. The specimens are subjected to uniaxial and multiaxial loading conditions [2]. Original fatigue tests, developed by PSA to load in-service critical regions, are carried out on cylinder heads. Systematic analyses of fatigue failure surface are conducted to obtain the statistics of critical defects at the origin of the failures for both specimens and structures.In parallel, critical regions and the associated local loading mode in the structure are characterized by an appropriate high cycle fatigue analysis. The latter, combined with the fatigue test data and the statistical analysis of the critical defects, leads to a discussion about the size effect and an approach is proposed for a relevant fatigue design procedure.

Influence of residual stress profile and surface microstructure on fatigue life of a 15-5PH

This works aims at identifying the influence of surface integrity parameters on fatigue life of a 15-5PH. The effect of residual stress profile, surface roughness and microstructure are investigated. Various cutting and superfinishing processes (turning, ball burnishing and belt finishing) are used so as to reach various engineered surface integrities and as a consequence to highlight the influence of each surface integrity parameter. Rotary-bending tests have been carried out on samples to determine the average fatigue strength at 2 million cycles. It is shown that the influence of a deep compressive sublayer is by far more important than the ones of surface roughness or microstructure. The so-called ‘white-layer’ brings also a slight improvement of fatigue resistance.

Assessment of the suitability of biodegradable rods for use in posterior lumbar fusion: An in-vitro biomechanical evaluation and finite element analysis.

Interbody fusion with posterior instrumentation is a common method for treating lumbar degenerative disc diseases. However, the high rigidity of the fusion construct may produce abnormal stresses at the adjacent segment and lead to adjacent segment degeneration (ASD). As such, biodegradable implants are becoming more popular for use in orthopaedic surgery. These implants offer sufficient stability for fusion but at a reduced stiffness. Tailored to degrade over a specific timeframe, biodegradable implants could potentially mitigate the drawbacks of conventional stiff constructs and reduce the loading on adjacent segments. Six finite element models were developed in this study to simulate a spine with and without fixators. The spinal fixators used both titanium rods and biodegradable rods. The models were subjected to axial loading and pure moments. The range of motion (ROM), disc stresses, and contact forces of facet joints at adjacent segments were recorded. A 3-point bending test was performed on the biodegradable rods and a dynamic bending test was performed on the spinal fixators according to ASTM F1717-11a. The finite element simulation showed that lumbar spinal fusion using biodegradable implants had a similar ROM at the fusion level as at adjacent levels. As the rods degraded over time, this produced a decrease in the contact force at adjacent facet joints, less stress in the adjacent disc and greater loading on the anterior bone graft region. The mechanical tests showed the initial average fatigue strength of the biodegradable rods was 145 N, but this decreased to 115N and 55N after 6 months and 12 months of soaking in solution. Also, both the spinal fixator with biodegradable rods and with titanium rods was strong enough to withstand 5,000,000 dynamic compression cycles under a 145 N axial load. The results of this study demonstrated that biodegradable rods may present more favourable clinical outcomes for lumbar fusion. These polymer rods could not only provide sufficient initial stability, but the loss in rigidity of the fixation construct over time gradually transfers loading to adjacent segments.

Compressive and fatigue behavior of beta-type titanium porous structures fabricated by electron beam melting

β-type titanium porous structure is a new class of solution for implant because it offers excellent combinations of high strength and low Young's modulus. This work investigated the influence of porosity variation in electron beam melting (EBM)-produced β-type Ti2448 alloy samples on the mechanical properties including super-elastic property, Young's modulus, compressive strength and fatigue properties. The relationship between the misorientation angle of adjacent grains and fatigue crack deflection behaviors was also observed. The super-elastic property is improved as the porosity of samples increases because of increasing tensile/compressive ratio. For the first time, the position of fatigue crack initiation is defined in stress-strain curves based on the variation of the fatigue cyclic loops. The unique manufacturing process of EBM results in the generation of different sizes of grains, and the apparent fatigue crack deflection occurs at the grain boundaries in the columnar grain zone due to substantial misorientation between adjacent grains. Compared with Ti-6Al-4V samples, the Ti2448 porous samples exhibit a higher normalized fatigue strength owing to super-elastic property, greater plastic zone ahead of the fatigue crack tip and the crack deflection behavior.

Compression fatigue behavior and failure mechanism of porous titanium for biomedical applications

Porous titanium and its alloys are believed to be one of the most attractive biomaterials for orthopedic implant applications. In the present work, porous pure titanium with 50–70% porosity and different pore size was fabricated by diffusion bonding. Compression fatigue behavior was systematically studied along the out-of-plane direction. It resulted that porous pure titanium has anisotropic pore structure and the microstructure is fine-grained equiaxed α phase with a few twins in some α grains. Porosity and pore size have some effect on the S-N curve but this effect is negligible when the fatigue strength is normalized by the yield stress. The relationship between normalized fatigue strength and fatigue life conforms to a power law. The compression fatigue behavior is characteristic of strain accumulation. Porous titanium experiences uniform deformation throughout the entire sample when fatigue cycle is lower than a critical value (NT). When fatigue cycles exceed NT, strain accumulates rapidly and a single collapse band forms with a certain angle to the loading direction, leading to the sudden failure of testing sample. Both cyclic ratcheting and fatigue crack growth contribute to the fatigue failure mechanism, while the cyclic ratcheting is the dominant one. Porous titanium possesses higher normalized fatigue strength which is in the range of 0.5–0.55 at 106 cycles. The reasons for the higher normalized fatigue strength were analyzed based on the microstructure and fatigue failure mechanism.

Porous Ti6Al4V alloys with enhanced normalized fatigue strength for biomedical applications

In this paper, porous Ti6Al4V alloys for biomedical applications were fabricated by diffusion bonding of alloy meshes. The compression–compression fatigue behavior was studied. It results that porous Ti6Al4V alloys show enhanced normalized fatigue strength which is in the range of 0.5–0.55 at 106cycles. The porosity has some effect on the absolute S–N curves but minor effect on the normalized S–N curves. The relationship between strain per cycle and number of cycles shows three distinct stages and the value of strain per cycle is constant in stage II. The reasons for the higher normalized fatigue strength of porous Ti6Al4V alloys are discussed based on the fatigue crack initiation and propagation.