Fatigue Life Improvement Research Articles

Fatigue Failure Analysis on Precracked 304 Stainless Steel Components Repaired by Laser with Addition of Nanocomposites

Abstract The service life of components can be extended by repairing fatigue crack with laser so as to improve resource utilization rate. However, the effect of laser repair may be influenced by the instability of laser treatment. To investigate the addition of nanocomposites on the laser repairing effect, fatigue test under uniaxial tension was carried out to evaluate the fatigue life of precracked 304 stainless steel components repaired by laser. According to the results obtained from fatigue tests, a probabilistic fatigue life distribution model considered the sampling size and discreteness of data was established based on the three-parameter Weibull distribution model and three-parameter stress-life equation. The results of probabilistic fatigue life distribution model show that the fatigue life of components repaired by laser with addition of WC nanocomposites can be enhanced significantly, especially the high cycle fatigue life. Moreover, the microstructure of repaired region show that the WC nanoparticles could refine the recrystallized grains and lead to the improvement of strength and toughness of material around crack tip, which are the main reason of improved fatigue life. Meanwhile, smooth surface of repaired region caused by WC nanoparticles can also prevent the initiation of microcracks.

Interaction Between Normal and Shear Stresses and Its Effect on Multiaxial Fatigue Behavior

Interaction between normal and shear stresses plays an important role in multiaxial fatigue damage. The aim of this study was to investigate this interaction effect on fatigue behavior of shear failure mode materials under multiaxial loading conditions. In order to model the influence of normal stress on fatigue damage, the present study introduces a method based on the idea that the normal stress acting on the critical plane orientation causes two types of influence, first by affecting roughness induced closure, and second, by a fluctuating normal stress affecting the growth of small cracks in mode II. The summation of these terms could then be used in shear-based critical plane damage models, for example FS damage model, which use normal stress as a secondary input. In order to investigate the effect of the method, constant amplitude load paths with different levels of interaction between the normal and shear stresses were designed for an experimental program. The proposed method was observed to result in improved fatigue life estimations where significant interactions between normal and shear stresses exist.

Rolling fatigue life improvement effect of zinc thin film formed with shot peening

Rolling fatigue life improvement effect of zinc thin film formed with shot peening

A Study for Brittle Crack Prevention and Fatigue Life Improvement using a Welded Stiffener

Recently, thickness of steels used in ships is increasing along the increased size of vessels, e.g. container carriers in particular. In container vessels, thick plates are applied to hatch side coaming areas, and safety is also becoming important as steel plate thickness increases. Many studies are conducted on the brittle crack arrest characteristics to prevent catastrophic failure in those areas. A number of experiments have been conducted for investigating propagation characteristics of brittle crack from classification societies. Especially, ESSO test method is commonly employed to analyze the BCA (Brittle Crack Arrest) toughness. In order to prevent brittle crack, arrest holes, weld line shift and insert plates are suggested.Various researches have been carried out to arrest brittle crack in BCA steel. However, the actual structure thickness has to be applied to the experiment, and it is difficult to carry out experiments due to the large scale specimens. Thus, in this study, crack retardation method is researched considering the thickness, length, and location of a welded stiffener. Finite element analysis was conducted on the hatch side coaming to compare the allowable crack length according to the effect of stiffener and to compare fatigue life based on the characteristics of BCA steel. Key words: Brittle crack arrest steel, Brittle crack arrest toughness, ESSO test, Fatigue crack growth rate, Stiffener

Fatigue life improvement on 20MnCr5 steel through surface modification for auto transmission application

This research represents a unique approach in improving the fatigue life of 20MnCr5 (89–91 HR15N hardness) shafts which were surface treated through gas carburizing process in a sealed quench furnace followed by double tempering. Crack initiation on 20MNCr5 transmission shafts always occurs at the end of spline location (Location X), propagates longitudinally and finally ruptures at stem location. This was confirmed through optical microscope and SEM (make:Jeol) images. The fatigue experiments were carried out at room temperature in 11,500Nm torque test machine (Model – MTS-663-144-01). The torque test was carried-out by applying a fully reversed cyclic load with the frequency of 5Hz for the torsional load of ±3000Nm. Effect of double tempering, surface roughness, carbon case depth, Microstructure such as retained austenite and non-martensitic transformation products (NMTP) have been investigated in this research. The outcome of the research shows that increase in case depth (CD) (out of increased gas carburising time) resulting in increased inter-granular oxidation (IGO) and NMTP in its microstructure. The presence of retained austenite on the surface of the shaft has not yielded any remarkable improvement in fatigue life of the shaft. Presence of tempered martensite with lesser percentage of retained austenite on the surface improved the fatigue life drastically from 12,000 cycles to greater than 35,000 cycles at high torque load of ±3000Nm.

Improvement of mechanical properties and fatigue life by shot peening process on ASTM A516 Grade 70 steel

ASTM A516 Grade 70 is widely used in the industrial sector as it provides very good mechanical properties in tough conditions. The main usage of this material is in moderate and low operating services. This paper focuses on the effect of shot peening process on ASTM A516 Grade 70 on improving the mechanical properties and fatigue life of the material. Samples have been shot peened with steel shot to induce compressive residual stress. Hardness, tensile and fatigue test as well as microstructure were done on the samples before and after shot peening process to study the effects on mechanical properties. The result shows that there is an increment in every test after shot peening process. There is a slight increment of 0.47% in hardness value, 0.39% increment in tensile strength and 6.78% increment in fatigue life of the material after shot peening process applied. The slight increment in every result was due to the low intensity of the shot peening process. Result also shows that the shot peening process compressed the molecules closer to each other as can be seen under SEM. Therefore it was proven that in this study, there is a very significant improvement in mechanical properties and fatigue life by shot peening process on ASTM A516 Grade 70 Steel.

Novel treatment methods for improving fatigue behavior of laminated glass

The use of laminated glasses (LG) is increasing day-by-day in the fatigue prone structures. In the present work, the pre- and post-breakage behavior of LG was improved for the fatigue loading via five novel thermo-chemical treatment methods. The fatigue life of LG samples was evaluated using the standard bending cyclic fatigue test. The prime goal of the treatment methods was to bring the surfaces of LG samples in the compressive state without causing melting or bubbling of interlayer having a melting temperature lesser than 150 °C. A comparison of treatment methods based on cyclic fatigue strength (CFS) of LG was reported. Further, a theoretical explanation for improved fatigue life was presented. The life of LG samples was increased by more than 40% by one of the treatment methods. The finite element (FE) model was developed to calculate the deformations and stresses for the corresponding number of cycles at which LG samples were fractured. Further, the FE model was extending to consider the residual stresses profile due to different treatments before modeling the effect of cyclic fatigue loading. The strain-life and stress-life relations were established using experimental and modeling results. The crack propagation and fracture of LGs were explained using previously established models and correlated with the effect of treatment. The statistical analysis was conducted for ensuring the validity of the results.

Investigation of the fatigue properties of asphalt mixture designed using vertical vibration method

In this work, the flexural tensile fatigue behaviours of AC-20 asphalt mixtures were studied using beams, which were designed by the vertical vibration method (VVTM) and Marshall method. Then an analysis of variance (ANOVA) was used to examine the differences in fatigue lives, and the Weibull distribution was used to analyze the fatigue lives and establish the fatigue equations of the mixtures. Subsequently, the fatigue lives were compared and analyzed under the same stress level and repeated load force. The results showed that the test accuracy of specimens produced using the vertical vibration compaction method (VVCM) exceeds 92%. Compared to the Marshall design, the asphalt mixture fabricated with the VVTM design improved crack resistance and rutting resistance by at least 40% and 30%, respectively. The ratio of the slopes (the values of the regression coefficient b) of fatigue equations of asphalt mixtures designed by the VVTM to that of Marshall method ranges from 0.883 to 0.965, and the ratio of the intercepts (the values of the regression coefficient a) ranges from 1.278 to 1.284. The results also showed that compared to the Marshall design, asphalt mixtures fabricated with VVTM design exhibited improved fatigue lives by at least 61% (at a stress level of 0.45) and 356% (with a repeated load force of 0.378 kN).

$${\mathcal {H}}_{2}$$ H 2 optimization and numerical study of inerter-based vibration isolation system helical spring fatigue life

This paper presents an optimization and numerical analysis of vibration-induced fatigue in a two degree-of-freedom inerter-based vibration isolation system. The system is comprised of a primary, e.g. source body, and a secondary, e.g. receiving body, mutually connected through an isolator. The isolator includes a spring, a dashpot and an inerter. Inerter is a mechanical device which produces a force proportional to relative acceleration between its terminals. A broadband frequency force excitation of the primary body is imposed throughout the study. The goal of the proposed optimization is to prolong the fatigue life of the ground connecting helical spring of the secondary body. The optimization is based on minimizing separately the displacement and velocity amplitudes. Both optimization criteria are compared with regard to spring fatigue life improvement for fair benchmark comparison. The inerter-based optimized systems, in which the $${\mathcal {H}}_{2}$$ index of the receiving body is minimized, are also compared with the optimized systems without inerter. Notable improvements are observed in inerter-based systems due to the inclusion of an optimally tuned inerter in the isolator. The proposed analytical vibration fatigue method optimization results are compared with the finite element method results, and a very good agreement is observed. Most accurate helical spring deflection and stress correction factors are discussed and determined. Furthermore, the inerter concept is successfully implemented into finite element-based dynamic solution.

Surface Treatment of Powder-Bed Fusion Additive Manufactured Metals for Improved Fatigue Life

High-cycle fatigue (HCF) tests were conducted on samples fabricated by two powder-bed additive manufacturing techniques. Samples were tested with as-produced surfaces and after various non-contact surface improvement treatments. Ti-6Al-4V samples were made using both electron beam melting (EBM) and selective laser melting (SLM), while Inconel 625 was fabricated using SLM. Ti-6Al-4V was treated with a commercial chemically accelerated vibratory polishing process, with target material removal of approximately 200 µm from each surface for EBM samples and 100 µm for SLM samples. This technique led to increases in both the number of cycles before failure at a given loading condition and endurance limit (at 107 cycles) compared to samples with as-produced surfaces. The results are interpreted as the reduction in elastic stress concentration factor associated with surface defects where fatigue cracks initiate. SLM 625 was treated with both an abrasive polishing method and laser surface remelting. Both methods led to improvements in surface roughness, but these did not lead to improvements in fatigue properties of SLM 625. For abrasive polished samples, the combination of improved measured surface roughness without fatigue property enhancement suggests that surface material is removed, but the roots of surface defects, where fatigue cracks initiate, were left intact. For laser treatment, the remelted surface layer retained a rapidly solidified microstructure that did not increase the number of cycles before crack initiation even though the surface was smoother compared to the surface prior to polishing.

The influence of low‐plasticity burnishing process on the fatigue life of friction‐stir‐processed Al 7075‐T6 samples

AbstractThe effects of low‐plasticity burnishing (LPB) on the fatigue life of friction‐stir‐processed (FSP) Al 7075‐T6 plates were examined experimentally and numerically. Aluminum samples were taken from plates to test fatigue response in the presence of heat‐affected zone (HAZ) at different loading magnitudes. Finite element method was employed to numerically evaluate fatigue life of FSPed samples by means of the Smith–Watson–Topper (SWT) model. Through numerical analysis, the FSP and its cooling procedure were modelled on the basis of the arbitrary Lagrangian–Eulerian technique, and then, the effect of the LPB to assess fatigue response of samples was examined. Aluminum samples undergoing friction‐stir process presented lower‐fatigue life as stresses were highly concentrated within FSP regions. Involvement of LPB regained fatigue durability through compressive residual stress induced on the aluminum samples. The higher applied force over the LPB promoted compressive residual stress on the sample surface and improved fatigue life of samples. The predicted life results were found twice more in magnitude than those of experimentally obtained.

Microstructure and mechanical properties of friction stir welded AA6092/SiC metal matrix composite

There is a need for improved understanding on the effects of friction stir welding (FSW) on the metallurgical and mechanical properties of aluminium matrix composite (AMC). In this study, AA6092/SiC/17.5p-T6 AMC joints were produced by using FSW with varying tool rotation and traverse speeds. The microstructural characterisation by scanning electron microscopy equipped with electron backscattered diffraction (EBSD) system revealed a substantial grain refinement and a homogeneous distribution of reinforcement particles in the nugget zone. The grain size of the nugget zone was greatly influenced by weld pitch, as a key indicator to control the amount of heat input, exposure time and cooling rate. Vickers microhardness profile across the welding zone revealed a significant difference in microhardness among the base metal, heat affected zone, thermo-mechanically affected zone and nugget zone. The tensile strength of the cross-weld specimens showed a high joint efficiency of about 75% of the base metal combined with relatively high ductility. Low-cycle fatigue properties were investigated in the axial total strain-amplitude control mode (from 0.3% to 0.5%) with R=εmin/εmax=−1. The results indicate that the fatigue life of the cross-welded joints varies with grain size in the nugget zone and it is lower than that of the base metal. A significant improvement of fatigue life is found to be related to the finer equiaxed grains dominated by high angle grain boundaries in the nugget zone.

From microstructural design to surface engineering: A tailored approach for improving fatigue life of additively manufactured meta-biomaterials

Recently, lattice titanium manufactured by additive manufacturing (AM) techniques has been utilized in various applications, including biomedical. The effects of topological design and processing parameters on the fatigue behaviour of such meta-biomaterials have been studied before. Most studies show that the fatigue life of additively manufactured lattice structures is limited. Post-processing techniques could play a major role in improving the fatigue of these promising biomaterials. This study aims to provide an in-depth investigation into the effects of heat treatments, hot isostatic pressing (HIP), sand blasting, and chemical etching on the microstructure, surface morphology, strength and fatigue resistance of selective laser melted titanium meta-biomaterials. It was found that the combination of microstructural design and surface engineering, induced by HIP and sand blasting respectively, allows to increase the endurance limit of these lattice meta-biomaterials by a factor of two. HIP treatment substantially decreased the internal porosity and transformed the microstructure to a more ductile mixture of α + β phases. Sand blasting allowed to eliminate surface imperfections and induced favourable compressive stress in the surface layer of the struts. Statement of SignificanceAdditively manufactured metallic meta-biomaterials are progressively being used as bone replacement orthopedic implants. While there is a great amount of research related to topological designs and their effect on mechanical (e.g. stiffness), physical (e.g. mass transport), and biological (e.g. osseointegration) properties, fatigue lifetime of such structures remains limited. This study provides fundamental investigation into the combined effect of microstructural design and surface engineering of titanium meta-biomaterial, enabled through various post treatment methods ranging from heat treatments to physical and chemical surface modifications. The findings show that fatigue life is significantly improved by applying developed herein novel method, which effortlessly can be used on other bone-mimicking metallic meta-biomaterials.

Experimental and numerical investigation into strength of bolted, bonded and hybrid single lap joints: Effects of adherend material type and thickness

In line with the developments in advanced engineering applications such as aerospace and automotive industries, the techniques of joining similar and dissimilar materials have become a crucial issue, i.e. the need for a stronger joint has significantly grown. Compared to conventionally used bolted, riveted and pinned joints, adhesively bonded joints have been increasingly used due to its improved fatigue life and damage tolerance and lower structural weight, especially the case when relatively thin adherends are used. Alternatively, hybrid joints, combination of two or more joining techniques, are presently investigated to create a joint with higher strength than those gained from one technique. In this study, we compared mechanical performance of bolted, bonded and hybrid single lap joints subjected to the tensile loading using three different adherend thicknesses and two different adherend materials with different mechanical behaviors, such as yield and tensile strength and ductility. To this end, a combined experimental and numerical study was performed. In finite element simulations, cohesive zone, ductile and shear damage models were used to model the damage initiation and evolution for the adhesive film layer (AF163-2K), aluminium adherend (AL6061 and AL7075) and the steel bolt materials, respectively. Force displacement curves, the amount of energy absorbed and failure history for each configuration tested, were analysed extensively to elucidate the strength of various joints.

Optimum parafibre length according to mechanical properties in hot mix asphalt

Fibre is additive to hot mix asphalt that can improve fatigue life, rutting resistance, durability of asphalt concrete mixtures, creep compliance, tensile strength, dynamic modulus, elasticity, viscoelasticity, and reflective cracking. The fibres can be useful in controlling the cracks. Previous studies on various fibres have mostly focused on changing and optimising fibre percentage. No study has been conducted on appropriate fibre length in the mixture. Therefore, the effect of various fibre lengths on mixture properties needs to be clearly determined. The aim of this study was to evaluate the effect of various parafibre lengths on mechanical properties of hot mix asphalt through Marshall strength, indirect tensile strength, and resilient modulus tests. To achieve this, we added 0.5% fibre by weight of the asphalt mixture with lengths of 3, 6, 9, 12, 15, and 18 mm to hot mix asphalt samples and performed the above tests on the samples. The results showed that fibre improves hot mix asphalt mechanical properties; however, fibre percentage needs to be increased as the length increases, so that there are sufficient fibres present in the mixture to act against cracks. Accordingly, in all three tests, 12 mm fibres at 1% fibre content created the highest values. Therefore, the optimal fibre length was 12 mm, and for this length the most affected properties were resilient modulus, indirect tensile strength, and Marshall strength, respectively.

Effects of Laser Shock Peening over Minimally Detectable Partial Through-Thickness Surface Cracks

Abstract Laser shock peening (LSP) has evolved as a viable alternative to other surface treatments (shot peening, burnishing, etc.) that induce beneficial residual stress in structural components. While fatigue life improvements have been recognized by the aerospace industry, in-service components may have surface flaws with sizes below current inspection limits. A concern is that the application of LSP over an existing crack may cause unintended detrimental consequences. To address this concern, two different LSP processes were applied over a 0.25 in. (6.35 mm) partial through-thickness surface fatigue crack in 7075-T651 aluminum. The resulting residual stresses were measured using combinations of included X-ray diffraction with layer removal, incremental center-hole drilling, and neutron diffraction. Both LSP processes resulted in a through-thickness residual stress profile in which compressive stresses at either surface were balanced by tensile stresses in a band centered in the mid-thickness of the specimen. Peened and unpeened (baseline) specimens were then tested under constant-amplitude cyclic loading to assess the fatigue response. The average fatigue life of the baseline specimens was about 49,000 cycles. The peened specimens (both treatments) survived to runout, exhibiting no crack growth when examined with an optical microscope even when the crack tip was either very close to or inside the region of tensile residual stress.

Micro-stiffener surface characteristics with belt polishing processing for titanium alloys

Titanium alloys with effective material characteristics are widely applied to the design of significant components of aero-engines, such as the blisk, blade, and commutator, and it is very important to create under requirement surface integrity to improve fatigue life by using advanced manufacturing technology, under the requirements of surface integrity for titanium alloy parts. In this paper, the method of micro-stiffener belt polishing (MSBP) is presented to realize anti-fatigue surface, which is based on the influence rule of the thin-plate stiffener on the bending life and the micro-crack principle. Thereafter, the surface characteristics and formation of titanium alloy MSBP are revealed by comparing analyses of the different polishing processes, using numerical simulation and advanced measuring. The belt polishing processes of reciprocating and high-speed rotation are used to form the micro-stiffener and smooth surface, respectively, while the different surface characteristics are obtained by adjusting the feeding speed and pressure. The formation rules of the surface characteristics are analyzed according to material removal under flexible contact conditions. The surface topography is expressed by the wavelet transform and polishing moving model, following which the surface topography is analyzed by means of comparison with the electron microscope test results. The workpiece surface residual stress is tested by an X-ray projector, and the distribution of different depth residual stresses is predicted by finite element analysis. The surface profile is obtained by a contour-graph, based on which the surface roughness distribution is obtained. The experimental results demonstrate that the surface characteristics, surface topography, surface roughness, and residual stress all meet the requirements. Furthermore, the micro-stiffener surface characteristics are superior to those of the smooth surface, particularly in terms of consistency. Therefore, the new method can be used to improve the surface integrity characteristics using the micro-stiffener for titanium alloy materials.

A study on fatigue life of RC bridge and its improvement methods

Fatigue is a crucial factor which affects the total life and serviceability of highway bridges. There are several factors that contribute to the development of fatigue in RC bridges. Among them, traffic loads, wind loads and corrosion of reinforcement are some of the important factors to name a few. In order to address these fatigue issues and improve the fatigue life of highway bridges, many improvement meth- ods have been suggested in the literature. This study attempts to investigate the effectiveness of various improvement methods suggested in the literature. The improvement methods that has been included in this study are externally bonded reinforcement and deck overlay. To accomplish the objective, an existing highway bridge was selected based on the structural details and traffic data and then various improvement methods were applied to analyze its improvement in fatigue life. For analysis, an embedded reinforcement modeling was carried out in ANSYS followed by a nonlinear analysis in ANSYS workbench. The results of the study were found to be in alignment with AASTHO LRFD (2007), IRC SP 60(2002), IRC 6 (2014), IRC SP 37 (2010) and IRC 37 (2012) codal provisions.

Linking microstructure and processing defects to mechanical properties of selectively laser melted AlSi10Mg alloy

Here we analyse the relationship between the monotonic and cyclic behaviour of cylindrical AlSi10Mg (CL31 AL) samples fabricated by Selective Laser Melting (SLM) to the build direction, the presence of manufacturing defects (pores, voids, oxides, etc.) and the beneficial effect of post-processing – T6 and hot isostatic pressing (HIP) – treatments. Correlative Computed Tomography (X-ray tomography, optical microscopy, electron backscatter diffraction, SEM and TEM) is used to characterise the microstructure and the three-dimensional (3D) structure of fatigue samples to shed light on the role of defects on the experimental fatigue behaviour. Pancake-shaped pores are observed in the plane of the deposited layers having a 130% higher volume fraction for the vertical layering deposition (VL) than for horizontal layered (HL) orientations and being larger and flatter.Further, while T6 treatment had relatively little effect on reducing porosity, the HIPping reduced the pore fraction by 44% and 65% for VL and HL samples, respectively. T6 + Hipping decreased the yield stress and the ultimate tensile strength considerably while increasing elongation and reduction of area accordingly. Our results suggest that the fatigue life seems to be dominated by the presence of these crack-like (pancake-like) defects perpendicular to the loading direction such that it is better to build samples transverse to the highest fatigue loads. T6 heat treatment appears to reduce the fatigue strength of the material regardless of the AM deposition scheme due to two main reasons: (i) a reduced tensile strength of the T6-treated alloy and (ii) the inability to reduce (or even increasing) porosity in both layering orientations. The presence of larger and flatter pores/voids (crack-like defects) after T6 were observed. From X-ray tomography, it was observed that T6 + Hipping resulted in improved densification and in smaller and blunter defects which resulted in improved fatigue lives.

A Comparison of Fatigue Life Improvement Methods for an Existing Offshore Jacket Platform Structure

Most offshore Jacket platforms are installed in shallow water, less than 300 meters for drilling and production of oil or gas. They are normally fixed to seabed and constructed as three dimensional frameworks with tubular members as structural elements. The surrounding environment around offshore platform is affected by various environmental loads that comprise of wind, waves, currents and earthquake. The major load in such structures is wave loading, repetitive in nature which causes time varying stresses that results global and or local fatigue damage on the offshore steel structure. This study includes fatigue assessment of a selected jacket structure using ANSYS software. This study concentrates mainly on the stress reduction at joints by various methods under the applied cyclic loading. The various methods considered in this study are providing: joint cans with branch stubs, grouted connections, FRP wrappings, stiffeners at the joint section. A comparison of various methods for reducing the hotspot stresses which increases fatigue life are studied and the most effective one is recommended.