Fatigue Life Improvement Research Articles

Fatigue life improvement and grain growth of gradient nanostructured industrial zirconium during high cycle fatigue

The effect of surface gradient nanostructure on the fatigue life of commercial pure (CP) Zr was investigated. Four point bending fatigue tests indicated that the fatigue limit of CP Zr with surface gradient nanostructure was increased by about 28.3 % compared with the original sample (annealed state). The microstructure evolution at different fatigue loading stages was characterized. The high strength of surface gradient nanostructure could increase the crack initiation resistance. Furthermore, electron back scattered diffraction (EBSD) analysis demonstrated that the surface nanocrystals grew and rotated gradually during the fatigue loading, which was beneficial to reducing stress concentration, inhibit fatigue crack initiation, and prolong crack initiation life. The stored distortion energy of CP Zr calculated before and after fatigue indicated that the stored distortion energy decreased dramatically during cyclic loading, which provided the driving force for grain growth. Besides, the growth of nanocrystals consumed the mechanical energy produced by the applied load to a certain extent, thus, slowing down the accumulation of fatigue damage. The coarse grains at the interior could deform plastically and reduce the crack growth rate. In addition, the compressive residual stress caused by USSP treatment reduced the local effective stress and the driving force of crack growth.

Robust design of ball bearings for an improved performance using genetic algorithm

The design of rolling bearings must be optimized to get an improved fatigue life. As the fatigue life is directly affected by the dynamic capacity, therefore the later is considered as an objective function, which is to be maximized using a robust design optimization. Such design procedures enhance the qualities and reliability of products by ensuring that they are insensitive to the source of variabilities or uncertainties present in them, without eliminating these sources and at the same time maintaining the design feasibility with probabilistic constraints. In the present work, deep groove ball bearings (DGBBs) are designed in such a way that it will have minimum variation in the objective function due to the variation in design variables. A nonlinear constrained optimization problem is formulated with one objective function, ten design variables and a set of total thirty-five constraints. The real-coded genetic algorithm (GA) is used as the optimization tool. The result obtained shows the optimized bearings have improved fatigue life. A convergence study is performed to ensure global optima in the design space and further a sensitivity analysis is performed. The analysis shows that the dynamic capacity is the most sensitive to the variation in the inner groove curvature radius and least sensitive to the bearing pitch diameter. This paper allows the designers to design the rolling bearings by optimizing the fatigue life along with the minimization of their variations, which are due to variations or having tolerances in input design variables during its manufacturing. Moreover, the proposed design methodology is not limited to the bearings only, but can also be enforced in the design of any machine components (like clutches, gears, cams, levers, springs, etc.) having substantial uncertainties in their geometry or material, such as the residual strain, modulus, thickness, density, etc.

Failure analysis of an in-vivo fractured patient-specific Ti6Al4V mandible reconstruction plate fabricated by selective laser melting

Fatigue fracture of titanium alloys fabricated by Selective Laser Melting (SLM) in engineering has been studied in recent years, but an underlying understanding of the in-vivo fatigue fracture of SLM-fabricated titanium patient-specific implants (PSIs) is rarely revealed due to the complex in-vivo biomechanical behaviour and biological environment combined. This paper provides a multidimensional and comprehensive analysis of the in-vivo fatigue failure mechanism of an SLM-fabricated Ti6Al4V mandible reconstruction plate, by macro-scaled Finite Element Analysis (FEA) of biomechanical behaviours, static force analysis and micro-scaled metallurgical investigation. FEA indicates maximal tensile stress of 392.6 MPa locally on the implant with high stress gradients around. Although the theoretical stress is far below the fatigue strength of SLM-fabricated Ti6Al4V material, indicating low potential of plate failure, the site with maximal stress and high stress gradients successfully determine the failure route of the plate, which coincides with the factual crack growth mode on the plate. The followed analysis of stress and force conditions theoretically confirms the failure condition on the implant. Metallurgical investigations further show β and α/α’ phases and tiny pores at implant (sub)surface, together with typical features (striations, cleavage facets and dimples) of metal fatigue failure in fractography analysis. Taken together these can conclude that the fatigue crack initiates from the implant surface subjected to surface defects and maximal stress caused by implant design, and propagates radially, showing a combined brittle/ductile fracture mode. The process is synergistically influenced by Ca/P/O deposits and hydrogen embrittlement under the biological environment. Practical measures for the improvement of in-vivo fatigue life of implants are finally provided.

Preparation and fatigue behavior of graphene-based aerogel/epoxy nanocomposites

Abstract In this research, the effect of adding graphene-based aerogel ((G)A) nanoparticles on the tensile and fatigue behavior of the epoxy polymer was investigated. Specimens of nanocomposites were prepared by adding 0.05, 0.1, 0.2, 0.5, 1, and 2 wt.-% (G)A nanoparticles to the epoxy polymer. Tensile tests revealed that the 0.1 wt.-% graphene-based aerogel/epoxy ((G)A/E) nanocomposites had the highest increase in tensile strength with 19 % growth compared to neat epoxy. Also, the tensile modulus increased by 15 % in the 0.5 wt.-% (G)A/E nanocomposites. A substantial improvement in fatigue life of the epoxy polymer was observed on adding 0.1 wt.-% (G)A nanoparticles. For instance, the fatigue life of (G)A/E nanocomposites improved by 236 % at a maximum stress of 35 MPa compared to the neat epoxy. Fractography in failure analysis of the test samples showed that the placement of (G)A porous nanoparticles on the epoxy polymer with crack twist or crack tilt prevent the formation of large and catastrophic cracks, resulting in delaying the fatigue failure.

Improving fatigue life of cold forging dies by finite element analysis: A case study

Cold forging tools fail mainly due to loss of dimensions (as a result of abrasive wear and plastic deformation) and fatigue failure. Since tooling costs represent a considerable share of the total cost in forging processes, improving fatigue life is a crucial aspect in the design of forging tools. The present study aimed to improve the fatigue life of a specific cold forging tool currently employed in the mass production of an automotive part. The stresses in the die were evaluated by means of finite element simulations, which were able to predict both the crack nucleation spot and its propagation direction, as observed in the tools employed in the production line. There is compelling evidence in the literature that fatigue life of a die is influenced by the amount of interference fit (preloading) to which it is subjected. Therefore, a slight alteration in the preloading profile was proposed, and the finite element analysis was repeated. Numerical results predicted a change from 597 MPa to 170 MPa in the highest principal stress. The design modification was then implemented in the production line and the expected mean life increased from 17,563 to 71,323 parts produced per tool. Overall, the procedure herein described may serve as a valuable tool for the design of cold working tools less prone to fatigue failure.

Random impact FEM simulation of irregularly-shaped media for parametric study of vibratory surface enhancement

Aerospace materials experience high levels of mechanical and thermal loading, high/low cycle fatigue, and damage from foreign objects during service, which can lead to premature retirement. Mechanical surface treatments of metallic components, for example, fan blades and blisks, are proven to improve fatigue life, improve wear resistance and avoid stress corrosion by introducing work hardening, compressive residual stresses of sub-surface, and surface finishing. Vibropeening can enhance aerospace materials’ fatigue life involving the kinetic agitation of hardened steel media in a vibratory finishing machine that induces compressive stresses into the component sub-layers while keeping a finished surface. Spherical steel balls are the most widely used shape among steel-based media and have been explored for decades. However, they are not always versatile, which cannot access deep grooves, sharp corners, and intricate profiles. Steel ballcones or satellites, when mixed with round steel balls and other steel media (diagonals, pins, eclipses, cones), works very well in such areas that ball-shaped media are unable to reach. However, a methodology of study the effect of irregularly-shaped media in surface enhancement processes has not been established. This paper proposes a finite element-based model to present a methodology for the parametric study of vibratory surface enhancement with irregularly-shaped media and investigates residual stress profiles within a treated area of an Inconel component. The methodology is discussed in detail, which involves a stochastic simulation of orientation, impact force, and impact location. The contrasting effects of a high aspect ratio, or an edge contact, as opposed to rounded and oblique contacts are demonstrated, with further analysis on the superposition of these effects. Finally, the simulation results are compared with actual residual stress measurements and was found to have a max percent difference of 34% up to 20 [Formula: see text]m below the media surface.

Simultaneous Improvement of Toughness and Fatigue Life in a Typical Ultrahigh Strength Steel by a New Deep Cryogenic Treatment Process

Simultaneous Improvement of Toughness and Fatigue Life in a Typical Ultrahigh Strength Steel by a New Deep Cryogenic Treatment Process

Effects of shot peening and artificial surface defects on fatigue properties of 50CrV4 steel

Shot peening processes are commonly used for improving the fatigue properties of steels. Shot peening introduces a compressive residual stress field in the near surface of steel, which can reduce or stop the growth of fatigue cracks and improve fatigue properties. This study experimentally investigated the effect of shot peening on the fatigue properties of 50CrV4 steel alloys with different artificial surface defects. Drilling tools were used to introduce different artificial defects with root radii of 0.585 mm and 0.895 mm on the surface of unpeened samples. The shot peening was applied to the drilled and undrilled samples. Scanning electron microscopy (SEM) observations, micro-hardness and X-ray diffraction residual stress measurements were conducted to analyse the characteristics of the shot-peened and unpeened samples. The results show that the shot peening leads to the transformation of the retained austenite to martensite in the near-surface microstructure. The hardness rates of the surface and near surface both increase by 8% after the shot peening. The peened samples exhibit compressive residual stresses with a high degree of isotropy in the near surface. The fatigue properties of samples were experimentally evaluated by conducting 3-point bending tests. The results indicate that the shot peening improves the fatigue life of drilled and undrilled samples. For the defects with the root radius of 0.895 mm, the shot peening leads to a 500% improvement in the fatigue life compared to unpeened samples regardless of defect depth. For the defects with the root radius of 0.585 mm, the improvement in fatigue life is 40% for the defect depth of 0.2 mm compared to unpeened samples. The improvement increases to 60% and 200% by increasing the defect depths to 0.4 mm and 0.6 mm. The fatigue properties are linked to the changes in the features of defects mainly caused by the deformation hardening and compressive residual stress after shot peening.

An improved fatigue life prediction model for shock absorber cylinder with surface roughness correction

PurposeThe purpose of this paper is to obtain a more accurate fatigue life of structures by introducing the surface roughness into fatigue life prediction model.Design/methodology/approachBased on the fatigue life prediction model with surface roughness correction, the shock absorber cylinder is taken as an example to verify the feasibility of the improved method. Based on the load of the shock absorber cylinder during driving, fatigue experiments are performed under longitudinal and lateral forces, respectively. Then, the fatigue life predicted by the modified model is compared with that predicted by the traditional model.FindingsBy comparing with the test results, considering the influence of mean stress, the Manson method is more accurate in life prediction. Then, the modified Manson-Coffin and Manson method with surface roughness is more accurate in life prediction under longitudinal force and lateral forces, respectively. This verifies the feasibility of the improved method with the surface roughness.Originality/valueThe research on the influence of surface roughness on fatigue life can lay the technical foundation for the life prediction of products and have great significance to the quality evaluation of products.

Effects of irregular loading patterns on fatigue life measured in indirect tensile fatigue test: a traffic-based study

Loading patterns used in pavement tests are responsible for modeling the traffic loads applied on the road. This paper suggests a framework to aggregate hourly distribution of daily traffic into 3-h loading blocks, keeping fixed the loading times across all blocks but allowing the rest periods to differ block to block. Difference in rest times among loading blocks is a consequence of different headways between heavy vehicles, better reflecting the influence of asphalt self-healing than when fixed rests are used. Field-specific loading patterns are generated by combining the loading blocks, providing irregular shapes compared to the classical loading pattern commonly used in labs. Traffic data of Tehran-Karaj freeway was obtained and analyzed to generate 3 irregular loading patterns representing the traffic conditions of 3 days with different temperatures. Indirect tensile (IDT) fatigue test was performed at the temperature of each experimental day using the classical loading pattern as well as the irregular one corresponding to that temperature. Moreover, for each temperature, the fatigue test was performed at 3 different stress levels predetermined by an Indirect tensile strength (ITS) test. Results show that by applying the irregular loading patterns instead of the classical one, the measured fatigue lives of specimens increase ranging from 19 to 29%. It should be further underlined that the average value of rest periods is constant under both loading conditions, and irregularity caused these improvements. Moreover, the larger percentages of fatigue life improvements are attributed to the tests with lower stress levels as they underwent more irregular loading patterns than the tests with higher stress level.

Laboratory investigation of the performance of binders and asphalt mixtures modified by carbon nano tube, poly phosphoric acid, and styrene butadiene rubber

This study evaluated the performance characteristics of binders and Stone Matrix Asphalt (SMA) Mixtures modified by Poly Phosphoric Acid (PPA), Carbon Nano Tube (CNT), and Styrene Butadiene Rubber (SBR). In order to investigate the rheological behavior of bitumens, linear Amplitude Sweep (LAS), rotational viscosity, Dynamic Shear Rheometer, Bending Beam Rheometer, Multiple Stress Creep Recovery (MSCR), and storage stability tests were implemented. Moreover, Resilient modulus (Mr), Indirect tensile strength (ITS), dynamic creep, wheel track, and Four-Point Beam Fatigue (FPBF) tests were performed to investigate the properties of asphalt mixtures. LAS and MSCR tests result revealed that the addition of SBR/PPA and CNT leads to improved fatigue life and rutting properties of binders at both strain levels. Additionally it was found that modification of SBR/PPA with 85–100 binder enhances the intermediate temperature behavior of bitumens. The results indicate that the addition of SBR/PPA enhances the Mr, rutting properties, ITS, fracture energy, and fatigue performance of specimens. By increasing the CNT contents, the rutting behavior improves.

Improvement of fatigue life of riveted joints in helicopter airframes

Using original cold-formed rivets in repairs of airframes of helicopters is difficult due to no access to inside parts of the airframe. Thus, the main aim of the study was to investigate the possibility to use the blind rivets or hybrid joints by verification the fatigue performance of such joints that must be better than with original rivets. Riveted and hybrid joints have been experimentally tested under static and fatigue loads. Furthermore, numerical calculations of stress distribution for strapped joint have been conducted. The test results covered fatigue life of lap joints and models of repaired airframe sheets using ordinary mushroom head rivets ref. 3558A-4-10, titanium driven blind bolts with pin, ref. MBF2110AB-05-150 and modified hybrid joints. Using titanium driven blind bolts with pin instead of ordinary hammer-bucked rivets, can improve the fatigue life of element made of aluminum alloy AW 2024T3. There are advantages of replacing riveted joints with modified hybrid (rivet & adhesive) joints in threefold increase in fatigue life of repaired airframe structures.

Modeling on contact fatigue under starved lubrication condition

This study develops a model for the description of surface breaking crack initiation, under starved lubrication condition. Varying from fully flooded to severely starved lubrication condition, impacts of important factors, including surface roughness, contact pressure, and lubricant viscosity, on fatigue life are examined. Surprisingly and interestingly, it is found a lower lubricant viscosity elongates fatigue life when severe starvation occurs, which is opposite to the EHL rule under fully flooded lubrication condition, stating higher viscosity leads to thicker lubrication film and improved fatigue life. However, it is in line with the experimental observation (Cann in Tribol Trans 42(4):867–873, 1999), where film thickness was shown to increase when moving from high viscosity base oil to a lower one under starvation condition.

Improvement of Fatigue Life for TiNi Shape Memory Alloy by Electrolytic Polishing and Thermal Nitridation Treatment

Improvement of Fatigue Life for TiNi Shape Memory Alloy by Electrolytic Polishing and Thermal Nitridation Treatment

Effect of Cavitation Peening on Fatigue Properties in Friction Stir Welded Aluminum Alloy AA5754

Friction stir welding (FSW) is an attractive solid-state joining technique for lightweight metals; however, fatigue properties of FSWed metals are lower than those of bulk metals. A novel mechanical surface treatment using cavitation impact, i.e., cavitation peening, can improve fatigue life and strength by introducing compressive residual stress into the FSWed part. To demonstrate the enhancement of fatigue properties of FSWed metal sheet by cavitation peening, aluminum alloy AA5754 sheet jointed by FSW was treated by cavitation peening using cavitating jet in air and water and tested by a plane bending fatigue test. The surface residual stress of the FSWed part was also evaluated by an X-ray diffraction method. It was concluded that the fatigue life and strength of FSWed specimen were improved by cavitation peening. Whereas the fatigue life at σa = 150 MPa of FSWed specimen was about 1/20 of the bulk sheet, cavitation peening was able to extend the fatigue life of the non-peened FSW specimen by 3.6 times by introducing compressive residual stress into the FSWed part. This is the first paper to demonstrate the improvement of fatigue properties of FSWed metallic sheet by cavitation peening.

Mechanical behaviors of electron beam welded titanium alloy up to very high cycle fatigue under different process conditions

The mechanical behavior of electron beam welded joints for titanium alloy has been investigated in static and cyclic loading up to very high cycle fatigue. It can be concluded that a higher welding voltage parameter and optimal heat treatment condition in combination will produce higher static and cyclic properties of titanium alloy welded joints. The metastable martensite α′ structures in the β grains of the fusion and heat-affected zones are wholly decomposed into small lamellar α phases, forming α + β phases through the post-weld heat treatment (PWHT). Hence, the welded joints by PWHT could transform the “concave” shape microhardness distribution into “convex” shape distribution. The fatigue cracks are initiated from the internal pores of the material in very high cycle fatigue regime. The higher voltage parameter applied to the welding process could reduce the density of pores in the material, which is in favour of the fatigue life of the welded joints. • A higher voltage and optimal heat treatment in combination produce higher mechanical properties of welded joints. • Microstructure and microhardness of welded joints were analyzed before and after heat treatment. • The higher voltage reduced the pores density in the welded joints, which improved fatigue life.

Effect of Heat Treatment and Cryotreatment on the Fatigue Crack Growth in Aluminium 7075 Alloy

Al 7075 is an aluminium alloy with zinc as major element. Al 7075 is commonly used in aircraft components, especially for critical parts like wing and fuselage components. In present investigation Al 7075 has been selected subjected to annealing, cryogenic treatment and the combination of annealing and cryogenic treatment for the study of fatigue crack growth subjected to fatigue load at constant amplitude. Specimens were subjected to fatigue crack growth as per ASTM E-647 standards and the results of fatigue crack growth were compared. NASGRO software was used to predict the fatigue crack growth (FCG) and the results are validated using experimental results. Microstructure analysis of metallic specimens was also carried out to study the micro structural changes in specimens undergoing heat treatment and cryotreatment. From the present investigation it has been observed that cryogenic treatment and heat treatments plays vital role in improving fatigue life of Al 7075 components.

Wellhead fatigue life improvements through BOP thetering system variables optimization

Wellhead fatigue life improvements through BOP thetering system variables optimization

A novel approach for design and analysis of an all-terrain vehicle roll cage

A roll cage is a structural framework of an All-Terrain Vehicle (ATV) which provides three-dimensional protection to the driver in case of sudden impact or rollover accidents and also to mount the various subsystems of the vehicle. Engineering world expects to satisfy the levied rules with substantiated quality of ATV which is compact in design, minimal members, lightweight, and high strength. The dynamically well-balanced ATV requires improvement of weldment strength and fatigue life of roll cage members to protect the driver and overcome terrain impacts. The objective of this work is to formulate a conceptual design using Solidworks and analysis by using Ansys for defined constraints like front, side, rear, and rollover impact on the roll cage of material of duplex S32205 steel with completion of weldment post-processing methods. The analysed set of data inputs experimentally made weld joint by Tungsten Inert Gas (TIG) welding under optimal process parametric condition and test on weldment is conducted for this study. This work involves post-processing method in which we accomplished good strength by utilizing the technique of shot peening where the small spherical shots were bombarded on the welded joints. As an outcome, we had achieved considerable decrease in residual compressive stresses on the weldment areas and increased its surface as well as fatigue strength. Before the strengthening techniques were put through, the weldment areas were spotted for inducting the shot peening method that enhances its strength further. To ensure the definiteness, test trials were done and value sets were taken for Finite Element Analysis (FEA) on optimal model with appropriate material. This proposed work provides the well-studied and optimal design approach on ATV roll cage to improve its acceleration by better weight reduction and enriched strength ratio for a long duration. Therefore, the weldment of roll cage which is shot peened has the better strength than that of unpeened sample.

Impact on Mechanical Properties and Microstructural Response of Nickel-Based Superalloy GH4169 Subjected to Warm Laser Shock Peening.

Laser shock peening (LSP), as an innovative surface treatment technology, can effectively improve fatigue life, surface hardness, corrosion resistance, and residual compressive stress. Compared with laser shock peening, warm laser shock peening (WLSP) is a newer surface treatment technology used to improve materials’ surface performances, which takes advantage of thermal mechanical effects on stress strengthening and microstructure strengthening, resulting in a more stable distribution of residual compressive stress under the heating and cyclic loading process. In this paper, the microstructure of the GH4169 nickel superalloy processed by WLSP technology with different laser parameters was investigated. The proliferation and tangling of dislocations in GH4169 were observed, and the dislocation density increased after WLSP treatment. The influences of different treatments by LSP and WLSP on the microhardness distribution of the surface and along the cross-sectional depth were investigated. The microstructure evolution of the GH4169 alloy being shocked with WLSP was studied by TEM. The effect of temperature on the stability of the high-temperature microstructure and properties of the GH4169 alloy shocked by WLSP was investigated.