Fatigue Life Improvement Research Articles

Improvement of titanium alloy TA19 fatigue life by submerged abrasive waterjet peening: Correlation of its process parameters with surface integrity and fatigue performance

Submerged abrasive waterjet peening (SAWJP) is an effective anti-fatigue manufacturing technology that is widely used to strengthen aeroengine components. This study investigated the correlation of SAWJP process parameters on surface integrity and fatigue life of titanium alloy TA19. SAWJP with different water pressures and standoff distances (SoDs) was conducted on the TA19 specimens. The surface integrity of the specimens before and after SAWJP with different process parameters was experimentally studied, including microstructure, surface roughness, microhardness, and compressive residual stress (CRS). Finally, fatigue tests of the specimens before and after SAWJP treatment with different process parameters were carried out at room temperature. The results highlighted that the fatigue life of the TA19 specimen can be increased by 5.46, 5.98, and 6.28 times under relatively optimal process parameters, which is mainly due to the improved surface integrity of the specimen after SAWJP treatment. However, the fatigue life of specimens treated with improper process parameters is decreased by 0.55 to 0.69 times owing to the terrible surface roughness caused by the material erosion. This work verifies that SAWJP can effectively improve the surface integrity and fatigue life of workpieces, and reveals the relationship between process parameters, surface integrity, and fatigue life, which provides support for the promotion of SAWJP in the manufacturing fields.

Optimization of S-N curve fitting based on neighborhood rough set reduction with improved firefly algorithm

S-N curve fatigue samples of titanium alloy welded joints have such a comparatively significant scatter, that results in the issue that the fatigue life prediction accuracy is not optimal. In this work, the titanium alloy welded joints' fatigue data is used as analysis data, and the neighborhood rough set reduction with improved firefly algorithm efficient method of fitting stress-life curves is set forth. The welded joint's fatigue decision system is built with fatigue data. The continuous iteration of the firefly algorithm is used as the search strategy, the neighborhood rough set is adopted to decrease attributes, and the major deciding elements of welded joints' fatigue life is identified. The fatigue characteristic domains are divided based on the neighborhood rough set reduction with improved firefly algorithm's key factor set, and the S-N curves can then be fitted to each domain individually. According to the goodness-of-fit analysis, the proposed approach can improve fatigue life accuracy and reduce sample scattering from fatigue.

Development of light weight sustainable pineapple/kevlar hybridized fiber and peanut husk cellulose toughened vinyl ester biocomposite for unmanned aerial vehicle applications

AbstractThe aim of this study is to develop a light weight hybrid biocomposite using pineapple and Kevlar fiber with peanut husk cellulose in vinyl ester resin for applications in unmanned aerial vehicles. This study focuses on how the silane treatment on fiber and cellulose particle influences the mechanical, fatigue and low velocity impact properties of this hybrid biocomposites. Using hand lay‐up technique, the biocomposite was prepared with cellulose loading ranging from 1 to 5 vol%. The results revealed that the 5 vol% of cellulose added composite had an improved tensile, impact, flexural, hardness and ILSS of 161 MPa, 224 MPa, 6.8 J, 84 shore‐D and 21.4 MPa. Moreover, the biocomposite with the inclusion of 3 vol% cellulose had an improved fatigue life count of 42 697, 29 821, 22 381 and 18 164 at 25%, 50%, 75% and 90% of UTS. Similarly, the 3 vol% cellulose reinforced composite showed an improved low velocity impact toughness of 12.36 J. The obtained results clearly indicated that these mechanically strengthened and highly toughened biocomposites could be used as working material for number of applications, especially in making of UAVs for the aerospace industry, automotive components for the transportation sector and structural material in domestic infrastructure.

Evaluation of the Effect of WCO/SBS-Modified RAB and RAP and Stiffness Recovery Procedure on Fatigue Performance of HMA

The use of reclaimed asphalt pavement (RAP) in asphalt mixtures is an expedient and cost-effective solution against rutting failure, but its performance against fatigue and thermal cracking is unsatisfactory and causes the pavement to deteriorate sooner than expected. Rejuvenators can improve RAP performance under various operating conditions through stiffness recovery and binder healing. In this paper, waste cooking oil (WCO) and styrene butadiene styrene (SBS) polymer were used to evaluate the rheological behavior of reclaimed asphalt binder (RAB). The modified RAB has been evaluated using chemical composition, linear amplitude sweeping (LAS), and bending beam rheometer (BBR) tests. The results of the LAS test showed that the fatigue life of RAB containing WCO was about 6 to 9 times longer than the control RAB specimen at the strain level of 2.5% and 5%. The results of the BBR test also indicated the good performance of the WCO. Thus, the WCO used in this study was expected to enhance the fatigue performance and thermal cracking of RAB. To evaluate the behavior of RAP containing RAB and additives on the fatigue response before and after self-healing, four-point bending beam test (4PBB) was employed. The results showed that besides improved fatigue life of hot-mix asphalt (HMA) produced with RAP- and RAB-containing additives, it had a longer fatigue life after self-healing. The specimen containing WCO had a better performance than the specimen to other samples.

Analysis of fatigue performance of austenitic stainless steels with bimodal harmonic structures based on multiscale model simulations

Austenitic stainless steel with a bimodal harmonic structure (HS) is a recently developed material that consists of a periodic arrangement of coarse-grained (CG) structures surrounded by a network of ultra-fine grains (UFG). This material is characterised by high strength and good ductility. In this study, we analysed the fatigue performance of HS materials by modifying our previous multiscale model to extend its applicability from only homogeneous materials to HS materials. A novel method was proposed to characterise the microstructural features of HS materials considering the distributions of CG and UFG. A microstructure model was developed to reproduce grain maps using these characterised microstructures. The multiscale model was successfully validated by experimental results for two types of austenitic stainless steels. The model was used to elucidate the governing microstructural factors of fatigue performance in HS materials through systematic simulations. Consequently, a higher UFG area fraction was confirmed to improve fatigue life, and a UFG area fraction of approximately 40% is suggested for optimal microstructural design to balance fatigue performance and fabricating costs. This study presents the first step in the qualitative design of the fatigue performance of HS materials to replace the expensive and inefficient experimental analyses used in previous studies.

Comparative evaluation of dynamic responses of RCC composite pavement under different moving axles

Roller-compacted composite pavement is considered as an alternative for traditional pavements that provides the desired benefits of both rigid and flexible pavements in a cost-effective manner. To evaluate the performance of roller-compacted concrete (RCC) composite pavement under moving loads, a three-dimensional finite element model (FEM) was developed. A realistic dynamic moving load was applied to the surface of the pavement via DLOAD subroutines developed by FORTRAN. The dynamic response of the pavement at critical locations induced by single, tandem, and tridem axles at speeds of 8 and 80 km/h was determined using the developed FEM model and utilized to estimate pavement performance. Sensitivity analysis of design factors considering variations of hot mix asphalt (HMA) and RCC thicknesses, RCC elastic modulus, axle configurations, and moving speed was conducted to evaluate their impact on fatigue life and rutting development of RCC composite pavement. The results show that the performance of the pavement is significantly affected by axle configurations and moving speed. Among different axle configurations, the tandem axle creates the highest tensile stress and shear stress in the RCC base and HMA layer, respectively. The fatigue life of the pavement prolongs by increasing in the speed of moving loads, so it is expected that the pavement withstands higher load repetition when it is subjected to higher speed traffic. Moreover, increasing layer thicknesses and stiffness of the RCC layer improve fatigue life and rutting performance of the pavement. Even though increasing the thickness of the HMA layer enhances the fatigue life of pavement, increasing it to over 15 cm increases the potential of rutting.

The Effect of Laser Shock Peening on Back Stress of Additively Manufactured Stainless Steel Parts

Abstract This work studies the use of laser shock peening (LSP) to improve back stress in additively manufactured (AM) 316L parts. Unusual hardening behavior in AM metal due to tortuous microstructure and strong texture poses additional design challenges. Anisotropic mechanical behavior complicates application for mechanical design because 3D printed parts will behave differently than traditionally manufactured parts under the same loading conditions. The prevalence of back-stress hardening or the Bauschinger effect causes reduced fatigue life under random loading and dissipates beneficial compressive residual stresses that prevent crack propagation. LSP is known to improve fatigue life by inducing compressive residual stress and has been applied with promising results to AM metal parts. It is here demonstrated that LSP may also be used as a tool for mitigating tensile back-stress hardening in AM parts, thereby reducing anisotropic hardening behavior and improving design use. It is also shown that the method of application of LSP to additively manufactured parts is key for achieving effective back-stress reduction. Back stress is extracted from additively manufactured dog bone samples built in both XY and XZ directions using hysteresis tensile. Both LSPed and as-built conditions are tested and compared, showing that LSPed samples exhibit a significant reduction to back stress when the laser processing is applied to the sample along the build direction. Electron backscatter diffraction (EBSD) performed under these conditions elucidates how grain morphologies and texture contribute to the observed improvement. Crystal plasticity finite element (CPFE) modeling develops insights as to the mechanisms by which this reduction is achieved in comparison with EBSD results. In particular, the difference in plastic behavior across build orientations of identified crystal planes and grain families are shown to impact the degree of LSP-induced back-stress reduction that is sustained through tensile loading.

Fatigue life improvement of similar and dissimilar aluminum friction stir welds by deep rolling

At the moment, high production costs prevent the friction stir welding (FSW) process from further industrial applications even if comparable high fatigue strength of the joints can be reached. Higher welding speed may reduce the production costs but decrease the fatigue strength of the manufactured FSW joints. A potential solution is a mechanical post weld treatment directly after welding to increase the fatigue strength of the FSW joint again. In this study, hydrostatic deep rolling was applied for fatigue strength improvement of similar and dissimilar FSW joints made of EN AW 5083 and EN AW 6082 alloys manufactured with different welding speed. Additionally, the fatigue strength was directly compared to conventional joints manufactured by metal inert gas (MIG) welding and to base material specimen made of EN AW 5083. The surface state in as-welded condition was characterized by surface roughness and residual stress measurements. Fatigue tests were performed to quantify the fatigue strength of the FSW joints. Similar compressive residual stresses were determined after deep rolling for similar and dissimilar joints. No fatigue life improvement was determined for deep-rolled similar joints made of EN AW 5083. However, in this condition, the fatigue life of the specimen was within the range of the base material. Thus, significant lower fatigue life and a high fatigue life improvement by deep rolling were reached for dissimilar joints. An increase of the welding speed from 300 to 800 mm/min strongly decreased the fatigue strength of dissimilar welded joints in the investigated case.

Process-performance-prediction integration for fatigue life improvement technologies: An implementation in cold expansion of hole structures

In this work, the process-performance-prediction integration (3P integration) is developed to improve the fatigue life and reveal the strengthening mechanisms of holed structures after a novel cold expansion process (CEP), where a specific dual-scale modelling approach is proposed to refine the close-loop system. In macro-scale simulation, the cyclic deformation behavior is addressed by an isotropic constitutive model, where the residual stress distribution extracted from CEP simulation is set as initial stress field. The local deformation histories in weak region calculated from the macro-scale simulation are supplied to micro-scale simulation as boundary conditions. The micro-scale simulation solves the local damage evolution of holed structures after CEP by using a modified crystal plasticity theory that grain size effect is considered. Excellent agreements between experimental data and predicted results in terms of fatigue life and damage mechanisms prove the robustness and accuracy of the proposed dual-scale modelling approach. Finally, a feedback link in 3P integration is established by the proposed approach combined with different residual stresses and plastic layers. The significant fatigue life improvement is mainly ascribed to residual stress, while plastic layer plays a synergistic role in life performance.

Fatigue experiments and life predictions of notched C-Mn steel tubes

Present work is aimed at performing fatigue tests on notched C-Mn steel tubes. Tests were conducted on different sizes of notches under remote axial strain-controlled conditions. Individual effects of peak equivalent strain amplitude and strain gradient are brought out on experimental fatigue life. The localized measured strains are compared with corresponding outcome of FE analyses.Fatigue life for notched specimens was predicted based on peak equivalent strain amplitude/existing critical plane model considering stress/strain information at peak/characteristic distance locations. The characteristic distance location with critical plane model and peak equivalent strain amplitude results in improved fatigue life predictions.

Fatigue Life Improvement of Weld Beads with Overlap Defects Using Ultrasonic Peening

Welding defects are common during the production of large welded structures. However, few studies have explored methods of compensating for clear welding defects without resorting to re-welding. Here, an ultrasonic peening method to compensate for the deteriorated mechanical properties of overlap weld defects without repair welding was studied. We experimentally investigated changes in the mechanical properties of defective welds before and after ultrasonic peening. The weld specimen with an overlap defect contained a large cavity-type defect inside the weld bead, which significantly reduced the fatigue life. When the surface of the defective test piece was peened, the fatigue life of the weld plate was restored, resulting in an equivalent or higher number of cycles to failure, compared to a specimen with a normal weld. The recovery of mechanical properties was attributed to the effect of surface work hardening by ultrasonic peening and the change in stress distribution. Thus, ultrasonic peening could compensate for the deterioration of mechanical properties such as tensile strength, fatigue life, and elongation due to overlap defects, without resorting to repair welding.

Effect of WC/C coated rolling element to improve bearing life

ABSTRACT The research is aimed to understand the effect of WC/C coating on the bearing rolling elements to improve fatigue life in surface distress conditions. The 1–1.5 microns optimized coating thickness was deposited on the steel substrate using the PVD process. The coating’s physical and chemical properties were analysed by X-ray diffraction, scanning electron microscope, contact profilometer, nanoindentation hardness, and micro scratch indenter. The wear properties were compared on a ball-on-disc tribometer with and without a coated specimen. The bearing tests were performed in contaminated and clean lubrication conditions to prove the coating’s effect on bearing life in surface distress conditions. Results show that coated roller bearings perform at least two times that of the standard bearing. The overall benefit of less friction and wear resistance was due to the polishing action and coating transfer mechanism of multi-layer coating which increases oil film thickness between contacts and hence improve bearing life.

Optimize Multiple Peening Effects on Surface Integrity and Microhardness of Aluminum Alloy Induced by LSP

Laser shock peening is a modernized surface enhancement performed methodically to improve fatigue life, enhance the hardness of the material and make coarse grains flat under the superficial layer. In this current study, the effect of varying optimized multiple laser shock peening (LSP) is studied on the surface integrity, microhardness, and mechanical properties. The results show that the LSP-treated specimens have visible signs of valleys, wavy and varying height distribution as well as dimples. However, the presence of non-uniformity and sharp protrusions was detected from the superficiality of the as-received specimen and this was so because of the SiC abrasive material used to polish the superficial layer of the specimen before the test experiment. Prior to LSP, the surface roughness was 2 μm, however, after LSP the roughness increased to 4 μm, 6 μm and 17 μm for 1, 2, and 4 impacts, respectively. High-density dislocation can also be observed close to the grain boundary because the grain boundary prevents the migration of dislocation which could lead to dislocation walls and dislocation tangles. The increase in impacts decrease the average grain size, nevertheless, the micro-strain increased after multiple impacts. Furthermore, coarse grains after LSP were transformed into finer grains. The increase in the number of impacts increases the micro-strain likewise the full-width half maximum (FWHM). Finally, the increase in microhardness increases as the LSP impacts increase.

Improved Fatigue Life of Slm-Produced Tc4 Alloy Treated by Pulsed Magnetic Field

Improved Fatigue Life of Slm-Produced Tc4 Alloy Treated by Pulsed Magnetic Field

Design and optimization of squirrel cage geometries in aircraft engines toward robust whole engine dynamics

In this work, an coupled end-to-end approach for the optimization of the rotor dynamic behavior of a dual-spool aircraft engine along with fatigue life optimization of squirrel cages (SQC) is presented. A realistic model to simulate the rotor dynamics is created, where the high-pressure (HP) rotor is supported by two squirrel cages. The aim of this work is to find a robust rotor dynamics design by shifting a critical speed to higher rotational speed, and at the same time improving the squirrel cage design with respect to fatigue life. Fully automatized coupled analysis process chain is implemented, allowing to compute the influence of the SQCs geometry variation onto the full rotor dynamics and structural performance of the SQC. Two global optimization techniques are employed to explore the SQCs design space and find optimal 3D geometries, using the aforementioned coupled process. Optimization results are compared and discussed in detail, indicating the importance of the numerical optimization to improve fatigue life of the squirrel cage. It is shown that optimized and non-optimized SQC designs, both fulfilling rotor dynamics goals, can have significantly different performance regarding their fatigue life. Moreover, the advantage of the coupled process is illustrated, allowing to find superior SQC designs by considering both disciplines simultaneously in comparison with a sequential (uncoupled) approach, when the target elastic properties of an SQC, selected only based on the rotor dynamics requirements, may lead to sub-optimal fatigue life.

鋼材組織制御による疲労き裂進展特性の改善とその効果の一検証

The fatigue crack propagation properties of newly-developed SM490 grade steels were investigated in comparison with a conventional steel of the same grade. The fatigue crack propagation rate of the developed steel in Region II of the da/dN-ΔK relationship was suppressed to about 1/2 that of the conventional steel, and its ΔKth value was more than twice as large as in the conventional steel. However, fatigue crack resistance for long crack propagation does not necessarily improve the fatigue life in a condition of increasing ΔK from a small defect, which is usually detected in practical fatigue damage in actual structures in service. The developed steels were subjected to surface crack propagation tests using specimens with artificial small defects to examine their potential under more practical conditions. The fatigue life of the developed steel was about three times longer than that of the conventional steel. A detailed analysis of the surface crack propagation revealed crack propagation below ΔKth only in the developed steels, which suggested the so-called “short crack regime” in a fatigue crack. The crack propagation from a surface defect that deviated from long crack behavior was convincingly explained by the corrected threshold using the R-curve model of a short crack proposed in the previous literature. Based on the experimental fatigue life improvement and its analytical estimation of the propagation resistance in the short crack regime, the effect of the ΔKth value for a long crack in the initial propagation stage just after fatigue crack initiation was discussed.

Determine the Response of Structural and Mechanical Properties to Heat Treatment For (PMMA) Matrix Reinforcement By (Al2o3) Powder

Poly (methyl methacrylate) (PMMA) was used as a matrix for fabrication samples of polymer matrix composites (PMCs). The matrix was reinforcement by (Al2O3) powder with a volumetric ratio (20%) and an average grain size (40 ±5) μm. The manufactured samples were treated after conducting the surface treatment operations thermally with different temperatures (50,70 and 90 ⁰C) and with different treatment times (2,4,6,8, and 10 hours). The samples were subjected to a set of mechanical and microscopic tests, whereby the effect of thermal treatment on the bending fatigue life of the manufactured samples, and the effect of thermal treatment on the ability of the material to resist shear stress, flexural strength, wear, compressive resistance and impact resistance. The results showed that there was an improvement in fatigue life, shear stress, flexural strength, compressive strength, wear resist, and impact resistance after heat treated samples. The best fatigue life, shear resistance, wear resist, and flexural strength occurred for 6 hours post cure time, while it was observed that the best improvement in compressive resistance and impact resistance occurred after the passage of (4hours) of the post cure time. The results also showed that the best thermal treatment temperature is (T=70⁰C).

On study of process induced defects-based fatigue performance of additively manufactured Ti6Al4V alloy

Additively manufactured Ti6Al4V are high strength alloys but associated with inferior ductility and inherent defects. These process-induced defects act as a stress raiser and deteriorates the mechanical and fatigue properties of the alloy. Post-processing is generally done to enhance the ductility and fatigue properties but at the expense of strength of the additively manufactured Ti6Al4V alloy. In this study, the effect of defects on mechanical properties and fatigue performance of additively manufactured (selective laser melting) Ti6Al4V alloy has been investigated based on experimentally calibrated defects-based model. The as-built specimens were subjected to different heat treatments in order to optimize their mechanical properties and the most optimum heat treatment was then selected for investigation of changes in fatigue properties. The beneficial effect of heat treatment on mechanical properties was mainly associated with the reduction in fraction of α ’-martensitic microstructure and increase in α -lamellae thickness. Most of the fatigue failures occurred from surface and sub-surface defects, owing to the high stress concentration factor associated with them, and improved fatigue lives as a result of optimum heat treatment. A process-structure–property (PSP) linkage has been established to predict the mechanical properties. Experimentally calibrated defects’ based fatigue model has been formulated to predict the fatigue performance using the experimentally observed data i.e. the process induced defects’ size and their locations. • Different post-processing of SLM-based AM Ti6Al4V was investigated. • Heat treatment was carried out, which improved both mechanical & fatigue properties. • A Process-Structure-Property (PSP) linkage was established. • A new defects-based model was proposed to predict the fatigue properties.

Numerical investigation on spall fracture in a metallic material caused by laser shock peening

Laser shock peening (LSP) is a very effective technology for metallic surface treatment, which has been widely used to improve fatigue life and the resistance of foreign object damage. However, when LSP is applied to a thin specimen, an undesirable result, spall fracture close to the free surface of the specimen, may occur. In this work, the spall fracture in Ti-17 alloy caused by LSP by numerical simulations is investigated. To this end, a three-dimensional (3D) finite element model is constructed, which consists of both finite elements and infinite elements in the commercial software Abaqus. In the simulation framework, the shock wave induced by laser is modeled as a pressure load varying in time and space, and the spall fracture criterion is a tensile failure model for high strain-rate dynamic problems. This simulation framework is verified and validated with the surface residual stress as the key result. Then, simulations of single-shock LSP process and multiple-shock LSP process are both conducted. The results show that spall fracture occurs within a very short time after the shock wave propagates into the specimen at a location very close to the free surface. Multiple layers of spall fracture occur inside the specimen after multiple shocks. The relation between the spall fracture and the specimen thickness, the peak pressure of shock wave and the shock number is thoroughly studied.

Investigations of Fatigue Damage in a Nitriding Low-Carbon Bainitic Steel for High-Performance Crankshaft

In the automotive environment, the need to increase the performance of materials requires extra engineering efforts. The possibility of developing new materials is strategically important. Indeed, alternative solutions in terms of material choice allow designers to optimise their projects and keep competitive production costs. Traditional quenched and tempered steels are usually used for highly stressed components, and possible alternatives could be important competitive opportunities. One possible substitute is using bainitic steels to exploit their economic advantages while maintaining acceptable mechanical performances. This paper explores the fatigue life behaviour of a new low-carbon bainitic steel for applications requiring case hardening treatment obtained by the nitriding process. A high-cycle fatigue (HCF) strength assessment is conducted through a test campaign to compare treated and untreated material. The improvement in fatigue strength is evaluated as well as the study of fracture surfaces, residual stress, and microhardness profiles to assess in detail the effectiveness of the nitriding process. It is found that the nitriding leads to an improvement in fatigue life but not as much as expected because of the low ductile behaviour of this steel, the high speed of stress application added, and the embrittlement of the nitriding treatment, as confirmed through fracture surface analysis.