Fatigue Life Extension Research Articles

An Efficient Reliability-Based Simulation Method for Optimum Laser Peening Treatment

A method is introduced for efficient reliability-based design of laser peening (LP) surface treatment to extend fatigue life of metal components. The method includes nonparametric probability density estimation, surrogate modeling using a new finite element (FE or FEA) approach, and reliability analysis with correlated random variables (RVs). Efficient LP simulation is achieved via a new technique termed single explicit analysis using time-dependent damping (SEATD), which reduces simulation times by a factor of 6. The example study of a three-point bend coupon reveals that fatigue life reliability significantly affects optimal LP design, as 52 laser spots are needed for 99% reliability versus 44 spots for 95%.

超音波衝撃処理による溶接継手の疲労寿命向上効果に及ぼす影響因子

超音波衝撃処理による溶接継手の疲労寿命向上効果に及ぼす影響因子

Boundary element analysis of edge cracked steel plates strengthened by CFRP laminates

This paper presents a numerical study on the fatigue behaviour of edge cracked steel plates strengthened with carbon fibre reinforced polymer (CFRP) laminates based on the boundary element method. The models were first validated by a comparison with experimental data in the literature, followed by a further investigation into the effects of initial damage degree, bond configuration and crack type on the fatigue behaviour of retrofitted specimens. Results indicated that CFRP overlays could effectively slow down crack growth and extend fatigue life of edge cracked steel plates, regardless of the initial damage levels. It seems better to adopt an early repair considering the total fatigue life. Double-sided repair showed significant superiority over single-sided repair when equivalent composite materials were used. For the repair configuration presented herein, the strengthening was more efficient for edge cracked steel plates than centre cracked steel plates.

INVITED] A review: Warm laser shock peening and related laser processing technique

This paper reviews the recent progress in warm laser shock peening (WLSP) and related laser processing technique. The process design, enhanced mechanical performance, and microstructure evolution of WLSP are discussed in details. The fundamental process mechanism is reviewed by building the processing-microstructure-property relationship. In particular, the precipitation kinetics during WLSP is discussed to study the effect of process parameters on the nucleation of nano-precipitates, and multiscale discrete dislocation dynamics (MDDD) simulation results are summarized to investigate the dislocation multiplication and propagation behaviors as well as the dislocation pinning effect. In addition, the research progress of thermal engineered laser shock peening (TE-LSP) technique is reviewed with a focus on the coarsening of precipitates, the extended fatigue life, and more importantly, the fundamental process mechanism.

An experimental study on fatigue characteristics of CFRP-steel hybrid laminates

This paper aims to investigate the fatigue characteristics of hybrid laminates consisting of wave carbon fiber reinforced polymer (CFRP) sheets and a thin stainless steel plate under the tension–tension loading. Different loading options (e.g. same stress and same force), layers of CFRP sheets, and lay-ups of laminates (single and double sides) were considered. A series of experimental tests were performed to determine the effectiveness of the CFRP bonding on prolonging fatigue crack initiation life, preventing fatigue crack propagation and extending fatigue life of the hybrid laminates. Three distinct failure modes, classified as delamination, delamination bending and fiber breakage, were observed in the tests. It is shown that the loading conditions and CFRP thickness are the critical parameters affecting the failure modes and fatigue resistance. The crack initiation life and fatigue life of fiber-metal laminates (FMLs) increase by factors ranging from 1.06 to 1.96 and 1.17 to 2.07, respectively, relative to monolithic steel plates under the same force condition; whereas decrease by factors ranging from 0.63 to 0.89 and 0.28 to 0.61 under the same stress condition. Moreover, the double-side bonded FMLs show better fatigue properties and more stable crack propagation than single-side counterpart with the same thickness of CFRP.

Effects of warm laser peening at elevated temperature on the low-cycle fatigue behavior of Ti6Al4V alloy

This study focused on the effects of warm laser peening (WLP) on the fatigue behavior of Ti6Al4V titanium alloy during low-cycle fatigue (LCF) tests. The Ti6Al4V specimens were treated by laser peening at room temperature (RT-LP) and WLP at elevated temperatures from 100°C to 400°C. The residual stress relaxation (RSR) tests and LCF tests were conducted subsequently. In addition, the microstructure analysis of fracture surfaces was performed using scanning electron microscope (SEM). Finally, the fracture mechanism of the untreated, RT-LPed and 300°C-WLPed samples during LCF was revealed. It is found that although the compressive residual stress (CRS) induced by WLP decreases at elevated temperatures, the depth and stability of CRS increase with the increasing treatment temperature, which help to retard the early fatigue crack initiation. Moreover, for the 300°C-WLPed specimens, the growth rate of effective cracks is decreased and the lengths of crack growth paths are increased by the induced high angle boundaries (HABs) and nano-precipitates. Therefore, specimens treated by WLP at 300°C are found to have a significantly extended fatigue life when subjected to low-cycle loads. This extended fatigue life is attributed to the great depth and stability of introduced CRS, as well as the enhanced fracture toughness. It can be concluded that 300°C is the optimal temperature for WLP of Ti6Al4V titanium alloy from the perspective of LCF improvement.

Effect of cellulose nano fiber (CNF) on fatigue performance of carbon fiber fabric composites

The effect of cellulose nano fibers (CNF): micro-fibrillated cellulose and bacteria cellulose fibers were investigated on the fatigue life of carbon fiber (CF) fabric/epoxy (EP) composites. Epoxy used as the matrix was physically modified with CNF in advance before fabricating the laminates. The high cycle fatigue strength was significantly improved at 0.3wt% CNF. There exists an appropriate CNF content which makes the fatigue life longest. An increase of adhesive strength between CF and matrix results due to physical modification with CNF. The adhesive strength much increases with increasing the CNF content. Almost no interfacial debonding occurs at 0.8wt% CNF content when CF breakage takes place. On the other hand, some debonding occurs along CFs from the breaking point at 0.3wt% CNF. Debonding is more significant in the case of no CNF addition to the matrix. An appropriate interfacial strength brought at 0.3wt% CNF is the key of fatigue life extension.

Cyclic performance of reinforced concrete T-beams strengthened in shear with fiber-reinforced polymer composites: Sheets versus laminates

Much existing reinforced concrete (RC) civil infrastructure worldwide is in need of shear strengthening and rehabilitation. The use of externally bonded (EB) carbon fiber-reinforced polymer (CFRP) sheets and laminates to strengthen deficient RC beams in shear is now an acceptable and cost-effective practice, particularly under static loads. However, due to its complexity, the cyclic (fatigue) behavior and performance of shear-strengthened beams is not fully documented. Recently, use of CFRP continuous sheets wrapped over the shear length for fatigue upgrade has been studied. This technique may present drawbacks related to surface preparation and FRP debonding. Therefore, when possible, prefabricated CFRP L-shaped laminates can be a cost-effective alternative because they require less surface preparation and do not peel off easily. The objective of this paper was to present the results of an experimental investigation that compared cyclic (fatigue) and static (post-fatigue) behavior of two EB CFRP techniques (sheets vs. laminates) for shear retrofit of RC T-beams. In total, six laboratory tests on full-size 4520-mm-long beams were conducted. The specimens were subjected to fatigue loading of up to 6 million load cycles at a rate of 3 Hz. Specimens that sustained 6 million cycles were then tested monotonically up to failure. The variables examined in the paper were: (1) the EB CFRP strengthening scheme, and (2) the presence and ratio (spacing) of internal shear reinforcement. The test results confirmed the effectiveness of using EB CFRP shear-strengthening methods under cyclic loading. They also revealed that the fatigue performance of RC T-beams strengthened with L-shaped laminates is significantly superior in extending fatigue life compared to corresponding T-beams strengthened with U-wrapped sheets. This was quantified in terms of deflection response (47% increase due to fatigue loading for specimens with L-shaped laminates compared to 90% for specimens with U-wrapped sheets), and the level of increase in steel-stirrups strain range (9–42% vs. 62–114%) and in EB CFRP (36–97% vs. 58–163%).

Predictive Crack Growth Technique for Laser Peening Process Development

Laser peening (LP) has shown excellent fatigue life extension in numerous tests with typical treatments garnering 2-4 times the fatigue performance of an untreated component. Initially, large test programs were implemented to determine the best LP parameters for a given scenario, eventually being augmented by physics-based modeling due to the large design space available to the LP process. Approval for these processes continues to be on a case-by-case basis, contingent on multiple factors: cost, applicability, time, % fatigue life extension, and ability to track crack growth. Because LP induces compressive residual stresses in the near surface region, the compensatory tensile residual stresses are shifted sub-surface. While an axial tensile load would be mitigated by surface compressive stresses, sub-surface a crack can propagate rapidly via tensile stresses. Current predictive methods lack the ability to track this sub-surface behavior, limiting the accuracy of fatigue crack growth predictions throughout the various design stages of an LP treatment. This work demonstrates a framework that incorporates user-defined geometry, material data, crack growth data, mechanical loading, and residual stresses to predict the crack front shape evolution in 3D solids. A baseline case with no residual stresses is simulated and compared with a closed form solution.

Steel fatigue life extension by pulsed electron beam irradiation

Extension (by a factor of 3.5) of fatigue life of steels of different structural classes has been established. The structural change, phase composition, defect substructure of the material surface layer at different scale levels (from micro to nano), and the suppression of processes contributing to the formation of zones of potential submicrocrack location have been analyzed by means of transmission electron microscopy.

Mixed mode fatigue crack initiation and growth in a CT specimen repaired by stop hole technique

The crack growth retardation and the location of fatigue crack initiation from stop-hole edge under different mode-mixities are examined in this paper by means of a developed fatigue code. Different loading conditions have been generated by using a mixed-mode CT specimen made of Al6061-T651. The numerical results reveal that in the presence of stop holes the reduction in the stress concentration becomes larger for mode-II loading conditions. A comparison between the reported experimental results and the obtained computational results shows that the fatigue life extension of repaired specimens can be well predicted by the numerical model developed in this study.

Three dimensional fatigue crack initiation and propagation analysis of a gear tooth under various load conditions and fatigue life extension with boron/epoxy patches

Three-dimensional finite element analyses were conducted to study both crack initiation and propagation in a gear tooth. A damage mechanics approach was used to model crack initiated on the surface of a gear tooth due to rolling contact fatigue (RCF). A finite element model was developed to study the effects of friction on the fatigue crack initiation life. After the location of crack initiation was identified, a fracture mechanics approach was employed to simulate mixed-mode fatigue crack propagation. Partial contact loading conditions were also considered based on the width of the contact load acting along the thickness.

Fatigue life extension by ICR treatment for cracked weld joints subjected to tensile or bending loads

Fatigue life extension by ICR treatment for cracked weld joints subjected to tensile or bending loads

Fatigue Life Extension Study in Cast Steel Railway Couplings Used in Freight Trains

In this paper, fatigue life extension results from pre-existing surface defects in cast steel ASTM A148 90-60 of railway couplings, used in service in Portugal for coal transportation, are presented using two crack growth relationships (Paris equation and modified Paris equation). Fatigue lives were obtained and the results were discussed in terms of the threshold value for non-propagation cracks (modified Paris law) the initial value of the starting crack. It was found that for both crack growth relationships significant crack extension lives can be assumed. Therefore for these components and in this application damage tolerant procedure can be used with safety and this component can be kept in service provided appropriate inspection procedures are applied to detect and measure fatigue cracks. This will avoid early retirement from service of these components, since extension life can be assumed with safety.

Fatigue life extension of epoxy materials using ultrafast epoxy-SbF5 healing system introduced by manual infiltration

The present paper is devoted to the verification of the capability of epoxy-SbF5 system as a healing chemistry for rapidly retarding and/or arresting fatigue cracks in epoxy materials at room temperature. Owing to the very fast curing speed of epoxy catalyzed by SbF5, epoxy monomer and the hardener (ethanol solution of SbF5-ethanol complex) are suc- cessively infiltrated into the fracture plane under cyclic loading during the tension-tension fatigue test. As a result, the mechanisms including hydrodynamic pressure crack tip shielding, polymeric wedge and adhesive bonding of the healing agent are revealed. It is found that the healing agent forms solidified wedge at the crack tip within 20 s after start of poly- merization of the epoxy monomer, so that the highest healing effect is offered at the moment. The epoxy-SbF5 system proves to be effective in rapidly obstructing fatigue crack propagation (despite that its cured version has lower fracture toughness than the matrix), and satisfies the requirement of constructing fast self-healing polymeric materials.

Evaluation of Stress Intensity Factor for CFRP Bonded Steel Plates

Recent studies on the application of carbon fibre reinforced polymer (CFRP) materials to defected steel structures have demonstrated the potential for significant reduction of stress intensity factor (SIF) values at crack tips leading to extended fatigue lives. However, most of the previous research relied on experimental and numerical methods, which were either expensive or time-consuming. In this paper, the SIF values at crack tips of steel plates strengthened with bonded composite materials were evaluated using linear elastic fracture mechanics. The analysis was based on the classical solution of SIF values of plain steel plates, considering load share effect and geometry correction factor change resulted from the overlay patch. The effect of different parameters were demonstrated and compared with experimental results, including initial damage degrees of specimens, geometric and mechanical properties of retrofitting materials and bond locations. Good agreement with the experimental data indicated that this approach could conservatively predict the SIF values with reasonable accuracy. A parametric study on variables including the CFRP modulus, the bond width and bond length was conducted based on this method to further investigate their effect on the SIF values.

Fatigue Crack Propagation of Notched Steel Rebar in RC Beams Repaired with Externally Bonded CFRP

AbstractThis paper presents the results of an experimental study on the fatigue performance of RC beams strengthened with different externally bonded carbon fiber-reinforced polymer (CFRP) systems. Seven specimens were fabricated; three had no CFRP, while the remaining four had one of two CFRP systems. The objective of the research reported in this paper was to determine the effect the CFRP repair had on the growth rate of a fatigue crack initiating from a notch in the tensile rebars. The steel reinforcement was selected for focus because similar studies have found this reinforcement to be the limiting fatigue component in RC beams. The experimental results reported in this paper showed an extended fatigue life and a slowed crack growth rate in specimens repaired with both CFRP systems. The crack growth rates were used to determine material constants for the Paris law, which describes growth of a stable fatigue crack. These results are then used to propose recommendations for design of CFRP repair systems...

Very high cycle fatigue behavior of SAE52100 bearing steel by ultrasonic nanocrystalline surface modification.

In this paper, the SAE52100 bearing steel contained large quantities of cementite dispersed in ferrite matrix was subjected to the ultrasonic nanocrystalline surface modification (UNSM) treatment that aims for the extension of fatigue life. The microstructure and fatigue life of the untreated and treated specimens were studied by using electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), and a developed ultra-high cycle fatigue test (UFT). After UNSM treatment, the coarse ferrite grains (- 10 μm) were refined to nanosize (- 200 nm), therefore, nanostructured surface layers were fabricated. Meanwhile, in the deformed layer, the number density and area fraction of cementite were increased up to - 400% and - 550%, respectively, which increased with the decrease in depth from the topmost treated surface. The improvement of hardness (from 200 Hv to 280 Hv) and high cycles fatigue strength by - 10% were considered the contribution of the developed nanostructure in the UNSM treated specimen.

Functional and structural fatigue of titanium tantalum high temperature shape memory alloys (HT SMAs)

Abstract Due to their high work output and good mechanical properties, actuators made from shape memory alloys (SMAs) are used in numerous applications. Unfortunately, SMAs such as nickel–titanium (Ni–Ti) can only be employed at temperatures up to about 100 °C. Lately, high-temperature shape memory alloys (HT SMAs) have been introduced to overcome this limitation. Ternary systems based on Ni–Ti have been intensively characterized and alloys are available that can operate at elevated temperatures. However, these alloys either contain substantial amounts of expensive noble elements like platinum and palladium, or the materials are brittle. The titanium–tantalum (Ti–Ta) system has been developed to overcome these issues. Binary Ti–Ta provides relatively high M S temperature combined with excellent workability, but it suffers from fast cyclic degradation. By alloying with third elements this drawback can be overcome: The ternary Ti–Ta–Al alloy shows overall promising properties as will be shown in the present work. In-situ thermo-mechanical cycling experiments were conducted and allowed for evaluation of the factors affecting the functional and structural fatigue of this alloy. Functional fatigue is dominated by ω-phase evolution, while structural fatigue is triggered by an interplay of ω-phase induced embrittlement and deformation constraints imposed by unsuitable texture. In addition, a concept for fatigue life extension proposed very recently for binary Ti–Ta, is demonstrated to be also applicable for the ternary Ti–Ta–Al.

Effect of matrix properties on the fatigue damage initiation and its growth in plain woven carbon fabric vinylester composites

The influence of matrix properties including the fracture toughness and carbon fiber (CF)/vinylester (VE) adhesion on the static tensile properties and tension–tension fatigue properties of CF/VE composites has been investigated. An appropriate chemical modification of VE resin improved the tensile strength up to 37.3% and extended the fatigue life of CF/VE composites greatly by several ten times than that using regular VE resin as matrix. Under static loading different failure modes appeared and strain at failure changed due to chemical modification of VE altering the fracture toughness and CF/VE adhesion. Based on TSA (thermoelastic stress analysis) and SEM (scanning electron microscopy) observation, it was revealed that the fracture toughness of matrix dominated the initiation and growth of matrix cracks in CF/VE composites at fatigue initial stage. Transverse cracks formed by linkage of individual matrix crack occurred at interface or matrix that depended on CF/VE adhesive strength. At fatigue middle stage, local delamination tended to initiate early in composites fabricated by resin with lower fracture toughness. High CF/VE adhesive strength also brought a positive effect on the resistance to delamination growth resulting in remarkable extension in fatigue life of composites. The coupling effect of fracture toughness of matrix and CF/VE adhesion on the mechanical properties especially fatigue performance of CF/VE composites was investigated.