Cycle Fatigue Characteristics Research Articles

Fatigue crack growth behavior of marine steel under variable amplitude loading-combining DIC technique and SEM observation

This paper experimentally studies the fatigue crack growth (FCG) behavior of AH36 grade marine steel under variable amplitude loading. FCG testing is conducted using compact tension specimens. Digital image correlation technique (DIC) is employed to study the evolution of strain distribution and cyclic fatigue characteristics. Fractographic analysis is performed to explore the overload retardation mechanism. Results indicate that the FCG rate varies significantly under overload application, leading to the extension of fatigue life by 1.03, 1.18 and 2.42 times under the overload ratios (OLRs) of 1.5, 2.0 and 2.5. Reasonable evaluation of the fatigue performance depends on the loading history. Extension of the fatigue life for periodic overload (POL) cases is not a simple superposition of that under two single ones, the interaction effect between the adjacent tensile overloads cannot be neglected. Greater plastic dissipated energy corresponds to more severe plastic deformation at larger OLRs and crack lengths. The larger strain accumulation at the crack tip should account for the greater FCG resistance and more evident overload retardation effect. The fresh fracture surface is destroyed at a certain region after overload due to the premature contact of the crack flanks, confirming that overload induced crack closure exists.

On the cyclic fatigue of adhesively bonded aluminium: Experiments and molecular dynamics simulation

Direct bonding of metals with resin plays a critical role in the jointing of dissimilar materials. The adhesion strength is known to be dependent on strain (loading) rate due to the strain rate sensitivity of polymeric resin. This study evaluated the adhesion durability of an epoxy resin adhesive on an aluminum alloy under cyclic impact loading. First, we used a repetitive laser shock adhesion test (LaSAT), which enabled us to evaluate the impact strength of interfacial fractures (i.e., adhesion and its durability). In this method, a pulsed YAG laser was used to generate strong elastic waves, which resulted in the interfacial fracture of the adhesive/aluminum alloy. We prepared two types of specimens with different curing temperatures (20 °C and 100 °C) and found that the specimen with the higher curing temperature showed higher adhesion strength and durability. We revealed that the adhesion strength showed cyclic fatigue characteristics, and that higher curing temperature improved fatigue strength. To elucidate this mechanism on the molecular level, we conducted a molecular dynamics (MD) simulation for the epoxy resin adhesive on aluminum alloy. The model featured an all-atom model of the Al2O3/epoxy resin interface, and we performed cyclic tensile deformation until delamination. We found that the number of loading cycles until delamination increased when the applied tensile stress was lower. We also found that the 400 K curing model had greater adhesion strength than the 300 K curing model. This tendency was confirmed by the results of the LaSAT experiments. Our comprehensive study incorporating LaSAT experiments and MD simulations evaluated adhesion strength and revealed its fracture mechanism.

Cyclic fatigue characteristics of rock failure using infrared radiation as precursor to violent failure: Experimental insights from loading and unloading response

AbstractIn this research paper, a new method is developed that can be used as a precursor of violent rock failure. It is based on measuring the characteristics of rock failure using infrared radiation (IR) under uniaxial cyclic loading and unloading. The loading–unloading response ratio (LURR) is determined based on IR indexes such as average infrared radiation temperature (AIRT) and infrared radiation temperature rate (IRTR). The results show that the LURR based on AIRT revealed an overall rising trend and mutation that occurred in the last cycle, which can be used as a precursor of the imminent rock failure. By contrast, the LURR based on IRTR has a noticeable ‘mutation‐rapid decrease’ in the middle and later periods of cyclic loading and unloading, which can also be considered as an early precursor of rock failure. Combined with the precursors of the imminent rock failure, the categorized warning of rock failure is recognized.

Cyclic fatigue characteristics of strong burst-prone coal: Experimental insights from energy dissipation, hysteresis and micro-seismicity

This paper documents investigations of the fatigue behaviour of strong burst-prone coal in terms of energy dissipation, hysteresis and micro-seismicity. The energy dissipation characteristics of the coal subjected to multi-level-frequency cyclic loading were studied. Two hysteresis indexes were proposed according to the stress-strain relation during cyclic loading. These two indexes are effective as twin precursors to predict the fatigue failure of coal samples. The effect of frequency on energy dissipation rate was studied. With the aid of an AE monitoring system and a high speed camera, 3D distribution of AE events and failure patterns were obtained. The damage evolution of coal samples subjected to cyclic loading is not homogenous due to the stiffness effect of the loading platen. The failure patterns captured by high speed camera are consistent with the results characterized by the spatial distribution of the AE events.

Cyclic Fatigue Fracture Behavior of Spray-Deposited SiCp/Al-Si Composite

The cyclic fatigue characteristics of spray-deposited SiCp/Al-Si composite were investigated in comparison with the unreinforced Al-Si alloy. The as-extruded specimens were cyclically deformed with fully reversed loading under a range of total strain amplitudes. The results show that the cyclic response characteristics for the reinforced and unreinforced materials are similar to each other. Both the composite and matrix alloys display cyclic hardening under total strain amplitude of 0.35-0.5%. Otherwise, the composite exhibits higher degree of strain hardening than that of the matrix alloy. Dislocation substructure developed during cyclic deformation was analyzed using transmission electron microscopy. The discrepancy between dislocation substructures obtained from processing compared to its development during cyclic strain loading is thought to give rise to the observed cyclic stress response behavior. Fractographic analysis shows that particle/matrix debonding and particle cracking are the main mechanisms of failure in the SiC particle-reinforced composite.

Fatigue life evaluation of 42CrMo4 nitrided steel by local approach: Equivalent strain-life-time

In this paper, the fatigue resistance of 42CrMo4 steel in his untreated and nitrided state was evaluated, using both experimental and numerical approaches. The experimental assessment was conducted using three points fatigue flexion tests on notched specimens at R = 0.1. Microstructure analysis, micro-Vickers hardness test, and scanning electron microscope observation were carried out for evaluating experiments. In results, the fatigue cracks of nitrided specimens were initiated at the surface. The fatigue life of nitrided specimens was prolonged compared to that of the untreated. The numerical method used in this study to predict the nucleation fatigue life was developed on the basis of a local approach, which took into account the applied stresses and stabilized residual stresses during the cyclic loading and the low cyclic fatigue characteristics. The propagation fatigue life was calculated using fracture mechanics concepts. It was found that the numerical results were well correlated with the experimental ones.

The tensile properties and high cyclic fatigue characteristics of Mg–5Li–3Al–1.5Zn-2RE alloy

The tensile and fatigue properties of the Mg–5Li–3Al–1.5Zn-2RE (LAZ531-2RE) alloy were presented. The alloy was prepared by hot extrusion after vacuum casting. The Portevin–Le Chatelier (PLC) phenomenon was observed during tensile deformation. The PLC effect was attributed to the dynamic interaction between solute atoms (Li) and dislocation movement. The abnormal negative strain rate sensitivity (SRS) in high strain rate regime can be explained by the dynamic strain ageing (DSA) mechanism. The higher ductility may be associated with finer-grained microstructure and Li addition. Fatigue crack propagation was characterized by striation-like features. The smaller spacing of fatigue striations and larger fatigue crack propagation zone at the lower total strain amplitude gave rise to a longer fatigue life.

An In Vitro Comparison of Torsional Stress and Cyclic Fatigue Resistance of ProFile GT and ProFile GT Series X Rotary Nickel-Titanium Files

The purpose of this in vitro study was to compare the torsional stress and cyclic fatigue characteristics of ProFile GT (Dentsply Tulsa Dental, Tulsa, OK) and ProFile GT Series X (Dentsply Tulsa Dental). Files of 0.04 and 0.06 taper, 25 mm in length, and ISO sizes of 20 and 30 tips were compared ( n = 25 per test group). Torque stress resistance was evaluated by measuring the torque in gram-centimeters (g-cm) and angle of deflection (degrees of rotation) required for instrument separation with use of a torsiometer instrument. Cyclic fatigue was determined by recording the time until breakage of a file rotating in a simulated canal with an applied 45° or 60° curve. The files were operated in a cyclic fatigue instrument that simulated clinical rotary file usage with a constant cyclical axial motion. There was no statistical difference (p > 0.05) when comparing the torque (g-cm) required to induce a torsional failure of ProFile GT and ProFile GT Series X files of identical file sizes. The angle of deflection (degrees of rotation) of ProFile GT was significantly greater (p ≤ 0.001) before separation than ProFile GT Series X for all file sizes tested except 20/.04 (p > 0.05). There was no statistical difference (p > 0.05) in cyclic fatigue failure for ProFile GT and ProFile GT Series X in a canal with a curvature of 45°. In the 60° canal curvature, ProFile GT was found to be significantly more resistant (p = 0.005) to fracture because of cyclic fatigue than ProFile GT Series X for file size 30/.06 and significant (p ≤ 0.001) for files sizes 20/.06 and 30/.04. There was no difference (p > 0.05) in cyclic fatigue resistance in the 60° canal for ProFile GT and ProFile GT Series X for file size 20/.04.

High cyclic fatigue characteristics of gravity cast AZ91 magnesium alloy subjected to transverse load

High cycle fatigue behavior of magnesium AZ91 alloy under transverse loading ambience is reported. An electro dynamic shaker system was used to apply transverse load on the specimen fabricated according to ASTM standard. The S–N curve for this novel test method for the alloy was generated and compared with the existing fatigue data. This assumes significance in the terrain of commercial applications, where the design of critical components is subjected to repeated transverse loads. Fatigue cracks were initiated at the pores in most of the samples. Considering the pores as the initial cracks, using linear fracture mechanics, critical stress intensity amplitude ( K cr) was estimated. Structure – fatigue property correlation was investigated by metallographic (optical and SEM) examination.

The effects of surface pretreatment on the cyclic-fatigue characteristics of bonded aluminium-alloy joints

The present work has used an accelerated test method, based upon cyclic-fatigue testing using a fracture-mechanics approach, to study the durability of organosilane-pretreated joints which were adhesively-bonded using a hot-cured epoxy-film adhesive. The silane primer investigated was γ -glycidoxypropyltrimethoxysilane (GPS). The cyclic-fatigue tests were mainly conducted in (a) a ‘dry’ environment of 25±2 °C with a relative humidity of 55±5% and (b) a ‘wet’ environment where the joints were fully immersed in distilled water at 28±2 °C. However, tests were also undertaken by varying the relative humidity of the environment. The loci of failure of the joints were identified using various surface specific techniques. The use of the GPS-pretreatment has been shown to be effective in increasing the joint durability, compared with a simple grit-blast and degrease (GBD) treatment and, indeed, gives comparable results to using a chromic-acid etch (CAE) pretreatment.

Torsional fatigue and endurance limit of a size 30.06 ProFile rotary instrument.

To evaluate the torsional cyclic fatigue characteristics and specifically the endurance limit (EL) of a nickel-titanium rotary instrument. Size 30.06 taper ProFile instruments were evaluated. The equipment was assembled according to the ANSI/ADA Specification No. 28. The motor was programmed to repeatedly rotate to a selected deflection angle (DA) and then return to zero (cycle). Testing started at 200 degrees and was continued at decreasing angles until 10(6) cycles were reached without instrument fracture. Ten instruments were tested at each DA. The mean log number of cycles to fracture and standard deviation were determined for each DA at which fracture occurred. The DA at which 10(6) cycles were reached without instrument fracture corresponded by definition to the EL. Analysis of variance and pairwise comparisons using Duncan's multiple range test were performed to detect significant differences among the mean log number of cycles of the different DA. Significance was determined at the 95% confidence level. Instruments cycled at larger DA consistently demonstrated fewer cycles to fracture than those cycled at smaller DA. The differences among the mean log number of cycles of the different DA were statistically significant (P < 0.001). Cycles of 10(6) were completed without instrument fracture at 2.5 degrees. A torsional fatigue profile was generated for a specific NiTi rotary instrument. The EL was 2.5 degrees.

Corrosion fatigue of bio-ceramic sapphire in isotonic sodium chloride solution

Alumina (sapphire) is considered as a candidate material for bio-ceramics, because of its superior strength characteristics, corrosion resistance and bio-inert properties. These biomaterials are used in aggressive corrosive environments such as the human body under severe mechanical conditions superposing cyclic loading on constant load. Therefore, it is necessary to evaluate the static and cyclic fatigue characteristics in an environment as severe as the human body. In this paper, the effects of human body environment corrosion upon cyclic fatigue characteristics of single crystal sapphire and polycrystalline alumina have been investigated using in vitro testing in isotonic sodium chloride solution, whose composition is closely similar to body fluid. Also, degraded corrosion morphologies and fracture surfaces of single crystal sapphire specimens were examined in detail by SEM and AFM. The fatigue limit of single crystal sapphire was found to be extremely degraded in the isotonic sodium chloride solution compared with that in air. The importance of environment assisted slip deformation is indicated.

Plasma and vacuum carburizing processes and mechanical properties of SCM 415 steel

The plasma and vacuum carburizing processes are innovative surface modification techniques for potential applications in manufacturing processes of vehicle components. In this study, the influence of this relatively new process on the surface hardness of SCM 415 low-alloy steel (0.15% C) was investigated under various process conditions involving gas composition, gas pressure, plasma current density, temperature and time. The effective plasma carburizing temperature of SCM 415 steel was found to be higher than 850°C, and the case depth was proportional to the square root of carburizing time under the same current density. Also, high cycle fatigue and dry wear characteristics of vacuum carburized SCM 415 steel were evaluated after various heat treatments. Comparing the fatigue and wear characteristics of vacuum carburized specimens to those of hot annealed or reheat quenched specimens, the wear resistant property as well as bending fatigue limits were found to be substantially improved. The optimum amount of retained austenite exists for the maximum fatigue strength in vacuum carburized steels. Through quantitative analysis of the retained austenite using X-ray diffractometer, this value was found to be about 4%.

Cyclic Fatigue Characteristics and Crack Extension of a Ceramic/Metal Joint.

Fatigue strength and crack propagation characteristics of ceramic/metal joints were investigated under cyclic bending loading. Fracture surfaces were examined by SEM and X-ray methods. Fatigue fracture occurred at the metal/interlayer interface as well as the ceramic/interlayer interface. Fatigue strength was independent of the fracture path. Fatigue crack initiated and grew on the ceramic/interlayer interface at the specimen corner. Under constant amplitude cyclic loading, the fatigue crack decelerated and finally halted. Under increasing amplitude cyclic loading, the final fracture occurred at the metal/interlayer interface. Even under cyclic compressive loading, fatigue cracks were initiated on the ceramic/interlayer interface.

Cyclic Fatigue Characteristics and Crack Extension of a Ceramic/Metal Joint.

Fatigue strength and propagation characteristics of ceramic/metal joints were investigated under cyclic bending loading. Fracture surfaces were examined by SEM and X-ray methods. Fatigue fracture occurred at the metal/interlayer interface as well as the ceramic/interlayer interface. Fatigue strength was independent of the fracture path. Fatigue crack initiate and grow on the ceramic/interlayer interface at the specimen corner. Under constant amplitude cyclic loading, the fatigue crack decelerated and finally halted. As a result of the increasing amplitude of cyclic loading, the final fracture occurred at the metal/interlayer interface. Even under cyclic compressive loading, fatigue cracks were initiated on the ceramic/interlayer interface. This study looks at the method to evaluate the fracture mechanics parameter for interface cracks on ceramic/metal joints.

High Cyclic Fatigue Characteristics of Austempered Ductile Iron. Effect of Surface-Finished Layer Removed by Electropolishing.

Rotating bending fatigue tests were conducted on finished (material A) and electropolished (material EA) austempered ductile iron. The S-N curve of material A represented combined fracture, whose starting points of fracture is on the surface layer for a small number of cycles and is an internal defect for a very high number of cycles. On material EA, the structural sensitivity is lower than on material A, and is of a similar degree to 90% pearlite ductile iron. In the horizontal transitional part, the crack does not propagate to a significant extent. Due to the difference of matrix structure, the fatigue strength of the bainitic matrix is 25% higher than that of the 90% pearlite matrix structure.

High Strain Cyclic Fatigue and Fracture Behavior of a SiC Reinforced Cast Aluminum Alloy Composite

The high strain cyclic fatigue characteristics of a cast aluminum alloy A356 discontinuously reinforced with 20 vol% silicon carbide particulates strained to failure over a range of strain amplitudes giving lives of less than 104 cycles to failure were studied. The specimens were cycled using tension-compression loading under total strain control. The as-cast material displayed cyclic hardening and eventual softening to failure at the higher cyclic strain amplitudes, and progressive cyclic softening at low strain amplitudes

各種セラミック材料における強度特性およびその欠陥寸法分布との関係 （第２報 疲労強度）

It is important to predict the S-N curves of ceramics under cyclic loading in their application to machine components. In order to study the cyclic fatigue characteristics of ceramics, the cyclic fatigue tests under 3-point bending were carried out at room temperature in air on the smooth specimens of sintered silicon nitride, partially stabilized zirconia and alumina as well as the precracked specimens of sintered silicon nitride with surface cracks introduced by Vickers hardness indentation. The effect of specimen geometry on the fatigue-life was also investigated by conducting ring compression tests under cyclic loading on the silicon nitride specimens at room temperature in air.The effect of specimen geometry on the fatigue-life was found to be small for the endurance limit at 107 stress-cycles of the case investigated in this study. A large scatter was observed in the fatigue-life of the smooth specimens as compared with that of the pre-cracked specimens. The scatter in the fatigue-life on the smooth specimens may be explained from the scatter of the inherent flaw size in the specimens.From the slow crack growth rate determined on the pre-cracked specimens, the S-N curves for the smooth specimens with different sizes of initial cracks were predicted. Furthermore, the S-N curves for the smooth specimens of the tested materials were predicted on the assumption that the distribution of flaw size of the specimens used for the fatigue tests were equal to that of the specimens for the static fracture tests.

Micromechanisms governing fatigue behaviour of lithium containing aluminium alloys

AbstractThe emergence of lithium containing aluminium alloys as potential light metals for aircraft structures has engendered an unprecedented widespread interest aimed at improving their mechanical properties and studying their cyclic fatigue characteristics. The phenomenon of fatigue is environment sensitive and the alternating moisture conditions experienced by this candidate airframe material can be a handicap when designing for enhanced fatigue resistance. In this paper, the fatigue properties, deformation characteristics, and cyclic stress response of the commercial Al–Li–Cu–Mn and the experimental Al–Li–Mn and Al–Li–Cu–Mg–Zr alloys cycled to failure are compared over a range of plastic strain amplitudes in various environments, spanning the inert to the very aggressive. Abnormal plastic strain–fatigue life behaviour was observed for all the alloy systems and is attributed to differences in the distribution of deformation as a function of plastic strain amplitude and to change in the relative amount...

飽和含水状態にある岩石の繰り返し疲労特性におよぼす間隙水圧の影響

In order to clarify the effect of pore pressure on cyclic fatigue characteristics of water saturated rocks under confining pressure, cyclic fatigue tests using 1 Hz sinusoidal wave of pulsating compressive stress were carried out under both confining pressure and pore pressure for Kimachi sandstone and Hirono sandy mudstone laying emphasis on endurance limit and the change of volumetric strain in cyclic fatigue process.Main results obtained in this study were summarized as follows.(1) Under the condition of constant pore pressure and constant confining pressure, endurance limit exists for both rocks. Pore pressure remarkably decreases both number of cycles to failure and endurance limit.(2) Stress ratio corresponding to endurance limit increases with b value in equation (1), which means that the lower the stress ratio corresponding to endurance limit is, the more number of cycles is required to reach the endurance limit.(3) Pore pressure promotes dilatancy in fatigue process of water saturated rocks, and the dilatancy due to the pore pressure lowers endurance limit and the number of cycles to failure.