Rotating bending fatigue tests have been conducted at room temperature in air using the specimens of medium carbon steel (S45C), low alloy steel (SCM435) and titanium alloy (Ti-6Al-4V) with HVOF sprayed coating of a cermet (WC-12%Co) and S45C with WFS sprayed coating of a 13Cr steel (SUS420 J2). The fatigue strength and fracture mechanisms were studied. The fatigue strength evaluated by nominal stress was strongly influenced by substrate materials and the thickness of sprayed coatings. Detailed observation of crack initiation on the coating surface and fracture surface revealed that microcracks initiated at the WC grain boundary coalesced, and then the crack grew rapidly in the coating. Cracks in the substrate were initiated by the stress concentration of the crack in the coating, which was modeled by finite element analysis. For the specimens tested in this study, the fatigue strength of sprayed specimens was dominated by that of sprayed coating. Thus, the fatigue strength could be evaluated uniquely by the true stress on the coating surface.
The effects of normal shot-peening (SP) and hard shot-peening (HSP) on the fatigue properties and material properties of SUS316L austenite stainless steel were investigated. The rotating bending fatigue tests at 50Hz in air and under water cooling conditions were carried out on the SP and HSP treated and the non-shotted SUS316L specimens. In air condition, the fatigue strength of the SP and HSP treated materials were improved, but the specimens became heated because of internal friction. The fatigue strength under water cooling condition was improved on the SP and HSP treated SUS316L. The fatigue limit of 108 and the fatigue strength in all lives of the HSP treated materials were highest in comparing with the SP and the non-shotted materials. X-ray diffraction measurement and hardness tests were also carried out on the SP and HSP treated SUS316L. The results showed that there existed the hardening layer caused by plastic deformation and the high compressive residual stress in the surface layers of the SP and HSP treated materials. Fatigue crack origins were transferred from the surface to the subsurface by the surface hardening and by the compressive residual stress. For the fatigue strength improvement, the HSP treatment of austenite stainless steel was very successful.
The effect of shot peening on the fatigue strength of Ti-6Al-4V alloy was investigated in the temperature range of 20°C to 450°C. In order to evaluate separately the individual effects of compressive residual stress, strain hardening and surface roughness induced by shot peening, the results obtained were discussed by the Murakami's √area parameter model which could predict fatigue strength using two parameters of defect size (√area) and Vicker's hardness (HV), and the effect of static mean stress (σm) could also be considered in the model. Shot peening increased fatigue strength at 20°C-350°C, but decreased at 450°C. The reduction at 450°C was due to surface roughness because compressive residual stress was decreased during fatigue loading. At 450°C, by comparing the fatigue strength of the rough surface specimens with that of the surface polished specimens, the defect size (√area) equivalent to the surface roughness was evaluated to be approximately 76μm. At 20°C-350°C, the fatigue strength observed could be regarded as that obtained by adding the contribution of compressive residual stress (static mean stress) to the fatigue strength of the specimens with the defect size of 76μm. The compressive residual stress predicted by the model showed reasonable values quantitatively. Drilling holes equivalent to √area of 93 and 185μm decreased fatigue strength markedly. The holes, however, were crushed by shot peening. For example, √area of 185μm was decreased down to about 55μm. Therefore, the fatigue strength of the drilled and shot-peened specimens increased drastically and was comparable to that of the shot-peened specimens without drilling holes but with the rough surface of 76μm.
The purpose of this study is to clarify the effects of various strengthening mechanisms on the fatigue properties of low carbon ferrite-pearlite hot-rolled sheet steels and to formulate the fatigue limit of the steels by considering the components of strengthening mechanisms. Low carbon ferrite-pearlite sheet steels strengthened by a solid solution of Si, the precipitation of TiC, an increased volume fraction of pearlite and various combinations of these, were prepared. Load and strain controlled fatigue tests were carried out. The results were discussed in terms of the relationship with tensile strength. The increase in fatigue limit of steel through the addition of various strengthening elements was found to be estimated by summing up all the increases in fatigue limit due to the individual strengthening mechanisms. The formula obtained here for evaluating the fatigue limit showed that a reduction of pearlite volume fraction and instead, the strengthening of ferrite by solid solution or precipitation resulted in a higher endurance ratio. The effects of various strengthening mechanisms on the fatigue limit were also discussed on the basis of detailed observations of the change in sub-microstructure during fatigue tests and fatigue crack initiation.
To investigate the effect of non-metallic inclusions on the fatigue properties of quenched and tempered 0.46% carbon steel (HV≅650) in the super-long life fatigue range (N≤5.0×108), tension-compression fatigue tests were carried out. The fatigue strength was discussed based on the √area parameter model. The results obtained are: (1) Fatigue fracture origins were mostly at a non-metallic inclusion. (2) However, the locations of the inclusion at fracture origin were not uniformly distributed over the specimen section due to the nonuniform distribution of residual stress induced by heat treatment. (3) The fatigue limit defined by the cycle N=5.0×108 can be predicted by the √area parameter model, i.e. with three parameters, the Vickers hardness, HV, of the matrix, the square root of the projected area of inclusions, √area, and residual stress. (4) The expected value of inclusion size √areamax of 0.46% carbon steel in a definite number of specimens can be estimated using the statics of extreme values. The lower bound of the scatter of fatigue strength was predicted with the combination of the √area parameter model and the value of √areamax.
Rotating bending fatigue tests were carried out on austempered ductile cast iron (ADI) in order to investigate the statistical properties of life distributions of crack initiation and propagation, and also the evaluation of fatigue life. The results are summarized as follows: (1) The size of crack initiation sites of the material was represented by a Weibull distribution without regarding to the kinds of crack initiation sites such as microshrinkage and graphite grain. The crack initiation life scattered widely, but the scatter became much smaller as soon as the cracks grew. (2) The crack propagation life Nac which was defined as the minimum crack propagation rate showed lower scatter than the crack initation life. (3) The fatigue life of the material was evaluated well by Nac and the propagation rate after Nac. It was clear that the fatigue life of ductile cast iron was goverened by the scatter of Nac.
In order to reveal the mechanical performance of powder metallurgical (P/M) alloys, several kinds of mechanical tests were carried out on three sorts of HIPed high speed steel P/M alloys. Three ingot metallurgical (I/M) alloys with almost the same chemical compositions as those of the P/M alloys were also used as reference materials. Dependence of mechanical properties on chemical composition and fablication process was discussed in details. The P/M alloys show higher bending strength than I/M alloys irrespective of the difference in additive content. Fracture toughness of both P/M and the I/M alloys depends on Co content. Fratigue strength of the P/M alloys is higher than that of the I/M alloys. Fatigue crack growth rate of P/M alloys shows clear dependence on additive content; the greater the additive content, the higher the crack growth rate. Such a crack growth behavior of the P/M alloys can be correlated well with the size and the density of carbides distributed in the matrix of each P/M alloy.
Fatigue crack growth behavior of a Ti-48Al-2Mn-2Nb (at%) alloy with lamellar microstructure has been investigated at room temperature in laboratory air. Single-edge-notched (SEN) specimens with different lamellar colony size and lamellar thickness were prepared by appropriate heat-treatments. The values of fatigue crack growth rate scattered largely for the specimens with large colony size. The highest crack growth resistance was obtained when the orientation of lamellar plates was aligned perpendicular to the crack growth direction. These results indicate that the lamellar orientation at crack tip dominates the fatigue crack growth behavior in the large colony sized specimens. For the specimens with small colony size, fatigue crack growth resistance appears to be averaged since the orientation of lamellar colony is random at the crack tip. Fatigue crack growth resistance in the air cooled specimens is higher than that in the furnace cooled ones. This may be due to the difference in lamellar thickness, type of lamellar boundary and/or quantity of α2 phase.
Uniaxial push-pull High Cycle Fatigue (HCF) tests as well as Low Cycle Fatigue (LCF) tests were conducted on a medium carbon steel under constant stress or strain amplitude. To examine the small crack growth behavior, a small drilled hole of 50μm in diameter and in depth was introduced on each specimen surface. A fracture mechanics parameter, suitable for characterizing fatigue crack growth rate in both fatigue regions was examined. The results showed that the cyclic crack tip opening displacement range (ΔCTOD), which can well be correlated with overall cyclic plastic strain range Δεp, was a suitable parameter to describe the fatigue crack growth rate in both fatigue regions.
The objective of the present paper is to evaluate the effect of variable stress amplitude, particularly those of the stress cycle ratio, number of load levels and axle size on the fretting fatigue crack initiation at press-fitted axle assembly. The rotational bending fatigue tests with two-level, three-level and ten-level loadings were conducted on the induction hardened press-fitted axles of 40mm in diameter. It was found that the modified Miner's damage (D) under the two-level variable load decreases as the stress cycle ratio increases, but D becomes constant when higher, lower and equivalent stress levels are the same respectively. For the size effect, D of 40mm diameter axle is less than 1.0, whereas D of full-scale axle is larger than 1.0. This size effect is caused by the fact that the locations of maximum stress, where cracks initiate, for respective load levels do not coincide in the full-scale axle.
In order to investigate the effects of environmental conditions, especially water vapor in the atmosphere and high temperature, on the strength behavior of alumina ceramics, fast fracture tests, static and cyclic fatigue tests were carried out in vacuum and at elevated temperatures. Cyclic fatigue tests were also carried out under three different varying load conditions to clarify the effect of load pattern on fatigue behavior. The results of cyclic fatigue tests at room temperature reveal the following fatigue behavior of alumina ceramics. The cyclic degradation is observed clearly in ordinary atmospheric environment, and the higher is the load repetition frequency the lower becomes the fatigue strength when the fatigue strength is evaluated from the view point of cumulative loading time to failure. Furthermore, the degradation disappears in vacuum. The results of high temperature cyclic fatigue tests indicate that the cyclic degradation also disappears at a certain temperature over 800°C as same as the case in vacuum. The above results indicate that the cyclic degradation in ordinary atmospheric environment results from an interaction of time-dependent degradation and cyclic-dependent one, which is assisted by water vapor in air.
The fracture mechanics fatigue testing technique for ceramics which had been developed by the authors was examined to improve the dynamic instability caused by relaxation of crack tip stress concentration at high temperatures. Double eccentric pulsating compressive loading applied by a computer controlled fatigue testing machine was combined with negative double cantilever loading by soft springs. The testing conditions were extended to include the negative R-ratio of K-value, and the stability of crack growth was statically increased. For measuring the crack growth by the quasi-elastic compliance method and controlling the crack growth rate, an on-line computer software was newly developed, which increased the dynamic stability of testing. Consequently, the fracture mechanics fatigue testing for stable crack growth in silicon nitride became possible beyond 1300K.
In order to characterize the fatigue strength of injection moulding advanced thermoplastic, such as Polyetheretherketon (PEEK), short glass fiber reinforced PEEK (GE/PEEK) and short carbon fiber reinforced PEEK (CF/PEEK), the fatigue tests were conducted under tension-tension cyclic loading (stress ratio, R=0.1-0.7) at 23±1°C, 50±5%RH. The effects of repeated stress amplitude, mean stress, characteristic of matrix and the kind of reinforced short fibers on the fatigue strength were investigated on the basis of the fractographic examination of fatigue fracture surface. The results are summarized as follows. (1) The influence of repeated stress amplitude and mean stress on the fatigue characteristic of PEEK series were revealed. (2) It was found that the effect of short fiber reinforcement on the fatigue strength characteristic is not high, the same as that on the static strength of PEEK series. (3) The short fiber reinforcement of those materials was also discussed on the basis of fractographic examination.
This paper describes the tensile and fatigue properties and the fracture mechanism of long glass fibre-reinforced polypropylene at ambient temperature in laboratory air. Tensile tests and pulsating-tension fatigue tests have been carried out using the smooth specimens of two grades of the material, with and without an acid modified polypropylene. Tensile and fatigue strengths were evaluated and the effects of interfacial strength on the mechanical properties were discussed on the basis of macroscopic and microscopic observations of fractured specimens and fracture surfaces. Interfacial strength showed a great influence on the tensile and fatigue strengths. By the addition of the coupling agent, both strengths were considerably improved and the predominant fracture mode changed from fibre pull-out to fibre breakage, suggesting effective load transfer from the matrix to the glass fibres due to improved interfacial strength. Close examination of the specimen surfaces during fatigue test revealed that a fatigue crack of approximately 500μm in length was generated at 70 to 80 percent of fatigue life in both materials. Before such macroscopic fatigue crack initiation, the percent of pulled-out and broken fibres was gradually increased and then saturated. Fatigue cracks were found to be initiated along fibre bundles at the corner of the specimens which made a large angle to the loading axis, and then grew with breaking fibres and with decohesion and fibre pull-out, respectively, in the materials with and without the coupling agent.
Constant ΔG tests were conducted to examine the effects of specimen width and fiber orientation on the Mode I delamination fatigue crack growth behavior of CFRP laminates. For every specimen width, the crack growth rate was lower for the larger angle of crack tip fiber orientation. For the lower angle of crack tip fiber orientation, the growth rate was higher for the wider specimen, while the rate was lower for the wider specimen for higher crack tip fiber orientation. From the microscopic observation by scanning electron microscopy, branching of crack tip, winding of crack path, and bridging of crack faces by fibers were found to be responsible for the effect of fiber orientation. The miss-arrangement of fibers and the anti-plane deformation by Poisson's effect may bring the effect of specimen width. The delamination fatigue crack growth rate for the unidirectional specimen wider than 10mm gave the upper limit value among every crack tip fiber orientation and specimen width.
A very small strain history recorder based on the rainflow method has been developed and was named the Mini Rainflow Corder. The Mini Rainflow Corder is installed with IC chips containing the Rainflow algorithm software, an amplifier circuit, automatic bridge balance, a standard small size memory card in which the decomposed strain history data is stored and batteries. The data obtained under service loading can be easily retrieved by a personal or laptop computer from the memory card. The performance of the Mini Rainflow Corder was tested using the fatigue testing machine in laboratory, and also by the specially designed cantilever type cyclic loading testing machine. The accuracy of data acquisition, reliability at low and high temperatures, and shock resistance were also examined. The advantage of the developed recorder in application to rotating machine parts without a slip ring is emphasized. The application to car wheels was demonstrated.