Fatigue Life Nf Research Articles

Effect of Stress Ratio on the Relationship between Fatigue Lives of Bovine Bone and Stress Frequency.

Completely reversed push-pull, tensile and compressive fatigue tests were performed on bovine bone under various stress frequencies to investigate an effect of stress ratio, R on a relationship between fatigue lives Nf and stress frequencies, f. Fatigue lives Nf under a constant stress amplitude in the reversed push-pull fatigue tests showed an independence of stress frequencies employed. On the other hand, fatigue lives in the tensile and compressive fatigue tests increased with an increase of stress frequency. So, it was clarified that the Nf vs. f relation of bovine bone was strongly affected by stress ratio. A viscoelastic feature of wet bone and a cyclic deformation behavior of bone specimen during a fatigue process are major factors which are concened with the above effect of stress ratio in the Nf-f relation.

封圧下における稲田花こう岩の疲労特性

Cyclic loading tests on Inada Granite were carried out under 0, 5 and 25 MPa of confining pressure to examine the next two effects: the effect of confining pressure on the characteristics of strength and deformation; and the effect of stress amplitude on fatigue life. The main results are as follows.1) The fatigue strength rises as the confining pressure is increased. However, the rate of the increase of the fatigue life Nf to the decrease of the maximum differential stress σDmax is almost identical despite of the different level of confining pressure. A same curve can represent the relationship between σDcσDmax (difference between static strength σDc and σDmax) and log Nf under the two levels of confining pressure (5, 25 MPa). That is, this curve can be used to predict the fatigue strength from the static strength.2) Three stages, namely. transient, steady and tertiary stage, are observed in fatigue deformation. Under the 25 MPa of confining pressure, the transient stage is shorter and the steady stage is relatively longer. As the confining pressure is increased, dilatant strain is restrained and the rate of the increase of volumatric strain per one cycle decreases.3) When the maximum differential stress is fixed at a constant value, the fatigue life increases as the stress amplitude decreases. This tendency becomes more evident as the value of the maximum differential stress becomes lower.4) When the maximum differential stress in fatigue tests and creep tests are the same, time to failure in fatigue is shorter than that in creep in the range of the maximum differential stress lower than a certain point.

焼なまし材と調質材の疲労寿命のばらつき特性

In order to investigate the physical basis of scatter in fatigue life, rotating bending fatigue tests of smooth specimens were carried out using two steels with different microstructures. 16 or 18 specimens were fatigued at each stress range, and the initiation and propagation behaviors of a crack which led to the fracture were examined for all the specimens. The fatigue data exhibited a large scatter in fatigue life Nf at a stress near the fatigue limit. The scatter in Nf is controlled by the behavior of a microcrack, namely it is controlled by the scatter in N0.3 for an annealed steel and N0.1 for a heat-treated steel, where N0.3 and N0.1 are the number of cycles to attain a crack length of 0.3 and 0.1mm, respectively. On the other hand, after the crack became 0.3 or 0.1mm, the propagation life exhibited small scatter, and its scatter was hardly affected by the stress range. To estimate the scatter characteristics quantitatively, the statistical treatment of fatigue data was performed by assuming a Weibull distribution.

Initiation and Propagation Behaviors of Surface Fatigue Cracks and Their Statistical Properties in Ferrite-Base Spheroidal Graphite Cast Iron.

Rotating bending fatigue tests were carried out on smooth specimens of ferrite-base spheroidal cast iron at room temperature. Initiation and growth behaviors of surface cracks as well as the statistical properties of lives of initiation and propagation were discussed by detailed observation of the specimen surface and fracture surface. Surface fatigue crack initiation was mainly governed by microshrinkage existing just under the surface. A negative correlation between the crack initiation life Ni and the square root of the projected area (√(area)) of the microshrinkage as the initiation sites was found, but the correlation factor between them was very low. Surface cracks propagated with a great deal of branching and coalescence during the fatigue process, and it was found to exhibit the remarkable repetition of increase and decrease in the crack prppagation rate da/dN during the range of small cracks. The distributions of the initiation life Ni, the propagation life Np and the fatigue life Nf in the main cracks which propagated to 2a=3000μm were well represented by a three-parameter Weibull distribution, and the dependence of the three parameters on the stress amplitude were discussed and compared with the results of other researchers and other findings of the authors.

高速度工具鋼の引張・圧縮疲労強度に関する研究 ＩＩＩ フィッシュアイを伴う丸棒試験片のき裂進展挙動

The behavior of crack propagation in the complete reversed tensile and compressive fatigue test piece of circular section with Fish eye of the heat treated high-speed steel was investigated by using an acoustic emission technique. Moreover, the property of crack propagation in the 1/2CT test piece was investigated.The results obtained are summerized as follows;(1) The number of stress cycles NAE for crack initiation and propagation and the number of stress cycles Nf for fracture were accurately estimated by measuring continuously AE total ringdown counts during the fatigue process.(2) The fatigue life was good accord with the value of Nf estimated by the acoustic emission technique. The life of fatigue crack propagation (Nf-NAE)Lab. was 25-50% of the fatigue life Nf and decreased with decreasing stress amplitude.(3) The life of fatigue crack propagation (Nf-NAE)cal. calculated from the property of crack propagation in the 1/2CT test piece showed good agreement with the value of (Nf-NAE)Lab. obtained by the acoustic emission technique.(4) The behavior of crack propagation in the complete reversed tensile and compressive fatigue test piece of circular section showed good corrrelation with the behavior of crack propagation in the 1/2CT test piece, so that it is suggested that the initiative mechanism of residual stress on the fracture surface with Fish eye can be discussed by using the 1/2CT test piece.

焼結モリブデンの低サイクル疲労強度に及ぼす炭化チタン被覆の影響

Low cycle fatigue tests of TiC coated and uncoated molybdenum materials were performed in a vacuum at room temperature 773 and 1, 073K in order to examine the effect of TiC coating on the fatigue strength. Moreover the low cycle fatigue behavior was investigated by fractography and crack observations. The relation between the fatigue life Nf and the plastic strain range ΔεaP was expressed by the equation ΔεapNfβ=C, where β and C are constants. At 773 and 1, 073K uncoated molybdenum fractured with ductile appearance on the plane of maximum shear stress, but it fractured in a brittle manner on the plane of maximum tensile stress at room temperature. The low cycle fatigue strength of TiC coated molybdenum decreased compared with that of uncoated one at 773K, where cracks were observed to be initiated at TiC coating layer and to extend into the molybdenum matrix by a notch effect. At 1, 073K, however. TiC coated molybdenum exhibited higher fatigue strength than the uncoated one since the TiC coating layer restricted a slip deformation of molybdenum matrix.

Evaluation of fatigue life for crack initiation and fracture of notched specimens.

The fatigue-crack-initiation life Nc and the total fatigue life Nf of S45C specimens having a wide range of notch geometries were investigated in the rotating bending fatigue tests. The experimental data were discussed on the basis of the concept of "linear notch mechanics" proposed recently by one of the authors. Nc is determined mainly by the maximum elastic stress repeated at a notch root σmax and the notch root radius ρ alone, irrespective of the notch depth t. On the other hand, Nf is determined mainly by σmax and ρ alone, except the case of a shallow notch (t=0.1 mm). This is because σmax obtained from the elastic calculation and ρ control the elastic-plastic behavior near the notch root.

オーステナイト系ステンレス鋼ＳＵＳ３２１の低サイクル疲労挙動に及ぼす温度，ひずみ速度の影響

A strain controlled low-cycle fatigue test was conducted on an austenitic stainless steel SUS 321 at 500 to 700°C under a fully reversed triangular wave form strain with a strain rate of 10-4 to 10-2 sec-1, to obtain fundamental information on the effects of temperature and strain rate on the low-cycle fatigue behavior of austenitic stainless steels at high temperatures. The test results were examined by observation of fracture surfaces and discussed from view-points of deformation mechanism and fracture morphologies by considering cyclic strain induced precipitation at high temperatures.The main results of the study are as follows:(1) The fatigue life is reduced with decreasing strain rate at a constant temperature and with increasing temperature at a constant strain rate.(2) At 500 and 550°C, the fatigue life Nf can be well correlated with the strain aging parameter, (T/ε)exp(-Q/RT), while not at 650° and 700°C.(3) The fatigue strength and cyclic strain hardening properties of the material are affected by dynamic strain aging, by the precipitation of Ti(C, N), M23C6 and a σ-phase and by creep deformation. At 550°C, dynamic strain aging plays an important role on the reduction of fatigue life caused by the decrease in strain rate, whereas at 650°C the fatigue life is reduced mainly by creep deformation.(4) The failure mode of the specimens tends to change from transgranular type to an intergranular as the temperature becomes high, the strain range large, or the strain rate slow.

端面一体巻きFW容器の静圧強度および疲れ強度

Two experimental investigations of end-closed filament-wound vessels are described in this paper. The first is on how tom ake filament-wound vessels that bear more than 80kg/mmmm2 maximum hoop stress at burst pressure. The second is to establish the relation between the cyclic internal pressure and the fatigue life of the vessels that hold 80kg/mmmm2 hoop stress.From the tests, the results were as follows.1) End-closed filament-wound vessels that had 80kg/mmmm2 maximum hoop stress were made.2) The relation between the cyclic internal pressure P and the fatigue life Nf was expressed asPNfα=Cwhere the values of the constants α and C were given as α=0.034 and C=178.

The Effect of Stress Sequence on the Fatigue Strength under Program Loading

The annealed S35C carbon steel was tested under the four-step-in-stress bending program loading. The effect of the stress sequence on the fatigue lives was investigated with respect to the effects by the initial stress level and the magnitude of stress levels as a whole. The number of cycles in one program block was n0=3600 cycles. The results show:(1) The effects of the initial stress level and the stress sequence on the fatigue lives can not be seen, because the value of n0 in this experiment is very small compared with the total number of cycles to failure.(2) When the maximum stress σ4 is large and all the stresses are overstresses, the fatigue lives nearly coincide with Nth's which are calculated by the use of the linear damage law. When σ4 becomes small, the fatigue life Nf also becomes smaller than Nth, and Nf reaches the smallest value at certain stress level. When the all stresses except σ4 are understresses, the fatigue life Nf becomes equal to or larger than Nth.