On a reliable equation for the second portion of the fatigue crack growth diagram

The fatigue crack growth kinetics on the second portion of the fatigue crack growth diagram is described using an equation that takes into account both the current crack tip opening displacement and the instant when the crack closure in a cycle vanishes and does not appear until the specimen failure. This instant corresponds to the critical effective SIF range, ΔK_fcl. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 83–87, May–June, 1999.     

The relationship between fracture surface roughness and fatigue load parameters

A procedure was sought for estimating fatigue loading information from roughness characteristics of fracture surfaces. Topographs of fracture surfaces produced in compact tension specimens of a titanium alloy in load-shedding and monotonically increasing ΔK tests were analyzed with a fast Fourier transform. The resulting elevation power spectrum density (EPSD) curve was correlated with the stress intensity range, ΔK, and the stress ratio, R.A plot of (EPSD)^1/2 vs ΔK showed three distinct regions, possibly reflecting the competition between surface roughening and surface flattening from partial crack closure as ΔK increased. The plot further indicated the effect of the stress ratio.Although the influence of crack closure complicated the relationship among (EPSD)^1/2, ΔK, and R, the EPSD curves and (EPSD)^1/2 values systematically changed as a function of ΔK and R. These systematic changes may serve as a base for extracting fatigue loading information from roughness characteristics of fracture surfaces.     

An examination of load shedding during fatigue fracture

The existence of fatigue markings across the entire fracture surface of certain failed components implies that load shedding had occurred during the course of the fracture process. In order to examine the role of load shedding, a sample containing three parallel load paths with the central path being cracked was tested in fatigue using fixed grips. Loads in the three load paths, measured with strain gauges, were observed to be in good agreement with values predicted by theoretical analysis, based upon compliance calculations. Measured fatigue crack propagation rates also agreed well with those predicted from theoretical calculations.     

Assessment of fatigue load spectrum from fracture surface topography

The possibility of deducing load spectrum parameters from fatigue failure surfaces is explored by applying innovative, three-dimensional topographic characterization and analysis techniques to failure surfaces in aluminum sheet. Precise, high-resolution elevation maps of fracture surfaces were obtained using confocal optics scanning laser microscopy. Elevation power spectral density curves resulting from a fast Fourier transform of the elevation data appear sensitive to stress intensity range and environment. A conjugate fracture surface matching procedure, FRASTA, can detect and may provide a way to quantify overloads.     

A study of fatigue within the framework of the kinetic concept of fracture

The times to failure for several materials, both metals and plastics, are compared for static and cyclic loadings. Their similarity leads to the conclusion that the fracture is caused by the same elementary processes, with their thermofluctuation nature being described by the Zhurkov formula.     

A fracture mechanics analysis of fatigue crack growth data for various materials

A large amount of previously published fatigue crack growth data obtained from 10 in. wide centre-cracked sheet specimens of various materials has been re-analysed in terms of the range of stress intensity factor (ΔK) and the results presented as master curves of crack growth rate against ΔK. In addition, data obtained previously from fatigue tests on edge-cracked plate specimens concerning the minimum cyclic stress that would just not cause a crack of a given length to grow have been similarly analysed.     

Effect of small notch and absorbed hydrogen on the fatigue fracture in two-step stress test within fatigue limit diagram

It is usually regarded as a common understanding that fatigue failure would not occur if all stresses were kept within fatigue limit diagram. However, it was shown that fatigue failure occurred in some special cases of variable amplitude loading condition even when all stresses were kept within fatigue limit diagram in the case of small-notched specimen. The cause of such a phenomenon was examined using two-step stress pattern for low alloy steel SCM440H. In the case of constant stress amplitude loading, non-propagating crack was formed only at low mean stress and not formed at high mean stress. However, in the case of two-step stress pattern in which the first step stress was chosen as  R  =−1 and the second step stress was with high mean stress, a non-propagating crack was formed by the first step stress. This crack functioned as a pre-crack for the second step stress with high mean stress. Consequently, fatigue failure occurred by the stresses within fatigue limit diagram. In this study, the effect of notch size and shape were examined. The effect of absorbed hydrogen was also investigated. Absorption of 0.3 ppm hydrogen caused more reduction of fatigue limit.     

Cyclic torsion very high cycle fatigue of VDSiCr spring steel at different load ratios

Cyclic torsion fatigue tests with superimposed static torsion loads are performed with VDSiCr spring steel with shot-peened surface in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Fatigue properties are investigated at load ratios R = 0.1, R = 0.35 and R = 0.5 up to limiting lifetimes of 5 × 10⁹ cycles with a newly developed ultrasonic torsion testing method. Increasing the load ratio reduces the shear stress amplitude that the material can withstand without failure. Fatigue cracks are initiated at the surface in the HCF regime. In the VHCF regime, cracks are preferentially initiated internally in the matrix, below the surface layer with compression residual stresses, and less frequently at the surface. Cyclic and mean shear stresses with 50% survival probability in the VHCF regime are presented in a Haigh diagram. Linear line approximation delivers a mean stress sensitivity of M = 0.33 for load ratios between R = −1 and R = 0.5.     

疲劳荷载对橡胶混凝土损伤和断裂性能的影响

虽然橡胶混凝土塑性和疲劳性能较好,但由于掺入橡胶,其在疲劳荷载下离散性增大,损伤过程及最终的断裂机制均不明确.为研究橡胶混凝土在疲劳荷载下的损伤和断裂性能,基于声发射开展了不同橡胶掺量的混凝土在疲劳荷载下的三点弯曲疲劳断裂试验.计算有效裂缝长度,分析疲劳荷载下不同橡胶掺量的混凝土裂缝长度a的变化规律,并利用裂缝长度a和...     

Basic diagram of fatigue fracture for 34KhN1M steel under conditions of phase transition in water

We construct a basic diagram of fatigue fracture for 34KhN1M steel aimed at computing the durability of a steam turbine disk working in the steam-water transition zone. In plotting this diagram, we took into account the service and extreme electrochemical conditions at the tip of a corrosion fatigue crack. The diagram was applied to estimate the serviceability of a disk with cracks of various types depending on the intensity of tensile stresses and the geometry of cracks.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 2, pp. 90–99, March–April, 1994.     

Fe-rich portion of the Fe-Cr phase diagram: electron microscopy study

Transmission electron microscopy and the corresponding selected-area electron diffraction patterns of Fe-(10;20;30;40;47) wt. pct. Cr alloys heat treated isothermally from 500°C up to 1500°C at 100°C was employed to construct the equilibrium Fe-rich portion of the Fe-Cr phase diagram. It has been found to be quite different from the generally accepted one. Two regions of phase separation are in the diagram at low and high temperatures. Two lines of the second-order phase transition ordering-separation are pointed also. The -phase region in the diagram is considered as a region containing two phases, namely, the -phase surface layer and the solid solution bulk.     

An empirical model for fatigue behavior prediction of glass fibre-reinforced plastic composites for various stress ratios and test frequencies

Current models used to predict the fatigue life of glass fibre-reinforced plastic composites do not accurately consider the effects of load stress ratios and load frequencies. These models usually require significant amount of experimental data to establish a set of characteristic fatigue curves for a given composite. This paper proposes a fatigue model for glass fibre-reinforced plastic composites that includes the non-linear effect of stress ratio and load frequency on the fatigue life. The model can be used to predict the fatigue behavior of a composite material using a well-defined minimum number of tests. Fatigue data from the literature and selected research laboratories were used to test the model. Predictions were found to be in good agreement with all experimental data adequately accounting for the influence of test frequency and stress ratio on the fatigue life of composites.     

On the use of the Goodman diagram for high cycle fatigue design

Materials in rotating machinery are typically subjected to vibratory loading from a number of sources which, in turn, is superimposed on mean stresses which result primarily from steady-state centrifugal loads. In addition, components subjected to vibratory stresses can sustain damage during manufacturing, break-in cycles, or during service such as from foreign objects, fretting, or other types of wear. The combination of vibratory and ‘steady’ stress levels can, for certain load levels, produce low cycle fatigue damage in addition to the damage produced from the high frequency (HCF) vibratory loading since the ‘steady’ stresses are actually low cycle fatigue (LCF) which results in one cycle for every startup and shutdown operation. Design for HCF is generally based on a Goodman diagram which takes into account the vibratory as well as the steady stress amplitudes for fatigue runout or fatigue under a given number of cycles. It does not, however, take into account the combined effects of LCF and HCF. In this investigation, the combined effects are demonstrated analytically by numerical examples which consider both the initiation and propagation phases of fatigue. In addition to the analysis of LCF/HCF interactions, considerations which must be accounted for in design are reviewed in light of a number of failures of components in service in U.S. Air Force fighter engines. A critical assessment of the concepts embedded in the use of the Goodman diagram is presented. Comments on the limitations on the use of a Goodman diagram for design are provided. Some suggestions are offered for the improvement of the design methodology for HCF which involve both damage tolerance considerations and methods for assessing and improving the margin of safety.     

互感器校验仪差压回路的分析及其附加负荷推算

为确定互感器校验仪差压回路负荷对电磁式电压互感器检定结果的影响,在给出国内检定规程对差压回路负荷技术要求的基础上,分析目前国内外常用的两种不同互感器校验仪的原理以及相应的差压回路附加负荷计算过程。分析结果表明:若忽略电流极性指示器所消耗的容量,两种互感器校验仪差压回路的容量相等;互感器校验仪差压回路附加负荷造成的测量误差,取决于标准和被检电压互感器二次侧阻抗压降;采用0.002级双级电压互感器对0.01~0.05级电压互感器进行检定时,附加负荷造成的影响可忽略不计。