Analytical Fatigue Research Articles

Modeling of cracked thick metallic structure with bonded composite patch repair using three-layer technique

A finite element analysis involving three-layers of two-dimensional Mindlin plate elements, to model cracked plate, adhesive, and composite patch, was developed to characterize fatigue crack growth behavior of a thick metallic panel repaired with an adhesively bonded composite patch. Also, fatigue experiments were conducted with 6.35 mm thick specimens with a pre-crack repaired asymmetrically with adhesively bonded unidirectional boron/epoxy patch. Fatigue crack growth rates on the unpatched and patched faces (PF) were measured along with debond of the composite patch. Stress intensity factors obtained from the analysis were combined with the fatigue crack growth relationship for the unrepaired cracked material to obtain the analytical fatigue crack growth rates. The experimental and analytical fatigue crack growth rates on the unpatched face (UPF) were in a good agreement with each other when the proper consideration of the effective crack length and debond were incorporated in the analysis. Thus, the three-layer technique was found to be capable of characterizing the fatigue crack growth behavior of the repaired thick panels as in the case of repaired thin panels shown in the previous studies.

Prediction of fatigue crack propagation life in notched members under variable amplitude loading

One of the interesting phenomenon in the study of fatigue crack propagation under variable amplitude load cycling is the crack growth retardation that normally occurs due to the application of a periodic overload. Fatigue crack growth rate under simple variable amplitude loading sequence incorporating period overloads is studied using single edge notched specimens of AISI304 stainless steel. Load interaction effects due to single and multiple overload have been addressed. Substantial retardation of fatigue crack growth rate is observed due to the introduction of periodic tensile overloads. Estimates of fatigue life have been obtained employing Wheeler model (using Paris and modified Paris equations) and Elber’s model. Analytical predictions are compared with experimental results. Results of these analytical fatigue life predictions show good agreement with the experimental fatigue life data. Fatigue crack propagation rates also have been evaluated from the fractographic study of fatigue striations seen on the fracture surface. Good agreement was found between the experimentally observed crack growth rates and the fatigue crack growth rates determined by the fractographic studies.

An analytical model for fatigue life prediction based on fracture mechanics and crack closure

Fatigue in steel structures subjected to stochastic loading is studied. Of special interest is the problem of fatigue damage accumulation and in this connection, a comparison between experimental results and results obtained using fracture mechanics. Fatigue test results obtained for welded plate test specimens are compared with fatigue life predictions using a fracture mechanics approach. In the calculation of the fatigue life, the influence of the welding residual stresses and crack closure on the fatigue crack growth is considered. A description of the crack closure model for analytical determination of the fatigue life is included. Furthermore, the results obtained in studies of the various parameters that have an influence on the fatigue life, are given. A very good agreement between experimental and analytical results is obtained, when the crack closure model is used in determination of the analytical fatigue lives. Both the analytical and experimental results obtained show that the Miner rule may give quite unconservative predictions of the fatigue life for the types of stochastic loading studied.

Effect of heat treatment on low cycle fatigue properties of zircaloy-4: Xiao, L. and Gu, H.C. Xiyou Jinshu Cailiao yu Gongcheng (Rare Metal Materials and Engineering) (Oct. 1993) 22(5), 61–66 (in Chinese)

Effect of cycling frequency on fatigue behavior of neat, talc filled, and short glass fiber reinforced injection molded polymer composites was investigated by conducting load-controlled fatigue tests at several stress ratios (R = −1, 0.1, and 0.3) and at several temperatures (T = 23, 85 and 120 °C). A beneficial or strengthening effect of increasing frequency was observed for some of the studied materials, before self-heating became dominant at higher frequencies. A reduction in loss tangent (viscoelastic damping factor), width of hysteresis loop, and displacement amplitude, measured in load-controlled fatigue tests, was observed by increasing frequency for frequency sensitive materials. Reduction in loss tangent was also observed for frequency sensitive materials in DMA tests. It was concluded that the fatigue behavior is also time-dependent for frequency sensitive materials. A Larson–Miller type parameter was used to correlate experimental fatigue data and relate stress amplitude, frequency, cycles to failure, and temperature together. An analytical fatigue life estimation model was also used to consider the strengthening effect of frequency in addition to mean stress, fiber orientation, and temperature effects on fatigue life.

Analytical fatigue evaluation of externally welded box girders

An analytical method is presented for evaluating the fatigue strength and life of externally welded box girders used in crane and bridge structures. In this method, fracture mechanics techniques are used to study the effect of lack of weld penetration by treating this defect as real crack. Crack initiation and propagation models are employed to study the effect of internal weld defects. The proposed method is illustrated by evaluating the fatigue strength and life of an externally welded box girder of a gantry crane.

A Mechanistic Model for Strength and Fatigue of Cement-Treated Soils

Abstract A mechanistic model for strength and fatigue of cement-treated soils is developed using the Griffith failure theory for the purpose of understanding the structural behavior of cement-treated bases in pavements. Failure under static loads is assumed to occur when the tensile stress at the tip of the most critically oriented flaw reaches a value characteristic of the material. This static failure model is used to study fatigue behavior under repeated stresses. Results of repeatedload triaxial tests on a clayey gravel soil cement are used to define fatigue failure criteria for cement-treated soils. The analytical fatigue model developed is used to derive a cumulative damage hypothesis. According to this hypothesis, fatigue behavior under compound loading depends on the sequence and magnitude of applied loads and the curing age of the cement-treated material. The hypothesis also suggests that Miner's rule could lead to unconservative predictions of fatigue life under random loading conditions.

Fatigue Life of Metallic Materials under Varying Repeated Stresses of Two Different Stress Waves

The fatigue tests under varying repeated stresses, which consisted of two different bending stresses (the first and second stress waves), were carried out by the specially designed testing machine, using notched specimens of 0.22 and 0.61 per cent carbon steels and super duralumin. These test results were compared with the calculated ones which predict the fatigue life of metallic materials subjected to such varying repeated stresses. And the following are concluded : (1) The analytical fatigue life of 0.22 per cent carbon steel and super duralumin under the varying repeated stresses has a good agreement with the test results. (2) In the case of 0.61 per cent carbon steel, the second stress wave has no influence on the fatigue life, when the ratio of the second stress wave to the first stress wave, λ, is smaller than about 0.8, and the analytical fatigue lives have a good agreement with the test results, only when λ is greater than about 0.8.

繰返変動応力に対する疲労強度

As is generally the case, a large number of machine members are subjected to repeated stresses of varying amplitude under the mean stress level which is varying continuously, and some parts are subjected to occasional overstresses (the stresses over the fatigue limit under constant stress amplitude). It is important in both academic and practical sense to estimate the fatigue strength, i.e. the fatigue life and the fatigue limit of metallic members under the stress condition like this.A number of workers have investigated this problem, but none have established a rational formula which predicts the fatigue life and the fatigue limit of metal under varying repeated stresses. For the accurate determination of the fatigue strength in these cases, the new criterion and formula have been established by employing the idea of the fatigue damage and the stress history.The functions of fatigue damage and stress history of metallic materials were established in this paper from the experimental investigations, considering the phenomena of work hardening caused by the repetition of the microscopic plastic strains of metals. The formulas which give the analytical S-N curves and the calculating methods to predict the fatigue lives and fatigue limits of metallic members under alternating stresses of varying amplitude, were obtained by employing the functions of fatigue damage.The fatigue test results under alternating stresses of varying amplitude carried out by Nishihara and Yamada, and those by Dolan and others, were analyzed by applying the calculating methods described.And it has been made clear that the analytical fatigue lives and fatigue limits under alternating stresses of varying amplitude had a good agreement with the test results. So it is concluded that the fatigue strength of metals subjected to varying repeated stresses like this can be determined by the analysis introduced in this paper.