Variable Amplitude Block Loading Research Articles

Intralaminar crack growth rates of a glass fibre multiaxial laminate subjected to variable amplitude loading

The literature shows that variable amplitude loading can be much more severe than constant amplitude loading in a fatigue damage context. However, the underlying physical explanations for this are still unknown. Therefore, this paper investigates if the local energy release rate at each intralaminar crack tip in a tension/tension fatigue loaded multidirectional laminate can be used to determine the crack growth rate of these cracks subjected to variable amplitude loading. The local energy release rate is investigated through a developed finite element model in which any number of cracks can be included to account for their interaction. The position of the cracks is determined based on previous experimental work. The experimental results from the previous work are compared to the numerical results in this work. At similar crack density levels the energy release rates increase in the variable amplitude tests compared to constant amplitude tests, but not enough to significantly impact the crack growth rate. It is concluded that less than 5% of the increased crack density rate and crack growth rate caused by variable amplitude block loading is due to increases in local energy release rate. The primary reason is more likely found on a microstructural level.

Cycle-by-cycle crack closure measurements and fatigue crack growth in CT specimen made of 7075-T7351 aluminium

In this study, an advanced piezoelectric strain sensor is applied to measure the crack opening loads of a compact tension specimen with a fatigue crack under variable amplitude block loading. The crack opening loads are subsequently utilised to modify the stress intensity factor range by taking into account the proportion of each load cycle for which the fatigue crack remained closed. This effective stress intensity factor is applied to collapse the growth rate curve, which initially displays a strong dependence on stress ratio, into a single master curve where the only variable governing crack extension is the effective stress intensity factor. The findings of this study provide strong evidence for the crack closure concept, which is important for the development of algorithms which can compress/truncate large spectrums. These algorithms rely on a detailed understanding of how much the crack is open or closed during each load cycle. Furthermore, the results indicate that the piezoelectric sensor could be used to study crack closure under realistic spectrum loading conditions, which will be the subject of future investigations.

Fatigue life prediction of needled ceramic matrix composite under variable amplitude loading

The fatigue loading parameter-life (S-N) curve of the needled composite was proposed by constant amplitude fatigue tests first, and then the variable amplitude fatigue tests of needled C/SiC ceramic matrix composite were completed. Combined with the rainflow counting method, the Goodman line, the constant amplitude S-N curve and the linear damage accumulation rule, the fatigue life of the needled C/SiC composite was predicted under tensile variable amplitude block loading. According to the ratio of the width containing needled fiber bundle to the specimen width, the influence of needled fiber bundle on the fatigue life dispersion was quantitatively analyzed for the specimens with different fatigue lives.

Effect of variable amplitude block loading on intralaminar crack initiation and propagation in multidirectional GFRP laminate

Studies have shown that the fatigue life of multidirectional laminated fibre composites is reduced for variable amplitude block loading and random load spectra compared to constant amplitude loading. One of the hypotheses in the literature for this is a cycle mix effect that causes extra damage each time the load increases i.e. load amplitude or mean load. However, no explanation for this effect has been provided in the literature. This work aims at identifying the reasons for the reduced fatigue life of variable amplitude block loading tests compared to constant amplitude loading tests. This is done through white light imaging which is used to detect when intralaminar cracks occur in the off-axis layers and from this to determine crack density evolution, crack density rates, as well as initiation and propagation of intralaminar cracks contributing to the total crack density. Combined, these parameters show that there is a slow transition from a high crack density rate in the high load blocks to a lower crack density rate in the low load blocks. The crack density rate in the low load blocks in a variable amplitude loading test is higher than in a constant amplitude loading test at similar damage states. Finally, the high load blocks initiate new cracks in each block but they also cause all existing cracks to continue their propagation in subsequent low load blocks at increased rates. In conclusion, there is evidence that the block changes from high to low loads cause increased crack propagation in subsequent low load cycles, which is an important contribution to explain the damaging effect of cycle mixing.

Predicting mode II delamination growth of composite materials to assist simplification of fatigue spectra by truncating non-damaging cycles validated with experiment

A mode II delamination growth rate model was constructed using constant amplitude fatigue data from mode II displacement-controlled testing, for the purpose of failure prediction. Unidirectional precracked composite coupons were tested using a fully reversible three-point bending fixture. Using a series of constant amplitude delamination growth rate data, the growth of delamination propagation under variable amplitude block loading was predicted and compared with experimental results. The model was then utilised to assist simplification of the variable amplitude FALSTAFF fatigue spectra by identifying non-damaging cycles to truncate. A 77% reduction in fatigue spectra length was achieved and confirmed through experimental fatigue testing.

Fatigue Strength of HFMI-treated High-strength Steel Joints under Constant and Variable Amplitude Block Loading

Lightweight-design of welded high-strength steel structures in cyclic service necessitates the use of post-treatment methods like the high frequency mechanical impact treatment (HFMI). Service loads during operation mostly consist of variable amplitudes, whereat recommendations are only available for the as-welded condition. Therefore, this paper deals with the effect of variable amplitude block loading on the fatigue strength of HFMI-treated T-joints. An evaluation of the real damage sum exhibits characteristic distinctions to constant amplitude test results in regard to the base material strength. The application of an equivalent stress range method by nominal and effective notch stress approach is finally presented.

Analysis of Variable Amplitude Fatigue Data of the P355NL1 Steel Using the Effective Strain Damage Model

This paper proposes an analysis of variable amplitude fatigue data obtained for the P355NL1 steel, using a strain-based cumulative damage model. The fatigue data consist of constant and variable amplitude block loading, which was applied to both smooth and notched specimens, previously published by the authors. The strain-based cumulative damage model, which has been proposed by D.L. DuQuesnay, is based on the growth and closure mechanisms of microcracks. It incorporates a parameter termed net effective strain range, which is a function of the microcrack closure behavior and inherent ability to resist fatigue damage. A simplified version of the model is considered, which assumes crack closure at the lowest level for the entire spectrum and does not account for varying crack opening stresses. In general, the model produces conservative predictions within an accuracy range of two on lives, for both smooth and notched geometries, demonstrating the robustness of the model.

Multiaxial fatigue behavior of Ni-based superalloy GH4169 at 650 °C

The fatigue behavior of a Ni-based superalloy GH4169 was investigated using thin tubular specimens under multiaxial loading at 650 °C. Fatigue testing included multiaxial constant amplitude loading and variable amplitude block loading. The test results showed that the characteristics of cyclic hardening or softening depended on the strain loading paths, the sequence of the loading paths, and the loading parameters. Additional hardening due to non-proportional cyclic loading was still present at 650 °C, which caused the fatigue life to decrease. The sequence of the loading paths has an obvious influence on fatigue life, as shown by the increase in loading strain amplitude. Creep effect was observed in the multiaxial fatigue process at high temperature, and an increase in the number of cycles gradually strengthened the creep effect. Fractography showed intergranular fracture phenomena, which reflects the fatigue–creep behavior under non-proportional cyclic loading at high temperature.

Fatigue analysis of thin AlMgSi welded joints under constant and variable amplitude block loadings

This paper reports the fatigue behaviour of thin AlMgSi1 aluminium alloy weldments and the improvement in fatigue strength due to post-weld treatments. Several fatigue tests were performed using two distinct types of thin welded joints, T and single lap, manufactured using a 6xxx series aluminium alloy, and the efficiency of fatigue life improvement techniques, such as post-weld heat treatment and weld toe’s burr dressing, was studied. Both the post-weld heat treatment and the burr dressing produced a fatigue strength improvement. This fact is discussed, for both geometries, in the context of the initiation and propagation phases contribution to total fatigue life. Finally, several fatigue tests were performed under variable amplitude block loading using T6 post-weld heat-treated single lap joints, and the correspondent fatigue lives were compared with the predictions of Miner’s rule.

ANALYSIS OF LOAD INTERACTION EFFECT FOR WELDED DETAILS UNDER VARIABLE AMPLITUDE FATIGUE

Influence of load interaction on fatigue behavior of highway bridge components under service loading condition is to be clarified for better understanding of variable amplitude fatigue. Fatigue crack growth analyses considering crack closure effect are carried out on non-load-carrying cruciform joints under periodic overload (POL), variable amplitude block loading (BL) and variable amplitude (VA) loading. The analytical results are compared with those of the tests carried out in the past. Crack closure effect exists in most cases, and load interaction effect is significant under POL and BL. However, load interaction effect is insignificant under VA loading.

VARIABLE AMPLITUDE LOADING OF SMALL FATIGUE CRACKS IN 6261‐T6 ALUMINIUM ALLOY

Abstract— Constant and variable amplitude (VA) loading fatigue studies were carried out on a 6261 aluminium alloy using cylindrical plain hour‐glass specimens. Crack growth was monitored via surface replication using cellulose acetate.Crack growth results at constant amplitude loading show the typical intermittent high and low periods of growth rate associated with crack‐microstructure interactions. Acceleration in growth rate during an overload block depends on crack length and stress amplitude ratio. It appears to pass through a maximum at a crack length corresponding to the first microstructural barrier. Microstructural‐based modelling is therefore required for small fatigue cracks, rather than solely closure‐based modelling. The Navarro‐de los Rios model of short fatigue crack growth appears able to provide good indications of crack growth rates under VA block loading, and gives reasonable life predictions.For short cracks (surface length < 80 μm) and a small overload ratio (6.7%), crack growth may show severe retardation during the overload block. This is ascribed to crack tip blunting being more important than the increase in stresses when closure is low. It appears from a Miner's rule type exercise, that VA block loading has its major effect on growth at a surface crack length of 20 μm. This means that the crack initiation period cannot be ignored in life prediction models for small fatigue cracks.

FATIGUE CRACK GROWTH RATES OF STRUCTURAL STEELS UNDER CONSTANT AND VARIABLE AMPLITUDE BLOCK LOADING

Fatigue crack growth rates of structural steels (JIS SS 41, SM 50, SM 58 and HT 80) were measured under constant amplitude and variable amplitude block loadings. The measurement was carried out using Compact-type specimens according to the ASTM E 647-78T. The fatigue crack growth rates were measured in the range of ΔK between 10MPa√m and 35MPa√m. Within the test data, the crack growth rates of all four steels were in good agreement. The variable amplitude block loading was represented by an equivalent stress range computed by the root-mean-cube method, and crack growth rates under variable amplitude block loading were in good agreement with that under constant amplitude loading.

Ｅｌｂｅｒの式に基づく疲労き裂伝ぱのシミュレーション

A simpler model than the one which we reported in the previous paper has been proposed to economize the computer time. It is essentially based on the following three assumptions:(1) When the applied stress range is held constant, the effective stress range ratio U follows Elber's equation, i. e., U=0.5+0.4R, where R is the stress ratio.(2) When the applied stress varies, there appears a transition period during which the crack opening stress is affected by the previous load. This transition period continues until the cyclic plastic zone formed by the present stress range will pass over the cyclic plastic zone formed by the previous large stress range.(3) The rate of crack propagation is given by the equation da/dN=C(ΔKeff)m, where ΔKeff is the effective range of the stress intensity factor and C and m are the material constants.Based on this model, a computer simulation of fatigue crack propagation has been carried out. The summary of the results is as follows:(1) The model can express the retarding and accelerating effects which appear in the variable amplitude block loading tests. A series of simulation results are in good agreement with the experimental results.(2) The model can be applied directly to the simulation of the random loading tests.(3) The model can be applied to the practical problems from the viewpoint of the computer time.

Analysis of Fatigue-Crack Growth in a High-Strength Steel—Part II: Variable Amplitude Block Loading Effects

To investigate load sequencing effects in fatigue-crack growth, surface flaw or part-through crack (PTC) specimens of a high-strength pressure vessel steel were subjected to variable-amplitude cyclic loading. Blocks of different load sequences and stress-ratio (R) patterns and cyclic lengths were employed. Crack-growth rate (da/dN) data were analyzed within the framework of linear elastic fracture mechanics using the stress-intensity range parameter, ΔK. The da/dN data were found to correlate well with a seauence-independent formulation of ΔK derived using nominal mean stress, σm, for the loading blocks studied. However, it proved necessary to utilize the normalizing relationship discussed in “Analysis of Fatigue-Crack Growth in a High-Strength Steel—Part I: Stress Level and Stress Ratio Effects at Constant Amplitude”, to account for stress-ratio effects observed under the variable-amplitude block loading. Only the sequential block pattern of high-low-intermediate produced a significant crackgrowth retardation. Predictions made using the sequence-independent normalizing procedure compare favorably with those using the Willenborg model. It is concluded that useful predictions for structural life under variable-amplitude cyclic loading sequences can be made for a wide variety of high-strength pressure vessel applications using sequence-independent analysis procedures.