Variable Amplitude Load Sequences Research Articles

LOAD INTERACTION EFFECTS DURING FATIGUE CRACK GROWTH UNDER VARIABLE AMPLITUDE LOADING—A LITERATURE REVIEW. PART I: EMPIRICAL TRENDS

A summary is given of reported trends in fatigue crack growth observed in variable amplitude fatigue tests on metallic materials, specifically on steels, under both simple and complex load histories. The effects of load variables, specimen geometry, material properties, microstructure and environment are considered. Attention is given to the threshold behaviour and small crack effects. The reviewed data suggest that, depending on a particular combination of load parameters, material, geometry and environment, variable amplitude load sequences of the same type can produce either retardation or acceleration in fatigue crack growth.

Transient fatigue-crack growth behavior following variable-amplitude loading in a monolithic silicon nitride ceramic

Transient cyclic fatigue-crack growth behavior following variable-amplitude loading sequences has been investigated in a hot-pressed, monolithic Si 3N 4 ceramic (NTK EC-141), for which fracture toughness (resistance-curve) and cyclic fatigue properties are well characterized. Following rapid changes in the applied stress-intensity range, Δ K, during both low-high and high-low block-loading sequences, transients were observed in the subsequent cyclic crack-growth rates, but only under specific conditions. When K min was suddenly varied with K max held constant, no transients were observed. However, when the load changes involved both Δ K and K max, subtle transients were detected in the form of retardations following high-low, and accelerations following low-high block-loading sequences. The transient growth rates were typically a factor of ∼2 different from the steady-state (baseline) growth rates for load excursions of ∼30–40%, and lasted for several hundred microns of crack extension before a steady-state growth rate was achieved. Such behavior is rationalized in terms of grain-bridging mechanisms active in the crack wake.

Variable-amplitude fatigue testing of a high hardness armour steel

This study investigated the effect of variable amplitude load sequences on the fatigue behaviour of a high hardness steel similar to that used in the manufacture of light armoured millitary vehicles. Fatigue testing was undertaken on specimens, notched in the L-T and T-L orientations, using service load spectra obtained from an army vehicle subjected to different road conditions. A three stage computer program was used to convert the raw stress data and generate variable amplitude analog signals for testing on a servo-hydraulic test machine. The study showed that the crack growth rate was greater in T-L notch orientation when compared with the L-T notch orientation. Of the two variable load spectra used, crack growth rate was greater under cross country spectrum compared with growth rate under rough spectrum. This paper describes the above results in terms of the inter-relationship between crack orientation, load spectra and the role of microstructure in influencing the crack growth process.

Application of a closure model to predict crack growth in three engine disc materials

This paper describes the crack-growth predictions made in an AGARD Structures and Materials Panel, Engine Disc Cooperative Programme [1, 2] using the life-prediction code FASTRAN II. The objective of the program was to compare various methods of modeling crack-growth behavior in engine disc materials. The experimental database included three titanium alloys (Ti-6A1-4V, IMI-685 and Ti-17) and two crack configurations (compact and corner-crack tension). Crack-growth-rate data were provided under constant-amplitude loading (R=0.1 and 0.7) and under a repeated spike overload sequence. Participants were to use these data and mechanical properties of the materials to determine the crack-growth-rate properties to be used in their analysis method. The crack-growth properties were then used to predict the number of cycles required to grow a crack from an initial size to a final size under three simple variable-amplitude load sequences and under four complex Turbistan sequences. Predicted results from FASTRAN II agreed well with the test data with an overall mean of 1.01 and standard deviation of 0.31 for all materials and loading conditions.

A process for improving the fatigue crack growth resistance: Bharti, A. and Saxena, V.K. Patent GB 2257163, 6 Jan 1993

Transient cyclic fatigue-crack growth behavior following variable-amplitude loading sequences has been investigated in a hot-pressed, monolithic Si3N4 ceramic (NTK EC-141), for which fracture toughness (resistance-curve) and cyclic fatigue properties are well characterized. Following rapid changes in the applied stress-intensity range, ΔK, during both low-high and high-low block-loading sequences, transients were observed in the subsequent cyclic crack-growth rates, but only under specific conditions. When Kmin was suddenly varied with Kmax held constant, no transients were observed. However, when the load changes involved both ΔK and Kmax, subtle transients were detected in the form of retardations following high-low, and accelerations following low-high block-loading sequences. The transient growth rates were typically a factor of ∼2 different from the steady-state (baseline) growth rates for load excursions of ∼30–40%, and lasted for several hundred microns of crack extension before a steady-state growth rate was achieved. Such behavior is rationalized in terms of grain-bridging mechanisms active in the crack wake.

Fatigue design of welded structures: Maddox, S.J. Proc. Conf. Engineering Design in Welded Constructions (Conception et calcul pour les constructions soudées), Madrid, Spain, 7–8 Sept. 1992 pp. 31–58

Transient cyclic fatigue-crack growth behavior following variable-amplitude loading sequences has been investigated in a hot-pressed, monolithic Si3N4 ceramic (NTK EC-141), for which fracture toughness (resistance-curve) and cyclic fatigue properties are well characterized. Following rapid changes in the applied stress-intensity range, ΔK, during both low-high and high-low block-loading sequences, transients were observed in the subsequent cyclic crack-growth rates, but only under specific conditions. When Kmin was suddenly varied with Kmax held constant, no transients were observed. However, when the load changes involved both ΔK and Kmax, subtle transients were detected in the form of retardations following high-low, and accelerations following low-high block-loading sequences. The transient growth rates were typically a factor of ∼2 different from the steady-state (baseline) growth rates for load excursions of ∼30–40%, and lasted for several hundred microns of crack extension before a steady-state growth rate was achieved. Such behavior is rationalized in terms of grain-bridging mechanisms active in the crack wake.

Strength, fracture, and fatigue behavior of advanced high-temperature intermetallics reinforced with ductile phases: Soboyejo, W.O., Rao, K.T.V., Ritchie, R.O. and Sastry, S.M.L. Metall. Trans. A (Mar. 1993) 24A (3), 585–600

Transient cyclic fatigue-crack growth behavior following variable-amplitude loading sequences has been investigated in a hot-pressed, monolithic Si3N4 ceramic (NTK EC-141), for which fracture toughness (resistance-curve) and cyclic fatigue properties are well characterized. Following rapid changes in the applied stress-intensity range, ΔK, during both low-high and high-low block-loading sequences, transients were observed in the subsequent cyclic crack-growth rates, but only under specific conditions. When Kmin was suddenly varied with Kmax held constant, no transients were observed. However, when the load changes involved both ΔK and Kmax, subtle transients were detected in the form of retardations following high-low, and accelerations following low-high block-loading sequences. The transient growth rates were typically a factor of ∼2 different from the steady-state (baseline) growth rates for load excursions of ∼30–40%, and lasted for several hundred microns of crack extension before a steady-state growth rate was achieved. Such behavior is rationalized in terms of grain-bridging mechanisms active in the crack wake.

CRACK PROPAGATION STUDIES ON Al 7475 ON THE BASIS OF CONSTANT AMPLITUDE AND SELECTIVE VARIABLE AMPLITUDE LOADING HISTORIES

Abstract— Fatigue crack propagation behaviour under variable amplitude loading is significantly influenced by load history induced sequence effects. Because the understanding of the causes of such sequence effects is still rather limited, crack propagation predictions become difficult. In the present investigation an exploratory test program with simple variable amplitude load sequences was performed, which was designed to improve the general understanding of sequence effects and to offer a basis both for an evaluation of existing crack propagation prediction models and for the development of improved methods.The investigations were performed on center cracked 8 mm thick Al 7475‐T7351 specimens. In the first part of the study constant amplitude tests were performed over a wide range of R‐ratios. The test results and the transition of the fracture mode during tests could be represented on the basis of the effective ΔK‐value.In the second part of the paper tests with simple systematically varied sequences of high loads, low loads and underloads were performed and the crack propagation behaviour was measured. In order to obtain detailed information on the crack propagation increments per cycle, fractographic analyses were also performed. It was found that also under variable amplitude loading the crack propagation approximately followed the effective ΔK‐values, except in one case.A linear (non‐interaction) prediction model could not describe the observed crack propagation behaviour. The results of the application of a model assuming a constant crack opening level are also given. Finally, some general viewpoints regarding further developments of models are outlined.

The Effects of Very Dry and Fully Water Saturated Air Environments on the Fatigue Life of D6AC Steel Under Constant and Variable Amplitude Loading Sequences

Abstract Axial load fatigue tests have been carried out on keyhole notched (Kt = 3.15) specimens of D6AC steel (UTS = 1600 MPa) in very dry air and in air fully saturated with water vapor under both constant-stress-amplitude and four-load-range program loading sequences. Estimates were made of the crack initiation lives (corresponding to cracks of 0.1 mm), crack propagation lives, and crack propagation rates under program loading from the “program markings” on the fracture surfaces. Under the program loading sequence used in this investigation, the total fatigue lives in a fully water saturated air environment were about 35% of those in very dry air—the crack initiation life was about half, and the propagation life to failure about one-third of that in dry air. Thus, the environment has a greater effect on the crack propagation phase than on the crack initiation phase of the fatigue process. Fatigue life predictions for program loading were made using the constant-stress-amplitude fatigue data and the sim...