M.T. Yu

The effect of overloads on threshold and crack closure

Abstract The effect of tensile and compressive overloads on the threshold stress intensity level and crack closure behaviour of one aluminium alloy and three steels has been investigated. A few tensile overloads significantly decreased the crack propagation rate and increased the threshold stress intensity. An initially decreased and then increased opening stress was mostly responsible for the delayed retardation and crack arrest. Intermittant compressive overloads significantly accelerated the crack propagation and decreased the threshold stress intensity which was a function of the frequency of overloading. The opening stress was decreased to below zero after a large compressive peak load, and it took >105 cycles for the opening stress to return to its stable level. During this period an initially high crack propagation rate also gradually decreased to the stable value.

The effective stress range as a mean stress parameter

Abstract This investigation examines the hypothesis that variations in the crack opening stress level of short cracks can account for the observed variations in fatigue strength with mean stress under constant-amplitude cyclic loading. Support for the hypothesis is provided by the experimental data generated for a 2024-T351 aluminium alloy and a SAE 1045 steel. Based on the observed variations in crack opening stress with mean stress an effective stress range is postulated as a mean stress parameter. The effective stress range successfully reduces the fatigue data for all levels of mean stress to a unique fatigue-life curve for each material.

Notch fatigue behaviour of SAE1045 steel

Abstract The effects of notch diameter, material heat treatment and applied stress ratio on the notch fatigue behaviour of an SAE1045 steel were investigated. The fatigue notch factor increases with notch diameter for sharp notches, but it decreases with notch diameter for blunt notches. The notch sensitivity of the quenched-tempered martensitic steels was higher than that of the as-received ferritic-pearlitic steel, especially for small notches. However, the notch sensitivity of the quenched-tempered materials was not significantly affected by the tempering temperature. The fatigue notch factor was found to be higher for R = −1 tests than for R = 0 tests for a given notch size in the as-received material. Short crack fracture mechanics was applied to predict the fatigue life for sharp and blunt notches. A reasonable agreement between the experimental and predicted data was observed.

The effect of compressive peak stress on fatigue behaviour

Abstract The effect of compressive peak stress on the maximum stress at the endurance limit, crack propagation rate, threshold stress intensity and crack closure was studied in a laboratory environment using two steels (SAE1045 and SAE1010) and two aluminium alloys (2024-T351 and 7075-T651). As the compressive peak stress, S cp, was increased in magnitude, the maximum stress at the endurance limit, S fa, decreased linearly. Compression-compression cycling did not initiate any cracks in a centre-notched SAE1010 steel specimen but initiated cracks, which gradually became non-propagating, in the notched 2024-T351 aluminium alloy specimens. In compression-tension tests, the crack propagation rate increased, and the threshold and the crack opening stress intensities decreased linearly with increasing compressive peak stress. During compression-compression cycling the load/displacement curves were not linear, indicating that the crack was not fully closed.

Effects of compression and compressive overloads on the fatigue behavior of a 2024-T351 aluminum alloy and a SAE 1045 steel

Smooth cylindrical specimens of 2024-T351 aluminum alloy and SAE 1045 steel were tested under constant amplitude cycling to study the effect of compressive stress on fatigue life. The results show a reduction in the fatigue life at constant maximum stress as the compressive portion of the stress cycle is increased. Tests performed to study the effect of a periodic compressive overload on the order of the yield strength show an increasing reduction in fatigue life at constant maximum stress as the frequency of application of the overloads is increased. The compressive overload cycle significantly increases the damage done by subsequent smaller cycles, including those far below the constant amplitude fatigue limit. The results indicate that the present techniques used for damage summation for variable amplitude service histories may give grossly unconservative fatigue life predictions, especially for histories containing large compressive load cycles accompanied by a relatively large number of small cycles. It is suggested that conservative life predictions for such histories can be made using the stress-life curve from constant amplitude tests with compression on the order of the yield stress accompanying every cycle.

The effect of microstructure on the mechanical behaviour of a low carbon, low alloy steel

Abstract The effect of microstructure on the mechanical behaviour of a low carbon, low alloy steel was studied. The hot-rolled ferrite-pearlite showed low monotonic and cyclic strengths with high ductility in terms of true fracture strain and a high shreshold stress intensity. The quenched and tempered low carbon martensite showed high monotonic and cyclic strengths and high ductility. However, the threshold stress intensity was significantly lower than that of the ferrite-pearlite. Both the strength and threshold stress intensity of an austempered bainite and a duplex ferrite-martensite are greater than those of the hot-rolled ferrite-pearlite. At a same strength level, the ductility and the threshold stress intensity of the low carbon martensite are higher than those of a medium carbon martensite.

MECHANISMS AND MECHANICS OF FATIGUE CRACK INITIATION AND GROWTH

ABSTRACT This paper reviews the mechanisms by which a fatigue crack grows and the metallurgical and loading variables that influence these mechanisms. Fatigue crack growth regimes for constant amplitude fatigue are characterized as a metallurgically short crack regime, a mechanically short crack regime and a long crack regime. In the metallurgically short crack regime the crack is short compared to metallurgical variables, such as the grain size, and growth is strongly affected by microstructure. In the second regime the crack is long enough that the resistance to growth by microstructural barriers is averaged out, but it is not long enough for crack closure due to the plastic wake of the crack to have reached a stable level. During this stage the crack grows faster than a long crack having a similar linear elastic stress intensity. In the final long crack regime crack closure has reached a steady state level and the crack growth rate can be characterized by the range of stress intensity and the stress ratio independent of crack length. The effect of stress or stress intensity level, mean stress or stress ratio, microstructure and crack closure on crack growth in each of these regimes is examined. The manner in which crack growth resistance to short cracks develops is shown to lead to short cracks in sharp notches which stop propagating at low stress levels, and to reduced fatigue concentration factors for small notches and defects. Work illustrating some of the ways in which variable amplitude loading changes crack closure, thresholds, and other characteristics of fatigue behaviour is reviewed. Particular attention is paid to closure changes that cause current fatigue analysis procedures to give unconservative life predictions.

Deformation and fatigue behaviour of a cold-rolled SAE 1010 steel

Abstract An SAE 1010 low-carbon steel was cold rolled to 22, 56 and 76% thickness reductions. Monotonic tensile tests, smooth and notched specimen fatigue tests and crack propagation tests were performed. The effect of loading direction on fatigue behaviour was examined. The monotonic and cyclic yield strengths increased and the ductility decreased as the degree of cold rolling increased. The increase in yield strenth and the decrease in ductility were more pronounced in the transverse direction than in the longitudinal direction. Under strain control, the fatigue life at high strain amplitudes was lower for loading in the transverse direction than for loading in the longitudinal direction. At low strain amplitudes the fatigue lives in both directions were approximately equal. The notched specimen fatigue strength was only slightly increased by cold rolling since two opposing factors, the smooth specimen fatigue strength and the notch sensitivity, were both increased by cold rolling. The threshold stress intensity decreased and the crack propagation rate, at a given stress intensity, increased as the degree of cold rolling increased.

Effect of carbon content and microstructure on near threshold crack propagation

Abstract Near threshold crack propagation rate was measured for SAE1010, SAE8620, SAE1045 and SAE1080 steels in various conditions. The crack propagation rate was lower and the threshold stress intensity was higher for the steels in the as-received hot rolled condition than for steels in the cold rolled or quenched and tempered conditions. Neither the as-received ferrite-pearlite nor the quenched and tempered martensite showed significant changes in crack propagation rate or threshold stress intensity with carbon content. Tempering temperature had a pronounced effect on tensile strength and ductility; however its effect on threshold stress intensity was small. There is no simple relationship between tensile properties and threshold stress intensity. The threshold stress intensity of an upper bainite was close to that of the as received ferrite-pearlite, while that of a lower bainite was close to that of the quenched and tempered martensite.

THE EFFECTS OF OVERLOADS AND THEIR INTERACTION ON A MILD STEEL

ABSTRACT Tensile and compressive overload tests were performed on a CSA G40.21 mild steel. A tensile overload (OR=2) arrested a crack at the near threshold region, whereas, at a high stress intensity only crack growth retardation, was observed. A compressive overload significantly decreased crack closure and caused a dormant crack to begin growing again. A compressive overload applied before a tensile overload had little effect on the tensile overload retardation. However, a compressive overload applied immediately after a tensile overload completely eliminated the tensile overload retardation. Periodic tensile overloads decreased crack propagation rates and increased the threshold stress intensity factor. Periodic compressive overloads increased crack propagation rates and decreased the threshold stress intensity factor. The crack propagation rate under periodic tensile-compressive overloads was higher than that under periodic compressivetensile overloads. The effect of both tensile and compressive overloads was most pronounced at low stress intensities and disappeared at high stress intensities. Fracture surface examination showed stretched tearing edges resulting from periodic tensile overloads, and flattened facets due to periodic compressive overloads, which are probably related to roughness induced crack closure.