A.K Vasudevan

Analysis of overload effects and related phenomena

Overloads and underloads perturb steady state fatigue crack growth conditions and affect the growth rates by retarding or accelerating the growth. Clear understanding of these transient effects is important for the reliable life prediction of a component subjected to random loads. The overload effects have predominately been attributed to either plasticity induced crack closure behind the crack tip, residual stresses ahead of the crack tip, or a combination of both. These effects are critically examined in the context of the Unified Approach proposed by the authors. Recent experimental and analytical evaluation of crack closure has confirmed its negligible contribution to crack growth and has demonstrated that changes in the stresses ahead of the crack tip are more important than closure behind the crack tip. It is shown that the overload effects and other transient effects arise due to perturbation of the stresses ahead of the crack tip, and these can be accounted for by the two parametric approach emphasized in the unified theory. It is shown that related phenomena including the role of Kmax, the existence of propagation threshold Kpr, and effects of overloads on Kpr and Kmax etc, are all accounted for by the Unified Approach.

Short crack growth and internal stresses

Abstract Short crack growth behavior has been examined using the two parametric approach developed earlier by the authors. It is commonly accepted that there is a lack of similitude in the description of the short crack growth behavior. Contrary to this understanding, it is shown that there is no anomaly in the short crack growth behavior. The apparent anomaly arises because of ignoring: (1) the second parameter associated with the threshold K ∗ max ; and (2) the existence of internal stresses in the crack tip field where the short cracks nucleate and grow. In the case of short cracks nucleating from notches the internal stresses can be pre-existing or can be generated in situ if they form at a free surface. In the latter case, formation of intrusions and extrusions at the persistent slip bands are the precursors for the crack nucleation providing the necessary internal stresses. Thus short cracks grow under a total force, consisting of both internal and applied stresses, satisfying the same two thresholds for long cracks. Examples are provided from the literature to illustrate the concepts. It is shown that the internal stresses decrease as a short crack grows out of the existing stress field. These internal stresses can be predicted using elastic or elastic-plastic continuum approximations. Application of this concept is extended to understand the role of: (1) residual stresses; and (2) the transformation induced internal stresses on the crack growth. Based on the analysis, we restate the similitude concept as: equal crack tip forces result in equal crack growth rates for the same crack growth mechanism, provided all the contributing forces are taken into consideration.

Near threshold fatigue crack growth in aluminium alloys at low and ultrasonic frequency: Influences of specimen thickness, strain rate, slip behaviour and air humidity

Abstract Near threshold fatigue crack growth in ambient air and in vacuum has been studied with ultrasonic fatigue testing equipment (cycling frequency 20 kHz) and with servo-hydraulic equipment (20 Hz). Aluminium alloy 2024-T3 and 7075, heat treated to promote planar slip (7075-UA) and homogeneous slip (7075-OA), respectively, have been tested at load ratio R=-1. In vacuum, no strain rate influences were found and similar crack growth rates were observed at both frequencies. Specimen thickness does not influence the threshold stress intensity amplitude (Kmax,th) determined at 3.5×10–13 m/cycle in vacuum whereas crack growth resistance increases with decreasing specimen thickness at higher growth rates. Kmax,th determined at 10-10 m/cycle in ambient air are similar at 20 Hz and 20 kHz and are 45–55% of those measured in vacuum. In ambient air and at higher stress intensities, fatigue cracks may propagate by a factor of 5–50 faster at the lower frequency. In vacuum, 7075-UA shows better crack growth properties than 7075-OA, more pronounced at higher growth rates. In ambient air, the planar slip materials show a plateau like regime in the (Δa/ΔN vs Kmax)-curves, where growth rates are hardly affected by the stress intensity.

Role of microstructures on the growth of long fatigue cracks

Abstract An analysis is presented to understand the role of microstructures on the two crack growth driving force parameters, K ∗ max and Δ K ∗ th , without invoking the extrinsic crack closure concepts. Microstructural variables considered are: grain size, precipitates and stacking fault energy. It is shown that K ∗ max is strongly affected by the scale of the microstructure, such as grain size or precipitate spacing. For each case, the mode of slip deformation and environment affects the fatigue resistance as represented by K ∗ max . However, the microstructures seem to have a smaller effect on Δ K ∗ th . Also, the enhanced planarity of slip from the reduction in stacking fault energy has a pronounced effect on K ∗ max when compared with the materials deforming under homogeneous slip.

Near threshold fatigue crack growth at positive load ratio in aluminium alloys at low and ultrasonic frequency: influences of strain rate, slip behaviour and air humidity

Abstract Fatigue crack growth near threshold has been studied in 2024-T3 and in over-aged 7075 (7075-OA) using servo-hydraulic equipment (cycling frequency 20 Hz) and ultrasonic equipment (20 kHz). Experiments have been performed at load ratio R =0.05 and R =0.5 in ambient air and in vacuum. Below 10 −9 m/cycle, fatigue crack growth in vacuum is not influenced by the cycling frequency and no strain rate effects are found. Air humidity accelerates near threshold fatigue crack growth, and threshold stress intensities at 10 −10 m/cycle in ambient air are approximately 50–60% of those measured in vacuum. Similar threshold stress intensities at 20 Hz and 20 kHz, however, higher crack growth rates at the lower frequency at stress intensities above threshold were found in ambient air, where the frequency influence was most pronounced for fully reversed loading. Near threshold 2024-T3 shows planar slip behaviour and crystallographic (stage I-like) fatigue crack growth in vacuum, whereas stage II fatigue crack growth is found in ambient air. 7075-OA shows homogeneous slip properties and stage II fatigue crack growth in both environments. No apparent influences of load ratio and cycling frequency on fracture surfaces produced at very low growth rates are visible.

Analysis of fatigue crack growth under compression–compression loading

Abstract It has been suggested that two driving forces ΔK and Kmax mainly govern the overall fatigue crack growth behavior of any material. This basic concept has been used to explain the damage in the tension–tension and tension–compression regime in a self-consistent manner. In the present paper, this concept has been extended to analyze the compression–compression fatigue behavior. The analysis indicates that to characterize accurately the entire crack growth region of loading from R 0, one must include both the internal stresses and the applied stresses. It is observed that there is an unique internal stress gradient that can be represented for the entire regime of crack growth. The basic criteria for a crack advance would then be the total driving force of both ΔK and Kmax has to exceed their corresponding threshold values for a long crack. In certain cases, there can be a deviation from this uniqueness of the internal stress gradient due to additional contribution from plasticity.

Application of unified fatigue damage approach to compression–tension region

Abstract A Unified Approach to fatigue crack growth developed earlier is extended to include the fatigue behavior over the entire load ratio (R) regime, from compression–tension to tension–tension. This approach provides a self consistent description of cyclic damage using the two loading parameters, ΔK and Kmax, applicable over the entire crack growth regime. Crack growth behavior in the R>0 regime has been classified into five distinct classes. This classification is extended here to include the negative R-ratio region. When compression loads are included with the tension cycle, each classification is further divided into three additional subsets, reflecting a characteristic change in crack growth mechanisms. The current overall classification provides a methodology for understanding the combined effects of tension and compression loads on crack tip forces and their effect on material crack growth resistance. Like wise the role of environmental effect will also be altered in the presence of compressive forces.

Influence of microstructure and load ratio on fatigue threshold behavior in 7075 aluminum alloy

Abstract The influence of microstructure and load ratio on fatigue crack propagation in the near-threshold regime has been investigated on the 7075-aluminum alloy. The alloy heat treatment was varied to give underaged and overaged microstructures with approximately the same yield strength properties. Crack growth measurements were performed at constant load ratios between R =−1 and +0.8 in ambient air. The influence of load ratio is discussed in terms of deformation slip, crack branching and fracture mode. Vacuum data from the literature are compared with the current air results. For both environmental conditions, it was observed that the crack growth and threshold properties of the underaged alloy were better than the overaged alloy. The overall fatigue behavior is discussed in terms of the two parameter concept namely Δ K and K max .

The microstructure and environment influence on fatigue crack growth in 7049 aluminum alloy at different load ratios

Abstract The role of microstructure and environmental influence on fatigue crack has been investigated on a high strength 7049 aluminium alloy, artificially aged to underaged (UA) and overaged (OA) microstructures with approximately the same yield strength properties, but differing in the mode of slip deformation. The UA alloy deforms by planar slip while the OA alloy by wavy slip. Crack growth measurements were performed at constant load ratios, between 0.8 and −1, in ambient air and vacuum. The influence of load ratio is discussed in terms of slip deformation mechanisms, microstructure, and the influence of environment using the two intrinsic parameters, ΔK and Kmax.

Analysis of high temperature fatigue crack growth behavior

Abstract High temperature fatigue crack growth has been examined in the light of the new concepts developed by the authors. We observe that the high temperature crack growth behavior can be explained using the two intrinsic parameters ΔK and Kmax, without invoking crack closure concepts. The two-parameter requirement implies that two driving forces are required simultaneously to cause fatigue cracks to grow. This results in two thresholds that must be exceeded to initiate the growth. Of the two, the cyclic threshold part Δ K ∗ th is related to the cyclic plasticity, while the static threshold K ∗ max is related to the breaking of the crack tip bonds. It is experimentally observed that the latter is relatively more sensitive to temperature, crack tip environment and slip mode. With increasing test temperature, the cycle-dependent damage process becomes more time-dependent, with the effect that crack growth is dominated by Kmax. Thus, in all such fracture processes, whether it is an overload fracture or subcritical crack growth involving stress corrosion, sustained load, creep, fatigue or combinations thereof, Kmax (or an equivalent non-linear parameter such as Jmax) remains as one essential driving force contributing to the final material separation. Under fatigue conditions, cyclic amplitude ΔK (or an equivalent non-linear parameter like ΔJ) becomes the second necessary driving force needed to induce the characteristic cyclic damage for crack growth. Cyclic damage then reduces the role of Kmax required for crack growth at the expense of ΔK.