in Vol. 16 - March Issue - Year 2015
Crack Growth Behaviour Of 2024-T351 Peened Aluminium Alloy
Figure. 1. Axial loading, constant amplitude S-N curves for peened and unpeened specimens of AA 2024-T351 (a)
Table 1. Shot peening conditions applied to the AA under study
Figure 2. Crack propagation behaviour for peened and unpeened specimens at different stress levels
Table 2. Specimen testing conditions for crack propagation measurements
(a) N = 220 000 cycles
(b) N = 230 000 cycles
(c) N = 240 000 cycles Figure 3. Crack coalescence between cracks from each side of the specimen in unpeened specimen (
Figure 4. Crack growth rate vs. half-surface crack length
Controlled shot peening is a cold working treatment in which small balls (usually of cast steel but sometimes of ceramic glass, casting or cut steel wire) collide with the material surface to be treated at high speed. The particles plastically deform and indent the surface. Such inhomogeneous plastic deformation give rise to: (i) roughening of the surface, (ii) an increase in the near surface of dislocation density (work hardening), and (iii) the development of compressive residual stresses (CRS) below the surface. The latter is beneficial to fatigue as it hinders the initiation of fatigue cracks and can slow down the propagation of small cracks because the CRS act as a crack closure stress on the crack flanks. However, propagation of cracks may be accelerated by virtue of the stress concentration effect (roughened surface). Strain hardening, in turn, will retard the propagation of cracks by increasing the resistance to crack tip plastic deformation.
Based on experimental results, particular attention is paid to the role of the peening modifications on the propagation and non-propagation of fatigue cracks. In this respect, peening conditions were selected as the optimum and worst conditions to be implemented in a fatigue testing . Equal weight was given to the residual stresses, microhardness and surface roughness, i.e. a balancing exercise of the surface changes was undertaken. The selected peening conditions used in this study were as indicated in Table 1.
Conventional uniaxial tension-tension fatigue tests were carried out using hourglass specimens as described elsewhere . The results of fatigue tests conducted for the AA are presented in the form of Wöhler stress versus cycles to failure (S-N) curves over a wide range of applied stress in Fig. 1. The fatigue endurance is defined as the endurance stress at or below which a specimen can sustain cycling for up to 7x106 cycles without failing.
Trends observed in the fatigue life curves for AA12024 revealed that shot-peened specimens have a marginally superior life compared to those unpeened, specifically at an intermediate zone of the low- and high-cycle fatigue regions. It was evident; however, that peening did not impair the fatigue performance of the material. Optimum peening conditions results, on the whole, were found to be better than those given by the worst conditions. Furthermore, there was a discernable improvement in endurance by peened specimens when testing at stresses around the 300 MPa. The fatigue endurance for the unpeened material was approximately 190 MPa, whilst for the peened at worst and optimum conditions were 190 and 225 MPa respectively. No discernable difference in fatigue life was observed in the low-cyclic fatigue region.
2. Crack growth
Crack propagation measurements were carried out by taking plastic replicas of specimens tested under conditions described in Table 2. This choice was based on the S-N data, so that an appreciable change in crack propagation rates would be expected.
The growth behaviour of cracks from detection to failure was recorded for each of the replicated tests in terms of the number of cycles to initiation (Ni) and the number of cycles to failure (Nf). Crack length visible on replica is plotted as half-crack length. Fig. 2 shows two plots of the kind, half-crack length versus number of cycles, for each of the replicated tests.
Surface replicas revealed that dominant cracks usually initiated at a corner edge and grew along both sides (width and thickness). Fatal cracks formed within the gauge area for both AA¡¦s in all cases. Very few isolated initiation sites at each of the applied stress levels were observed in peened specimens, whilst an increased number of such sites were observed in unpeened specimens. However, it was observed that only one crack dominated fatigue life. Furthermore, as the test progressed, coalescence was often observed in both unpeened and peened specimens as shown in Fig. 3.
3. Crack growth rate
Crack propagation rate was calculated using the secant method from half-surface crack length measurements. The average growth rate between two crack length measurements of the same crack in two consecutive replicas, taken at known cycle lifetimes, is expressed as:
where ai, ai+1 are crack lengths at two successive stages, and Ni, Ni+1 are the number of cycles corresponding to crack lengths ai, ai+1, respectively. The mean growth rate
Plots of crack propagation rates against the mean crack length are given in Fig 4. Accelerations and decelerations were evident until steady fatigue crack growth was reached. Coalescence is manifested by sudden jumps in crack growth rates as shown in Fig 4 (a). In Fig. 4 (b), crack growth rate is approximately the same for both unpeened and peened specimens.
Some researchers like Wohlfahrt  have recognised that peening conditions for optimum fatigue strength should result from achieving a surface hardening as high as possible, compressive residual stresses as large as possible, and with a surface roughness as low as possible. Hence, selection of the optimum and worst conditions implemented in the fatigue stage were given in line with the above. From the overall fatigue results, the use of the finest shot did provide the best fatigue performance in AA 2024-T351. This suggests that this AA is likely to be roughness-sensitive or surface damage-sensitive. The consistent degradation of fatigue life as peening media size increases may be due to one or more of these factors:
Broken or highly deformed media and embedded particles, which give rise to deeper surface penetration and hence, more likely to cause crack nucleation by stress concentration.
Unevenness in the plastic deformation layer, 100% coverage with the finest shot is entirely impinged with some overlapping. Variations of the plastically deformed layer when peening with larger media size may be quantitatively greater than with smaller media.
The tendency of a larger media is to leave larger indentations and, thus, material around the indentation displaced a greater distance would indent to a greater depth and or induce impact extrusion of material parallel to the surface. It has been reported by many researchers [3, 4] that such surface damage known as peened surface extrusion folds (PSEF) or simply folds are responsible for acceleration of the crack nucleation by the stress concentration.
1. Effect Of The Shot Peening Process On The Surface Changes In The 2024-T351 Aluminium Alloys. Metal Finishing News. Vol. 9. March issue. 2008. pp. 58-61.
2. Wohlfahrt, H., (1987), "Practical aspects of the application of shot peening to improve the fatigue behaviour of metals and structural components". In: Shot Peening- Science-Technology-Application. Third International Conference on Shot Peening (ICSP-3). Germany. Eds. H. Wohlfahrt, R. Koop and O. Vohringer. Deutsche Gesellschaft fur Metallkunde e.V.: pp. 563-584.
3. Simpson, Roger S., 1984, "Development of a mathematical model for predicting the percentage fatigue life increase resulting from shot peened components, phase I." Airtech Precision Shot Peening, Inc. Flight dynamics laboratory. Air force wright aeronautical laboratories, air force systems command wright-patterson AFB, Ohio 45433. Final report for period September 1983- April 1984: pp. 1-37.
4. Mutoh, Y., Fair, G. H., Noble, B. and Waterhouse, R. B., (1987), "The effect of residual stresses induced by shot-peening on fatigue crack propagation in two high strength aluminium alloys". Fatigue and Fracture of Engineering Materials and Structures. Vol. 10, No. 4: pp. 261-272.
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