Vol. 10
July Issue
Year 2009

Science Update

in Vol. 10 - July Issue - Year 2009
A Note On the Detrimental Effects of Peened Aluminium Alloys

Figure 1: SEM fractograph for 2024-T351 Aluminum Alloy peened to particular conditions showing initiation site (indicated by the square frame) at stress level of smax= 300 MPa; R=0.1 and constant amplitude sinusoidal loading with a frequency of 20-25 Hz at ambient temperature. The magnified picture (right) depicts the initiation site right below a peening indentation as indicated by the white arrow.

Figure 2: SEM fractograph for 2024-T351 Aluminum Alloy peened to particular conditions showing initiation site (indicated by the square frame) at stress level of smax= 300 MPa; R=0.1 and constant amplitude sinusoidal loading with a frequency of 20-25 Hz at ambient temperature. This picture illustrates a subsurface crack initiation indicated by the arrow, which is located at 150-200

Table 1: Influence of the hardness of each alloy on the plastic deformation, defined in terms of the indentation radius (a) to the shot size (D) and its corresponding depth (h). It should be noted that the indentation radius was measured after one pass of the peening stream on particular coupons (designed exclusively for this purpose). The coverage in this case is less than 100%.

Figure 3: Shot penetration effect on the fatigue resistance for the AA

Figure 4: Effect of surface roughness on the crack arrest conditions for an AA 2024-T351 peened at predicted optimum conditions (S110, 200%, 45

The Applied Stress

There is an apparent influence of the level of applied stress on the site of crack initiation for peened specimens, i.e. the higher the applied stress the closer the crack initiation is to the surface. This behaviour has been largely reported both for ferrous and non-ferrous peened metals [1-3]. The accepted explanation of surface crack initiation for shot peening treated materials is that cyclic deformation relaxes the surface residual stresses [4]. This adverse effect is known to readily take place in the low cyclic fatigue regime, where the magnitude of the applied stresses lies closely to or even above the yield strength of the cyclically loaded material. The magnitude of the plastic strain amplitude governs the extent of macro residual stress relaxation in fatigue-loaded materials, as concluded by Schulze, et al. [5]. Furthermore, concerning the evolution of compressive residual stress (CRS) during fatigue, Schütz [6] observed that steel was much more stable than aluminium, which is consistent with the general assertion that the stability of CRS depends on the hardness of the material.

As a result, the magnitude of residual stress decreases when the applied fatigue stress increases. High tensile stresses are set up at stress concentration points, such as at the root of the indentations resulting from the peening treatment, or at any other irregularity existing immediately below the surface as illustrated in Fig. 1. Consequently, fatigue cracks are initiated and propagated in surfaces without the beneficial effect of CRS. In short, the fatigue limit for surface initiation should be determined by the magnitudes of the surface residual stress and that of the stress concentration of the applied stress.

In subsurface initiation (Fig. 2), the fatigue crack forms where the outer compressive stress field is balanced by the residual tensile stress field. The transition between the compressive and tensile residual stress fields and the depth of the plastically deformed layer roughly corresponds to the crack initiation sites. Cracks, however, may occur due to inner defects (inclusions, pores, etc.). Although crack origin is located below the surface, they soon break through the surface and become relatively larger cracks. Accordingly, the fatigue life of the subsurface crack becomes equal to that of a surface crack.

Surface Stress Concentration

Some researchers like Wohlfahrt [7] 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 conditions implemented in the fatigue stage has to be given in line with the above. From experimental work carried out [8], the use of the finest shot did provide the best fatigue performance in both 2024-T351 and 7150-T351 AA’s. It therefore, suggested that both AA’s are 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:

1. Broken or highly deformed media and embedded particles, which give rise to deeper surface penetration (table 6.1.1) and hence, more likely to cause crack nucleation by stress concentration.
2. 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.
3. 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 [9,10] 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.

With the aim of further support for these statements, a sensitivity analysis was undertaken to assess the effect of surface degradation (in terms of the surface penetration, termed h in table 1) on the fatigue resistance by using the following peening conditions: (a) S110 (shot type), 200% (coverage), 90º (incidence angle); (b) S330, 200%, 30º; (c) S330, 200%, 90º. Fatigue testing was done using a maximum stress of 300MPa under similar experimental conditions as indicated. The effect of increasing the penetration depth was found to be more detrimental on the fatigue resistance as seen in Fig. 3 (a,b). Similar findings were reported recently by Dörr et al. on peened aluminium and magnesium alloys [11] [255].

Theoretical Stress Concentration

The effect of surface roughness considered as micro-notches (dents) is introduced utilising the Navarro & Vallellano’s formulation [256], which provide a solution to a notch fatigue crack growing through the microstructure of the material. According to their work, the nominal stress in a notched member is given by “formula can not be displayed online”

where “xxxx” is the applied stress, “xxxx”    
is the distribution of the nominal stress ahead of the notch root as a function of the distance from the notch i, mapped as i=2a/D, and Zi is the fatigue notch factor given by “formula can not be displayed online”

and i=1,3,5,…
The parameters  ”xxxx”  and  “xxxx” represent in a dimensionless form the notch depth ? and the notch half width ß . The parameter D represents the distance between two successive barriers. In the case where grain boundaries are considered as being the dominant barrier, D is regarded as the grain diameter.
According to “formula can not be displayed online”, the effect of both the crack closure stress and surface roughness on the ability for a peened component to arrest cracks is given by “formula can not be displayed online”
From this later relationship, the boundary conditions that would enable the residual stress to compensate totally for the notch effect (roughness caused by the shot indents) on the crack arrest capacity of the peened material is given by, “formula can not be displayed online”

Application of the above equation renders the effect of roughness (notch) on the crack arrest capacity for a peened 2024-T351 AA under optimum conditions as shown in Fig. 4.
Examination under arrest conditions of the peened materials depicted in Fig. 4 revealed a discernable influence of the stress concentration on each peened material in relation to the stress required by a crack to overcome the i-th barrier in the notch zone. A more pronounced effect of Kt in the AA 2024-T351 is predicted, as shown by the lower decrease in percentage terms of the peening arrest capacity when incorporating a notch effect. The crack arrest capacity for the peened AA 2024-T351 to optimum conditions decreases rapidly as Kt increases. Strictly speaking, for a=25µm and ß=75µm, i.e. Kt= 1.33 (experimentally determined for AA 2024-T351, S110, 200%, 45º) no appreciable peening improvement is predicted, as the calculated inherent arrest capacity of the unpeened material is higher. Thus, the stress gradient associated with the dents at the first barrier (grain size) dominates over the closure stress effect. However, the stress gradient due to the notch will decrease with distance from the notch tip, and therefore, there will be a barrier further away which would either delay the growth of a crack or arrest it. This may be the reason for achieving a marginal fatigue improvement in the peened AA 2024-T351 when compared to the unpeened material, particularly at low fatigue applied stresses.

Concluding Remarks

These results support the previously experimental observations regarding the sensitivity analysis, namely, that the effect of increasing the penetration depth or varying the notch geometry was found to be detrimental to the fatigue resistance, as seen in Fig. 3(a,b).
The strain hardening effect has been intentionally neglected because: a) the induced strain hardening developed by shot peening is usually overcome by the intrinsic hardening of cyclic harden materials, b) strain hardening is commonly limited to a depth close to the free surface and c) the above results are provided as a conservative analysis.


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Author: J. Solis-Romero (PhD)