J. Solis Romero Instituto Tecnológico de Tlalnepantla, Tlalnepantla Edo. de Méx., 54070, México. Instituto Tecnológico y de Estudios Superiores de Monterrey (ITESM), campus Edo. de Méx., 52926, México.

Introduction

Engineering surface treatments such as shot peening have been known to provide a highly effective, versatile and relatively inexpensive method for reducing damage caused by fatigue in metallic materials. Today, the parameters that control the performance of shot peening, i.e. media, intensity, incidence angle and coverage, are better understood and the new designation, that of controlled shot peening (CSP), has emerged. Peening parameters and the induced changes on the surface and subsurface of a metallic component caused by a peening process are graphically illustrated in Fig. 1.
A common practice in shot peen forming within the aerospace industry is to peen the components to give the required geometry, then in areas of likely stress concentration and crack initiation, to peen again to saturation levels to maximise the fatigue resistance. Likely areas for this treatment are in the proximity of rivets, welds, and joins between structural sections (Fig. 2).
The work carried out in this project was concerned with Aluminium Alloy 2024-T351 (AA2024), a material commonly used in peen formed within the aerospace industry. The results of this project should provide information regarding the properties of shot peened components made from AA2024, and help quantify the effects of current peening treatments used, as well as investigating the effects of varying different peening parameters.

Duplex peening

The theory behind Duplex peening is that the two different stress distributions given with each shot will have a combined effect of inducing a thicker compressive layer, thus providing more opposition to crack propagation because of the increased time required for the crack to overcome the residually stressed layer, resulting in improved fatigue resistance (see Fig. 3).

Previous work investigating the effects of duplex, or double shot peening [1], concentrated on the maximisation of the surface residual stress for steel, which is desirable for increased fatigue life of vehicle components. However, little attention was given to the thickness of the residually stressed layer. By comparing specimens that have been shot peened using fine shot, coarse shot, and the duplex technique, this study aims to investigate any possible advantages that may be gained using this technique on AA2024.

Experimental Procedure

Details of test specimens and shot peening media
The specimen type used in this work was a standard coupon for the residual stress, hardness and roughness measurements as shown in Fig. 4. The existence of a fatigue curve prediction model will predict the S-N curve for AA2024, given the required residual stress, hardness and roughness parameters. The basic layout of the model is detailed in [2], together with the material parameters relevant to the fatigue life that exist in the model. The fatigue simulation is that of four-point bending, with stress ratio R=0.1. The model has been proven to show good correlation with experimental results [3], so it can be expected to give an accurate prediction of the likely fatigue performance of 4-point bending specimens with the properties that are input into the model. Bearing in mind the experimental scatter that would almost certainly occur, the model will serve as a good guide for what to expect. Two different sizes of shot media were used during the project, chosen because of their use by the aircraft industry, and following previous investigation into optimisation of the shot peening process [3-5].
The optimum configuration was identified as S110-200%-45°. This method of notation (A-B%-C°) is used as standard throughout this work, where A is the shot type, B is the percentage surface coverage, and C is the nozzle angle. Although this configuration was identified as optimal, no work was carried out, varying the coverage around 200% whilst keeping shot type and nozzle angle constant.  Therefore the work in the first part of this project involved optimisation of the coverage around this level. Three different coverage percentages were used; 100%, 200% and 400%.  The notations for each coupon are detailed as follows: Coupon 2: S110-100%-45°; Coupon 3: S110-200%-45°; Coupon 4: S110-400%-45°.

Saturation peening
The coarsest shot available (S330) is used to represent the peen-forming process, and the finest grade (S110) was used for the saturation peening. Two different coverages were investigated for the peen-forming & saturation investigation:  50% and 100%.  Two coupons were used for each of the coverages, one for just the peening with S330, and one for the peening with S330 followed by saturation peening with S110.  The notations used for each coupon are detailed as follows: 50% Coverage: Coupon 5: S330-50%-90°; Coupon 6: S330-50%-90° + S110-200%-45°SAT*: 100% Coverage:  Coupon 7: S330-100%-90°; Coupon 8: S330-100%-90° + S110-200%-45°SAT*. From the previous optimisation work, 90° was found to be the best nozzle angle for S330, with regards to residual stress, hardness and surface roughness. 

Duplex peening
The method of duplex peening is to peen with two different types of shot to maximise the thickness of the residually stressed layer; coarse shot (S330) and fine shot (S110).  For each shot type, the peening parameters used were the optimum configurations identified for each shot: 100% coverage and 90° nozzle angle for S330, and 200% and 45° for S110. The order of peening was also reversed on a second coupon to investigate any different effects this may give with regards to the measured properties of a peened specimen.  Coupon 9: S110-200%-45° + S330-100%-90°; Coupon 10: S330-100%-90° + S110-200%-45°. It can be seen that coupon 10 undergoes identical treatment to coupon 8 in the saturation peening work, therefore there was no need to repeat the process.

Results

The results can be divided into three sections: coverage optimisation, saturation peening, and duplex peening.

Unpeened control coupon
For an unpeened coupon, to act as a control for the experiments, the residual stress and hardness profiles can be seen in Fig 5. The resulting S-N curves for the prediction using the experimental results, and the theoretical parameters contained in the model are shown in figure 6.

Coverage Optimisation
Three coupons were used in this part of the project (coupons 2-4 ); S110 at 45°, peened to 100%, 200%, and 400% coverage.

Saturation Peening
Four coupons (5-8) were used in this part of the study.

Duplex Peening
Two coupons were used in this part, coupons 9 and 10.

The results of the work carried out in this project give a valuable insight into the effects of the various shot peening operations used and optimum peening conditions, the results help quantify the effects that the commonly used practice of saturation peening may have on structural components. Finally, the work has looked at a new technique for maximising fatigue resistance, duplex peening, to investigate the benefits it may bring to components made from AA2024.

Unpeened control coupon
Compared to all the other fatigue curves for the peened coupons, the improvements are apparent, with greater cycles to failure at each stress, and much higher fatigue limits.  Some of the curves are shown in Fig. 7 for comparison. The optimum performance is given by S110-200%-45o, however an interesting observation is the curve for S330-100%-90o, which at higher stress levels has reduced fatigue performance compared to the unpeened coupon. This can be attributed to the large value of the maximum residual stress depth and small value of residual stress penetration, together with the high Rt and Kt values. Thus at high stress levels the crack is more likely to initiate, and travel quickly due to the longer distance to reach the high compressive stress layer.  At low stresses, however the overall greater levels of residual stress compared to the unpeened coupon lead to a much-improved performance, with a much higher fatigue limit; 280 MPa compared to 220 MPa.

Conclusions

200% is predicted as the optimal coverage for shot peening with S110 at 45o, this gives superior results over all the other peening variable combinations carried out in this project.  This conclusion supports previous optimisation work [4, 6], with regards to the maximisation of fatigue life. The industrial process of saturation peening with fine shot, in areas of likely stress concentration, after peen-forming with the standard coarse shot, brings definite improvements in fatigue resistance compared to just the peen formed condition, seemingly irrespective of the coverage used in the actual peen-forming stage.
Duplex peening does bring improvement in fatigue performance compared to just peening with coarse shot, however these results show that peening with fine shot only (S110-200%-45o) is still optimal in comparison.
The level of the peak compressive stress is the prime factor in increasing fatigue performance; however other residual stress profile parameters, hardness parameters, and surface roughness parameters also have some effect.
Alternative methods for residual stress calculation should be investigated to try and quantify any inaccuracy in the incremental hole-drilling technique used in conjunction with the differential method of stress calculation. The integral method is suggested, as it has been previously shown to give good agreement with other techniques such as X-Ray and neutron diffraction.
Shot peening research and associated measurement techniques are subject to error, as are all experimental processes. Repeated testing is recommended in all stages, especially residual stress measurement which is a highly important factor with regards to the fatigue life.

References

1. Ishigami, H., et al., A Study on stress, reflection and double shot peening to increase compressive residual stress. Fatigue and Fracture of Engineering Materials and Structures, 2000. 23: p. 959-963.
2. de.los.Rios, E.R., M. Trull, and A. Levers, Modelling fatigue crack growth in shot peened components of Al 2024-T351. Fatigue and Fracture of Engineering Materials and Structures, 2000(23): p. 209-216.
3. Solis-Romero, J., Optimisation of the shot peening process in terms of fatigue resistance, in Mechanical Engineering. 2002, The University of Sheffield: Sheffield, U.K.
4. Solis-Romero, J., et al. Optimisation of the shot peening in terms of the fatigue resistance. in Shot Peening: Present and Future. The 7th Conference on Shot Peening (ICSP-7). 1999. Warsaw, Poland.: Institute of Precision Mechanics <IMP>.
5. Solis-Romero, J., et al. Toward the optimisation of the shot peening process in terms of fatigue resistence of the 2024-T351 and 7150-T651 aluminium alloys. in Surface treatment V: Computer methods and experimental measurements for surface treatment effects. 2001. Spain: WIT press, Southampton, U.K.
6. Karuppanan, S., A theoretical and experimental investigation into the development of coverage in shot peening, in Mechanical Engineering. 2000, The University of Sheffield: Sheffield, U.K.

Instituto Tecnológico de Tlalnepantla, Tlalnepantla Edo. de Méx., 54070
México. Instituto Tecnológico y de Estudios Superiores de Monterrey (ITESM), campus Edo. de Méx., 52926, México.
E-mail: solis_jose@infosel.net.mx