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Vol. 6
July Issue
Year 2005
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Science Update


in Vol. 6 - July Issue - Year 2005
Importance of Coverage in Shot Peening on Fatigue of Al 2024-T4






Authors: Prof. Lothar Wagner (left) and Dipl.-Ing. Tomasz Ludian (right), Institute of Materials Science and Technology Clausthal University of Technology, Germany


a) LC (16 %)


b) IC (53 %)


c) FC (100 %)
Fig 1: Examples of coverage degrees in Al 2024-T4


Fig. 2: Development of coverage in Al 2024-T4


Fig. 3: Residual stress-depth profiles after shot peening











Fig. 4: Roughness profiles of the various surface conditions


Fig. 5: Degree of coverage vs. fatigue life in Al 2024-T4


Fig. 6: Crack nucleation at individual impacts (LC)


Fig. 7: S-N curves of the various surface conditions


The new multifunctional shot peening machine at TU Clausthal

It is often anticipated that coverage of 100 % or more in shot peened surfaces is required to achieve full benefit in terms of near-surface high dislocation densities, residual compressive stresses and fatigue performance [1, 2]. However, prolonging the exposure time over 100 % can drastically decrease the fatigue life as shown in work on titanium alloys [3]. Since the residual stress field caused by a single impact is much greater than the size of the indentation, it can also be argued that 100 % coverage may not be required [4, 5]. On the other hand, there are shot peening specifications that require even more than 100 %, e.g., 200 % coverage. The present work was aimed at evaluating the effect of coverage in shot peening on the fatigue performance of the well known aircraft alloy Al 2024-T4. Particular emphasis was given to the effect of coverage on the resistance to fatigue crack nucleation.

Experimental

The age-hardening aluminum alloy Al 2024 was tested in the solution treated and naturally aged (T4) condition. The tensile properties  were as follows: Young’s modulus E = 74 GPa, yield stress 0.2 = 415 MPa, tensile strength UTS = 595 MPa, tensile elongation El = 18.9 % and true fracture strain F = 0.23.
Shot peening was done using a gravity induction system and spherically conditioned cut wire (SCCW14) having an average shot size of 0.36 mm. Small slits were applied in the shot feed system to reduce the mass flow to 45 g/min in order to enable low coverage peening at reasonable exposure times with sufficient reproducibility. The Almen intensity was measured on conventional A-type and N-type Almen test strips. In addition, strips of Al 2024-T4 having thicknesses of 1.3 mm and 2.0 mm were machined with the same dimensions in length and width as the Almen strips. These aluminum strips were taken to enable measurements of the degrees of coverage as well as deflections as a function of exposure time on the test material itself. Quantitative metallography (Image C analysis) was used to determine coverage degrees.
The surface roughness was measured by a profilometer.
Shot peening-induced residual stresses were evaluated with the incremental hole drilling technique as described elsewhere [6].
Fatigue tests were performed on hour-glass shaped specimens with a gage diameter of 3.6 mm in rotating beam loading (R = -1) at a frequency of 100 Hz. Before shot peening to various degrees of coverage, all specimens were electropolished (100 m removed from surface). The electrolytically polished condition (EP) was also taken as the baseline to which the various shot peened conditions (SP) will be compared.

Results and Discussion

Examples of various coverage degrees of shot peened Al 2024-T4 are illustrated in Fig. 1a-c. In each case, optical micrographs (left side) are shown together with image C converted pictures (right side).
Peening with SCCW14 for exposure times of 4, 20 and 80 s resulted in coverage degrees of 16 % (low coverage, LC), 53 % (intermediate coverage, IC) and 100 % (full coverage, FC), respectively. The development of coverage with exposure time is shown in Fig. 2.
The coverage first strongly increases with time and then gradually levels off when approaching 100 % (Fig. 2). From parallel work [7] it is known that this saturation in Al 2024-T4 is reached faster than in the conventional Almen strips, this result  also reported in [8].
The shot peening-induced residual stress-depth profiles in Al 2024-T4 are given in Fig. 3 comparing the results for the conditions LC, IC and FC.
With an increase in the degree of coverage from LC to IC and FC, the magnitude of the near-surface residual compressive stresses clearly increases. Although not demonstrated in the results from the (integrating) incremental hole drilling procedure (Fig. 3), the local residual stresses in LC and IC may largely vary due to incomplete coverage [9, 10]. The surface roughness profiles (Fig. 4) clearly reflect the various degrees of coverage. Starting with the very low roughness of EP, an increase in coverage from LC to IC increases the frequency of occurrence of roughness peaks. At full coverage (FC), the roughness profile tends to saturate with regard to maximum roughness and frequency of occurrence as also reported in [11, 12].
The effect of the degree of coverage in shot peening on the fatigue life at stress amplitudes of 300, 250 and 238 MPa is illustrated in Fig. 5. Starting with EP, the fatigue life at the various stress amplitudes first decreased at low coverage followed by marked improvements as the coverage increased. No further improvements in fatigue life were observed by increasing the coverage beyond 100 % (Fig. 5).
While the beneficial effect of increasing coverage on fatigue life was anticipated, the significant decrease in fatigue life from EP to LC (Fig. 5) was unexpected.
Optical microscopy revealed that in LC, fatigue cracks nucleated even earlier than in EP. These cracks nucleated at single impacts and then propagated through un-peened regions (Fig. 6).
From the results shown in Fig. 6, the conditions EP, LC, IC and FC were taken for further fatigue testing. The S-N curves of these conditions are illustrated in Fig. 7.
The fatigue performance of LC is by far the worst exhibiting a drop in the 107 cycles fatigue strength from 225 MPa (EP) to 150 MPa. This poor fatigue performance of LC (Fig. 7) can be explained firstly by the stress concentration at the fairly irregular indentations caused by the SCCW 14 shot which obviously favors early crack nucleation (Fig. 6). Secondly, the residual compressive stresses in LC are not sufficiently high (Fig. 3) and are not homogeneously distributed to markedly retard early crack growth. Comparing the fatigue behavior of LC with EP, it is seen that the HCF performance is more detrimentally affected than the fatigue life at high or intermediate stress amplitudes (Fig. 7), this again indicating the loss in resistance to fatigue crack nucleation resulting from the stress concentration at the single impacts. Neglecting residual stress effects in LC and taking the ratio a107 (EP)/a107 (LC), this stress concentration in fatigue amounts to 1.33.
By increasing the coverage degree from LC to IC, the residual compressive stresses increase (Fig. 3) and likely become more homogeneously distributed. Thus, micro-crack growth can now effectively be hindered. As a result, the negative effect of early crack nucleation is more than compensated resulting in improvements in fatigue life relative to EP.
The retardation effect of residual compressive stresses on micro-crack growth is fully developed in FC resulting in the most marked overall improvements in life (Fig. 6) and highest fatigue strength (Fig. 7). Similar results are reported in [1]. It is argued that this improvement in fatigue performance is mainly caused by the full development of residual compressive stress fields in the surface area of FC.

Summary

The results of the present investigation on Al 2024-T4 can be summarized as follows:
Shot peening to low (16%) coverage can markedly decrease the HCF strength. This effect is caused by early crack nucleation at individual shot indentations. Residual compressive stresses are barely developed and thus, can hardly hinder micro-crack growth from the surface into the interior.
Shot peening to intermediate (53%) coverage already leads to improvements in fatigue performance. The development of residual compressive stresses is able to significantly hinder micro-crack growth which over compensates for   early crack nucleation and increases the fatigue life.
Shot peening to full (100%) coverage leads to a full development of residual compressive stresses. Therefore, micro-crack growth from the surface to the interior is most markedly suppressed resulting in highest improvements in fatigue life and fatigue strength.

Acknowledgements

Thanks are due to Dr. M. Hilpert of Otto Fuchs Metallwerke for providing the Al 2024 alloy. The experimental assistance of O. Fischer in residual stress measurements, M. Pohl, A. Allwardt and H. Boeckels in shot peening and mechanical testing is gratefully acknowledged.

References

1. A. Tange and H. Okada: Shot Peening and Coverage, Shot Peening (L. Wagner, ed.) Wiley-VCH (2003) 516.
2. K. Tosha, Y. Ueno and K. Iida: Effect of Shot Peening on Fatigue Strength of Phosphor Bronze C5191, Shot Peening (J. Champaigne, ed.) Proceedings of ICSP7 (1996) 344.
3. L. Wagner and G. Luetjering: Influence of the Shot Peening Parameters on the Surface Layer Properties and the Fatigue Life of Ti-6Al-4V, Shot Peening (H. O. Fuchs, ed.) ASPS (1984) 195.
4. P. S. Prevey and J. T. Cammett: The Effect of Shot Peening Coverage on Residual Stress, Cold Work and Fatigue in a Ni-Cr-Mo Low Alloy Steel, Shot Peening (L. Wagner, ed.) Wiley-VCH (2003) 295.
5. J. Cammett, N. Nayaraman and P. S. Prevey: The Effect of Shot Peening on Residual Stress, Cold Work and Elevated Temperature Fatigue in a Ni-Based Superalloy, Shot Peening (A. Niku Lari, ed.), 2005 (in press)
6. T. Schwarz and T. Kockelmann, VDI Report 940 (1992) 99 (in German).
7. T. Ludian and L. Wagner, Coverage effects in shot peening of Al 2024-T4, Shot Peening (A. Niku Lari, ed.), 2005 (in press)
8. S. Karuppanan, J. S. Romero, E. R. de los Rios, C. Rodopoulos and A. Levers: A Theoretical and Experimental Investigation into the Development of Coverage in Shot Peening, Shot Peening (L. Wagner, ed.) Wiley-VCH (2003) 101.
9. D. Kirk and R. C. Hollyoak: Relationship between Coverage and Surface Residual Stress, Shot Peening (A. Niku Lari, ed.), 2005 (in press)
10. S.A. Meguid and M. S. Klair: Finite Element Analysis into Incomplete Coverage in Shot Peening, , Shot Peening (H. O. Fuchs, ed.) ASPS (1984) 306.
11. W. Koehler and K.-P. Hornauer: Selected Examples on the Topography of Shot peened Metal Surfaces, Shot Peening (H. Wohlfahrt, R. Kopp and O. Voehringer, eds.) DGM (1987) 269.
12. W. Koehler: Influence of Shot Peening with Different Peening Material on the Stress Corrosion and Corrosion Fatigue of a Welded AlZnMg-Alloy, Shot Peening (H. O. Fuchs, ed.) ASPS (1984) 126.




Author: Prof. L. Wagner, Dipl-Ing. T. Ludian

For information:

Authors
Prof. L. Wagner
E-mail: lothar.wagner@tu-clausthal.de

Dipl-Ing. T. Ludian
E-mail: tomasz.ludian@tu-clausthal.de

Institute of Materials Science and Engineering
TU Clausthal, Agricolastrasse 6, Germany
Tel: +49.5323.72 2770, Fax: 72 2766