Linkoping University, Department of Mechanical Engineering Division of Engineering aterials, S Linkoping, Sweden

Transcription

1 Torsion fatigue tests were carried out at constant torque amplitudes on the as-machined specimens, and on the as-machined and shot peened specimens. All fatigue tests were conducted on a servo-hydraulic machine designed for torsional loading. Two stress ratios R=-1 (reversed torsion, zero mean stress) and R=O (pulsating torsion, non zero mean stress) were appllied. The test frequency was about 10 HZ. Load cycling continued until an angular deflection of 1.2' was attained during a fatigue test and the corresponding Linkoping University, Department of Mechanical Engineering Division of Engineering aterials, S Linkoping, Sweden Abstnct Torsional fatigue behaviour of an aluminium alloy, AA 7010-T7651, in the shot peened and unpeened conditions have been studied. Distributions of residual compressive stresses with depth of a shot peened specimen have been determined by using x-ray diffraction technique and step-wise electropolishing. Some improvement in fatigue life may be realised by shot peening. Monotonic torsion test properties of the alloy have been evaluated. - Keywords Torsion fatigue, aluminium alloy, fracture mode, compressive stress, residual stress distribution. shot peening, fatigue life, residual Introduction It is well established that compressive residual stresses increase the resistance to fatigue failure, especially in hard materials. One way of introducing the compressive residual stresses is shot peening, i.e. balls made of steel, glass, ceramics etc. are thrown onto the work piece. Beneficial effect of shot peening on high strength aluminium alloys is well documented [I-51. However, there is no study known to. the authors which systematically investigate the effect of shot peening on torsional fatigue behaviour of aluminium alloys. Although, some work has been done on torsional fatigue of shot peened steels [6-81. The residual compressive stresses are of significance with respect to the fatigue life of a component only if they remain stable in service. The overal objective of this work is to quantitatively study the effect of resid ual stresses on the torsion fatigue behaviour. Also investigate the stability of the residu.al stresses during torsion cyclic loading. This is ongoing work, results from the fmt part are summarised in this paper. - Ex~erimental details Hour glass shaped (Fig. 1) specimens were made from an high strength aluminium alloy, AA 7010-T7651, based on the Al-Zn-Mg-Cu system. The specimens were machined from a 25 mm thick plate. The chemical composition of the alloy plate is shown in table 1. Table 1. Alloy composition (in weight percent)

2 MAT-TEC 91 I number of cycles were taken as a measure of fatigue life. The specimens in this study were shot peened to an Almen intensity of (mm) A using ceramic shots, Z 425 ( %, Si02 31 %). The nominal shot diameter was in the range of mm. The shot peening was carried out by Metal Improvement Company (Montargis). Residual stresses introduced by shot peening were measured by x-ray diffraction technique using CuKa radiation and employing sin2y method. These measurements were made at Saab Aircraft Division in Linkoping. The static torsion test data were determined from the applied torque (T) versus shear strain curves. In case of torsional loading, the maximum of the shear stress (T,) and the principal normal stresses (01 and 03) at k45o have the same magnitude (see Fig. 2). For the determination of shear strain (y), strain gauges with measuring grids at 245' to the specimen axis were mounted. The shear modulus (G) was determined from the followig fundamental relationships: Where Ro is the radius of the specimen. During the static torsion tests angle of twist between the grips was also measured as afunction of applied torque. Rollinq direction _

3 SHOT PEENING Fig. 2 State of stress in torsion. Results and Discussion Static Tests Fig. 3, shows an example of applied torque versus shear strain (a) and applied iorque versus angle of twist (b) curves obtained during the monotonic torsion testing. The static test data based on the average of three tests are summarised in Table 2. Both nominal shear stress value and shear mess corrected for non-linear stress distribution resulting from yielding are shown. The correction for yielding was made using an equation according to Nadai [9]... Nominal value of stress, Stress corrected for yielding Table 2 Monotonic torsion test data Shear strain (%) Angle of twist 0 (deg) (a) (b) Fig. 3 Static torsion test curves of AA 7010-T7651 alloy specimen, Torque (Nm) versus shear strain (%) (a) and Torque (Nm) versus angle of twist (I (deg) (b).

4 A change in the fatigue-fracture mode was observed. In the short-lifehigh-stress regime the fracture mode was of shear type (mode I1 on the surface and mode 111in the interior). The shear fracture was normally along the radial planes for stress ratio R=-1 and along longitudinal planes for stress ratio R=O. In the long-lifebow-stress regime the fracture mode was of tensile type (mode I). The tensile fracture was on a plane normal to the principal tensile stress, i.e. at 45' to the specimen axis which results in helical failures. The shot peening affected the shift in fracture mode by suppressing the tensile fracture further towards the longer fatigue lives. This change was most obvious for the R=-1 loading condition. Development of cracks on the specimen surfaces fatigued at the various maximum shear stress levels is sketched in'fiij. 5. T-TEC 9 1 Fatigue Tests Torsional fatigue tests results for the shot peened specimens and unpeened specimens are shown in Fig. 4 as shear stress amplitude versus number of cycles to failure. The both type of specimens were tested at the two stress ratios R=-1 and R=O. Within the linear elastic loading range, the nominal alternating shear stress (za) was calculated from the alternating torque (Ta) and using eq. (2). In the case of plastic deformation the alternating shear stress was calculated using eq. (4) and the applied torque amplitude. A significant improvement in the fatigue life of shot peened specimens over unpeened specimens can be seen in Fig. 4. Shot peened (stress ratio RE-1) 1 0 Unpeened (stress ratio R=-1) A shot peened (stress ratio R=O) 0 Unpeened (stress Ratio R-0) Cycles to failure (N) Fig. 4 S-N relationships o f AA 7010-'I7651 alloy under torsion loading in the shot peened and unpeened conditions (stress ratios R=-1 and R=O).

5 SHOT PEENING1 u Unpeened specimens Shot peened specimens,, T = 300,250 r, = ,, T = 300 T,, = zma, = 200,150 *,ax = 250,200,150 - Specimen axis Fig. 5 Torsional fatigue crack orientation for the various maximum shear stress loading. Residual stresses induced by shot peening were measured using x-ray diffraction technique. The distributions of residual compressive stresses with depth of a shot peened specimen for the axial, 45' and tangential directions are shown in Fig. 6. Residual compressive stresses are present to a depth of about 0.25 mm. Note: No corrections are made for the influence of stress gradient and of removed layer on measured stress profiles. Changes in the surface residual stress state after torsion fatigue at the various nominal shear stress amplitudes and after specified number of loading cycles have been monitored and extended results will be published somewhere else. A Axial 0 45" 0 Tangential I 1 I I I I I Depth below surface (pm) Fig. 6 Residual stress-depth profiles of a shot peened AA 7010-T7651 alloy specimen.

6 f Summary and Conclusions - Monotonic and cyclic behaviour of an aluminium alloy AA 7010-T7651 have been' studied under to&ional loading. Observation of failure modes have revealed that at short fatigue lives the fracture mode was of shear type while at longer lives the fracture mode was tensile. The distributions of residual compressive stresses induced by shot peening have been determined. - Some improvement in fatigue life can be achieved by shot peening. Acknowledgements This work has been supported.as part of COST 506 projects - Industrial Applications of Light Metal Alloys. The authors would like to thank Dr. B. Jaensson, Saab Aircraft Division, for the measurement of residual stresses. The technical assistance of Mr. B. Skoog is gratefully acknowledged. References a G. S. Was and R. M. Pelloux, Metall. Trans. A, 1979, Vol. 10A, pp T. Hirsch, 0. Vohringer, E. Macherauch, Proc. 2nd Int. Conf. on-shot Peening, Chicago, May, 1984, pp , H. 0. Fuchs, ed. A. Niku-Lari, D. Gillereau, Proc. 2nd Int. Conf. on Shot Peening, Chicago, May, 1984, pp , H. 0. Fuchs, ed. B. Jaensson, Advances in Surface Treatments, Vol. 3, A. Niku-Lari, ed., Pergamon Press, 1986, pp Y. Mutoh, G. H. Fair, B. Noble and R. B. Waterhouse, Fatigue Fract. Engng. Mater. Struc., Vol. 10, No. 4, pp , A. Bignonnet, Shot Peening, DGM, 1987, pp H. Jiawen, Wu Yusheng, Yu Yonghe, J. F. Flavenot, Shot Peening, DGM, 1987, pp M. Desvignes, PhD thesis, ENSAM, Paris, A. Nadai, Theory of Flow and Fracture of Solids, Vol. 1, McGraw-Hill, New York, 1950, p. 349.