Investigation of shot peening as a forming process for aircraft wing skins

The technique of generating curvatures in thin sheets by shot peening is well established for forming airplane wing skins to controlled contours. Success used to depend on the peening machine operator’s skills; therefore, a systematic experimental investigation was conducted into the effects of workpiece geometry and process variables on resulting curvatures. Most of the tests were conducted on 6 x 24 in. (152 x 610 mm) 2024-T3 and 7075-T6 aluminum alloy sheets of up to 0.5 in. (12.7 mm) thickness. Shot of 0.023 to 0.066 in. (0.6 to 1.7 mm) diam was used in a wheeltype machine at speeds up to 250 ft/s (80 m/s). The radius of curvature Rof a peened specimen can be expressed as R = A + B/Swhere Sis the shot density (weight per unit area) and Aand Bare parameters which are functions of other process variables. Specimens with a length-to-width ratio of L/W =1 have equal stiffness in the Land Wdirections but developed a predominant curvature the direction of which was affected by secondary variables such as the rolling direction in the sheet. However, transverse and longitudinal radii of curvature were almost identical in thicker specimens of L/W =1/4 and could be used to predict the transverse curvatures of long workpieces peened under production conditions, as confirmed by productionscale experiments.