Relaxation of Shot Peening Residual Stresses in the 7050-T7451 Aluminium Alloy after Heat Cycles for Adhesive Bonding

Author:  Roth, Wortman
Source:  Conf Proc: ICSP-8 Sept. 16-20, 2002 Garmisch-Partenkirchen, Germany
Doc ID:  2002029
Year of Publication:  2002
Authors Martin Roth, C. Wortman, Department of National Defence, Ottawa, Ontario, Canada Introduction Shot peening, cold working of holes, and adhesive bonding of metallic or composite doublers are oftern used singly or in combination in order to increase the fatigue life of critical aerospace components. A doubler, applied to a highly loaded area, provides an alternate load path and can reduce the propensity for fatigue cracking. Adhesive bonding of a doubler requires a number of steps involving heat and pressure cycles where the temperature of the part might exceed the maximum temperature to which shot peened aluminium alloys should be subjected to, because of the possibility of thermal stress relaxation. The maximum exposure temperature ranges from 90 C to 121 C for aluminium alloys, depending on the shot peening specification used. [1] Slow relaxation of shot peening stresses in the 7075-T6 aluminium alloy heated to 225 F (107 C) has been reported. [2] The following steps have been followed by the Canadian Forces to successfully adhesively bond a doubler to an aluminium alloy part, starting from a coating free surface, whether shot peened or not: -grit blasting using aluminium oxide (220 grit) to produce a chemically active surface for the silane treatment and a rougher surface with greater surface area for the bond, -application, and curing of a silane coupling agent (30 minutes at 175 F (79 C)), -application and curing of a corrosion inhibiting primer (i.e. Cytec BR127) (1 hour at 250 F (121 C) preferred, or 30 minutes at 250 F (121 C)), -application of the adhesive (i.e. Cytec FM73M) and doubler, and curing of adhesive (2 hours at 250 F (121 C) preferred, or 8 hours at 185 F (85 C) at a pressure of 14.7 psi (101 kPa)). [3] The preferred cycles are preferred for bonding reasons only and not based on the effects on a shot peened article. The Aerospace Materials Specification AMS-S-13165 states in section 3.3.10: "When peened parts are heated after shot peening ... the temperature employed shall be limited as follows: Aluminum alloy parts 200 F (90 C) maximum". [4] This is qualified by section 6.13: "Processing or service temperatures of shot peened parts shall be limited to the temperatures in 3.3.10 unless test data for specific applications support the satisfactory use of higher temperatures". An experimental program was set up by the Canadian Forces to investigate: -the effects of the low and high temperature cycles used for adhesive bonding on the relaxation of the shot peening residual stresses in the 7050-T7451 aluminium alloy, -the effects of the grit blasting, -the effects of a 2 hour exposure at 163 C, as some areas of a part can reach that temperature in order to achieve the desired bondline temperature of 121 C, -and the effects of the low and high temperature cycles on fatigue crack initiation of shot peened 7050-T7451 aluminium alloy under a Canadian CF188 (F/A-18) spectrum loading.

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