Direct viscoelastic strain
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(3) Heat the indented coupons in a constant temperaturecontrolled environment. In this study, a constant temperature oil bath was used, and the samples were protected in plastic bags. After heat treatment, cool coupons by immersion in ice water. (4) After annealing, sputter coat with gold to a thickness of approximately 300 ~k. The gold coating is required to provide the highly reflective specimens necessary for successful interference microscopy. (5) Photograph indentations with a two-beam interference microscope. In this study, a coherent sodium lamp (A = 589 nm) was used for illumination. (6) Throughout this test procedure, store the samples in a refrigerator between postindentation steps to minimize room-temperature strain relaxation. Strain relaxation data were obtained on both surfaces of an experimental prepainted 1005 sheet steel shown schematically in Figure 1. The steel was coated with a multilayered black coating on one side and a multilayered white coating on the other. The black side consisted of a 10-/xm-thick pigmented black urethane primer, a 15-/xm-thick nonpigmented black polyester, and a 13-/zm-thick nonpigmented clear fluorocarbon topcoat. The white side consisted of an identical black urethane primer layer and an 18-/zm-thick pigmented white polyester topcoat. It should be noted that the presence of the pigmentation in the white layer significantly alters the relaxation behavior, as shown subsequently. Furthermore, the initial indentation depth in the white layer is less than for the black layer, approximately 14.1 /zm vs 15.6 ~m. This indicates that the TiO2 particles in the white layer provide an increment of strengthening. The samples were indented, as shown schematically in Figure 1, and an example of the resulting interference micrograph for the black side in the as-indented condition is shown in Figure 2. The fringe pattern indicates two distinct regions: a square portion, which delineates material in contact with the indenter, and a circular portion, which indicates distortion in the surrounding matrix. Since illumination was achieved via a coherent sodium lamp, the spacing of two bright or dark fringes
(1) Obtain coupons of the prepainted sheet material. (2) Indent each coupon surface with a DPH hardness indenter operating at 100 grams of force for 10 seconds. At this load, observation with a scanning electron microscope (SEM) indicated that indentations were sufficiently deep without splits.
P.J. KYED, Graduate Research Associate, and D.K. MATLOCK, Armco Foundation Fogarty Professor, are with the Advanced Steel Processing and Products Research Center, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript submitted February 5, 1993. 1678--VOLUME 24A, JULY 1993
Fig. 1 --Schematic of the experimental prepainted sheet steel. METALLURGICAL TRANSACTIONS A
s o l i d 14-6'8] is assumed to apply to the coatings shown in Figure 1 with strain directly proportional to the indentation depth, d (i.e., e a d). With these assumptions, upon r
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