Stress relaxation testing of service exposed IN738 for creep strength evaluation

Standard size and miniature specimens of IN738 were taken from a service ex posed turbine blade and vane for comparative stress relaxation testing at 8 00C, 850C, and 900C. Base darn taken from root section material were used t o construct stress versus creep rare parametric curves which could be used directly in design. Up to five decades in creep rates were obtained at each temperature from rests lasting less than one day, The darn were also prese nted in the form of stress versus predicted times to 0.5 percent creep whic h compared well with available long rime creep data. Differences were noted in specimens taken from different locations in the airfoil regions which p robably resulted from differences in grain size or orientation, Based on th ese measurements it was concluded that there was no significant effect of s ection size on creep strength as defined by this test, and that the alloy w as quite insensitive to prior deformation and thermal exposures. A life man agement procedure. using a combination of creep strength evaluation based o n the stress relaxation test and a separate fracture evaluation measurement , is outlined in which end of useful life is defined in terms of minimum ac ceptable performance levels.