The strain gage blind hole-drilling technique may be used to determine residual stresses at and below the surface of components. In this paper, the hole-drilling analysis methodology for thick plates is reviewed, and experimental data are used to evaluate the methodology and to assess its applicability to thin plates. Data on the effects of gage pattern, surface preparation, hole spacing, hole eccentricity, and stress level are also presented.
A failure analysis was performed to establish the cause and prevent the recurrence of a 2014-T6 aluminum compressor blade failure in a large NASA-Ames Research Center wind tunnel. A metallurgical failure analysis showed that a 0.13 mm (0.005 in.) deep scratch in the shank of the blade had acted as an initiation site for a fatigue crack. The crack subsequently grew by Stage II fatigue across most of the blade's 22.9-cm (9-in.) base transition region before final fracture occurred by unstable crack growth. Extensive fractographic characterization of both the blade and laboratory specimens was performed using a scanning electron microscope. Surface morphology, including fatigue striations, was used to estimate the vibration stress levels, the time to grow the crack, and the magnitude of the local mean stress present.
A structural failure investigation was conducted to determine the resonance vibration condition which made the blade sensitive to a scratch. Studies of the blade natural frequencies were made to determine the mode responsible for the vibrations which grew the crack. The effect of the crack on modal frequency was measured and computed. Tunnel measurements were used to determine the vibration resonance tunnel speed, magnitude, and source. These data were used to compare with the crack growth data from the fractographic studies and to make recommendations to avoid future blade failures.
Summary
The effective elastic parameter (EEP) was determined for three material and surface conditions of ASTM 516 grade 70 steel and found to differ by as much as 19 percent. This difference occurred between data for plate with a machined surface and that for a chemically milled surface.
In these tests, the same EEP appeared to apply for specimens both before and after exposure to yield level strains. This suggests that the X‐ray calibration data may be used to determine stresses in welds exposed to yield‐level residual strains as long as the appropriate EEP values are used for the weld material and surface condition.
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