Characterization of residual stress relaxation in welded steel plate using TAP-NDE and wavelets

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to digital@library.tamu.edu, referencing the URI of the item.Includes bibliographical references (leaves 110-115).Issued also on microfiche from Lange Micrographics.This thesis presents the characterization of residual stress relaxation in a welded ASTM 1018 steel plate by using the Thermo-Acousto-Photonic Nondestructive Evaluation (TAP-NDE) technique and the Gabor Wavelet Transform (GWT) which together produce effective wave dispersion characteristics of the digitized surface ultrasonic wave at desired resolution. The welded steel plate was cured in an oven at certain set temperatures and time durations to accelerate the relaxation of thermally induced surface residual stresses. Immediately after each oven-curing procedure, laser-generated surface acoustic waves (SAW) were initiated in the welded steel plate using a Q-switched Nd:YAG (neodymium-doped yttrium aluminum garnet) pulsed laser and then acquired using a fiber-optic guided interferometer known as the Fiber-Tip interferometer (FTI) running on a continuous-wave He-Ne laser. Residual stress relaxation was quantitatively established by investigating the wave dispersion of the laser-induced ultrasonic wave initiated to interrogate the welded plate. Deviation of dispersion behavior identified in the joint time-frequency domain using the GWT and obtained at different stages of residual stress relaxation using the laser induced surface acoustic wave were compared with residual stress readings measured using strain gauges. The established correspondence allows the residual stress to be quantified as a function of wave dispersion. Two empirical equations, one for the heat-affected zone and the other for the fusion zone, correlate surface residual stresses with the temporal arrival of certain frequency components. This research confirms the feasibility and applicability of the TAP-NDE technique to the determination of heat-induced residual stresses in steel plates. In summary, once the relationship of wave dispersion characteristics and the corresponding stress level is established, the surface stress states of any engineering material that support the propagation of broadband ultrasound can be remotely determined using the non-contact, non-invasive TAP-NDE technique