Fatigue Crack Growth Behavior in Dissimilar Metal Weldment of Stainless Steel and Carbon Steel
Constant amplitude fatigue crack growth (FCG) tests were performed on dissimilar metal welded plates of Type 316L Stainless Steel (SS) and IS 2062 Grade A Carbon steel (CS). The plates were welded by TIG welding using SS E309 as electrode. FCG tests were carried on the Side Edge Notch Tension (SENT) specimens of 5 mm thickness, with crack initiator (notch) at base metal region (BM), weld metal region (WM) and heat affected zones (HAZ). The tests were performed at a test frequency of 10 Hz and at load ratios (R) of 0.1 & 0.6. FCG rate was found to increase with stress ratio for weld metals and base metals, where as in case of HAZ, FCG rates were almost equal at high ΔK. FCG rate of HAZ of stainless steel was found to be lowest at low and high ΔK. At intermediate ΔK, WM showed the lowest FCG rate. CS showed higher crack growth rate at all ΔK. However, the scatter band of data was found to be narrow. Fracture toughness (Kc) was found to vary in different locations of weldments. Kc was found lowest for the weldment and highest for HAZ of stainless steel. A novel method of characterizing the FCG behavior using an Infrared thermography (IRT) camera was attempted. By monitoring the temperature rise at the fast moving crack tip region, the amount of plastic deformation was estimated. Keywords—Dissimilar metal weld, Fatigue Crack Growth,
Notch-root plastic response by temperature measurement
It is an experimental fact that the majority of the irreversible work of plastic deformation in metals is converted to heat. Under adiabatic conditions the spatial distribution of heating is indicative of the spatial distribution of plastic work. Tiny thermocouples welded directly to a metal surface were found to provide adequate measurement speed and sensitivity to detect the temperature change associated with plastic deformation. An array of thermocouples was used with some success in determining plastic-zone size. The heating response was used to study notch-root plastic deformation following overloads and following various post-overload heat treatments. The notch-root plastic response was correlated directly with the notch-root residual-stress level. The degree of stress relief as a function of time and temperature found in the notched members studied agrees well with predictions made using the Larson-Miller parameter and relaxation data on other steels. The deformation response in notched-member tests following stress relief is in qualitative agreement with that predicted by mechanics analysis of cracked bodies done by others.
The detection of fatigue damage by exoelectron emission
Plastic deformation of metal enhances the photoelectron emission producing so‐called exoelectrons In our experiments the metal surface is scanned by a small spot of ultraviolet light and the electron emission is recorded as a function of position of the light spot. Results for aluminum and steel demonstrate that fatigue deformation produces exoelectron emission after less than 1% of the fatigue life. The regions of emission are very localized; some are beyond the resolution of the apparatus (15 μm). The localized emission increases throughout the fatigue life, and failure finally occurs in the region of most intense emission. The regions of intense emission are attributed to the development and propagation of fatigue cracks.