Fatigue Damage Assessment of Face Centred Cubic (FCC) Metals & Alloys

Abstract

Cyclic fatigue tests were conducted at low and high temperatures on annealed and polished samples of polycrystalline Copper, Alpha-brass, Aluminium, Mild steel (0.25 %C) and Stainless steel (18Cr-8Ni) with grain sizes of 341.2 um, 150.0 um, 72.5 um, 63.5 um and 45.0 um respectively. Prior to fatigue test, non-destructive testing (NDT) was performed to check for the presence of any surface and internal flaws. To obtain data on ductility and strength of the specimens, impacts tests were performed to evaluate the different materials and set up limits of stresses to be applied. The endurance limit. oe, obtained from the Wohler S-n curves at room temperature for Cu, Cu-30^oZn, Al, Mild steel and Stainless-steel samples were 62.9 MPa, 97.0 MPa, 173.3 MPa, 228.1 MPa and 303.5 MPa respectively. It was observed that the fatigue strength of the materials show a decrease with temperature increase. Optical micrographs of deformed specimens revealed the formation of cavities and coalescence of micro-cracks into macro-cracks. For the interpretation of the experimental results, the model for damage based on Paris law,

da — = C (/_\ K)”' dn

was analysed, where da/dN is the crack growth rate (i.e. per cycle), m is the crack growth rate exponent, /_\ K is stress intensity C is the crack growth rate coefficient. The fatigue damage assessment show that: (i) fatigue limit is directly related to the inverse half -power of grain size; (ii) over the range 10'7-I0'2 mm/Cycle, the Paris law produces stable crack growth for stainless steel and aluminium; (iii) fatigue deformation results from residual stresses which cause crack opening and propagation and (iv) the specimens failed by cup and cone fracture.