EFFECT OF RIPPLE LOAD ON THE STRESS-CORROSION CRACKING IN STRUCTURAL STEELS
The presence of small ripple loading can, under certain circumstances, significantly reduce time-to-failure and threshold stress intensity for stress-corrosion cracking (SCC) of steels. A predictive framework for such ripple-loading effects (RLE) is developed from concepts and descriptors used in SCC and corrosion fatigue characterisation. The proposed framework is capable of defining critical conditions required for the occurrence of RLE and predicting time-to-failure curves. The agreement between the predicted and laboratory data is excellent.
RIPPLE-LOADING EFFECTS ON STRESS-CORROSION CRACKING IN STEELS
Stress-corrosion cracking (SCC) under ripple-loading conditions was in­ vestigated in three quenched-and-tempered steels ranging in yield strength from 860 to 1210 MPa (125 to 175 ksi). These studies involved precracked cantilever-beam specimens exposed to 3.5 percent NaCl aqueous solutions at room temperature and at two electrode potentials, freely corroding and -1.0 V versus Ag/AgCl. Ripple-loads were applied at a cyclic frequency of 0.1 Hz and load ratios (R = minimum load/maximum load) between 0.90 and 0.975. Measured in terms of time-to-failure as a function of initial Mode I crack­ tip stress-intensity (Kr), the primary responses to ripple-loading were no effect in one instance and strongly detrimental in another. Where ripple­ loading was observed to significantly affect sec, threshold values under ripple-loading appeared to agree with the fatigue crack growth rate threshold (~Kth) for comparable loading and environmental conditions. These observa­ tions suggest that ripple-loading effects on sec in steels may be primarily mechanical in origin.
Determination of Optimal Ball-Burnishing Parameters for Plastic Injection Moulding Steel
The objective of this study is to determine the optimal plane ball-burnishing parameters for plastic injection moulding steel PDS5 on a machining centre by utilising the Taguchi’s orthogonal array method. The design and manufacture of a burnishing tool are described. Four burnishing parameters, namely the ball material, burnishing speed, burnishing force and feed, were selected as the experimental factors in Taguchi’s design of experiments to determine the optimal burnishing parameters which have the dominant influence on surface roughness. The optimal burnishing parameters were found after carrying out the experiments specified by Taguchi’s L18 orthogonal table, by the analysis of variation, and by a full factorial experiment. The optimal plane burnishing parameters for the plastic injection moulding steel PDS5 were a combination of a tungsten carbide ball, a burnishing speed of 200 mm/min, a burnishing force of 300 N and a feed of 40 μm. The surface roughness Ra of the specimen can be improved from about 1 μm to 0.07 μm by using the optimal burnishing parameters for plane burnishing.