A new fitting method for P-S-N curve of ultra-high-strength sucker rod

P-S-N curve is one important basis for analyzing the mechanical performance of ultra-high-strength sucker rod, and its fitting method is generally referred to that of conventional sucker rods.Based on experimental data, however, ultra-high-strength sucker rod can't satisfy the assumptions of P-S-N curve fitting model which is used for conventional sucker rods.A three-parameter P-S-N curve fitting model based on Box-Cox variable transformation method was developed to search for a fitting method suitable for the P-S-N curve of ultra-high-strength sucker rod.By means of Box-Cox variable transformation method, fatigue life can satisfy normal distribution in a certain cyclic stress.The three-parameter P-S-N curve model is more accordant with the trend of typical S-N curve.It is indicated from experimental calculation results that by virtue of this new method, the fitting degree of P-S-N curve is higher with relative error only 1.46%-4.12%, which is just 11%-30% of the conventional value.The fitting based on this new method can embody the mechanical performance of ultra-high-strength sucker rod more accurately.