Creep-fatigue life prediction of perforated copper-alloy plate by using digital image correlation method

Abstract

LE-9 rocket engine for H3 launch vehicle is being developed for realizing high performance, low cost and high reliability. Thermal fatigue life prediction on a combustion chamber structure with multiple cooling holes is important for design of rocket engine. In this paper, focusing on very low cycle conditions below 200 cycles, strain-controlled fatigue and creep-fatigue tests have been conducted at 558℃, using perforated copper-alloy plate specimens simulating cooling holes of combustion chamber for a rocket engine. Creep-fatigue life was defined based on 25% stress drop (N25) from steady state, where about 2mm-length cracks were found. Strain measurement using digital image correlation method was performed during fatigue tests for confirming strain distribution in the test specimen. Strain distribution from inelastic FEM almost agreed with that from the measurement. Creep-fatigue life prediction was carried out for perforated plate specimen, combining inelastic FEM with both the linear damage rule based on creep rupture time fraction and the strain range partitioning method. Life prediction accuracy was discussed based on both the definition of creep-fatigue life depending on detectable crack length and the choice of stress-strain behavior. Under the very low cycle conditions, the predicted life using the averaged stress-strain behavior within a gage length of 2mm, i.e. detected crack length, was within a factor of 2 when compared with experimental life.

Keywords

• creep-fatigue
• digital image correlation method
• perforated plate
• cu-cr-zr alloy
• inelastic fem
• linear damage rule
• strain range partitioning method