Ultrasonic Nondestructive Evaluation Of Thermal Fatigue Damage

Die making is very important in industry development field. Hot die steel plays a veryimportant role in automotive, machine, farm equipment industry and the demand is increasingday after day.Inactivation of hot die steel can do a lot of damage to commercial run. And it containsthree dimensions: thermal fatigue, thermal abrasion and abruption. Hereinto, thermal fatigueis primary. Therefore, it is important and necessary to evaluate the thermal fatigue damage.Traditional ways to evaluate the damage caused by thermal fatigue are almost destructive.This paper is based on NDE. According to the relationship between E and velocity of soundE=[3-(1/((c_l/c_t)~2-1))]ρc_t~2, measuring longitudinal velocity (c_l) and transverse velocity (ct) indifferent specimens which thermal fatigue times are different. Young's modulus E andeffective modulus of elasticity (?) are available. So damage parameter D=1-((?)/E) isavailable, and UNDE of thermal fatigue damage can achieve. Under this experimentalcondition, damage parameter D has increased to 0.1 after 300 times of thermal fatigue. Theresult shows that the purpose of this paper has been realized.The experiment results show that:1,Radial c_l and c_t decrease when thermal fatigue times are increased, and radial c_ldecreases more than axial c_l. c_l increases from centre to surface in one sample. The reasonof why sonic velocity changed has been discussed.2,Density increases from centre to surface in one sample, it is anastomotic with thechange trend of c_l. Density and strength of extension decrease when thermal fatigue timesare increased.3,Fracture pattern of tensile sample has been observed. Found that plastic naturedecreases and the crack density increases when thermal fatigue times are increased.4,Texture variation and micro crack of thermal fatigue samples have been observed.