An investigation of dynamic failure events in steels using full field high-speed infrared thermography and high-speed photography
Abstract (Summary)An infrared (IR) imaging system has been developed recently at Caltech for measuring the temperature increase during the dynamic deformation of materials. The system consists of an 8x8 HgCdTe focal plane array, with 64 parallel preamplifiers. Outputs from the 64 detector/preamplifiers are digitized using a row-parallel scheme. In this approach, all 64 signals are simultaneously acquired and held using a bank of track and hold amplifiers. An array of eight 8:1 multiplexers then routes the signals to eight 10MHz digitizers, acquiring data from each row of detectors in parallel. The maximum rate is one million frames per second. Crack tip temperature rise during dynamic deformation is known to alter the fracture mechanisms and consequently the fracture toughness of a material. However, no direct experimental measurements have ever been made to determine the same because of limited diagnostic tools. Further, the temperature rise in the vicinity of the crack tip could potentially be used as a direct measure of loading and could serve as a diagnostic tool in order to extract appropriate fracture parameters. By transcending the existing experimental limitations, this investigation presents detailed, real time evolution of the transient crack tip temperature fields in two different steels (C300 and HY100 steels), using the 2-D high speed IR camera. The crack tip temperature rise at initiation in C300 steel was found to be about 55K. In case of HY100, the crack tip temperature rise was above 90K and was seen to be a strong function of loading rate. HRR elastic-plastic singular field has been used to extract J integral evolution from the measured temperature field. Critical value of J integral at initiation was seen to increase with loading rate. An experimental investigation has been conducted to study the initiation and propagation characteristics of dynamic shear bands in C300 maraging steel. Pre-fatigued single edge notched specimens were impacted on the edge under the notch to produce shear dominated mixed mode stress fields. The optical technique of coherent gradient sensing (CGS) was employed to study the evolution of the mixed mode stress intensity factors. Simultaneously, the newly developed 2-D high speed infrared (IR) camera was employed to obtain the temperature field evolution during the initiation and propagation of the shear bands. A criterion for shear band initiation is proposed in terms of a critical mode II stress intensity factor. The IR images, for the first time, revealed the transition of crack tip plastic zone into a shear band and also captured the structure of the tip of a propagating shear band. These thermographs support the notion of a diffuse shear band tip and reveal "hot spots" distributed along the length of a well developed shear band.
Advisor:A. J. Rosakis
School Location:USA - California
Source Type:Master's Thesis
Date of Publication:09/15/2000