PSI - Issue 43

Raghu V. Prakash / Procedia Structural Integrity 43 (2023) 190–196 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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material. This radiant energy if picked up by the IR sensor in a passive manner can provide information about temperature response of a material due to the application of stress. It may be noted that while a material is loaded in tension below the yield stress, there is cooling of the specimen due to thermo-elastic effect and once the specimen crosses the yield stress, due to plastic deformation, the specimen temperature rises. Volume dilatation takes place in elastic region during tensile loading of material as the atoms get pulled apart slightly and as a consequence, there is a slight increase in the bond length, hence, there is a gross decrease in temperature in this case of elastic tension by a tensile stress [Pandey and Chand, 2003]. IRT has been used by several researchers to understand the yield point in metals during tensile testing and post-yield temperature rise to predict the failure zone during tensile testing [Venkatraman et. al, 2004]. Interestingly, when the stress is reversed in the specimen, the resultant specimen temperature is higher than the starting temperature, even when the specimen is loaded and unloaded within the elastic limits. This is due to the irreversibility of deformation in the material. This irreversibility provides a clue to use the IR technique to study change in material thermal response during cyclic loading. Figure 3 provides a photograph of the test set-up that was used for infrared thermal imaging during fatigue cycling. Shown in Fig. 4 is the temperature response during first cycle of loading and unloading on a stainless steel SS 304 flat dog-bone specimen. The temperature of the specimen does not return to its original value even after complete reversal of stress. Stress controlled fatigue tests were conducted on SS304 specimens at a peak stress of 250 MPa with a stress ratio (ratio of minimum to maximum stress) of 0.1 below its yield stress (of 325 MPa) at a cyclic frequency of 5 Hz under sinusoidal waveform. Figure 5 presents the temperature response captured using a FLIR IR camera with a temperature range of 273-393 K with a noise equivalent temperature difference of 0.05 K. It can be seen that the average temperature is increasing with the number of cycles. The temperature response is found to be linear and consistent with stress cycling. Figure 6 presents the average temperature profile of the test specimen till failure. The thermal profile comprises of three stages, viz., primary stage where the average temperature increases rapidly with cycles, which is followed by a steady state response and near specimen failure, a tertiary stage of rapid increase in temperature. This is analogous to the classical damage progression of materials and suggests a good correlation between the damage progression and temperature response of the material.

Fig. 3 – Photograph of test set-up used for IR thermography and fatigue testing.

Fig. 4 – Typical temperature response of stainless steel SS 304 during a single cycle of loading and unloading.

In addition to continuous fatigue cycling till failure, a few experiments were conducted with periodic interruptions (5000 cycles) of fatigue cycling. During the interruption period, the specimen was allowed to cool to room temperature by soaking in lab air conditions for several hours prior to restart of the experiment. Figure 7 presents the temperature response of the specimens after interruptions. It can be seen that the temperature slopes during the primary response regime is dependent on the prior fatigue cycling history. Figure 8 presents the same