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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propagates as macro-crack prior to final failure. Several non-destructive test methods such as dye-penetrant test, X ray radiography, ultra-sonic and newer techniques such as X-ray computed tomography (CT) are used to detect damage in a component, primarily in the form of discontinuities, cracks. Non-destructive techniques are used as quality assurance protocol while a new component is inducted into service or during routine inspection. Considering that fatigue damage initiates at microstructure level as dislocation pileups, trans-granular fracture or inter-granular fracture prior to formation of micro-cracks, it is essential to study the fatigue damage progression in materials through advanced non-destructive test techniques. Figure 1 presents the schematic of changes that occur at the microstructural level in case of copper as well as in a polymer composite, which suggests the development of micro cracks in materials. Interestingly, even though the damage is at the microstructural level, one can identify the presence and degradation of material through conventional fatigue tests, which suggests change in elastic modulus as a function of cycles of loading as shown in Figure 2. Based on this, several cumulative damage models have been developed for understanding the damage progression. If one were to consider change in elastic modulus as an indicator, it opens up several avenues for studying the damage progression, through higher order ultrasonic measurements (which is based on change in wave velocity due to deformation in material), or through change in thermal, electrical or inductive response of materials. Infra-red thermal imaging technique is based on change in thermal emissivity of material due to dislocation pile-up and dislocation movement. Similarly eddy current technique can serve as a tool for understanding damage progression in materials. Potential drop techniques are used to study crack propagation during fatigue crack growth rate testing. Acoustic emission technique, which relies on sensing of low amplitude, high frequency sound waves during cracking of a material, is another technique that can be employed for diagnosis of fatigue damage progression.

Fig. 1 – Damage in Copper (presence of micro-cavities) and in polymer composites (micro-cracks in carbon fiber-epoxy laminate) [Lemaitre and Desmorat, 2005]

Fig. 2 – Schematic of change in elastic modulus during cyclic loading and evolution of damage in materials. When the cumulative damage reaches critical damage (D c ), failure takes place [Lemaitre and Desmorat, 2005].

This paper presents an overview of the use of Infrared thermography and X-ray computed Tomography to study the fatigue damage progression in structural materials, both in metals and in polymer composite materials. The stiffness degradation as a function of cycles of fatigue loading has been considered as another index of damage esp. in polymer composite materials. 2. Experimentation 2.1. Infrared Thermography When metals deform, due to the work of deformation, heat is generated, which if it can be measured using a non contact passive measurement technique, can help in understanding the thermal-mechanical coupling in the material. Infrared thermal (IR) imaging, also referred to as infrared thermography (IRT), is a non-contact, non-destructive measurement technique, which works by detecting the radiant energy from the surface of the mechanically loaded