PSI - Issue 58

K N Pandey et al. / Procedia Structural Integrity 58 (2024) 122–129 K. N. Pandey and G. Singh / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Most of the engineering systems such as ships, aeroplanes, automobiles etc. acts under extreme loading conditions and therefore, continuous monitoring is required to check the initiation and subsequent propagation of cracks. Electro mechanical impedance (EMI) technique is one of such methods for checking the in-service performance. This technique uses a piezo-electric ceramic (PZT) sensor to detect incipient damage at an early stage and operates at higher frequency range. This method was first applied on a prototype truss by Sun et al.,1995 and subsequently by Ayres et al. 1998 on an actual size truss. Now it has been successfully applied to various applications such as civil structural components (Soh C.K. et al., 2000), Structures with bolted joints (Lopes V. et al., 1999), thin structural elements (Abe M. et al., 2002), aircraft structures (Isabela Iuriko Campos Maruo et al., 2015), components or structures under fatigue loading conditions (Park et al., 2006; W. Yan et al., 2008; Yee Yan Lim and Chee Kiong Soh., 2011; Bhalla et al., 2004, Palomino et al., 2011; Yee Lan Lim, Chee Kiong Soh., 2014, Reetesh K Shukla et al, 2022) etc. The studies related to fatigue loading conditions were mostly under constant amplitude loading. Although there are many works related to determine the effect of block loading and load sequence on life of a component, (BorregoL.P. et al., 2008; Carvalho et al., 2009), but EMI technique is not applied to monitor the damage for AA6061 aluminium alloy (Reetesh K Shukla et al, 2022). The present study attempts to determine the effect of load sequence on AA6061 alloy with the help of EMI technique.

Nomenclature EMI

Electro-Mechanical Impedance Lead Zirconate Titanate

PZT

CT Compact Tension COD Crack Opening Displacement CA Constant Amplitude

Lo-Hi Low High Hi-Lo High Low

2. Material and experimental set-up 2.1. Material Al-6061 aluminium alloy with chemical composition as shown in Table 1 was used for the experiments. The ultimate strength of this alloy was 299 MPa.

Table 1. Chemical composition of Al-6061 Aluminum alloy. Material Al Cu Mg Mn

Cr

Si

Ti

Zn

Al-6061

95-97.5

0.1

0.45-2.5

0.1

0.1

0.2-0.8

0.1

0.1

Compact tension (CT) specimens of AA-6061 aluminium alloy were prepared as per ASTM-E647 to perform the fatigue crack growth rate, Fig. 1 (a). The straight notch on the CT specimen was prepared by wire EDM, Fig. 1(b). The length, breath and thickness of the specimen was 48 mm, 46.5 mm, and 6.2 mm respectively, as shown in Fig. 1(a). To convert the notch into a sharp crack, the CT specimen was subjected to fatigue pre-cracking. Under this process a cyclic load with maximum load of 4 kN was applied on the CT Specimen till a sharp crack of 1 mm length was created. After this, fatigue crack growth tests were performed under constant amplitude and block loading conditions (Borrego et al., 2008; Carvalho et al., 2009).