PSI - Issue 60

Rakesh Bhadra et al. / Procedia Structural Integrity 60 (2024) 149–164 Bhadra et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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between the non-dimensional force-interference results obtained from the model developed using the present method and the results published by Ahmed et al. (2020). In Ahmed et al.'s study, the indentation process of a CNT based nanocomposite block with a CNT wall thickness of 0.034 nm was examined using a Berkovic indenter. The comparison between the two curves reveals a reasonable level of agreement, with similar shapes. This comparison provides strong support for the validity of the current model, indicating its ability to accurately represent the behavior of the system being investigated. 3.3. FE analysis The primary objective of this analysis is to explore how material gradation influences the contact behavior of the CNT-FGM nanocomposite. Furthermore, the study investigates the impact of the wall thickness of carbon nanotubes (CNTs) during the indentation process. The indentation procedure initiates with the indenter establishing initial contact with the surface to be indented. Following this, the indenter gradually descends, reaching a predefined indentation depth. The indenting process is then followed by a gradual withdrawal of the indenter to disengage it from the workpiece. The movement of the indenter towards the work piece is referred to as the loading process, while the withdrawal process is termed unloading. For the analysis, three different values of the elastic gradation fact or (γ e ) are considered: +2, 0, and -2. Additionally, three different values of CNTs' wall thickness are also taken into account: 0.034 nm, 0.102 nm, and 0.170 nm. These variations in γ e and CNTs' wall thickness are studied to understand their respective effects on the contact behavior of the nanocomposite.

(a) The correlation between contact force and indentation depth (0.78 nm), with varying CNTs wall thickness (0.034 nm, 0.102 nm, 0.170 nm, solid CNT, and FGM), while keeping the elastic gradation parameter constant. ( γ e = 2).

(b) The relationship between contact force and indentation depth, maintaining a constant CNTs wall thickness of 0.102 nm, while allowing the elastic gradation parameter to vary ( γ e = +2, 0, -2).

Fig. 6 Contact force plots.

To gain insights into the contact characteristics, the contact force is plotted against indentation depth, considering variations in both the elastic gradation parameter and CNTs' wall thickness. Fig.6.(a) displays the contact force versus indentation depth for different CNTs' wall thickness values (0.034nm, 0.102 nm, 0.170 nm, solid CNT, and FGM) while keeping the elastic gradation parameter constant (γ e = +2). The results clearly indicate that the contact forces improve when CNTs are incorporated into the FGM as the reinforced material, especially when their thickness is higher (above 0.034 nm). However, the contact force reduces when CNTs' wall thickness is 0.034 nm. One intriguing observation is that all the plots coincide up to a certain indentation depth. This behavior arises because, at very low indentation depths, the CNTs remain unaffected, causing all the composite behaviors to resemble that of the reinforced material alone. However, as the thickness-to-radius ratio of the CNTs increases, their stiffness also increases, leading to a situation where the stiffness of thinner CNTs falls below that of the matrix material. This phenomenon explains the reduction in contact force during the indentation of the nanocomposite