PSI - Issue 68

Minghua Cao et al. / Procedia Structural Integrity 68 (2025) 828–834 M. Cao et al. / Structural Integrity Procedia 00 (2025) 000–000 # =∫ $%& !"# %& !"# '(,%&̇ !"# + =1 , (1) where # is a state variable that increases monotonically. At each increment, the state variable must satisfy the following condition: ∆ # = ∆%& !"# %& !"# ((,%&̇ !"# ) ≥0 , (2) where ̅ # /0 * , ̅̇ # /0 . is the plastic strain at the onset of damage and ̅̇ # /0 is the plastic strain rate. 2.3. Loading and boundary conditions CGI used in automotive engineering operates under high-temperature conditions, often exceeding 400 °C (Tang et al. (2022)). To investigate the impact of elevated temperatures on the mechanical behaviour of CGI at the microscale, pure thermal loading was applied to all unit cells, with temperature rising form 20 °C to 500 °C, and decreasing back to 20 °C. Proper modelling of an RVE requires ensuring continuity of both displacements and tractions across its adjacent edges. This is accomplished by applying periodic boundary conditions (PBCs) at the RVE edges, allowing for precise simulation of the deformation field (Drago and Pindera (2007)). In this study, PBCs were applied to the side surfaces, while the top surface was left free to deform. The bottom surface was constrained with U1=U2=UR3=0, permitting deformation of the nodes in the XY plane only. 3. Results and discussion 3.1. Damage in graphite inclusions The effect of the morphology of graphite particles play a crucial role in the thermomechanical behaviour of CGI. The temperature at the onset of damage in graphite inclusions was evaluated for both C-CTE and T-CTE cases. A normalized magnitude in terms of the applied thermal load range during the heating process was used for comparison. In the case of spherical graphite (Nod_N), damage initiation occurred at lower temperature than for the vermicular graphite due to a stronger interaction between the spherical graphite particles and the matrix caused by its larger volume than that of the vermicular ones. Additionally, when using lower values of temperature-dependent CTE (T CTE) of graphite, damage in all types of particles initiated at a lower temperature compared to the case of higher values with C-CTE was used. Since the CTE of the metallic matrix was higher than that of graphite, lower T-CTE values caused a greater mismatch between the graphite and the matrix. As a result, graphite with T-CTE experienced damage initiation at lower temperatures. 831 4

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Fig. 4. Normalised temperature excursion of onset of graphite damage.