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2 2023

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Study on the stress intensity factor of a compact specimen under the pre-compressed load condition

Wu Dongquan, Liu Zixiang, Li Dinghe, Zhang Zhiqiang, Chen Jianguo

Journal of Theoretical and Applied Mechanics

2023

Vol. 61 nr 1

37--47

Structural components are often operated under combined stress conditions (primary and secondary stresses), but the stress levels generated by residual stress (or secondary stress) is hardly ever evaluated. Hence, stress intensity factors at the crack tips of a compact tension (CT) specimen under a pre-compressed load condition are analyzed using the finite element method. Then, the average residual stress intensity factor is calculated and analyzed. As the crack length α0/W increases, the average residual stresses σave/σ0 grows under the same pre-compression load. σave/σ0 increases rapidly at a low range of the pre-compression load but tends to a constant in a high range of the load. The distribution of the average residual stress intensity factors Kave and Κave/σ0 of the CT specimen with same crack length under different pre-compression loads have the same tendency. Additionally, the distribution of Κave and KFEM under different pre-compression loads are also similar. Nevertheless, Kave estimated by the average residual stress is too conservative and not accurate, and the method is complex, which depends on the analysis of simulation. Therefore, a simple method for calculating Mode I stress intensity factor K for this model is presented. A group of examples is presented to verify the accuracy of the method.

Experimental study of the fracture of CT specimens printed in PLA as a function of the raster width

Aourik O., Othmani M., Chouaf A.

Archives of Materials Science and Engineering

2023

Vol. 122, nr 2

78--85

Purpose: The FDM (Fused Deposition Modelling) additive manufacturing process is characterised by a large number of process variables that determine the mechanical properties and quality of the manufactured parts. When printing layer by layer, the filaments constituting the layer are welded on the one hand between them in the same layer and on the other hand between the superimposed layers, this welding develops on the contact surfaces (raster width) along the deposited filaments. The quality of this welding determines the resistance to crack propagation between filaments and between layers. This article aims to study the effect of the width of the raster on the resistance to crack propagation in a structure obtained by FDM. Design/methodology/approach: We have developed an experimental approach from CT specimens to determine the tensile strength of polylactic acid (PLA) polymers, considering the J-Integral method. And given the complexity of the problem, three cases of raster width (l=0.42 mm, l=0.56 mm and l=0.68 mm) have been treated. Findings: According to the results obtained (J, ∆a), the resistance to crack propagation in the parts printed by FDM seems to be better when the width of the filament is small. Indeed, the energy necessary to break the specimen is relatively greater than in the case of a larger width. This finding was confirmed by comparing the values of J for a given advancement of the crack for the three cases studied. Research limitations/implications: In order to present an exhaustive study, we focused on the effect of raster widths (including 0.42 mm, 0.56 mm to 0.68 mm) on the crack propagation of printed PLA. This study is in progress for other printing parameters. To highlight the cracking mechanisms, microscopic observations will be developed in greater depth at the SEM. Practical implications: Our analysis can be used as decision support in the design of FDM parts. In effect, we can choose the raster width that would provide the resistance to crack propagation desired for a functional part. Originality/value: In this article, we analysed the damage mechanism of CT specimens printed by FDM. This subject represents a new direction for many lines of research. For our study, we used the J-Integral theoretical approach to study the fracture behaviour of these parts by determining the resistance curves (J-∆a).