The main objective of this research is to obtain a comprehensive insight into the influence of infill density on the fracture toughness results of PLA material, used in FDM technology. This paper presents four infill cases (100%; 70%; 40%; 10%) with 0.1 mm layer height. The whole research includes cases from 10 to 100% infill with a 10% increment on three different layer heights (0.3; 0.2; 0.1 mm). The FDM-grade PLA material is brittle hence, the fracture toughness of this material was estimated using the ASTM D5045-14 standard on SENB specimen geometry. This standard requires the plane-strain state near the crack tip to be met to obtain valid results. Three specimens were tested per infill group, all of which met the plane-strain condition. However, when looking at the KIC results from the highest to the lowest infill in all three considered layer configurations, the SENBs start to fail at 50%- onward due to the apparent presence of plasticity. This is evident from the maximum load value in the lower infill cases, where it exceeds the conditional value by more than the limit value of 10%. Therefore, for a better assessment of current results, the J integral approach was also used.

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