Issue 41

A. A. Ahmed et alii, Frattura ed Integrità Strutturale, 41 (2017) 252-259; DOI: 10.3221/IGF-ESIS.41.34 253 conventional “subtractive” technologies, with this holding true especially in the presence of complex shapes that would be very difficult to be manufactured by using traditional techniques. If attention is focused specifically on plastics, examination of the state of the art suggests that acrylonitrile butadiene styrene (ABS) and polylactide (PLA) are the two materials that are commonly employed along with low-cost 3D printers, where polymers can be additively manufactured by melting/extruding powders, wires and flat sheets. As mentioned earlier, the unique feature of additive manufacturing is that parts with complex geometries can be fabricated very easily, with the obtained objects being characterized by a remarkable level of accuracy in terms of both shape and dimensions. This makes it evident that 3D-printed objects can contain very complex geometrical features that are likely to act as local stress concentrators. Therefore, accurate and simple design methods are needed to allow structural engineers to effectively assess the static strength of 3D-printed materials by taking explicitly into account the presence of stress raisers of all kinds. As far static assessment of notched components is concerned, examination of the state of the art suggests [1-9] that the Theory of Critical Distances (TCD) is the most powerful candidate to be used to design additively manufactured components for the following reasons: (i) the TCD assesses the detrimental effect of notches independently from their shape and sharpness; (ii) it models explicitly the material morphology through ad hoc critical lengths; (iii) the required local stress fields can be estimated without adopting complex non-linear constitutive laws; (iv) the TCD can be applied along with the results from linear-elastic FE models, where the same solid models can be used also to inform the manufacturing process. In this challenging scenario, this paper aims to investigate whether the linear-elastic TCD is successful in performing the static assessment of 3D-printed notched PLA subjected to in service static loading. To conclude, it is worth observing that the present investigation was based on PLA since this material is a biodegradable, absorbable and biocompatible thermoplastic aliphatic polyester that is widely used to manufacture biomedical components having complex shape [10]. Further, thanks to its specific features, PLA is one of those polymers that can be additively manufactured at a relatively low-cost by employing commercial 3D-printers. S TATIC ASSESSMENT ACCORDING TO THE T HEORY OF C RITICAL D ISTANCES ccording to the TCD, the static breakage of notched materials subjected to Mode I loading is avoided as long as a critical distance based effective stress, σ eff , is lower than the material inherent strength, σ 0 [1], i.e.:    0 eff (1) An important feature of the TCD is that it assesses static strength by directly post-processing the linear-elastic stress fields damaging the material in the vicinity of the notch being designed, with this holding true independently from the ductility level of the material under investigation [1-4, 6]. This can be done successfully provided that material inherent strength σ 0 is determined consistently [3, 4]. The procedure being recommended to be followed to determine σ 0 experimentally will be reviewed in what follows. These considerations suggest that the TCD is bi-parametrical design method where the critical distance and the inherent material strength are the two material parameters being used. Under static loading, the TCD’s length scale parameter is recommended to be estimated according to this well-known relationship [1, 11]:          2 0 1 Ic K L (2) where K Ic is the plane strain fracture toughness. As soon as material length L is known, the required effective stress, σ eff , can then be calculated directly according to either the Point Method (PM) or the Line Method (LM) as follows [1, 11]:             0, 2 eff y L r Point Method (PM) (3) A