Static assessment of plain/notched polylactide (PLA) 3D‐printed with different infill levels: Equivalent homogenised material concept and Theory of Critical Distances

Abstract

A novel approach based on the equivalent homogenised material concept and the theory of critical distances is formulated to perform static assessment of plain/notched objects of polylactide (PLA) when this polymer is additively manufactured with different infill levels. The key idea is that the internal net structure resulting from the 3D‐printing process can be modelled by keeping treating the material as linear elastic, continuum, homogenous, and isotropic, with the effect of the internal voids being taken into account in terms of change in mechanical/strength properties. This idea is initially used to assess the detrimental effect of the manufacturing voids on the static strength of the plain (ie, unnotched) material. This is done by addressing this problem in a Kitagawa‐Takahashi setting via the Theory of Critical Distances. Subsequently, this approach is extended to the static assessment of notched components of 3D‐printed PLA; ie, it is used to take into account simultaneously the effect of both manufacturing voids and macroscopic geometrical features. The accuracy and reliability of this design methodology were checked against a large number of experimental data generated by testing, under axial loading, plain specimens, as well as notched samples (including open notches) of PLA. These specimens were manufactured by making the infill level vary in the rage 10% to 90%. This validation exercise allowed us to demonstrate that the proposed approach is highly accurate, returning estimates falling within an error interval of ±20%. This remarkable level of accuracy strongly supports the idea that static assessment of 3D‐printed materials with complex geometries and manufactured with different infill levels can be performed by simply post‐processing conventional linear elastic finite element (FE) solid models, ie, without the need for modelling explicitly the detrimental effect of the manufacturing voids.

Recent advances on notch effects in metal fatigue: A review

“…Specifically, a new method to assess the equivalent stress inside the spherical material control volume defined by VM is developed, and the strategy using the averaging stress tensor associated to the mesh nodes contained in the sphere for model calibration provides more precise predictions for several types of test samples. Ahmed and Susmel formulated a novel approach for static assessment of plain/notched polylactide 3D‐printed with different infill levels. By combining the equivalent homogenized material concept with the TCD, their methodology provided sound correlations with experimental fatigue data, returning estimations falling within ±20% error interval.…”

Reference strength values to design against static and fatigue loading polylactide additively manufactured with in‐fill level equal to 100%

“…Whilst AM is conceived as a simple and innovative way to process PLA, machine settings in the form of process parameters have been considered by experimental evaluations to study consequential effects on the material mechanical behaviour. The structural response of AM PLA under static loading is affected by the following parameters: layer thickness, infill percentage, nozzle size, manufacturing orientation (Figure ), filling pattern, filling rate, and fill temperature. Important manufacturing variables that affect printing resolution and integrity include the shell thickness, which is recommended to be set to a value equal to a multiple of the nozzle diameter to effectively reduce, in the bulk material, the formation of manufacturing voids and defects …”

“…The diagram of Figure A makes it evident that both angle θ R and shell thickness t s had little influence on σ UTS , with all the data being within two standard deviations, S D , of the mean. This tells us for a direct CAD to FEA design interoperability, where AM PLA can be treated as a homogenous and isotropic material …”

Fatigue strength of additively manufactured polylactide (PLA): effect of raster angle and non-zero mean stresses

“…In order to use components of additively manufactured (AM) PLA in situations of engineering interest, one of the key aspects is performing both static and fatigue assessment by always reaching an adequate level of accuracy and, therefore, of safety. In this context, the available technical literature makes it evident that in recent years the international scientific community has focussed its attention mainly on the mechanical behaviour and strength of AM PLA when this polymer is subjected to static loading (see Refs [3][4][5][6][7][8][9][10] and the references reported therein). In contrast, just a few studies dealing with the fatigue behaviour of 3D-printed PLA have been published so far [10][11][12][13][14][15].…”

“…As far as AM PLA's mechanical response under static loading is concerned, much experimental evidence suggests that its ultimate tensile strength (UTS) depends primarily on the following technological variables: layer thickness, infill level, filling pattern, filling rate, nozzle diameter, raster angle, feed rate, printing speed, and manufacturing temperature [2][3][4][5][6][7][8]. In this context, both the UTS and Young's modulus, E, of AM PLA are seen to increase as the raster angle decreases [3,4], the UTS vs. E relationship being just a simple linear function [4].…”