Issue 41

A. A. Ahmed et alii, Frattura ed Integrità Strutturale, 41 (2017) 252-259; DOI: 10.3221/IGF-ESIS.41.34 257 The stress vs. strain diagrams reported in Fig. 3 show the mechanical behaviour as measured by testing the un-notched specimens. These charts make it evident that the stress vs. strain response of the additively manufactured material being tested was characterised by a mechanical behaviour that was predominantly linear up to the maximum stress value being recorded during testing. In terms of reference mechanical properties, the results generated by testing the plain samples returned the following average values: Young’s modulus, E, equal to 3296 MPa, 0.2% proof stress, σ 0.2% , equal to 40.3 MPa and ultimate tensile strength, σ UTS , equal to 42.5 MPa. The charts reported in Figs. 4a and 4b summarise the influence of manufacturing angle θ p on both E and σ UTS . These diagrams make it evident that the infill direction had little effect on the overall mechanical behaviour of the additively manufactured material being investigated. In particular, the experimental results expressed in terms of both E and σ UTS in Figs. 4a and 4b, respectively, are seen to be all within two standard deviations, S D , of the mean. The results generated by testing specimens with crack-like notches are summarised instead in the K C (for t=4 mm) vs. θ p diagram of Fig. 4c. The values for K C shown in this chart were estimated according to Linear Elastic Fracture Mechanics (LEFM), i.e. by using the following standard relationship [15]:       C f K a (5) In Eq. (5) α is the shape factor (estimated as recommended in Ref. [16]), σ f is the nominal failure stress referred to the gross area, and a is the crack-like notch depth. Since, according to Fig. 4c, the experimental values for K C are within two standard deviations of the mean, it is possible to come to the conclusion that also the fracture resistance of the additively manufacture polymer under investigation was marginally affected by manufacturing angle θ p . To conclude, it is worth observing that, due to such a high level of consistency, the notched specimens being tested were manufactured by setting angle θ p solely equal to 0º, 30º, and 45º. Figure 5 : Linear-elastic stress fields in the incipient failure condition and determination of critical distance L. V ALIDATION BY EXPERIMENTAL DATA y considering notched specimens of polymethylmethacrylate (PMMA), in 2004 Taylor et al. [17] observed that the inherent strength, σ 0 , of this material takes on a value that is equal to about 2·σ UTS . Accordingly, the standard formalisation of the TCD considered in the present paper can be used to perform the static assessment of PMMA as long the notches being assessed result in stress concentration factors that are larger than σ 0 /σ UTS ≈2. Having recalled these important aspects, in the preliminary investigating summarised in the present paper the accuracy of the TCD in estimating the static strength of notched additively manufactured PLA was checked by forming the following simplifying hypotheses: 0 50 100 150 200 250 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0  1 [MPa] Distance, x [mm] Linear-elastic stress fields Sharp Intermediate Blunt  0 =85.0 MPa Error= +20% Error= -20% L/2=0.467 mm  UTS =42.5 MPa B