Investigation by Digital Image Correlation of Mixed Mode I and II Fracture Behavior of Metallic IASCB Specimens with Additive Manufactured Crack-Like Notch

Abstract

This work investigates the fracture behavior of maraging steel specimens manufactured by the selective laser sintering (SLS) technology, in which a crack-like notch (sharp notch) was directly produced during the additive manufacturing (AM) process. For the evaluation of the fracture toughness, the inclined asymmetrical semi-circular specimen subjected to three points loading (IASCB) was used, allowing to cover a wide variety of Mode I and II combinations. The effectiveness of manufacturing crack-like notches via the SLS technique in metals was evaluated by comparing the obtained experimental results with the ones obtained with pre-cracks induced by fatigue loading. The investigation was carried out by using the digital image correlation (DIC) technique, that allowed the evaluation of the full displacement fields around the crack tip. The displacement field was then used to compute the stress intensity factors (SIFs) for various combinations of Mode I and II, via a fitting technique which relies on the Williams’ model for the displacement. The SIFs obtained in this way were compared to the results obtained with the conventional critical load method. The results showed that the discrepancy between the two methods reduces by ranging from Mode I to Mode II loading condition. Finally, the experimental SIFs obtained by the two methods were described by the mixed mode local stress criterium.

Investigation by Digital Image Correlation of Mixed-Mode I and II Fracture Behavior of Polymeric IASCB Specimens with Additive Manufactured Crack-Like Notch

“…By plotting the J-integral as a function of the mode mixity ratio M e , it can be observed that the curve has a minimum at a mixity ratio equal to 0.77 and then it increases up to pure mode II. Generally, in the literature [ 4 ], higher fracture toughness values are experimentally observed for mode II with respect to mode I. This is due to a wider stress state distribution for mode II than mode I, which leads to higher fracture toughness values.…”

“…Figure 2b shows an example of a strain field correlated by the DIC software (y-strain, 10-42-42 test configuration). Following the approach adopted in [4], the crack-like notch was initiated in two different ways: 12 samples had the notch embedded during the additive manufacturing process (named AM specimens), while the other 3 samples had the notch initiated manually by a razor blade (named SM specimens, in which SM stands for subtractive manufacturing). The AM specimens were manufactured with an angle α of 0 • and 10 • and tested, changing the support spans S 2 , while S 1 was kept at 42 mm for all the tests to obtain different mode I/II mixity.…”

“…The possibility of sintering artificial crack-like notches during the SLS process was investigated by T. Brugo et al [ 3 ], showing the feasibility of the approach. The same method was then successfully applied in [ 4 ] to study the fracture behavior of high-strength metallic material. A similar approach was also successfully applied to predict fracture loads of 3D-Printed ABS specimens, in which a pre-crack is embedded in the additive manufacturing process, this time using fused-deposition modeling (FDM) technology [ 5 ].…”

Fracture mechanics of additive manufactured crack-like notches by digital image correlation

“…The metallic specimens were manufactured by the EOSINT M 280 SLS machine (EOS GmbH), using the EOS Maraging Steel MS1 steel powder, with a chemical composition corresponding to the European 1.2709 classification. Other manufacturing details and material properties can be found in [21].…”

“…by means of the Ramberg-Osgood model, as it is done in [20]). In this paper the application of the two DIC based methods is shown in the case of two different materials, one Martensitic steel which can be considered linear elastic [21] and a Polyamide [22] which exhibits marked plasticity.…”