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

Abstract

In this work, the fracture mechanics properties of polyamide (PA) specimens manufactured by the selective laser sintering (SLS) technology are investigated, in which an embedded crack-like notch was inserted in the design and produced during the additive manufacturing (AM) phase. To cover a wide variety of mode I/II mixity levels, the inclined asymmetrical semicircular specimen subjected to three points loading (IASCB) was employed. The investigation was carried out by analyzing the full displacement field in the proximity of the crack tip by means of the digital image correlation (DIC) technique. To characterize the material, which exhibits a marked elastic-plastic behavior, the quantity J-integral was evaluated by two different methods: the first one exploits the full fields measured by the DIC, whereas the second one exploits the experimental load–displacement curves along with FEM analysis. The DIC methodology was experimentally validated and proposed as an alternative method to evaluate the J-integral. It is especially suited for conditions in which it is not possible to use the conventional LDC method due to complex and possibly unknown loading conditions. Furthermore, results showed that the AM technique could be used effectively to induce cracks in this type of material. These two aspects together can lead to both a simplification of the fracture characterization process and to the possibility of dealing with a wider number of practical, real-world scenarios. Indeed, because of the nature of the additive manufacturing process, AM crack-like notches can be sintered even having complex geometry, being three-dimensional and/or inside the tested structure.

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

“…Like in the previous case the DIC based technique tends to slightly overestimate the fracture mechanics parameter, but for all the Mode Mixity values. The J-integral is significantly higher in Mode II as it is well-known in literature [22].…”

“…In this section some of the results of [21] and [22] are reported for sake of comparison of the two different DIC-based methodologies, the complete set of experimental data can be retrieved from the aforementioned papers.…”

“…Moreover, it would be tricky to make the crack propagate exactly in the desired direction. References [21,22] show the difference with respect to an actual crack in Mode I, which is more marked in the case of materials having higher-yielding strength.…”

“…The polymeric specimens were produced by an EOS Formiga P100 SLS machine (EOS GmbH), using fine polyamide PA 2200 powder purchased from Prototal Plastic Design and Service (Götene, Sweden). The laser-sintered specimens were manufactured from mixed powder, recycled and new in equal proportions [22].…”

“…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.…”