PSI - Issue 28

2

D. Rigon et al. / Procedia Structural Integrity 28 (2020) 1655–1663 Rigon et al. / Structural Integrity Procedia 00 (2019) 000–000

1656

Nomenclature FDM

Fused Deposition Modeling

IM

Injection Moulding

vABS rABS

virgin Acrylonitrile Butadiene Styrene recycled Acrylonitrile Butadiene Styrene

vPP rPP

virgin Polypropylene

recycled Polypropylene vPP-GF30 virgin short glass fiber reinforced Polypropylene (30%wt) rPP-GF30 recycled short glass fiber reinforced Polypropylene (30%wt) E Young’s modulus [GPa] R load ratio (σ min / σ max ) Δσ range of nominal stress (σ max - σ min ) [MPa] ε B strain at fracture [%] σ UTS engineering ultimate tensile strength [MPa] σ Y engineering tensile proof stress [MPa]

1. Introduction The interest in using additive manufacturing or 3D printing from both industries and research organizations has been increasing in the last decades. Many polymer materials being available, the development and fabrication of products are continuously increasing with technological advancement (Dizon et al. (2018)). As additive manufacturing becomes more widely used, parts must withstand mechanical stresses and environmental conditions that occur during in-service use. Understanding the required strengths for material produced by a specific process and subjected to specific loading conditions is of paramount importance for any load-bearing application. In particular repetitive cyclic loads even lower the yield stress of material may result in fatigue damage, i.e. an accumulation of microstructural damage which may lead to initiation and spreading of one or more cracks, eventually leading to failure. Polymers are sensitive to fatigue at applied stresses below yield, which can cause microcracking and final failure of the part (Sauer and Hara (1990)). More precisely, fatigue failure in polymers occurs by two mechanisms: (i) thermal failure due to softening and melting from hysteretic energy and (ii) mechanical failure from crack initiation and propagation. Thermal failure is caused by the fraction of hysteretic energy converted into heat, leading to a temperature increase accompanied by stiffness loss. From a macroscopic viewpoint, the process of fatigue failure in polymers is similar to that of metals, where initial micro-cracks typically starts from the surface and at stress concentrators and subsequently grow into macroscopic crack (Crawford and Benham (1975); Hertzberg et al. (2012)).

Figure 1. Fused Deposition Modelling process.