PSI - Issue 13

Ralf Lach et al. / Procedia Structural Integrity 13 (2018) 34–38

35

Author name / Structural Integrity Procedia 00 (2018) 000 – 000

2

1. Introduction

Additive manufactured polymer materials produced by using selective laser sintering and related techniques are used more and more for mechanically loaded parts in industrial applications. However the long-term mechanical performance and the lifetime of such parts are almost unknown. Based on this point of view a new testing method, the rolling ring test using cyclically loaded notched ring specimens was developed to assess the lifetime of additive manufactured polymer materials. Generally, investigations dealing with the crack propagation and fracture behavior are hardly available not only for laser-sintered polymer materials but also for other polymers prepared by other rapid technologies as well, for laser-sintered polymers see Van Hooreweder et al. (2013), Blattmeier et al. (2012) and Grießbach et al. (2009). The notched impact strength of laser-sintered polyamide 12 (PA12) has been found to be linearly dependent on the normalized density (as a measure of the porosity), compared to the density of injection molded PA12, see Grießbach et al. (2010, 2012). Van Hooreweder et al. (2014, 2013, 2010) analyzed the lifetime (cycles to failure) under fatigue loading by measuring Wöhler curves for both notched and unnotched laser-sintered PA12 samples (for Wöhler curves of PA12 see also Munguia and Dalgarno (2015, 2014)). Blattmeier et al. (2012, 2010) investigated the fatigue crack growth behavior of laser-sintered PA12 as a function of the layer orientation using linear-elastic fracture mechanics. Compared to conventional (Wöhler curves) and fracture mechanics fatigue testing which are time-consuming and in need of an expensive testing machine, the rolling ring tester is not expensive, easily to handle and allows relatively fast data acquisition.

Nomenclature E 0

low cycle deformation

E x

deformation

E  N 0 N B

fatigue deformation limit

material-dependent parameter in Equation (1)

the number of cycles at break

PA12 polyamide 12

2. Experimental

2.1. Materials und specimens

The polymeric laser sintering powder was composed of polyamide 12 (PA12) of the grade PA 1800 provided by VG Kunststofftechnik GmbH, Chemnitz (Germany). For laser sintering of the specimens a laser sintering machine Vanguard HiQ-HS of 3D Systems was applied where the energy density was varied in the range of 148 – 174 Ws/cm 3 (the most used standard energy density is 160 Ws/cm 3 ), for further information see Grießbach et al. (2012, 2008). As test specimens notched ring specimens manufactured at VG Kunststofftechnik GmbH have been used. The ring specimens having an outer diameter of 80 mm and a cross-sectional area of 4  10 mm 2 are notched four times all 90° (depth and width of the U-shaped notches: 1.5 mm and 0.5 mm, respectively) and can be manufactured at the same time together with such parts which are used for real applications. The specimens were tested directly after manufacturing, dry as prepared without conditioning. 2.2. The rolling ring test The rolling ring test using cyclically loaded notched ring specimens – as described in Chapter 2.1 – was developed to assess the lifetime of additive manufactured polymer materials, especially ones prepared by laser sintering, but also for other polymers. The rolling ring testing device constructed in close cooperation with the company Festo enables a relatively simple and fast analysis of the mechanical performance and lifetime without testing the real parts built in the same powder bed. The device of the rolling test with clamped rolling ring is shown in Figure 1. During testing the rolling ring is fixed between three bearings, a horizontally movable roll (to vary deformation E x )