Stiffness Degradation of Digital Polypropylene Under Fatigue Loading:Investigations via 3-Dimensional Polyjet Printed Coupons

The capability of additive manufacturing (AM) for making monolithic, multi-material structures allows the fabrication of complex parts with varying mechanical properties, material combinations, and phases. The utilization of additively manufactured parts is gaining popularity in a functional application. Polymer-based additive manufacturing parts are utilized in a variety of engineering applications for automotive, aerospace, and energy. AM printed parts being the newer class of materials that have the potential to eliminate difficulties in the current traditional technologies. However, the structural performance of these materials is not understood completely, precisely there is currently a lack of published work regarding the performance of Polyjet printed parts under fatigue loading.

This paper investigates the stiffness degradation of homogenous 3-Dimensional test coupons caused by tension-tension fatigue behavior of additive manufacturing components, fabricated by the Polyjet printer. This homogeneous 3-Dimensional test configuration eliminates the limitations of the traditional ASTM D638 2D fatigue test coupon. In the traditional 2D test configuration, one direction is thinner than the other two directions, which may not account for non-uniformity in material deposition in all three directions during printing. Further, the effect of normal strain, which is a compressive strain in thinner directions, is not effectively captured.

To study the fatigue behavior of polymeric materials, the general approach is to conduct extensive experiments and then use these experimental data to develop a model for prediction of fatigue life or damage.  An empirical model of effective elastic modulus and an analytical model of the accumulated damage state, as defined on the basis of stiffness degradation during cyclic loading are presented. Further, the actual damage accumulation due to cyclic loading with the predicted model is compared. Modeling of the S-N diagram provides a better estimation of fatigue life and fatigue life modeling of AM printed test coupons is done by performing linear regression analysis of experimental data with high correlation coefficient R² (0.9971). The analytical model of the accumulated damage state is based on the stiffness degradation and this model is derived from the regression analysis of experimental data of stiffness degradation at different loading percentages assuming a polynomial of degree 4. The AM printed parts show the reverse trend of stiffness degradation as compared to composite materials and this behavior is due to strain hardening. Therefore, during fatigue of AM parts at higher loading, strain hardening strengthens the polymer that results in minimum stiffness degradation as compared to fatigue at lower loading. These studies finding provide insight into the fatigue damage state and cyclic performance of Polyjet printer additively manufactured parts.