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

The utilization of additively manufactured parts is gaining popularity in functional applications. Polymer-based additive manufacturing (AM) parts are utilized in a variety of engineering applications for automotive, aerospace, and energy. AM printed parts are however a newer class of materials, and the structural performance of these materials is not fully understood completely, and very limited exists currently on the performance of Polyjet printed parts and associated digital materials under fatigue loading. This paper investigates the stiffness degradation under tension-tension fatigue loading of digital polypropylene using homogenous 3-Dimensional test coupons formed using PolyJet printing. The homogeneous 3-Dimensional test configuration employed in the present study eliminates the process-induced limitations of traditional ASTM D638 2D fatigue test coupons for AM processed materials. Fatigue data is analyzed to present 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. 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 and is obtained via 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 is derived from the regression analysis of experimental data of stiffness degradation at different loading percentages assuming a polynomial of degree 4. The present study provides insight into the fatigue damage state and cyclic performance of digital polypropylene from Polyjet printing.