Study of Flexural Strength and Fracture of Additive Manufactured Parts With Stiffeners

Abstract

The authors have developed trans-tibial prosthetic sockets using additive manufacturing. These sockets made with mono-material thermoplastics such as Acrylonitrile Butadiene Styrene (ABS) and Polylactic Acid (PLA) at lower thicknesses were found to fracture within a few days of use by the amputees. The fracture was repeatedly found to occur at specific locations such as the lobe corners and the socket’s lower one-third zone. The most probable causes of crack initiation are lack of fusion (LOF) sites and voids. The causes of crack propagation are the lower interlayer bond strength compared to intra-layer bond strength. However, no scientific work exists that clearly explains these phenomena and methods to prevent such potential crack initiation sites and arrest the propagation of such fracture in additively manufactured polymeric structures. Therefore, in the present work, the investigation was carried out into possible enhancement in the resistance to fracture by different strength-enhancing post-processing techniques. In the first technique, the placement of stiffener features at selected locations on the socket was investigated. Three-point bending tests were carried out on D790 standard ABS specimens with different stiffeners introduced on the bottom face. The study focused on fracture characteristics in the stiffener-based topologically optimized geometric design of plate structures made by Fused Deposition Modeling (FDM) under flexural loading. The D790 three-point bending specimens were provided with differently shaped stiffeners, namely, triangular, prismatic, cuboidal, and pyramidal, extending all along the specimen’s length and spread with differential gaps in the width direction. In the second method, thermosetting epoxy resin coatings were applied on the three-point bending specimens of ABS, and the effect of the coating on the flexural strength was investigated. Bending tests were done on three specimens, the first specimen without any coating, the second specimen with only the epoxy resin coating, and the third specimen with two different coating layers. The first of the two coating layers on the third specimen was with primer and the second layer was with epoxy resin. Scanning electron microscope (SEM) and energy dispersion spectroscopy (EDS) scanning analyses were conducted on the fractured specimens. The scanning images indicated that both the primer and resin materials showed a tendency to diffuse into the substrate of ABS, thereby weakening the extreme fibers of material on the specimen’s tension side, resulting in premature crack initiation and propagation. Significant gain in the flexural strength was observed in both the strength enhancement techniques compared to plain specimens.