Stochastic fracture of additively manufactured porous composites.

Extrusion-based fused deposition modeling (FDM) introduces inter-bead pores into dense materials, which results in part-to-part mechanical property variations, i.e., low mechanical reliability. In addition, the internal structure of FDMed materials can be made porous intentionally to tailor mechanical properties, introduce functionality, reduce material consumption, or decrease production time. Despite these potential benefits, the effects of porosity on the mechanical reliability of FDMed composites are still unclear. Accordingly, we investigated the stochastic fracture of 241 FDMed short-carbon-fiber-reinforced-ABS with porosity ranging from 13 to 53 vol.% under tensile load. Weibull analysis was performed to quantify the variations in mechanical properties. We observed an increase in Weibull modulus of fracture/tensile strength for porosity higher than ~40 vol.% and a decrease in Weibull modulus of fracture strain for an increase in porosity from 25 to 53 vol.%. Micromechanics-based 2D simulations indicated that the mechanical reliability of FDMed composites depends on variations in bead strength and elastic modulus of beads. The change in raster orientation from 45°/-45° to 0° more than doubled the Weibull modulus. We identified five different types of pores via high-resolution X-ray computed tomography. A 22% and 48% decrease in carbon fiber length due to extrusion was revealed for two different regions of the filament.

Grip forces of the fingertips.

OBJECTIVE: A model has been developed for the power grip on a cylindrical handle. The model gives estimates of the forces acting on the fingertips as functions of total grip force, diameter of the handle, and hand size. DESIGN: The data in the literature were used to estimate the parameters of the model. An experiment was conducted to verify that the distribution of the forces is independent of the force level. BACKGROUND: In the authors' opinion, the outcomes of the various experiments described in the literature can be represented in a compact model without too much information loss. METHODS: The forces acting on the fingertips of 10 subjects were measured with three different handle diameters and five grip force levels. RESULTS: The force sharing turned out to be independent of the grip force and the diameter of the handle. CONCLUSIONS: It is advantageous to shape the handle so that the local diameters at the fingers are proportional to the finger lengths. RELEVANCE: Knowing the force distribution of the power grip is important in design of handles. Experimental data presented in a concise form is needed also in the development of biomechanical models.