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The fatigue behaviour of continuous wire polymer composite (CWPC) fabricated by fused filament fabrication was investigated. Four compositions were examined: polylactic acid (PLA), PLA with copper wire (Cu), thermoplastic polyurethane (TPU), and TPU with Cu wire. Residual properties were measured after different sets of number of cycles (10 , 10 , 10 ). Residual strengths were 89.736% and 70.464% of the ultimate tensile strength of the original material after 10 cycles for PLA CWPC and TPU CWPC, respectively. A one-way analysis of variance (ANOVA) statistical tests showed insignificant changes in the residual strengths of PLA-based materials after an increasing number of cycles and significant changes for the TPU-based materials. CWPC electromechanical properties under fatigue test demonstrated reverse piezoresistivity behavior. A strain-controlled fatigue life analytical model was compared to the experimental results showing good agreement. This study demonstrates the applicability of FFF technique to print sensors with continuous integrated wire with tunable properties.

This work studies the fatigue behavior of biodegradable composites of thermoplastic starch (TPS) reinforced with different amounts of date palm fibers (DPF) (20, 50, and 70 wt%). Composite with 50 wt% fiber content had higher static and fatigue strengths compared to other composites. The alternating stress at which the composites lasted for 10 7 cycles was about 16% of the flexural strength. Mathematical models following the assumptions of D’Amore et al. (1996) and Mao and Mahadevan (2002) were shown to give a good agreement with experimental results of the residual strengths under various fatigue cycles and of the fatigue damage index at different levels of stresses.

In this study, the electromechanical properties of two different three-dimensional (3D) printed continuous wire polymer composites (CWPC) were characterized and compared. The two composite materials were copper wire polylactic acid (PLA) composite (rigid material) and copper wire polyurethane (PU) composite (flexible material). The electromechanical measurements were based on piezoresistive properties of the sensor at which the mechanical strain and the electrical resistance were correlated under a uniaxial loading condition. Both types of materials exhibited a direct linear relationship between the two quantities, indicating the ability of CWPC to be used for strain sensing applications. The gauge factor (GF) sensitivity was compared for the two types of materials. It was found that there is no statistical significance difference between the GF of PLA CWPC (1.36 ± 0.14) and PU CWPC (1.29 ± 0.07)); therefore, the sensing property depends mainly on the wire integrated into the 3D-printed structure rather than the matrix. Thus, different matrices can be used to fit different applications. An analytical model for GF showed agreement with the experimental results for both materials. PU CWPC showed significant improvement in both Young’s modulus (E) and ultimate tensile strength (UTS) (210.5 % and 31.86 %, respectively), compared with pure PU, while the change in Poisson’s ratio (ν) was insignificant. Young’s modulus of PLA CWPC was significantly increased by 80.3 % compared with PLA, while UTS and ν did not significantly change. The experimental mechanical properties showed good agreement with data from the analytical models. The outcome of this study focused on the manufacturing of 3D-printed functionalized structure for strain sensing applications with improved mechanical properties. The wide range of attained strain allowed their use in different applications based on the range of strain needed, such as rigid sports equipment and flexible wearable sensors.

Biodegradable composites of starch–date-palm fibers were prepared by first plasticizing corn starch and chemically treating the fibers before being formed by compression molding. The effect of fiber content on mechanical properties was examined and it was found that tensile strength and Young's modulus for 50 weight percent (wt%) fiber composite improved by 7 and 12.5 times, respectively, compared to thermoplastic starch. Impact strength showed similar behavior and improved by 4.3 times for 50 wt% fiber composites. At higher fiber content the matrix was insufficient to cover the fibers, causing the mechanical properties to deteriorate. The results also showed that exposure to moisture resulted in progressive decrease in mechanical properties with increasing moisture absorption. It was found that after reaching moisture saturation, the retained tensile strengths were about one-third the starting values and the retained impact strengths were about two-thirds the starting values.