Tensile fracture and failure behavior of thermoplastic starch with unidirectional and cross-ply flax fiber reinforcements

Thermoplastic starch (MaterBi(R)) based composites containing flax fibers in unidirectional and crossed-ply arrangements were produced by hot pressing using the film stacking method. The flax content was varied in three steps, viz. 20, 40 and 60 wt.-%. Static tensile mechanical properties (stiffness and strength) of composites were determined on dumbbell specimens. During their loading the acoustic emission (AE) was recorded. Burst type AE signal characteristics (amplitude, width) were traced to the failure mechanisms and supported by fractographic inspection. The mechanical response and failure mode of the composites strongly depended on the flax content and the flax fiber lay-up. It was established that the tensile strength increases until 40 wt.-% flax fiber content but stays almost constant above this value. In the case of 40 wt.-% unidirectional fiber reinforcement, the tensile strength of the composite was 3 times greater than that of the pure starch matrix. The flax fiber reinforcement increased the tensile modulus of the pure starch by several orders of amplitude.