The effect that the infill orientation angle has on the strain-rate dependence of the yield stress for material
extrusion additive manufactured (ME-AM) PolyLactic Acid (PLA) material was investigated. Symmetric angleply stacking sequences were used to produce ME-AM tensile test samples. Measured yield stresses were
compensated for the voided structure, typical of ME-AM components. Furthermore, molecular orientation and
stretch was macroscopically assessed by a thermal shrinkage procedure. Additionally, hot-press compression
molded (CM) samples were manufactured and mechanically characterized in uniaxial tensile and compression
in order to determine the material’s isotropic bulk properties. Initial model parameters for the Ree–Eyring
modification of the Eyring flow rule were determined using CM data. According to SEM fractography, all
samples showed microscopically brittle fracture behavior. Notwithstanding, contrary to CM samples, ME-AM
specimens showed macroscopically ductile stress–strain behavior and a transition from a regime with only
a primary 𝛼-deformation process, at low strain rates, to a regime with 2 deformation processes (𝛼 + 𝛽), at
high strain rates. These effects are an influence of the processing step and are attributed to the molecular
orientation and stretch of the polymer chains, provoking anisotropic mechanical properties. As a consequence,
a deformation-induced change of the Eyring rate constants is needed to adequately describe the strain-rate
dependence of the ME-AM yield stress behavior, leaving the initial activation volumes unchanged. Taking this
deformation-dependence of the rate constants into account, yield stresses as a function of infill orientation
angle can be appropriately predicted.
