Low-cycle compression-compression fatigue behavior of MEX-printed PLA parts

Abstract

Due to cyclic loads, additively manufactured (AM) parts can collapse at lower stress values than the strength corresponding to static loads. To date, there is limited research on the fatigue life of AM polymers. The present work investigates the fatigue behavior of material extrusion (MEX) printed Polylactic acid (PLA) parts. In this regard, the low-cycle fatigue (LCF) domain was studied and a compression-compression fatigue test setup was adopted. Initially, before the experimental tests, the MEX-printed samples were subjected to physical assessments. The printed fatigue samples presented a high accuracy with dimensional relative errors below 0.04%, while 6.66% was noted as their mass relative error. Subsequently, quasi-static and cyclic tests were performed in order to determine the mechanical behavior of the AM parts, with special focus on the low-cycle domain of the S-N (or Wöhler) curve. The quasi-static compression behavior of the PLA samples is typical of cellular materials, the samples highlighting a shear band as a failure mechanism. Five load levels were used to plot the low-cycle domain of the S-N (or Wöhler) curve with its 95% level confidence bands. The tests were performed in force-controlled mode with the stress asymmetry ratio R = 0.1 and at the test frequency of f = 10 Hz. An exponential relationship was also found between the strain at break and the number of cycles until failure. The failure of the samples in fatigue load occurred suddenly.