Compression Behavior of EBM Printed Auxetic Chiral Structures.

In this study, the cyclic compression and crush behavior of chiral auxetic lattice structures produced from titanium alloy (Ti6Al4V) metallic powder using electron beam melting (EBM) additive manufacturing technology is investigated numerically and experimentally. For material characterization and understanding the material behavior of EBM printed parts, tensile and three-point flexural tests were conducted. Log signals produced during the EBM process were investigated to confirm the stability of process and the health of the produced parts. Furthermore, a compressive cyclic load profile was applied to the EBM printed chiral units having two different thicknesses to track their Poisson's ratios and displacement limits under large displacements in the absence of degradation, permanent deformations and failures. Chiral units were also crushed to investigate the effect of failure and deformation mechanisms on the energy absorption characteristics. Moreover, a surface roughness study was conducted due to high surface roughness of EBM printed parts, and an equation is offered to define load-carrying effective areas to prevent misleading cross-section measurements. In compliance with the equation and tensile test results, a constitutive equation was formed and used after a selection and calibration process to verify the numerical model for optimum topology design and mechanical performance forecasting using a non-linear computational model with failure analysis. As a result, the cyclic compression and crush numerical analyses of EBM printed Ti6Al4V chiral cells were validated with the experimental results. It was shown that the constitutive equation of EBM printed as-built parts was extracted accurately considering the build orientation and surface roughness profile. Besides, the cyclic compressive and crush behavior of chiral units were investigated. The regions of the chiral units prone to prematurely fail under crush loads were determined, and deformation modes were investigated to increase the energy absorption abilities.

Monitoring the Damage State of Fiber Reinforced Composites Using an FBG Network for Failure Prediction.

A structural health monitoring (SHM) study of biaxial glass fibre-reinforced epoxy matrix composites under a constant, high strain uniaxial fatigue loading is performed using fibre Bragg grating (FBG) optical sensors embedded in composites at various locations to monitor the evolution of local strains, thereby understanding the damage mechanisms. Concurrently, the temperature changes of the samples during the fatigue test have also been monitored at the same locations. Close to fracture, significant variations in local temperatures and strains are observed, and it is shown that the variations in temperature and strain can be used to predict imminent fracture. It is noted that the latter information cannot be obtained using external strain gages, which underlines the importance of the tracking of local strains internally.