Malociklično utrujanje avksetičnih celičnih struktur : doctoral disertation

slovenian

doktorska dela

2021

Univerza v Mariboru, Fakulteta za strojništvo

The doctoral thesis presents the development and validation of the computational model for predicting the fatigue life of 2D auxetic cellular structures, characterised with a negative Poisson's ratio. The presented computational model is based on the energy approach and consists of damage initiation and damage evolution period up to final failure. In this study, research was focused on the mechanical characterisation of aluminium alloy 5083-H111, the development and validation of the appropriate computational model and, finally, the numerical and experimental analysis of the given auxetic cellular structures. Experimental testing of the analysed aluminium alloy included the quasi-static and dynamic testing in the low-cycle fatigue regime. Dynamic tests were performed in a strain control at the strain ratios R=-1 and R=0 at different amplitude strain levels. The experimental results were served as a basis for determining the material constants of the energy approach (c1, c2, c3 and c4) and the material parameters of the constitutive material model, which was then used in the subsequent computational analysis using the Simulia Abaqus software. In the computational model for fatigue life prediction, the algorithm of direct cyclic analysis integrated into the Simulia Abaqus software was used to accelerate the numerical simulation and determination of the fatigue life of the analysed samples. In the proposed computational model, the flat and CT specimens' geometries were analysed first. Both geometries were used also in the previous experimental testing to obtain the mechanical characterisation of the base material. The computational model for fatigue life prediction was successfully validated, based on the comparison of numerical and experimental results for flat and CT specimens. Furthermore, the validated computational model was used as a basis for determining the fatigue life of the chiral and re-entrant auxetic structure. The auxetic structure's fatigue life was defined by the failure of one cell strut, while the total fatigue life consists of the number of cycles required for damage initiation and for damage evolution up to final failure. Computational analyses of auxetic specimens were performed in a strain control at the strain ratio R=-1 at different amplitude strain levels. Furthermore, the LCF experimental tests of the chiral and re-entrant auxetic structure were also performed. Due to different stiffness of the specimens, the experimental results of the LCF tests were presented in three various forms of endurance curves, on the basis of which it was found that both auxetic structures, despite different sample stiffness at the same average deformation energy per cycle, achieve a similar number of cycles up to failure of first cell strut. Finally, a comparison between computationally and experimentally obtained fatigue lives of the chiral and re-entrant auxetic structure was performed. Based on this comparison it was concluded that the developed computational model is suitable for fatigue life prediction of chiral and re-entrant auxetic structures.

08.08.2023