Christian N. Della

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Multi-step micromechanics-based models are developed to predict the overall effective elastic moduli of porous ceramic with randomly oriented carbon nanotube (CNT) reinforcements. The presence of porosity in the ceramic matrix that has been previously neglected in the literature is considered in present analysis. The ceramic matrix with porosity is first homogenized using a classical Mori-Tanaka model. Then, the homogenized porous ceramic matrix with randomly oriented CNTs is analysed using two micromechanics models. The results predicted by the present models are compared with experimental and analytical results that have been reported in literature. The comparison shows that the discrepancies between the present analytical results and experimental data are about 10% for 4 wt% of CNTs and about 0.5% for 8 wt% CNTs, both substantially lower than the discrepancies currently reported in the literature.

A micromechanics method is developed to investigate the effects of porosity in the matrix and the polarization orientations of the piezopolymer matrix and the piezoceramic fibers on the performance of 1–3 piezoelectric composites. The Mori-Tanaka (MT) method is first used to homogenize the porous piezopolymer matrix, and then the MT method for piezoelectric composites is used to analyze the porous piezopolymer matrix with embedded piezoceramic fibers. Results show that the performance of the composites is significantly enhanced by the presence of pores in the matrix and further enhanced by the opposite polarization orientation of the matrix and the fibers.

A new multi-axial fatigue model for fiber-reinforced composite laminates based on Puck’s criterion is proposed in this article. In the fatigue model, fatigue master curves from the ATM are used to determine the uniaxial ply fatigue strengths and the multi-axial fatigue failure is then determined by Puck’s criterion with the fatigue strengths at the ply level. The fatigue master curves from ATM are generated with limited uniaxial fatigue tests and can be applied to fatigue loading conditions with various frequencies and stress ratios. Both uniaxial and multi-axial S-N curves can be derived from thefatigue model. Fatigue failure envelopes are also generated from the model to better interpret the multi-axial fatigue failure in multi-axial stress spaces. The proposed multi-axial fatigue model is based on ply-level predictions, but it can beextended to laminate-level predictions with the CLT or numerical methods such as the FEM. Multi-axial fatigue failures caused by either local or global multi-axiality can be predicted by the model. Both uniaxial and biaxial fatigue experimentswere carried out to provide test data for establishing and validating the proposed fatigue model. The application oftheproposed multi-axial fatigue model is demonstrated with predictions of S-N curves and fatigue failure envelopes of unidirectional laminates and multi-directional laminates with typical lay-up configurations. The predictions from the proposed fatigue model are also compared with various experimental results and reasonably good agreement is observed.

Analytical solutions are developed to study the free vibrations of multiple delaminated beams under axial compressive loadings. The Euler—Bernoulli beam theory and both the `free mode' and the `constrained mode' assumption in delamination buckling and vibration are used. In addition, a parametric study is performed to study the influence of the axial compressive load on the vibration characteristics of the multiple-delaminated beam. Results show a monotonic relation between the natural frequency and the axial compressive load. Furthermore, the axial compressive load is shown to significantly influence the first vibration mode shape of the beam. However, the influence on the second vibration mode shape is less significant. The present `free mode' solution is also used to investigate the first mode and the second mode buckling of the delaminated beams. For certain delamination lengths and locations, the second mode buckling load is lower that the first mode.

In this research, a comparative study of the hydrostatic performances of 1-3 piezoelectric composites with a porous matrix is presented. The piezoelectric fibers PZT-5H and PZT-7A are considered in the present study. The micromechanics based Mori-Tanaka model is used. Results of the study show that PZT-5H/Aradite D composite have better hydrostatic performance than PZT- 7A/Aradite D composite, and this advantage of PZT-5H/Aradite D composite over PZT-7A/Aradite D composite increases with the increase of porosity in the matrix.