Ronald F. Gibson

Use of Nanoindentation, Finite Element Simulations, and a Combined Experimental/Numerical Approach to Characterize Elastic Moduli of Individual Porous Silica Particles

This article describes the use of a combination of experimental nanoindentation and finite element numerical simulations to indirectly determine the elastic modulus of individual porous, micron-sized silica (SiO2) particles. Two independent nanoindentation experiments on individual silica particles were employed, one with a Berkovich pyramidal nanoindenter tip, the other with a flat punch nanoindenter tip. In both cases, 3D finite element simulations were used to generate nanoindenter load–displacement curves for comparison with the corresponding experimental data, using the elastic modulus of the particle as a curve-fitting parameter. The resulting indirectly determined modulus values from the two independent experiments were found to be in good agreement, and were considerably lower than the published values for bulk or particulate solid silica. The results are also consistent with previously reported modulus values for nanoindentation of porous thin film SiO2. Based on a review of the literature, the authors believe that this is the first article to report on the use of nanoindentation and numerical simulations in a combined experimental/numerical approach to determine the elastic modulus of individual porous silica particles.

Experimental and Numerical Characterization of Relaxation in Bolted Composite Joints

This paper reports on experimental and numerical studies of the effects of bolt preloads, viscoelasticity, and external applied static and dynamic loads on bolt load relaxation in a unidirectional carbon/epoxy composite bolted joint. Experimental measurements of bolt-connected joints in three-point bending specimens were employed in the studies, and relaxation was observed to depend on the initial preload and external dynamic applied loads. It was observed that for any magnitude of external load the bolt load relaxation decreases with increasing initial preload. These findings emphasize the importance of the magnitude of the preload. It was concluded that only about 1/3 of the bolt force relaxation in the composite joints could be attributed to viscoelastic behavior of the polymer matrix in the composite, and the remaining 2/3 of the relaxation is likely caused by other mechanisms such as bolt thread slip, plasticity and/or external excitation. This paper also briefly reviews some relevant relaxation studies found in the literature for mechanically fastened composite and hybrid joints, as well as the effects of environmental conditions such as temperature and moisture on joint relaxation, and points out some gaps where more research needs to be carried out to understand the behavior of such joints.

Characterization of particle diameter and interphase effects on Young's modulus of SiO2/epoxy particulate composites

This study involves the investigation of spherically shaped filler diameter and interphase effects on the Youngs modulus of micro and nano size silicon dioxide (SiO2) particle reinforced epoxy composite materials. Specifically, 10μm and 80nm size SiO2 particles and Epon 862 epoxy are chosen as fillers and a matrix material, respectively. While 10μm and 80nm SiO2 particles are dispersed in the epoxy through a direct shear mixing method, nano-composites are fabricated with hardener at desirable ratios. Both micro- and nano-composites are prepared at 2 different particle loading fractions for tensile testing. It is observed that the nano-composites show significant increase in Youngs modulus over micro-composites, showing a linear increase as particle volume fraction increases. This could indicate that for nano-composites, the interphase region between the particle and matrix can considerably affect their mechanical properties. Here, we develop a finite element analysis (FEA) model to investigate the interphase effect on the Youngs modulus of both micro- and nano-composites. This model demonstrates how to estimate the effective volume fraction of a particle as filler using a combined experimental/numerical approach. The effective volume fraction is shown to be important in predicting the mechanical response of nano-scale particles reinforced composite materials.

Characterization of Fatigue Behavior of Composite Sandwich Structures at Sub-Zero Temperatures

This chapter summarizes recent studies of the flexural fatigue characteristics of foam core carbon/epoxy and glass/epoxy composite sandwich beams over the temperature range from 22°C to −60°C. Core shear was found to be the dominant fatigue failure mode for the test specimens over this temperature range. Significant increases in the useful fatigue life with brittle type core shear failure were observed at low temperatures by comparison with the corresponding room temperature behavior. Fatigue failure at the subzero temperatures was catastrophic and without any significant early warning, but the corresponding failures at room temperature were preceded by relatively slow but steadily increasing losses of stiffness. Two different approaches were used to investigate stiffness reductions during fatigue tests, and both approaches led to the same conclusions. Static finite element analyses confirmed the experimentally observed locations of fatigue crack initiation.