Elastic and Thermal Expansion Constants of Two-Phase Composites
Date Issued
2005
Date
2005
Author(s)
Hsieh, Chin-Lung
DOI
en-US
Abstract
In the present study, a unit cell model is proposed to predict the elastic constants (Poisson’s ratio, Young’s, bulk and shear moduli) and thermal expansion coefficient of two-phase composites. The feasibility of the present model is verified by comparing the model predictions with experimental model.
Comparing with many monolithic materials, many more factors can affect the elastic and thermal properties of two-phase composites. For examples, the shape of the second phase, composition, the size and distribution of pores, interface integrity, etc., can all affect the resulting properties of composites. Apart from these microstructural complexities, the transfer of stress within the composite is also unknown. Therefore, it is almost impossible to deliver a precise estimation on the elastic constants and thermal expansion coefficient of composites. Instead, a pair of upper and lower bounds to cover the possible variation on the elastic and thermal constants is proposed in the present study. We further assume that the model is applied only to the fully dense composites in which the components are strongly bonded. Therefore, it is not possible for the two components to separate at their interfaces when the composite is loaded or heated. The discussion also restricts itself to macro-composites, namely, to those in which the scale of the second phase is large to microstructure so that composite properties can be modeled by using continuum mechanics without resort to atomic and dislocation mechanics. Furthermore, the size of specimen is much larger than the size of second phase so that the properties are an appropriate average of those of the components in the composites.
The present model tend to offer predictions over the entire composition range of the two-phase composites, namely, the amount of the second phase varies from 0 to 100 vol.%. The experimental data for the composites with the composition covers the entire composition range are sparse. Therefore, the Al2O3-NiAl composites with the NiAl content varied from 0 to 100 vol.% are prepared by hot-pressing. The elastic constants of the composites are measured by employing an ultrasonic technique, the thermal expansion coefficient by using a thermal mechanical analyzer.
As the amount of second phase is large, the second phase particle tends to interconnect with each other to form an interpenetrating microstructure. To reflect such microstructural feature, a unit cell model with elongated second phase is proposed in the present study. The stress-strain coupling is also taken into account. The model prediction on elastic modulus and Poisson’s ratio shows strong dependence on the ratio of elastic modulus of matrix to that of second phase. The thermal expansion coefficient of composite also shows strong dependence of the elastic modulus ratio. It demonstrate that the thermal expansion of a composite as it is under a temperature change is in-fact an elastic problem.
The model predictions are compared with the experimental data of the Al2O3-NiAl composites. A comprehensive collection on the experimental data for other two-phase composites has also been conducted. The comparison between the model predictions and all available experimental data demonstrates the validation of the model. The model prediction is also compared with other theoretical models, for example, the Hashin-Shtrikman model for elastic constants and Kerner and Schapery models for thermal expansion coefficient. The model proposed in the present study shows similar precision on the properties of composites as the elastic modulus ratio is lower than 10. However, the elastic modulus ratio is higher than 20, the model proposed in the present study shows improved prediction on the properties of two-phase composites.
Subjects
熱膨脹係數
Al2O3-NiAl
雙相複合材料
單元體模型
彈性常數
泊松比
unit cell model
thermal expansion coefficient
Elastic constants
two-phase composites
Type
thesis
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