Guo-Quan Lu

Volume fraction effects on interfacial adhesion strength of glass-fiber-reinforced polymer composites

Abstract The performance of fiber-reinforced composites is often controlled by the properties of the fiber–matrix interface. Good interfacial bonding (or adhesion), to ensure load transfer from matrix to reinforcement, is a primary requirement for effective use of reinforcement properties. Thus, a fundamental understanding of interfacial properties and a quantitative characterization of interfacial adhesion strength can help in evaluating the mechanical behavior and capabilities of composite materials. A large number of analytical techniques have been developed for understanding interfacial adhesion of glass-fiber-reinforced polymers. Among these techniques, the vibration damping technique has the advantage of being non-destructive as well as highly sensitive for evaluating the interfacial region, and it can allow the materials industry to rapidly determine the mechanical properties of composites. In the present study, a simple optical system was contributed for measuring the damping factor of uniaxial fiber-reinforced polymer composites in the shape of cantilever beams. The interfacial damping factors in glass-fiber-reinforced epoxy resin composites were correlated with transverse tensile strength, which is a qualitative measurement of adhesion at the fiber–matrix interface. Four different composite systems were tested in this study. In each system, three different surface treatments of glass-fiber at three different volume fractions were evaluated. The experimental results show an inverse relationship between damping contributed by the interface and composite transverse tensile strength.

A model for predicting micromechanical interfacial adhesion in polymer composites

The mechanical behavior of fiber-reinforced composites is largely determined by adhesion at the fiber-matrix interface. Thus, a fundamental understanding of the interfacial region and a quantitative characterization of the level of interfacial adhesion can contribute to an evaluation of the mechanical behavior and performance of composite materials. Among numerous techniques for interface characterization, the vibration damping method has attracted continually more attention because it provides sensitive and nondestructive detection of the interfacial region in composites. In the research presented here, a new optical system for measuring vibration damping was introduced, and a model for evaluating the interfacial adhesion between fiber and matrix from a damping parameter was developed. A quantitative relationship between the dynamic (vibration damping) and static (interfacial shear strength) adhesion measurements was established. The experimental data from glass-fiber-reinforced epoxy resin composites with different interfacial treatments showed good agreement with the theoretical model.

Correlation of fiber pull-out strength and interfacial vibration damping techniques by micromechanical analysis

Adhesion between fiber and matrix in fiber-reinforced polymer composites plays an important role both in controlling mechanical properties and in the overall performance of composites. This suggests that analytical and experimental methods to characterize the interface can be used to predict the mechanical performance of the material. To this end, vibration damping techniques have been used as a non-destructive method to evaluate interfacial effects on composites. According to the theory of energy dissipation, the quality of the interfacial adhesion can be evaluated upon separating the predicted internal energy dissipation associated with perfect adhesion from the measured internal energy dissipation of a composite system; this enables the quantification of interfacial adhesion. A micromechanics-based model for evaluating the adhesion between fiber and matrix from the damping characteristic of a cantilever beam was developed that shows an inverse relationship between the damping contributed by the interface and its adhesion strength. A simple optical system was used to measure the damping factor of unidirectional fiber-reinforced-polymer composites. Cantilever beam specimens containing either a single glass fiber or three types of single metallic wires embedded in an epoxy resin matrix were tested. A correlation was found between the measured interfacial adhesion strength directly from microbond pull-out tests and the micromechanics-based calculations from vibration damping experiments.

Non-destructive characterization of fibre-matrix adhesion in composites by vibration damping

Adhesion at the fibre-matrix interface in fibre-reinforced composites plays an important role in controlling the mechanical properties and overall performance of composites. Among the many available tests applicable to the composite interfaces, the vibration damping technique has the advantages of being non-destructive as well as highly sensitive. An optical system was set up to measure the damping tangent delta of a cantilever beam, and the damping data in glass fibre-reinforced epoxy-resin composites were correlated with transverse tensile strength which are also a qualitative measurement of adhesion at the fibre-matrix interface. Four different composite systems containing three different glass fibre surface treatments were tested and compared. Our experimental results showed an inverse relationship between damping contributed by the interface and composite transverse tensile strength.

Densification and crystallization kinetics of cordierite glass-ceramic coatings on rigid substrates

The authors report their findings on the sintering kinetics of slip-cast glass-ceramic coatings on rigid substrates at temperatures between 900 C and 1000 C, the glass-ceramic crystallizes into cordierite at sintering temperatures above 900 C. Changes in coating thickness during sintering were monitored in situ using an optical setup, and the crystallization of the coatings was determined by x-ray diffraction. The coatings went through a rapid shrinkage due to initial densification, followed by an expansion stage caused by the transformation of the glass into a crystalline phase with a lower density. The rate and extent of the expansion were more noticeable at sintering temperatures above 900 C. The authors also found that wetting between the glass-ceramic and the substrate was important to the sintering kinetics and sintered microstructure of the coatings. For coatings formed on silicon substrate, growth of large pores was observed at the glass-ceramic/silicon interface. Using wafer-curvature measurement technique, the authors found a significant tensile stress build-up in the coatings during the initial stage of sintering. They believe that the combination of poor wetting and the tensile stress build-up led to the observed desintering phenomenon at the glass-ceramic/silicon interface.

Nondestructive Characterization of Fiber-Matrix Adhesion in Composites by Vibration Damping

Adhesion at fiber-matrix interface in fiber-reinforced composites plays an important role in controlling the mechanical properties and overall performance of composites. Among the many available tests applicable to the composite interfaces, vibration damping technique has the advantages of being nondestructive as well as highly sensitive. We set up an optical system to measure the damping tangent delta of a cantilever beam, and correlated the damping data in glass-fiber reinforced epoxy-resin composites with transverse tensile strength which is also a qualitative measurement of adhesion at fiber-matrix interface. Four different composite systems containing three different glass-fiber surface treatments were tested and compared. Our experimental results showed an inverse relationship between damping contributed by the interface and composite transverse tensile strength.

A Dimple-Array Interconnect Technique for Power Semiconductor Devices

This paper discusses a new method for interconnecting power semiconductor devices: the DimpleArray Interconnect technique (DAI). This technique employs arrays of dimples on copper sheet that allow the semiconductor devices to have metallurgical solder joint with these dimples. Some attractive features of this technique include reduced parasitic noises, more distributed heat flows, capacity of handling high current, minimized thermo-mechanical stresses, and a low cost processing compared to other three-dimensional packaging techniques. In this paper, the implementation of DAI in prototype power modules was described, initial electrical and thermal test results were discussed, and its thermo-mechanical performance was studied using FEM.