Su-Seng Pang

Four-phase sphere modeling of effective bulk modulus of concrete

Abstract A four-phase sphere model, extended from Christensen and Lo’s three-phase sphere model for two-phase composite materials, was proposed to estimate the effective bulk modulus of three-phase concrete. The formulations were developed by reducing the four-phase sphere model to an equivalent three-phase sphere model and an equivalent two-phase sphere model. A distinctive characteristic of the proposed model is that, in addition to considering other physical-mechanical parameters, it is able to evaluate the effect of the maximum aggregate size and aggregate gradation on the effective bulk modulus of concrete. Reasonable agreement was found between the calculated effective Young’s modulus and the experimental results from the literature. This suggests that the proposed four-phase sphere model is suitable for estimating the effective elastic modulus of concrete. It is found that the maximum aggregate size, aggregate gradation, and the interfacial transition zone have a significant effect on the effective modulus of concrete.

Experimental Study of Cement-Asphalt Emulsion Composite

An experimental investigation was conducted on a three-phase cement-asphalt emulsion composite (CAEC), in which asphalt was introduced as a cushion layer in between coarse aggregates and cement mortar matrix by dispersing asphalt emulsion-coated coarse aggregates into cement mortar matrix. Laboratory tests on fatigue, strength, rigidity, temperature susceptibility, and stress-strain relationship were implemented to evaluate the mechanical properties of the CAEC. The preliminary test results showed that CAEC possessed most of the characteristics of both cement and asphalt, namely the longer fatigue life and lower temperature susceptibility of cement concrete, and higher toughness and flexibility of asphalt concrete. This experimental study suggested that CAEC might be an alternative way of a base course material in pavement.

Effective Young's modulus estimation of concrete

A two-step analytical procedure is proposed to evaluate the quantitative influence of the maximum aggregate size and aggregate gradation on the effective Youngs modulus of concrete. In the first step, the effective Youngs modulus of a specified basic element, which is composed of an aggregate coated with interfacial transition zone and again covered with cement paste, is obtained based on a proposed four-phase sphere model. The theory of elasticity and Eshelbys equivalent medium theory are used to achieve the goal. In the second step, the rule of mixture method is used to estimate the effective Youngs modulus of concrete. Following the two-step procedure, the maximum aggregate size and aggregate gradation are included in the formulations for the effective Youngs modulus of concrete. The calculated results are compared with experimental results from the literature. The comparison results show a reasonable agreement when isostrain is assumed for every basic element in the second step. Parameters influencing the effective Youngs modulus of concrete are discussed via calculated results.

Impact response of single-lap composite joints

Abstract The low-velocity impact of adhesive-bonded single-lap composite joints has been studied using a spring-mass model. In this quasi-static model, the impact response is represented by a time-dependent force, and the target joint is represented by an equivalent mass with equivalent stiffness. An analytical model has been developed to determine the equivalent mass and stiffness of the joint. The laminated anisotropic plate theory was used in the derivation of the governing equations of the two bonded laminates. The entire coupled system, as well as the assumed peel stress, were solved using both the joint kinematics and suitable boundary conditions. With the combination of a spring-mass equilibrium system and the developed joint model, a relationship between the impact force and the duration has been established. Adhesive stresses, which are believed to be the cause of failure, were predicted from the impact force. Impact tests of single-lap composite joints with different sample thicknesses and overlay lengths have been conducted to verify the proposed model.

Buckling Analysis of Thick-Walled Composite Pipe under External Pressure

Composite pipe has been used in transporting corrosive fluid in many chemical processes in the petrochemical and pulp and paper industries. It is often subjected to external pressures due to process conditions (vacuum), water hammer, pump suction, and differences in elevation head. Most composite pipe designers are currently using traditional metallic pipe standards which do not take into account the anisotropic nature of composite materials. An analytical model for the buckling of orthotropic composite pipe under external pressure has been developed in this study. In this study, first-order laminated anisotropic plate theory was used to construct models of the kinematic and constitutive behavior of the composite cylinders. The Ritz Method was then applied to determine the buckling load under external pressure. Results obtained from the developed model were compared with the theory derived by Flügge for the case of metallic pipe. Due to the inclusion of shear deformation, the new model gave lower buckling loads than Flügges results especially for the case of thick-walled pipe. Good agreement was found when comparing the developed model with experimental results provided by Fibercast Company following ASTM D 2924 for centrifugally cast composite pipe.

Analytical modeling of particle size and cluster effects on particulate-filled composite

Abstract A three-layer built-in model, extended from Christensen and Lo’s three-phase sphere model for particulate-filled composites (PFC) containing no clusters [R.M. Christensen and K.H. Lo, J. Mech. Phys. Solids, 27 (1979) 315.], is proposed to evaluate the particle size and cluster effect on the mechanical properties of PFC. Different from the self-consistent model used by Corbin and Wilkinson [S.F. Corbin and D.S. Wilkinson, Acta Metall. Mater., 42 (1994) 1311], which gives only average stress–strain distribution, this model can be used to estimate the point-by-point stress–strain distribution induced by either external force or temperature variation. Particles that are harder and softer than matrix are studied. It is found from the calculated results that reducing cluster and particle size, using less scattered particles, reinforcing the bonding strength at the interface of particles and matrix, enhancing the deformability of matrix, and employing particles with a coefficient of thermal expansion smaller than that of matrix are efficient methods to resist damages of PFC. In addition, reducing cluster concentrations and increasing particle contents are preferred for PFC containing hard particles and have negative effect for PFC involving soft particles. The selection of particle rigidities should be based on a balanced comparison between strength and rigidity requirements of PFC.

Stress-strain analysis of adhesive-bonded single-lap composite joints under cylindrical bending

Abstract An analytical study is proposed to develop an advanced model for determining the stress and strain distributions of adhesive-bonded composite single-lap joints under cylindrical bending. The anisotropic laminated plate theory and mechanics of composite materials are first used to derive the governing equations of the two bonded laminates. The entire coupled system is then obtained through assuming the peel stress between the two laminates. With the Fourier series and appropriate boundary conditions, the solutions of the system are obtained. Based on the proposed model, the stress and strain distributions of the adherends and the adhesive can be predicted. The coupling effects between tension and bending for asymmetric laminates are also included in this analysis. With the predicted stress distribution, the maximum peel and shear stresses, which are believed to be the most critical criteria on the joint strength, can be located and their values can be determined. An existing FEA code, “ALGOR”, is used as a comparison with this proposed analytical model. Based on the proposed analytical model, the maximum adhesive stress for different overlay lengths is predicted. An optimal overlay length is found to minimize the adhesive stress at the ends of the overlay. This study will be of interest to the aircraft industry since many advanced composite materials and adhesive-bonded lap joints are widely used in this field.

A three-layer built-in analytical modeling of concrete

Abstract It has been accepted in recent years that concrete can be treated as a three-phase composite, consisting of a cement mortar continuous phase, a coarse aggregate dispersed phase, and interface transition zone (ITZ)– an interphase between cement mortar and coarse aggregates. A large number of experiments have been conducted to investigate the effect of every phase on the mechanical properties of concrete; however, the analytical modeling studies have not been conducted as extensively as experimental studies have. In this paper, a three-layer built-in model was developed by assuming coarse aggregates as spheres and embedding a spherical coarse aggregate coated with an ITZ-like interphase and cement mortar layer with uniform thickness into equivalent concrete media. The preliminary calculation results show that, for the three-phase concrete, each phase has several effective factors for the improvement of the mechanical properties of concrete. For overall evaluations, the effective factors are: 1) reducing the elastic modulus of cement mortar and ITZ-like interphase and increasing the elastic modulus of coarse aggregates, 2) decreasing the coefficient of thermal expansion of each phase, 3) enhancing the tensile strength of cement mortar and reinforcing the deformability of ITZ-like interphase, 4) reducing the volume fraction of cement mortar and increasing the concentration of ITZ-like interphase and coarse aggregates phase, 5) decreasing the maximum grain size of coarse aggregates, and 6) adopting open-graded coarse aggregates.

Force-indentation study of transversely isotropic composite materials using a conical-tip indenter☆

Abstract An experimental and analytical study was conducted on the force-indentation relations of composite materials with a rigid, conical-tip indenter. The analytical analysis was developed based on two models: transversely isotropic elastic, and perfectly plastic. In both cases, the force was proportional to the square of the indentation. The proportional constants were also evaluated for the elastic and plastic models. The elastic proportional constant was found to be much larger than that of the plastic at a given load. Scotchply composite oaminae were experimentally evaluated. Comparison of analytical models and the experimental results indicated that the square-power relation was satisfied. The experimentally determined proportional constant was found to be between the elastic and plastic models, indicating that the actual case is a combination of both models. The plastic model showed the lower bound, while the elastic model showed the upper bound for the proportional constants. As the force increased, the results approached the plastic model and deviated substantially with the elastic model.

Deflection and stress analysis of stiffened orthotropic skew panels under uniform transverse loading

Abstract Skew plates with different orientations behave in a manner quite different from those of rectangular plates. An analysis has been carried out for the deflections and stresses of plates with various skew angles subjected to uniformly distributed transverse loads. It has been thought that the excess deflection at the rear tip of a cantilever plate can be reduced by applying stiffeners along the length of the plate. These stiffeners can also reduce the stresses at the root drastically. The skin-stringer connections in aircraft can be treated as a problem of this type. An investigation has been carried out on the stress and deflection characteristics of stiffened parallelogramic plates with different skew angles. The numerical solution with assumed displacement function was developed using a finite element analysis. Experiments using aluminum and Scotchply composite laminates were conducted to verify the results. Cantilever and simply supported boundary conditions were included in the analysis, and an optimized angular stiffener for a particular swept-back panel was achieved.