X.L. Liu

Anisotropy related spring-in of angled composite shells

During the manufacture of curved or angled shell composite structures, the enclosed angle of such structures tends to be reduced after cure. This phenomenon is referred to as “spring-in”. It is believed that such distortion is caused mainly by the significant difference between the in-plane coefficient of thermal expansion (CTE) and the through-thickness CTE. This might result in a larger out-of-plane contraction than the in-plane contraction during the time that a composite structure is cooling down from the curing temperature. In this paper, a 3-D Finite Element Analysis procedure was developed to predict “spring-in” resulting from anisotropy for both thin and thick angled composite shell structures. The results of the FE analysis were evaluated together with those from the analytical study and experimental investigation conducted by Jain at the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) in Australia. It was concluded, based on these results, that the FE model gives more accurate results than the analytical model, particularly for thicker composite shells. The corner radius effect, shell thickness effect, and lay-up effect on “spring-in”, together with the effect of tool/part interaction on the total distortion were also discussed. Comparison was made with the experimental results reported by Radford and Rennick.

Modelling orthotropic viscoelastic behaviour of composite laminates using a coincident element method

In this paper a finite element procedure for modelling the viscoelastic behaviour of orthotropic composites is presented. The procedure uses the commercially available finite element package ABAQUS and requires no code development. The procedure utilises two coincident shell elements to model the orthotropic viscoelastic behaviour of a composite laminate. The first element exhibits isotropic and viscoelastic behaviour, while the second element exhibits orthotropic and elastic behaviour. The elements are superimposed in such a way that they are coincident. That is, the two shell elements share the same nodes and hence deform together. Consequently, it is expected that this combined hybrid model will exhibit combined orthotropic and viscoelastic behaviour. Numerical examples are presented to illustrate the capability and accuracy of the new procedure.

Modelling of thermal response of oil-heated tools due to different flow rates for the manufacture of composite structures

Abstract Aimed at optimising the heat-transfer process during the cure of resin, a series of numerical simulations on thermal responses of an oil-heated tool for the manufacture of composite structures were performed and results of the study were discussed by authors [Compos. Struct. 47 (1999) 491]. There it was found that the temperature response and distribution of the tool was highly dependent on the oil flow rate. Hence, control of thermal response of an oil-heated tool might be achieved by controlling the flow rate of the oil. To verify the numerical simulations, an oil-heating test rig was built at the Department of Mechanical Engineering, Monash University, Australia, and a series of experimental investigations using different flow rates of the thermal oil of BP product – Transcal-N – were conducted. The results of the experimental study presented in this report were found to be in a very good agreement with the results of corresponding numerical simulations. Based on these findings, analytical and empirical models were developed to describe the heat-transfer process using oil-heated tools, including the time period of the transient heat transfer and the maximum temperature of the tools. The results predicted by the alternative models agreed well with those from the experimental and numerical investigations.

Numerical investigation on thermal response of oil-heated tool for manufacture of composite products

In this paper, the thermal response of an oil-heated tool is studied using a commercial FEM package ABAQUS and the numerical results for laminar flows and fully turbulent flows in the oil are presented. The study is focused on the influence of the oil flow rates on the oil temperature distributions along the oil flow path and the thermal responses of the tool surface. It was determined that the higher a flow rate was, the shorter a transient heat transfer process took. Consequently, it is concluded that the heating process of an oil-heated tool can be controlled by the control of the thermal oil flow rates. Furthermore, since the oil-heated tool can also be used to cool the composite products by circulating cold oil through the tool, instead of hot oil, the cooling processes were also simulated. The results confirm that cooling can be similarly controlled by flow rates.

Glass transition and viscoelastic behaviour of partially cured composites

Abstract For this study, two tests were conducted in order to investigate the cure monitoring of composite parts utilizing XMTM-49 (carbon/epoxy composite) as the specimen. The first test involved the use of a dynamic mechanical thermal analyzer to investigate the nature of the loss modulus while the second test incorporated a differential scanning calorimeter to evaluate the degree of cure of the composite. From the results of the research, it was found that the loss modulus is an extremely sensitive cure monitoring indicator for composites beyond 70% cured. This is a significant finding since traditional ultrasonic procedures could only be effective in monitoring the cure of composite structures when the degree of cure reaches approximately 70% but decreases when the cure reaches 80% or more. Therefore, it is recommended that future developments should focus attention on a non-contact technique for measuring loss modulus for cure monitoring.

In-plane mechanical properties of a carbon-epoxy composite laminate at elevated temperatures

This paper deals with the experimental determination of the in-plane mechanical properties of a carbon-epoxy composite laminate in the temperature range of 100 °C to 150 °C. Focus has been on the in-plane shear modulus of the laminate. A time-dependent constitutive model for the in-plane shear modulus has been developed. Material constants for the model were derived from the creep compliance of the laminate, obtained experimentally at various temperatures. The results show that the in-plane shear property of the composite laminate is linear viscoelastic within the temperature range and loading level investigated. The constitutive model was validated experimentally.