Material characterization and residual stresses simulation during the manufacturing process of epoxy matrix composites

doi:10.1016/j.compositesa.2009.09.025

Composites Part A: Applied Science and Manufacturing

Volume 41, Issue 1, January 2010, Pages 108-115

https://doi.org/10.1016/j.compositesa.2009.09.025 Get rights and content

Abstract

A thermal, rheological and mechanical material characterization of an aeronautic epoxy resin from commercial prepreg is reported in this article. The kinetic of the crosslinking reaction of the resin is characterized and modeled. The specific heat, the glass transition temperature, the thermal expansion coefficients, the chemical shrinkage coefficients and the thermo-mechanical properties have been investigated as a function of temperature and degree of cure. Dynamic mechanical measurements are used to determine the gel point. Finally, the residual stresses developed during the curing process are calculated using a finite element simulation, taking into account the material properties evolutions according to proposed models. The results highlight the importance of the characterization accuracy and the associated models.

Introduction

The use of composite materials is growing rapidly especially for aeronautic structures. Nevertheless, the curing of thick composite parts, for example, remains a challenging task because it can lead to temperature and degree of polymerization gradients through the thickness. These phenomena are due to the combination between the low thermal conductivity of the composite and the large heat of reaction released during the crosslinking reaction. The temperature and degree of cure gradients, the chemical shrinkage, the evolution of properties of the matrix during the curing process and the material anisotropy lead to the development of residual stresses in the manufactured part. For example, Bogetti and Gillespie [1] have proposed a methodology for predicting the residual stress development during the curing of thick pieces and that by adding the effect of volumetric shrinkage. The residual stresses can have a significant effect on the mechanical performance of composites structures by inducing delaminations, distortions and initiating cracks [2], [3] especially for the complex shape parts.

Generally speaking, studies concerning residual stress formation in thermosetting composite materials take into account only the thermal effect [4], [5]. Moreover, uniform temperatures and properties through the thickness of the piece are often supposed. The residual stresses are determined by calculating the temperature difference between the cure and ambient temperatures assuming that no stress occurs before the cooling.

Another mechanism contributing to the development of residual stress is the chemical shrinkage of the resin during the cross-linking polymerization reaction. Some authors have emphasized the importance of the volumetric shrinkage [6], [7]. Others have studied the influence of this phenomenon on the curing stress development for thick composite pieces [1], [8].

During the curing process, the thermal, physical, rheological and mechanical properties of the resin vary making the analysis difficult. The modeling has to be improved for a better representation of the variations of the properties and advances in experimental characterizations, especially for the new resin generation, have to be made.

In this paper, an appropriate characterization and modeling of the reaction kinetic of an aeronautic epoxy resin from prepreg (the commercial name cannot be revealed for confidentiality reason) is presented. The variations of the specific heat, the volumetric variation, the glass transition temperature (T_g) and the mechanical properties of the resin with cure are also determined. T_g is identified as a key factor: when the temperature is below T_g the reaction rate is strongly reduced and the material behavior change from rubbery state to glassy state. In addition, a rheological study is performed to correctly identify the gel point from which residual stresses are developed, and a new model is proposed to represent the cure-dependent variations of the mechanical properties of the resin. Finally, a finite element approach is used to simulate the residual stresses generated by the curing process of composite materials.

Section snippets

Characterization of the kinetic of reaction

The reaction kinetics of the resin is characterized with a Pyris-1 Differential Scanning Calorimeter (DSC) from Perkin-Elmer^® and a specific mold (named PVTα mold and described in previous publications [9], [10]) developed in our laboratories. To estimate the time-dependent conversion degree α(t) of the resin, it is necessary to measure the total heat generated ΔH_tot and the instantaneous heat released ΔH(t) during the reaction $α (t) = \frac{Δ H (t)}{Δ H_{tot}}$ The polymerization rate $\frac{d α}{dt}$ is determined as the

Rheological characterization

For the characterization of the gel point, dynamic rheological measurements are performed, with an ARG2, parallel plates imposed-stress rheometer (supplier: TA Instruments). We prepare a 550 mg resin sample, which represents a sufficient quantity for the gap (0.8 mm) between the rheometer parallel plates (diameter of 25 mm). Tests are carried out at different isotherms (between 160 °C and 210 °C). For each of them, frequency scans are performed at different times during the resin curing. Four

Thermo-mechanical characterization

The resin mechanical properties vary with polymerization and temperature. The knowledge of these variations is essential for the estimations of the residual stresses and final properties. In this work, a dynamic mechanical thermal viscoanalyser (Metravib^® DMA 150) is used for the determination of the mechanical properties (the elastic and viscous modulus) variations with the temperature and conversion degree.

Some authors [1], [26] have reported a linear correlation between the conversion degree

Residual stresses simulation

A study of process-induced stresses is presented in this section. Lamina properties are highly dependent on the fiber and resin properties, and fiber volume fraction. Properties of the fiber are assumed constant and independent of curing.

The instantaneous spatially varying properties, for the effective composite plies, are calculated from a micromechanical model [8]. A thermo-chemically-elastic model is used for determining the residual stresses due to the thermal strains

and the chemical

Conclusion

A kinetic, rheological and mechanical characterization of an aeronautic epoxy resin is presented. The use of a specific mold permits to determine the heat flux, the specific heat, and the volumetric variation of representative samples. In addition, an experimental study has shown that an extensive use of DSC can lead to non-reproducible results for the new resin generation. A new model is proposed to represent the cure-dependent variations of the elastic modulus and a comparison with

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Material characterization and residual stresses simulation during the manufacturing process of epoxy matrix composites

Abstract

Introduction

Section snippets

Characterization of the kinetic of reaction

Rheological characterization

Thermo-mechanical characterization

Residual stresses simulation

Conclusion

Compos Part A – Appl Sci

Compos Sci Technol

Compos Part A – Appl Sci

Compos Sci Technol

Polymer

Compos Struct

Compos Struct

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J Compos Mater

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J Compos Mater

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J Compos Mater

Process-induced residual stresses analysis of AS4/3501-6 composite material

Mech Compos Mater Struct

Analysis and modelling of PVTX diagram of an unsaturated polyester resin, thermoplastic additive, and mineral fillers blend

J Appl Polym Sci

Kinetics and thermal characterization of thermosets cure

Polym Eng Sci

Differential scanning calorimetry of the epoxy cure reaction

Polym Eng Sci