Micromechanics of the progress induced damage evolution in thermosets (original) (raw)
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Stress Evolution during Thermoset Cure
MRS Proceedings, 1998
ABSTRACTThe residual stress evolution in an epoxy during isothermal curing process has been determined experimentally. It was found that for a certain curing temperature range in which the characteristic time for the molecular motions leading to volume recovery is shorter than the time scale of the experiment, an incremental elastic constitutive equation can be used to describe the mechanical response of the epoxy. It was also found that appreciable residual stresses are developed in a threedimensionally constrained epoxy resin system within the rubbery state.
Thermomechanical analysis of a toughened thermosetting system
Mechanics of Composite Materials, 2008
This paper reports experimental results of viscoelastic mechanical tests, at five different levels of conversion, for a thermoset composite matrix system toughened with an appropriate percentage of thermoplastic polymer. Results from static tests are used to build master curves at specific degree of cure while shift factors are compared with corresponded values from dynamic experiments in order to assess the validity of time temperature superposition for each conversion. Neat resin plates have been accurately cured, according to the full kinetics model for dynamic and isothermal temperature regime; conversion gradient within the plane and through the thickness was assessed by thermal analysis of samples taken from different locations before extracting the samples from the plates.
Journal of Materials Science
The use of thermoset composites has increased remarkably during the recent past in naval, automobile and aeronautical applications. Despite superior mechanical behaviour, certain problems, e.g. shape distortion, fibre buckling and matrix cracking, are induced in composite part, especially during fabrication due to the heterogeneous nature of such materials. Excellent control of the curing process is required for production of a composite part with required shape and properties. For an accurate simulation of the curing process, exact knowledge of cure-dependent polymer properties and heat transfer is needed. Several instruments are required to identify these parameters, which is time consuming, and costly. In the present study, results on the simultaneous characterization of bulk modulus, chemical shrinkage and degree of cure of vinylester resin using PVT-α device are presented. Determination of cure and temperature-dependent thermal conductivity of the matrix using the same device is also discussed. The obtained results are compared with the available literature results.
Cure kinetics modeling and cure shrinkage behavior of a thermosetting composite
Polymer Engineering & Science, 2010
In this work, the cure kinetics and through-the-thickness cure shrinkage upon curing of a carbon fiber-epoxy composite (AS4/8552) were studied. The study is composed of two major parts. Firstly, dynamic and isothermal Differential Scanning Calorimeter (DSC) scans were performed to develop a new cure kinetics model. The most appropriate kinetic model that produces a nearly perfect fit of all data sets corresponds to a process with two single-step parallel autocatalytic reactions with diffusion control. Multivariate kinetic analysis was used to evaluate the parameters. In the second part of the study, the coefficients of thermal expansion (CTEs), the glass transition temperatures (T g), and the through-the-thickness cure shrinkage strain values of the partially cured unidirectional and cross-ply composite samples were measured by using a dynamic mechanical analyzer (DMA). Cure strains were measured throughout the Manufacturer's Recommended Cure Cycle (MRCC) with the same method. Results indicate that glass transition temperatures of partially cured samples can be measured very closely by the two methods (DSC and DMA). The methods proposed were proved to be very reliable to predict the degree of cure and to measure the through-the-thickness strains during the cure cycle.
Structural micromechanical characterization of the interphase region between two thermoset resins
The local interactions at the interface between two thermoset resins are investigated using microscopic, chemical and micromechanical characterization techniques. The novelty of this work lies in the combination of physico-chemical measurement techniques to investigate polymer interfaces. The characterization techniques include scanning electron microscopy, Raman spectroscopy and nano-indentation. The results allow a quantitative analysis of the concentration profile in the interface region. The observation of the interfacial area by Transmission Electron Microscopy also highlights a gradient of morphological structure. This morphological gradient can be correlated to the concentration profile obtained by the other techniques. Based on the analyses, an estimated interphase length of 800 µm is found for the resins studied in this paper. The concentration gradient in resin/prepreg model systems could only be measured by energy dispersive X-ray microscopy. In this case, the length of the interphase was reduced to 500µm.
Chemical shrinkage characterization techniques for thermoset resins and associated composites
Journal of Materials Science
Control and optimization of curing process is very important for the production of high quality composite parts. Crosslinking of molecules of thermoset resin occurs in this phase, which involves exothermy of reaction, chemical shrinkage (Sh) and development of thermo-physical and thermo-mechanical properties. Exact knowledge of the evolution of all these parameters is required for the better understanding and improvement of the fabrication process. Sh is one such property of thermoset matrix, which is difficult to characterize due to its coupling with thermal expansion/contraction. A number of techniques have been used to determine volume Sh of thermoset matrix, which later on has been used to find tensor of Sh for the simulation of residual stresses and shape distortion of composite part, etc. Direct characterization of volume Sh of composites has also been made by some authors. Though not much, but some work has also been reported to determine the Sh of composite part in a specific direction. In this article, all the techniques used in the literature for the characterization of Sh of resin and composite are reported briefly with their respective advantages, disadvantage and important results.
Review Monitoring the Cure State of Thermosetting Resins
2013
The propagation of low intensity ultrasound in a curing resin, acting as a high frequency oscillatory excitation, has been recently proposed as an ultrasonic dynamic mechanical analysis (UDMA) for cure monitoring. The technique measures sound velocity and attenuation, which are very sensitive to changes in the viscoelastic characteristics of the curing resin, since the velocity is related to the resin storage modulus and density, while the attenuation is related to the energy dissipation and scattering in the curing resin. The paper reviews the results obtained by the authors' research group in the last decade by means of in-house made ultrasonic setups for both contact and air-coupled ultrasonic experiments. The basics of the ultrasonic wave propagation in polymers and examples of measurements of the time-evolution of ultrasonic longitudinal modulus and chemical conversion of different thermosetting resins are presented. The effect of temperature on the cure kinetics, the comparison with rheological, low frequency dynamic mechanical and calorimetric results, and the correlation between ultrasonic modulus and crosslinking density will be also discussed. The paper highlights the reliability of ultrasonic wave propagation for monitoring the physical changes taking place during curing and the potential for online monitoring during polymer and polymer matrix composite processing.
Experimental determination and modelling of volume shrinkage in curing thermosets
This work deals with the characterisation and modelling of the curing process and its associated volume changes of an epoxy based thermoset resin. Measurements from differential scanning calorimetry (DSC) define the progress of the chemical reaction. The related thermochemical volume changes are recorded by an especially constructed experimental setup based on Archimedes principle. Information on measuring procedure and data processing are provided. This includes investigations on compensation of environmental influences , long-term stability and resolution. With the aim of simulating the adhesives curing process, constitutive models representing the reaction kinetics and thermo-chemical volume changes are presented and the model parameters are identified. Keywords Archimedes principle · curing adhesive · expansion and shrinkage measurement · thermoset · epoxy resin · real-time measurement
Reaction kinetics and heat transfer studies in thermoset resins
Chemical Engineering Journal, 1999
The differential scanning calorimetry technique is used to study the polymerization kinetics of thermoset resins (bone cement), both under isothermal and dynamic conditions. A phenomenological kinetic model, which takes into account the diffusion effect, is proposed. The model, coupled with an energy balance, is used to predict the fractional conversion, the conversion rate and the temperature pro®le for different thicknesses of the resin. In isothermal conditions, the conversion is less than 1, giving proof of the presence of unreacted monomer in the resin. The results obtained under dynamic conditions indicate that the resulting temperature increase is responsible for the higher conversion and a better cure of the resin. Bone cement is intended for use in ®lling the gap between bone and metal prosthesis. For such cases, we have also noted that the prosthesis absorbs much heat and effectively cools the resin, whereas bone does not support a strong temperature elevation and is only locally in contact with hot cement at the bone±cement interface, and for a short time period. In this respect, the proposition of a workable model for cement polymerization may help in determining proper limits for cementation techniques. #