Numerical analysis and fiber Bragg grating monitoring of thermocuring processes of carbon fiber/epoxy laminates

A three-dimensional differential viscoelastic model combining the effects of curing degree, thermal expansion, chemical shrinkage and stress relaxation for composite laminates was established and well verified. The evolution of strain and stress of composite laminates during cure was numerically simulated using the validated model. Also, fiber Bragg grating temperature and strain sensors were adjacently embedded in the composite laminate to in situ monitor the temperature and strain evolution. The monitored strain was evaluated by comparison with the corresponding simulated strain. The results reveal that the monitored temperature can reflect the actual temperature evolution in composite laminates, whereas the monitored strain cannot accurately characterize the actual strain evolution in composite laminates at the early stage of cure. However, when the resin/grating interfacial bond strength increases enough to transfer resin strain effectively, the changes in the monitored strain can match well with the actual strain changes.     

Thermochemical and physical properties of printed circuit board laminates and other polymers used in the electronics industry

Theories behind four thermal analysis techniques are reviewed, and relevant case studies are used to illustrate the application of these techniques to measure; various parameters relevant to printed circuit board laminates and engineering polymers.Thermogravimetric analysis (TGA) is used to determine the filler content of polymers and composites and when combined with Fourier transform infrared (FTIR) spectroscopy can be used for the chemical analysis of evolved gases.Differential scanning calorimetry (DSC) is used to measure the melting point of polymers and the degree of cure of prepregs, laminates and adhesives.Thermomechanical analysis (TMA) is used to measure the coefficient of thermal expansion (CTE) and the glass transition temperature (Tg) of laminates, and dynamic mechanical analysis (DMA) is used to measure the storage modulus, loss; modulus and Tan δ of polymers.     

High‐performance aromatic thermosetting resins with hydroxyl and ethynyl groups

A novel hydroxyl‐ethynyl‐arene (HEA) resin was synthesized via Aldol condensation and Sonogashira reaction. The structure of the obtained resin was confirmed by the techniques of mass spectroscopy (MS), gel permeation chromatography (GPC), proton nuclear magnetic resonance spectroscopy (¹H‐NMR), Fourier transform infrared spectroscopy, (FT‐IR) and elemental analysis (EA). Differential scanning calorimetry (DSC) results showed an exotherm at the temperature range of 187°C–245°C, attributable to crosslinking reaction of the acetylene groups. After thermal cure, the obtained cured resin possessed excellent thermal stability. Thermal gravimetric analysis (TGA) in nitrogen showed the Td₅ (temperature of 5% weight loss) was about 400°C, and the char yield in nitrogen was about 78% at 900°C. The laminate composite of HEA resin was prepared and its mechanical and thermal properties were determined. The usefulness of the HEA resin as matrix for ablative composite was evaluated. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermo-mechanical characterisation of in-plane properties for CSM E-glass epoxy polymer composite materials – Part 1: Thermal and chemical strain

The in-plane thermo-mechanical properties and residual stresses of a CSM E-glass/Epoxy material are characterised through the use of DSC and TMA. The measured data is used to generate material models which describe the mechanical behaviour as a function of conversion and temperature. The in-plane thermal expansion coefficient (α) of the composite material decreases above the glass transition temperature (T_g), which is compensated by a higher out of plane deformation above T_g. Comparison of α and chemical shrinkage measurements suggests that chemical bonds between the polymer matrix and the glass fibres are formed prior to shrinkage of the epoxy matrix, i.e., at an early processing stage. This suggests that production of composites with low residual stresses requires focus on reactivity between the matrix and the sizing rather than the matrix cure properties. As a consequence, residual stresses in the composite material are mainly a result of restricted cure shrinkage rather than mismatch between thermal expansion coefficients.     

Improving the interlaminar fracture toughness of carbon fiber/epoxy composites using clustered microcapsules

The composite laminates are susceptible to delamination between reinforcing plies during their long-term service. In this paper, we propose a modified carbon fiber/epoxy composite laminate with embedded clustered dual-component microcapsules in order to increase the interlaminar fracture toughness of the lamina. The details of microcapsules were illustrated using scanning electron microscope (SEM). The modified CF/EP composite laminates were fabricated using hot-compaction technique. Mode I interlaminar fracture tests were conducted using double cantilever beam specimens, then the values of opening fracture toughness G_IC were calculated to evaluate the toughening effect of modified laminates. The toughening mechanism was revealed and discussed through micrographs of the fracture surfaces obtained by ultra-depth microscope and SEM. The results show that clustered microcapsules after polymerization are equal to special Z-pinning, significantly enhancing the ability of crack arrest, and largely and roundly improved the G_IC values of resultant composite laminates. Meanwhile, the clustered microcapsules and matrix resin formed a second-phase material layer, which also absorbed the fracture energy and suppressed the expansion of cracks.     

Measurement of (post-)curing strain development with fibre Bragg gratings

Curing shrinkage of polymer matrices is a significant source of residual strain formation in thick composite products manufactured with liquid resin infusion. The goal of this paper is to investigate the contributions of cure shrinkage and postcure to residual strain development in a thermosetting polyurethane system suitable for resin infusion of thick composites using fibre Bragg gratings. The results showed that around half of the total shrinkage that contributes to residual strain build-up is due to chemical shrinkage, whereas the other half comprises thermal contraction from the vitrification point. The postcure treatment was found to relax internal strains significantly. The strain-free temperature was found below the postcure temperature and, therefore, the postcure treatment did not induce additional chemical or thermal strains.     

Thermal expansion of aromatic polyamideimide composite films reinforced with aromatic polyamide fibers

Aromatic polyamideimide (PAI) films were reinforced by aromatic polyamide fibers as a unidirectionally oriented composite (Type I) and bidirectionally oriented laminate composite (Type II). The thermal expansion of the composite films was investigated with respect to the direction of fiber orientation. The thermal expansion behavior was anisotropic in the unidirectionally reinforced composite films, and considerably isotropic in the bidirectionally reinforced composite film. The thermal expansion coefficients based on thermoelastic models are calculated and compared with experimental data.     

Enhancement on interlaminar shear strength and water‐lubricated tribological performance of high‐strength glass fabric/phenolic laminate by the incorporation of carbon nanotubes

High‐strength glass fabric (HSGF)/phenolic laminates modified with different contents of carbon nanotubes (CNTs) were fabricated by hot‐compression technique. The effects of CNTs on the interface of HSGF/phenolic, interlaminar shear strength (ILSS) and water‐lubricated tribological performance of HSGF/phenolic laminate were investigated. The ILSS of the laminates were tested on a universal testing machine (DY35), and the tribological properties were evaluated by a block‐on‐ring tribo‐tester. The interfaces of HSGF/phenolic and the worn surfaces of the laminates were analyzed by scanning electron microscope. The results showed that the moderate incorporation of CNTs improved the interface of HSGF/phenolic and accordingly enhanced the ILSS of the laminate. Besides, the friction coefficient of HSGF/phenolic laminate sliding against stainless steel in water can be remarkably stabilized and lowered by the incorporation of CNTs due to the better water lubrication induced by added CNTs and the intrinsic self‐lubrication of CNTs which were further graphitized during the friction and wear process. And the wear rate of the laminate can be accordingly reduced by 1 order of magnitude. The results indicate that CNTs have excellent potential in enhancing both ILSS and tribological fabric/polymer laminate composite, which will greatly improve the current situation of deterioration on mechanical properties by adding traditional solid lubricants. Copyright © 2014 John Wiley & Sons, Ltd.     

Cure study of addition-cure-type and condensation-addition-type phenolic resins

By the incorporation of propargyl and methylol groups on to novolac backbone, a series of addition-curable phenolic resins and condensation-addition dual-cure type phenolic resins (novolac modified by propargyl groups referred as PN, and novolac modified by propargyl and methylol groups simultaneously referred as MPN) were synthesized. The processing characteristics, thermal cure and catalytic cure behavior for both resins were investigated mainly by means of viscosity measurement and non-isothermal differential scanning calorimetry (DSC) techniques. The effect of propargyl and methylol content of PN and MPN, the molecular weight and the configuration of the parent novolac, on the processing and cure behavior was studied in details. Processing parameters and curing kinetic parameters were obtained. Both resins exhibit excellent processing properties. Thermal cure of PN resins possessed one cure mechanism and that of MPN resins possessed two cure mechanisms according to DSC analysis. The dual-cure-type mechanism made MPN resins superior to PN resins in terms of a mild and controllable cure process. Compared with thermal cure, catalytic cure of PN resins showed lower initiation temperature and cure temperature by about 60 °C. These novel resins have a bright prospect of application as matrix for thermal-structural composite materials.     

On the strain measurement and stiffness calculation of carbon fibre reinforced composites under quasi-static tensile and tension-tension fatigue loads

The applicability of different strain measurement techniques for carbon/epoxy laminates under quasi-static tensile and tension-tension fatigue loads was studied. Strain gauges, mechanical extensometers, digital image correlation and 2 D camera systems were applied on laminates tested at angles of 0°, 45°, 60°, 90° and ±45°. In addition, displacements recorded by the servo-hydraulic piston were monitored and compared to local strain measurement techniques. Representative examples that illustrate characteristics and limits of each technique in quasi-static and fatigue tests are discussed. Influences of the respective method of strain measurement, the specimen surface, fibre direction and processes in the specimens during tests on the recorded stress-strain behaviour and on the calculated stiffness are presented. Recommendations for accurate strain measurement of anisotropic laminates based on the results are made.     

Measurement of coefficient of thermal expansion of films using digital image correlation method

Applications of the digital image correlation method (DIC) for the determination the coefficient of thermal expansion (CTE) of films is investigated in this paper. A heating chamber was designed for applying thermal load and DIC provides the full-field thermal deformation fields of the test film sample due to temperature changes. The average normal strains in the x and y direction from the region of interest are then extracted for the determination of CTE. The influence of unavoidable small rigid body rotation is discussed and a method to eliminate it to show the pure thermal expansion of the test film is demonstrated. For validation, the CTE of a pure copper sample is determined and compared with the textbook value, confirming the effectiveness and accuracy of the proposed technique. Finally, the CTE of Polyimide (PI) composite film in the temperature range of 20–140 °C is measured. The results reveal that the DIC is a practical and effective tool for full-field thermal deformation and CTE measurement of films.     

Matrix properties and composite failure

The influence of matrix extensibility on the properties of a composite was studied using two glassy polymers of almost identical chemical structure but differing crosslink densities. The lower crosslink density gave a 73 % increase in tensile elongation at break and a 56% increase in specific fracture energy. Unidirectional laminates of glass, carbon, and Kevlar^® fibres were prepared with these two polymers and tested for shear strength, transverse tension, and dynamic fatigue.The shear strengths of the polymers were found to be almost independent of crosslink densities (about 100 MPa). The interlaminar shear strengths of the carbon fibre laminates corresponded to those of the matrix polymers (Kevlar^® fibre laminates failed at 60 %). In accordance with Griffith's equation the more extensible polymer and its laminates performed better in tensile tests transverse to the fibres due to improved fracture energy. Failure criteria based on strain magnification were useful in the case of glass fibre laminates, but proved inadequate for laminates based on anisotropic fibres such as carbon and Kevlar^®.The dynamic fatigue strengths of the two matrix polymers were unaffected by the difference in crosslink densities. Almost the same fatigue strengths were obtained for the matrix polymers as for the laminates (carbon, glass) transverse to the fibres. A lack of processability of the polymer with high functionality was identified as a source of deteriorating effects.     

Impact behaviour of Dyneema® fabric-reinforced composites with different resin matrices

This paper presents an experimental study on the impact behaviour of composite laminates made of a Dyneema^® woven fabric and four different resin matrices. Three thicknesses of each kind of resin laminate were subjected to impact by a spherical steel projectile in a velocity regime ranging from 100 to 200 m/s. The results revealed that the laminates having flexible matrices performed much better in perforation resistance and energy absorption, but had a greater extent of deformation and damage than the counterparts with rigid matrices. It was found that the matrix rigidity played a crucial role in controlling the propagation of transverse deformation, and thereby the local strain and perforation resistance of laminates. The more rigid matrix restrained the laminate's transverse deformation to a smaller area at a given time, which led to higher local strain and lower perforation resistance. Fibre failure in tension was identified as the dominant failure mechanism for the tested laminates.     

Investigation on interlaminar delamination tendency of multidirectional carbon fiber composites

In this investigation carbon fiber reinforced laminates with different orientation layups are prepared and studied under tensile loading condition. Multiple strain measurement techniques, namely, resistive strain gauges, embedded optical sensors and digital image correlation are used to analyze stress-strain behavior simultaneously through the thickness of composite materials, and to determine the sequence of failure in different plies. Inconsistencies of strains measured through different methods is correlated with the tendency for interlaminar delamination, therefore demonstrating the ability of multi-instrument approach to describe damage progress through the thickness of multidirectional laminates. Complementary analysis through acoustic emission methods reveals that the angle of off-axis surface plies can influence the sequence of failure under tensile loading condition, and damage monitoring capabilities of acoustic emission system is directly affected by delamination tendency of surface plies. Remarkably, the delayed failure of off-axis plies is shown to be related to reorientation of these layer towards loading direction using infrared thermography method.     

Viscoelastic response and interlaminar delamination resistance of epoxy/glass fiber/functionalized graphene oxide multi-scale composites

Graphene oxide (GO) was functionalized using three different diamines, namely ethylenediamine (EDA), 4,4′-diaminodiphenyl sulfone (DDS) and p-phenylenediamine (PPD) to reinforce an epoxy/glass fiber (EP/GF) composite laminate, with the aim of improving the overall composite mechanical performance. Different mechanical characterization techniques were used to determine the mechanical performance, including: tensile stress strain, double cantilever beam (DCB) mode-I fracture toughness and dynamic mechanical thermal analysis (DMTA). Scanning electron microscopy (SEM) was used to support the results and conclusions. The results demonstrated remarkable enhancements in the mechanical performance of EP/GF composite laminates by incorporation of functionalized graphene oxide (FGO) nanofiller, whilst the mechanical performance of the GO reinforced composite only improved marginally. Finally, the mechanical performance of the EP/GF/FGO multi-scale composites was found to be dependent on the type of FGO functional groups; of which EDA exhibited the highest performance. These investigations confirmed that the EDA-FGO-reinforced EP/GF composites possess excellent potential to be used as multifunctional engineering materials in industrial applications.     

Determination of thermal expansion coefficient of a monofilament polyamide fiber using digital image correlation

Coiled polymer actuators are a type of artificial muscles that are a promising development in the field of smart materials. The coefficient of thermal expansion of monofilament polyamide fibers is a crucial parameter for understanding the actuation of coiled fibers. The main purpose of this work is to develop a new methodology for estimating the coefficient of thermal expansion and the transition temperature of monofilament polymer fibers. In the experimental procedure, axial deformations of monofilament polyamide fiber samples were induced by temperature variations using a controlled thermal system. These deformations were determined from images of polyamide samples using the digital image correlation method. Two different approaches based on distinct temperature conditions were conducted. An alternative model with three parameters, including the coefficient of thermal expansion, was introduced to describe the thermal-mechanical behavior of monofilament polyamide fibers. Moreover, polyamide samples were also characterized using four conventional methodologies. Results indicated that the coefficient of thermal expansion changed of a modest negative value to a large negative value and this transition occurred around the glass transition temperature of the polyamide. The thermal expansion curves demonstrate good repeatability and all estimated parameters were in accordance with literature, indicating that the proposed approach can be suitable for the proposed study. This investigation may help in understanding of the intrinsic thermal-mechanical behavior of polymeric monofilaments employed as actuators.     

碳布／聚芳醚酮热塑性复合材料的破坏允限

本文研究了碳布增强的聚芳醚酮复合材料的破坏允限。用二碘甲烷增强的x-射线照相法对试件内部的冲击缺陷进行观察,结果表明,其破坏类型可分为分层、基体开裂、纤维断裂和脱胶。用三点弯曲方法测定了损伤试件的剩余弯曲强度与冲击能量的关系。     

Three-phase cyanate ester composites with fumed silica and negative-CTE reinforcements

Three-phase cyanate ester adhesives have been developed using a bisphenol E cyanate ester resin, fumed silica, and negative-CTE (coefficient of thermal expansion) reinforcements: short carbon fiber or zirconium tungstate (ZrW₂O ₈ ). Fumed silica was used to impart thixotropic behavior on the resin and decrease settling in the adhesives. The cured composites were evaluated using various thermal analysis techniques for their thermal-mechanical properties. Composites with short carbon fiber showed enhanced modulus and decreased thermal expansion (70% reduction for 20 vol%) and showed little phase separation. While settling of the dense ceramic particles could not be completely eliminated for the zirconium tungstate composites through rheological modification of the adhesive with added fumed silica, a reduction in CTE of 84% was achieved in the composite (58 vol%) compared to the neat resin. In addition, the effect of thermal history on the cure and temperature induced ZrW₂O₈ phase transitions, and their corresponding influence on thermal strains vs. temperature, are examined by thermomechanical analysis.     

Scaling effect on the tensile properties of [±45/0/±45/0/±45] Polypropylene/Twaron laminates

The effects of scaling on the mechanical response under tension of balanced nonsymmetrical laminates were investigated for a thermoplastic composite: Polypropylene reinforced with Twaron^® fibers. The composite baseline was an 8-ply laminate which consisted of unidirectional plies arranged in the sequence [±45/0/±45/0/±45]. The influence of specimen size on the tensile properties was studied for one (thickness), two (in-plane) and three (volume) dimensional scaling. The stress-strain curves suggested some variation in laminate behavior owing to the dimensional scaling; nevertheless, a further analysis with the classical lamination theory demonstrated that the observed effect was due to small variations in the fiber volume fraction of the laminates. It was concluded that the mechanical properties of these thermoplastic laminates do not exhibit scaling effects. The failure mechanism of the laminates was studied at macroscopic level; a scale effect of the fracture mechanism was observed.     

Experimental investigation of thermal properties and fire behavior of carbon/epoxy laminate and its foam core sandwich composite

According to structural characteristics, composites are classified as laminated structure and sandwich structure. Carbon/epoxy laminate and foam core sandwich composite are the most commonly used laminate and sandwich structure material in the aircraft industry. The flammability of epoxy resins and foam core material is an inherent hazard. Many previous studies focused primarily on their mechanical properties, while the studies on the thermal and fire properties of carbon/epoxy laminate and its foam core sandwich composite have rarely conducted. Therefore, to characterize their thermal and fire properties, a comprehensive experimental investigation and theoretical analysis were carried out in this work using thermogravimetric analysis, cone calorimeter, vertical/horizontal burning tests, limiting oxygen index and scanning electron microscope tests. Several typical characteristic parameters were obtained and analyzed, such as pyrolysis temperature, heat release rate, mass loss, flaming spread rate and limiting oxygen index. These experimental data coupled with theoretical analysis can provide support for fire risk assessment and fire protection design in aircrafts. The carbon/epoxy laminate and foam core sandwich composite are both characterized as the thermally thick materials. The ignition models and mass loss rate models were obtained. Foam core material negatively affects most of the thermal and fire properties of sandwich composite, but the foam core sandwich composite has self-extinguishing behavior during horizontal burning tests, whose LOI is higher than that of carbon/epoxy laminate. Thus, an important conclusion was reached that the ignition position and flame spread direction have critical effect on the fire behavior of foam core material.