Aging of a polymer core composite conductor: Mechanical properties and residual stresses

Polymer core composite conductor specimens were aged in atmospheric conditions at 140 and 180 °C and then tested under four point bending. When aged up to a year at a temperature of 140 °C no detrimental effect on flexural performance of the composite was observed, as opposed to aging at 180 °C, which had a very negative effect on the properties. A finite element model was developed to characterize the residual stress in the composite on a micro scale using representative volume elements (RVE). The residual stresses developed after aging at 140 °C for a year were minimal. However, at temperatures higher than 160 °C significant increases in the stresses were observed. The effect of chemical aging on the failure process of the rods was not considered but could result in the rapid reduction in the loads at failure for the rods tested at 180 °C for up to a year.     

Fabrication and characterization of three-dimensional PMR polyimide composites reinforced with woven basalt fabric

A PMR polyimide composite reinforced with three-dimensional (3D) woven basalt fabric is fabricated for medium high temperature applications. The PMR polyimide matrix resin is derived from 4,4′-methylenediamine (MDA), diethyl ester of 3,3′,4,4′-oxydiphthalic (ODPE) and monoethyl ester of Cis-5-norbornene-endo-2,3-dicarboxylic acid (NE). The rheological properties of the PMR polyimide matrix resin are investigated. Based on the curing reaction of the PMR type polyimide and the rheological properties, an optimum two-step fabrication method is proposed. The three dimensional fabric preforms are impregnated with the polyimide resin in a vacuum oven at 70 °C for 1 h followed by removing the solvent and pre-imidization. The composites are then consolidated by an optimized molding procedure. Scanning electron microscopy analysis shows that needle shaped voids are generated in yarns and the void volume fraction is 4.27%. The decomposition temperature and the temperature at 5% weight loss of the composite post-cured at 320 °C for 24 h are 440 °C and 577 °C, respectively. The dielectric constant and the dielectric loss of the composite are measured by circular cavity method at 7–12 GHz. The tensile strength and the modulus in the warp direction of the composite are 436 MPa and 22.7 GPa. The composite shows a layer-by-layer fracture mode in three-point bending test. The flexure strength and modulus in the warp direction of the composite are 673 MPa and 27.1 GPa, respectively.     

Thermal behavior of E-glass fiber-reinforced unsaturated polyester composites

The aim of this work is to study the behavior of E-glass fiber unsaturated polyester composites, subjected to moderate and high temperatures. The obtained results show that the chemical, physical and mechanical properties of the resin and the composite change with the rise of the temperature. A thermogravimetric analysis (TGA) revealed that the thermal degradation of the composite occurs in two steps: the first between 130 and 200 °C and the second between 250 and 440 °C.The characterization of the resin and the composite, after heating, revealed that at moderate temperatures (lower than 100 °C) an improvement of the properties of materials is observed. For high temperatures but lower than the temperature of decomposition (Td), the mechanical strength of the resin does seem to be very affected, even improved for certain cases. For these temperatures, the composite presents some fractures of the fiber–matrix interfaces, which causes losses in strength and ductility.When the temperature reaches the temperature of decomposition (Td), a fall of the mechanical properties was recorded for both resin and composite.     

An efficient healing agent for high temperature epoxy composites based upon tetra-glycidyl diamino diphenyl methane

Poly(ethylene-co-methacrylic acid) (EMAA) as a thermally activated healing agent in a high performance, high temperature tetra-glycidyl methylene dianiline (TGDDM)/diethyl toluene diamine (DETDA) mendable epoxy composite is reported for the first time. Despite curing above EMAAs melting point (T_m = 85 °C), healing occurred by incorporating a preliminary low temperature curing step of 5 h at 80 °C, prior to cure at 177 °C. Healing occurred via the pressure delivery mechanism derived from tertiary amine catalysed surface condensation reactions between EMAA and hydroxyl groups from the epoxy resin. Healing efficiencies of 36%, 55% and 105% were achieved after heating at 150 °C, 200 °C and 230 °C respectively, but decreased rapidly with continued healing. Healing at 150 °C and 200 °C revealed significant healing despite remaining in the glassy state. In addition, EMAA enhanced mode I interlaminar fracture toughness by more than 270% for both the DETDA and 4,4-DDS networks.     

The effect of post-cure duration on the mode I interlaminar fracture toughness of glass-fibre reinforced vinylester

The effect of different post-cure conditions on the mode I fracture toughness of a vinylester resin and its glass-fibre reinforced composite counterpart has been studied. Two sets of parameters were investigated. The first was the post-cure duration at constant temperature; 90°C for 1, 4 and 24 h. The second, for resin toughness only, was a combination of post-cure temperature and duration; 90°C/4 h, 80°C/8 h and 70°C/16 h. The results show that the post-cure increases toughness. This trend is consistent between the pure resin and the fibre composite for all treatments, except at 90°C/24 h. It is believed that the prolonged post-cure duration of 24 h has weakened the bond strength at the fibre–matrix interface, thus reducing the effectiveness of toughness transfer from the matrix to the composite. The study concludes that the post-cure enhances the toughness of the glass-fibre/vinylester composite, mainly due to the increase of resin toughness.     

Accelerated thermal ageing of epoxy resin and 3-D carbon fiber/epoxy braided composites

This paper reports the accelerated thermal ageing behaviors of pure epoxy resin and 3-D carbon fiber/epoxy braided composites. Specimens have been aged in air at 90 °C, 110 °C, 120 °C, 130 °C and 180 °C. Microscopy observations and attenuated total reflectance Fourier transform infrared spectrometry analyses revealed that the epoxy resin oxidative degradation only occurred within the surface regions. The surface oxidized layer protects inner resin from further oxidation. Both the resin degradation and resin stiffening caused by post-curing effects will influence the compression behaviors. For the braided composite, the matrix ageing is the main ageing mode at temperatures lower than glass transition temperatures (T_g) of the pure epoxy resin, while the fiber/matrix interface debonding could be observed at the temperatures higher than T_g, such as the temperature of 180 °C. The combination of matrix degradation and fiber/resin interface cracking leads to the continuous reduction of compressive behaviors.     

Ultra-high-temperature tensile properties and fracture behavior of ZrB2-based ceramics in air above 1500 °C

Nearly fully dense ZrB₂–SiC–graphite composites were fabricated from commercially available powder at 1900 °C by hot pressing. The tensile strength of ZrB₂-based ceramics was measured in air up to 1750 °C, which is the first reported tensile strength measurement in air above 1500 °C. A mechanical testing apparatus capable of testing material in ultra-high temperature under air atmosphere was built, evaluated, and used. Tensile strength was measured as a function of temperature up to 1750 °C in air. The respective average values of the tensile strength measured at 1550 °C, 1650 °C, and 1750 °C are 58.4, 44.8, and 21.8 MPa, which are 49.4%, 37.9%, and 18.4% of their room-temperature strength (118.2 MPa), respectively. Moreover, the tensile fracture behaviors and mechanism of ZrB₂-based ceramics at different testing temperatures were discussed based on microstructure characterization.     

Effects of thermal treatment on the mechanical properties of poly(p-phenylene benzobisoxazole) fiber reinforced phenolic resin composite materials

Thermal degradation behaviors of the poly(p-phenylene benzobisoxazole) (PBO) fiber and phenolic resin matrix were investigated. The unidirectional PBO fiber reinforced phenolic resin composite material laminates were fabricated and exposed in a muffle furnace of 300 °C, 550 °C, 700 °C, and 800 °C for 5 min, respectively, to study the effects of thermal treatment on mechanical properties of the composites. After undergone thermal treatments at 300 °C, 550 °C and 700 °C for 5 min, the flexural strength was reduced by 17%, 37% and 80%, respectively, the flexural modulus was decreased by 5%, 14% and 48%, respectively, and the interlaminar shear strength (ILSS) was lowered by 12%, 48% and 80%, respectively. Thermal treatment at 300 °C, the phenolic resin began to pyrolyze and shrink resulted in the irreversible damage of the composites. After 550 °C thermal treatment, the phenolic resin pyrolyzed mostly but the PBO fiber had no obvious pyrolyze, the interface had sever broken. After 700 °C thermal treatment, the phenolic resin formed amorphous carbonaceous and PBO fiber pyrolyzed mostly so the mechanical properties dropped dramatically. At being heated at 800 °C for 5 min, the fiber was nearly totally pyrolyzed and and kept fibrous carbonaceous although the specimen became too brittle to stand any load thereon.     

Synthesis and properties of sandwiched films of epoxy resin and graphene/cellulose nanowhiskers paper

Graphene (GN)-based composite paper containing 10 wt.% cellulose nanowhiskers (CNWs) exhibiting a tensile strength of 31.3 MPa and electrical conductivity of 16 800 S/m was prepared by ultrasonicating commercial GN powders in aqueous CNWs suspension. GN/CNWs freestanding paper was applied to prepare the sandwiched films by dip coating method. The sandwiched films showed enhanced tensile strength by over two times higher than the neat resins. The moduli of the sandwiched films were around 300 times of the pure resins due to the high content of GN/CNWs paper. The glass transition temperature of the sandwiched films increased from 51.2 °C to 57.1 °C for pure epoxy (E888) and SF (E888), and 49.8 °C to 64.8 °C for pure epoxy (650) and SF (650), respectively. The bare conductive GN/CNWs paper was well protected by the epoxy resin coating, which is promising in the application as anti-static materials, electromagnetic interference (EMI) shielding materials.     

Effect of environment atmosphere on thermal shock resistance of the ZrB2–SiC–graphite composite

The thermal shock resistance of the ZrB₂–SiC–graphite composite was evaluated by measuring the retention of the flexural strength after the electrical resistance heating to the temperature ranging from 1000 °C up to 2500 °C. The experiment was operated in two different environment atmospheres (pure oxygen and low oxygen partial pressure which mixed O₂ and Ar with 1:9) at total pressure 2000 Pa. The residual strength for the specimen decreased gradually as the temperature increased up to 2200 °C, and it was slightly higher when heated in low oxygen partial pressure environment than in pure oxygen. In contrast to the specimen heated in low oxygen partial pressure environment, the residual strength for the specimen in pure oxygen increased steeply as the temperature increased from 1600 °C up to 1800 °C. The analysis of the SEM observations combined with EDS confirmed that the surface oxidation played a positive role in the thermal shock resistance of the ZrB₂–SiC–graphite composite with different environment atmospheres. The results here pointed out a potential method for charactering the effect of environment atmosphere on thermal shock resistance of the ZrB₂–SiC–graphite composite.     

Characterization of melded carbon fibre/epoxy laminates

‘Melding’ is a novel in situ method for joining thermosetting composite structures, without the need of adhesives. Laminate joining is achieved using uncrosslinked resin matrix of the pre-preg. This study used Hexply914C pre-preg material to characterize melded CFRP structures produced using the melding method. A designated area of a laminate was maintained at temperatures below 40 °C retaining uncured (B-staged) material, while the remainder of the laminate was cured at 175 °C. After a 2.5 h cure cycle, the cured region showed a high degree of cure (0.88) and glass transition temperature (176 °C). The uncured area of the same laminate was cured in a second stage, simulating an in situ melded joint. By controlling the temperature and duration of the intermediate dwell and affecting minimum viscosity values prior to final cure, low values of porosity (<0.5%) were achieved. The mechanical properties of the resulting joint were consistent throughout the melded laminate. Flexural strength (1600 MPa), flexural modulus (100–105 MPa) and short beam strength (105–115 MPa) values observed where equivalent or greater than those found in the recommended autoclave cured control specimens. After the entire laminate was post cured, glass transition temperatures of 230 °C (peak tan δ) were observed in all areas of the laminate.     

Retention of mechanical performance of polymer matrix composites above the glass transition temperature by vascular cooling

An actively cooled vascular polymer matrix composite containing 3.0% channel volume fraction retains greater than 90% flexural stiffness when exposed continuously to 325 °C environmental temperature. Non-cooled controls suffered complete structural failure through thermal degradation under the same conditions. Glass–epoxy composites (T_g = 152 °C) manufactured by vacuum assisted resin transfer molding contain microchannel networks of two different architectures optimized for thermal and mechanical performance. Microchannels are fabricated by vaporization of poly(lactide) fibers treated with tin(II) oxalate catalyst that are incorporated into the fiber preform prior to resin infiltration. Flexural modulus, material temperature, and heat removal rates are measured during four-point bending testing as a function of environmental temperature and coolant flow rate. Simulations validate experimental measurements and provide insight into the thermal behavior. Vascular specimens with only 1.5% channel volume fraction centered at the neutral bending axis also retained over 80% flexural stiffness at 325 °C environmental temperature.     

Cure path dependency of mode I fracture toughness in thermoplastic particle interleaf toughened prepreg laminates

The effect of cure cycle on fracture behaviour of a commercial thermoplastic particle interleaved prepreg system was investigated. Laminates were manufactured at 700 kPa in an autoclave using eight different thermal cycles that included both raising the cure temperature above the standard 180 °C cure cycle and incorporating an intermediate dwell stage between 150 and 170 °C prior to reaching the 180 °C cure temperature. Double cantilever beam tests were conducted on specimens from the cured laminates. The stick–slip crack behaviour, observed in samples manufactured using the standard cure cycle, changed to stable crack growth when processing deviated by 10 °C. The mode I fracture toughness values were reduced by 11–22% when incorporating an intermediate dwell stage before the final cure temperature. Scanning electron microscopy inspection of the fracture surfaces showed differences between samples made by standard cure cycles and those made using process deviations.     

The compressive responses of glass fiber composite pyramidal truss cores sandwich panel at different temperatures

A new method for fabricating glass fiber composite sandwich panel with pyramidal truss cores was developed based on the vacuum assisted resin transfer molding technology. The microstructure and organizations of fabricated sandwich panels were examined by the scanning electron microscope. The out-of-plane compressive tests of composite sandwich panels were performed throughout the temperature range from −60 °C to 125 °C. Then the effects of temperature on the compressive strength, compressive modulus and failure mechanism were investigated and analyzed. Our results indicated that cryogenic temperature resulted in the increasing of the compressive modulus and strength, while high temperature caused the degradation of the compressive modulus and strength. The effect of temperature on failure mode of composite sandwich panel was also observed. Analytical expressions were presented to predict the compressive modulus and strength of composite sandwich panels at different temperatures.     

Temperature effects on the time dependent viscoelastic behaviour of carbon/epoxy composite materials: Application to T700GC/M21

In this study, strain rate and low temperature dependencies of the viscoelastic behaviour of the T700GC/M21 composite material are characterised and analysed. Dynamic tests for various environmental temperatures are performed on hydraulic jack equipped with an environmental chamber. Three speeds, between 8.33 · 10⁻⁴ m s⁻¹ and 0.5 m s⁻¹, at three temperatures (20 °C, −40 °C and −100 °C) are tested. The increase of the shear modulus with the decrease of the temperature is more pronounced between −40 °C and −100 °C than between 20 °C and −40 °C. Complementary DMA (Dynamic Mechanical Analysis) tests are performed on the M21 epoxy resin to characterise the viscoelastic behaviour of the matrix which contributes to the viscoelastic behaviour of the laminate. DMA tests highlight a low temperature transition called β transition (−67 °C for the 1 Hz test) which is responsible of the larger increase of the storage modulus, for the epoxy matrix, between −40 °C and −100 °C. Consequently the β transition could also be at the origin, for the composite, of the observed larger increase of the shear modulus with respect to the strain rate, for strain rates higher than 10 s⁻¹.     

Cure behaviour and void development within rapidly cured out-of-autoclave composites

This study investigates the effect of slow and fast heating rates (1.5 and 10 °C/min) on the formation of voids during the out-of-autoclave curing process of an aerospace composite (HexPly 8552). The cure cycles were interrupted at pre-defined stages for each heating rate enabling the in situ behaviour of the resin, void content and growth of voids to be studied. It was found that the morphology and content of voids remained unchanged up to the second heating stage of the cure cycle, regardless of heating rate. Thereafter, differences to minimum resin viscosity for the faster heating rate (5.2 Pa s compared to 32.5 Pa s) and a higher gelation temperature (177 °C compared to 160 °C) caused a slight increase to void growth for the rapid curing conditions. The causes of voids were the result of moisture volatiles contained within the prepreg, identified by mass spectrometry. Overall, the faster heating rate reduced the cure cycle time by 32% without any effect on the final degree of cross-linking (88.4%) or overall void content, which remained below 2%.     

Interfacial bond strength and fracture energy at room and elevated temperature in titanium matrix composites (SCS-6/Timetal 834)

Push-out experiments in the temperature range from room temperature to 530 °C have been performed. On the basis of the measured maximum load to break the fibre/matrix bond and the load to overcome frictional shear stresses finite element analyses are performed to derive an interfacial shear strength and interfacial fracture energy for the different test temperatures. The thus derived material properties decrease at increasing test temperature. In the analyses the in-homogeneity of the (shear) stress distribution plays an important role for the thermal as well as for the mechanical stress distribution. Although only the energy concept requires principally an initial debond length, it is shown that without an initial defect the shear strength concept leads to unrealistic results. The results presented are valid if it can be accepted that during specimen preparation an initial debonded zone of 12–18 μm at the surface is introduced.     

Thermal shock behavior of nano-sized SiC particulate reinforced AlON composites

Aluminum oxynitride (AlON) has been considered as a potential ceramic material for high-performance structural and advanced refractory applications. Thermal shock resistance is a major concern and an important performance index of high-temperature ceramics. While silicon carbide (SiC) particles have been proven to improve mechanical properties of AlON ceramic, the high-temperature thermal shock behavior was unknown. The aim of this investigation was to identify the thermal shock resistance and underlying mechanisms of AlON ceramic and 8 wt% SiC–AlON composites over a temperature range between 175 °C and 275 °C. The residual strength and Young's modulus after thermal shock decreased with increasing quenching temperature and thermal shock times due to large temperature gradients and thermal stresses caused by abrupt water-quenching. A linear relationship between the residual strength and thermal shock times was observed in both pure AlON and SiC–AlON composites. The addition of nano-sized SiC particles increased both residual strength and critical temperature from 200 °C in the monolithic AlON to 225 °C in the SiC–AlON composites due to the toughening effect, the lower coefficient of thermal expansion and higher thermal conductivity of SiC. The enhancement of the thermal shock resistance in the SiC–AlON composites was directly related to the change of fracture mode from intergranular cracking along with cleavage-type fracture in the AlON to a rougher fracture surface with ridge-like characteristics, crack deflection, and crack branching in the SiC–AlON composites.     

Microstructure and mechanical properties of MoSi2–MoSi2 joints brazed by Ag–Cu–Zr interlayer

The present work investigates joining of two MoSi₂ parts through Cusil/Zr/Cusil interlayer with Cusil being a commercial eutectic of Cu–Ag alloy. The joining operation was implemented in an inert gas tube furnace by brazing. The brazing temperature ranged from 800 to 930 °C while the operation lasted for 60 min. Evaluation of joints strength through shear loading identified the maximum strength 60.31 MPa for the brazed sample at 830 °C. Interfacial microstructure was studied by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD) techniques. Applying the temperature of 830 °C was led to a uniform dense joint consisting of various phases with excellent bonding within the interfaces. XRD and EDS results revealed different phases such as Mo₅Si₃, Ag-rich solid solution and Cu₁₀Zr₇ at the interface. At higher brazing temperatures the amount of intemetallic compounds and residual stresses increased and therefore, mechanical properties of the joint degraded. The fracture analysis by SEM revealed various fracture path and morphology for different brazing temperatures.     

Seawater effect on impact behavior of glass–epoxy composite pipes

The effect of seawater immersion on impact behavior of glass–epoxy composite pipes is experimentally investigated. Glass–epoxy pipes with [±55°]₃ orientation were fabricated using filament winding method. Composite pipes were selected for four different diameters as 50 mm, 75 mm, 100 mm, and 150 mm. The pipes were immersed in artificial seawater having a salinity of about 3.5% for 3, 6, 9, and 12 months in laboratory conditions. At the end of the conditioning period, the specimens were impacted at three distinct energy levels as 15 J, 20 J, and 25 J at ambient temperature of 20 °C. The comparisons between the dry and immersed cases were carried out by using contact force, deflection and absorbed energy data of the impact tests. Results show that moisture absorption, salt in seawater, diameter of specimen and residual stresses produced by manufacturing process of the composite pipe have significant effect on maximum contact force, maximum deflection, absorbed energy and failure of composite pipes according to exposure time to seawater.