Rapidly cured epoxy/anhydride composites: Effect of residual stress on laminate shear strength

The drive towards rapid cure thermosetting composites requires a better understanding of the residual stresses that develop during curing. This study investigates the impact of residual stresses on the interlaminar shear strength of resin-infused epoxy/anhydride carbon-fibre laminates. The magnitude of the residual stress was varied by changing the initial injection cure temperature between 75 °C and 145 °C. The corresponding cycle times and the final glass transition temperature of the resin were also measured. The experimentally measured chemical shrinkage and thermal expansion properties of the resin after vitrification were used as inputs to a finite element analysis to calculate the peak residual stresses in the composite. An increase in the initial cure temperature from 85 to 135 °C resulted in an increase of 25% in the residual stress, which led to an experimentally measured reduction in the composite’s short beam shear strength of approximately 16% (8 MPa), in good agreement with model prediction.     

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.     

Thermal aging of fiberglass/carbon-fiber hybrid composites

Thermal oxidation of a unidirectional carbon-fiber/glass-fiber hybrid composite was investigated to determine oxidation kinetics and degradation mechanisms. The epoxy composite rods were comprised of a carbon-fiber core and a glass-fiber shell. A reaction–diffusion model was developed for each of the two hybrid sections to obtain the oxygen-concentration profile and the thickness of the oxidized layer (TOL) within the composite rods. The TOL was measured experimentally for samples exposed at 180 °C and 200 °C for up to 8736 h, and measured values were similar to the modeling predictions. The glass-fiber shell functioned as a protective layer, limiting the oxidation of carbon-fiber core. A relationship was derived relating TOL to tensile strength of the hybrid composite. The tensile strength remained essentially unchanged by thermal oxidation after 52 weeks of exposure. Inspection of thermally aged capped rods showed no cracking after long-term exposures.     

Effects of thermal exposure on mechanical behavior of carbon fiber composite pyramidal truss core sandwich panel

An experimental study was performed to investigate the effect of high temperature exposure on mechanical properties of carbon fiber composite sandwich panel with pyramidal truss core. For this purpose, sandwich panels were exposed to different temperatures for different times. Then sandwich panels were tested under out-of-plane compression till failure after thermal exposure. Our results indicated that both the thermal exposure temperature and time were the important factors affecting the failure of sandwich panels. Severe reductions in residual compressive modulus and strength were observed when sandwich panels were exposed to 300 °C for 6 h. The effect of high temperature exposure on failure mode of sandwich panel was revealed as well. Delamination and low fiber to matrix adhesion caused by the degradation of the matrix properties were found for the specimens exposed to 300 °C. The modulus and strength of sandwich panels at different thermal exposure temperatures and times were predicted with proposed method and compared with measured results. Experimental results showed that the predicted values were close to experimental values.     

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.     

Research on heat treatment of TiBw/Ti6Al4V composites tubes

Heat treatment with different parameters were performed on the hot-hydrostatically extruded and swaged 3.5 vol.% TiBw/Ti6Al4V composites tubes. The results indicate that the primary α phase volume fraction decreases and transformed β phase correspondingly increases with increasing solution temperatures. The α + β phases will grow into coarse α phases when the aging temperature is higher than 600 °C. The hardness and ultimate tensile strength of the as-swaged TiBw/Ti6Al4V composite tubes increase with increasing quenching temperatures from 900 to 990 °C, while they decrease with increasing aging temperatures from 550 to 650 °C. A superior combination of ultimate tensile strength (1388 MPa) and elongation (6.1%) has been obtained by quenching at 960 °C and aging at 550 °C for 6 h. High temperature tensile tests at 400–600 °C show that the dominant failure modes at high temperatures also differ from those at room temperature.     

Effects of steam environment on creep behavior of Nextel™720/alumina–mullite ceramic composite at elevated temperature

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1000 and 1100 °C in laboratory air and in steam. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel?720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. The presence of steam accelerated creep rates and dramatically reduced creep lifetimes. The degrading effects of steam become more pronounced with increasing temperature. At 1000 °C, creep run-out (set to 100 h) was achieved in all tests. At 1100 °C, creep run-out was achieved in all tests in air and only in the 87.5 MPa test in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.     

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.     

The paint–bake response of three Al–Mg–Zn alloys

The aging behaviors of three Al–Mg–Zn alloys have been investigated under conditions similar to the paint–bake cycle currently used in automotive manufacturing. The three alloys contain Mg in atomic concentrations from one to two times those of Zn. Natural aging at 25 °C after solutionizing is found to produce a linear increase in hardness with logarithmic time for times of up to 1 year. Hardnesses in naturally and artificially aged conditions are found to increase with Mg content. Artificial aging at 175 °C for 30 min, which simulates the automotive paint–bake cycle, produces increases in hardness of 15–36% over the solution-treated conditions. Peak hardness from artificial aging at 175 °C is produced in all alloys after approximately 8 h. Natural aging for 10 days prior to artificial aging at 175 °C does not produce significant changes in hardness compared with artificial aging alone. Natural aging for 1 year after simulated paint–bake aging increases hardnesses by 41–78% over those after simulated paint–bake aging alone. The precipitation strengthening mechanism in these alloys is consistent with η′ formation. Increases in hardness and strength with increasing Mg content are consistent with increased solid–solution strengthening, which is retained even after artificial aging.     

Effect of (α + β) heat treatment on microstructure and mechanical properties of (TiB + TiC)/Ti–B20 matrix composite

For the first stage, a metastable β titanium alloy, Ti–3.5Al–5Mo–4V–2Cr–2Sn–2Zr–1Fe reinforced with trace amounts of TiB whiskers and TiC particles was fabricated by vacuum arc melting process and hot forging followed by heat treatment at 780 °C/740 °C, then by aging at 500 °C, 550 °C, 570 °C and 600 °C. For the second stage, the unreinforced titanium alloy was also fabricated by the same process. The microstructural characteristics were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Traces of TiB whiskers and TiC particles (2.2 vol.%) with a volume ratio of 2:3 synthesized in situ exerted a hybrid reinforcing effect on the β titanium alloy. The reinforcements were uniformly distributed in the matrix and the elastic modulus was improved about 25 GPa. Ultimate tensile strength and yield strength achieves about 1625 MPa and 1500MPa respectively, with ductility at 7% when the aging temperature is 500 °C. The ductility of (TiB + TiC)/(Ti–3.5Al–5Mo–4 V–2Cr–2Sn–2Zr–1Fe) matrix composite could be enhanced by increasing the aging temperatures. After 780 °C followed by aging at 570 °C, excellent strength and plasticity properties were obtained (ultimate tensile strength of matrix alloy is 1350 MPa with elongation of 18% and ultimate tensile strength of composite is 1500 MPa with elongation of 13%).     

Size-controlled preparation of α-calcium sulphate hemihydrate starting from calcium sulphate dihydrate in the presence of modifiers and the dissolution rate in simulated body fluid

Different-sized α-calcium sulphate hemihydrate (α-CSH) rods were hydrothermally prepared by converting calcium sulphate dihydrate at 110–140 °C in the presence of MgCl₂, sodium citrate (CANa), and sodium dodecyl benzene sulfonate (SDBS) as the modifiers. The α-CSH rods with the average diameters and the average lengths in the ranges of 2.6–5.2 and 17.5–33.1 μm, respectively, were tunably prepared. The presence of the modifiers favoured the formation of small-sized α-CSH rods. The effect of the modifiers on decreasing the diameters of α-CSH rods was in an order of SDBS > CANa > MgCl₂. The dissolution rates of the different-sized α-CSH rods prepared at 140 °C in simulated body fluid were in an order of α-CSH (CANa) > α-CSH (MgCl₂) > α-CSH (reference) > α-CSH (SDBS). The naked and small-sized α-CSH rods had high dissolution rates. The adsorption of SDBS on the surfaces of α-CSH rods decreased their dissolution rates.     

Experimental investigation of high-strain rate properties of 3-D braided composite material in cryogenic field

This paper reports the high-strain rate properties of 3-D braided basalt/epoxy composite materials at 26 °C, −50 °C, −100 °C and −140 °C with strain-rate range from 1300 s⁻¹ to 2100 s⁻¹ by experimental study. A simple and effective cryogenic device was applied to the SHPB system to create the low-temperature field of the samples. It was found that the compression modulus, peak stress, failure strain and specific energy absorption of the 3-D braided basalt/epoxy composite materials had different sensitivity to temperatures and strain rates. In the out-of-plane impact, there were two failure modes, namely, compression-failure mode and shear-failure mode. Fracture of fiber tows was irregular with abundant pull-out of fiber and much finely-divided fragmentation of resin among fibers at room temperature. In cryogenic field, the fracture of fiber tows was neat and tidy with few pull-out of fiber and few finely-divided fragmentation of resin. However, in the in-plane impact, there was only compression failure mode. And there was no fracture of fiber tows and no big difference among samples tested under different gas pressures. Because of the function of squeezing and buckling, split-off separation of the composite could be blocked by the tangled fiber tows. As a whole, the reinforcement could still keep its structural integrity.     

Characterization and properties of transparent cellulose nanowhiskers-based graphene nanoplatelets/multi-walled carbon nanotubes films

Transparent cellulose nanowhiskers (CNW)/graphene (GN) and CNW/multi-wall carbon nanotube (MWCNT) films were obtained by ultrasonication assisted mechanically stirring followed by solvent casting methods. GN has more significant influence on the properties of CNW film than MWCNT does because GN exhibits strong interaction with CNW by its adsorption on the surface of GN. Thermal behaviors of CNW-based composite films were greatly affected by addition of GN or MWCNT. The melting peak and initial degradation temperature increase by 23.5 and 24 °C, and by 78 °C and 94 °C for the composite films containing 5 wt% MWCNT and 5 wt% GN, respectively. The composites show the contact angles of 61.9° for GN included film and 46.9° for MWCNT included film, which is higher than that of pure CNW film (42.8°).     

Effect of heat treatment on microstructure and mechanical properties of a new β high strength titanium alloy

Microstructure and mechanical properties of a new β high strength Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe titanium alloy were investigated in this paper. Both the α/β and β solution treatment and subsequent aging at temperatures ranging from 440 °C to 560 °C for 8 h were introduced to investigate the relationship between microstructures and properties. Microstructure observation of α/β solution treatment plus aging condition shows that the grain size is only few microns due to the pinning effect of primary α phase. The β solution treatment leads to coarser β grain size and the least stable matrix. The size and volume fraction of secondary α are very sensitive to temperature and strongly affected the strength of the alloy. When solution treated at 775 °C plus aged at 440 °C, the smallest size (0.028 μm in width) of secondary α and greatest volume fraction (61%) of α resulted in the highest yield strength (1624 MPa). And the yield strength decreased by an average of 103 MPa with every increase of 40 °C due to the increase of volume fraction and decrease of the size of secondary α. In β solution treatment plus aging condition, tensile results shows that the strength if the alloy dramatically decreased by an average of 143 MPa for every increase of 40 °C because of larger size of secondary α phase than α/β solution treated plus aged condition.     

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.     

Microstructure and tribological characteristics of aged Co–28Cr–5Mo–0.3C alloy

In this study, an attempt to investigate the role of isothermal aging on the microstructure and tribological characteristics of Co–28Cr–5Mo–0.3C alloy was made. Regarding the results, isothermal aging at 850 °C and 950 °C for at least 16 h contributed to the formation of lamellar-type carbides at the grain boundary regions. Moreover, at higher aging times (over 16 h), the amount of lamellar-type carbides decreased. The wear properties of as-cast and heat treated samples were determined at 0.5 ms⁻¹ speed several under normal applied loads such as 50, 80, and 110 N. At the lowest applied load (50 N), the samples were isothermally aged at 850 °C for 8 and 16 h and also the ones were aged at 950 °C for 16 h had higher wear resistance probably due to more volume fraction of lamellar-type carbides when compared to as-cast and the other aged samples, but, at higher applied loads (80 and 110 N) due to the formation of adhesive oxide layer on the as-cast sample surface, the wear rate of as-cast samples is lower compared with all heat treated ones.     

Effect of coprecipitation temperature on the properties and activity of fibroblast growth factor-2 apatite composite layer

Fibroblast growth factor-2 (FGF-2) apatite composite layers were formed on anodically oxidized titanium (Ti) rods at temperatures lower than the previously used 37 °C to reduce the risk of the inactivation of FGF-2 in calcium phosphate solution. A two-step procedure was used to coprecipitate FGF-2 apatite composite layers on Ti rods. Continuous and homogeneous carbonate-containing low-crystalline apatite layers incorporating FGF-2 were formed on the surface of the Ti. The amounts of apatite and FGF-2 coprecipitated on the Ti rod surface decreased with decreasing coprecipitation temperature. When the coprecipitation temperatures were 15 and 20 °C, the amounts of FGF-2 precipitated on the Ti rods were 0.19 ± 0.03 and 0.20 ± 0.03 μg/cm², respectively. A cell proliferation assay for evaluating the mitogenic activity of FGF-2 immobilized in the layer showed that when FGF-2 was coprecipitated at 15 and 20 °C, the number of NIH3T3 cells cultured with FGF-2 extract was significantly larger than that at 4, 10, 25 and 30 °C. To obtain the largest amount of active FGF-2 on a Ti surface, a temperature of 15 or 20 °C should be used for coprecipitating FGF-2.     

Numerical simulation of residual stresses fields of DD6 blade during laser shock processing

The object of this paper was to address the effect of laser shock processing (LSP) with single and multiple impacts on the residual stresses of aeroengine blades manufactured by a type of thick DD6 alloy of [0 0 1] orientation at 980 °C. The finite element method (FEM) model of the DD6 blade was established during LSP with round laser spot, and LS-DYNA and ANSYS are employed to simulate the residual stresses fields of the DD6 blade by numerical computation. The first four modal shapes of the DD6 blade of [0 0 1] orientation at 980 °C were given. Moreover, the validity of the model was verified by numerical computation and LSP experiments. As a result, the distribution rules of the compressive residual stress with different impacts multiplicity were described on the basis of discussing the measurement method of peak pressure. Results showed that the impacts number corresponding to the state of uniform stress was not the same as that related to the maximum compressive residual stress which might occur at lower number of shots. For the DD6 blade of [0 0 1] orientation at 980 °C, the best compressive residual stress could be achieved by three impacts.     

Colour shift and mechanism investigation on the PMMA diffuser used in LED-based luminaires

PMMA material is widely used in LED-based luminaires due to several advantages such as excellent optical transparency, durability against radiation, surface hardness (scratch free), rigidity and strength and can be completely recycled. However, few studies have been reported on the colour shift and failure mechanisms caused by this type of material. This paper experimentally investigated PMMA materials with different aging conditions. The following conclusions could be drawn. (1) Discolouration was not observed for any sample subjected to aging of 85 °C for 5000 h, or with additional blue light irradiation for 5000 h, or with additional humidity of 85%RH for 5000 h, or even with aging of 100 °C for 3000 h. (2) The specimen subjected to aging of 150 °C for 360 h has a surface discoloration and has a significant wavelength dependent degradation in the transmission spectrum caused by oxidation. The specimen with aging of 100 °C for 3000 h has a less oxidation, although no significant transmission spectrum reduction was observed. (3) Using such aged specimen as a diffuser mounted on a LED-based luminaire, the radiant flux peak intensity in the blue light area has a more severe reduction than that in the yellow light area, which results in a reduction of the radiant flux intensity ratio of blue light to yellow light and hence induces the colour shift to yellow. The colour shift investigated is 0.005, very close to the general failure criterion of 0.007, while the lumen decay is 10.2%, far less than the failure criterion of 30%.     

Effect of Zr addition on properties of Al–Mg–Si aluminum alloy used for all aluminum alloy conductor

The effects of Zr addition on mechanical property in the aged Al–Mg–Si alloy exposed to thermal-resistant treatment (180–250 °C) have been studied by using both Brinell Hardness tests and tensile tests. The softening process at 180 °C and 230 °C has been investigated by transmission electron microscope (TEM). The Arrhenius Model is introduced to simulate the strength evolution in the thermal-resistant treatment. The results show that tensile strength and thermal-resistant property are improved by addition of Zr, and both the Brinell Hardness and Tensile Strength could maintain no less than 90% of their initial values when the alloy is exposed to heat treatment at 180 °C for 400 h and 230 °C for 2 h. The presence of rod-shaped phases and coarsening particles results in decreasing the hardness of the sample. The relationship between thermal-resistant life and temperature is derived by the Arrhenius Model. When the Al–Mg–Si–Zr alloy is heated at 130 °C, the duration described in the Arrhenius plot could reach to 40 years.