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.     

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.     

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.     

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.     

Fabrication of diamond/aluminum composites by vacuum hot pressing: Process optimization and thermal properties

Diamond/Al composites were prepared by vacuum hot pressing (VHP) to get high thermal properties. The sintering temperature, pressure and time in the VHP process were optimized. Microstructures, thermal properties, interface reaction product and its effect on the properties of the composites were investigated. The result shows that the sintering temperature and time are key parameters to get high thermal property of the composites. The composites with 20–55 vol% diamond sintered at 650 °C for 90 min under a pressure of 67 MPa exhibit thermal conductivities of 320–567 W/mK, over 90% of the theoretical predictions by the differential effective medium (DEM) scheme. The high thermal conductivity is attributed to the favorable interface conductance, while the formation of aluminum carbide at diamond–Al interface is found to be negative.     

Sol-hydrothermal synthesis and characterization of lead zirconate titanate fine particles

A novel sol-hydrothermal route to prepare lead zirconate titanate (PZT) fine particles is presented, which combined the conventional sol–gel process and the hydrothermal method. The effects of experimental parameters including sol/(sol + water) ratio, reaction temperature and reaction time on the product powders were investigated. The prepared PZT powders were characterized by XRD, SEM/EDS, and Raman techniques. The results indicated that the optimal synthesis conditions were sol/(sol + water) = 20%, 200 °C, 8 h. The obtained powders with an average particle size of 700 nm, were phase pure perovskite PZT with cubic-shaped morphology, and the reaction temperature was as low as 150 °C, which was comparatively lower than that synthesized by the normal sol–gel route. It was supposed that the sol precursor and hydrothermal environment played important roles in the formation of the PZT fine powders at low temperature and low mineralizer concentration.     

Failure and formability studies in warm deep drawing of Ti–6Al–4V alloy

Deep drawing experiments have been performed in order to study formability of Ti–6Al–4V alloy sheet at temperature ranging from room temperature to 400 °C. It is found that below 150 °C, formability of the material is very poor and above 150 °C till 400 °C, limiting drawing ratio (LDR) is found to be 1.8 which is substantially lesser than other structural alloys. For better understanding of failures in failed cups, failure regions have been identified in neck and wall which are validated using finite element (FE) simulations. Fractured surface has been examined with scanning electron microscope (SEM) which reveals different types of shallow dimples indicating predominantly ductile failure. Additionally, in the properly drawn cups, thickness distribution has been studied over a temperature range of 150–400 °C and blank diameter 50–54 mm. In order to optimize blank diameter and temperature to obtain uniform thickness distribution of drawn cups, artificial neural network (ANN) and genetic algorithm (GA) have been employed. Thickness distribution for optimized parameters is validated using FE simulation.     

High temperature compression properties of open-cell Ni–20Cr foams produced by impregnation

The mechanical properties of open-cell Ni–20Cr foams were studied by uniaxial compression tests, which were performed at a strain rate of 10^− 3 s^− 1 at ambient temperature, 250 °C, 400 °C and 550 °C respectively. Then, the effect of temperature on the mechanical properties of these foams was analyzed. It was found that both the compressive strength and energy absorbed by Ni–20Cr foam at elevated temperatures became lower than those at room temperature. The compressive strength changed with temperatures in a form of parabola, which satisfied the model derived from Gibson–Ashby model combined with Kocks' kinetic model. A multi-parameter phenomenological elasto-plastic constitutive model was modified to take the temperature effect into consideration. The relationships between model parameters and temperature were determined. The fitting stress–strain curves of the model were in good agreement with the experimental results.     

Experimental study on a two-stage rotary desiccant cooling system

The objectives of this study were to evaluate the performance of a novel two-stage rotary desiccant cooling (TSRDC) system and to obtain useful data and experiences for practical application. Newly developed compound desiccant (silica gel–haloids) was adopted in the system. An experimental set-up was built and used to test the system performance under three typical environmental conditions. System performances were evaluated in terms of moisture removal D and thermal coefficient of performance COP_th. It has found that the required regeneration temperature of TSRDC system is low and COP_th of the system is high. Regeneration temperatures from 65 °C to 80 °C, 65 °C to 75 °C and 80 °C to 90 °C were recommended for each environmental condition. In addition, the effects of some important operating parameters, such as inlet temperature and humidity ratio of process and regeneration air, on system performance were also investigated in this study.     

The effect of processing temperature and placement rate on the short beam strength of carbon fibre–PEEK manufactured using a laser tape placement process

The ability of a modern near infra-red laser tape placement system to produce high-quality laminates is investigated by performing short beam strength tests on samples manufactured at different process temperatures from 400 °C to 600 °C at placement rates of 100 mm/s and 400 mm/s. The temperature history in tape placement is highly dynamic and the correlation between the process control temperature, laser power and the consolidation temperature is not well understood. The complete temperature history was therefore estimated with a previously developed optical-thermal model and validated using long wave infra-red imaging. Short beam strengths equivalent to conventional manufacturing methods were found for placement rates of 400 mm/s. Failure modes of the samples were elucidated by scanning electron microscopy of the fracture surfaces. Signs of degradation were observed on samples prepared with a 600 °C process temperature at 100 mm/s, however none was evidenced at 400 mm/s for the same process temperature.     

Modeling and experimental study of long term creep damage in austenitic stainless steels

One of the main challenges for some reactors components in austenitic stainless steels at high temperature in-service conditions is the demonstration of their behavior up to 60 years. The creep lifetimes of these stainless steels require on the one hand to carry out very long term creep tests and on the other hand to understand and to model the damage mechanisms in order to propose physically-based predictions toward 60 years of service. Different batches of austenitic stainless steels like-type 316L with low carbon and closely specified nitrogen content, 316L(N), are subjected to numerous creep tests carried out at various stresses and temperatures between 525 °C and 700 °C up to nearly 50 • 10³ h.Interrupted creep tests show an acceleration of the creep deformation only during the last 15% of creep lifetime, which corresponds to macroscopic necking. The modeling of necking using the Norton viscoplastic power-law allows lifetime predictions in fair agreement with experimental data up to a transition time of about ten thousand hours which is temperature dependent. In fact, one experimental result together with literature ones, shows that the extrapolation of the ‘stress–lifetime’ curves obtained at high stress data leads to large overestimations of lifetimes at low stress. After FEG–SEM observations, these overestimates are mainly due to additional intergranular cavitation as often observed in many metallic materials in the long term creep regime. The modeling of cavity growth by vacancy diffusion along grain boundaries coupled with continuous nucleation proposed by Riedel is carried out. For each specimen, ten FEG–SEM images (about 250 observed grains) are analyzed to determine the rate of cavity nucleation assumed to be constant during each creep test in agreement with many literature results. This measured constant rate is the only measured parameter which is used as input of the Riedel damage model. Lifetimes for long term creep are rather fairly well evaluated by the lowest lifetime predicted by the necking model and the Riedel model predictions. This holds for experimental lifetimes up to 200,000 h and for temperatures between 525 °C and 700 °C. A transition time as well as a transition stress is defined by the intersection of the lifetime curves predicted by the necking and Riedel modelings. This corresponds to the change in damage mechanism. The scatter in lifetimes predicted by the Riedel model induced by the uncertainty of some parameter values is less than a factor of three, similar to experimental scatter. This model is also validated for various other austenitic stainless steels such as 304H, 316H, 321H (creep rupture data provided by NIMS). A transition from power-law to viscous creep deformation regime is reported in the literature at 650 °C–700 °C for steel 316H. Taking into account the low stress creep rate law, it allows us to predict lifetimes up to 200,000 h at very high temperature in fair agreement with experimental data.     

Reduction of Cr(VI) facilitated by biogenetic jarosite and analysis of its influencing factors with response surface methodology

In this paper, the facilitating role of biogenetic jarosite in the reduction of Cr(VI) by sulfide and its mechanism were investigated through batch experiments and analysis of X-ray photoelectron spectrum (XPS). To study the effects of operational parameters on the reduction of Cr(VI) by sulfide, four operational parameters (pH of solution, operation temperature, loading of jarosite and reaction time) were considered as input variables for response surface methodology (RSM). Graphical response surfaces and contour plots were used to evaluate the effect of interaction between operational parameters on the reduction of Cr(VI). The results suggest that a cycle process of converting Fe(III) to Fe(II) occurred on the surface of jarosite and markedly accelerated the reduction of Cr(VI) by sulfide. For example, the efficiency of Cr(VI) reduced by sulfide increased from 20.5% to 100% when jarosite (1 g/L) was added to the homogenous reaction system at pH = 8 within 40 min. The analysis of variance (ANOVA) revealed a high coefficient of determination (p-value < 0.0001, R² = 97.99%, Adj-R² = 95.98%) between experimental Cr(VI) removal efficiency and predicted one by RSM developed model. The Pareto analysis results demonstrated that the pH of solution was the most significant term of the developed model. Operation temperature, loading of jarosite and reaction time exhibited synergistic effects on the reduction of Cr(VI), and the effect of interaction between independent factors on the response factor can't be ignored.     

Experimental investigation and prediction of wear properties of Al/SiC metal matrix composites produced by thixomoulding method using Artificial Neural Networks

In this study, the wear properties of the SiC particle reinforced aluminium (A356) composite materials (MMCs), produced with thixomoulding method, were investigated both by experimental and Artificial Neural Network (ANN) model in order to determine the weight loss after the wear tests. Two different temperatures (590 °C and 600 °C) were used in production of the MMCs containing 5%, 10%, 15% and 20% SiC (vol%). The samples of MMC were tested at 2 ms⁻¹ constant sliding speed under 30 N and 60 N loads against four different sliding distances (500 m, 1000 m, 1500 m, and 2000 m). The results indicated that by increasing the production temperature increased the grain size of the MMCs was increased, but the hardness was decreased. The MMCs produced at 590 °C were found to have lower weight loss as compared with ones produced at 600 °C. In the theoretical prediction model of the MMCs, weight loss, SiC per cent, production temperature, applied weight and sliding distance were used as input values. After comparing the experimental results and the ANNs predicted data it was observed that R² was 0.9855. This shows that the developed prediction model has a high level of reliability.     

Powder synthesis and properties of LiTaO3 ceramics

The LiTaO₃ powders with sub micrometer grade grain size have been synthesized successfully using a molten salt method. Lithium tantalate began to form at 400 °C reaction temperature and transformed to pure phase without residual reactants when it was processed at 500 °C for 4 h in static air. The undoped LiTaO₃ ceramics with a Curie temperature about 663 °C were obtained by pressureless sintering at 1300 °C for 3 h. The relative dielectric constant (ɛ_r) increases from 50 to 375 at temperature ranging from 30 to 663 °C and then decreases quickly as the temperature increases above 663 °C. The ceramics shows a relative dielectric constant of 49.4, a dielectric loss factor (tan δ) of 0.007, a coercive field (Ec) of 28.66 kV/cm and a remnant polarization (Pr) of 32.48 μC/cm² at room temperature.     

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.     

Effect of heat treatment on microstructure,hardness and rollability of V55Ti30Ni15 alloy membranes

The effect of heat treatment on the microstructure, hardness and rollability of V₅₅Ti₃₀Ni₁₅ alloy membranes has been investigated in this study. The microstructure resulting from different heat treatment conditions has a great influence on hardness. Fine NiTi particles precipitate from the supersaturated V-matrix solid solution at temperatures above 600 °C, increase in quantity until 800 °C, then dissolve back into the V-matrix with a further increase in temperature up to 950 °C. The resultant hardness decreases with temperature until 800 °C, and then increases from 800 to 950 °C. In the present study, a comparison has been made between the rollability of the as-cast and the heat treated state selected for deformation at different rolling temperatures. The percent reduction in thickness of the heat-treated alloy (800 °C/18 h) has been found to be up to 30% higher than that of the as-cast alloy, even at room temperature (cold rolling).     

Influence of process parameters on mechanical performance and bonding area of AA2024/carbon-fiber-reinforced poly(phenylene sulfide) friction spot single lap joints

The effects of friction spot joining process parameters on the bonding area and mechanical performance of single lap joints were investigated using full-factorial design of experiments and analysis of variance. On one hand, the main process parameters with significant influence on the bonding area were joining pressure, tool rotational speed and joining time. On the other hand, tool rotational speed and joining pressure displayed the highest influence on the lap shear strength of the joints followed by tool plunge depth, whereas the joining time was not statistically significant. The interaction between the rotational speed and joining time was the only interaction with a significant effect on the mechanical performance. Joints with ultimate lap shear forces varying between 1698 ± 92 N and 2310 ± 155 N were obtained. It was observed that generally a larger bonding area as a result of higher heat input leads to an increased mechanical performance of the joints. The generated regression model by the analysis of variance was used to identify an optimized set of parameters for increasing the lap shear strength of the joints to 2280 ± 88 N. Furthermore, the process temperature was monitored, which varied in the range of 370–474 °C.     

Kinetics of thermal gradient chemical vapor infiltration of large-size carbon/carbon composites with vaporized kerosene

Large-size samples of carbon/carbon composites were prepared using thermal gradient chemical vapor infiltration with kerosene precursor at 950, 1020, 1100, 1180 and 1250 °C. The temperature gradient, kinetics and density distribution of these samples were studied and the microstructure of pyrolytic carbon was examined by polarized light microscopy. The results show that the initial infiltration rate increased from 5.81 to 21.32 g min^?1 by increasing deposition temperature from 950 to 1250 °C. The densification kinetics relied on deposition temperature and competition between reaction and diffusion, and the diffusion mechanism transformed from bulk to Knudsen diffusion regime. The calculated apparent activation energy is about 68.2 kJ mol^?1. The temperature range 1020–1100 °C is appropriate for fabricating composites with high final bulk density due to high degree of pore filling and the density of sample S3 infiltrated at 1100 °C is the highest among all investigated samples.     

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%.     

High-capacity 60 K single-stage coaxial pulse tube cryocoolers

A high-capacity single-stage coaxial pulse tube cryocooler operating at around 60 K has been developed to provide the appropriate cooling for the next-generation very-large-scale long wave infrared focal plane arrays under development. The application background and cooler design process are described, and the performance characteristics are presented. At present, the cooler typically provides 4.06 W at 60 K with the input power of 180 W at 300 K reject temperature. 4.72 W can also be achieved when the input power increases to 200 W, and over 9.4% of Carnot efficiency at 60 K has been realized. The larger pulse tube diameter of 14.2 mm is used and the evident orientation sensitivity is observed in the range of 55–65 Hz. The experiments also observe the obvious reject temperature dependence.