Accelerated thermo-oxidative aging for carbon fiber reinforced polycyanate under high pressure atmosphere

The aim of this study is to accelerate thermo-oxidative aging in carbon fiber reinforced plastics (CFRP) submitted to high temperature atmospheric pressure. In this study, long-term thermo-oxidative aging test was conducted to examine the effect of high pressure atmospheric environment on the weight and mechanical properties of CFRP to accelerate the rate of thermo-oxidative degradation. The effect of thermo-oxidative environment under high atmospheric pressure on the change in weight, strength, and shrinkage behavior of a CFRP with polycyanate (FSD-M-08178) was investigated. The aging test was conducted at 180 °C up to 8,000 h under atmospheric pressure and the test result was used as a reference data. Other aging tests were also conducted under elevated pressure conditions (0.3 and 0.5 MPa) at the same exposure temperature up to 2,000 h and the test results were treated as the accelerated aging test data. Unidirectional laminates [0]₈ and [90]₈ for tensile specimens and angle ply laminates [±45]_2S for off-axis tensile specimens were aged in oven, and then mechanical tests were carried out on them. The results indicated that elevated pressure would cause faster and larger weight gain in earlier aging period. The outcomes of these experiments were changes in weight, strength, and local shrinkage deformation were changed quickly under elevated atmospheric pressure environment. Same micro damage onset and extension irrespective of pressure during thermo-oxidative aging and subsequent loading test were observed. The test results indicate that the change in weight, strength, and local shrinkage deformation could be accelerated by elevated pressure without change in thermo-oxidative degradation mechanism. Therefore, the elevated pressure testing would be a better method to accelerate thermo-oxidative degradation. On the other hand, the change in strength under elevated pressure conditions is faster than local shrinkage deformation. Equivalent acceleration rate of degradation could not be obtained.     

Effect of long-term high-temperature atmospheric exposure on damage progress and mechanical properties for carbon fiber/polycyanate 90° unidirectional composites

This study focuses on the effect of long-term exposure to high-temperature atmosphere on mechanical properties of carbon fiber reinforced plastic (CFRP) laminate with polycyanate resin system. Transversely oriented unidirectional laminate of carbon fiber/polycyanate (T700SC/FSD-M-08178) CFRP was employed on isothermal aging at 180 °C in air up to 4000 h. Matrix crack extension during isothermal aging was observed by soft X-ray radiography. Tensile and compressive tests were conducted on both non-aged and aged (500, 1000, 2000, and 4000 h) specimens with an acoustic emission (AE) measurement in order to evaluate mechanical properties and damage behavior due to thermo-oxidative degradation. After tensile tests, the fracture surface observation with a scanning electron microscopic (SEM) apparatus was also carried out to characterize damage extension due to thermo-oxidative degradation. The results show that thermo-oxidation induced matrix cracks preferentially in perpendicular area along the fiber direction and the matrix crack propagated along the fiber direction. Due to the crack onset, tensile strength was sharply dropped, even though compression strength was slightly changed. From the AE measurement result, the failure mode change was detected for the tension test. Comparing the soft X-ray radiography result with the SEM observation result, the fracture for tension test did not occur at the maximum crack length location for the longer thermo-oxidative aged specimen.     

The effect of water on the critical stress intensity factor of unsaturated polyester resins

The effect of water on the critical stress intensity factor,K _IC, of a highly unsaturated isophthalic polyester resin has been measured for periods of immersion of up to 2200 h in water at temperatures of 35, 70 and 80° C.K _IC decreased from 0.84±0.1 MN m^–3/2 to 0.24±0.03 MN m^–3/2 after immersion for 2200 h in water at 80° C. The observed changes in the critical stress intensity factor indicate that a constant value is eventually reached. This is consistent with the decline being attributable to the leaching of extractable matter from the polyester resin.     

Improvement of tensile properties and toughness of an epoxy resin by nanozirconium-dioxide reinforcement

Zirconium dioxide (ZrO₂) nanoparticles were systematically added as reinforcement to a diglycidyl ether of bisphenol A (DGEBA)-based epoxy resin. A series of composites with varying amounts of nanoparticles was prepared and their morphology and mechanical properties were studied. The obtained nanocomposites were characterized by tensile tests, dynamic mechanical thermal analysis, and fracture toughness (K_IC) investigations; by standardized methods, to define the influence of the nanoparticle content on their mechanical and thermal properties. The morphological analysis of the composites shows that nanoparticles form small clusters, which are uniformly distributed into the matrix bulk. The tensile modulus (E) and the K_IC of the epoxy matrix increase at rising zirconia content. Improvements of more than 37% on modulus and 100% on K_IC were reached by the nanocomposite containing 10 vol.-% ZrO₂ with respect to the neat epoxy (E_o = 3.1 GPa, K_ICo = 0.74 MPam^0.5). The presence of nanoparticles produces also an increment on glass transition temperature (T _g). The epoxy resin added with 8 vol.-% ZrO₂ records a T _g approximately 8% higher than the unmodified matrix (T _go = 100.3 °C).     

A simplified method for determining K IC in glass with liquid solutions impregnation

In order to determine K _IC in a simplified form, rectangular pieces of glass were scratched with a glazier's diamond, considering this scratch as the crack tip. The sides of the crack were made by sticking two rectangular pieces of proper thickness with epoxy resin as was made in a previous work. The K _IC was determined in glass in which the cracks are impregnated with liquid solutions of kerosene, sodium hydroxide and sodium silicate. For the samples impregnated with kerosene the mean value of K _IC remains constant, whereas for the other two solutions a decrease in the mean value of K _IC is observed, a 10% in samples impregnated with sodium hydroxide and 16% in samples impregnated with sodium silicate, when comparing them with their respective groups of samples of glass without impregnating. The corresponding Weibull diagrams were made and Weibull functions of three parameters were obtained. The respective parameters of the probability distribution functions of K _IC were estimated by means of the graphic method of nomograms.     

Equilibria,kinetics and mechanism for the degradation of the cytotoxic compound L-NG-nitroarginine

L-N^G-nitroarginine (LNNA), an analog of L-arginine, is a competitive inhibitor of nitric oxide synthase which causes the selective reduction of blood flow to tumor cells. Despite the potential of LNNA to function as an adjuvant in cancer therapies, its poor solubility and stability have hindered the development of an injectable formulation of LNNA that is suitable for human administration. This work, for the first time, details a systematic study on the determination of equilibrium K_a constants and the rate law of LNNA degradation. The four K_a values of LNNA were determined to be 1.03, 1.10?×?10^?2, 2.51?×?10^?10, and 1.33?×?10^?13 M. From the kinetic and equilibrium studies, we have shown that the deprotonated form of LNNA is the main form of LNNA that undergoes degradation in aqueous media at room temperature. The rate law of LNNA degradation was found to be first order with respect to OH^? concentration and first order with respect to LNNA^? concentration. The rate constant at 25?°C and 1?atm was determined to be 0.04453 M^?1min^?1. A base catalyzed mechanism of LNNA degradation was proposed based on the kinetic study. The mechanism was found to be very useful in explaining the discrepancies and changes of the rate law at different pH values. It is thus recommended that LNNA should be formulated as a concentrated solution in acidic conditions for maximum chemical stability during storage and be diluted with a basic solution to near physiological pH just before administration.     

Experimental characterization of damage behavior in polycyanate CFRP laminates under high temperature atmospheric exposure

This study focuses on the experimental characterization of damage behavior due to thermo-oxidative-induced matrix shrinkage in carbon fiber reinforced plastics (CFRP) with polycyanate ester. To investigate the effects of laminate configuration on matrix shrinkage behavior, [90]₈ and [0]₈ unidirectional laminates, [±45]_2S angle ply laminates, and [45/0/–45/90]_3S quasi-isotropic laminates were exposed to high temperature atmospheric environment at 180 °C to analyze matrix shrinkage up to 2000 h. These samples were removed from convection oven to observe sample side surface changes. The thermo-oxidative-induced matrix shrinkage was measured on the side surface of CFRP sample by confocal laser microscopy. The results suggested thermo-oxidative-induced matrix shrinkage depended on aged hours, fiber-to-fiber distance, and fiber orientation angle. The matrix shrinkage coefficient could be calculated with a tensorial transformation and empirical formula. The model can predict matrix shrinkage tendency of the 45° intra-lamina layer in quasi-isotropic laminate using the data of 0° and 90° matrix shrinkage in the quasi-isotropic laminates.     

The role of thermo-oxidative aging at different temperatures on the crystal structure of crosslinked polyethylene

In order to investigate the role of thermo-oxidative aging on the crystal structure of crosslinked polyethylene (XLPE) at temperatures below and above the melting point, thermo-oxidative aging was conducted on XLPE cable insulation at 100, 120, 140 and 160 °C respectively. The crystal structure of the fresh sample and aged samples was characterized by X-ray diffraction, differential scanning calorimetry (DSC) and scanning electron microscopy tests. The concentration of antioxidants in XLPE samples was measured by the DSC method. It was deduced by the obtained results that the thermo-oxidative aging process could be divided into the physical aging stage and the chemical aging stage depending on the concentration of antioxidants in XLPE. In the physical aging stage, re-crystallization process led to the improvement of crystal structure and the increase in crystallinity. In the chemical aging stage, the breakage of intramolecular and intermolecular bonds led to drastic chains scission, resulting in the decrease in crystallinity. At 100 °C, the thermo-oxidative aging mainly affected the amorphous regions and had a limited effect on the crystalline regions. At 120, 140 and 160 °C, the thermo-oxidative aging affected the molten spherulites, resulting in decreased lamellae, increased lamellar spacing and the destruction of spherulites.     

New approaches for modelling subgrade nonlinearity in thin surfaced flexible pavements

The primary objective of this paper is to present new methods in characterising flexible pavements that possess nonlinear subgrade behavior using deflection data from falling weight deflectometer (FWD). The two techniques to be introduced are Simplified Deflection Modeling and Deflection Ratio (DefR) approach. FWD deflection data can be modelled accurately using an exponential curve in a mathematical form of Y = K₁ exp (?r/K₂). K₁ is equal to deflection at D₀ in micron and K₂ is the structural parameter at the respective sensor location. K₂ parameter is found to have a direct relationship with the material constant, value of the subgrade and it is taken as a measurement of the nonlinearity of the pavement layer. As K₂ increases and approaches 500, the pavement structure is observed to possess linear elastic behaviour. In the second method, the DefR is defined as the ratio of the FWD deflection of a sensor divided by the deflection of the preceding sensor. For pavements that exhibit nonlinear subgrade behavior, the DefR shows an increasing trend for FWD sensors located at 300mm and beyond. The two techniques have provided alternative approaches for modelling subgrade nonlinearity.     

The grain size dependence of fracture toughness in an Al-6.0% Zn-2.5% Mg alloy

The grain size dependence of the fracture toughness (K _IC) of an aged Al-6.0% Zn-2.5% Mg alloy was studied experimentally. K _IC depended strongly upon grain size (L _G) in two ways. In the small grain size region K _IC decreased with increasing average grain size. In contrast, K _IC increased with increasing average grain size for large grain sizes. The increase in K _IC with increasing grain size arose as a result of the presence of abnormally large grains compared to the average grain size in the large-grained specimens.     

Effects of test procedures and lithology on estimating the mode I fracture toughness of rocks using empirical relations

Empirical estimation is a common method for getting mode I fracture toughness K_IC of rock. By collecting data from tests in this study and literature, 204 sets of K_IC and tensile strength σ_t test data are obtained for new empirical K_IC‐σ_t relations regression. The empirical relations make the estimation of K_IC values from σ_t conveniently, but test procedures and lithology will influence its reasonability and reliability. Results indicate that the empirical K_IC‐σ_t relations obtained from the four different suggested K_IC test methods are all in good but obviously different linear relationship. The analyses show that cracked chevron notch Brazilian disc specimen (CCNBD) test‐based empirical relation is more accurate for estimating K_IC than the other three test‐based empirical relations. As to different lithology, isotropic rocks such as sandstone and carbonatite may be more appropriate for the application of empirical estimation method. However, for coarse grained or anisotropic rocks such as granite and marble, estimation method should be applied carefully because of possibly weak K_IC‐σ_t relations.     

The effect of oxygen pressure on the rate of non-isothermal thermo-oxidation of low density polyethylene

The results of non-isothermal kinetic analysis of the thermo-oxidative degradation in air and in oxygen of the low density polyethylene (LDPE) are presented. It was observed that similar processes take place at thermo-oxidative degradation in air and in oxygen. The first process, which leads to solid products, is followed by thermo-oxidative degradation with generation of volatile products. It was shown that the first process of thermo-oxidation occurs at lower temperatures in oxygen than in air. The kinetic analysis of this process, performed using Kissinger method, shows that the pre-exponential factor (A) depends on the partial pressure of oxygen (P) according to the relationship: A = A ₀ P , where A ₀ and are material constants.     

Application of artificial neural network for predicting plain strain fracture toughness using tensile test results

A back‐propagation neural network was applied to predicting the K_IC values using tensile material data and investigating the effects of crack plane orientation and temperature. The 595 K_IC data of structural steels were used for training and testing the neural network model. In the trained neural network model, yield stress has relatively the most effect on K_IC value among tensile material properties and K_IC value was more sensitive to K_IC test temperature than to crack plane orientation valid in the range of material data covered in this study. The performance of the trained artificial neural network (ANN) was evaluated by comparing output of the ANN with results of a conventional least squares fit to an assumed shape. The conventional linear or nonlinear least squares fitting methods gave very poor fitting results but the results predicted by the trained neural network were considerably satisfactory. This study shows that the neural network can be a good tool to predict K_IC values according to the variation of the temperature and the crack plane orientation using tensile test results.     

Aloe vera vs. poly(ethylene)glycol-based synthesis and relative catalytic activity investigations of ZnO nanorods in thermal decomposition of potassium perchlorate

The green synthesis approach using ecofriendly biological precursors has gained world-wide popularity, reputation and recognition in the synthesis of several inorganic nanomaterials. This work demonstrates that a proper selection of biological precursor from the sustainable natural resources can effectively replace the commercial surfactant for fabrication of nanomaterials. Through this work, the green biotemplate Aloe vera plant extract has emerged as a better substitute of industrial surfactant poly(ethylene)glycol of molecular weight 8000 (PEG8000) in synthesis of ZnO nanorods using a simple sonoemulsion route. The colloidal growth of ZnO nanorods in PEG8000/Aloe vera -assisted sonoemulsion route has been elaborated in the context of relative supremacy of ultrasonic-assisted self-aggregation rate with steric-hindrance effect imposed by PEG8000/Aloe vera . The relative catalytic activity of PEG8000/Aloe vera synthesized ZnO nanorods, Co₃O₄ nanobelts and CuO nanorods in thermal decomposition of potassium perchlorate has been studied by thermo-gravimetric analysis and differential thermal analysis of pure potassium perchlorate and its mixture with nanoscale ZnO/Co3O4/CuO by 2% weight. The ZnO nanorods formulated through Aloe vera route demonstrated higher catalytic activity than that of ZnO nanorods prepared through PEG8000 route. The relative order of catalytic effect of nanoscale metal oxides in thermal decomposition of potassium perchlorate was found in descending order as CuO nanorods > Co₃O₄ nanobelts > ZnO nanorods.     

A fractographic criterion for subcritical crack-growth boundaries in hot-pressed alumina

The percent intergranular fracture (PIF) was measured along radii extending from fracture origins in hot-pressed alumina specimens, fractured at various loading rates and temperatures, and plotted versus estimates of stress intensity factors (K _I) at the various crack lengths. Minima in PIF occur at values ofK _I that are close to the critical stress intensity factors (K _IC) for cleavage on various crystal lattice planes in sapphire. The subcritical crack-growth boundary (K _I=K _IC of the polycrystalline material) occurs near the primary minimum in PIF suggesting that this minimum can be used as a criterion for locating this boundary. In addition, it was noted that the polycrystallineK _IC (4.2 MPa m^1/2) is very close to theK _IC for fracture on {¯1 ¯1 2 6} planes which is 4.3 MPa m^1/2. These observations suggest that critical crack growth begins when increased fracture energy can no longer be absorbed by cleavage on these planes. There is a secondary minimum atK _I>K _IC that appears to be associated with theK _IC necessary for fracture on combinations of planes selected by the fracture as alternatives to the high fracture-toughness basal plane.     

Observations on the effect of calcium segregation on the fracture behaviour of polycrystalline alumina

The influence of calcium segregation to the grain boundaries of polycrystalline alumina on room temperature fracture behaviour has been investigated. In a commercial high-density single-phase alumina containing less than 5 ppm calcium by weight, thermal treatments were employed to achieve equilibrium segregation from 0.6 to 1.6 at % calcium without detectable changes in grain size (18m) or porosity distribution. Room temperature SENB test results revealed an inverse dependence of K _IC on calcium segregation levels in the range examined. Fractures were primarily intergranular in all specimens. Qualitatively, the relationship between K _IC and calcium segregation would be predicted from a consideration of the effect of such an ion on the interatomic spacing at the boundary. However, quantitative agreement with the model is poor, the measured effect being much greater than predicted. A relatively high K _IC value was achieved in a fine grained (2m) hot-pressed alumina containing very low levels of segregated impurities. This material exhibited substantial amounts of cleavage fracture. The higher fracture toughness of this alumina is discussed in terms of both increased intergranular and transgranular fracture stresses promoted by the relatively clean grain boundaries and small grain size, respectively.     

Miscibility, morphology and fracture toughness of tetrafunctional epoxy resin/poly (styrene-co-acrylonitrile) blends

Poly(styrene-co-acrylonitrile) (SAN) was found to be miscible with the tetraglycidylether of 4,4'-diaminodiphenylmethane (TGDDM), as shown by the existence of a single glass transition temperature (T _g) over the whole composition range. However, SAN was found to be immiscible with the 4,4-diaminodiphenylmethane (DDM)-cured TGDDM. Dynamic mechanical analysis (DMA) shows that the DDM-cured TGDDM/SAN blends have two T _gs. A scanning electron microscopy (SEM) study revealed that all the DDM-cured TGDDM/SAN blends have a two-phase structure. The fracture toughness K _IC of the blends increased with SAN content and showed a maximum at 10 wt% SAN content, followed by a dramatic decrease for the cured blends containing 15 wt% SAN or more. The SEM investigation of the K _IC fracture surfaces indicated that the toughening effect of the SAN-modified epoxy resin was greatly dependent on the morphological structures.     

A fractographic criterion for subcritical crack growth boundaries at internal fracture origins in hot pressed silicon nitride

Using the elliptic integral method, stress intensity factors (K _I) were estimated at boundaries defined by fracture features observed at various distances from internal fracture origins in H.P. silicon nitride. The fracture origins are surrounded by regions of transgranular fracture. At the outer boundaries of these regionsK _I is less thanK _IC showing that these are regions of subcritical crack growth. Regions of hummocks and depressions were observed surrounding the regions of transgranular fracture.K _I was calculated at the elliptical boundary determined by the outer edge of the nearest of these features to the fracture origin. At this boundary,K _I K _IC. Therefore, these features can be used to locate the subcritical crack growth boundary.     

A new method to estimate the plane strain fracture toughness of materials

Although the testing method for fracture toughness K_IC has been implemented for decades, the strict specimen size requirements make it difficult to get the accurate K_IC for the high‐toughness materials. In this study, different specimen sizes of high‐strength steels were adopted in fracture toughness testing. Through the observations on the fracture surfaces of the K_IC specimen, it is shown that the fracture energy can be divided into 2 distinct parts: (1) the energy for flat fracture and (2) the energy for shear fracture. According to the energy criterion, the K_IC values can be acquired by small‐size specimens through derivation. The results reveal that the estimated toughness value is consistent with the experimental data. The new method would be widely applied to predict the fracture toughness of metallic materials with small‐size specimens.     

Evaluation by indentation of fracture toughness of ceramic materials

A transition fracture mode from Palmqvist to median has been observed in a number of ceramic materials. A new expression to determine the fracture toughness (K _IC) by indentation is presented. The K _IC values calculated by this formula are independent of the crack profile (median or Palmqvist) and of the applied load. This formula has been obtained by modifying the universal curve of Evans and Charles to incorporate Palmqvist and median cracks over a wide range of loads in the case of brittle materials with different mechanical properties (elastic properties: E, v, K _IC).