The effect of moisture on the physical and mechanical integrity of epoxies

The effect of specific combinations of moisture, heat, and stress on the physical structure, failure modes, and tensile mechanical properties of diaminodiphenyl sulphone (DDS)-cured tetraglycidyl 4,4diaminodiphenyl methane (TGDDM) epoxies [TGDDM-DDS (27 wt% DDS)] are reported. Sorbed moisture plasticizes TGDDM-DDS epoxies and deteriorates their mechanical properties in the range 23 to 150° C. Studies of the initiation cavity and mirror regions of the fracture topographies of these epoxies indicate that sorbed moisture enhances the craze initiation and propagation processes. The effect of tensile stress-level, applied for 1 h on dry epoxies, on the subsequent moisture sorption characteristics of the epoxies was also investigated. Such studies indicate that the initial stages of failure that involve both dilatational craze propagation and subsequent crack propagation enhance the accessibility of moisture to sorption sites within the epoxy to a greater extent than in the latter stages of failure which involve crack propagation alone. The amount of moisture sorbed by TGDDM-DDS epoxies is enhanced by 1.6 wt% after exposure to a 150° C thermal spike, as a result of moisture-induced free volume increases in the epoxies that involve rotational—isomeric population changes.Research sponsored in part by the McDonnell Douglas Independent Research and Development Program and in part by the Air Force Office of Scientific Research/AFSC, United States Air Force, under Contract No. F44620-76-C-0075. The United States Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright notation hereon.     

Fracture mechanisms of the Strombus gigas conch shell: implications for the design of brittle laminates

Flexural strength, crack-density evolution, work of fracture, and critical strain energy release rates were measured for wet and dry specimens of the Strombus gigas conch shell. This shell has a crossed-lamellar microarchitecture, which is layered at five distinct length scales and can be considered a form of ceramic plywood. The shell has a particularly high ceramic (mineral) content (99.9 wt%), yet achieves unusually good mechanical performance. Even though the strengths are modest (of the order 100 MPa), the laminated structure has a large strain to fracture, and a correspondingly large work of fracture, up to 13 kJ m^–2. The large fracture resistance is correlated to the extensive microcracking that occurs along the numerous interfaces within the shell microstructure. Implications of this impressive work of fracture for design of brittle laminates are considered.     

On the factors affecting strength of portland cement

This paper reports mechanical property measurements for Portland Cement paste free from fabrication artifacts (e.g. bubble-type voids), and compares them to published results both for normal and new high strength cement. Removal of large voids (above 100m) by vacuum de-airing leads to an increase of 15% in mean flexural strength and a small decrease in fracture toughness. This increase in flexural strength is predictable from the tied-crack model previously proposed to explain the notch-sensitivity behaviour of hardened cement paste, and for which direct experimental evidence was obtained. It is suggested that factors such as moisture content are at least as important as large voids in controlling mechanical properties. It is concluded that the much increased strength of the new polymer-containing cements must result from improvements to the microstructure other than the simple elimination of voids.     

Dielectric properties of hydrated Nafion-(SO3K) membranes: thermally stimulated depolarization currents

Dielectric studies have been carried out on hydrated perfluorosulfonate polymer membranes in potassium salt form, 1190 g equivalent-1 by means of thermally stimulated depolarization current (TSDC) techniques in the temperature range 77–300 K. Three dispersions I–III, are observed in a TSDC thermogram, and their positions depend upon the water content. We employ thermal sampling (TS) to analyse the three relaxation process experimentally, into approximately single responses and to determine the spectra of activation energy, E(T), on a sample with a water content of 0.9 H2O/SO3K. Dispersion I is analogous to the -relaxation mechanism, which was observed in mechanical relaxation studies, with an activation energy between 0.55 and 0.68 eV. Measurements with different electrode configuration reveal different aspects of the dynamics of the relaxation mechanism and allow distinction between dipolar and interfacial polarization contributions of Dispersion II. Dispersion II is analogous to the -relaxation mechanism that was observed in mechanical relaxation studies on hydrated membranes. Dispersion III may be due to a phase or structural change of the material in this temperature range. However, a contribution to this peak from electrochemical effects at the electrodes cannot be excluded. © 1998 Chapman & Hall     

Deformation and fracture of synthetic α-quartz

The deformation and fracture properties of synthetic -quartz crystals were measured using hardness, pre-cracked bend and Hertzian tests over the range 20 to 545° C in air and dry nitrogen. The resistance to fracture decreases significantly with increase in temperature and is orientation-dependent (becoming very small as the inversion temperature is approached), while the microhardness remains relatively insensitive to temperature and environment. These effects are interpreted in terms of the moisture content of the environment and the intrinsic water content of the crystals.     

Dry versus hydrated collagen scaffolds: are dry states representative of hydrated states?

Collagen composite scaffolds have been used for a number of studies in tissue engineering. The hydration of such highly porous and hydrophilic structures may influence mechanical behaviour and porosity due to swelling. The differences in physical properties following hydration would represent a significant limiting factor for the seeding, growth and differentiation of cells in vitro and the overall applicability of such hydrophilic materials in vivo. Scaffolds based on collagen matrix, poly(DL-lactide) nanofibers, calcium phosphate particles and sodium hyaluronate with 8 different material compositions were characterised in the dry and hydrated states using X-ray microcomputed tomography, compression tests, hydraulic permeability measurement, degradation tests and infrared spectrometry. Hydration, simulating the conditions of cell seeding and cultivation up to 48?h and 576?h, was found to exert a minor effect on the morphological parameters and permeability. Conversely, hydration had a major statistically significant effect on the mechanical behaviour of all the tested scaffolds. The elastic modulus and compressive strength of all the scaffolds decreased by ~95%. The quantitative results provided confirm the importance of analysing scaffolds in the hydrated rather than the dry state since the former more precisely simulates the real environment for which such materials are designed.

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Environmental sensitivity and mechanical behavior of boron-doped Fe–45at.%Al intermetallic in the temperature range from 77 to 1000 K

The tensile properties and fracture behavior of a coarse-grained (grain size 420 μm) Fe–45at.%Al intermetallic doped with 0.05 at.% boron were examined at ambient temperature in air, argon and vacuum as well as in the 77–1000 K temperature range in liquid nitrogen, dry ice and air. Before testing the alloy was low temperature annealed (vacancy annealed) in order to remove all the retained vacancies. At ambient temperature ductility increases accordingly to decreasing water vapor (moisture) content in each environment. The mixed transgranular cleavage (TGC)+intergranular failure (IGF) mode in vacuum, which is associated with the highest elongation (6%), exhibits around 40% of IGF and the mixed fracture mode in argon, which is associated with the second highest elongation (3.2%), exhibits 15% of IGF. The TGC fracture mode in air is associated with the lowest elongation (1%). Elongation in the cryogenic temperature range from 77 to 213 K is very low being in the range from 0.6 to 2.8%, and is associated with a mixed transgranular+intergranular fracture mode. Elongation increases gradually from 300 to 800 K attaining a ductility peak at 800 K and then decreases rapidly with increasing temperature. At 600–800 K, the yield strength of Fe–45Al–0.05B exhibits anomalous temperature dependence with the yield strength peak at 800 K. The mode of fracture from 300 to 700 K is predominantly TGC and that at the ductility peak is ductile rupture with very deep dimples. At temperatures above 800 K the mode of fracture changes to a typical intergranular creep (fibrous) failure with numerous flat dimples (voids/cavities) at the grain boundary facets, which is associated with a tensile ductility drop. Fine particles (borides) are observed at the grain boundary facets, which assist the development of intergranular creep fracture.     

A tough nut to crack

A study of the microstructure and mechanical properties of macadamia nutshells reveals these to behave like an isotropic wood. Their dry density is 1.3×10³kgm^–3, their Vickers hardness is 180±30 MPa, their fracture strength is 25 to 80 MPa and their fracture toughness is 1 to 2MPam^1/2. Their Young's modulus probably lies nearer the highest (6GPa) than the lowest (2GPa) of the present measurements and their work of fracture lies in the range 100 to 1000 Jm^–2. None of the mechanical properties is obviously dependent on the water content of the shell. The results demonstrate quantitatively why these shells have a reputation for being difficult to crack.     

Effects of long-term aging on moisture sensitivity of foamed WMA mixtures containing moist aggregates

Long-term aging of an asphalt mixture is complicated, but can be simulated in the laboratory. The objective of this study was to investigate the influence of long-term aging on moisture susceptibility of foamed warm mix asphalt (WMA) mixtures containing moist aggregate. Weight loss, indirect tensile strength (ITS) of dry and conditioned specimens, and deformation (flow) were measured for all mixtures. The experimental design included two aggregate moisture contents (0 and ~0.5% by weight of the dry mass of the aggregate); two lime contents (1 and 2% lime by weight of dry aggregate) and one liquid anti-stripping agent (ASA); one foaming WMA additive (Asphamin) and two foaming water contents (2 and 3%); and two aggregate sources. A common long-term aging procedure was used in this study. A total of 64 mixtures were evaluated and 256 specimens were made and tested in this study. The test results indicated that long-term aging improved the moisture resistance of WMA mixtures regardless of the ASA and moisture conditioning. In addition, aggregate source significantly affected the moisture resistance regardless of the foaming technology, ASA, and aggregate moisture content. The mixture with various hydrated lime contents exhibited similar moisture resistance under dry and wet conditions. The liquid ASA used in the WMA mixture showed a weaker resistance to the moisture damage in comparison with hydrated lime.     

Composite theory and the effect of water on the stiffness of horn keratin

-keratin is a hydrogen-bond dominated composite material. The dry -keratin (0% regain) of the horns of an oryx has a stiffness of 6.1 GPa. Water interacts only with the amorphous matrix of -keratin to break down structural hydrogen bonds and reduce stiffness to 4.3 GPa at 20% regain and 1.8 G Pa at 40% regain. The effect of water on the stiffness of horn -keratin is not modelled by the Voigt estimate at high to moderate regains. Water interacts probably with the disordered regions within the fibres which reduces the effective fibre length. As a result the reinforcing effect of the fibres is reduced and the stiffness and strength of hydrated horn keratin are less than that predicted by the simple Voigt estimate. If the Voigt estimate is modified to take into account a short fibre length of 40 nm, the stiffness and tensile strength of horn -keratin can be modelled successfully.     

The yield behaviour of poly(phenylenebibenzimidazole)

The compressive yield behaviour of poly(2,2-m-phenylene-5,5-bibenzimidazole) (PBI) has been studied over a wide range of temperatures. The tensile behaviour was also studied under superimposed hydrostatic pressure. In both cases wet and dry samples were examined and the results revealed the considerable effects of moisture on the mechanical properties of PBI. The results of all these tests show that PBI has a remarkably high shear yield stress at room temperature. This results in a very high compressive yield stress and a very high tensile yield stress observed under superimposed hydrostatic pressure when brittle failure from surface flaws is prevented. It is concluded on the basis of quantitative analysis that the yield mechanism in PBI at room temperature is initiation controlled, as in a metal or ceramic, rather than a velocity controlled, thermally activated, viscoelastic process which is generally considered applicable in polymers.     

Quality Assurance of Rice and Paddy Moisture Measurements in Thailand

A bilateral comparison in moisture measurement between the National Institute of Metrology Thailand (NIMT) and the Central Bureau of Weights and Measures (CBWM) was organized for quality assuring of rice and paddy moisture measurement in Thailand. The bilateral comparison was conducted by using the same batch of sample and moisture meter as transfer device. It consisted of two parts: moisture measurement in rice and in paddy. A rice moisture meter belonging to CBWM and rice standards prepared at the nominal moisture content of 10 %, 12 %, 14 % and 16 % at NIMT, were used for rice moisture comparison, while a paddy moisture meter belonging to NIMT and paddy standards prepared at the nominal moisture content of 12 %, 14 %, 16 % and 18 % at CBWM, were used for paddy moisture comparison. Both laboratories measured the moisture content of a sample by using the standard method in ISO 712 and used that sample to calibrate a moisture meter by means of the method based on ISO 7700-1. Since the moisture content of the sample can change during the comparison, correction values in moisture content between the standard value and the reading value from the moisture meter are used as calibration results for the comparison evaluation. For the rice moisture comparison, differences in the correction value measured by the two laboratories vary from 0.18 % to 0.46 %, with their combined comparison uncertainty of 0.37 % (\(\hbox {k}= 2)\). The main contribution to the difference comes from the standard values from both laboratories differing from 0.27 % to 0.53 %, as the rice standard was found to drift in moisture content less than 0.05 %. Similarly to the rice moisture comparison, differences in the correction value for the paddy moisture measurement range from 0.08 % to 0.56 % with the combined comparison uncertainty of 0.38  % (\(\hbox {k} = 2)\), whereas the stability in moisture content of the paddy sample at NIMT was found to be within 0.12 %.     

The strength of brass/Sn-Pb-Sb solder joints containing 0 to 10% Sb

The strength of brass/Sn-Pb-Sb solder joints has been determined for solders containing 0 to 10% Sb and the fracture process examined by microscopial techniques. Intermetallic formation by interdiffusion in the Cu-Zn/8n-Pb-Sb and Cu-Zn/Pb-Sb systems were also investigated. As the amount of antimony in the solder increases the thickness and hardness of the intermetallic layer increases, and at and above 4% Sb cuboids of SnSb form in the solder. Antimony causes solid-solution strengthening of the joint up to 3% Sb and a ductile fracture occurs at the solder/intermetallic interface. At and above 4% Sb there is a fall in strength with a cleavage type of fracture associated with SnSb in the solder, and at the 8 and 10% Sb levels with fracture in the intermetallic layer. Additions of up to 10% Sb in the solder reduce the shear strength by only 10%, but cause the joint to have o variable and occasionally very low fatigue resistance. Interdiffusion studies show that the Cu-Sn intermetallic phase formed with the antimony-containing solder is not based on the -phase, and is probably based on the -phase in the Cu-Sn-Sb system.     

On brittle fracture in polycrystalline iridium

The brittle intercrystalline fracture (BIF) in iridium and low plasticity of this refractory fcc-metal in polycrystalline state for wide temperature range, give occasion to supposition that a metal having fcc lattice can be brittle substance like silicon due to specific features of interatomic bonds. On the other hand, experience has shown that such behavior of fcc-metal should be impurity induced effect. High plasticity of iridium single crystals (by means of octahedral slip), which however cleave under tension, does not support idea on classic inherent brittleness of this metal. High sensibility of its mechanical properties to impurities (10 ppm is critical level for carbon!) has been severe restriction for experiments aimed to searching of inherent fracture mode for polycrystalline iridium. In present paper fracture behavior of high purity polycrystalline iridium and iridium, where grain boundaries (GBs) have been contaminated, are considered. Fine grained impurity-free metal exhibits 100% brittle transcrystalline fracture (BTF) and cracked GBs on the fracture surface and this is an inherent fracture mode of polycrystalline iridium. Re-crystallization in vacuum leads to appearance of BIF regions, whose total square can reach 100% of the fracture surface in coarse grain aggregates. Contaminated iridium mainly fails on grain boundaries, at that portion of BIF on the fracture surface (50 ÷ 100%) does not depend on grain size.     

Properties of saccharides and saccharide derivatives in the dry and partially hydrated states

This study investigates the correlation (if any) between the surface properties of dry mannitol, sorbitol and glucose, as determined using inverse gas chromatography (IGC) and other techniques, with microwave dielectric analysis of the molecular polarisability of water in the hydrated material. Microwave dielectric analysis of the hydrated material (using a circular resonant cavity) showed a transition (h _t) in the gradient of imaginary permittivity ( at 2.2 GHz) vs. water content for all materials. This transition reflects the appearance of another population of water with enhanced mobility. The gradient below h _t (m ₁) reflects the reorientation mobility of water close to the surface of the solid, and therefore provides one measure of the strength of interaction with each material. Estimates for h _t and m ₁ parallel the IGC data by giving a rank order of affinity as glucose mannitol sorbitol. This link suggests that the molecular properties of water and the strength of interaction with the hydrated material are most certainly governed by the acceptor/donor properties of the surface (as determined by IGC). Moreover, it can be inferred that the presence of mobile water (up to the relatively high levels defined by h _t) does not change the surface energy of the material.     

Fractography of unidirectional graphite-epoxy as a function of moisture,temperature and specimen quality

The tensile failure surfaces of (0°)₈ T300/5208 graphite-epoxy specimens were examined using both optical and scanning electron microscopy. Fractography was used to determine how moisture content and temperature as well as specimen preparation technique, prepreg batch and cure condition affected the failure mode. A distinctive low-energy failure morphology was found in defective specimens and also in those whose edges were poorly prepared. This morphology was predominant in failures at elevated temperature or moisture content for specimens which had been made from one suspect batch of prepreg. This finding combined with unusual end-tab failures from such specimens indicated that this batch was indeed defective, but that such defective batches could in the future be identified by tests under hot, wet conditions. For specimens made from good prepreg, temperature or moisture appeared to decrease flaw sensitivity and thus increase strength, even though moisture also seemed to increase interfacial debonding between filament and matrix. When combined, moisture and temperature appeared to degrade performance by increasing interfacial debonding and making the epoxy matrix more prone to fracture.     

Interaction mechanism of cement and asphalt emulsion in asphalt emulsion mixtures

The interaction characteristics of cement asphalt composite mastic (CAM) and performance properties of cement asphalt emulsion mixtures (CAEM) were evaluated in this work using chemical and mechanical test methods to investigate the effect of the presence of cement on asphalt emulsion mixtures (AEM). The chemical composition of the CAM was obtained through use of X-ray diffraction, Fourier-transform infrared spectroscopy, and environmental scanning electron microscopy (ESEM) as a means to describe the interactions between the cement and asphalt in the composite materials. Test results demonstrated that cement can hydrate with the water phase of the asphalt emulsion. Asphalt droplets can simultaneously enclose cement particles and delay the hydration reaction process of cement. The interaction mechanism of cement particles or hydration products and residual asphalt is a physical compound process. The influence of these findings on asphalt emulsion mixture design and performance properties was assessed using varying mix design components and conducting laboratory-based mechanical test methods for rutting resistance and moisture susceptibility. Mix design components varied including added water content, emulsion content, and cement dosage levels. The optimum fluids content was determined based on the dry indirect tensile strength. It was found that the cement content significantly impacts the optimum fluids content for both added water and emulsion. Furthermore, the presence of cement improves the dry tensile strength, rutting resistance, and moisture susceptibility. Based on microstructural analysis of CAM and CAEM, the mechanism by which cement improves the performance of AEM is attributed to the ability of hydration products to increase both the stiffness of the asphalt binder and the adhesion at the mastic–aggregate interface. In practical applications, this study recommends a mix design method for cement-modified asphalt emulsion mixes (CAEM) based on selection of optimum cement and emulsion contents using indirect tensile strength and verification of the design through evaluation of the moisture susceptibility and rutting resistance of the CAEM mix. Threshold values of CAEM mix mechanical properties to determine the quality of the design are proposed.     

Stability of Freeze-Dried Tablets at Different Relative Humidities

The purpose of the study was to evaluate the stability of two different freeze-dried tablet formulations at different relative humidities (RHs). The tablets contained 25 mg hydrochlorothiazide (HCT) as a model drug and were prepared by freeze-drying a suspension and an oil-in-water (o/w) emulsion. Formulation A was a rapidly disintegrating tablet and consisted of 80 mg of maltodextrine DE38; 8 mg of polyethyleneglycol (PEG 6000), 8 mg of xanthan gum, and 25 mg of HCT. Formulation B was a lyophilized dry emulsion tablet that consisted of 160 mg of Miglyol^® 812, 80 mg of maltodextrin DE38, 16 mg of methylcellulose (Methocel^®) A15LV, and 25 mg of HCT. Tablets were packaged in different packing materials: polyvinylchloride (PVC)/aluminum blister packs, PVC-polyvinylidenechloride (PVDC)/aluminum blister packs, closed containers with a dessicant tablet, and open containers. The tablets were stored at three relative humidities (45%, 60%, and 85% RH) and were characterized on mechanical strength, residual moisture, porosity, content uniformity, and scanning electron microscopy (SEM) during a period of 6 months. After 1 month at 60% and 85% RH, a strong increase in moisture content (from 2.7% to 6.8%) was seen for the tablets packed in the open and closed containers and for the PVC/aluminum blistered tablets. This increase was higher for formulation A compared to formulation B since B contained 160 mg of triglycerides and was more hydrophobic. This increase in water content was correlated with a decrease in mechanical strength. The tablets also showed a change in microstructure and porosity. At a moisture content of 7.2%, formulation A showed a structural “collapse” since water acts as a plasticizer for the amorphous glass, lowering the glass transition temperature T_g. This phenomenon even occurred in PVC/aluminum blister packs at 85% RH. The structural collapse was associated with a complete loss of microstructure as detected by porosimetric analysis and SEM. For the PVC-PVDC/aluminum blistered tablets, the increase in moisture content and decrease in mechanical strength at 85% RH occurred much slower, and the water uptake and strength loss were less intensive. No significant breakdown of HCT could be observed in both formulations with all of the packing materials. Packaging of freeze-dried tablets with PVC/aluminum blister packs, PVC/PVDC/aluminum blister packs, or closed containers did not offer protection against moisture uptake, mechanical strength loss, and structural collapse.     

New evaluation technique of interfacial properties in GFRP

A new method was proposed to evaluate the mechanical properties at the interface between the fibres and the matrix in composites using an embedded single fibre coupon test. A mechanical parameter at the interface (called the interfacial transmissibility, ) was derived from the fibre strength and the apparent stress of the fibre immediately before the first fracture of embedded fibre, _fa. This parameter indicated the degree of the mechanical transmission from the matrix to the fibre through the interface. This avoided some complicated problems such as the stress distribution along fibre fragments and the critical fragment state in a typical single-fibre test. This new method was tried to determine the -values for a fibre glass/epoxy resin with different amounts of a coupling agent at the interface. In order to measure the stress at the first fracture, the fracture process was monitored with a video camera during the single fibre test. The stress values at the first fracture for many coupons were analysed as a function of the three-parameter Weibull distribution. The resulting average stress and its coefficient of variation indicated that the reliability of the measurement for the stress at the first fracture was not less than that obtained by the usual single-fibre test. The change of interfacial transmissibility with amount of the coupling agent revealed the existence of an optimal interface.     

Mechanical and statistical analyses of fracture of whisker-reinforced ceramic-matrix composites

This paper analyzes the fracture toughness of short-fiber reinforced ceramic-matrix composites (CMC). The effects of crack deflection and fiber pullout on matrix cracking are examined using a combination of mechanical and statistical models. First, the stress intensity factors of a deflected crack subjected to closure stress due to fiber pullout are analyzed based upon the mechanical model. Distributed dislocation method is used for the elastic analysis. Since the deflected crack is subjected to biaxial loading, a mixed mode fracture criterion in linear elastic fracture mechanics is applied to calculate the fracture toughness. Secondly, the number of pullout fibers on the fracture surface is treated as a random variable, and the statistical distribution of these fibers has been determined. The pullout force acting on a deflected crack is also obtained as a random variable by assuming a simple mechanism of fiber pullout. The probability of failure of CMC can thus be estimated from the strength characteristics of the fiber and matrix as well as the interface between these two.