Search

Stress rupture is an important failure phenomenon in composite overwrapped pressure vessels, which is highly unpredictable other than on a statistical basis. Even then, there are several statistical models, with varying bases in composite micromechanics and molecular failure mechanisms. Choosing among these models is not trivial, even when micromechanical details of the failure process are reasonably well appreciated, and one has available a reasonably large database of strength and lifetime data. As a result, there is little in the way of guidance to choose the most appropriate model. One important issue is that accurate predictions are desired at relatively low service loads compared to the strength, and low probabilities of failure that are far less, e.g., 10-6, than can be directly confirmed using the data itself. In essence, one needs a robust and accurate statistical model free of inconsistencies associated with such low stress levels and probabilities of failure. This paper performs an in-depth comparison of several current models, which have varying physical bases. The models compared differ in the number of parameters to be estimated from data. The results of this study, however, show that over a broad range of parameter values these models give surprisingly similar failure probability predictions. While practitioners may have a preference for one model over another, the basis for such a choice is not easily established, given the fidelity of typical data. [ABSTRACT FROM AUTHOR]

This paper aims to statistically estimate the dynamic fatigue strength in brittle materials under a wide range of stress rates. First, two probabilistic models were derived on the basis of the slow crack growth (SCG) concept in conjunction with two-parameter Weibull distribution. The first model, Model I, is a conventional probabilistic delayed-fracture model based on a concept wherein the length of the critical crack growth due to SCG is enough larger than the initial crack length. For the second model, Model II, a new probabilistic model is derived on the basis of a concept wherein the critical cracks have widely ranging lengths. Next, a four-point bending test using a wide range of stress rates was performed for soda glass and alumina ceramics. We constructed fracture probability-strength-time diagrams (F-S-T diagrams) with the experimental results of both materials using both models. The F-S-T diagrams described using Model II were in good agreement with plots of the fracture strength and the fracture time of both materials more so than Model I. [ABSTRACT FROM AUTHOR]

It is of great interest for economical and security reasons to understand the compressive properties of underground cemented paste backfill. In this paper, the stress–strain behaviours of cemented paste backfill (CPB) subjected to uniaxial compression and conventional triaxial tests are presented and discussed. The effect of CPB basic components, strength, ageing and confining pressure on the deformation behaviour of CPB are evaluated and discussed. The results show that the stress–strain behaviour of CPB is strongly influenced by the confinement, the age and strength of CPB, and its components. The increase in confining pressure leads to a change in the mode of failure, in the stiffness, and an increase in the strength. [ABSTRACT FROM AUTHOR]

The mechanical properties of cold-drawn pearlitic wires are controlled largely by the microstructure developed during processing and, to some extent, by the residual stresses during drawing. The advent of powerful computers and the availability of equipment to perform diffraction experiments, have made possible numerical predictions and accurate measurements of residual stresses. This paper—a review of work done by the author and collaborators—shows how stress-relaxation losses, environmental assisted cracking and fatigue life of cold-drawn pearlitic wires are influenced by residual stresses. The role of pre-stretching loads, or of stress relieving treatments, on stress-relaxation can be understood when the profile of residual stresses is known. Some awkward results in times to fracture during hydrogen embrittlement tests can be explained if accurate values of residual stresses near the surface are known, and the same is true of fatigue life. In this context numerical simulations and measurements performed on cold-drawn pearlitic wires, with different profiles of residual stresses, have shown very good quantitative agreement. [ABSTRACT FROM AUTHOR]

In this paper, yield and fatigue behavior of a polypropylene nanocomposite and a polyamide-6 nanocomposite has been studied. The Eyring equation was used to model the temperature and strain rate sensitivity of the yield strengths of these two nanocomposites. Both activation volume and activation energy of the polypropylene nanocomposite were higher than those of the polyamide-6 nanocomposite. The fatigue strength of the polyamide-6 nanocomposite was higher than that of the polypropylene nanocomposite. However, the ratio of maximum fatigue stress to yield strength of the polypropylene nanocomposite was higher than that of the polyamide nanocomposite. The fatigue failure in both composites was initiated at agglomerated nanoparticles. [ABSTRACT FROM AUTHOR]

The strength of a continuous-fiber-reinforced ceramic composite is directly related to the strength distribution of the reinforcements. Therefore, it is essential to understand how processing and service conditions affect the filament strength distribution. The objective of this paper was to determine the strength distribution of Nextel™ 610 alumina filaments, a potential reinforcement in oxide-oxide ceramic composites, and to characterize the defect population as a function of exposure temperature. The filament strength was measured in single filament tests and decreased as the exposure temperature increased. The defect population was quantified via fractography, and eight distinct defect types were identified. Grain growth and filament-to-filament sintering were identified as critical microstructural changes during the heat treatments, and both affected the development of grain boundary grooves, either from thermal etching or from the breaking of the filament-to-filament sintering bonds. The initial crack path was intergranular but quickly changed to a mixture of transgranular and intergranular fracture. During catastrophic crack growth, the crack path changed back to intergranular. The critical crack size, as calculated from linear elastic fracture mechanics, was found in many cases to be greater than the size of the strength limiting defect suggesting the presence of subcritical cracking. [ABSTRACT FROM AUTHOR]

This paper presents an overview of fatigue fracture modes in selected structural alloys employed in gas turbine engines. These include the mechanisms of fatigue crack growth in the near-threshold, Paris and high-δK regimes obtained from Ti-6A1-4V, Inconel 718 and PWA 1472 (a single crystal nickel-based superalloy of similar chemical composition to Inconel 718). Fatigue fracture modes in these materials are shown to be strong functions of the stress intensity factor range, AK, and the maximum stress intensity factor, Kmax. Fatigue mechanism maps are also presented to show the parametric ranges of ΔK and Kmax corresponding to the different fatigue fracture modes. [ABSTRACT FROM AUTHOR]

The moisture content is known to influence the properties of wood. While mechanical parameters like stiffness and strength decrease with increasing moisture content, investigations on the influence on fracture properties have been concentrated on linear elastic fracture mechanics. In this paper, the wedge splitting method is used to investigate also the non-linear fracture behaviour of sprucewood at different moisture contents. Complete load-displacement curves at four different moisture levels are evaluated. While the initial slope and the maximum stress state are decreasing with increasing moisture content the specific fracture energy is increasing. It is shown that this behaviour can be attributed to an increase in ductility which increases both the dissipated energy during crack initiation and the one in the crack propagation phase. [ABSTRACT FROM AUTHOR]

The radial direction of wood is reinforced by an additional tissue called rays. These rays are one of the reasons for the anisotropy of wood in the transverse plane. In this paper the influence of rays on the mechanical properties like tensile strength as well as on fracture mechanical parameters like critical stress intensity factor and specific fracture energy is shown. By investigating two deciduous tree species with a similar wood structure in general but a different ray characteristic in particular the importance of this radial reinforcement of the wood structure could be demonstrated. The relevance for the living tree is discussed. [ABSTRACT FROM AUTHOR]

Attempts have been made to discuss the fibre axial tensile and compressive behaviour of several carbon fibres prepared from different precursors, and surface- and/or sizing-treatments. With all fibres, the number of breaks increased with increasing tensile or compressive strain, and remained constant beyond a certain strain. The constant number of breakages based on the precursor differed remarkably in compression, whereas those based on the surface- and/or sizing-treatments differed remarkably in tension. In tension, the PAN-based fibre could be broken with a somewhat greater ease than the pitch-based fibre, while in compression, the pitch-based fibre could be broken with somewhat greater ease than the PAN-based fibre. © 1998 Chapman & Hall [ABSTRACT FROM AUTHOR]

Multi-layered braided structures are formed as a result of over-braiding the previously formed braids and they are increasingly being used for numerous applications ranging from hoses to energy absorbing composites. In this research work, a series of multi-layered braided structures were prepared on circular braiding machine for obtaining various combinations of braid angles of 30° and 45° in inner and outer layers. Subsequently, the tensile properties of multi-layered braided structures were analysed and it was found that the braid angle in the outer layer has significantly affected the stress-strain behaviour. A simple analytical model for predicting the tensile behaviour of multi-layered braided structures has also been proposed based on the previously developed model of 'braid-elastic core' system. A clear distinction has also been made between the helix and braid angles. Furthermore, a comparison has been made between theoretical and experimental values of braid angle, toughness and stress-strain characteristics of multi-layered braided structures. [ABSTRACT FROM AUTHOR]

The present study investigated the influences of aging temperature and time on the tensile properties and quality indices of 359-type Al-9%Si-0.5%Mg casting alloys. These alloys are still not widely used by most foundrymen in spite of the fact that they are extremely promising for several fields of engineering applications because of their superior strength. For the purposes of validating their use in industrial applications, a solid data base was created, based on the present study, correlating the tensile properties and the quality indices of these alloys with the aging parameters. Quality charts were used as an evaluation tool for selecting the optimum aging conditions for developing high strength and optimum quality in 359 casting alloy. Aging at a low temperature was observed to produce the greatest strength and optimum quality in the 359-type castings compared to aging at higher temperatures. The peak-strength for 359 alloy was observed to be attained after shorter aging times on condition that the aging temperature is increased. The aging times required for reaching peak-strength in 359 alloys are 32, 24, 1 h, 30, and 10 min when applying aging temperatures of 155, 170, 195, 220, and 245 °C, respectively. Aging treatment at higher temperatures is accompanied by a reduction in the tensile properties and quality index values of the castings; however, it also introduces the possibility of a significant economical strategy for minimizing the time and the cost of this treatment. Depending on the required level of tensile properties and based on the quality charts developed, it is possible to make a rigorous selection as to the most suitable aging parameters to be applied to the 359 alloy so as to obtain the best possible cost-effective compromise between alloy strength and quality. [ABSTRACT FROM AUTHOR]

Due to the typical large variability in the measured mechanical properties of flax fibres, they are often employed only in low grade composite applications. The present study aims to investigate the reasons for the variability in tensile properties of flax fibres. It is found that an inaccuracy in the determination of the cross-sectional area of the fibres is one major reason for the variability in properties. By applying a typical circular fibre area assumption, a considerable error is introduced into the calculated mechanical properties. Experimental data, together with a simple analytical model, are presented to show that the error is increased when the aspect ratio of the fibre cross-sectional shape is increased. A variability in properties due to the flax fibres themselves is found to originate from the distribution of defects along the fibres. Two distinctive types of stress-strain behaviours (linear and nonlinear) of the fibres are found to be correlated with the amount of defects. The linear stress-strain curves tend to show a higher tensile strength, a higher Young's modulus, and a lower strain to failure than the nonlinear curves. Finally, the fibres are found to fracture by a complex microscale failure mechanism. Large fracture zones are governed by both surface and internal defects; and these cause cracks to propagate in the transverse and longitudinal directions. [ABSTRACT FROM AUTHOR]

An investigation has been carried out on the adiabatic shear band (ASB) in a Ti-3Al-5Mo-4.5V (TC16) alloy deformed at high strain rate by a split Hopkinson pressure bar (SHPB). ASB in TC16 alloy is a “white” band with a width of about 13 μm. Microhardness of the ASB is larger than that of the matrix. The elongated cell structures of width about 0.2–0.5 μm with thick dislocation exist in the boundary of the shear band. Results suggest that the fine equiaxed grains with α-phase and α″-phase coexist in the shear band. The “white” band is a transformation band. Calculation of the adiabatic temperature rise indicates that the maximum temperature within ASB is about 1,069 K that is above the phase transformation temperature. Finally, formation of an ASB in the TC16 alloy and its microstructure evolution are described. [ABSTRACT FROM AUTHOR]

Carbon/carbon composites are well suited to high-friction applications due to their excellent mechanical and thermal properties. Since interfacial shear strength is critical to composite performance, characterization of fiber/matrix interface is a crucial step in tailored design of composites. This article presents a hybrid experimental/analytical study to evaluate the interfacial shear strength (IFSS) of PAN-fiber-reinforced carbon matrix composites. Microstructure was studied by light and high-resolution transmission electron microscopy (HRTEM). A series of push-out tests were conducted to examine the fiber/matrix debonding process. The residual fiber displacement was confirmed by scanning electron microcopy (SEM). The validity of the calculated IFSS value was demonstrated by a simplified analytical approach, where the components contributing to the measured displacement were analyzed considering the mechanics of the indentation. The method described in this article has been successfully used for determining the IFSS of PAN-fiber-reinforced carbon matrix composites. [ABSTRACT FROM AUTHOR]

ZrO2 porous nanosolid has been successfully prepared by a novel hydrothermal hot-press (HHP) method, using ZrO2 nanoparticles as the starting material. Furthermore, a kind of O, O-donating chelating regent, morin was assembled into the pores of ZrO2 porous nanosolids, and a morin/ZrO2 porous nanocomposite was obtained. Because of the interaction between morin molecules and the surface of ZrO2 porous nanosolid, a blue-shift of the photoluminescence (PL) peak was observed in ZrO2/morin nanocomposite by comparing with that of morin. [ABSTRACT FROM AUTHOR]

Microstructure and fracture behavior of brazed joint between commercially pure titanium and low carbon steel using silver (Ag–34Cu–2Ti) and copper (Cu–12Mn–2Ni ) based alloys have been characterized to determine the effect of brazing parameters and chemical composition on the strength of brazed joints. It is found that the shear strength of brazed joints strongly depends on the lap width. Furthermore, the fracture path and the value of shear strength significantly changed with the type of filler alloy. The two filler metals showed metallurgical interaction with steel and titanium forming different kinds of intermetallic compounds such as CuTi, Cu2Ti, and FeTi with silver based filler and Ti2Cu, FeTi and TiCuFe with copper based filler. [ABSTRACT FROM AUTHOR]

The breaking force during transversal loading of fibre-cement corrugated roofing sheets was measured on several test samples from a serial production. The results were statistically analyzed assuming the 2-parameter Weibull statistics. In addition, Monte Carlo statistical simulations were made by using a computerised built-in random-number generator. While smaller sample data groups, mostly containing up to 50 samples, were studied in the literature, we extended their size up to 400 samples. We showed that some trends in the evaluation of statistical parameters which hold for smaller data groups, apply well to larger data groups. In particular, we confirmed that the statistical distribution of the Weibull parameters obtained from repeated Monte Carlo simulations is log-normal. Furthermore, we considered the influence of the measurement uncertainty on the statistical parameters. [ABSTRACT FROM AUTHOR]

An accurate solution for a rotating functionally graded annular disk is presented. Material properties of the present annular disk are assumed to be graded in the radial direction according to a simple exponential-law distribution. The inner surface of the disk is pure metal whereas the outer surface of the disk is pure ceramic. The boundary condition of rigid casing is considered herein, that is the vanishing of the radial displacement at the outer surface. The boundary condition at the inner surface of the disk is taken to be vanishing either radial displacement or radial stress. Analytical solutions for the elastic deformation of the rotating functionally graded annular disks subjected to these boundary conditions are obtained. Numerical results for radial displacement, circumferential and radial stresses are presented. Comparisons between the different rotating homogeneous and functionally graded annular disks are made at the same angular velocity. The results show that distributions of stresses and displacement through the radial direction of the rotating annular disk vary with different parameters. [ABSTRACT FROM AUTHOR]

Polycrystalline CdTe thin films were irradiated with 80 MeV oxygen (O6+) ions for various fluences and its effect on the composition, structure, surface topography and optical properties have been investigated. The as-grown films are found to be slightly Te-rich in composition and there is no significant change in the composition after irradiation. X-ray diffraction analysis shows a high degree of crystallite orientation along the (111) plane of cubic phase CdTe. Upon irradiation a large decrease in intensity of the (111) plane and a small shift in the peak position has been resulted. The shift in the peak position is correlated with the change in the residual stress. The surface roughness of the films get increased after irradiation. A decrease in the grain size was observed after irradiation due to ion-induced recrystallization. The optical band gap energy decreased from 1.53 eV for as-grown film to 1.46 eV upon irradiation. The photoluminescence (PL) spectrum is dominated by the defect band and the effect of irradiation has been discussed and correlated with the observed change in the XRD peak position and optical band gap. [ABSTRACT FROM AUTHOR]

A theory is provided for piezoresistivity in carbon fiber reinforced cement (with and without embedded steel reinforcing bars) under flexure (three-point bending). The phenomenon, which involves the reversible increase of the tension surface electrical resistance and the reversible decrease of the compression surface electrical resistance upon flexure, allows strain sensing. The theory is based on the concept that the piezoresistivity is due to the slight pull-out of crack-bridging fibers during crack opening and the consequent increase in the contact electrical resistivity of the fiber-matrix interface. This work is an extension of prior theory, which concerns the effect of uniaxial loading on the volume resistance. The extension requires modeling the surface resistance and its change under flexure. The theoretical results on the piezoresistivity, both with and without rebar, are in good agreement with prior experimental results. Differences between theoretical and experimental results are probably due to minor damage and rebar debonding during flexure. [ABSTRACT FROM AUTHOR]

Diametral compression is usually used to determine the mechanical resistance of cylindrical ceramic specimens. This work deals with the possibility of employing diametral compression in order to evaluate the mechanical response of ceramic rings, by testing two sets of rings (used as pump seals) that had two different sizes. The rings were characterized by different techniques: qualitative X-ray diffraction, apparent density and porosity measurements, determination of Vickers hardness, surface roughness, and microstructural analysis. α-alumina was identified as the majority crystalline phase in both types of rings. The porosities were rather similar, even though the observed mean grain size of the large rings was slightly larger. Significant differences were observed in the average roughness. Diametral compression tests at room temperature were carried out on a statistical number of each ring set. The fracture features were analyzed by ocular inspection and SEM observation of the fracture surfaces. Several approaches were used to estimate the fracture strengths: three analytical formulae with and without an empirical constant, and a finite element calculation. The simplest approach, an analytical formula that only requires the knowledge of the geometrical magnitudes of rings besides de fracture loads, gave a conservative estimation of the mechanical strength of rings and a limited explanation of fracture features. On the other hand, the numerical model being the most complex and informative of the approaches, gave the complete stress distributions. [ABSTRACT FROM AUTHOR]

The basic mechanical and chemical properties of fly-ash-based inorganic polymer concretes (IPC) have been studied widely, but, key engineering and structural properties of the material for instance modulus of elasticity, compressive, tensile, flexural strengths and bonding strength of the material to reinforcement have received little attention. Structural applications of reinforced IPC depend on the bond performance of the material to the reinforcement. Due to their difference with ordinary Portland cement (OPC) based concrete in terms of chemical reaction and matrix formation it is not known whether IPC exhibit different bonding performance with the reinforcement. Simply relying on compressive strength of the material and extrapolating models and equations meant for OPC based concrete may lead to unsafe design of structural members. To that end, 27 beam-end specimens, 58 cubic direct pullout type specimens and number of laboratory test specimens were tested to evaluate bonding performance of IPC with reinforcement. The results of beam-end specimens and direct pullout type specimens correlate favourably, although the results of direct pullout tests are in general more conservative than those of beam-end specimens. Overall, it can be concluded that bond performance of IPC mixes are comparable to OPC based concrete and therefore IPC and steel can be used as a composite material to resist tension in addition to compression. [ABSTRACT FROM AUTHOR]

The compressive strength of ice was measured at high strain rates of 103 s−1 order of magnitude. Since ice compressive strength is known to be strongly dependent on strain rate, properties corresponding to high strain rates are needed for engineering predictions of the behavior of ice under dynamic crushing scenarios. The split Hopkinson pressure bar (SHPB) apparatus was used to successfully measure compressive strength over a strain rate range of 400–2,600 s−1. Strain rate variation was achieved by adjusting the specimen length and the velocity of the SHPB striker bar; increased velocity and reduced specimen length produced higher strain rates. Since the compressive strength was found to be nearly uniform over the measured strain rate range, an average value of 19.7 MPa is reported. However, when comparing the present results with data in the existing literature spanning several orders of magnitude in strain rate, a trend of continuously increasing strength for strain rates beyond 101 s−1 can be observed. [ABSTRACT FROM AUTHOR]

Cellular materials from hollow bronze spheres were prepared and investigated. The main aim was to determine the effective Young’s modulus of cellular materials by means of the dynamic resonant method, and the next goal was to demonstrate the possibility of using sandwich samples for such determination. For this purpose, two kinds of samples were prepared: quasihomogeneous rods consisting entirely of a cellular material, and sandwich rods containing the cellular material only in a core. Resonant frequencies of the fundamental flexural modes of all these samples were measured, and corresponding flexural rigidities were evaluated. Obtained values of flexural rigidities were subsequently utilized for calculation of the effective Young’s modulus of cellular material. A reasonable agreement between the modulus values determined by means of quasihomogeneous and sandwich samples was obtained. This indicates that the sandwich samples could be used for measuring the material properties of particular layers. [ABSTRACT FROM AUTHOR]

Austenitic stainless steel specimens have been examined by positron-lifetime measurements at various stages until failure during fatigue tests at constant stress or plastic strain amplitudes. A positron-source-detector assembly has been mounted on the servohydraulic testing machines that allowed truly non-destructive positron annihilation studies without removing the specimens from the load train. Positrons were generated by a 72Se/72 As source with a maximum activity of 0.9 MBq (25 μCi). The average positron lifetime has been determined by a β+ − γ-coincidence applying a simplified data evaluation procedure. Under constant stress or plastic strain amplitudes early stages of fatigue damage could be detected. The strong increase of the average positron lifetime already during the first 10% of fatigue life could be related to the fatigue life of the specimens. Issues of lifetime prediction by positron annihilation measurements are discussed. [ABSTRACT FROM AUTHOR]

The determination of the mechanical properties of fabrics in biaxial tension and in-plane shearing is made from 3D finite element analyses of the unit woven cell. Compared to experimental tests these virtual tests have several advantages. They can easily be carried out for sets of varied parameters, they provide local information inside the woven material and above all they can be performed on woven materials that have not yet been manufactured. The 3D computations are not classical analyses because the yarns are made up of several thousands of fibres and their mechanical behaviour is very special. Several specific aspects of the analysis are detailed, especially the use of a hypoelastic law based on an objective derivative using the rotation of the fibre which allows a strict evolution of the directions of orthotropy according to the fibre direction. Examples of analyses are presented in biaxial tension and in-plane shear for woven reinforcements and in the case of the biaxial tension of a knitted fabric. The results obtained are in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

The effect of thermal cycling on the stress-strain behavior of polyamide (nylon) and polyphenylene-sulfide (PPS) based injection molded Nd-Fe-B magnets was investigated after test specimens were cycled between -40 and 150°C for 50, 500, or 5000 repetitions. It was found that PPS based magnets exhibit higher ultimate strengths, higher modulus and lower toughness than nylon based magnets. Furthermore, formulations containing platelet morphology particles exhibited higher strengths and modulus than those containing spherical morphology particles, with increases in particle volume fraction leading to a decrease in strength. Differences in strength, modulus, and toughness were attributed to the degree of bonding between the matrix and the magnet powder in the various formulations, the degree of crosslinking, along with the effects of powder morphology. Additionally, it was found that while the stiffness of these materials increased with thermal cycling, their toughness decreased significantly, by as much as 99%. The extent of these effects was found to be dependent on the polymer matrix, powder morphology, and volume fraction of powder in the magnet. Finally, it was found that the PPS composites showed less relative change due to thermal cycling than the Nylon composites. [ABSTRACT FROM AUTHOR]

The small strain (below yielding) tensile loading-unloading tests were carried out on the low-density polyethylene (LDPE) and polypropylene (PP) at low strain rate and room temperature. The experiments unambiguously indicate to a remarkable decrease in residual strains in comparison with those predicted by conventional viscoelastic models. These deviations cannot be explained without taking into account structural transformations of semi-crystalline polymers. As long as small deformations cannot result in significant change in content and texture of crystalline and amorphous components, it was assumed that such transformations should include disintegration of connectivity in crystallite clusters. This structural rearrangement is supposed to be caused by the strain-induced decrystallization of narrow (and thus highly stressed) "bridges" connecting domains of conjugated crystallites or inside crystallites. A simple 1D modelling of the deformation processes supports this expectation. The disconnection in polymer morphology is simulated by small portions of amorphous ligaments appearing between neighbouring crystallites in the course of deformation. In spite of simplicity of the model a precise fitting of the stress-strain diagram is obtained along with small variations in structural and material characteristics (crystallinity degree, effective rigidity and plastic ability) of the concerned polymers. [ABSTRACT FROM AUTHOR]

Low cycle fatigue tests for a hot extruded Nickel base alloy tube material have been performed at room temperature and at 204°C. The alloy shows a normal hardening and softening cyclic stress-strain response at room temperature. At 204°C, however, the cyclic stress-strain response shows a strain hardening first followed by a relatively stable stress and finally a secondary cyclic strain hardening. This stable stress disappears with increasing strain amplitude. The mechanisms of the secondary cyclic strain hardening have also been investigated by transmission electron microscopy (TEM). Besides dislocation multiplication, interactions between stacking faults and moving dislocations and between interstitial atoms and moving dislocations could also contributed to this secondary cyclic strain hardening. The formation of micro-twins during cyclic loading at 204°C and its influence on the cyclic stress-strain response were also discussed. [ABSTRACT FROM AUTHOR]

The bipolar electric fatigue behaviour of antiferroelectric ceramics with the composition of Pb0.88Ba0.10La0.02 (Zr0.55Sn0.35Ti0.10)O3 was investigated under various cycling fields. The material exhibits a degradation in the maximum field induced strain, a diffuse AFE-FE phase transition and an enhancement in the diffusion character of the FE-AFE phase transition due to electric cycling. Those variations increase with cycle number, indicating a logarithmic fatigue up to 108 cycles. There is no indication for the variations to be recovered, and the symmetry of the negative and positive parts of the strain hysteresis loops still remains. Higher cycling field results in a stronger deterioration of the maximum field induced strain and a larger extent of diffusion in AFE-FE and FE-AFE phase transitions. The normalized maximum strain shows nearly the same scale of degradation when the materials are cycled at various electric fields. After a heat treatment at 500°C for 1 h, the variations in the AFE-FE and FE-AFE phase transition due to cycling disappear, whereas the maximum strain resumes almost to its original value. Electrochemical variations are considered to contribute to the main fatigue mechanism for the material under investigation. [ABSTRACT FROM AUTHOR]

Precise estimation of local stress profiles in individual phases of a fiber reinforced metal matrix composite is a crucial concern for design of composites. Stress profiles are significantly affected by plastic relaxation of soft matrix. In this work, an analytical model was developed to compute local stress profiles in individual phases of fibrous metal matrix composites. To this end, embedded cell cylindrical composite model was applied in which a layered concentric cylinder consisting of a fiber-, matrix- and homogenized composite layers was used. Mean field micromechanics was integrated into the conventional elasticity solution process so that micro-macro dual scale analysis could be performed. The algorithm was formulated in an iterative incremental structure which was able to perform plastic analysis. This also allows temperature dependence of flow stress to be considered. Taking copper-SiC system as a reference composite, stress profiles were obtained for mechanical and thermal loading cases. For comparison, independent finite element analyses were carried out for two different unit cell models. Excellent agreement between analytical and numerical solutions was found for the mechanical loading case even for plastic range. In the case of thermal loading, however, plastic solutions revealed notable difference in quantity, especially for the axial stress component. [ABSTRACT FROM AUTHOR]

In recent years, ruby fluorescence spectroscopy has been demonstrated as a powerful technique for monitoring residual stress evolution in the thermally grown oxide scale in thermal barrier coating (TBC) systems. The measured residual stresses, in turn, can be used to monitor evolution of damage in the coatings. Effective use of this technology for real-time damage monitoring requires the identification of trends in measured stresses that can be used as indicators of damage evolution. The present work focuses on studying the evolution of residual stresses in TBC systems during long-term exposure to turbine operating conditions. The coatings are electron beam physical vapor deposited (EBPVD) and atmospheric plasma sprayed (APS) zirconia. The stress evolution in both EBPVD and APS coatings is analytically modeled by an approach that takes into consideration contributions due to both thermal mismatch and oxide growth. Microstructural changes in the TBC system are correlated with measured stress trends through comparison with the modeled stresses. The stress measurements and modeling provide insight into failure modes and mechanisms, and to identify critical features in the measured stress data that can be used as indicators of failure in TBCs. [ABSTRACT FROM AUTHOR]

The tensile behaviour of different geothermal well cement formulations was investigated in order to determine how these materials perform under such loading and how tensile capacity can be improved. The influence of latex, perlite and fibres on the load-displacement relationship of the cements was measured on notched cylinders. The fracture surfaces were examined to further elucidate failure mechanisms. Unreinforced cements exhibited linear elastic behaviour to different degrees and failed in a brittle manner. Cements reinforced with either carbon (150 μm) or steel (1–3 mm) microfibres required greater loads for failure. However, the microfibres did not provide any major improvement in ductility. Addition of 13 mm steel fibres to the cements resulted in both strengthening and transition to ductile behaviour. Inclusion of the types of fibres studied in this work in cements offers potential benefits in maintaining the integrity of geothermal wells when tensile stresses are involved. [ABSTRACT FROM AUTHOR]

LPCVD polysilicon films exhibit tensile or compressive residual stresses and stress gradients, depending on deposition temperature. The stresses, which are a result of an “intrinsic” growth eigenstrain, ℇg, produce undesired curvatures in released structures. A combined experimental/computational design procedure is presented for controlling the curvature of thin films comprised of tensile layers, deposited at 570°C, alternated with compressive layers deposited at 615°C. Experimental measurements are first used to calculate the through-the-thickness variation of ℇg for both temperatures. This information is in turn incorporated into a mechanical model that predicts, for prescribed parameters that define the geometry, elastic moduli, and eigenstrain distribution, the stress distribution before release, and the curvature upon release, of multilayered films. Comparisons of predicted and measured average stress before release of a number of films, and of the curvature upon release of a circular micromirror device, provide a preliminary assessment of the semi-empirical model, which when combined with optimization algorithms can be used to develop recipes (thicknesses of the individual layers of a multiplayer device) that will achieve prescribed curvature according to given constraints. [ABSTRACT FROM AUTHOR]

A series of helical ceramic springs were manufactured from MgO partially stabilized zirconia to investigate their mechanical properties. Nine springs were machined from zirconia tubing, initially one inch in length, with a rectangular pitch of 16, 14, or 12 turns per inch. An experimental apparatus that both supports and equalizes the applied loads on springs was developed. The spring deflection versus applied load was measured using an optical sight mounted on a micrometer. Deflection data on each spring were collected, plotted and successfully modeled using Hooke's Law. A more extensive model was used to calculate the shear modulus of rigidity and shear stress. This model incorporates the spring dimensions, pitch, applied load, and deflection and provides insight into the effects of the materials of construction and manufacturing technique on the effective shear modulus of spring. A specific manufacturing effect was observed in the initial deflection resulting from the mass of the spring as the pitch was increased. [ABSTRACT FROM AUTHOR]

Induced damage in brittle materials due to two interacting Vickers indentations at various orientations was investigated using a three-dimensional finite element model. The model considers `tensile cracking and compressive yielding' behavior of ceramics. Damage evolution due to both the simultaneous and sequential double indentations was studied. The simulation results indicated that the induced damage zone patterns are strongly a function of the relative orientation of the two indenters. The existence of another nearby indentation reduces the crack size on the side closer to the first indentation but increases the overall median damage zone size. These results were validated by sequential Vickers indentation experiments on borosilicate glass. The evolved damage patterns were further rationalized based on Bousinesq and blister field stress fields. Finally, the implication of these results on material removal mechanisms due to simultaneous interaction of several grits in a ceramic grinding process is discussed. [ABSTRACT FROM AUTHOR]

It is well established that the indentation hardness of metallic alloys shows a reasonable correlation with their yield strength or ultimate strength. Experiments illustrate that such a unique correlation is nonexistent for discontinuously reinforced metal matrix composites, even when the indentation size is much greater than the reinforcement size. For aluminum alloys reinforced with silicon carbide particles, the same composite yield strength and tensile strength with different reinforcement fractions do not lead to similar hardness, or vice versa. Finite element analyses are carried out to rationalize the experimental findings. The modeling utilizes a two-dimensional plane-strain formulation. Discrete particles are included in the material model, and the overall stress-strain response and the indentation response are numerically simulated. The results confirm the lack of unique correspondence between the composite hardness and strength. The alteration of local heterogeneity in the composite is found to affect the indentation response. Effects of the geometrical arrangement of particles and thermal residual stresses on the indentation response are also investigated numerically. [ABSTRACT FROM AUTHOR]

Thermal stresses have been measured in single carbon fibre polypropylene matrix composites using micro Raman spectroscopy. The study focuses on the influence of a transcrystalline interlayer on the thermal residual stress distribution in composites. A series of three experiments were made in order to systematically study the influence of thermal history, transcrystalline interlayer thickness and matrix supermolecular structure. From the experiments it was shown that the transcrystalline interlayer results in higher thermal residual shear stress. This was explained by anisotropy in the transcrystalline interlayer resulting in higher radial thermal expansion and thus higher radial stress. [ABSTRACT FROM AUTHOR]

The reinforced and toughened PP/PA66 in situ composites were prepared via extrusion-drawing-injection method. The relationship among composition, phase morphology and mechanical properties, together with their functional mechanism, was investigated. The results show that in the range of PA66 weight fraction (fw) from 0 to 20% and under the experimental processing conditions, the main changes in phase morphology of the composites with fw are that the number of in situ formed PA66 microfibers and remained PA66 particles increases with fw whereas the fiber transverse size and its dispersity decrease till fw = 15% and then increase. This can be attributed to the combined effect of break-up, coalescence and deformation of the PA66 phase in the PP phase in the course of extrusion and drawing. The tensile strength of composites has a maximum value at fw of 15% and Young's modulus increases with fw up to a plateau level, while impact strength continuously rises with fw, an effect which can be ascribed to the distinct dependence of these properties on the phase-morphological factors mentioned above. [ABSTRACT FROM AUTHOR]

Non-ideal bundle models that predict the effect of filament slack, misalignment and bonding were compared to the predicted ideal bundle stress-strain response and the measured response of an Alumina tow (Nextel™ 610) as a function of exposure temperature. The ideal bundle model assumes that the filaments are perfectly aligned, independent and filament strength is characterized by single filament tests; the non-ideal bundle models relax these assumptions. The Nextel™ 610 tow was found to behave ideally until the sintering bonds between filaments were strong enough to resist fracture during testing. When bonded filament clusters existed throughout testing, the weakest filament in the cluster likely caused failure of the entire cluster. Therefore, the assumption of independence was not valid, and the measured bundle strength was lower than the ideal bundle prediction. The assumption that the strength distribution of the bonded filaments was the same as the filaments removed from bonded filament clusters also appears to be invalid. It is hypothesized that the effective decrease in strength of bonded filaments is a result of induced shear stresses that form along the bond line when bonded filaments are either bent or twisted. However, within a composite this motion is limited. The modeling indicated that 10% clustering can be tolerated with little degradation in composite properties as long as the clusters remain independent from their neighbors. [ABSTRACT FROM AUTHOR]

This study has successfully incorporated Al2O3, SiC particulates into the 7005Al alloy matrix by using a drag-push method. The reinforced particulates are uniformly distributed in the matrix. This study also discusses the influence of aging treatment on the friction welding properties of 7005Al/10 wt%, 15 μm and 6 μm SiC(p) composites and 7005Al/10 wt%, 15 μm Al2O3(p) composites joint system. Experimental results show that after peak aging treatment was performed, if SiC particulates were used in the strengthening phase, the heat-affected zone (HAZ) had higher density of strengthening particulate, this resulted an increase in the hardness and stress concentration at the fully plasticized zone (Zpl) of the HAZ region, but a decrease in the width of the Zpl zone and the welding strength. And the welded fracture surface morphology had a low-ductile fracture. [ABSTRACT FROM AUTHOR]

For the antiferroelectric crystals PbZrO3 and PbHfO3 pure and doped with small amount of PbTiO3 the measurements of electrostrictive strain e have been performed in paraelectric phase as a function of temperature, frequency (40–400 Hz) and strength (100–700 kV/m) of an applied electric field. In particular, compliance with quadratic relation between strain e and the electric field E ( e = ME2) in different frequency regions was examined. The observed effects for PbZrO3 and PbHfO3 were compared and discussed with previously investigated low frequency dielectric dispersion found in those antiferroelectric materials. The experiments showed that in the frequency range where the dipolar relaxation exists, the departure from the quadratic relation ( e = ME2) can be clearly observed for PbZrO3 and PZT single crystals. In all investigated single crystals (PbZrO3, PZT and PbHfO3) the dependence of strain versus electric field obeys the electrostrictive relation in the frequency range below and above dielectric relaxation observed. For frequencies beyond dielectric dispersion the electrostrictive coefficients Q11 for all samples are temperature independent and take values typical for ferroelectric and antiferroelectric materials (i.e. Q11 ≅ 2 × 10−2 − 3 × 10−2 m4/C2). [ABSTRACT FROM AUTHOR]

The tensile behavior of two magnetically soft alloys, Fe-49Co-2V and Fe-27Co, has been characterized as a function of testing temperature, grain size, and degree of long-range order. Several trends in the yield strength of the two alloys have been noted and possible mechanisms for their occurrence discussed. Ordering is found to markedly lower the yield strength of the Fe-49Co-2V. Both alloys exhibit three distinct regions in their yield strength vs. temperature curves. At lower and higher temperatures, i.e. regions I and III, the yield strength shows the normal drop with increasing temperature. In the intermediate temperature range (region II), however, Fe-49Co-2V with a B2 ordered structure demonstrates an anomalous strengthening with increasing temperature while the yield strength remains constant in the disordered Fe-27Co. Both alloys exhibit a Hall-Petch type relationship in their yield strength as a function of grain size and show a decrease in the strain hardening coefficient with increasing grain size. [ABSTRACT FROM AUTHOR]

Metal laminates of copper/aluminium were prepared by roll bonding at 430°C with various rolling strains. The effect of the rolling strain on the interface development and bond strength of the laminates was examined. It was found that as the rolling strain increased the bond strength of the laminates was generally enhanced in both as-rolled and sintered conditions. Critical post-rolling sintering conditions were found to exist for achieving optimum bond strengths of the laminates. It is evident that the development of optimum strength for the laminates is related to the formation of various intermetallic phases at the interface which is in turn determined by the diffusion activity of the metallic elements in the area. The greatest strength enhancement was generally observed to develop in the 60% rolled samples, suggesting that rolling strain of the roll bonding process may impose great influence on diffusion of the metallic elements. A higher copper content, without significant Kirkendall void formation, was found to build up in the interface area of the material, leading to development of strong interfacial phases. It is believed that a higher rolling strain of the roll bonding process has provided a greater area of physical contact between the bonded metals and imposed diffusion enhancement of the metallic elements across the interface. [ABSTRACT FROM AUTHOR]

Most of adhesively bonded joints are under complicatedly distributed triaxial stress in the adhesive layer. For the estimating of the strength of adhesively bonded joints, it is crucial to clarify behavior of yield and failure of the adhesives layer under triaxial stress conditions. Two types of the adhesively bonded joints were used in this study: One is the scarf joint which is under considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied with scarf angle. The other is the butt joint with thin wall tube in which considerably uniform pure shear can be realized in the adhesive layer under torsional load conditions. These joints can cover the stress triaxiality in adhesive layers of most joints in industrial application. The effect of stress triaxiality on the yield and fracture stresses in the adhesive layer were investigated using the joints bonded by three kinds of adhesives in heterogeneous and homogeneous systems. The results showed that both the yield and failure criterion depend on the stress triaxiality and that the fracture mechanism of the homogeneous adhesive is different from that of the heterogeneous one. From these experimental results, a method of estimating the yield and failure stresses was proposed in terms of a stress triaxiality parameter. [ABSTRACT FROM AUTHOR]

Poly(phenylene sulfide) (PPS) was melt blended with Nylon66 and the mechanical properties and corresponding fracture morphologies were investigated. The thermal distortion temperature (HDT) of PPS/Nylon 66 blend showed that the inherent thermal stability of pure PPS can be maintained up to 30 wt% Nylon66, but then it started to decrease linearly thereafter to that of pure Nylon66 based on the rule of mixtures relationship. Tensile tests of PPS/Nylon66 blends at testing temperatures of −30, 25, 75, and 150°C showed that the maximum stress decreased up to 30 wt% Nylon66, and started to increase thereafter. Strain at break showed little change at low nylon content regardless of testing temperature, however, a large strain at break increase could be observed at more than 30 wt% Nylon66 and at 150°C testing temperature. At the same testing temperatures, the impact strength of PPS/Nylon66 blends was investigated, and it was found that an impact strength increase at all testing temperatures could be observed at more than 30 wt% Nylon66. [ABSTRACT FROM AUTHOR]

Ceramic workpiece integrity and residual surface stresses generated by single pass diamond grinding were evaluated for three flaring cup wheels and four machine-loop stiffnesses. Stresses in silicon nitride bars ground on one face were characterized by X-ray diffraction, strength by four-point bending, and grinding damage depth by scanning electron microscopy. A custom-built workpiece holder was used to tune the grinding machine-loop stiffness. Electrolytic in-process dressing was applied to one of the wheels to provide stable cutting conditions. The experimental results indicate machine stiffness does not have significant influence on flexural strength, but rather affects the depth of cut. All ground surfaces have some degree of damage and residual stress, and differences are revealed between wheel bonds and grit sizes. The competing phenomena of strength enhancement due to residual stress and strength degradation due to damage are discussed. [ABSTRACT FROM AUTHOR]

This work focuses on the modification of the tensile yield strength of CaCO3 filled HDPE brought about by the incorporation of SEBS elastomers. Two types of SEBS elastomers were used, grafted and ungrafted with maleic anhydride functions. The grafted elastomer encapsulates the filler particles in-situ and creates an adhesive interphase. The tensile yield stress was increased with increasing content of grafted elastomer until a maximum value. The influence of the interfacial area and the volume fraction of filler were studied. It was shown that the relative increase in tensile yield stress when increasing amount of interfacial agent was added, both depends on the volume of filler and the interfacial area. [ABSTRACT FROM AUTHOR]

The effect of thermal treatment on the properties and structure of carbon-silicon alloy fibres produced from a novel silicon-containing carbon precursor is reported. The precursor, containing about 22 wt% Si, was melt spun into fibres and then oxidatively stabilized under different conditions to render the fibres infusible. The fibres were pyrolysed and heat treated to 1600 °C in inert atmosphere. The extent of stabilization was found to be critical to the development of mechanical strength of the fibres which varied with heat treatment temperature, showing a maximum at 1200 °C when the strength was 1.2–1.4 GPa. Moduli were low because of the lack of orientation of the carbon layer planes along the fibre axis. The maximum strength and the thermal stability at high temperatures is considerably reduced if the fibres are excessively oxidized at the stabilization stage. Optimally stabilized fibres show a drop in strength at 1300 °C but this stabilizes at about 600 MPa over the range 1300–1600 °C. These strengths are remarkably good considering the low modulus which is due to the quite high failure strains. The fibres can show excellent resistance to oxidation if given an initial short exposure to oxygen at high temperature. This is considered to be due to an imperceptible layer of silica. [ABSTRACT FROM AUTHOR]