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The tension-tension fatigue behavior and damage mechanism of basalt fiber-reinforced epoxy polymer (BFRP) composites at different stress ratios are studied in this paper. The fatigue experiments were performed under stress ratios, R = σmin/σmax of 0.1 and 0.5, while the lifetime and the stiffness degradation were monitored and analyzed to investigate the effect of stress ratios. The damage propagation during fatigue loading was periodically monitored by using an in situ scanning electron microscope (SEM). The results show that the fatigue life decreases and the fatigue life degradation rate increases with the decrease of stress ratio for examined BFRP composites. The stiffness degradation is also sensitive to different stress ratios, showing a greater stiffness loss before failure at lower stress ratio. From the SEM images, it is indicated that the micro-damage mode shifts from interface debonding and matrix cracking into fiber breaking with decreasing stress ratios. [ABSTRACT FROM AUTHOR]

The objectives of this paper are to present the results of experimental study and analysis on the behaviour and the strength of reinforced geopolymer concrete slender columns. The experimental work involved testing of twelve columns under axial load and uniaxial bending in single curvature mode. The compressive strength of concrete for the first group of six columns was about 40 MPa, whereas concrete with a compressive strength of about 60 MPa was used in the other six columns. The other variables of the test program were longitudinal reinforcement ratio and load eccentricity. The test results gathered included the load carrying capacity, the load-deflection characteristics, and the failure modes of the columns. The analytical work involved the calculation of ultimate strength of test columns using the methods currently available in the literature. A simplified stability analysis is used to calculate the strength of columns. In addition, the design provisions contained in the Australian Standard AS3600 and the American Concrete Institute Building Code ACI318-02 are used to calculate the strength of geopolymer concrete columns. This paper demonstrates that the design provisions contained in the current standards and codes can be used to design reinforced fly ash-based geopolymer concrete columns. [ABSTRACT FROM AUTHOR]

The paper focuses on modeling the strain fatigue lives of three commonly used cable insulation polymers, namely (1) polyvinyl chloride, (2) crosslinked polyethylene, and (3) polyphenylene ether under selected strain and temperature ranges. On the basis of results obtained from their fatigue tests, Coffin–Manson model, mean/maximum strain fatigue model, and a set of new semi-empirical equations were applied to establish the relationship between fatigue lives and strains. The unified strain model, herein we name it the Wei–Wong model, is developed to predict the fatigue lives of three polymers studied and their prediction capability was examined using our experimental data. It was found that the proposed Wei–Wong model can provide a better life prediction compared to the experimental data and other methods in the literature at selected temperatures, namely −40, 25, and 65 °C. [ABSTRACT FROM AUTHOR]

Wood fibres constitute the structural framework of e.g. wood, paper, board and composites, where stiffness and dimensional stability are of importance. An analytical modelling approach has been used for prediction of hygroelastic response, and assessment of the stresses in thick-walled cylinder models of wood fibres. A wood fibre was idealised as a multilayered hollow cylinder made of orthotropic material with helical orientation. The hygroelastic response of the layered assembly due to axisymmetric loading and moisture content changes was obtained by solving the corresponding boundary value problem of elasticity. A simple solution scheme based on the state space approach and the transfer matrix method was employed. This was combined with an analytical ultrastructural homogenisation method, used to link hygroelastic properties of constituent wood polymers to properties of each layer. Predicted hygroelastic response captured experimentally measured behaviour. Fibres that were constrained not to twist showed a stiffer response than fibres allowed twisting under uniaxial loading. It was also shown that the ultrastructure, i.e. the microfibril angle, will control the hygroexpansion in the same way as it controls the compliance of the cell wall. Qualitative failure trends comparable with experimental observations could be established with stress analysis and a simple plane-stress failure criterion. [ABSTRACT FROM AUTHOR]

In recent years, important efforts have been focused on rapid production of tools using Rapid Prototyping and Manufacturing (RP&M) technologies such as the Stereo-Lithography Apparatus (SLA). One of the applications is the development of rapid polymer tooling such as dies for injection molding. For these applications, optimal thermal as well as mechanical properties of final tools are of significance. In order to characterize the mechanical response of materials made by SLA, a standard set of material tests, including uniaxial tension and compression tests under different strain rates and different temperatures, was conducted for both silica filled and non-filled resin. In this paper, the mechanical response of the non-filled SL5170 cured resin is discussed in terms of an elastic-viscoplastic material model. Further, a new model for silica filled SL5170 cured resin was developed to estimate the stress-strain relationship of the composite. This composite model is an extension of the elastic-viscoplastic model for non-filled resin to include the elastic deformation of the silica particles. The stress-strain curves predicted by the models under homogeneous deformation show good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Knitted fabrics are known for their excellent stretching and draping properties due to their curved fabric architecture. In recent years knitted fabrics have been used as reinforcement network for polymer sheet thermo-processing. This paper describes a method for evaluating the product height, product profile and strain distribution of a flat polymer matrix sheet, reinforced with knitted fabric, undergoing the sheet thermoforming process. The availability of a knitted fabric reinforced PMC to stretch is proposed to be equal to the least ratio of curved loop length to its projected length. The product profile is then obtained from the availability of the material to stretch and the tool geometry involved. Finally a mapping scheme is proposed to evaluate the severity of deformation in terms of strain distribution. A reasonably good agreement is observed between the model and experimental results. An understanding of the severity of deformation distribution would shed light on the location of highest degree of deformation as well as its magnitude and should therefore assist the product designer in dealing with the thermoforming process of this new material. [ABSTRACT FROM AUTHOR]

Biological tissues are made from nano-composite materials and given the recent interest in manufacturing synthetic nano-composites an analysis of natural nano-composites seems a worthwhile exercise. There is also potential for extracting natural nano-fibres and using them as reinforcements in other materials. In this paper a hierarchical mechanical model is formulated to describe potato tuber tissue and the model is used to back calculate the properties of cell wall nano-fibres. The model contains two structural levels, the cell structure and the cell wall structure. Material properties are assigned at the level of cell wall microfibrils (nano-composite fibres). Force deflection data from the compression of cubes of potato tissue were fed into the model and the properties of the cell wall microfibrils predicted. The modulus was found to vary with strain, but had a maximum value of 130 GPa, which is close to predictions from theoretical chemistry for the stiffness of cellulose microfibrils. At 8% wall strain (the value at which failures were suspected to begin), the stress was predicted to be 7.5 GPa which is also close to theoretical chemistry predictions for the strength of cellulose microfibrils. The large strains and decreasing stiffness indicate the influence of polymers other than cellulose. [ABSTRACT FROM AUTHOR]

The mechanical behaviour of poly(vinyl butyral) (PVB) at small (<0.1%) and large strains (>200%) is investigated experimentally over a range of strain rates to provide data for the development and validation of constitutive models. The small-strain response is investigated using dynamic mechanical analysis at frequencies from 1 to 100 Hz and temperatures from −80 to 70 °C. It is found that a generalised Maxwell model adequately describes the material behaviour in the small-strain regime. The large-strain response is investigated using a high-speed servo hydraulic test machine at strain rates from 0.2 to 400 s. It is found that the PVB response is characterised by a time-dependent steep initial rise in stress followed by a hyperelastic type response until failure. No current material model completely captures this time-dependent behaviour at large strains. [ABSTRACT FROM AUTHOR]

We studied the stress-dependent piezoresistive behavior of tunneling-percolation systems. Starting from percolation-like power law of resistivity, a model relating piezoresistive stress coefficient to the applied stress has been developed by considering the stress-induced changes in the critical exponent and that in the concentration of conducting phases. It is found that the coefficient exhibits reverse and logarithmic dependence on the applied stress in the different stress ranges. We show that the experimental data, obtained from carbon-reinforced polymer composites and polymer-derived ceramics, follow the theoretical prediction of the model very well, indicating that the stress effects described by the model indeed exist in tunneling-percolation systems. [ABSTRACT FROM AUTHOR]

The toughening behavior of short glass fiber reinforced toughened polymers was studied using fracture mechanics and microscopic techniques. The essential work of fracture (EWF) analysis shows that the inclusion of short glass fibers not only provided a stiffening effect but also a toughening influence. It was observed that rubber-related toughening and fiber-related toughening were competitive in nature for the reinforced, toughened nylon 6,6. When the matrix stress was substantially reduced by the presence of short fibers via the load-shedding mechanism, rubber toughening was severely curtailed. At higher fiber volume fractions, fiber pull-out work contributed significantly to the enhancement of the specific essential fracture work. Fiber-end plasticity was evident under microscopic examination. [ABSTRACT FROM AUTHOR]

Single edge notched polycarbonate (PC) specimens of thickness 0.175 mm were pulled to complete fracture at temperatures between 25°C and 100°C and at loading rate values of 2, 5 and 50 mm/min. A duckbill-shaped yielded zone was formed ahead of the crack tip in all the specimen tested. Propagation of the crack within the yielded zone was always stable. The method of essential work of fracture (EWF) was used to study the effects of temperature and loading rate on fracture toughness. The specific essential work of fracture, we, was found to be independent of both temperature and loading rate. The non-essential work of fracture, β wp, increased with increasing temperature but showed no systematic variation with respect to loading rate. Moreover, plastic constraint factor, m, also increased with increasing temperature. A linear temperature dependence was obtained for both β wp and m giving the extrapolated values of β wp = 0 and m = 0.5 at −23°C. [ABSTRACT FROM AUTHOR]

The effects of elemental powder characteristics, binder content and its composition, as well as some additives on pressure change and drainage phenomenon of pastes during extrusion have been mainly investigated. The pastes consisted of a powder, zirconia or stainless steel, and a water-based binder, an aqueous solution of water-soluble polymer (hydroxypropyl methylcellulose). The drainage phenomenon has been found in extrusion of the stainless steel pastes with lower binder contents, while the zirconia pastes show a small probability of drainage in the range of the binder contents used in this investigation. It is shown that broadening particle size distribution by mixing powders with different average particle sizes has a significant effect on decreasing extrusion pressure and restraining occurrence of the drainage phenomenon, thus improving the extrudability of pastes. It is effective to increase the binder content in pastes, raise the mixing fraction of HPMC in binder and add plasticizer like glycerol, in order to reduce occurrence of the drainage phenomenon during extrusion of the stainless steel pastes. [ABSTRACT FROM AUTHOR]

The stress-strain behaviour of partially processed leather which has been dried under a range of uni-axial strains has been investigated. It has been found that the dependence of the tensile modulus on the strain applied during drying is non-linear, increasing slowly at first then more rapidly later on. A two-dimensional microstructural model based on an idealised fibre network can describe this non-linear relationship and account for differences between samples. High values of set are produced by drying under strain and some of this set is retained even after soaking in water. It is suggested that this is due to the formation of stable crosslinks between the chemically modified collagen fibres that comprise leather. [ABSTRACT FROM AUTHOR]

In this part of the series, the impact behaviour of the PBT and PC blends without impact modifier was studied. Failure mechanism of the blends under various conditions was discussed. It was found that the key toughening process, i.e. interfacial debonding-cavitation, was disabled when the blends were subjected to impact loading. Hence, the fracture of the thick PBT/PC specimens with strong interface occurred under plane-strain condition. Their impact toughness obeys the rule of mixtures and synergistic toughening could not be achieved. When thinner specimens were tested, the fracture took place under non-plane-strain condition. But, the toughness of the blends was much lower than the value predicted by the rule of mixtures. Negative blending effect was obtained. Study on the strain rate effect suggests that under impact loading, the PC domains in the blends are subjected to an additional plastic constraint imposed by the neighboring PBT matrix, which is more rigid at a higher strain rate. Since fracture of the PC is highly sensitive to the plastic constraint at the crack-tip, the PBT imposed high plastic constraint promotes brittle fracture of the PC, leading to a deteriorated impact resistance. Evidences from TEM, SEM and OM studies support the mechanism proposed. Based on this mechanism, some suggestions on the selection of polymer components and design of microstructure for rigid-rigid polymer blends are also given. [ABSTRACT FROM AUTHOR]

The energy for complete fracture in double edge-notched tension test specimens has been measured for a wide range of polymer films. Results indicated that the variation of the total specific work of fracture, wT, with ligament length, L, can be described by two straight lines, both of the form wT = we + β wpL, thus giving upper and lower intercept values at zero ligament length (i.e. we) for each film. The first term, we, is the energy absorbed per unit area of fracture, whereas the second term, wp, is the energy absorbed per unit volume of plastic deformation remote from the fracture surface. The lower we value was obtained from the extrapolation of the data within the mixed mode region (plane-stress/plane-strain) where the maximum net-section stress exceeded 1.15 times that of the tensile yield stress, σy, of the material, and the upper value was ascertained by extrapolating the data within the plane stress region where the net-section stress was 1.15 σy. It appears that the transition from plane stress to plane strain mode of fracture in thin films occurs at a ligament length much greater than 5B, where B is the specimen thickness. Moreover, it was found that the linearity of the data within the plane-stress region was not affected when ligament length values exceeded the plastic zone size. Moreover, variation of the extension to break with ligament length, for both pure plane stress and the mixed mode regions, was also linear; and the extrapolation values at zero ligament length were identified as crack opening displacements. Essential work estimated from the crack opening displacement agreed reasonably well with the extrapolated values. [ABSTRACT FROM AUTHOR]