Mechanical behavior of cement-based materials reinforced with sisal fibers

Fiber-reinforced cement composites were produced in Brazil using blast furnace slag cement reinforced with pulped fibers of sisal originated from agricultural by-products. Thin pads were produced by slurring the raw materials in water, followed by de-watering and pressing stages. Studies of mechanical behavior included observations of stable crack growth behavior under monotonic loading (resistance-curve behavior), followed by scanning electron microscopy (SEM) analysis of the fracture surfaces. Reinforcement with cellulose fibers resulted in improved fracture toughness, even after 9 months in laboratory environment. Microscopic analysis indicated a considerable incidence of crack bridging and fiber pull-out in the composite. The shielding contributions from crack bridging are estimated using a fracture mechanics model, before comparing with the measured resistance-curve behavior.     

Mechanical properties of nanocrystalline materials

The mechanical properties of nanocrystalline materials are reviewed, with emphasis on their constitutive response and on the fundamental physical mechanisms. In a brief introduction, the most important synthesis methods are presented. A number of aspects of mechanical behavior are discussed, including the deviation from the Hall-Petch slope and possible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the tendency for shear localization, the fatigue and creep responses. The strain-rate sensitivity of FCC metals is increased due to the decrease in activation volume in the nanocrystalline regime; for BCC metals this trend is not observed, since the activation volume is already low in the conventional polycrystalline regime. In fatigue, it seems that the S-N curves show improvement due to the increase in strength, whereas the da/dN curve shows increased growth velocity (possibly due to the smoother fracture requiring less energy to propagate). The creep results are conflicting: while some results indicate a decreased creep resistance consistent with the small grain size, other experimental results show that the creep resistance is not negatively affected. Several mechanisms that quantitatively predict the strength of nanocrystalline metals in terms of basic defects (dislocations, stacking faults, etc.) are discussed: break-up of dislocation pile-ups, core-and-mantle, grain-boundary sliding, grain-boundary dislocation emission and annihilation, grain coalescence, and gradient approach. Although this classification is broad, it incorporates the major mechanisms proposed to this date. The increased tendency for twinning, a direct consequence of the increased separation between partial dislocations, is discussed. The fracture of nanocrystalline metals consists of a mixture of ductile dimples and shear regions; the dimple size, while much smaller than that of conventional polycrystalline metals, is several times larger than the grain size. The shear regions are a direct consequence of the increased tendency of the nanocrystalline metals to undergo shear localization.The major computational approaches to the modeling of the mechanical processes in nanocrystalline metals are reviewed with emphasis on molecular dynamics simulations, which are revealing the emission of partial dislocations at grain boundaries and their annihilation after crossing them.     

Mechanical behavior of fire-resistant biocomposite

Biocomposites are typically formed by binding natural fibers derived from plants or cellulose using organic binders. The fibers that are used are normally industrial by-products and, hence, they are abundant and inexpensive. One such material is sawdust, and varieties of composite boards are being manufactured utilizing sawdust as filler material. Two major drawbacks of this system are their vulnerability to fire and very low bending strength. Both the matrix and the sawdust are flammable and this paper deals with using an inorganic matrix to improve the fire resistance. The inorganic matrix can resist temperatures up to 1000 °C and it provides protection to sawdust for short durations. The strength of these boards was increased by reinforcing with a very low percentage of high strength glass and carbon fibers. Since these fibers provide up to a fifteen-fold increase in strength, the cost increase is justifiable. Prisms were made using various proportions of sawdust ranging from about 11% to 38% by mass. The prisms were tested in compression and flexure to obtain the basic mechanical properties and determine the optimal sawdust content. Prisms with optimal sawdust content were also strengthened with glass or carbon fiber reinforcements to increase flexural capacity. The results indicate that it is possible to manufacture and engineer these types of composite beams to obtain a specified strength without using any specialized equipment, heat, or pressure, thus, producing an environmentally conscious biocomposite material.     

Mechanical behavior of cracked solids

A general relation between the apparent fracture toughness and the crack growth distance developed earlier is used to study mechanical behavior of a micro-cracked solid in this paper. A one-dimensional model is constructed in which the micro-crack length is regarded as an internal variable. Following the argument in traditional damage mechanics, an incremental form of the constitutive relationship between stress and strain is derived. Numerical results of the theory are used to illustrate the influence of the material parameters appearing in the model.     

Mechanical behavior of unstable emulsions

Results of experimental studies of the rheologic properties of unstable macroemulsions of the water-oil type are presented. A region of non-Newtonian behavior of the test emulsions is identified.Translated from InzhenernoFizicheskii Zhurnal, Vol. 30, No. 3, pp. 467–472, March, 1976.     

Mechanical properties of some plant materials

A study of mechanical properties of some plant materials, particularly vegetable flesh and cultural plant stalks is reported. It is shown that the tensile (compressive) strength, _m, of these and other plant materials is controlled by a relatively close exponential regression relation _m=0.2E ^0.75, where E is Young's modulus (r=0.975). More significant deviations from this relation are explained by the participation of buckling strength in the deformation by compression of the materials consisting of large thin-walled cells filled with air. A marked dependence of Young's modulus and strength of plant tissues on the crude fibre content is also demonstrated.     

Mechanical properties of dental investment materials

Measurement of the elastic modulus (E) of investment materials has been difficult because of their low strength. However, these values are essential for engineering simulation and there are many methods available to assess the elasticity of materials. The present study compared two different methods with one of the methods being non-destructive in nature and can be used for specimens prepared for other tests. Two different types of investment materials were selected, gypsum-and phosphate-bonded. Method 1 is a traditional three-point bending test. Twelve rectangular bars with dimension of (70 x 9 x 3 mm) were prepared and placed on supports 56.8 mm apart. The test was conducted at a cross-head speed of 1 mm/min by use of a universal testing machine. The load applied to the test specimen and the corresponding deflection were measured until the specimen fractured. The E value was calculated from a linear part of the stress-strain plot. Method 2 is an ultra micro-indentation system to determine near surface properties of materials with nanometer resolution. The measurement procedure was programd such that the specimens were indented with an initial contact force of 5 mN then followed by a maximum force of 500 mN. Measurement consisted of 10 indentations conducted with a spherical stainless steel indenter (R = 250 m) that were equally spaced (500 m). The E value rose asymptotically with depth of penetration and would approach the three-point bending test value at approximately four times maximum contact depth for both materials. Both methods are practical ways of measuring the E of investment materials.     

Fatigue behavior of composite materials

While various expected and accepted interpretations of the term fatigue behavior exist for homogeneous materials, especially metals, fatigue behavior of composite materials is not well defined—or understood. Generally speaking, one can discuss that topic by addressing the change in residual strength, life, and stiffness during cyclic (or otherwise variable) load histories. The present discussion will begin by defining the problem of fatigue behavior of composite materials, after which a representative spectrum of physical observations will be cited and models of the behavior presented. Both unnotched and notched behavior will be discussed. A brief section will deal with interpretations of data and statistical data handling schemes. Frontiers of the field will be identified. While the paper is presented in the spirit of a discussion of physical behavior rather than a review of the literature, representative work of numerous investigators is cited.

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Résumé Bien qu'il existe des interprétations variées et largement acceptées du terme de comportement en fatigue dans le das des matériaux homogènes, notamment les métaux, le comportement en fatigue des matériaux composites n'est pas bien défini voire compris. D'une manière générale, on peut aborder ce sujet en prenant en considération le changement de résistance résiduelle, la vie, et la rigidité au cours de Mistoire des contraintes cycliques ou tout du moins variables. La présente discussion commence à définir le problème du comportement en fatigue des matériaux composites suivant lequel un spectre représentatif d'observations physiques peut être invoqué et des modèles de comportement présentés. On discute le comportement à l'état non entaillé et à l'état entaillé. Un chapître spécial est relatif à l'interprétation des données et les schémas de traitement des données statistiques. Bien que le mémoire soit présenté dans l'esprit d'une discussion sur un comportement physique plutôt que dans 1'esprit d'une revue de la littérature, les travaux de nombreux chercheurs sont cités sous référence.

     

Mechanical behavior of cellulignin based composites

A powder derivative of acid pre-hydrolysis processing of cellulosic wastes, known as cellulignin, was used as filler in resin matrix composites. The flexural mechanical properties of cellulignin-polyester, epoxy or urea-formaldehyde matrix composites was evaluated. The results obtained show that the urea-formaldehyde and epoxy based composites can be used as alternative materials for low cost and low strength applications. Their advantages over the common wooden agglomerates or composites are presented and are based on the fact that cellulignin can be obtained virtually from any cellulosic waste.     

Mechanical behavior of mycelium-based particulate composites

This work investigates the mechanical behavior of mycelium composites reinforced with biodegradable agro-waste particles. In the composite, the mycelium acts as a supportive matrix which binds reinforcing particles within its filamentous network structure. The compressive behavior of mycelium composites is investigated using an integrated experimental and computational approach. The experimental results indicate that the composite mimics the soft elastic response of pure mycelium at small strains and demonstrates marked stiffening at larger strains due to the densification of stiff particles. The composite also exhibits the characteristic stress softening effect and hysteresis under cyclic compression previously observed for pure mycelium. To gain further insight into the composite behavior, a three-dimensional finite element model based on numerical homogenization technique is presented. Model validation is performed by direct comparison with experiments, and a parametric study of the effect of mycelium density and particle size is discussed.     

Mechanical and fracture behaviour of porous materials

The elastic moduli of porous materials represented as a combination of spherical, cylindrical or disk shaped holes or solid elements was calculated using a self consistent method. A yield criterion could be found by stating that the elastic distortion energy evaluated with these moduli was equal to a critical value. Another yield criterion could be approximated from the results of finite elements computations using the homogenization technique. These criteria possess the same homogeneity as the yield criterion of the dense material. In the presence of a secondary population of smaller cavities the yield criterion was found to be different from the one obtained by simply adding the two volume fractions of holes. The hole growth rate deduced from these calculations was proportional to the equivalent strain rate and to the stress triaxiality ratio, inkeeping with the evolution of damage proposed by Lemaitre. This hole growth rate was enhanced by a population of small secondary cavities. In general the strain hardening and the damage are coupled. However for sintered porous nickel the introduction of an initial damage parameter proportional to the porosity sufficed to describe the strain hardening behaviour. The condition for hole coalescence could not be connected to either a critical value of the porosity, or of the damage parameter, or a necking condition. The yield criterion modified by replacing the yield stress by the fracture stress could describe experimental results. Yet a critical local strain criterion also gave a good fit. The fracture toughness of syntactic foams could be explained by the local stress distribution in the glass spheres. In the case of ductile porous nickel the COD at initiation decreased as the fracture strain. This material exhibited a large tearing modulus whose decrease when the porosity was increased could be taken into account by the damage parameter. Fatigue crack propagation rates could also be rationalized with the use of the damage parameter and by reducing the surface of the material to be fractured.     

Mechanical properties and design of sandwich materials

Sandwich materials consisting of a low density core with stiff skins offer considerable potential for weight saving in panel applications, where the main loads are flexural. Sandwich materials of interest for car and van body panels, seat shells, etc, include steel/plastic laminates, integral skinned plastic foams and glass fibre-reinforced polyester skins with foamed plastic cores. In this paper, basic design formulae for the flexural stiffness and strength of such sandwich materials are reviewed and a method for designing optimum sandwich structures for least weight or cost is given. Mechanical property data are presented on a range of sandwich materials of potential interest for vehicle panel applications. It is then shown how use of the least-weight design method enables core and skin thicknesses to be determined and gives a means of improving the flexural properties of existing sandwich constructions.     

Magnetic behavior of heterogeneous magnetic materials

The magnetic susceptibility change in heterogeneous magnetic materials as a function of the magnetic matter characteristics (grain size, chemical composition, concentration) and also of the sample shape is studied. Techniques for measuring weak susceptibilities are described. They are used to show the existence of macroscopic demagnetizing fields in rod specimens beyond a certain volume of magnetic matter