Mechanical properties of discontinuous fiber reinforced thermoplastics. II. Random-in-plane fiber orientation

The mechanical properties are presented for a series of discontinuous fiber-reinforced thermoplastic composites made with random-in-plane fiber orientation. The matrix and fiber materials were chosen to provide a wide range of strength, modulus, ductility and adhesive properties. In many cases strong, rigid, yet tough composites were fabricated. Strength levels of over 20,000 psi and modulus values over 1,000,000 psi were reached in several systems reinforced with short Kevlar-49 and graphite fibers. A strong dependence of composite strength and modulus on fiber strength and modulus was noted indicating good transfer of load from matrix to reinforcement. Fiber efficiency factors for modulus and strength were calculated for the experimental composite systems and averaged 0.19 and 0.11 respectively. Data were analyzed using basic composite theory. Properties of the experimental composites could not be predicted from constituent properties.     

Effect of fiber loading and orientation on mechanical and erosion wear behaviors of glass–epoxy composites

For a composite material, its mechanical behavior and surface damage by solid particle erosion depend on many factors. One of the most important factors is the fiber content. Similarly, these properties are also greatly affected by the fiber orientation. In this work, a series of experiments were carried out to investigate the influence of fiber loading and fiber orientation on mechanical and erosion behavior of glass fiber‐reinforced epoxy composites. The composites were fabricated with three different fiber loadings (20, 30, and 40 wt%) and at four different fiber orientations (15°, 30°, 45°, and 60°). The conclusions drawn on the basis of the experimental findings are discussed, and composite with 30° fiber orientation shows better microhardness compared with other fiber orientations irrespective of fiber loading. Similar observations are also noticed for other mechanical properties of the composites, such as tensile strength, flexural strength, interlaminar shear strength, impact strength, etc. Finally, the morphology of eroded surfaces is examined using scanning electron microscopy (SEM), and possible erosion mechanisms are identified. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Mechanical behavior and fracture toughness characterization of high strength fiber reinforced polymer textile composites

The mechanical and fracture behavior of polymer composites are the subject of great interest from many years and still interesting among the researchers. Composites are extremely used for their superior mechanical, thermal and fracture toughness properties in various sectors such as automobile, aerospace and defense applications. In this article, unidirectional and woven high strength glass, carbon and Kevlar fiber reinforced polymer textile composites are taken into consideration for the comprehensive review of mechanical behavior and fracture toughness characterization. Current review work began with the introduction to polymer textile composites with its manufacturing stages, processing techniques and factors affecting the performance under mechanical loading. The mechanical behavior of high strength fiber reinforced polymer (HSFRP) textile composites was discussed in tension, compression, flexural, low velocity and high velocity impact loading with the recent numerical and experimental characterization studies. Textile geometrical modeling and CAE tools are also described for numerical characterization. Under the influence of mechanical loading on composites, failure occurs actually due to the crack initiation and propagation, so it is also required to characterize. Significant elements of fracture mechanics are well described for the better understanding of fracture toughness characterization. Mode-I, Mode-II, Mode-III interlaminar and Mode-I intralaminar fracture toughness characterization are widely explained by considering the effect of filler content, fiber orientation and fiber volume fraction. Fracture toughness characterization techniques and research summery are uniquely presented by considering various factors under one umbrella for better understanding of fracture behavior. Statistical Weibull distribution is also presented for the failure prediction of composites.

     

Techniques for the evaluation of the drawing behavior of experimental fibers

Equipment and techniques are described which have been developed for determining the drawing behavior of fibers from novel polymers which are available only in very small quantities. Emphasis is placed upon information obtained from the stress–strain diagrams of the fibers during drawing. A parameter, the reinforcement factor, is described which reflects the degree of orientation induced by drawing short lengths of fiber in a discontinuous manner. This parameter exhibits maxima at various combinations of temperature and draw rate, and the relationship between the corresponding temperatures and draw rates is shown to be logarithmic. The parameter is used to predict temperatures and draw rates which will give fibers having a high degree of orientation and good tensile properties when drawn continuously. The application of these techniques to give acceptable fibers from poly(m-phenylene adamantane-1,3-dicarboxamide) is described. This material is of interest as one possessing a high degree of thermal stability, and the best fibers produced to date have a tenacity of 4.0 g per denier, an initial modulus of 60 g per denier, and a break extension of 16.0%.     

On the strength and stiffness of planar reinforced plastic resins

The recent history of planar reinforced plastic resins, including glass flake, high modulus ceramic flake, and continuous vapor coated film composites, is reviewed. The theoretical mechanics of both continuous (film) and discontinuous (flake and ribbon) reinforcements are summarized in simple form. A novel set of design curves is presented from which the lower bound requirements for the flake composite constitutents may be read directly. At the same time, the dependence of the composite ultimate strength on the shear strength of the plastic resin matrix is demonstrated. The mechanical properties of experimental film and flake composites representative of recent work are reported and compared with the theoretical predictions. In conclusion, the potential of planar reinforced plastic resin composites is discussed and found to be significant for applications where low weight and high isotropic stiffness are required, for example in aero-structural, airfoil, or blade components.     

The nature of phosphogypsum impurities and their influence on cement hydration

The effects of phosphogypsum on mortar and concrete with respect to setting time and compressive strength are presented. The influence of the phosphogypsum impurities is discussed, and synthetic surrogate phosphogypsums are studied to duplicate the behavior noted with phosphogypsum. A mechanism for the action of these impurities is postulated, and proposals for beneficiation of the impure material are presented.     

INTEGRATED LINEAR AND NONLINEAR VISCOELASTICITY OF DISCOTIC LIQUID CRYSTALS

Theory and simulation of steady, small-amplitude, and large-amplitude oscillatory capillary Poiseuille flow of discotic mesophases are presented, discussed, and used to provide an integrated framework to characterize the rheology of precursors used in the manufacturing of carbon super-fibers. The underlying microstructural changes responsible for the non-Newtonian rheology are presented and discussed, taking into account dynamic couplings between orientation and velocity fields. Proper scaling of viscoelastic material functions leads to superposition and hence provides a better fundamental understanding of flow processes in steady and oscillatory flows. The relations between shear-thinning behavior in steady flow, elastic storage in small-amplitude oscillatory shear, and flow enhancement in pulsatile flow are established.     

INTEGRATED LINEAR AND NONLINEAR VISCOELASTICITY OF DISCOTIC LIQUID CRYSTALS

Theory and simulation of steady, small-amplitude, and large-amplitude oscillatory capillary Poiseuille flow of discotic mesophases are presented, discussed, and used to provide an integrated framework to characterize the rheology of precursors used in the manufacturing of carbon super-fibers. The underlying microstructural changes responsible for the non-Newtonian rheology are presented and discussed, taking into account dynamic couplings between orientation and velocity fields. Proper scaling of viscoelastic material functions leads to superposition and hence provides a better fundamental understanding of flow processes in steady and oscillatory flows. The relations between shear-thinning behavior in steady flow, elastic storage in small-amplitude oscillatory shear, and flow enhancement in pulsatile flow are established.     

Tensile behavior of tetragonal zirconia micro/nano-fibers and beams in situ tested by push-to-pull devices

The tensile mechanical behavior of tetragonal zirconia micro/nano-fibers and beams was studied with push-to-pull (PTP) devices equipped in an in situ nanoindenter. The small-volume ceramics generally experienced linear elastic deformation before fracture. Polycrystalline and oligocrystalline micro/nano-fibers exhibit a tensile strength of ∼0.9–1.4 GPa, while single-crystal beams exhibit a much higher tensile strength (∼2.1–3.2 GPa). The tensile strength of the small-volume zirconia is found comparable to the corresponding compressive strength, which indicates the large discrepancy between the tensile and compressive strength observed in bulk zirconia becomes insignificant at micro/nano-scales. No martensitic transformation induced shape memory strain was detected in the zirconia fibers and beams. Further variation in dopant concentration and crystal orientation was explored for single-crystal beams and their significance in controlling the tensile strength was discussed. Our work offers a new insight into the mechanical behavior of tetragonal zirconia-based ceramics at small scales.     

Stability of X‐ray cellulose crystallite orientation parameters in native cotton with change of location and year of growth

In this article, data on cellulose crystallite orientation parameters measured in terms of the Hermans orientation factor, average angle of orientation (α_m), and 40, 50, and 75% X‐ray angles in respect to the same 13 cotton cultivars grown at different agroclimatic locations and in different crop years in India are presented and discussed. It was observed that whereas the average values of the X‐ray orientation parameters are different for different varieties they remain practically invariant within individual varieties with change of the location of growth. The orientation parameters, therefore, appear to be genetic in origin and independent of the agroclimatic conditions of growth. It is believed that these results can be suitably exploited by cotton breeders in evolving varieties with an increased strength of fibers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 269–276, 1999     

Blocking effect of twin boundaries on partial dislocation emission from void surfaces

Recent discovery that nanoscale twin boundaries can be introduced in ultrafine-grained metals to improve strength and ductility has renewed interest in the mechanical behavior and deformation mechanisms of these nanostructured materials. By controlling twin boundary spacing, the effect of twin boundaries on void growth is investigated by using atomistic simulation method. The strength is significantly enhanced due to the discontinuous slip system associated with these coherent interfaces. Atomic-scale mechanisms underlying void growth, as well as the interaction between twin boundaries and the void, are revealed in details.     

The mechanical properties of oriented discontinuous fiber-reinforced thermoplastics. I. Unidirectional fiber orientation

The mechanical properties of a series of thermoplastics reinforced with unidirectionally oriented short fibers are reported. Both organic and inorganic fiber reinforcements were used in fiber volume fractions of 0.10 to 0.50. A number of these composites were found to have excellent strength and stiffness properties combined with good toughness and low density. The dependence of composite properties on the properties of the constituent materials is discussed. Fiber efficiency factors for strength and modulus are presented and models for predicting composite mechanical behavior are reviewed.     

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part I. The Effects of Loading Rate, Test Temperature, Fiber Sizing and Global Strain Level

The effects of loading rate, fiber sizing, test temperature and global strain level on the adhesion strength between carbon fibers and a thermosetting epoxy (Epon 815) are studied using the single fiber fragmentation test procedure. Analytical methodology describing the viscoelastic behavior observed is also presented. The possibility of rate-temperature-interphase thickness superposition for the interfacial strength function is illustrated based on the analytical models discussed. Experimental data are discussed using Weibull statistics and also presented in the form of percent relative frequency histograms for the fiber fragments in a collective fashion. The use of histograms allows for interpretation of the skewness in the data population.     

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part I. The Effects of Loading Rate,Test Temperature,Fiber Sizing and Global Strain Level

The effects of loading rate, fiber sizing, test temperature and global strain level on the adhesion strength between carbon fibers and a thermosetting epoxy (Epon 815) are studied using the single fiber fragmentation test procedure. Analytical methodology describing the viscoelastic behavior observed is also presented. The possibility of rate-temperature-interphase thickness superposition for the interfacial strength function is illustrated based on the analytical models discussed. Experimental data are discussed using Weibull statistics and also presented in the form of percent relative frequency histograms for the fiber fragments in a collective fashion. The use of histograms allows for interpretation of the skewness in the data population.     

The anisotropy of creep behavior in oriented thermoplastics

Detailed studies have been carried out on the anisotropy of creep and creep rupture behavior of thermoplastics oriented by the imposition of a large permanent deformation. This deformation is usually such as to produce simple fiber symmetry within the specimen. Experimental techniques have been devised for the accurate measurement of all three principal strains during tensile creep on small samples which are cut from the oriented specimens at various angles to the symmetry axis. In this way a full characterization of the creep behavior up to strains of 5 percent has been obtained at room temperature. Results are presented for work on rigid poly (vinyl chloride), poly(methyl methacrylate), and low density polyethylene. The results are discussed in terms of the time dependence and nonlinearity of the anisotropy. Creep rupture results on similar specimens are also presented and discussed. Anisotropy due to orientation is shown to be important in determining engineering properties and in understanding structure-properties relationships.     

Direct measurement of polymer segment orientation and distortion in shear: semi-dilute solution behavior

The shear induced backbone segment orientation and deformation of the polymeric chromophore diacetylene 4-butoxycarbonylmethylurethane (4BCMU) in semi-dilute solution has been measured with an extended dichroism technique. At low shear rates the random coil, visco-elastic polymer shows orientation in the flow direction. At higher shear rates a reduction in the average conjugation length is observed with an increase in the number of segments orienting perpendicular to the flow direction. Novel behavior, which is not consistent with standard models, is observed for this visco-elastic polymer at high shear rates. The results presented are discussed in view of prior experimental and theoretical work.     

Dynamic mechanical analysis of fibers. II. Estimation of fiber orientation and characterization of heat set PET yarns

A drawn PET yarn was heat set (annealed) at temperatures between 110°C and 245°C in an unconstrained mode and the samples characterized by dynamic mechanical analysis (DMA). A method for estimating the fiber orientation factor (α) is proposed using DMA and shown to be more sensitive than the crystalline orientation (x-ray diffraction) or total orientation (birefringence) measurements of the heat set yarns. The extension/shrinkage behavior of the heat set yarns has been discussed in the light of morphological changes, e.g., degree of orientation and the micro-crystallite formation. Unlike in the unconstrained mode, heat setting under constraint does not lead to the formation of micro-crystallites as revealed by differential scanning calorimetry. As a consequence, although the modulus and degree of orientation increase upon constrained annealing, the thermal stability, i.e., loss of orientation (reflected by shrinkage) could not be improved.     

Structural relationship between thermomechanical history and strength of polymeric solids

Ultimate properties in polymeric solids strongly depend on thermomechanical history. We have shown that the polymer structure which depends on thermomechanical history can be quantitatively described by the relative amounts of enthalpy and entropy. The excess enthalpy decreases upon annealing and increases under the tensile stress. The increase in excess enthalpy reduces the relaxation time; thus a local brittle-to-ductile transition may be induced by stress concentration. It follows that in a well-annealed material, this transition is more difficult to induce. Effects of molecular weight and orientation are also discussed in terms of dissipation of strain energy as the condition required for the strength of polymeric solids.     

Tensile Behavior of Cement-Based Composites with Random Discontinuous Steel Fibers

In this paper, the tensile properties of cement-based composites containing random discontinuous steel fibers are reported. Direct tensile tests were performed to study the effects of fiber length (hence fiber aspect ratio), interfacial bonding, and processing conditions on composite properties. Composite tensile strength and ductility are highlighted and discussed.     

Fettsäurehydrierung: Kontinuierlich oder diskontinuierlich?

Hydrogenation of Fatty Acids: Continuous or Discontinuous? Costs of investment, operation and auxiliaries are compared for a continuous and a discontinuous process of hydrogenation. Savings in the operation of a continuous hydrogenation plant with a capacity of 80 t/24 h over a discontinuous plant are presented, and advantages and disadvantages of the individual processes with respect to the other are discussed. At sufficiently high daily capacity and adequately large batches of fatty acids, continuous process of hydrogenation can offer great advantages.