Tensile Stress-strain Behavior Research Articles

Toughening PET by blending with a functionalized SEBS block copolymer

Toughened poly(ethylene terephthalate) (PET) was obtained by blending with 1–5 wt% of a triblock copolymer with styrene end blocks and a functionalized ethylene/butylene midblock. The midblock was grafted with 2 wt% maleic anhydride. The blends were characterized for melt viscosity, domain morphology, and tensile stress-strain behaviour at two strain rates. Blending increased the fracture strain of PET by more than a factor of 10. The fracture strain was affected to some extent by both blend composition and processing conditions. However, neither blend composition nor processing conditions strongly influenced the melt viscosity of the blend or the particle size in the blend. These observations are consistent with in situ formation of a graft copolymer by reaction of PET hydroxyl end groups with maleic anhydride. The graft copolymer acted as an emulsifier to decrease the interfacial tension and reduce the tendency of dispersed particles to coalesce, and promoted adhesion between the phases in the blend. Evidence for the presence of a graft copolymer was obtained by i.r. analysis of blend extracts.

Magnetic losses and mechanical properties of Fe-4 to 7.8 wt% Si rapidly quenched alloys

Magnetic and mechanical properties have been investigated in Fe-4 to 7.8 wt% Si rapidly quenched and annealed ribbons. The roles played by composition and microstructure on the magnetic energy losses and the tensile stress-strain behavior have been put in evidence, by carrying out the related experiments as a function of grain size. The energy losses attain a minimum value at the 6.7% Si composition at all frequencies in the investigated range d.c.-10 kHz. The vanishing of a demonstrably important magnetostriction-related coercivity contribution, which can be singled out in these stress-free samples, is recognized as the leading cause of loss minimization. The composition and grain size dependent stress-strain curves are found to obey a Hall-Petch law for the yield stress. While the elastic limit increases with the Si content, a decrease of the strain at fracture is correspondingly observed.

Fracture process of thermally shocked discontinuous fibre-reinforced glass matrix composites under tensile loading

Fracture mechanisms of discontinuous carbon-fibre-reinforced glass matrix composites were experimentally studied for specimens with initial damage induced by thermal shock. First, matrix cracking due to thermal shock was observed using both optical microscopy and scanning acoustic microscopy (SAM) to reveal the damage state. Secondly, tensile stress-strain behaviour and acoustic emission during tensile tests were measured for specimens with and without thermal shock. The progress of microscopic damage during tensile loading was also investigated using both replica and in-situ SAM techniques. Finally, macroscopic transient thermal stresses during thermal shock were calculated using finite-element analysis. It is proved that the fracture process of the composite specimen with thermal-shock-induced cracks is different from that of the virgin specimen. This difference in fracture processes is attributed to the difference in the evolution of matrix cracking, which is affected by pre-existing microcracks in the matrix.

Characterization of microstructure and tensile behavior of a cross-woven CSiC composite

The microstructure of a cross-woven CSiC composite and its effect on tensile behavior was studied in terms of damage and damage development. Microstructure characterization revealed that the composite contained processing-induced cracks and nonuniform fiber bundle sections with different fiber-matrix distributions. The processing-induced cracks and the “soft” matrix-discontinuous bundle sections were found to be two of the main reasons for the low composite modulus. A largely nonlinear tensile stress-strain behavior was observed together with a continued decrease of the modulus in the composite. This nonlinear tensile behavior was caused by the propagation of the processing-induced cracks, which led to low proportional limit, the multiplication of matrix cracks, and other damage modes such as fiber breaking, ply delamination and bundle splitting. The tensile fracture of the composite was found to occur at the “weak link”-bundle crossover sections, which showed very ragged fracture surfaces with limited fiber pull out.

高分子系複合材料 ＣＦ／Ｅｐｏｘｙの非線形引張応力‐ひずみ挙動に及ぼす温度の影響

This paper presents the effect of temperature on the nonlinear tensile stress-strain behavior of carbon fiber reinforced epoxy composites (CF/Epoxy). A two-dimensional elastic-plastic constitutive equation is developed using both a fourth-order complimentary energy function and a second-order one-parameter plastic potential, assuming that linear elastic deformation occurs in the fiber direction and anisotropy parameters change with plastic deformation. Monotonic and loading/unloading tensile tests on the on-axis and off-axis specimens at various temperatures are carried out to determine the elastic constants and plastic parameters of the effective stress-effective plastic strain relation. It is proved that nonlinearity in the transverse direction and the transverse-shear coupling is due to plastic deformation, while nonlinearity in the shear direction is ascribed not only to plasticity but also to nonlinear elasticity. The elastic constants in the transverse and shear directions and the plastic parameters are significantly affected by temperature. The off-axis tensile stress-strain curves predicted by using the present model show better agreement with the experimental results than those predicted by using Sun and Chen's model.

Cyclic tensile stress-strain behaviour of SiC/SiC composites

Cyclic tensile stress-strain behaviour of SiC/SiC composites

Influence of processing damage on performance of fiber-reinforced composites

The effect of processing-induced fiber damage, or equivalently the effect of fiber length in discontinuouslyreinforced composites, on the tensile stress-strain behavior of a fiber-reinforced ceramic or metal matrix is determined as a function of the extent of initial fiber damage and the pristine fiber strength distribution. The analysis combines a generalization of the analysis employed for the undamaged fiber problem [Curtin, W. A. (1991) Theory of mechanical properties of ceramic-matrix composites. J. Am. Ceram. Soc. 74, 2837–2845] and numerical simulations to predict the stress-strain curve, ultimate tensile strength and strain, fiber pullout, and work of pullout in terms of the underlying micromechanical material parameters. The results show that the characteristic scale of initial damage required to weaken the composite substantially is always of the order of one fiber break per length δ c , where δ c is the critical fiber slip length in the undamaged composite (typically ~ 1 mm). Overall, the effect of damage on tensile strength, failure strain, and pullout is found to be fairly small, which is attributed to the load carrying capacity of broken fibers due to the sliding resistance of the fiber/matrix interface. However, the detailed stress-strain behavior of the composite is modified. The analysis is applied to a Nicalon/CAS composite in which “premature” fiber damage has been observed. The inclusion of initial fiber damage into the theory accurately accounts for the excess strain, lower tangent modulus, and lower ultimate strength (relative to the predictions in the absence of initial fiber damage) observed in this material.

Fracture Toughness by Small Punch Testing

AbstractThe small punch test, currently being used to empirically estimate the material fracture appearance transition temperature (FATT) of a range of components in fossil power plant service, has been further developed for direct estimation of KIc and JIc in a manner that is analytical, material-independent, and requires no prior knowledge of material mechanical properties. The procedure follows an approach that is based on the continuum material toughness concept wherein the criterion for fracture is defined and measured via the continuum stress-strain deformation properties of the material. The procedure specifically involves computing the “local” strain energy density accumulated at the location and time of crack initiation in the small punch test specimen using large-strain finite element (FE) analysis. Since the procedure also includes estimation of the material uniaxial tensile stress-strain behavior from the small punch load-displacement curve, both the fracture toughness and the uniaxial tensile behavior are determined from a single test. The procedure has been developed in order to take advantage of the miniature sample removal techniques that can be applied “nondestructively” to operating components. Results are presented on a range of steels.

Nonlinear Stress-Strain Behavior of PbZrO3–PbTiO3 under Various Temperatures

This study deals with the tensile stress-strain behavior of piezoelectric ceramics under various environmental temperatures. The material used is commercial piezoelectric ceramics, PbZrO3–PbTiO3 (PZT). The tensile test was performed under various temperatures in the environmental chamber for poled and nonpoled samples having a specially designed specimen geometry, in order to discuss the effects of temperature on the stress-strain behavior. It was clarified that the stress-strain response of poled and nonpoled samples was nonlinear independent of test temperature, and the nonlinearity was stronger with the increase of test temperature. Moreover it was clarified that the stronger the nonlinearity is, the higher the fracture strength is.

Matrix cracking of cross-ply ceramic composites

A micromechanics study is presented of the matrix cracking behavior of laminated, fiber-reinforced ceramic cross-ply composites when subject to tensile stressing parallel to fibers in the 0° plies. Cracks extending across the 90° plies are assumed to exist, having developed at relatively low tensile stresses by the tunnel cracking mechanism. The problem addressed in this study is the subsequent extension of these initial cracks into and across the 0° plies. Of special interest is the relation between the stress level at which matrix cracks are able to extend all the way through the 0° plies and the well known matrix cracking stress for steady-state crack extension through a uni-directional fiber-reinforced composite. Depending on the initial crack distribution in the 90° plies, this stress level can be as large as the uni-directional matrix cracking stress or it can be as low as about one half that value. The cracking process involves a competition between crack bridging by the fibers in the 0° plies and interaction among multiple cracks. Crack bridging is modeled by a line-spring formulation where the nonlinear springs characterize the sliding resistance between fibers and matrix. Crack interaction is modeled by two representative doubly periodic crack patterns, one with collinear arrays and the other with staggered arrays. Material heterogeneity and anisotropy are addressed, and it is shown that a homogeneous, isotropic average approximation can be employed. In addition to conditions for matrix cracking, the study provides results which enable the tensile stress-strain behavior of the cross-ply to be predicted, and it provides estimates of the maximum stress concentration in the bridging fibers. Residual stress effects are included.

Dynamic mechanical behavior of HTPB dummy composite propellant

The uniaxial dynamic tensile properties of HTPB (hydroxy-terminated polybutadiene) dummy solid composite propellant at high strain rates of about 103 S-1, strongly demanded but never obtained hitherto, was acquired using the KHKK (Kawata-Hashimoto-Kurokawa-Kanayama) one bar method of block-to-bar type established for the purpose of evaluating dynamic tensile stress-strain behavior up to breaking strain. Quasi-static to medium strain rate tensile tests at strain rates ranging from 10-3 to 10-1 s-1 were also performed and compared with dynamic behavior. The dependence of mechanical properties on strain rate is clarified. Particularly, the tensile strength and modulus increase drastically at dynamic strain rates of the order of 103 s-1. High velocity impact-absorbing ability is revealed, although the dynamic breaking strain shows values of an order similar to the static results. Fractured surfaces of filled HTPB dummy propellant after testing were also examined. The difference of fracture appearances in stat...

Effect of fiber length variation on tensile properties of carbon-fiber cement composites

Unlike other fiber types, carbon fibers tend to possess a distribution of fiber lengths even before mixing into a cementitious matrix. During mixing, some amount of fiber rupture may be expected. Along with other fiber parameters, fiber length distribution may be expected to play an important role in governing the composite properties. This paper describes the results of an experimental program which measures the length distribution of carbon fibers both before and after mixing. A database of composite tensile stress-strain behavior is established for a number of commercially available carbon fibers in a cementitious matrix. Special emphasis is placed on pseudo strain-hardening behavior and the associated ultimate tensile strength beyond first cracking. A theoretical model of this ultimate strength which explicitly account for fiber length distribution is developed and shown to account for the reduced tensile strength of experimentally observed results reasonably well. Results of this research suggest that for the purpose of brittle cementitious matrix reinforcement, carbon fibers with higher strain capacity may be more important than high modulus typically demanded for polymer matrix composite reinforcement.

Isotactic polypropylene/hydrogenated oligo(cyclopentadiene) blends: 2. Tensile stress-strain and morphological behaviour of anneled samples

Mechanical tensile tests and morphological analyses were carried out on samples of isotactic polypropylene/hydrogenated oligo(cyclopentadiene) (iPP/HOCP) blends. The blends were obtained by melt mixing in a Brabender-like apparatus. Sheets of such blends were prepared by moulding the material under pressure, quenching in iced water and then annealing them in an oven at 140°C under vacuum for 24 h. Stress-strain tensile tests, previously obtained at room temperature, were performed at higher temperatures (80°C and 100°C). Morphology observations were performed by scanning electron microscopy after treatment of the blends with two different etching solvents (n-heptane and chloroform) at their boiling temperatures for various times in order to dissolve the HOCP component. An acid solution with a strong oxidizing action was used as well. In all the investigated blends (from pure iPP to up to 80% HOCP in the blend) the iPP molecules provide the material interconnection, as evidenced by fibre formation. The transition from ductile to brittle in the mechanical behaviour depends on temperature and composition.

Tensile stress-strain behavior of lightly cemented sand

Tensile stress-strain behavior of lightly cemented sand

Tensile stress-strain behaviour of cementitious composites at high loading rates

The influence of loading rate on the tensile stress-strain behaviour of cementitious composites was studied experimentally. The project was undertaken to obtain an insight into the possible relation between internal structure parameters of composites and their loading-rate sensitivity. Five different types of cementitious composites were applied. Composite structure data were obtained by testing porosity and by quantitative observation of fracture surfaces. Direct tensile tests were performed at four different loading rates within the range 0.001–1000 MPa s−1. The tensile stress-strain behaviour was significantly influenced by the loading rate and structure parameters of composites. The relative tensile strength increase due to an increase of loading rate was found to be higher for composites with higher total porosity. Recorded stress-strain diagrams obtained at various loading rates are presented and discussed with the aid of continuous damage mechanics.

Rate and environmental effects on fracture of a two-phase TiAl-alloy

The influence of strain rate and environment on the fracture behavior of a two-phase TiAl-alloy, Ti-47Al-2.6Nb-2(Cr + V), heat-treated to a nearly fully lamellar microstructure has been studied by performing conventional tensile, compression, and fracture toughness tests in air, argon, and vacuum at 25 °C and 800 °C. Both tensile and compression tests were conducted at strain rates of 1 × 10−3 and 1 × 10−5 s−1, and fracture toughness tests were performed under displacement rates of 0.25 to 2.5 mm/min. In addition,in situ fracture toughness tests were conducted at slow rates both in vacuum and in air. The results indicated that both strain rate and environment affected the tensile stress-strain behavior and ductility and the fracture resistance of the TiAl-alloy at 800 °C. In contrast, neither the tensile ductility nor the fracture toughness was significantly affected by the environment at ambient temperature. For compression in air, the stress-strain behavior was insensitive to both strain rate and test temperature within the conditions tested. Studies of fracture surfaces revealed that low tensile ductility in this alloy at ambient temperature is associated with the tendency to delaminate alongγ/γ andγ/α 2 interfaces.

Mechanical properties of complexed α,ω‐diamino polybutadiene

Abstractα,ω‐Dipiperazino polybutadiene (PBD) was complexed with the hydrated forms of copper chloride, iron chloride, and nickel chloride in methanol. The polymers were recovered and compression molded into films and properties investigated. In particular, the tensile stress‐strain behavior as well as stress relaxation behavior were monitored with time at ambient conditions. It was observed that the polybutadiene material showed a strong association with copper providing a pseudo network behavior. The association for the iron and nickel components were considerably weaker than that of copper. Comparisons of the data obtained from these systems were also made with earlier data obtained from the same laboratory on carboxylated or sulfonated telechelic ionomers based on polyisoprene and polyisobutylene. It was concluded that the copper complex with the α,ω‐dipiperazino polybutadiene has comparable or better behavior than some of the carboxylate systems. © 1992 John Wiley & Sons, Inc.

The effect of temperature on the Deformation and Fracture of SiC/Ti-24Al-11Nb

The tensile stress-strain behavior and failure mechanisms of Ti-24Al-11Nb and a SiC/ Ti-24Al-11Nb composite with continuous SCS-6 fibers oriented parallel to the loading direction have been examined over a range of temperatures from 23 °C to 815°C in air. Failure in Ti- 24Al-11Nb occurred at strains of approximately 4 pct soon after crack initiation at low tem- peratures. Ductility increased with temperature up to a maximum of 20 pct elongation at 600 °C, as surface-initiated cracks did not propagate readily at intermediate temperatures. At higher temperatures, the onset of grain boundary and interfacial void nucleation limited ductility. Com- posite failure appeared to be controlled by fiber fracture at all temperatures; for practical en- gineering purposes, composite failure occurred at 0.8 pct strain at all temperatures. At temperatures of 425 °C and less, fiber fractures occurred at intervals along the lengths of the fibers and appeared to be cumulative, while at temperatures of 650 °C and greater, fiber fractures were only observed locally to the fracture surfaces. The decreased radial residual stresses, interfacial strengths, and matrix properties at 650 °C and 815 °C allowed the composite to unload at 0.8 pct strain, due to fiber fractures, followed by a reloading in which fibers pulled out and the matrix failed, resulting in composite failure. The decreasing residual stresses with increasing temper- ature determined from an elastic-plastic concentric cylinder model were shown to affect the stress-strain response of the composite and were consistent with the measured decreasing inter- facial shear stresses, the increased fiber pullout with temperature, and the circumferential de- bonding observed around the fibers at higher temperatures.

Characterisation of atomized nickel-copper powders : L.Salgado et al. (Cidade University, Spain.) Materials Science and Engineering A, Vol A133, 1991, 692–697.

This work attempts to develop superelastic TiNi wires with spatially varied shape memory characteristics along their length. A short section of the wire was over-aged by electrical resistance heating, which resulted in the deterioration of its superelasticity, whilst the reminder of the wire length remained superelastic. The tensile stress–strain behaviour of such treated wires is characteristic of two discrete stress plateaus during the stress-induced martensite transformation, corresponding to the original and over-aged sections.

Ultrasonic and magnetic analyses of porosity in iron compacts : Metallugical Transactions A, Vol. 20A, No. 4, pp. 571–578 (Apr. 1989)

This study showed that the tensile stress-strain behavior and ductility of porous iron compacts containing nearly spherical pores could be related solely to the initial poosity of the iron. Results are presented for these compacts on the use of ultrasonic wave propagation procedures for measuring porosity and on the effect on porosity on magnetic behavior, magnetization, and acoustic and magnetic Barkhausen signals. The successes, failures, and limitations of these nondestructive evaluation procedures for characterizing porosity and, therefore, the strength and ductility of porous iron are discussed. Wave velocity measurement techniques appear the most promising for characterizing porosity in porous iron compacts.