The influence of matrix properties on randomly oriented glass fiber epoxy composites has been studied. It is shown that an increased ductility (flexibility) of the matrix does not result in greater elongation to failure of the composite under tensile and flexural loads. The tensile (and flexural) strength and the modulus of elasticity are decreased as the ductility of the resin is increased. It is concluded that since the matrix material is subjected to a triaxial state of stress when the composite specimen is subjected to uniaxial loads, the effect of matrix modulus, Poisson's ratio, and yield strength are more important than the matrix ductility measured under uniaxial stress. The effect on mechanical properties of various surface treatments applied to the fibers is also investigated. Finally, scanning electron micrographs are presented showing matrix cracks, fiber debonding, and fiber pull-out. 相似文献
Impact performance is a primary concern in many applications of polymers. In this paper, finite element analysis (FEA) and ABAQUS/Explicit are used to simulate the deformation and failure of polymers in the standard ASTM D3763 multiaxial impact test. The specimen geometry and loading mode in this multiaxial impact test provides a close correlation with practical impact conditions. A previously developed constitutive model (“DSGZ” model) for polymers under monotonic compressive loading is generalized and extended for any loading mode and takes into account the different behavior of polymers in uniaxial tensile and compression tests. The phenomenon of thermomechanical coupling during plastic deformation is also included in the analysis. This generalized DSGZ model, along with thermomechanical coupling and a failure criterion based on maximum plastic strain, is incorporated in the FEA model as a coupled‐field user material subroutine to produce a unique tool for the prediction of the impact behavior of polymeric materials. Load‐displacement curves from FEA simulations are compared with experimental data for two glassy polymers, ABS‐1 and ABS‐2. The simulations and experimental data are in excellent agreement up to the maximum impact load. It is shown that not accounting for the different behavior of the polymer in uniaxial tensile and compression tests and thermomechanical coupling effects leads to an overestimation of the load and impact energy, especially at large displacements and plastic deformations. Friction also plays an important role in the impact behavior. If one neglects the friction between the striker and polymer disk, the predicted impact loads are lower as compared with experimental data at large displacements. 相似文献
Critical stresses for the initiation of the tetragonal-to-monoclinic phase transformation in 9Ce-TZP zirconia materials with five different grain sizes have been studied. The influence of the grain size on the critical transformation stresses has been investigated in multiaxial stress states, namely, in four-point bending, biaxial bending and torsion. It was found that phase transformation occurs as a homogeneous phase transformation with a transformation strain increasing continuously with increasing applied stress and also as an autocatalytic phase transformation with the autocatalytic formation of transformation bands normal to the maximum principal stress. An investigation of the critical transformation stresses under different multiaxial loads in the tensile regime, i.e. with positive hydrostatic stress, showed that both the homogeneous and the autocatalytic transformation do not follow the shear-dilatant criterion investigated in multiaxial compressive testing. The experiments showed that under multiaxial loading the onset of both transformation types can be predicted with the maximum principal stress transformation criterion, with the difference between the critical stresses of both transformation mechanisms strongly decreasing with grain size. 相似文献
A quantitative investigation was conducted on the effect of micromechanical stress concentrations on the strength of two-phase brittle-matrix ceramic systems. The materials consisted of a continuous brittle matrix containing dispersions with elastic properties different from those of the matrix. A soda borosilicate glass was used as the matrix and the dispersions consisted of spherical alumina particles 60μ in diameter and spherical pores 60μ in diameter. Stress concentrations were varied by measuring the strength of the composite under uniaxial and biaxial tensile stress conditions. The experimental results showed that micromechanical stress concentrations strongly affect the macroscopic strength of the composite. Under biaxial tensile stress, additions of either alumina microspheres or spherical porosity to the glass matrix resulted in a decrease in strength equal to the maximum calculated stress concentration factor. Under uniaxial tensile stress conditions, however, the reduction in strength for the glass-alumina system was negligible. The glass-porosity system gave a reduction in uniaxial strength which was not equal to the maximum calculated stress concentration factor. Experimental results suggest that differences in strength of brittle multi-component systems under uniaxial and biaxial stress states can be attributed in part to micro-structural features. On the basis of the experimental work, a hypothesis is developed relating the relative size of the region in the glass matrix over which stress concentrations act to the size of the Griffith flaws responsible for failure. This hypothesis is extended to the effect of porosity on the strength of polycrystalline brittle ceramic materials. 相似文献
Fatigue is one of the most common yet complicated failures that can cause damage to mechanical structures. Structural adhesively bonded joints are not exempt from this deleterious phenomenon and have to be assessed under vibration loads. In this work, fatigue characteristics of single-lap joints (SLJ) made of steel and carbon fibre reinforced plastic (CFRP) laminates under vibration loads are primarily investigated by experiments. The aim of this work is to analyze the changes in the ultimate load of the SLJ under vibration loads. The experimental results showed that SLJ will face cohesive failure after the uniaxial tensile loading test. In addition to the increase of vibration cycles, the ultimate load and failure displacement gradually decrease. In order to model the adhesive between joint components and simulate the damage propagation, a new traction–separation law called the embedded process zone (EPZ) and a damage factor are introduced and developed within the framework of cohesive zone Modeling (CZM) techniques. Meanwhile, the stress variations in the adhesive layer of SLJ in different vibration cycles are researched using the finite element method in ABAQUS. 相似文献
Combined compression-torsion tests were performed on the thermal-treated and as-machined silicon nitride ceramics to investigate their fracture behavior under multiaxial stress states. The thermal-treated samples showed considerable high strength and low anisotropy to the grinding direction in flexure tests compared to the as-machined samples. Under combined compression and torsion stress states, the thermal-treated samples showed considerably higher tensile strength than that of as-machined samples at low compressive stress states and weakening with increasing compression stress. The as-machined samples showed little decrease in tensile strength with increasing compression stress and comparable tensile strength with the thermal-treated samples under a highly compressive stress state. The behavior of thermal-treated samples were well described by the statistical theory of multiaxial fracture for volume-distributed flaws combined with a mixed-mode fracture criterion with the shear sensitivity constant of 1.75 and 1.65 for Shetty’s criterion and the ellipsoidal criterion, respectively. 相似文献
Tetragonal ferroelectric/ferroelectric BaTiO3/PbTiO3\hbox{BaTiO}_3/\hbox{PbTiO}_3 superlattice under uniaxial tensile stress along the c axis is investigated from first principles. We show that the calculated ideal tensile strength is 6.85 GPa and that the superlattice
under the loading of uniaxial tensile stress becomes soft along the nonpolar axes. We also find that the appropriately applied
uniaxial tensile stress can significantly enhance the piezoelectricity for the superlattice, with piezoelectric coefficient
d33 increasing from the ground state value by a factor of about 8, reaching 678.42 pC/N. The underlying mechanism for the enhancement
of piezoelectricity is discussed. 相似文献
Creep behaviour of ordinary refractory ceramics is evidently asymmetric under uniaxial tension and compression. In service, they are often exposed to multiaxial stress states. In the present paper, the modified shear test specimens were applied for a creep study in the shear-compression zone of the p-q diagram, and the pure shear creep parameters following the Norton-Bailey strain hardening equation were inversely identified in combination with a weighting function between pure shear and uniaxial compressive conditions. The weighting function was implemented in an in-house asymmetric creep constitutive model. The experimental curves can be well predicted with identified parameters of the asymmetric creep constitutive model. It shows that the shear creep of ordinary refractory ceramics is evidently different to uniaxial compressive/tensile creep. Consideration of shear creep in the thermomechanical modelling of industrial vessels increases the accuracy of simulation results and supports the lining concept optimization investigation. 相似文献
The Tsai-Wu Quadratic Failure Criterion provides satisfactory strength predictions for fiber-reinforced composite materials but requires five experimental tests to determine the strength parameters. This paper presents two modifications of this criterion, which employ micromechanics to determine these parameters. Experiments on uniaxial and multiaxial vinyl ester/fiberglass composite laminates show that the first modified failure criterion, which is based solely on fiber and resin properties, predicts strength within an average absolute error of 25.4% in comparison to the experimentally determined strength. The addition of a single longitudinal tensile test to the modified expression (second modified failure criterion) reduces the average absolute error to 15.9%. This compares well with the Tsai-Wu Failure Criterion, which gives an average absolute error of 9.4%. The proposed modified criteria are shown to provide satisfactory failure predictions, while greatly reducing the amount of testing required. 相似文献
Triaxial tests were performed on 100 mm × 100 mm × 100 mm cubic specimens of plain high-strength and high-performace concrete (HSHPC) at all kinds of stress ratios after exposure to normal and high temperatures of 20, 200, 300, 400, 500, and 600 °C, using a large static-dynamic true triaxial machine. Friction-reducing pads, using three layers of plastic membrane with glycerine were placed between the compressive loading plate and the specimens; the tensile loading planes of concrete samples were processed by an attrition machine, and then the samples were glued-up with the loading plate with structural glue. The failure mode characteristic of the specimens and the direction of the crack were observed and described. The three principally static strengths in the corresponding stress state were measured. The influence of the temperatures and stress ratios on the triaxial strengths of HSHPC after exposure to high temperatures was also analyzed. The experimental results showed that the uniaxial compressive strength of plain HSHPC after exposure to high temperatures does not decrease completely with the increase in temperature, the ratios of the triaxial to its uniaxial compressive strength are dependent on the brittleness-stiffness of HSHPC after different temperatures and the stress ratios. On this basis, a new failure criterion with the temperature parameters is proposed for plain HSHPC under multiaxial stress states. It provides the experimental and theoretical foundations for strength analysis of HSHPC structures subject to complex loads after subjected to a high temperature environment. 相似文献
In this experimental study the deformation behavior of two thermoplastics under uniaxial and biaxial loadings has been investigated. The experiments were conducted on thin walled tubular specimens of nylon-6, a semicrystalline polymer, and polymethyimethacrylate (PMMA),:an amorphous polymer. These specimens were subjected to tension, torsion and combined tension-torsion loadings until very large deformations were produced or specimen failure took place. The results are presented in the form of generalized stress-generalized strain curves which are applicable to all types of loadings, provided the generalized stress is monotonically increasing. In addition, the results show that the generalized stress-generalized strain relationship for nylon-6 is parabolic in nature, whereas that for PMM A can be represented by a bilinear curve. This characterization of the response of the thermoplastics under a biaxial stress field can be used to obtain a more realistic stress interaction between fiber and matrix in composite materials, since until now all theoretical studies on this aspect have assumed an elastic or elastic-perfectly plastic behavior of the matrix. 相似文献
AbstractThe composition-property relationships of LDPE-PP binary blends have been investigated. Young's modulus, yield strength and flexural strength of the blends varied monotonically with composition, whereas the elongation at failure and the true ultimate tensile strength for the blends with 30-50 wt-% PP exhibited synergetic effects. These blends showed strong cold draw hardening, and their elongations at failure and the true ultimate tensile strengths were much higher than those of the neat components. Meanwhile, impact strength showed an abrupt reduction with increasing PP content and the failure mode changed from ductile to brittle in the composition range of 30-50 wt-% PP. Failure mechanisms are discussed, addressing interfacial adhesion between LDPE and PP and the non-uniform shrinkage of the component domains upon cooling. 相似文献
The present work investigated the mechanical behavior of porous La0.6Sr0.4Co0.2 Fe0.8O3−δ LSCF under uniaxial compression. The porous (LSCF) samples with the same grain size but different porous structures with 1.5–41% of porosity were prepared using three different pore formers. All the samples had ferroelastic domains and exhibited ferroelastic mechanical behaviors under uniaxial compression. Initial and loading moduli as well as critical stress monotonically decreased and remnant strain increased with increasing the porosity. The initial modulus can be determined by the actual porosity regardless of porous structure or grain size, whereas the other properties were more sensitive to experimental condition such as loading rate and maximum applied stress. Compressive fracture strength could be significantly influenced by porous structure. 相似文献
Isotactic poly(propylene) (iPP) is one of a few polymers that show homoepitaxy, i.e., the formed crystals act as nuclei for so‐called daughter crystals that are formed nearly perpendicular to the backbones of the primary crystallized mother crystals. Lightly cross‐linked isotactic poly(propylene) (x‐iPP) offers the opportunity to form nearly all mother crystals in stretching direction and simultaneously allow formation of daughter crystals. Since crystals in polymers act as reinforcement along chain direction, this unique behavior allows multiaxial reinforcement in iPP induced only by stretching in one direction. In this study the influence of applied uniaxial strain is explored on the resulting multiaxial crystal orientations and Young's moduli parallel, perpendicular as well as under an angle of 45° to the stretching direction of the sample. It is shown that the occurring multiaxial orientations strongly depend on applied strain during crystallization and cause significantly improved Young's moduli parallel as well as perpendicular to the prior stretching direction while that under an angle of 45° is slightly decreasing. The here described technique to obtain multiaxially oriented morphologies is not restricted to thin films but can be efficiently applied also to bulk samples.