Transverse strength of metal matrix composites reinforced with strongly bonded continuous fibers in regular arrangements

The composite limit flow stress for transverse loading of metal matrix composites reinforced with a regular array of uniform continuous fibers is calculated using the finite element method. The effects of volume fraction and matrix work hardening are investigated for fibers of circular cross section distributed in both sqyare and hexagonal arrangements. The hexagonal arrangement is seen to behave isotropically with respect to the limit stress, whereas the square arrangement of fibers results in a composite which is much stronger when loaded in the direction of nearest neighbors and weak when loaded at 45° to this direction. The interference of fibers with flow planes is seen to play an important role in the strengthening mechanism. The influence of matrix hardening as a strengthening mechanism in these composites increases with volume fraction due to increasing fiber interaction. The results for a power law hardening matrix are also applicable to the steady state creep for these composites. The influence of volume fraction on failure parameters in these composites is addressed. Large increases in the maximum values of hydrostatic tension, equivalent plastic stain, and tensile stress normal to the fiber-matrix interface are seen to accompany large increases in composite strength.     

Micromechanical modeling of unidirectional continuous sigma fiber-reinforced Ti-6Al-4V subjected to transverse tensile loading

A micromodeling analysis of unidirectionally reinforced Ti-6-4/SM1140+ composites subjected to transverse tensile loading has been performed using the finite-element method (FEM). The composite is assumed to the infinite and regular, with either hexagonal or rectangular arrays of fibers in an elastic-plastic matrix. Unit cells of these arrays are applied in this modeling analysis. Factors affecting transverse properties of the composites, such as thermal residual stresses caused by cooling from the composite processing temperature, fiber-matrix interface conditions, fiber volume fraction, fiber spacing, fiber packing, and test temperature are discussed. Predictions of stress-strain curves are compared with experimental results. A hexagonal fiber-packing model with a weak fiber-matrix interfacial strength predicts the transverse tensile behavior of the composite Ti-6-4/SM1140+ most accurately.     

Application of the cruciform specimen geometry to obtain transverse interface-property data in a high-fiber-volume-fraction SiC/Titanium alloy composite

A combined experimental and computational methodology was used to determine the relevant strength and residual-stress parameters in a manufactured, high-fiber-volume-fraction multiply metal matrix composite (MMC). The method was similar to that previously demonstrated on single-fiber composites, which had an extremely low fiber volume fraction. Variabilities in residual stresses and debond strengths in high-fiber-volume-fraction multiply composites, as well as current demands on the micromechanics-based computational prediction and validation of complex composite systems, necessitated the establishment of the test methodology described here. The model material chosen for this investigation was a plasma-processed six-ply, unidirectional Sigma-1240/Ti-6Al-2Sn-4Zr-2Mo (wt pct) MMC containing 32 vol pct continuous fibers. Room-temperature transverse tensile experiments were conducted on cruciform specimens. In addition, rectangular specimens were also evaluated in order to verify their applicability in obtaining valid interfacial property data. Debonding events, evaluated at different positions within a given specimen geometry, were captured by stress-strain curves and metallographic examination. Analytical and finite-element stress analyses were conducted to estimate the geometrical stress-concentration factors associated with the cruciform geometry. Residual stresses were estimated using etching and computational procedures. For the cruciform specimens, the experimental fiber-matrix debond strength was determined to be 22 MPa. Separation occurred within the carbon-rich interfacial layer, consistent with some previous observations on similar systems. Thus, the cruciform test methodology described here can be successfully used for transverse interfacial-property evaluation of high-fiber-volume-fraction composites. For the rectangular specimens, the strain gages at different positions along the specimen width confirmed that the interface crack had initiated from the free edge and propagated inward. Hence, rectangular specimens cannot be used for valid interface strength measurements in multiply composites.     

Finite-element method simulation of effects of microstructure, stress state, and interface strength on flow localization and constraint development in Nb/Cr2Nb in situ composites

The effects of volume fraction of particles, stress state, and interface strength on the yield strength, flow localization, plastic constraint, and damage development in Nb/Cr₂Nb in situ composites were investigated by the finite-element method (FEM). The microstructure of the in situ composite was represented in terms of a unit rectangular or square cell containing Cr₂Nb particles embedded within a solid-solution-alloy matrix. The hard particles were considered to be elastic and isotropic, while the matrix was elastic-plastic, obeying the Ramberg-Osgood constitutive relation. The FEM model was utilized to compute the composite strength, local hydrostatic stress, and plastic strain distributions as functions of volume fraction of particles, stress state, and interface strength. The results were used to elucidate the influence of volume fracture of particles, stress state, and interface property on the development of plastic constraint and damage in Nb/Cr₂Nb composites.     

Influences of matrix ductility,interfacial bonding strength,and fiber volume fraction on tensile strength of unidirectional metal matrix composite

A trial to predict the influences of ductility of matrix, interfacial bonding strength, and volume fraction of fiber on the tensile strength of unidirectional metal matrix composites was attempted by means of a Monte Carlo computer simulation method. The main results are summarized as follows. (1) The strength of strongly bonded composites increased with increasing ductility of matrix and then remained nearly constant. (2) When the matrix was ductile, the strength of composite increased with increasing interfacial bonding strength and then remained nearly constant. When the matrix was not ductile, the strength increased but then decreased with interfacial bonding strength. In this case, there was an optimum bonding strength, for which the strength of composite was highest. (3) Concerning the strength of composite as a function of volume fraction of fiber, there arose the case where it is approximately described by the rule of mixtures and also the case where it is not described by this rule, depending on the ductility of matrix, interfacial bonding strength, and scatter of strength of fiber.     

不锈钢/碳钢耐蚀海水带肋钢筋轧制复合工艺

摘要：海洋工程用带肋钢筋要求有耐氯离子腐蚀能力，但选用双相不锈钢生产成本过高，不锈钢 碳钢轧制复合钢筋则可兼顾耐蚀性和低成本。覆层采用2205不锈钢，基材为低合金钢20MnSi，用有限元方法模拟钢筋的热轧复合过程，分析轧制过程尤其是成品孔中轧件的变形规律。有限元仿真发现，矩形组合坯料无孔型轧制时，其角部复合困难，而成品孔轧制时，钢筋横肋根部的应变最大，覆层在此位置减薄显著，应选择合适的复合坯覆层厚度。在实验室采用焊接、真空处理和热轧方法制备了直径为16mm的复合钢筋，屈服强度为485MPa，抗拉强度为701MPa，断后伸长率约为37.1%，复合界面剪切强度为317.5MPa。复合钢筋呈良好的冶金结合，Fe和Cr的扩散层厚度约为40μm。该工艺生产的复合带肋钢筋成本较不锈钢降低50%以上。     

Thermal expansion of morphologically textured short-fiber composites

We studied, both experimentally and theoretically, the thermoelastic response of short-fiber composites with a preferred orientation of the short fibers, i.e., a morphological texture. Our theoretical efforts are general, being applicable to any composite system with constituents that exhibit linear thermoelastic response. We propose a relatively simple micromechanics model to predict the thermal expansion coefficient (CTE), and give simple, easily used, results for orientation distributions of practical significance. We also present a convenient approach to represent the effects of texture on thermal expansion of short fiber composites, namely, a texture map. Our experimental efforts focus on a series of extruded SiC/Al short-fiber composites that we have fully characterized. This includes measurement of the complete set of elastic constants, the volume fraction, and the fiber orientation distribution function (ODF) using neutron diffraction. We measured the axial and transverse components of the overall CTEs of these composites using a quartz-rod dilatometer. Predictions are in good agreement with measurements.     

Effects of thermal residual stresses and fiber packing on deformation of metal-matrix composites

The combined effects of thermal residual stresses anmd fiber spatial distribution on the deformation of a 6061 aluminum alloy containing a fixed concentration unidirectional boron fibers have been analyzed using detailed finite element models. The geometrical structure includes perfectly periodic, uniformly spaced fiber arrangements in square and hexagonal cells, as well as different cells in which either 30 or 60 fibers are randomly placed in the ductile matrix. The model involves an elastic-plastic matrix, elastic fibers, and mechanically bonded interfaces. The results indicate that both fiber packing and thermal residual stresses can have a significant effect on the stress-strain characteristics of the composite. The thermal residual stresses cause pronounced matrix yielding which also influences the apparent overall stiffness of the composite during the initial stages of subsequent far-field loading along the axial and transverse direction. Furthermore, the thermal residual stresses apparently elevate the flow stress of the composite during transverse tension. Such effects can be traced back to the level of constraint imposed on the matrix by local fiber spacing. The implications of the present results to the processing of the composites are also briefly addressed.     

Theoretical Analysis on Tensile Strength of ZA22/Al2O3 Short Fiber Composite Added with Ce

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Strengthening effects in AC8A/Al2O3 short-fiber composites as a function of temperature and strain rate

The strengthening aspect of AC8A/Al₂O₃ short-fiber composites is examined under the framework of a modified shear lag model over the range of 298 to 723 K and 10⁻³ to 10³ s⁻¹. The strength sustained by the composite at high temperatures is much higher than for the alloy. As the strain rate rises, the portion of strength that the composite or alloy can sustain is drastically increased. Also, the composite shows a lower strain rate sensitivity, likely to be caused by the higher tendency of fiber damage and local stress concentration. As the temperature is higher, the strain rate sensitivity becomes considerably higher. The composite strength can be theoretically calculated using the Friend and Modified Tsai-Hill formulas. By closer examination, the experimental data agree better with the prediction of the Modified Tsai-Hill 2D (min) or 2D (max) model. Nevertheless, all of the predictions give quite reasonable strength values as well as the trend as a function of temperature and strain rate. Overall, test temperature governs the strengthening efficiency. High temperatures give the best efficiency. Influence from strain rate exists, but is less significant. It is observed that the strengthening effect is more pronounced when the matrix strength is lower, such as at higher temperatures and lower strain rates. Calculations from the critical fiber volume fraction V _crit and load transfer coefficient α both show an increasing trend with increasing temperature and decreasing strain rate, also suggesting that the strengthening effect by adding short fibers into the matrix is more apparent and efficient at high temperatures and low strain rates.     

The multiple yield phenomenon in single-fiber composites of iron in copper

Repeated yielding has been observed in single fiber composites of iron in copper. During each yield event, a Luders band forms in the iron, propagates a short distance, and stops. The number of separate yield events decreases with increasing volume fraction of fiber until a critical volume fraction, above which only one yield event per specimen occurs. A model based on interfacial shear and a consequent reduction in Luders band propagation stress has been developed to explain this behavior. The results of the proposed model agree well with the observations. Finally, it is shown that the theories of the volume fraction dependence of composite strength after yield point drops and after fiber failure are quite similar in nature.     

Prediction of creep-rupture life of unidirectional titanium matrix composites subjected to transverse loading

Titanium matrix composites (TMCs) incorporating unidirectional fiber reinforcement are considered as enabling materials technology for advanced engines which require high specific strength and elevated temperature capability. The resistance of unidirectional TMCs to deformation under longitudinally applied sustained loading at elevated temperatures has been well documented. Many investigators have shown that the primary weakness of the unidirectional TMC is its susceptibility to failure under very low transverse loads, especially under sustained loading. Hence, a reliable model is required to predict the creep-rupture life of TMCs subjected to different transverse stress levels over a wide range of temperatures. In this article, we propose a model to predict the creep-rupture life of unidirectional TMC subjected to transverse loading based on the creep-rupture life of unidirectional TMC subjected to transverse loading based on the creep-rupture behavior of the corresponding fiberless matrix. The model assumes that during transverse loading, the effective load-carrying matrix ligament along a row of fibers controls the creep-rupture strength and the fibers do not contribute to the creep resistance of the composite. The proposed model was verified using data obtained from different TMC fabricated using three matrix compositions, which exhibited distinctly different types of creep behavior. The results show that the creep-rupture life of the transverse TMC decreases linearly with increasing ratio of the fiber diameter to the ply thickness. The creeprupture life is also predicted to be independent of fiber spacing along the length of the specimen.     

Degradation of residual strength in SCS-6/Ti-15-3 Due to fully reversed fatigue

Little attention has been given to residual strength degradation in titanium matrix composites (TMCs) after exposure to fatigue loading. To address this problem, fatigue tests on SCS-6/Ti-15-3 were performed to investigate the fatigue life and residual strength behavior of TMCs with different fiber volume fractions. Results indicate that fiber volume fraction seems to have an effect on both of these quantities. Lower fiber percentages result in a material where the characteristics of the matrix, such as hardening or cracking, play a much larger role in the composite response. Fatigue lives were not affected by fiber volume fraction at higher strain ranges, but lower fiber volume fractions resulted in shorter fatigue lives at lower strain values. Also, a slight increase in residual strength occurred up to 75 pct of fatigue life, for the lower-fiber volume fraction material. Despite these distinctions between specimens with different fiber contents, all specimens tested retained the majority of their strength prior to failure. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.     

Theoretical prediction of tensile strength of fibers as a function of thickness of brittle zones on fiber surfaces

A theoretical model of longitudinal strength of fibers as a function of the thickness of brittle zones on fiber surfaces was presented. Unifying parameters, corresponding to a formation of a notch due to a premature failure of the brittle zone, to an extension of the notch and to an interfacial debonding between the fiber and the brittle zone, were derived by modifying Weibull theory, fracture mechanics and shear lag analysis of Dow, respectively. A framework for interpreting the interrelation between the magnitudes of these unifying parameters and the failure mode of the fiber/brittle zone composites was carried out. It was shown that, if the thickness of the brittle zone is thicker than a critical value and if the interfacial bonding is stronger than a critical value, the fiber strength is reduced. Some examples of combinations of these unifying parameters given as a function of the thickness of the brittle zone were presented and the failure mode of the fiber/brittle zone composite was described schematically also as a function of the thickness of the zone.     

Fibre uniformity and cavitation during the consolidation of metal-matrix composite via fibre-mat and matrix-foil diffusion bonding

Fibre uniformity is important for ensuring overall mechanical properties of a composite. The conditions for achieving uniform fibre distribution in solid-state consolidated composites are quantitatively analysed. It is shown that the uniformity is influenced by initial fibre spacing, fibre packing and foil thickness before consolidation and matrix flow during consolidation. A graphical technique is presented to determine optimum pre-consolidation arrangement of fibres and foils for a given volume fraction. Residual cavities in partially bonded composites are observed in both hexagonally and rectangularly packed fibre arrays. Foil bending is mainly the cause for the cavities found in the former case, whereas in the rectangular array an ear defect is observed after an intermediate stage of bonding and is believed to be responsible for the voids which form during matrix flow in this case. The initiation of the ear defect is determined by matrix volume incompressibility and quantitatively analysed with respect to different conditions of the flow constraint imposed during consolidation. These predictions are compared favourably with experimental results.     

Experiments on fracture behavior of single fiber-brittle zone model composites

Single fiber-brittle zone model composites were prepared by the electroless plating method, in order to know the effect of the interfacial bonding strength between the fiber and brittle zone on the fracture strength of the composites. For the case of weak inter-facial bonding, the notch formed by the fracture of the brittle zone at an early stage of deformation was unable to extend into the fiber due to a premature interfacial debonding. Therefore no deleterious effect of the brittle zone on the fiber strength was found. For the case of strong interfacial bonding, the formed notch extended into the fiber, resulting in loss of the fiber strength. The strength of such a composite was well explained by the theory recently proposed by Ochiai and Murakami. The important parameters to describe the fracture behavior of the fiber coated with the brittle zone, the strain energy release rate of the fiber and the permissible thickness of the brittle zone, below which the strength of the fiber is not reduced, were experimentally determined.     

Tensile properties of short fiber-reinforced SiC/Al composites: Part II. Finite-element analysis

The tensile properties of aluminum matrix composites containing SiC whiskers or particulate were investigated analytically and compared to experimental results. Two finite-element models were constructed and used for elastoplastic analysis. In both models, the SiC fibers are represented as longitudinally aligned cylinders in a three-dimensional array. The cylinder ends are transversely aligned in one model and staggered in the other. Using the models, the sensitivity of the predicted composite properties to the deformation characteristics of the matrix alloy was examined, and the general behavior of the models was validated. It was determined that both models are necessary to predict the overall composite stress-strain response accurately. The analytic results accurately predict: the observed composite stress-strain behavior; the experimentally observed increase in Young’s modulus and the work-hardening rate with increasing fiber volume content and aspect ratio; and the decrease and subsequent increase in proportional limit as the SiC volume fraction is increased. The models also predict that the transverse material properties should be insensitive to fiber aspect ratio. In addition, the model predicts the location of initial yielding and the propagation of the plastic region. These results offer insights into the deformation mechanisms of short fiber-reinforced composites.     

Influence of the fiber arrangement on the mechanical and thermo-mechanical behavior of short fiber reinforced MMCs

A unit cell approach is used to investigate the influence of the fiber arrangement (periodic and perturbed periodic) on the thermo-mechanical behavior of aligned short fiber reinforced MMCs. Two configurations are compared, staggered and unstaggered fibers, with the first one describing a more realistic composite. Whereas the global behavior of the composite is not strongly influenced by the arrangement, the local microstresses were found to differ significantly in the investigated arrangements. The failure behavior is considered at the microlevel via a damage indicator.     

Thermal expansion of boron fiber-aluminum composites

The linear thermal expansion of silicon carbide coated boron (BORSIC^®) aluminum composites was measured as a function of volume fraction fiber and angle with respect to the fiber axis. The measurements were made with a standard quartz tube-type dilatometer at a heating rate of 150°C per hr. Measurements were made between 25° and 300°C on 2024 aluminum alloy-BORSIC and 1100 aluminum alloy-BORSIC composites in the 0 and 90 deg fiber orientation as a function of volume fraction fiber. The axial test results are compared with several models found in the literature which predict composite thermal expansion. These predictions yield values which are higher than the measured expansion coefficients of the 0 deg composites. The discrepancy is assumed to be related to yielding in the matrix. The 90 deg composites are found to agree with the transverse thermal expansion coefficient relationship of Schapery (also Levin) which employs the Poisson ratio for each phase and the composite. The expansion coefficients of 2024 aluminum alloy-BORSIC composites containing 54 pct by volume fiber are given for fiber orientations of 0, 15, 30, 45, 60, 75, and 90 deg.     

Principles of the design of highly porous layered composites working in the bending mode

A comparative analysis is made of the spesific mechanical characteristics of a sandwich-type laminated porous composite under bending. The external layers of the composite are compact while the internal layer consists of a highly porous material made by using a pore-forming agent. The specific stiffness, strength, and yield load as a function of the volume fraction of pores Θ and the porous/compact-layer thickness ratio λ are considered. The stiffness, strength, and yield load are shown to be affected by variations of Θ and λ when the weight of the composite is constant. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(412), pp. 70–78, March–April, 2000.