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.     

Three-dimensional modelling of thermal residual stresses and mechanical behavior of cast SiC/A1 particulate composites

Thermal residual stresses developed during casting of SiC/aluminum particulate-reinforced composites were investigated as a function of cooling rate and volume fraction of particles using thermo-elastoplastic finite element analysis. The phase change of the matrix during solidification and the temperature-dependent material properties as the composite is cooled from the liquidus temperature to room temperature were taken into account in the model. Further, the effect of thermal residual stresses on the mechanical behavior of the composites was also studied. Based on the study, it was found that the matrix undergoes significant plastic deformation during cool down and has higher residual stress distribution as the cooling rate increases. The model which does not include the solidification of the matrix tends to overestimate the residual stresses in the matrix and underestimate the tensile modulus of elasticity of the composites. In addition, the presence of thermally induced residual stresses tends to decrease the apparent modulus of elasticity and increase the yield strength of the composites compared to those without residual stresses.     

Deformation behavior of continuous-fiber metal-matrix composite materials

The deformation behavior of continuous-fiber, metal-matrix composites was studied in terms ofin situ deformation behavior of the matrix and the fibers. X-ray diffraction techniques were employed to monitor the stress-strain behavior of the composite components (matrix and fibers) as a function of total composite stress-strain behavior. This experimental technique provided a unique approach to the study of metal-matrix composites since the deformation response of the components could be measuredin situ while the composite was under load. Furthermore, the influence of residual stress, component mechanical properties, and stress interactions between the matrix and the fibers could be incorporated into the analysis of composite deformation behavior. The study was conducted on composites of 2024 aluminum reinforced with tungsten fibers, and composites of 2024 aluminum reinforced with boron fibers. Composites were tested on a specially-designed, tensile device which served as a diffractometer specimen holder such that diffraction experiments could be performed while the specimen was incrementally deformed in uniaxial tension. Experimental results indicated that, except for the residual stress effects, the composites exhibited rule-of-mixture s behavior in the stage I, II, and in deformation regions. Measurements obtained perpendicular to the fiber and tensile axis during the tensile tests indicated that negligible stresses were developed as a result of Poisson’s ratio differences between the matrix and the fibers. Composite yield behavior was significantly influenced by residual stresses present in the individual components. Residual stresses parallel to the fiber axis could be included in the rule-of-mixtures analysis by considering the amount of prestrain which was present in each component. Formerly, Graduate Student, Department of Metallurgical Engineering, Drexel University, Philadelphia, Pa. This paper is based on a thesis submitted by H. P. CHESKIS to the Graduate Faculty in Materials Engineering at Drexel University in partial fulfillment of the requirements of the degree of Doctor of Philosophy.     

On the toughening of intermetallics with ductile fibers: Role of interfaces

The role of fiber debonding and sliding on the toughness of intermetallic composites reinforced with ductile fibers is examined. The toughness is shown to be a function of the matrix/fiber interface properties, residual stresses and the volume fraction, size and flow behavior of the fibers. Mechanical testing and in situ microstructural observations were carried out on a Ti-25at.%Ta-50at.%Al intermetallic matrix reinforced with W-3Re fibers. The fibers were coated with a thin oxide layer in order to induce debonding and prevent interdiffusion between the fiber and the matrix. The ductility, high strength and debond characteristics of coated tungsten-rhenium fibers promote a large increase in toughness. However, the mismatch in thermal expansion coefficients is the source of large residual tensile stresses in the matrix that induces spontaneous matrix cracking. Matrix cracking and composite toughness are examined as a function of the interfacial properties, residual stresses and properties of the fiber.     

Geometric factors affecting the internal stress distribution and high temperature creep rate of discontinuous fiber reinforced metals

The creep deformation behavior of metal-matrix composites has been studied by a continuum mechanics treatment utilizing finite element techniques. The objective of the work has been to understand the underlying mechanisms of fiber reinforcement at high temperatures and to quantify the importance of reinforcement phase geometry on the overall deformation rate. Internal stress distributions are presented for a material that consists of stiff elastic fibers in an elastic, power law creeping matrix. Results indicate that large triaxial stresses develop in the matrix, and that these stresses have a strong effect on reducing the creep rate of the composite. Reinforcement phase geometry, as measured by the fiber volume fraction, aspect ratio, separation, and overlap, greatly influences the degree of constraint on the flowing matrix material and the overall deformation rate. Theoretical predictions from this modeling are compared to experimental results of creep deformation in metal-matrix composite systems with varying degrees of agreement.     

In-situ observations of titanium metal-matrix composites under transverse tensile loading

The transverse stress-strain behavior of several titanium metal-matrix composites (TiMMCs) has been studied in-situ. Debonding of 1140+/Ti-6-4 composites occurs over a range of stresses. The sharpness of the first “knee” is affected by the fiber volume fraction and by the relative moduli of the matrix regions and the reinforced composite. It has been observed that debonding occurs mainly at the interface between two sublayers of carbon/carbon coatings in 1140+/Ti-6-4 composites and mainly at the interface between the carbon/reaction zone in the as-processed and peak-aged 35 pct SCS-6/Tiβ21s composites. At surface positions, this process starts at very low stresses (≥50 MPa) from the positions with sharp changes of curvatures (or undulations), voids, or debris at the periphery of the interface. Cracking of the outermost carbon sublayer and of the reaction zone in the 1140+/Ti-6-4 composites and the reaction zone in the SCS-6/Tiβ21s composites occurs during elastic deformation of the matrix. This has been directly observed in a field-emission gun (FEG)-scanning electron microscope (SEM) under incremental loading. Although these cracks are arrested and blunted by the matrix material, they cause local stresses and, thus, stimulate local plastic deformation of the matrix and subsequent development of a second knee on the stress-strain curve. The in-situ observations are discussed in terms of the effects of fiber volume fraction and fiber type on the loci and dynamic processes of interfacial debonding, cracking of carbon coatings and reaction zones, and plastic deformation of the matrix.     

An analytical study of residual stress effects on uniaxial deformation of whisker reinforced metal-matrix composites

Finite element modeling was utilized to simulate the stress-strain response of discontinuous SiC whisker reinforced aluminium-matrix composites, accounting for the thermal residual stresses (TRS) generated during solution treatment. The contributions of various micro-mechanisms to overall composite strengthening and deformation, and the impact of TRS on each mechanism were explicitly evaluated. It was inferred that constrained matrix plastic flow and matrix-to-fiber load transfer are the predominant sources of strengthening, with enhanced matrix dislocation density playing a secondary role. Residual stresses were found to significantly affect each of the operative strengthening mechanisms and hence the composite properties. Comparison with experiments revealed that the trends predicted by the model are generally consistent with actual composite behavior, although the model overpredicts work hardening rate. A parametric study of the effects of whisker volume fraction, aspect ratio and spacing on tensile and compressive deformation was also conducted. The results showed that increasing volume fraction, close end-to-end spacing and large aspect ratios result in greater strength and stiffness.     

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.     

Thermomechanical behavior of TiNi shape memory alloy fiber reinforced 6061 aluminum matrix composite

The processing and thermomechanical behaviors of TiNi shape memory alloy (SMA) fiber-reinforced 6061 Al matrix smart composites are investigated experimentally and analytically. Optimum processing conditions of hot pressing temperature and pressure are identified. Composite yield stresses are observed to increase with an increase in the volume fraction of TiNi fiber and prestrain given to the composites. An analytical model for thermomechanical behavior of the composites is developed by utilizing an exponential type of SMA constitutive model. The model predicts an increase in the composite yield stress with an increase in prestrain. It is found that the key parameters affecting the composite yield stress are the fiber volume fraction, prestrain, and matrix heat treatment. The predictions are in a reasonably good agreement with the experimental results.     

Thermomechanical behavior of TiNi shape memory alloy fiber reinforced 6061 aluminum matrix composite

The processing and thermomechanical behaviors of TiNi shape memory alloy (SMA) fiber-reinforced 6061 Al matrix smart composites are investigated experimentally and analytically. Optimum processing conditions of hot pressing temperature and pressure are identified. Composite yield stresses are observed to increase with an increase in the volume fraction of TiNi fiber and prestrain given to the composites. An analytical model for thermomechanical behavior of the composites is developed by utilizing an exponential type of SMA constitutive model. The model predicts an increase in the composite yield stress with an increase in prestrain. It is found that the key parameters affecting the composite yield stress are the fiber volume fraction, prestrain, and matrix heat treatment. The predictions are in a reasonably good agreement with the experimental results.     

The influence of residual stress on the yielding of metal matrix composites

A systematic numerical study of the effect of residual stresses on the yielding behavior of composites comprised of elastic particles well bonded to a ductile matrix is carried out. The calculations are made within the framework of continuum plasticity theory using cell models. An investigation is made into the roles volume fraction, particle shape, and hardening play in this interaction. A slight transient softening of the composite in both tension and compression is found, but the limit stress of the composite is unaffected by the residual stress. Thus the limit stress-strain response is symmetric in tension and compression for strains greater than a few times the matrix yield strain. A qualitative connection is made between the transient reduction in stiffness and the extent to which there was prior plastic deformation in the matrix due to residual stresses.     

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.     

Determination of residual stresses in thin sheet titanium aluminide composites

Residual stresses in Ti₃Al/SiC composites have been measured using two methods. The compressive residual stresses in the fibers were inferred from measurements of the change in their length when the matrix was entirely removed by etching. The stresses were found to vary substantially from fiber to fiber. The longitudinal and transverse stresses in the matrix were measured by X-ray diffraction. Repeated measurements were made as the outer layer of alloy was removed by electropolishing as far as the first row of fibers. In one composite of lower fiber volume fraction, the matrix stresses were thus found to be approximately uniform throughout the specimen. In a higher volume fraction material, on the other hand, the matrix stresses increased significantly with depth from the outer surface: the longitudinal matrix stresses among the fibers were found to be about 60 pct larger than they were on the specimen surface. The implications of these measurements for processing and reliability of thin sheet titanium aluminide composites are discussed.     

Optimization of the range of elastic behavior of unidirectional composites by prestraining

The effect of tensile prestrain on the stage I tensile yield stress has been studied both analytically and experimentally for composites whose stress-strain curves obey the rule of mixtures. The mathematical analysis provides a means for calculating the optimum amount of prestrain, the residual stresses (in the direction of the fibers) in the matrix and fiber materials after unloading from the prestraining, and the stage I yield stress in the composite after the prestrain treatment. It is shown that the improvement in stage I yield stress by prestraining is due to the development of negative residual stresses in the matrix. The stage I yield stress in composites with negligible residual stresses in the as-fabricated condition can usually be improved by a factor of two by prestraining; the amount of improvement is even greater if the as-fabricated composites have the usual state of residual stress,i.e., tension in the matrix. Experimental studies on 2024 aluminum-tungsten composites (filament-wound; hot-pressed) having tungsten fiber volume fractions between 0.08 and 0.40 verified the mathematical analysis. The stage I yield stresses measured in these composites after a prestrain of 4.2 × 10⁻³ were in good agreement with predicted values. Improvements of up to a factor of six were found in the stage I yield stress as a result of prestraining.     

苎麻纤维增强聚乳酸复合材料性能研究

通过密炼?注塑成型工艺制备了不同苎麻纤维含量的聚乳酸基复合材料,研究了纤维含量对复合材料性能的影响规律,并揭示了纤维增强机理。研究表明,苎麻纤维的添加提高了复合材料的耐热性能,尤其是当纤维质量分数为40%时,复合材料的热变形温度提高了10.5%。此外,苎麻纤维均匀地分散在基体中,由于纤维与聚乳酸的界面强度较弱,断面上有大量的纤维拔出和纤维孔洞;差示扫描量热仪测试表明高含量的纤维限制了聚乳酸分子链的运动,促进复合材料形成更加致密完善的晶核;同时,流变行为也表明苎麻纤维含量的增加有助于提高复合材料的黏弹响应和复合黏度;最后,苎麻纤维的加入提高了复合材料的拉伸和弯曲强度,且随纤维含量的增加而增大。与聚乳酸相比,当纤维质量分数为40%时复合材料的拉伸和弯曲强度分别提高了30%和21.9%。      

An experimental and numerical study of deformation in metal-ceramic composites

The deformation characteristics of ceramic whisker- and particulate-reinforced metal-matrix composites were studied experimentally and numerically with the objective of investigating the dependence of tensile properties on the matrix microstructure and on the size, shape, and distribution of the reinforcement phase. The model systems chosen for comparison with the numerical simulations included SiC whisker-reinforced 2124 aluminum alloys with well-characterized microstructures and 1100-o aluminum reinforced with different amounts of SiC particulates. The overall constitutive response of the composite and the evolution of stress and strain field quantities in the matrix of the composite were computed using finite element models within the context of axisymmetric and plane strain unit cell formulations. The results indicated that the development of significant triaxial stresses within the composite matrix, due to the constraint imposed by the reinforcements, provides an important contribution to strengthening. Systematic calculations of the alterations in matrix field quantities in response to controlled changes in reinforcement distribution give valuable insights into the effects of particle clustering on the tensile properties. The numerical results also deliver a mechanistic rationale for experimentally observed trends on: (i) the effects of reinforcement morphology and volume fraction on yield and strain hardening behavior of the composite, (ii) the pronounced influence of reinforcement clustering on the overall constitutive response, (iii) ductile failure by void growth within the composite matrix, (iv) the insensitivity of the yield strength of the composite to changes in matrix microstructure, and (v) the dependence of ductility on the microstructure of the matrix and on the morphology and distribution of the reinforcement. The predictions of the present analyses are compared and contrasted with current theories of elastic and plastic response in multi-phase materials in an attempt to develop an overall perspective on the mechanisms of composite strengthening and of matrix and interfacial failure.     

Matrix strengthening mechanisms of an iron fiber-copper matrix composite as a function of fiber size and spacing

Copper matrix-iron fiber composites of fiber diameters from 10 to 5 × 10⁻³ mils and volume fractions from 0.03 to 0.97 were fabricated in order to study the dependence of mechanical properties on these variables. Composite elastic moduli agreed well with the predictions of the rule of mixtures. However, matrix and composite yielding and plastic flow were quite dependent on fiber diameter and spacing, exhibiting positive deviations from the simple rule of mixtures by factors of more than five in some cases. Yielding behavior may be explained by a combination of dislocation extrusion and pileup models for low volume fractions of fiber. Triaxiality generated by the difference in Poisson coefficients of the phases inhibits matrix yielding in higher volume fraction composites, allowing matrix flow only when the fibers also yield. Formerly with the Department of Metallurgy and Materials Science, M.I.T., Cambridge, Mass. Formerly with the Department of Metallurgy and Materials Science, M.I.T.     

Tertiary compression creep of long-fiber composites: A model for fiber kinking and buckling

The uniaxial compression-creep behavior of unidirectionally reinforced continuous-fiber composite materials was investigated for the case where both the matrix and the fiber underwent plastic deformation by creep. The creep behavior of NiAl composites reinforced with 5 to 20 vol pct tungsten fibers was characterized at 1025 °C. The NiAl-W composites exhibited a three-stage creep behavior, with distinct primary, secondary, and tertiary creep. Microstructurally, tertiary creep was characterized by one of the following fiber-deformation mechanisms: brooming, bulging, buckling, or kinking. The composite tertiary creep is modeled by solving for global or local kink-band evolution, with composite deformation contributing, respectively, to fiber buckling or kinking. The model predicts (1) the critical strain for the onset of the tertiary stage to be most sensitive to the initial kink angles, while being relatively insensitive to the initial kink-band heights and (2) the critical strain to vary inversely with the volume fraction of fiber in the composite. Reasonable agreement between model predictions and experiments is obtained.     

The tensile deformation and fracture of Al-“Saffil” metal-matrix composites

In this paper we have studied the tensile deformation and fracture of aluminium alloy composites containing “Saffil” δ-Al₂O₃ fibres. The Bauschinger effect has been used to measure internal stress and shows that the tensile behaviour of these materials is determined by the development of these stresses due to the plastic flow of the matrix, differences in elastic constants of the two phases and residual thermal stresses developed during fabrication. Good agreement is obtained with the theoretical predictions derived in an earlier paper. Fracture is observed to occur by the growth of cracks from failed fibres until a crack large enough to nucleate catastrophic failure is formed.     

An experimental and theoretical investigation of the rapid consolidation of continuously reinforced,metal-matrix Composites

The feasibility of the rapid consolidation of T-14Al-21Nb/SCS-6 foil/fiber/foil composites using a forging approach was established as an alternative to slower and more expensive processes such as those based on hot isostatic pressing (HIP) or vacuum hot pressing (VHP). A firm basis for the technique was developed through theoretical analyses of temperature transients, forging pressures, and fiber fracture. These analyses demonstrated that there exists an optimal forging speed at which the consolidation stresses are a minimum. It was also shown that the flow stress of the encapsulation material relative to that of the densifying layup is an important consideration in achieving full consolidation during forging. Specifically, the difference in flow stress between the two materials influences the magnitude and sign of the in-plane (secondary) stresses that are developed during forging and therefore the rate of pore closure during the latter stages of the process. With regard to fiber fracture, analyses were performed to estimate the axial and tangential stresses during rapid consolidation. The theoretical work was validated by experimental trials using the Ti-14Al-21Nb matrix/silicon carbide fiber system. Measured forging pressures were in good agreement with predictions. Fiber fracture observations indicated that tangential tensile stresses developed in the fiber control failure; a forging window to avoid such failures was thus developed. Finally, it was demonstrated that matrix microstructures and mechanical properties similar to those of conventionally consolidated Ti-14Al-21Nb/silicon carbide composites can be achieved by the forge-consolidation technique.