Prashant Karandikar

Characterization and modeling of microcracking and elastic moduli changes in nicalon/cas composites

Abstract The damage progression in silicon carbide fiber (Nicalon) reinforced calcium alumino-silicate (CAS) glass-ceramic composites subjected to monotonic uniaxial tensile loading has been studied. A unidirectional composite and three cross-ply laminates were investigated. Microcrack densities were recorded at regular intervals of applied stress. Youngs modulus and the major Poissons ratio were also measured at the corresponding stress levels. Both longitudinal and transverse stress/strain relations show significant nonlinearity with the onset and development of damage. Matrix microcracking in a direction perpendicular to the applied load reaches a critial level, and the Youngs modulus and Poissons ratio start decreasing. On further loading, cracking increases and reaches a saturation level. A shear lag model was used to predict the variation of Youngs modulus with applied stress based upon the experimentally measured crack densities. Model predictions suggest that at the saturation of matrix cracking, load transfer from the fibre to the matrix occurs only through frictional stresses. In the cross-ply laminates the two major modes of damage are transverse cracking in the 90° plies and matrix cracking in the 0° plies. The initiation strain and saturation density of transverse cracks increase as the 90° layer thickness decreases. The initiation strain and the saturation density of matrix cracks in the 0° plies of the cross-ply laminates are comparable to those of matrix cracks in the unidirectional composite. A shear lag model was adapted to calculate the reduced Youngs modulus of the cracked 90° plies. The shear lag model for the unidirectional composite was used to calculate the reduced Youngs modulus of the 0° plies. The reduced moduli of both 0° and 90° plies were input to the rule of mixtures for predicting the Youngs moduli of the cross-ply laminates. Here again, model predictions suggest thay at the saturation of matrix cracking, load transfer from the fiber to the matrix occurs only through frictional stresses.

Characterization of aluminium-matrix composites made by compocasting and its variations

Compocasting (semisolid-semisolid, SS) and its two variations: SL (semisolid-liquid) and LL (liquid-liquid) process routes are used to make 2024Al reinforced with 3 mm and 12 mm long FP-alumina fibres. Squeeze-casting is used as a complementary casting technique. The effect of processing route on microstructure and the mechanical properties of these composites is studied. The SS route produces composites with uniform fibre distribution, but casting is difficult due to the high viscosity of the slurry. The SL route gives good fibre distribution and the casting is easy. The LL route allows addition of a large amount of fibres but gives castings with a non-uniform fibre distribution, which lowers the failure strains and reduces the strength of the composites drastically. The addition of alumina fibres to 2024Al increases its modulus of elasticity considerably. The observed modulus values show good agreement with the theoretical predictions. The strength values are somewhat lower than the theoretical predictions. This is because the composites failed at strains slightly lower than the fibre failure strain. Absence of fibre pull-out indicates that a good fibre matrix bond has been produced in each case.

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized.

Microstructural Characteristics of Reaction-Bonded B 4 C/SiC Composite

A detailed microstructural investigation was performed to understand structural characteristics of a reaction-bonded B4C/SiC ceramic composite. The state-of-the-art focused ion beam & scanning electron microscopy (FIB/SEM) and transmission electron microscopy (TEM) revealed that the as-fabricated product consisted of core-rim structures with -SiC and B4C cores surrounded by - SiC and B4C, respectively. In addition, plate-like -SiC was detected within the B4C rim. A phase formation mechanism was proposed and the analytical elucidation is anticipated to shed light on potential fabrication optimization and the property improvement of ceramic composites.

Fracture Mechanism in Randomly Oriented Discontinuous Carbon Fiber Reinforced Borosilicate Glass Matrix Composites.

Fracture mechanisms in randomly oriented discontinuous carbon fiber reinforced borosilicate glass matrix composites are experimentally characterized. Two types of composite specimens with identical constituents but different microstructure were produced. The first type shows the three-stage behavior in the tensile stress-strain curve. In the second type which has the initial cracks, the stress-strain response is almost linear, but the initial Youngs modulus is lower than that of the first type. Monotonic and static cyclic tensile and four point bend tests are conducted on those two specimens to measure the microcrack density, Youngs modulus and Poissons ratio as a function of applied strain. The experimentally determined crack density is used to estimate the interfacial shear stress. In the bend tests, evolution of microcracking on the tensile surface and through the thickness has been studied. Acoustic emission (AE) during the tests is monitored to understand the fracture mechanisms. It is proved that the difference in fracture mode between the two specimens is due to the difference in microstructure and the presence of initial defects. The AE behavior shows good correlation with the evolution of microcracking and can explain the fracture processes. Youngs modulus can be a damage parameter to estimate the microcrack density. Microcracking during the flexural tests begins on the tensile surface and the cracks progress gradually towards the neutral axis.