Anil Saigal

Residual strains in titanium matrix high-temperature composites

Abstract Residual thermal strains and stresses that developed during cooling of a silicon-carbide-fiber-reinforced titanium matrix high-temperature composite were determined by an experimental neutron diffraction technique. The results were compared with those obtained by finite element analysis. The study was conducted over the temperature range 20–950 °C. As a result of thermal expansion mismatch, compressive residual strains and stresses were generated in the fibers during cool-down. The axial residual strains and stresses were highly tensile in the matrix, and the matrix underwent plastic deformation. Average transverse residual strains in the matrix were compressive. The measured data compare fairly well with finite element method predictions. The effects of fabrication procedures and thermal processing, such as liquid nitrogen dipping and thermal cycling, on the residual strains were also studied.

Analysis of stresses in aluminum–silicon alloys

Abstract Two-phase aluminum–silicon-based alloys are widely used for premium quality castings for aerospace and automotive applications. While it is clear that silicon improves fluidity in the molten state, providing excellent castability to the alloy, and increases the tensile strength of the alloy, much needs to be done to improve the understanding of the structure–property relationships in castings. This paper deals with the application of a microstructural finite element method and the OOF program to study the effect of size and shape of silicon particles on the stresses in the silicon particles and the aluminum matrix. The highest stress in the matrix increases with increasing particle size for a given volume fraction of silicon particles. Therefore, the yield strength of a microstructure containing coarse particles would be lower than one containing fine particles. Once the silicon particles with large aspect ratios crack or the microstructure containing large silicon particles yield, the effective stiffness of the aluminum matrix decreases which significantly increases the average stress in the silicon particles and the highest stresses in both the silicon particles and the aluminum matrix. This indicates that once the matrix yields, the potential for particle cracking increases dramatically.

Electrical response during indentation of a 1-3 piezoelectric ceramic-polymer composite

The electrical response during indentation of a piezoelectric ceramic-polymer 1-3 composite has been investigated. The current (quasistatic charge increment) induced in the indentor due to the polarized layer on the contact surface increases with load as the contact area increases. Good agreement was found between the measured currents as a function of load with those predicted using an analytical model. In addition, the current increases with increasing indentation velocity and indentor diameter. It uses known analytical results to develop a new tool for characterizing the electrical response of piezoelectric composites. As such, linear elastic indentation with simultaneous measurement of load and electric current is shown to be a new, fast, and nondestructive technique that can be used for quality assurance and to study the effect of aging and development/presence of damage/microcracking in monolithic piezoelectric and 1-3 piezoelectric composites.

Effect of interface properties on microcracking of iron titanate

The orthorhombic pseudobrookites have served as model material systems for investigating the role of thermal expansion anisotropy on the microcracking behavior in single phase ceramics. Among those typically studied are MgTi_2O_5, Fe_2TiO_5 and Al_2TiO_5. Fe_2TiO_5 is anisotropic in both thermal expansion and paramagnetic susceptibility. Fe_2TiO_5 has an orthorhombic crystal structure and belongs to the Bbmm space group. It belongs to the class of titanates under investigation as second-phase toughness enhancers and as coatings for engine manifolds. The coefficient of thermal expansion of Fe_2TiO_5 is 10.1 × 10^(−6) K^(−1), 16.3 × 10^(−6) K^(−1) and 0.6 × 10^(−6) K^(−1) in the a, b and c directions, respectively. The goal of the present work is to examine residual stresses and study the effect of interface properties on fracture and propensity toward microcracking of Fe_2TiO_5 using finite element analysis.

Thermal residual stresses and mechanical behavior of cast SiC/Al composites

Abstract Thermal residual stresses developed during casting of SiC/aluminum composites were studied as a function of cooling rate and volume fraction of fibers. Thermo-elastoplastic finite element analysis was used in the investigation. The model accounts for the phase change of the matrix and the temperature-dependent material properties as the composite is cooled from the liquidus temperature to room temperature. Further, the effects of thermal residual stresses and fiber cross-sectional geometry on the mechanical behavior of the composites were also studied. Based on the study, it can be concluded that the matrix undergoes significant plastic deformation during cool-down and the residual stress distribution is a function of the cooling rate. In addition, the presence of thermally-induced residual stresses tends to decrease the aggregate modulus of elasticity and increase the yield strength of the composites. The fiber cross-sectional geometry affects the constitutive response of the composites in orientations transverse to the fiber axes.

Heat treatment optimization of alumina/aluminum metal matrix composites using the taguchi approach

The paper describes the use of the Taguchi approach for optimizing the heat treatment process of alumina-reinforced Al-6061 metal-matrix composites (MMCs). It is shown that the use of the Taguchi method makes it possible to test a great number of factors simultaneously and to provide a statistical data base that can be used for sensitivity and optimization studies. The results of plotting S/N values versus vol pct, solutionizing time, aging time, and aging temperature showed that the solutionizing time and the aging temperature significantly affect both the yield and the ultimate tensile strength of alumina/Al MMCs. 11 refs.

Taguchi analysis of heat treatment variables on the mechanical behavior of alumina/aluminum metal matrix composites

Abstract Alumina-reinforced aluminum 6061 alloy composites containing 10, 15 and 20 volume percentage particulates were tested using the standard tensile and Charpy impact toughness tests. A statistical approach, known as the Taguchi method, was implemented to identify heat treatment conditions for improved tensile and impact toughness properties. Modulus of elasticity, yield strength, and ultimate tensile strength all increased while impact toughness decreased as a function of the volume percentage of reinforcement for the alumina/aluminum composites in the T6 heat treated condition. Taguchi analysis results indicate that, on average, a 4% increase in yield strength and a 7% increase in ultimate tensile strength over the standard T6 heat treatment can be attained using a heat treatment of 6 h of solutionizing at 529°C and 12–18 h of aging at 160°C. Similarly, using a heat treatment of 2 h of solutionizing at 529°C and 12 h of aging at 127°C results in an impact toughness improvement of more than 87% over the standard T6 heat treatment.

High-temperature ultrasonic characterization of Ag-clad superconductor tapes

An ultrasonic nondestructive evaluation technique was developed to monitor liquid-phase evolution during heat treatment of high-T/sub c/ superconductors. The liquid phase is essential for microstructural and phase development of superconductors and is important in fabricating conductors with high critical current density, J/sub c/. Tapes were fabricated by a powder-in-tube process and then placed in a controlled-atmosphere furnace. During heat treatment, a magnetostrictive transducer launches a 140-kHz wave into the sample, and the relative change in acoustic velocity is then measured. A significant decrease in velocity was seen at the incongruent melting temperatures of Bi-based superconductors. In addition, Ag-clad tapes with Al/sub 2/O/sub 3/ and NaCl cores were used to validate the measurement technique. Advantages of this approach over other thermal analysis methods include in-situ analysis of final tape form, monitoring of isothermal liquid evolution, control of volatile species such as Tl and Pb.<<ETX>>

Thermal residual strains and stresses in silicon carbide-fiber-reinforced silicon nitride composites

Abstract Neutron diffraction was used to measure thermal residual strains in SiC-fiberreinforced reaction-bonded silicon nitride composites as a function of volume fraction of fibers. The measured values are in agreement with those estimated by finite-element modeling. Due to the anisotropy of the thermal expansion coefficient of SiC, residual axial strain (stress) is tensile and residual transverse strain (stress) is compressive in the fibers. The variation in the measured residual axial strain in the matrix can be related to the density of the composite. In addition, neutron diffraction was used to study the effects of fiber coatings and processing on the thermal residual strains.

Predictive force model for haptic feedback in bone sawing

Bone sawing simulators with force feedback represent a cost effective means of training orthopedic surgeons in various surgical procedures, such as total knee arthroplasty. To develop a machine with accurate haptic feedback, giving a sensation of both cutting force and rate of material removal, algorithms are required to forecast bone sawing forces based on user input. Presently, studies on forces generated while machining bone are not representative of the high cutting speeds and low depths of cut common to the bone sawing process. The objective of this research was to quantify sawing forces in cortical bone as a function of blade speed and depth of cut. A fixture was developed to simulate linear bone sawing over a range of speeds comparable to surgical reciprocating and oscillating (sagittal) bone saws. A single saw blade tooth was isolated and used to create a slotted cut in bovine cortical bone. Over a range in linear sawing speed from 1700 to 7000 mm/s, a t-test (α=0.05) revealed there was no statistically significant effect of blade speed on either cutting or thrust force. However, an increase in depth of cut from 2 to 10 μm resulted in a 30% increase in thrust force, while cutting force remained constant. The increase in thrust force with depth of cut was relatively linear, R(2)=0.80. Using a two factor, two level design of experiments approach, regression equations were developed to relate sawing forces to changes in blade speed and depth of cut. These equations can be used to predict forces in a haptic feedback model.