A. Wolfenden

Impulse excitation technique for dynamic flexural measurements at moderate temperature

The impulse excitation technique (IET), which is presently a precise and reliable technique for measuring dynamic moduli at room temperature, has been adapted to measure dynamic flexural modulus at temperatures in the range of 25° to 300 °C. This modified technique involves a sensitive microphone and electronics to record and analyze the sound waves emitted from a specimen vibrating in the fundamental flexural mode. The fundamental resonant frequency and geometry of the specimen are used to obtain the modulus. The location of the microphone relative to the specimen is critical and is a major factor once the specimen is placed within the heated environment. Problems were identified and solved, and test data for aluminum are presented to support the modification of the IET for use at elevated temperatures.

Mechanical damping and dynamic modulus measurements in alumina and tungsten fibre-reinforced aluminium composites

Simultaneous measurements of mechanical damping, or internal friction (Q−1 ), and dynamic Youngs modulus (E) were made near 80 kHz and at strain amplitudes (ε) in the range 10−8 to 10−4 on small specimens of continuous or chopped fibre-reinforced metal matrix composites (MMCs): 6061 aluminium reinforced with alumina (Al/Al2O3) and 6061 aluminium reinforced with tungsten (Al/W). Baseline experiments were also done on 99.999% aluminium (pure Al). The strain amplitude dependence of damping and the temperature dependence of dynamic modulus were of particular interest in this study. The temperature (T) dependence of the modulus from room temperature up to 475° C was determined for the Al/Al2O3 and pure Al specimens and a highly linear decrease in modulus with increasing temperature was observed. The rate of modulus loss (dE/dT ≈ −80 M Pa° C−1 ) was the same for both materials and the reduction in modulus of the Al/Al2O3 was attributed to the reduction in modulus of the alu minium matrix, not the alumina fibres. The size, type, and amount of fibre reinforcement were found to have a significant effect on the strain amplitude dependence of the damping in both MMCs. Unreinforced aluminium exhibited classical dislocation damping trends with a region of strain amplitude independent damping at low strains (less than 10−5) followed by a non linear, strain amplitude dependent region at higher strains. The addition of alumina fibres (chopped or continuous), while increasing stiffness, resulted in a significant reduction in damping capacity for the MMC relative to that for aluminium and near complete suppression of the amplitude dependent response. The damping levels increased as the volume fraction of fibre, and therefore, the amount of fibre/matrix (FM) interface decreased, indicating that the matrix, not factors such as increased dislocation densities at the FM interface, was the dominant influence on the damping. Analysis of the Al/Al2O3 results by Granato-Lücke (GL) theory indicated that dislocation densities were increased relative to those in aluminium, but the dis locations were well pinned and unable to increase damping levels effectively. Analysis of the Al/W results by GL theory also revealed high dislocation densities, but, unlike the Al/Al2O3 specimens, the Al/W specimens (continuous fibres) exhibited strong amplitude dependent damping (starting near strain levels of 2 × 10−6) with damping levels approximately twice those of pure aluminium. Trends showed increased damping with increased fibre diameter, not with increased FM interface area. There was some evidence that it was the tungsten fibre itself that dominated the damping behaviour in Al/W composites, not the aluminium matrix or the FM interface.

Measurement and analysis of elastic and anelastic properties of alumina and silicon carbide

Measurements of dynamic Youngs modulus, E, and damping as a function of temperature, T, were made for alumina and silicon carbide. The Youngs modulus data were compared with some from the literature, and analysed in terms of a theoretical framework relating the Debye temperature, θD, with the elastic constants. For both materials this analysis yielded a ratio T0/θD which was near 0.4, where T0 is an empirical fitting constant for the plot of (E(0)−E)/T versus 1/T (E(0) is the value of E at 0 K). The analysis of the damping data in terms of an Arrhenius type dependence led to effective activation energies near kT, where k is Boltzmanns constant.

Temperature dependence of dynamic Young's modulus and internal friction in LPPS NiCrAlY

The piezoelectric ultrasonic composite oscillator technique (PUCOT), operating near 80 kHz, was used to measure the temperature dependence, in the range 23–1000 °C, of dynamic Youngs modulus,E, and internal friction,Q−1 in three compositions of low-pressure plasma-sprayed NiCrAlY: Ni-15.6Cr-5.2Al-0.20Y (16-5), Ni-17.2Cr-11.6Al-0.98Y (17−12), and Ni-33Cr-6.2 Al-0.95 Y (33−6). Ambient temperature (23 °C) dynamic Youngs moduli for the three alloys were 205.0, 199.8, and 231.0 GPa, respectively. In each case, dE/dT was found to be — 0.06 GPa °C−1 over temperature ranges 23–800, 23–400 and 600–900, and 23–700 °C, respectively. Internal friction was essentially independent of temperature to about 600 °C (700 °C for the 16−5 alloy), at which point a temperature dependence of the formQ−1 =A exp (C/RT) was observed. The constantA for the three alloys was determined to be 62.7, 555, and 2.01 × 106, respectively. The constantC for the three alloys was determined to be 82.8, 111, and 170 kJ/mol−1, respectively. While the physical mechanism is not fully understood, both the pre-exponential constantA and the activation energyC correlate with durability in thermal barrier coatings (TBCs) wherein these alloys are used as bond coats.

Elastic constants of silver as a function of temperature

The temperature dependence of the elastic constants of silver single crystals has been determined over the range 300–1173 K with the piezoelectric ultrasonic composite oscillator technique (PUCOT). From a comparison of the present results with those available from the literature, it is deduced that the PUCOT and hence other standing-wave techniques are adequate for measuring compliances, but these techniques may have complications for computations of stiffnesses.

A rheometer to measure the viscoelastic properties of polymer melts at ultrasonic frequencies

A rheometer which uses a piezoelectric ultrasonic composite oscillator has been developed to measure the viscoelastic properties, G’(ω) and G‘(ω), of polymer melts at ultrasonic frequencies. The storage and loss moduli of an ethylene–tetrafluoroethylene copolymer melt were measured at six frequencies in the range 0.25×106–1.28×106 rad/s (between 40 and 200 kHz nominal frequency) and at a temperature of 280 °C. This ultrasonic instrument operates at strain amplitudes ranging from about 10−8 to 10−3 and is therefore ideally suited for linear viscoelastic property measurements.

The relation of dynamic elastic moduli, mechanical damping and mass density to the microstructure of some glass-matrix composites

Dynamic elastic moduli and mechanical damping were measured with the PUCOT (piezoelectric ultrasonic composite oscillator) technique at room temperature for ceramic-matrix composites (CMCs) of the following compositions: PRD-166 (fibres)/N51A glass (matrix), PRD-166 fibres coated with SnO2/glass, Nextel 480 fibres/glass, Nextel 480 fibres coated with SnO2/glass, and Nextel 480 fibres coated with BN/glass. The fibres were continuous, and the volume fractions varied from 0.24 to 0.43. Some of the mechanical-property measurements correlated with the thickness of one of the coating materials, and with microstructural observations of the misorientation angle of the fibres and normalized fibre length. With increasing volume fractions of fibres, the fraction of broken fibres increased. For the PRD-166/glass and PRD-166/SnO2/glass, a substantial fraction of the fibres were misoriented by angles of up to 15 °. Assessments were made of the measured properties in terms of the rule of mixtures and other theoretical estimations.

A single quartz crystal to measure dynamic elastic moduli at several ultrasonic frequencies

An instrument for measuring the elastic moduli at several ultrasonic frequencies, using a single quartz crystal, has been developed. In the past an array of pairs of oscillators was used as part of the piezoelectric ultrasonic composite oscillator technique to measure the dynamic Young’s modulus and shear modulus at several ultrasonic frequencies. Experimentation was done using the conventional composite and the new single crystal systems at 40, 80, and 160 kHz on brass and aluminum samples, both in the longitudinal and torsional modes. The good agreement between these systems is indicative of the validity and applicability of the new design.

Measurement of elastic and anelastic properties of reaction-formed silicon carbide-based materials

The dynamic Youngs modulus and the strain amplitude dependence of damping, at room temperature as well as at elevated temperatures, were determined for reaction-formed SiC (RFSC) ceramics, and the results are compared with those for other SiC materials. The method used was the piezoelectric ultrasonic composite oscillator technique (PUCOT). Five specimens were studied: NC 203 (a commercially produced SiC by Norton, Co.); RFSC No. 1 and RFSC No. 2 (each containing residual Si); RFSC No. 3 and RFSC No. 4 (both containing residual Si and MoSi2). Metallographie observations showed that the microstructure of the RFSC is essentially isotropic with a uniform distribution of phases. The “rule of mixtures” calculations cannot be used to predict accurately the elastic modulus of the RFSC, but they can be used to predict the density to within 5%. It was determined that for the RFSC, the dynamic Youngs modulus decreases as temperature increases, in a manner similar to that for other SiC materials. It was also found that the damping of the RFSC is generally independent of strain amplitude and is weakly affected by temperature. The activation energy was determined for the change in damping with change in temperature of RFSC No. 2 and RFSC No. 3.

Internal friction study of AISI 410 stainless steel

From this study of the internal friction at low stresses of AISI 410 stainless steel in four different conditions of heat treatment the following conclusions are made: 1. 1. Youngs modulus of the material is influenced by heat treatment and varies in the range 211 to 218 GPa. 2. 2. The internal friction is amplitude dependent and varies in the range 2 to 8 × 10−4 for the different heat treatments and range of strain amplitudes investigated. 3. 3. The break away strain amplitude of the internal friction depends on the heat treatment of the specimen and varies in the range 10−6 to 3 × 10−5. Moreover, the trend of the break away strain amplitude with heat treatment is the inverse of that for the trend of Youngs modulus. 4. 4. When the internal friction data for 10−5e−1 < 1 are analyzed in terms of the Granato-Lucke (GL) theory of dislocation damping, values of the minor loop length Lc of the vibrating dislocation line and of the mobile dislocation density Λ could be extracted for the specimens. 5. 5. The values of Lc (13 to 46 nm) and of Λ (1010 to 1012 m−2) obtained from the analysis are acceptable when interpreted in terms of TEM observations on carbide precipitates and dislocations in turbine blade materials similar to the material used in this study. 6. 6. The mobile dislocation density under conditions of amplitude dependent internal friction is less than the estimated total dislocation density. 7. 7. The internal friction results suggest that the value of K (the variable in the GL theory which depends on crystallographic orientation and specimen material) for the specimens used should be near 0.5, and that the value of LN (the major dislocation loop length) should be near 10−7m.