The eddy-current decay method developed by Bean for electrical resistivity measurements is well-suited for bulk metal characterization studies. The technique can be applied to investigations of low temperature plastic strain in pure aluminum.

Nuclear magnetic resonance pulse techniques are used in-situ during creep of single crystals of NaCl to evaluate the contribution of mobile dislocations to spin relaxation. 23Na spin-lattice relaxation rates were measured in the rotating frame (T1ρ) during compression creep of single crystals of NaCl along [110] direction at 473K at an applied stress of 20 MPa. The relaxation rates are evaluated from the spin-echo height following π/2, locking and 67° pulse sequence. The height of the free induction decay decreased as soon as the load is applied followed by a gradual increase until the steady-state is reached, at which point a saturation value is observed corresponding to the constant steady-state creep-rate. The mean jump distance of the mobile dislocations, evaluated from the ratio of the signal heights without deformation and during creep, decreased with time/strain reaching a constant value during steady-state creep regime. The results are compared with the dislocation-dislocation spacing, subgrain size as well as the jump distance predicted from creep models. The effects of dilvalent Ca and solid solution with LiCl are examined.

Synchrotron radiation and microradiographic techniques were used to study the development of creep damage in notched tensile samples. The creep damage in these samples was recorded using microradiography. The density and distribution of creep damage was measured from the microradiographs using an image analysis system. The results from the image analysis can be compared to damage predictions from finite element models of the damage process to determine the quality of these models.

Notched tensile samples of copper, iron and a low alloy steel were subjected to slow strain rate tensile tests at 500°C or 700°C. The tests were interrupted after various fractions of the creep lives had been expended. 1 mm thick longitudinal sections were then removed from the center of each sample for microradiography using electro-discharge machining.

Creep damage in the copper alloy was concentrated in a fairly narrow band around the plane of minimum cross-section in the samples. This is in stark contrast to the results from iron and the low alloy steel. The creep damage in these materials developed at fairly sharp angles to the notch or crack plane. These results show that the damage process in iron and this steel is controlled by the equivalent stress while the formation of damage in copper is controlled by the maximum principal or hydrostatic stress.

A complete understanding of the time-temperature dependent, viscoelastic mechanical properties of polymeric materials is essential for their successful application to engineering designs. We describe a computer-based system for acquiring, reducing and storing the mechanical properties of polymers, from which the useful forms of the mechanical characteristics may easily be reproduced and compared. We provide several examples illustrating the variety of ways in which these properties are utilized to forecast the performance of polymeric components of engineering designs under extreme field conditions.

This paper reviews recent advances at LaRC in quantitative measurement science applied to characterizing materials in a nondestructive environment. Recent demands on NDE have resulted in new thrusts to achieve measurements that represent material properties rather than indications or anomalies in a background measurement. Good physical models must be developed of the geometry, material properties, and the interaction of the probing energy with the material to interpret the results quantitatively.

In this paper are presented NDE models that were used to develop measurement technologies for characterizing the curing of a polymer system for composite materials. The procedure uses the changes in ultrasonic properties of the material to determine the glass transition temperature, the degree of cure, and the cure rate. A practical application of this technology is a closed feedback system for controlling autoclave processing of composite materials.

An additional example is in the area of thermal NDE. Thermal diffusion models combined with controlled thermal input/measurement have been used to determine the thermal diffusivity of materials. These measurements are remote, require no contact with the material under test and thus have interesting promise for NDE applications.

The techniques used to evaluate HV cable aging during service are reviewed. It is concluded that none gives a satisfying evaluation of cable condition.

Recent developments in the area of microelectronics now enable the fabrication of microdielectric sensors that can follow the drying, curing, and diffusion phenomena in thin films. This paper first investigates the response of the rricrodielectric sensor to cure in a thin epoxy film. The multi-frequency loss factor data are reduced to a single “ion-viscosity” response curve representative of the change in ion mobility during the cure reaction. The dynanic dielectric data are compared to Tg as a function of time as determined by differential scanning calorimetry (DSC) on several partially cured samples. The dielectric response is shown to be extremely sensitive to the entire cure process. A similar epoxy film is exposed to alternately wet and dry environments while the dielectric response is monitored. The diffusion coefficient of water in the cured resin is estimated through the dynamic change in permittivity using a simple Fickian model and the results are shown to be in good agreement with the literature values. Finally, changes in dielectric properties of a photoresist during UV exposure are presented.

The formation of well characterized poly-dimethylsiloxane networks has been studied by means of an acoustic interferometer. The hydrosilation reaction used to form these networks was followed by infra red spectroscopy. The relative changes in velocity of the ultrasonic longitudinal waves propagating through the system are found to be very sensitive to gelation and to the density of crosslinks. Due to our simultaneous kinetic and ultrasonic studies it was possible to relate the changes in acoustic properties of the curing system directly to the cure state.

Polymers absorb large quantities of gas under high pressure. This process is usually accompanied by a significant volumetric strain and also by changes in the mechanical properties of the sample. We will describe how these properties may be studied by using an ultrasonic technique. The sorption and volumetric changes are progressive in nature following the application of an incremental increase or decrease in ambient pneumatic stress. These techniques are also used to follow the time dependent changes in both the linear strain and the mechanical properties of the system. Data will be cited for a range of nitrogen gas pressures up to 27 MPa at 20°C for a virgin Poly-dimethylsiloxane (PDMS) elastomer and for its composite with glass filler to exemplify the experimental and analytical procedures. The final section of the paper shows how the same techniques may be used to monitor the inception and progressive development of internal cracks during certain ambient gas decompression profiles.

The information obtained provides a basis for better material selection and for the specification of the service life of particular materials in hostile environments.

An ultrasonic technique is described for the simultaneous measurement of specific volume, V, sound velocity, v, and attenuation, a, at frequencies between f = 0.5 and f = 15 MHz, in a wide range of temperature (- 150 to + 400°C) and pressure (up to 2 kbars). The results (V,v,a) are translated into a complex modulus, M* = M’ + iM” and analyzed in terms of the thermodynamic state of the material. Typical results for amorphous and semi-crystalline polymers are presented which show that the technique is a probe of the fundamental features of these materials (glass transition, crystallization, melting, molecular structure) which determine processability and end use properties. The method should prove of great interest for quality and process control.

The local elastic properties of a semi-crystalline polymer (isotactic polypropylene) have been investigated with the use of scanning acoustic microscopy (SAM) techniques. The operating frequency was 775 MHz and the temperatures ranged between 25 and 60°C. A new and effective signal processing method was developed to process the material signature V(z) curve. This method avoids complicated calibration procedures involved with operating the SAM lens at different temperatures. Longitudinal velocities VL(α) and VL(β) in individual α and β-type spherulites of isothermally crystalized samples have been measured. Acoustic velocity of a quenched sample was also obtained. It was found that vL(α) and VL(β) were nearly equal but the acoustic impedances of α and β-spherulites were unexpectedly very different.

This paper deals with the use of pulsed ultrasonic waves to monitor microstructural changes and degradation of ceramics in the field of high temperature. Two types of devices are described. One is a low frequency system for the measurement of Young's modulus at temperatures up to 1800°C. The second is an ultrasonic spectroscopy system for the evaluation of damage in structural ceramics during thermal fatigue experiments. Applications concern phase changes, porosity evolution or microcracking induced by thermal stresses when heating monolithic or composite ceramics.

Using theoretical models, we consider the sound velocities and elastic constants of ceramics containing pores. As an example, we consider alumina. However, the approach applies to all ceramics. As a point of departure, we consider spherical pores. For all the usual elastic constants–Young modulus, shear modulus, bulk modulus, Poisson ratio–we give relationships for both the forward and inverse cases: predicting the porous ceramic properties and estimating the pore-free ceramic properties. Following a suggestion by Hasselman and Fulrath that sintering or hot pressing can produce cylindrical pores, we derive a relationship for the elastic constants of a distribution of randomly oriented long cylinders (c/a + ∞, the prolate-spheroid limit). This model predicts elastic constants lower than for spherical pores, but well above measurement. We obtain agreement with observation by assuming the pores are oblate spheroids. For alumina, the necessary aspect ratio equals oneninth. Besides pore aspect ratio, the model requires only the pore-free alumina elastic constants. It contains no adjustable parameters.

We present the results of a study of glass-bonded Al2O3 with controlled composition and particle size using photoacoustic thermal wave nondestructive evaluation. Experimental results are related to particle size, density, porosity, and thermal properties of the materials.

The propagation of a thermal wave into a thin film or coating depends on the thermal properties of the material. Consequently, thermal wave generation and detection can be used to obtain the heat conductivity of the material. The method is also useful because thermal wave propagation is sensitive to inhomogeneity, porosity, inclusions, voids, and delaminations. The results of two specific applications of the thermal wave technique are presented, the heat resistance of oxide coatings and of diamond films.

The sol/gel process has recently received attention because it enables the production of pure and homogeneous glass without external heating, eliminating the high processing temperatures needed in conventional methods. A disadvantage of the process is that if the rate of evaporation during syneresis is too high, cracking due to mechanical shrinkage can be extensive. A slower evaporation rate produces less cracking, but results in a longer processing time. The key to decreasing processing time while simultaneously eliminating shrinkage cracks lies in monitoring the change of the mechanical properties during the evaporation process. Ultrasonic techniques are ideal for this purpose.

To monitor the process, an ultrasonic signal was passed through the material. Spectrum changes were measured and then converted to changes in modulus and viscosity, while shrinkage and density changes were monitored using an air-type transducer and an electronic balance. The results show no observable change up to the beginning of the gelation process, followed by rapid increase in the modulus and viscosity after the onset of shrinkage. This finding was confirmed using a viscometer and laser light scattering measurements. Because of the effectiveness of this ultrasonic technique in detecting changes in the material, such as the onset of cracking, it may be used in controlling and improving the process.

An ultrasonic method was developed for real-time monitoring of cake thickness during the casting of an alumina slip. The technique uses pulse-echo ultrasound to measure the time of flight and hence the thickness of the cast layer. Variations on the method are applicable to both production process control and to the fundamental studies of slip casting process kinetics.

Neutron diffraction is a powerful probe of the properties of condensed matter. In recent years neutron diffraction has been applied to the non-destructive evaluation of mechanical characteristics of engineering components. This paper presents examples of applications to ceramic composites including the measurement of position dependence of residual strain, grain size and minority phase concentration. In addition, an example of volume-averaged crystallographic texture is presented.

Results are presented from several studies where microfocus radiography has been applied to provide feedback necessary to improve reliability and performance of materials and components. Improving the reliability of advanced silicon nitride ceramics through the use of NDE was studied by exploring the relationship between process-related defects, radiography results, and fracture of test samples. Research in the areas of structural and electronic ceramic joining has been aided by monitoring the effects of process modifications with real-time microfocus radiography and computer- based image processing.

Microfocus projection radiography was used to nondestructively examine a large quantity of silicon nitride modulus of rupture test bars for internal defects. Failure stress prediction was attempted using a fracture mechanics model and quantitative NDE data, and compared to actual failure stress.

In-process NDE of silicon nitride ceramic to Incoloy 909 metal brazed test samples was performed. Correlation between NDE results, optical microscopy, and destructive mechanical strength levels of samples is discussed.

Microfocus x-ray imaging was used to monitor the process of bonding microwave power transistors to metallized BeO ceramic substrates. Nonuniform distribution of the eutectic bond was readily detectable, thereby providing feedback for process improvement. Comparison is made between x-ray, optical, and infrared images.

A rapid non-destructive test method for ceramic multilayer capacitors made from piezoelectric materials such as barium titanate or lead containing materials is described and evaluated. The test method is based on the internal excitation of standing acoustic waves in the capacitors. The standing waves are severely dampened by defects such as delaminations and pores. An undampened resonance is a good indication of a defect free ceramic multilayer capacitor. This finding was used in a nondestructive test set-up to evaluate about 1,000 capacitors. The test set-up has the potential to test over 100,000 capacitors an hour. Through metallographic (cross section) examination a sorting accuracy of 2% false accepts and 8% false rejects was determined for the high speed test set-up.