Characterization of Refractory Materials Using Nondestructive Acoustic Testing
Refractory and carbon materials are heterogeneous materials containing pores, cracks and multiphase aggregates. Such materials are generally exposed in service to thermomechanical conditions such as thermal shock, mechanical impact, abrasion and erosion, which provoke microstructural changes that affect material properties and consequently their behavior during operation.
Nondestructive acoustic testing is commonly used to characterize the microstructure of materials such as fine ceramics and metals. However, it is usually difficult to apply this technique to heterogeneous materials due to multiple acoustic wave and microstructure interactions, which favor high acoustic wave attenuation and multiple acoustic wave propagation modes. Many different techniques have been used to characterize refractories, but they do not allow high-temperature measurement of elastic properties including elastic modulus (under both flexural and longitudinal conditions), shear modulus and the Poisson's ratio. MQC Technology's new family of SonicByte acquisition and software systems not only can measure elastic properties at both room and at high temperatures, but also it can estimate material strength and critical flaw size using specially designed algorithms. Moreover, the system allows testing large material units, because unlike most conventional commercial acoustic equipment, it is not limited to high frequency-range measurements.
Acoustic testing principle
Sonic, or acoustic, testing is based on time-varying deformation or vibration in materials. In solids, sound waves propagate under different modes, which are based on the way the particles oscillate. Sound can propagate as longitudinal, shear, surface and (in thin materials) plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in sonic testing.
When an elastic material is struck on the surface, it resonates at a given natural frequency, which is a function of its elastic properties; i.e., E (elastic or Young's modulus), G (shearing or Coulomb's modulus) and v (Poisson's ratio). Knowing a material's elastic properties is of prime importance because they not only reflect the extend of bonding in the material, but also allow characterizing the material's behavior under stress.
Sonicbyte nondestructive acoustic system
Elastic constants are determined using the SonicByte nondestructive acoustic system by measuring the material's acoustic resonance periods under flexural, torsional and longitudinal modes. Under the flexural mode (when applicable), measurements are made along two orthogonal directions with respect to the material's longitudinal axis. The two resonance periods thus obtained allow distinguishing the effect of uniformly and nonuniformly distributed microstrutural defects in the material on its elastic properties, as well as on its mechanical strength. This technique also allows considering separately the effect of geometrical discontinuities within the material.
Nondestructive analysis of laboratory samples
A SonicByte XB-1000 signal analyzer is the main device used for nondestructive analysis of laboratory samples (Fig. 1a). The XB-1000 uses high-frequency response microphones specially designed to capture fast audio impulses generated from hammer impacts (Fig. 1e). The device analyzes the resonance frequencies collected by the microphones using a technique similar to FFT (Fast Fourier Transform). The signal is electronically processed using an ultrafast analog-to-digital converter featuring very accurate timing. Mathematical calculations are performed using carefully built algorithms to quickly output (<100 ms) the prominent resonance period, which corresponds to the average value from up to 2,000 consecutive periods collected by the system during each impact.
SonicByte Microsoft Excel-based software uses the period readings and displays the results on the computer monitor and also stores repetitive results and maintains a data bank for subsequent analysis (Fig. 1d). The software allows viewing of the raw signal (Fig. 1b) and the period spectrum after each impact (Fig. 1c).
A serial RS232 computer port is used to communicate with the XB-300 Hammer Control Box (Fig. 1f), which provides computer-controlled, single and repetitive hammer impacts on the sample. Using a controlled hammer instead of a manual hammer greatly increases the probability of repetitive measurements because a repetitive reading is, among other factors, sensitive to the impact location. The system can be used manually, but it is more convenient to use a maximum of four specially designed electric hammers to repetitively strike the sample at exactly the same spot every time at four distinct locations.
Hammers and microphones are located around a sample on a mounting table for room temperature testing (Fig. 1e), and a specially designed furnace and set-up is used for high-temperature testing (Fig. 1g and h). In the latter case, ceramic waveguides are used to collect the audio signals from the sample following impacts from specially designed electric hammers coupled with ceramic wire. Figure 2 shows the relative position of the hammers and microphones for both room and high-temperature testing on parallelepiped-shaped samples. Software calculates and reports elastic modulus under longitudinal and flexural (in two orthogonal directions) modes, shear modulus and Poisson's ratio. As mentioned previously, the measurement of the flexural resonance period along two orthogonal directions is the basis of a novel technique used by the SonicByte system to estimate the critical flaw size in a sample.
Nondestructive quality control of industrial materials
A SonicByte EP-1000 signal processor (see lead photo on page 40) and EP Exel-based software is used to test industrial material for the purpose of quality control. The signal processor is coupled to a digital filtering system unit that allows testing materials having various shapes, dimensions and sizes, which generally tend to generate undesirable vibrations such as harmonics and surface waves after impact. The system allows manual testing of industrial material for quality control using manual hammers, or automatic testing using a robotic system to control electric hammers and microphones.
Industrial quality control applications include refractory bricks, precast fired and non-fired refractory shapes, refractory ramming mixes, industrial ceramics, carbon materials, metals and composites. Among the precast fired and non-fired refractory shapes that can be tested are tap hole blocks, straight and conic thermocouple shields, floor tiles, straight and curved launder sections, spouts and grid plates (Fig. 3). Carbon materials that can be tested include green and fired anodes and cathodes, such as those used in aluminum electrolytic cells.
The SonicByte system was used to characterize the microstructure of four sets of laboratory monolithic refractory samples.
Castable samples containing equidistant, interacting notches: Refractory castable samples contained 15 equidistant, interacting notches (Fig. 4). The notches were produced using a 1-mm thick diamond saw and varied in length (Lo) up to 1.1 in. (20.74 mm). Sample dimensions were 6 x 1 x 1 in. (152 x 25 x 25 mm).
The flexural elastic moduli of the material's active zone for the five samples are nearly the same, which in theory should be the case. Notch length and the equivalent and uniform crack length values for the samples correlate very well as shown in Fig. 5.
Castable samples containing equidistant, noninteracting macroscopic cracks: Castable samples containing from 0 to 5 equidistant, noninteracting macroscopic cracks (Fig. 6) were first tested using the MQC technique and then subjected to a three-point bending test to measure their modulus of rupture at room temperature. Cracks were produced by inserting 0.004 in. (0.1 mm) thick plastic sheets in the material during the forming process to achieve crack lengths up to 1 in. (25.4 mm). Sample dimensions were 6 x 1.5 x 1.5 in. (152 x 38 x 38 mm).
Two methods used to calculate the relative strength values including one based on the modulus of rupture (MOR) measured values and one based on the active zone and crack length values according to the specially designed algorithm included in the SonicByte software. Figure 7 shows there is perfect correlation between both measured and estimated strength values. Moreover, there is very good correlation between Lo (the real crack length) and the estimated critical flaw size.
Unfired castable samples: Figure 8 shows the results of flexural elastic modulus measurements of unfired castable samples during sintering.
Unfired ramming-mix samples: Longitudinal an flexural elastic modulus measurements were made at room temperature on seven unfired cylindrical ramming mix samples having different sizes. Figure 9 shows that although the acoustic wave attenuation in such materials is very high, the SonicByte system produces relevant results. IH