Impulse excitation technology
The pulse excitation technique is a method for the non-destructive determination of elastic material properties . A test object to be measured is excited non-destructively and by a single elastic shock in a defined oscillation. The vibration signal is recorded and analyzed by a suitable medium. A measuring device specially developed for this method analyzes the vibration and displays the frequency of the specifically excited vibration. Theoretically, there are no limits to the shape and size of the test specimen, as long as the test specimen can be reproducibly excited in the desired vibration.
history
In the mid-1960s, a method was developed at the University of Leuven to classify the properties of grinding tools based on their hardness. The aim of the research was to improve productivity and quality in the grinding process by precisely determining the grinding wheel hardness. It was recognized that the modulus of elasticity is a valid criterion for determining the properties of grinding wheels: the higher the modulus of elasticity of a grinding wheel, the harder it is.
Due to the fact that the modulus of elasticity of a body is a function of form factor, weight and the natural frequency of the body, Lemmens NV developed the GrindoSonic measuring device for the University of Leuven to measure the natural frequency precisely, quickly and non-destructively using the pulse excitation technology of a test object measure up . In grinding technology as well as in many other material areas, the reading value has established itself over the years as an important measure for the test objects. This reading value or "R" value is the period of two oscillations of the test body in µs as shown by the GrindoSonic. Since the late 1980s, the oscillation has also been shown as a function of the frequency.
application areas
In principle, using the pulse excitation technique, test bodies can be measured that can be reproducibly excited in the respective desired vibration mode. A wide variety of sample geometries and complex shapes are possible. The frequency bandwidth to be processed by the measuring system is between approx. 40 Hz and approx. 100 kHz. Experience has shown that it is smaller when using microphones.
Measurement method
Three different types of vibration are measured, bending, torsion and longitudinal vibration. The test item is excited manually or automatically with minimal energy and an elastic impulse. The oscillation is recorded with a piezo detector or with a microphone. With very small sample shapes, a microphone is advantageous because it picks up the vibration without contact. In normal cases, a piezoelectric transducer is recommended, as it specifically filters undesired vibrations through its directional sensitivity and is not disturbed by ambient noise.
- Vibration modes
Bending vibration
Torsional vibration
Longitudinal or longitudinal vibration
Measurable materials
In principle, all materials that vibrate and whose values are reproducible are to be measured. For the individual materials, formulas for calculating the elasticity and shear modulus have been developed in standards committees, research institutes and industry.
- Wood
- stone
- Concrete / cement
- Ceramics
- Abrasive materials
- Refractory products
- Metal / steel / alloys
- Plastics
- Glass
- graphite
- Composite materials
Quality control
With the impulse excitation technique, a wide variety of geometries and specimens of different weights can be classified. The measuring method can be used industrially, the duration of a measurement is less than two seconds - the method is therefore also used in automated systems for 100% quality control. The determined measured value serves as the result and reference value. “Good” and “bad” parts are empirically classified.
Material development
With defined shapes, the frequency of the natural oscillation is used to calculate the modulus of elasticity, shear modulus and Poisson's constant - provided that the test object is homogeneous and isotropic. This applies to the following forms:
- Cuboid
- cylinder
- Discs
- Washers with holes
- Tubes
To calculate the modulus of elasticity, dimensions, weight, Poisson's constant and the natural frequency of the flexural fundamental oscillation are required. The Poisson number is chosen from the literature. For the determination of the shear modulus (G-module) and the calculation of the Poisson's number, the measurements of a second vibration mode are required. Standards and universities have included the process of pulse excitation technology in standards, e.g. B. ASTM.
Determination of elasticity at high and low temperatures
The fast and non-destructive measuring method makes it possible to measure a test object under changing temperatures. A specially developed furnace allows continuous measurements and elastic modulus determinations on the test item up to 1200 ° C. Measurements down to −265 ° C were carried out in the laboratory.
Web links
Individual evidence
- ↑ J. Peters, R. Snoeys, A. Decneut: Classifying Grinding Wheels. Leuven, 1968
- ↑ A. Decneut: Improvement of Productivity and Quality in the grinding operation. 1979
- ↑ H. Föllinger: The importance of the modulus of elasticity of grinding wheels for the behavior in the grinding process. 1985
- ↑ W. Hönscheid: Delimitation of material-specific grinding conditions for titanium alloys. 1975
- ↑ Standard Test Method for Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio by Impulse Excitation of Vibration. ASTM
- ↑ J. Zhang, A. Nyilas, as fruit: New technique for measuring the dynamic Young's modulus between 295 and 6K . In: Cryogenics . tape 31 , no. 10 , 1991, pp. 884-889 .