Impulse excitation technique

The impulse excitation technique is a method for non-destructive determination of elastic material properties. A test sample is to be measured non-destructively and excited by unique elastic Stross in a defined vibration. The vibration signal is recorded and analyzed by a suitable medium. A specially developed for this method analyzes the vibration meter and displays the frequency of the targeted excited vibration on. Shape and size of the specimen are theoretically no limits as long as the device under test is reproducible to excite the desired mode.

History

Mid-1960s, was designed at the University of Leuven, a method to classify properties of grinding wheels according to their hardness. The aim of the research was, by precise determination of the grinding wheel hardness to improve the productivity and quality in the grinding process. It has been found that the modulus of elasticity is a valid criterion for property determination of grinding wheels is that the higher the modulus of elasticity of a grinding wheel, the harder it.

Due to the fact that the modulus of elasticity of a body is a function of the form factor, weight and the natural frequency thereof, the developed Lemmens company NV for the University of Leuven, the Grindosonic meter to precisely the natural frequency to measure quickly and non-destructively using the impulse excitation technique of a test specimen. In the milling technology as well as in many other material regions has established itself over the years until today the Reading value as an important measure of the specimens. Reading this value or " R" value is represented by the Grindosonic period of vibrations of the specimen in two microseconds. Since the late 1980s, the vibration is also shown as a function of frequency.

Areas of application

Principle specimen can be measured that can be reproducibly stimulated in the desired vibration mode respectively according to the pulse excitation technique. The substance may have a wide variety of sample geometries and complex forms. The frequency to be processed by the measuring system bandwidth is between about 40 Hz and about 100 kHz. It is according to experience smaller when using microphones.

Measurement methods

Are measured three different types of vibration: the flexurale, torsional and longitudinal vibration. The sample is non-destructive, excited manually or automatically with minimal energy and an elastic pulse. The oscillation is received by a piezoelectric detector or a microphone. For very small samples forms a microphone is advantageous since it receives the non-contact vibration. Normally, a piezoelectric transducer is recommended because it filters out unwanted vibrations targeted by its directional sensitivity and is not disturbed by ambient noise.

  • Vibration modes

Torsional vibration

Longitudinal vibration

Measurable materials

To measure are basically all materials that vibrate and their values ​​are reproducible. For the individual materials formulas have been developed to calculate the elasticity and shear modulus in standards committees, research institutes and industry.

  • Wood
  • Stone
  • Concrete / Cement
  • Ceramics
  • Abrasive materials
  • Refractory products
  • Metal / Steel / Alloys
  • Plastics
  • Glass
  • Graphite
  • Carbon fiber materials
  • Composites

Quality control

With the impulse excitation technique can be classified a variety of geometries and specimens of different weights. The measurement method is industrially applicable, the duration of a measurement is less than two seconds - the procedure is thus also in automated plants for 100 - used % quality control. As a result, and the reference value, the measured value is. And "bad" -: - " good" parts are empirically classified.

Material development

In defined shapes, the frequency of the natural vibration to calculate elastic modulus, shear modulus and Poisson constant is used - on the condition that the test object is homogeneous and isotropic. This applies to the following forms:

  • Cuboid
  • Cylinder
  • Slices
  • Discs with hole

To calculate the modulus of elasticity, weight, Poisson's constant and the natural frequency of the fundamental oscillation flexuralen are required. The Poisson's ratio is chosen here from the literature. For determining the shear modulus (G- module) and the calculation of the Poisson's ratio, the measurements of a second oscillation mode is required. Standards and universities have incorporated the method of impulse excitation technique standards, such as ASTM.

Elasticity determination at high and low temperatures

Due to the rapid and non-destructive measurement method, it is possible to measure an examinee under changing temperatures. A specially designed furnace allows continuous measurements and modulus of elasticity rules on the test specimen to 1200 ° C. In the laboratory measurements were realized to -265 ° C.

Katholieke Universiteit Leuven Grinding (abrasive cutting)
410722
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