Ennio Curto
11-09-2007, 01:45 PM
NDT PORTABLE TESTER MEASURAMENT RESIDUAL STRESS
TECHNICAL DESCRIPTION
Preface:
For internal tensions come identified: 1) the exercise stresses, tension to which the materials are submitted and calculated. 2) the residuals stresses of the material due to heat treatments or tensions due to forging, welding etc. The equipment measure the applied and residual stress that equilibrate oneself to the superficies of the material, and in a relatively great volume to times of the same order of magnitude the entire structures, these tensions re-enter in the elastic field of the metal and have a guideline space triaxial.
Theory :
Elastic oscillations (also called vibrations) of a material consist of elastic and alternate movements of all elementary masses, which compose the material itself, around respective positions of equilibrium; due to these movements a transformation from potential energy into kinetic energy is produced. This phenomenon occurs because of the reactions (elastic forces) which the above elementary masses oppose to elastic movements; these reactions are proportional -by the Law of Hooke -to the movements themselves. The elastic waves, which are produced, propagate according to a fixed velocity, which depends on how fast the elementary masses go into oscillation.
The elastic waves of this type are called 'permanent progressive'; they propagate at a constant velocity that is absolutely independent on the velocity according to which the elementary masses move during their oscillating movement, and therefore on the respective accelerations. It is easily verified that the elastic oscillations of a material point P (in which it is supposed that the elementary mass m) are harmonic. Actually, due to the fact that in each moment the elastic force that applies on P is proportional to the distance x of the point from its equilibrium position 0, also the acceleration of P (due to the proportionality between the forces and the corresponding accelerations) is proportional to x; this is verified in the harmonic motion. The impulse I generates in the metallic mass a harmonic oscillation (vibration) characterized by a specific frequency f and with an amplitude equal to the movement x of the related mass. If we produce a constant impulse on to the metallic material, an elastic oscillation is generated from the point P which on its turn will produce a sinusoidal wave with specific amplitude, pulsation. period and velocity. This wave is longitudinal when the direction of the vibration is the same as the motion of the point P, or transversal when this motion is at right angle to the same direction. In both cases the resulting values are identical; the only difference is a vibration phase.
Elastic deformation energy
The elastic energy subordinate to a force impulse is:
Ei = Ek + Ed + Ep Ei = Impulse energy Ek = Kinetic energy
Ed = Elastic deformation energy Ep = Plastic deformation energy
Ed = ½ K dx² = ½ m ω² dx² = ½ m X dx
K = Constant elastic of material dx = displacement X = acceleration
K/X = φ constant depending from metal physic property.
Behaviour elastic metal, new discovery equations :
Technique:
The tester, through accelerometer fitted with magnetic base, gives the value of vibration acceleration generated from the impact device with constant energy, on metal. The acceleration value, combined with other parameters allows to get the exact value of the residual stress or of the loads applied at the point .This value appears in the LCD display directly in N/mm². For non-magnetic metals are using wax or gel to fix the accelerometer to metal. The application of this non-destructive method, gives the possibility to measure the residual stress under the action of loading service is very easy to utilize, efficient and allows measuring the residual stresses in welded and existent structures for all types of metals with NDT method .
Surface quality
Residual stress testing method require smooth surfaces free of oxide scale, paint, lubricants, oil, plastic coating due to corrosion protection, or metal coating for
better conductivity. The indentation depth should be large in comparison to the surface roughness. If surface preparation is necessary, care must be taken not to alter the surface by over heating or strain-hardening. More practical results can be achieved by using a battery-driven, high-speed (>12000 rpm) handheld grinder.
Ennio Curto (Italy)
TECHNICAL DESCRIPTION
Preface:
For internal tensions come identified: 1) the exercise stresses, tension to which the materials are submitted and calculated. 2) the residuals stresses of the material due to heat treatments or tensions due to forging, welding etc. The equipment measure the applied and residual stress that equilibrate oneself to the superficies of the material, and in a relatively great volume to times of the same order of magnitude the entire structures, these tensions re-enter in the elastic field of the metal and have a guideline space triaxial.
Theory :
Elastic oscillations (also called vibrations) of a material consist of elastic and alternate movements of all elementary masses, which compose the material itself, around respective positions of equilibrium; due to these movements a transformation from potential energy into kinetic energy is produced. This phenomenon occurs because of the reactions (elastic forces) which the above elementary masses oppose to elastic movements; these reactions are proportional -by the Law of Hooke -to the movements themselves. The elastic waves, which are produced, propagate according to a fixed velocity, which depends on how fast the elementary masses go into oscillation.
The elastic waves of this type are called 'permanent progressive'; they propagate at a constant velocity that is absolutely independent on the velocity according to which the elementary masses move during their oscillating movement, and therefore on the respective accelerations. It is easily verified that the elastic oscillations of a material point P (in which it is supposed that the elementary mass m) are harmonic. Actually, due to the fact that in each moment the elastic force that applies on P is proportional to the distance x of the point from its equilibrium position 0, also the acceleration of P (due to the proportionality between the forces and the corresponding accelerations) is proportional to x; this is verified in the harmonic motion. The impulse I generates in the metallic mass a harmonic oscillation (vibration) characterized by a specific frequency f and with an amplitude equal to the movement x of the related mass. If we produce a constant impulse on to the metallic material, an elastic oscillation is generated from the point P which on its turn will produce a sinusoidal wave with specific amplitude, pulsation. period and velocity. This wave is longitudinal when the direction of the vibration is the same as the motion of the point P, or transversal when this motion is at right angle to the same direction. In both cases the resulting values are identical; the only difference is a vibration phase.
Elastic deformation energy
The elastic energy subordinate to a force impulse is:
Ei = Ek + Ed + Ep Ei = Impulse energy Ek = Kinetic energy
Ed = Elastic deformation energy Ep = Plastic deformation energy
Ed = ½ K dx² = ½ m ω² dx² = ½ m X dx
K = Constant elastic of material dx = displacement X = acceleration
K/X = φ constant depending from metal physic property.
Behaviour elastic metal, new discovery equations :
Technique:
The tester, through accelerometer fitted with magnetic base, gives the value of vibration acceleration generated from the impact device with constant energy, on metal. The acceleration value, combined with other parameters allows to get the exact value of the residual stress or of the loads applied at the point .This value appears in the LCD display directly in N/mm². For non-magnetic metals are using wax or gel to fix the accelerometer to metal. The application of this non-destructive method, gives the possibility to measure the residual stress under the action of loading service is very easy to utilize, efficient and allows measuring the residual stresses in welded and existent structures for all types of metals with NDT method .
Surface quality
Residual stress testing method require smooth surfaces free of oxide scale, paint, lubricants, oil, plastic coating due to corrosion protection, or metal coating for
better conductivity. The indentation depth should be large in comparison to the surface roughness. If surface preparation is necessary, care must be taken not to alter the surface by over heating or strain-hardening. More practical results can be achieved by using a battery-driven, high-speed (>12000 rpm) handheld grinder.
Ennio Curto (Italy)