Residual stress
RESISUAL STRESS NDT PORTABLE UPDATE
Introduction
This type of nondestructive method NDT provides the possibility to measure residual stress and the effect of the service load with an impact device and a vibration measurement sensor. Internal stresses are to be considered as the following: 1) Operational strains referring to loads that the material is subject and calculated 2) Residual stresses in the material caused by heat treatments or stresses caused by welding, forging, casting, etc. The new technique is able to measure the applied load and residual stress that are balanced on the surface of the material, and in a relatively large volume, at times even the same size as the entire structures. This stress is part of the metal’s elasticity field and has a three axis spatial orientation. The system works through the accelerometer mounted with a magnetic base to generate the acceleration value of the vibrations created by the device impacting on the metal surface. The acceleration value, in combination with other parameters, permits obtaining the exact value of the residual stress or load applied in the desired point. This value will appear on the display directly in N / mm ². For nonmagnetic metals, wax or gel will be used to mount the accelerometer.
This new system, for buildings, bridges steel inspection is very simple for to use , portable ,measure exact values of residual stress due to welding and the applied loads. After many years of research and tests, and between e discover about elastics behavior in field of metal steel now is very practice inspection point to point building and bridge constructions. This new system, for buildings, bridges steel inspection is very simple for to use , portable ,measure exact values of residual stress due to welding and the applied loads.
Internal stresses are to be considered as the following: 1) Operational strains referring to loads that the material is subject and calculated 2) Residual stresses in the material caused by heat treatments or stresses caused by welding, forging, casting, etc. The new technique is able to measure the applied load and residual stress that are balanced on the surface of the material, and in a relatively large volume, at times even the same size as the entire structures. This stress is part of the metal’s elasticity field and has a three axis spatial orientation.
Description
Elastic oscillations (also called vibrations) of an elastic material consisting of elementary masses alternately moving around their respective balance positions; these movements cause a transformation of the potential energy into kinetic energy. This phenomenon takes place due to reactions (elastic forces) that the aforementioned masses produce in opposition to elastic movements; these reactions are proportional according to Hooke’s Law to the same movements. The elastic waves that are produced propagate according to a fixed speed that depends on how rapidly the elemental masses begin to oscillate.
Elastic waves of this type are called “permanently progressive”, and they propagate at a constant speed which is absolutely independent of the speed with which the elemental masses move during the oscillating motion, and therefore also their respective oscillations. It is easy to verify that the elastic oscillations, from a material point P (in which the elemental mass m is supposedly concentrated) are harmonic. In reality, due to the fact that in any moment the elastic force that is applied to P is proportional to the distance x of the point from its position of balance 0, P acceleration (caused by the proportionality between the forces and the corresponding accelerations) is also proportional to x; this is demonstrated in the harmonic movement. The impulse creates in the metallic mass a harmonic oscillation (vibration) which is characterized by a specific frequency f and by a width equal to dx (movement of the relative mass). If a constant impulse is produced in the metallic material, the elastic oscillation generated in the P point will also produce a sinusoidal wave with specific width, acceleration, speed and period values. This wave is longitudinal when the direction of the vibration is equal to the P point movement, or is trasversal, and in both cases the values of the results are identical; the only difference is the ¼ delay of the phase.
Analyse impact energies
Impact with the metallic surface results an elastic deformation plus a plastic deformation
Ed = Ei – ( Er + Ep )
Ei = Impact energy Er = Rebound energy
Ed = Elastic deformation energy Ep = Plastic deformation energy
Ed = ∫ ½ K dx² K = metal elastic Module
The elastic energy generate from the impulse generator, on the surface of the metal, varies with the hardness of the metal itself. This elastic energy, however, remains unchanged at each corresponding hardness.( see diagram )
The impulse of force gives an elastic volumetric deformation equal to:
Ee = ∫ ½ K dx Vol.
Vol = Volume deformation
K. = Elastic Module (volumic reaction.)
dx = Displacement.
At the same time gives rise to a transverse vibration forced with energy equal
Ev = ∫ ½ m a dx
m = inertial mass
a = acceleration of vibration.
therefore
Ee = ∫ ½ Vol. K dx = Ev = ∫ ½ m a dx = constant
The value of residual stress, with the elastic energy constant, depends on the change from the module K and inverse dx (other values are fixed: π, D, ρ).
The variation of K therefore corresponds to a proportional variation of:
a (vibratory acceleration).
The system works through the accelerometer mounted with a magnetic base to generate the acceleration value of the vibrations created by the device impacting on the metal surface. The acceleration value, in combination with other parameters, allow to obtain the exact value of the residual stress or load applied in the desired point. This value will appear on the display directly in N / mm ². For nonmagnetic metals, wax or gel will be used to mount the accelerometer. The system doesn’t recognize the compressive from tensile stress.
Conclusion
Application of this type of nondestructive method NDT provides the possibility to measure residual stress and the effect of the service load in a very rapid and simple way on any point of the metallic surface. Precision depends on the roughness of the surface.
This technology has demonstrated its validity over years of mechanical experimentation and has confirmed its theoretical basis.
About residual stresses
The residual stress in a metal doesn’t depend on its hardness, but from the elasticity module or Young module and from its chemical composition. The hardness of a metal indicates its ability to absorb elastic an plastic energy, but through it not possible to determine the value of residual stress. In a metal with the same hardness we will have different values of this stress. The residual stresses tend to equilibration themselves in the surface of the material. The measurement made with all the major methods, Xray, string gauge (destructive), optical etc. the residual stress is determined between the measuring the displacement of the equilibrium point the reticule crystalline. The method discovered analyse the value of frequency and vibratory acceleration generated by an impulse with the subsequent reaction elastic (elastic field) from the metal.
You will realize the convenience of this technique.
1) Portable system easy to use and very swift.
2) NDT nondestructive test.
3) Repeatable in unlimited number of points.
4) All metals type (amagnetic) and surface and inclination.
5) Don’t expensive. Effective for welding, hardened treatments, vessels control,
bridges, pipes line, aeronautics, NDT inspection for every metal
.p.i Ennio Curto.
With SADT cn. collaboration
And University of Science and Technology Beijing (Rayx).[/font][/size]
