NEW TECHNIQUE FOR RESIDUAL STRESS MEASUREMENT NDT
Introduction
This type of non-destructive
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 non-magnetic 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 transversal, and in both cases the values
of the results are identical; the only difference
is the ¼ delay of the phase.
Analyze impact
energies
Impact with the metallic surface
results an elastic deformation energy.
Ed
= Ei – ( Ek + Ep )
Ei = Impact energy Ek
= Kinetic energy
Ed = elastic deformation
energy Ep = plastic deformation energy + lost energy
Ed = ½ K dx² = ½ m ω² dx² K = constant elastic
material (stiffness)
Conclusion
Application of this type of non-destructive
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. The testing method requires smooth surfaces
free of oxides, paint, lubricants and oil. 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 or 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 equilibrate themselves in the surface
of the material. The measurement made with all the
major methods, X-ray, 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
analyzes 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 non-destructive test.
3) Repeatable
in unlimited number of points.
4) All metals
type (a-magnetic) and surface and inclination.
5) Don’t expensive. Effective for welding, hardened
treatments, vessels control,
bridges, pipes
line, aeronautics, NDT inspection for every metal
types.
p.i Ennio Curto.
more info:
www.scribd.com/doc/6067883/New-Techniqu ... ement-NDT-
