Introduction
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 ù² 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.
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)
Behavior elastic metals, due to new discovery
Fig. 1
Fig.2
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.
The system doesn't recognize the compressive from tensile stress.
Fig .3
Quality of surface
The test method requires smooth surfaces free of oxides, paint,
lubricants, oil. The indentation deep and the accurately of the test
depend from the roughness of the surface. For the preparation of the
surface, is necessary, must be careful not to alter the surface over
certain values of heating or hardening. More practical results can be
realized by using a high-speed grinder (> 12000 rpm).
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.
The new system provides a full-field, large area inspection, in real
time to point-by-point inspection too rapid and easy
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)
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.
Via E. di Velo, 84
36100 Vicenza (Italy)
T. 0039+0444+511819
E. enniocurto@astwebnet.it
Web site: www.scribd.com/doc/6067883/New-Technique-for-Residual-Stress-Measurement-NDT-
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