NEW TECHNIQE FOR RESIDUAL STRESS MEASUREMENT NDT
p.i. Ennio Curto
TECHNICAL DESCRIPTION
Preface:
For internal stresses are considered: 1) Strains to which the materials are submitted and calculated. 2) Residual stresses of material due to heat treatment or forging, welding, etc. The equipment measure all stress types than equilibrate themselves in the surface of the material, and in a relatively large volume sometimes the same order of entire structures, these stresses are within the elastic metal field and have a spatial orientation, triaxial.
Theory:
Oscillations elastic (also called vibration) in an elastic material, consist in alternately movement around the respective equilibrium positions of the elementary masses, cause of these movements generate a conversion of potential energy into kinetic energy. This phenomenon occurs due to reactions (elastic forces) that the elementary masses, above views, opposed to elastic movements, these reactions are proportional for the Hooke law to the displacement. The elastic waves produced, propagate themselves with a fixed speed, which depends on how quickly the masses elementary enter into oscillation. The elastic waves of this type are called 'permanent progressive, which spreads to a constant speed is absolutely independent of the speed by which the elementary masses move during their oscillation, and hence the respective accelerations. Is easily to verify that the oscillations of material point P. (on which suppose concentrated elementary mass m.) are harmonics. In any instant the elastic force, applies to P, is proportional to distance x from its position of equilibrium 0, the acceleration of P (for proportionality between the forces and the corresponding accelerations), this has occurred in harmonious motion. The impulse creates in a metal mass a harmonic oscillation (vibration), characterized by a specific frequency and displacement (movement of its mass) acceleration and speed.
This wave is longitudinal, when the direction of vibration is the same motion of point P, or transversal, in both cases the resulting values were identical, the only difference is a delay of 1 / 4 phase.
Elastic energy, as subordinate to a impulse energy, 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 = Ed = ½ m X dx
K = constant elastic material (stiffness)
dx = displacement Ẍ = acceleration Ẋ = speed ν = frequency
Behaviour elastic metals, due to new discovery:
Technique
The tester, through accelerometer mounted with magnetic base, gives the value of acceleration of the vibrations generated by the impact device with constant energy, on metal. The acceleration value combination with other parameters gives the exact value of residual stress or loads applied at the point tested. This value is displayed in the LCD directly in N/mm². For non-magnetic metal, will be using wax or gel to attach the accelerometer at the metal. The application of this non-destructive method, gives the opportunity to measure the residual stress in every surface point and more time. It is
very easy to use, efficient and allows for the measurement of residual stress in welded elements, and existing structures, for all types of metals with NDT method.
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).
Residual stress.
The residual stress in a metal doesn’t depend on its hardness, but from the module of elasticity or Young module and from its chemical composition.
The hardness of a metal indicates its ability to absorb energy elastic, plastic or kinetic 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. Therefore, the problem will be to measure the values that vary so exactly proportional to the residual stress values in a given metal.
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 to constant energy with the subsequent reaction elastic (elastic field) from the metal. The equipment don’t recognize the value of compression from that of tensile stress. It gives the value of the space resulting or volume but with the applying of a triaxial accelerometer we can have forces values axis x, y, z.
You will realize the convenience of this apparatus.
1) Portable and easy to use and very rapid.
2) NDT non-destructive test (strain gauges).
3) Repeatable in unlimited number of times.
4) Not complicate (X-ray).
5) All metals type a-magnetic (Barkhausen)
6) Don’t expensive. Effective for welding, hardened treatments, vessels control, bridges, pipes line, aeronautics, NDT inspection for every metal types.
Ennio Curto |
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