Non-destructive testing with ultrasound for materials management

Ultrasound is a mechanical vibration with a range greater than that audible by the human ear that is transmitted through a physical medium and is oriented, recorded and measured in Hertz with the help of a device created for that purpose.

Sound ranges:

Infrasonic = 1 - 16 Hz Sonic or audible = 16 Hz to 20 KHz Ultrasonic = 20 KHz onwards

For the ultrasound test on metallic materials it is 0.2 to 25 MHz.

non-destructive-ultrasound-tests

§ Principles

Acoustic impedance is the resistance that materials oppose to the passage of an ultrasonic wave. Acoustic waves equal to sonic waves. Transmission of energy between particles that promotes oscillation. The number of oscillations are according to the type of wave in question. They propagate in all elastic media where there are fractions of matter (atoms or molecules capable of vibrating). The vibration depends on the separation of the particles.

§ Applications

Detection and characterization of discontinuities Measurement of thickness, extension and degree of corrosion Determination of physical characteristics Characteristics of bonding between materials.

§ Advantage

The test is carried out more quickly obtaining immediate results. There is greater accuracy when determining the position of the internal discontinuities; estimating its dimensions, orientation and nature High sensitivity to detect small discontinuities High penetration capacity, which allows locating discontinuities at great depth of the material Good resolution that allows to differentiate two discontinuities close to each other Only requires access from one side of the object to be inspected It does not require special security conditions.

§ Limitations

Low inspection speed when using manual methods Requires personnel with good technical training and great experience Difficulty inspecting parts with complex geometry, very thin thickness or irregular configuration Difficulty detecting or evaluating near-surface discontinuities on the surface ultrasound is introduced, requires calibration and reference standards, is affected by the structure of the material (grain size, type of material) High equipment cost Coupling agent required.

§ Physical principles

Amplitude (A).- It is the maximum displacement of a particle from its zero position. Frequency (F).- It is defined as the number of times a repetitive event (cycle) occurs per unit of time. Its united Hertz. Wavelength (l).- It is the distance occupied by a complete wave and is equal to the distance through which the wave moves per cycle period. Velocity of propagation or acoustic velocity (V).- It is the speed of transmission of sound energy through a medium. Acoustic impedance (Z).- It is the resistance of a material to the vibrations of ultrasonic waves. It is the product of the maximum vibration speed and the density of the material.

Wave types

Longitudinal waves.- Their displacements of the particles are parallel to the propagation of the ultrasound. Transverse waves.- The displacements of the particles are perpendicular to the direction of the ultrasonic beam. Surface waves.- Are those that move over the surface of the material and penetrate to a maximum depth of one wavelength.

The main parameters that must be controlled in an ultrasonic system are:

Sensitivity. It is the ability of a transducer to detect small discontinuities.

Resolution. It is the ability to separate two nearby signals in time or depth.

Central frequency. The transducers should be used in their specified frequency range for optimal application.

Beam attenuation. It is the loss of energy of an ultrasonic wave when moving through a material. The main causes are dispersion and absorption.

Transducers

It is the means by which electrical energy is converted into mechanical energy (sound waves) or vice versa. It operates due to the piezoelectric effect, which is that certain crystals when stressed, become electrically polarized and generate electrical voltage between opposite surfaces. This is reversible in the sense that when a voltage is applied across the faces of a crystal, a deformation of the crystal occurs. This microscopic effect is caused by the symmetry properties of some crystals.

Piezoelectric Materials

Quartz. It is obtained from natural crystals. It has excellent thermal, chemical and electrical stability characteristics. It is very hard and resistant to wear as well as aging. Unfortunately, it suffers from interference in conversion mode and is the least efficient of acoustic power generators. It requires high voltage for its handling at low frequencies. It must be used at temperatures lower than 550 ° C, since above this it loses its piezoelectric properties.

Lithium sulfate. This material is considered one of the most efficient receptors. Its main advantage is its ease of obtaining optimal acoustic damping which improves the resolving power, does not age and is little affected by interference in conversion mode. Its disadvantages are that it is very fragile, soluble in water and must be used at temperatures below 75 ° C.

Polarized ceramics. They are obtained by sintering and are polarized during the manufacturing process. They are considered as the most efficient ultrasonic power generators when operating at low excitation voltages. They are practically not affected by humidity and some can be used up to temperatures of 300 ° C. Their main limitations are: relatively low mechanical resistance, in some cases there is interference in the conversion mode, they tend to age. They also have less hardness and resistance to wear than quartz.

CHARACTERISTICS OF THE MATERIALS USED AS TRANSDUCERS IN THE PROBE

Material	Efficiency as a transmitter	Efficiency as a receiver	Sensitivity	Resolving power	Mechanical characteristics
Quartz	Bad	Median	Limited	Optima	Good
Lithium sulfate	Median	Good	Good	Optima	Soluble in water
Barium titanate	Good	Median	Optima	Median	Fragile
Barium methaniobate	Good	Median	Optima	Optima	Good
Lead Titanate Zirconate	Good	Median	Optima	Median	Good

Choice of transducer

Crystal class. By choosing each type of crystal, the resolving power and sensitivity of the transducers can be varied. Crystal diameter. The larger the diameter of the crystal, the greater the depth of penetration, the greater the length in a near field and the less divergence. Frequency. With the choice of a higher frequency, a greater possibility is obtained for the identification of small discontinuities, greater length of near field, greater resolving power, less depth of penetration and minimum divergence.

PIEZOELECTRIC MATERIALS

MATERIAL	ADVANTAGE	DISADVANTAGES
QUARTZ	* Obtained from natural crystals. * It has excellent thermal, chemical and electrical stability characteristics. * It is very hard and resistant to wear as well as aging.	Suffers interference in conversion mode It is the least efficient of the acoustic power generators. It requires high voltage for its handling at low frequencies. It must be used at temperatures lower than 550 ° C, since above this it loses its piezoelectric properties.
LITHIUM SULPHATE	* More efficient receiver. * Ease of obtaining optimal acoustic damping. * Better resolving power. * Does not get old. * It is little affected by interference in conversion mode.	* It is very fragile * Soluble in water * It should be used at temperatures below 75 ° C.
POLARIZED CERAMICS	* They are obtained by sintering and are polarized during the manufacturing process. * They are considered the most efficient ultrasonic power generators when operating at low excitation voltages. * They are practically unaffected by humidity * Some can be used up to temperatures of 300 ° C.	* Relatively low mechanical resistance, * In some cases there is interference in the conversion mode. * They have a tendency to aging. * They also have less hardness and wear resistance than quartz.
BARIUM TITANATE	* It is a good emitter due to its high piezoelectric modulus.	* Coupling and damping problems. * Its use is limited to frequencies less than 15 MHz, due to its low mechanical resistance and high acoustic impedance. * Presents interaction between various modes of vibration. * The temperature of its curie point is 115 - 150 ° C.
BARIUM METHANIOBATE	* It presents a high piezoelectric module which qualifies it as a good emitter. * It has excellent thermal stability, similar to quartz, which allows it to be used at high temperatures. * It has a high internal damping coefficient, which is why it is considered the best material to generate short impulses.	It has a low fundamental frequency and poor mechanical resistance, so it is mainly applied at high frequencies. It presents interaction between various modes of vibration.
ZIRCONATE LEAD TITANATE	It is considered the best emitter due to its high piezoelectric modulus.	However, it is the most difficult to dampen due to its high coefficient of deformation. Its use is recommended when there are penetration problems.

Types of Probes

Contact probe. It is placed directly on the test surface by applying pressure and a coupling medium. It is manufactured for straight beam inspections. To protect the transducer from abrasion, it is covered with a hard material such as aluminum oxide.

Straight beam styli. It emits longitudinal waves with frequencies from 0.5 to 10 MHz. It is generally used for the inspection of parts in which the test unit can be placed directly on the area of interest, the discontinuities are parallel to the contact surface. It is also useful in detecting discontinuities and measuring thickness.

Angular incidence probes. Generates shear, surface and plate waves. It is built by coupling a straight beam unit to one of the faces of a plastic shoe, to which it has a certain angle of refraction. It is used in pulse echo equipment and its application is almost exclusively in the detection of discontinuities oriented perpendicular to the test surface.

Types of angle probes. According to its size, frequency, shape, type and interchangeability of the shoe. The angle of refraction of the sound within the test material is marked on the shoe, the commercial angles for steel are 35, 45, 60, 70, 80, 90 degrees.

COUPLING

More or less viscous liquid that is used to allow the passage of the waves from the transducer to the piece under examination, since the frequencies used for metallic materials are not transmitted in the air.

Coupling Liquid Characteristics:

(capable of wetting the surface and the stylus) Adequate viscosity, low attenuation. (100% transmission of sound) Low cost Non-toxic Non-corrosive Adequate acoustic impedance

Types of couplings:

Water Oil Fat Glycerin Vaseline

REFLECTION

Amount of ultrasonic energy that is reflected when striking an acoustic interface.

Law of reflection. The angle of the reflected wave is equal to the angle of the incident wave of the same species.

REFRATION

It is carried out when an ultrasonic beam passes from one medium to another, its velocity of the medium being different from each other and changing the direction in relation to the direction of incidence.

Law of refraction. The change in direction of the refracted wave, approaching the normal to its separation surface of both media, depends on the speed of sound in the second medium is less or greater than in the first medium.

V ₁ = Velocity of medium 1 a = Angle of incidence

V ₂ = Velocity of medium 2 q = Angle of refraction

CHARACTERISTICS OF THE OSCILOGRAM

LONGITUDINAL BEAM PROBE OF A GLASS

DIRECT CONTACT INSPECTION METHOD

CALIBRATION BLOCK

The reference standards can be a block or set of blocks with known artificial discontinuities and / or thicknesses. Which are used to calibrate ultrasound equipment and to evaluate indications of discontinuities in the inspected sample

The calibration blocks must have the same physical, chemical and structural properties as the material to be inspected.

Through the calibration blocks you can:

Verify that the system consisting of the transducer, coaxial cable and the equipment is working correctly. Set the gain or sensitivity with which discontinuities equivalent to a specified size or greater will be detected.

COMMENTS

The practice of ultrasonic tests is carried out under inappropriate operating conditions that can vary the precision and veracity of our measurements. It is necessary that activities be carried out with which deficiencies can be corrected or modified during the development of the practice. Some of these are:

Have an appropriate laboratory to perform the test, as we know the different materials can change their physical, chemical or mechanical properties according to the environment in which they are found.

Have the necessary equipment and material, since you do not have all the necessary elements to perform the test. As a particular case, we will mention that the coupling (oil) was taken directly from the container that contained it and applied with the fingers, since there was no oil can. It is worth mentioning that this can cause contamination of the parts that are going to be subjected to the ultrasound test.

The distribution of the students to observe the development of the practice is not adequate since due to the characteristics of the oscilloscope, it is not possible to observe what happens in practice and the graphs that the oscilloscope generates.

CONCLUSIONS

With the development of this practice, the following could be observed:

All the necessary theoretical information about the non-destructive testing of the ultrasound was studied by means of the emission of a high frequency sound that indicates the defects of our material through the screen of an oscilloscope. Ultrasound to our "test tubes" could be observed physically which is the procedure to follow to carry out the test. Likewise, the defect that our test tube presented was determined, being able to determine the distance at which it was. It was physically observed which equipment is necessary to perform the ultrasound test being these: A power source, an oscilloscope, a probe, coupling and the The relevant calculations were also performed to determine the following parameters: Wavelength, angle of divergence,near field and diameter of the defect. It was also observed that if the frequency increases the resolving power will be lower and the dead field of the background echoes will decrease. The materials subjected to the ultrasound test must be of regular shape and of non-porous materials.. On the other hand, we found that both ferrous and non-ferrous materials can be subjected to this test.The ultrasound test allows us to locate internal defects such as: pores, cracks, reluctances, welding defects, etc. Some of the advantages of this test are: It is used in any type of material, a paper record can be obtained, internal and subsurface defects are determined Some of its disadvantages are: It requires qualified personnel, high initial cost due to the type of equipment necessary to perform the proof.

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