Objective 08: Ultrasonic Testing Flashcards
At what frequency is sound audible?
10 - 20 000 Hz
At what frequency is vibration ultrasonic?
Above 20 000 Hz
What principle is Ultrasonic Testing based on?
The fact that solid materials are good conductors of sound waves. The waves are not only reflected at the interfaces but also by internal flaws, e.g. material separations and inclusions
What is the basis of Ultrasonic Testing?
The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy
What is the heart of the Transducer used for Ultrasonic Testing?
The Active Element
Active Element for Ultrasonic Testing
A piece of polarized material with electrodes attached to 2 of its opposite faces
What does the Active Element do for Ultrasonic Testing?
It converts electrical energy to acoustic energy and vice versa
Electrostriction
When an electric field is applied across the material, the polarized molecules align themselves with the electric field producing induced dipoles within the molecular or crystal structure of the material. This alignment of molecules causes the material to change dimensions.
Piezoelectric Effect
A permanently polarized material produces an electric field when the material changes dimensions as a result of an imposed mechanical force.
How does Ultrasonic Testing work?
- A disc of piezoelectric materials is attached to a block of steel either with cement or a film of oil (couplant) and a high-voltage electrical pulse is applied to the piezoelectric disc
- A pulse of ultrasonic energy is generated in the disc and propages into the steel
- This pulse of waves travels through the metal and the waves are reflected or scattered at any surface or internal discontinuity in the specimen
- This reflected or scattered energy is detected using a suitably-placed second piezoelectric disc on the metal surface wherein a pulse of electrical energy is generated
How is the distance of a discontinuity from the surface determined in Ultrasonic Testing?
The time interval between the transmitted and reflected pulse
How is the size of a flaw determined in Ultrasonic Testing?
The intensity of the return pulse
What are different types of transducers that may be used in Ultrasonic Testing?
- Cylindrical crystal wafers are most commonly used
- Small Diameter, High Frequency transducers are used to locate small discontinuities
- Large Transducers can generate more energy, allowing the inspection of thicker specimens
- Paintbrush transducers up to 150 mm wide can be used to detect defects quickly in a specimen with a large surface area, after which a smaller transducer is used to find the specific location and size
How many crystals can be used in a single transducer?
- 1 crystal can be used to act as both the sending and receiving unit
- 2 crystals can be used with 1 to act as sender and the other to act as receiver
What does the orientation of the Transducer for Ultrasonic Testing determine?
The angle at which the pulse strikes the specimen
How is Longitudinal Wave Testing peformed for Ultrasonic Testing?
The transducer is located flat on the specimen
How is Shear Wave Testing performed in Ultrasonic Testing?
The transducer is mounted on a plastic wedge which is part of the probe so that the pulse enters the specimen at the desired angle
What is Longitudinal Wave Ultrasonic Testing used for?
- Measuring the thickness of materials
2. Locating discontinuities directly underneath the transducer
Critical Angle for Ultrasonic Testing
When the longitudinal wave from the transducer upon
a wedge travels parallel to the surface
How is frequency chosen for Ultrasonic Testing?
The size of the discontinuity to be located. Short wavelengths (high frequencies) are used to locate small discontinuities.
Why is a Couplant between the Transducer and the Specimen important?
The acoustic impedence of the medium affects how much energy travels into the specimen, e.g. air has a low acoustic impedence
What are solutions to Low Impedence for Ultrasonic Testing?
- Immersing the specimen in a water tank
2. Covering the specimen with oil or grease to bond the transducer
What are important characteristics of a Couplant used in Ultrasonic Testing?
- Does not react with or contaminate the specimen
- Easy to remove
- Does not leak away during the test
What Couplant is typically used for relatively flat, smooth surfaces for Ultrasonic Testing?
A mixture of glycerin and water
What Couplant is typically used for rough surfaces for Ultrasonic Testing?
Light motor oil with a wetting agent. As the surface temperature increases, heavier oils are used
Why is a void in the specimen a better reflector than a metallic inclusion?
The impedence mismatch is greater between air and metal than between metal and another metal
Why is Signal-to-Noise ratio a good measure of detectability of a flaw?
The surrounding material has competing reflections
Signal-to-Noise Ratio
A measure of how the signal from the defect compares to other background reflections i.e. “noise”
What is the minimum signal-to-noise ratio required for Ultrasonic Testing?
3:1
What factors affect the absolute noise level and the absolute strength of an echo from a “small” defect for Ultrasonic Testing
- The probe size and focal properties
- The probe frequency, bandwidth and efficiency
- The inspection path and distance (water or solid)
- The interface (surface curvature and roughness)
- The flaw location with respect to the incident beam
- The inherent noisiness of the metal microstructure
- The inherent reflectivity of the flaw which is dependent on its acoustic impedance, size, shape, and orientation
What can obscure the revelation of cracks and volumetric defects from Ultrasonic waves?
- Many cracks are “invisible” from 1 direction and strong reflectors from another
- Multifaced flaws tend to scatter sound away from the Transducer
What are the advantages of Ultrasonic Testing?
- It is sensitive to both surface and subsurface discontinuities
- The depth of penetration for flaw detection or measurement is superior to other NDE methods
- Only single-sided access is needed when the pulse-echo technique is used
- Its high accuracy in determining reflector position and estimating size and shape
- Minimal part preparation required
- Electronic equipment provides instantaneous results
- Detailed images can be produced with automated systems
- It has other uses such as thickness measurements in addition to flaw detection
What are the limitation of Ultrasonic Testing?
- Surface must be accessible to transmit ultrasound
- Skill and training required is more extensive than with some other methods
- It requires a coupling medium to promote transfer of sound energy into test specimen
- Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect
- Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise
- Linear defects oriented parallel to the sound beam may go undetected
- Reference standards are required for equipment calibration and characterization of flaws
What are the 2 areas of Ultrasonic Testing that Code covers?
- Welds
2. Parts, components, materials, and thickness determinations
What are the categories of requirements laid out by Code for Ultrasonic Testing?
- Extent of examination and volume to be scanned
- Personnel qualifications
- Certification requirements
- Examination system characteristics
- Acceptance criteria
- Necessary records and documentation
- Report requirements
- Procedure requirements
What points are covered by Ultrasonic Testing wriiten procedures?
- Ultrasonic instrument types
- Description of calibration including blocks and techniques
- Technique (straight and angled beam, contact, and/or immersion)
- Search unit type with frequency and transducer size
- Special search units (wedges, shoes or saddles)
- Angles and modes of wave propagation in the material
- Directions and extent of scanning
- Couplant type and brand name
- Weld and material types
- Configurations to be examined (thickness dimensions) and form (casting, forging, plate)
- The surfaces from which the examination is completed
- Condition of the surface
- Data to be recorded
- Alarms
- Rotating, revolving or scanning mechanisms
- Post examination cleaning
What types of materials does Code provide instructions for in Ultrasonic Testing?
- Plate
- Forgings and Bars
- Tubular Products
- Castings
- Bolting Materials (studs and nuts)
- Pumps and Valves
- Inservice Examination
Is a report required for each Ultrasonic Testing?
Yes
