The Benefits of Using Ndt Flaw Detector
Flaw Detector: A Complete Guide | High-quality NDT Machines
Flaw Detector: A Complete Guide
What is Flaw Detector'
- Ultrasonic Flaw Detectors
- Magnetic Particle Flaw Detectors
- Eddy Current Flaw Detectors
- Radiographic Flaw Detectors
Magnetic particle flaw detectors, also known as magnetic particle inspection (MPI) equipment, are devices used for non-destructive testing (NDT) to detect and evaluate surface and near-surface flaws in ferromagnetic materials. Magnetic particle flaw detectors utilize magnetic fields and magnetic particles to identify and visualize indications of defects.
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Principle
Magnetic particle flaw detection is based on the principle of magnetic flux leakage. When a magnetic field is applied to a ferromagnetic material, such as iron or steel, the magnetic field lines should flow smoothly through the material. However, when there is a surface or near-surface defect, such as a crack or discontinuity, the magnetic field lines are disrupted, resulting in magnetic flux leakage around the defect.
Working Mechanism
Magnetization
The material being inspected is magnetized by either direct magnetization or indirect magnetization. In direct magnetization, a magnetic field is applied directly to the material using a magnetic yoke or an electromagnetic coil. In indirect magnetization, the material is magnetized by passing an electric current through it.
Particle Accumulation
The magnetic particles are attracted to areas of magnetic flux leakage caused by surface or near-surface defects. They accumulate and form visible indications, creating a contrasting pattern against the background surface.
Particle Application
Magnetic particles, either dry or suspended in a liquid carrier, are applied to the surface of the magnetized material. These particles are typically iron-based and have magnetic properties.
Inspection and Evaluation
The inspector examines the surface of the material under suitable lighting conditions, such as using a black light for fluorescent particles or white light for visible particles. The accumulated particles create indications that help identify and assess the location, size, and nature of the flaws.
Applications
Weld Inspection
Magnetic particle flaw detection is extensively used for the inspection of welds in industries such as construction, manufacturing, and pipelines. It helps detect surface-breaking defects like cracks, lack of fusion, and incomplete penetration in welded joints.
Casting and Forging Inspection
Magnetic particle flaw detectors are employed to inspect castings, forgings, and other manufactured components made from ferromagnetic materials. They help identify defects such as shrinkage, cracks, laps, and porosity.
Power Generation
Magnetic particle flaw detectors are employed in the power generation sector for inspecting critical components in power plants such as turbines, generators, and transformers. It aids in detecting surface defects and ensuring the reliability of these components.
Aerospace Industry
Magnetic particle flaw detection is crucial in the aerospace industry for inspecting components like landing gear, turbine blades, and aircraft structures. It helps detect surface cracks and other defects that could compromise safety and performance.
Automotive Industry
Magnetic particle inspection is used in the automotive industry to detect defects in critical components such as engine blocks, crankshafts, and gears. It ensures the integrity and reliability of these components.
Oil and Gas Industry
Magnetic particle inspection is used in the oil and gas industry for the inspection of pipelines, storage tanks, and pressure vessels. It helps identify surface defects and corrosion, ensuring the integrity and safety of these assets.
Advantages and Limitations
Advantages:
- Effective in detecting surface and near-surface defects in ferromagnetic materials.
- Relatively simple and cost-effective inspection method.
- Rapid inspection process, providing immediate results.
- Offers visual indications that aid in defect identification and evaluation.
- Can be applied to both ferromagnetic materials and ferromagnetic coatings.
Limitations:
- Limited to ferromagnetic materials, such as iron and steel.
- Surface preparation is crucial for accurate inspection results.
- Accessibility limitations in complex geometries or areas with restricted access.
- Requires proper lighting conditions and trained inspectors for accurate interpretation.
- The size and shape of defects may influence the sensitivity of detection.
Ultrasonic flaw detectors utilize the principle of ultrasonic waves, which are high-frequency sound waves above the range of human hearing (typically above 20 kHz). These waves are generated by transducers and are sent into the material being inspected. The waves propagate through the material, and when they encounter a boundary or defect, such as a crack, void, or inclusion, a portion of the wave is reflected back to the transducer.
Main components
Transducer
The transducer generates ultrasonic waves and also serves as a receiver to detect the reflected waves. It converts electrical energy into ultrasonic waves and vice versa.
Pulser/Receiver
The pulser generates a high-voltage electrical pulse that excites the transducer to emit ultrasonic waves. The receiver amplifies and processes the signals received by the transducer after they have interacted with the material.
Display and Controls
The flaw detector features a display screen that shows the ultrasonic waveform and any indications of flaws. It also includes controls and settings for adjusting the inspection parameters, such as gain, timebase, and frequency.
Data Storage and Analysis
Many modern ultrasonic flaw detectors have built-in memory for storing inspection data. They may also offer data analysis capabilities, such as signal processing, A-scan display, B-scan imaging, and data reporting.
Main Operation
Calibration
The instrument is calibrated using reference standards or test blocks to ensure accurate measurement and interpretation of signals.
Scanning
The transducer is moved along the surface of the material, or a phased array transducer may be used to electronically steer the beam. Ultrasonic waves are emitted and received, and the reflected signals are displayed as a waveform on the screen.
Coupling
A coupling medium, such as a gel or water, is applied between the transducer and the material being inspected. This medium helps to transmit the ultrasonic waves from the transducer into the material.
Analysis
The inspector interprets the ultrasonic waveform to identify indications of flaws, such as echoes or signal reflections from defects within the material. The characteristics of the indications, such as amplitude, time of flight, and shape, are analyzed to determine the size, location, and nature of the flaws.
Applications
Weld Inspection
Ultrasonic flaw detectors are extensively used for weld inspection in industries such as manufacturing, construction, and pipelines. They can detect internal and surface defects in welds, including cracks, lack of fusion, porosity, and incomplete penetration.
Material Characterization
Ultrasonic testing helps in assessing the properties of materials, such as grain structure, elasticity, and anisotropy. It is used for determining material composition, identifying alloys, and evaluating the integrity of materials.
Thickness Measurement
Ultrasonic flaw detectors are employed for non-destructive thickness measurement in materials. They can determine the thickness of pipes, plates, tanks, and other structures, making them useful for monitoring corrosion and assessing structural integrity.
Aerospace Industry
Ultrasonic testing is crucial in the aerospace industry for inspecting critical components like aircraft wings, turbine blades, and engine parts. It ensures the detection of defects, such as cracks or delaminations, that could compromise the safety and performance of aircraft.
Automotive Industry
Ultrasonic flaw detectors are used in the automotive industry to inspect components like engine blocks, transmission parts, and suspension systems. They help identify defects, such as voids or inclusions, that could affect the performance and reliability of automotive parts.
Oil and Gas Industry
Ultrasonic flaw detectors play a vital role in the oil and gas industry for inspecting pipelines, storage tanks, and pressure vessels. They help detect defects such as corrosion, pitting, and weld anomalies, ensuring the integrity and safety of these critical assets.
Advantages and Limitations
Advantages:
- Allows for non-destructive examination of materials and structures
- Can detect internal and surface defects
- Provides real-time results for immediate evaluation
- Helps prevent failures, accidents, or material breakdowns
- Supports quality control and assurance processes
Limitations:
- Requires trained and skilled operators for accurate interpretation
- Different flaw detection methods are suitable for specific materials and defect types
- Accessibility limitations may arise for certain inspection areas or complex geometries
- Some methods, such as radiographic testing, involve radiation safety considerations
Eddy current flaw detectors are instruments used for non-destructive testing (NDT) to detect and evaluate surface and near-surface flaws in conductive materials. Eddy current testing relies on the principle of electromagnetic induction and the interaction between alternating current and the material under inspection. Here's an overview of eddy current flaw detectors:
Principle
Eddy current testing involves inducing small, localized electrical currents known as eddy currents in a conductive material. These eddy currents generate their own magnetic fields, which interact with the material's electromagnetic properties. Any variations in the material's conductivity, magnetic permeability, or geometric features can cause changes in the eddy currents, thus producing detectable signals that can be used to identify surface or near-surface flaws.
Working Mechanism
Coil Excitation
An alternating current is passed through a coil or probe, which generates a changing magnetic field.
Interaction with Material
The eddy currents interact with the material's properties, such as electrical conductivity and magnetic permeability, as well as any defects or variations present in the material.
Inspection and Evaluation
The inspector scans the probe over the surface of the material, examining the signals on the screen for indications of flaws. The characteristics of the signals, such as amplitude, phase, frequency, and signal response, are analyzed to determine the presence, location, size, and nature of the flaws.
Induction of Eddy Currents
The changing magnetic field induces eddy currents in the conductive material being inspected. The eddy currents circulate within the material, creating their own magnetic fields.
Detection and Analysis
The eddy current flaw detector measures and analyzes changes in the electrical impedance or phase shift of the coil caused by the interaction with the material. These changes are displayed as signals on the instrument's screen and can be interpreted to identify and characterize surface defects, such as cracks, corrosion, or variations in thickness.
Applications
Manufacturing Quality Control
Eddy current testing is utilized in manufacturing processes to ensure product quality and identify surface defects in a wide range of materials, such as metal parts, tubes, wires, and coatings.
Tube and Pipe Inspection
Eddy current flaw detectors are commonly used to inspect heat exchanger tubes, boiler tubes, and pipes for defects like cracks, pitting, corrosion, and wall thickness variations.
Non-Ferromagnetic Material Inspection
Eddy current testing is particularly suitable for inspecting non-ferromagnetic materials, such as aluminum, copper, titanium, and their alloys, due to their high electrical conductivity.
Aerospace Industry
Eddy current testing is commonly used for inspecting aircraft components, such as airframes, engine parts, and landing gear, to detect cracks, corrosion, and material degradation.
Automotive Industry
Eddy current flaw detectors are employed to inspect automotive components, including engine parts, gears, and suspension systems, for surface cracks, defects, and material inconsistencies.
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Conductive Coating Thickness Measurement
Eddy current flaw detectors can be used to measure the thickness of conductive coatings, such as paint, plating, or anodizing, on metallic substrates.
Advantages and Limitations
Advantages:
- Fast and efficient inspection technique that provides immediate results.
- Can detect surface and near-surface defects in conductive materials.
- Suitable for inspecting non-ferromagnetic materials and thin conductive layers.
- Portable and handheld instruments are available for on-site inspections.
- Can be integrated into automated inspection systems for high-volume production environments.
Limitations:
- Limited inspection depth compared to other techniques like ultrasonic testing.
- Surface preparation and probe positioning are crucial for accurate results.
- Sensitivity to the orientation and alignment of flaws with respect to the probe.
- Limited ability to detect subsurface defects or significant variations in thickness.
Radiographic flaw detectors, also known as radiographic testing or industrial radiography equipment, are used for non-destructive testing (NDT) to detect and evaluate internal flaws in materials using X-rays or gamma rays. Radiographic flaw detectors produce an image of the object being inspected, allowing for the identification and characterization of defects.
Principle
Radiographic flaw detectors work on the principle of attenuation, where X-rays or gamma rays pass through the material being inspected, and the resulting radiation is captured on a film or digital detector. The intensity of the radiation reaching the detector is influenced by the material's density, thickness, and the presence of flaws. Defects in the material attenuate or scatter the radiation differently, creating variations in the recorded image.
Working Mechanism
Radiation Source
Radiographic flaw detectors use a radioactive source that emits X-rays (X-ray radiography) or gamma rays (gamma radiography). Common radioactive isotopes used include iridium-192, cobalt-60, and selenium-75.
Image Formation
A detector, such as a photographic film or a digital sensor, is placed on the opposite side of the material. The transmitted radiation exposes the detector, creating an image that captures the variations in radiation intensity caused by the material and any internal defects.
Exposure
The radioactive source is positioned outside the material being inspected, and the radiation is directed towards the object. The radiation passes through the material, and the intensity of the transmitted radiation is influenced by the material's density, thickness, and the presence of any internal flaws.
Processing and Interpretation
The exposed film is developed, or the digital image is processed, to enhance the visibility of defects. The resulting radiographic image is examined by a trained inspector who interprets the image to identify and evaluate the size, shape, and location of internal flaws.
Applications
Weld Inspection
Radiographic testing is commonly used for inspecting welds in industries such as construction, manufacturing, and pipelines. It can detect internal defects such as lack of fusion, incomplete penetration, cracks, and porosity.
Casting and Forging Inspection
Radiography is employed to inspect castings, forgings, and other manufactured components for internal defects such as shrinkage, porosity, inclusions, and cracks.
Power Generation
Radiography is employed in the power generation sector for inspecting components such as boilers, turbines, and heat exchangers. It helps identify internal defects and degradation that may impact performance and safety.
Aerospace Industry
Ultrasonic testing is crucial in the aerospace industry for inspecting critical components like aircraft wings, turbine blades, and engine parts. It ensures the detection of defects, such as cracks or delaminations, that could compromise the safety and performance of aircraft.
Structural Inspections
Radiographic flaw detectors are used in structural inspections of buildings, bridges, and other infrastructure. They aid in identifying internal defects, such as corrosion, voids, and material degradation.
Oil and Gas Industry
Ultrasonic flaw detectors play a vital role in the oil and gas industry for inspecting pipelines, storage tanks, and pressure vessels. They help detect defects such as corrosion, pitting, and weld anomalies, ensuring the integrity and safety of these critical assets.
Advantages and Limitations
Advantages:
- Ability to detect internal flaws and provide detailed imaging of the inspected object.
- Effective for inspecting a wide range of materials, including metals, composites, and some plastics.
- Provides permanent records (film or digital images) for documentation and further analysis.
- Can detect both surface and subsurface defects, depending on the material's thickness and radiation energy.
Limitations:
- Potential health and safety hazards due to the use of ionizing radiation.
- Requires specialized training and expertise to interpret radiographic images accurately.
- Slower inspection process compared to some other NDT methods.
- Limited portability and accessibility in some inspection scenarios.
Table of Advantages and Limitations
Here's the table comparing the advantages and disadvantages of the four methods (Eddy Current Testing, Ultrasonic Testing, Magnetic Particle Testing, and Radiographic Testing):
Testing MethodAdvantagesDisadvantagesEddy Current Testing1. Detects surface and near-surface defects in conductive materials.<br>2. Fast and efficient.<br>3. Suitable for non-ferromagnetic materials.<br>4. Can measure coating thickness.1. Limited inspection depth.<br>2. Requires precise positioning and alignment between the defect and the probe.<br>3. Cannot detect sub-surface defects or defects with significant thickness variations.Ultrasonic Testing1. Can detect internal defects and changes in materials.<br>2. Can measure defect size and location.<br>3. Applicable to most material types.<br>4. Provides greater inspection depth.1. Requires trained operators for interpretation and analysis.<br>2. Inspection results are influenced by material sound velocity and geometric shape.<br>3. Requires high surface finish and cleanliness of the material.Magnetic Particle Testing1. Can detect surface and near-surface defects in magnetic materials.<br>2. Fast and relatively simple.<br>3. Suitable for complex shapes and large-sized components.<br>4. Provides visual indication of defects.1. Limited to magnetic materials.<br>2. Requires direct contact with the surface of the tested object.<br>3. Requires high cleanliness and magnetization of the material.<br>4. Cannot detect sub-surface defects.Radiographic Testing1. Can detect internal defects and changes in materials.<br>2. Provides detailed imaging for defect assessment.<br>3. Applicable to a wide range of material types.<br>4. Can detect both surface and sub-surface defects.1. Requires trained operators for safe operation.<br>2. Potential radiation hazards necessitate strict safety measures.<br>3. Slower inspection process.<br>4. Limited accessibility in some cases.WHAT IS THE BEST NDT INSPECTION METHOD?
There is no one-size-fits-all answer to determine the 'best' flaw detector as the choice depends on several factors such as the specific application, material being inspected, the type of defects to be detected, inspection requirements, and available resources. Each flaw detection method has its advantages and limitations.
Eddy Current Testing: It is suitable for detecting surface and near-surface defects in conductive materials. It is commonly used for heat exchangers, tubing, and conductivity measurements. It offers fast inspection and can detect small defects. However, it may not be suitable for inspecting non-conductive materials or detecting sub-surface defects.
Ultrasonic Testing: It is versatile and widely used for detecting internal defects in a wide range of materials. It provides accurate defect sizing and localization. Ultrasonic testing is commonly used in weld inspections, aerospace, and critical component inspections. However, it requires trained operators, and surface preparation is crucial for optimal results.
Magnetic Particle Testing: It is effective for surface and near-surface defect detection in ferromagnetic materials. It is commonly used in industries like manufacturing, automotive, and aerospace. Magnetic particle testing is relatively simple and provides immediate visual indications of defects. However, it is limited to magnetic materials and requires direct surface contact.
Radiographic Testing: It provides detailed imaging of internal defects and is suitable for a wide range of materials. It is commonly used in weld inspections, castings, and infrastructure inspections. Radiographic testing can detect both surface and sub-surface defects. However, it requires trained operators, strict safety protocols due to radiation hazards, and can be a slower inspection process.
The selection of the best flaw detector depends on the specific requirements of your application. It is often beneficial to consult with experts or NDT professionals who can evaluate your needs and recommend the most suitable method based on their expertise and experience.
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An Introduction to Ultrasonic Flaw Detection
1. Basic Theory: Sound waves are simply organized mechanical vibrations traveling through a medium, which may be a solid, a liquid, or a gas. These waves will travel through a given medium at a specific speed or velocity, in a predictable direction, and when they encounter a boundary with a different medium they will be reflected or transmitted according to simple rules. This is the principle of physics that underlies ultrasonic flaw detection.
Frequency: All sound waves oscillate at a specific frequency, or number of vibrations or cycles per second, which we experience as pitch in the familiar range of audible sound. Human hearing extends to a maximum frequency of about 20,000 cycles per second (20 KHz), while the majority of ultrasonic flaw detection applications utilize frequencies between 500,000 and 10,000,000 cycles per second (500 KHz to 10 MHz). At frequencies in the megahertz range, sound energy does not travel efficiently through air or other gasses, but it travels freely through most liquids and common engineering materials.
Velocity: The speed of a sound wave varies depending on the medium through which it is traveling, affected by the medium's density and elastic properties. Different types of sound waves (see Modes of Propagation, below) will travel at different velocities.
Wavelength: Any type of wave will have an associated wavelength, which is the distance between any two corresponding points in the wave cycle as it travels through a medium. Wavelength is related to frequency and velocity by the simple equation
λ = c/f
where
λ = wavelength
c = sound velocity
f = frequency
Wavelength is a limiting factor that controls the amount of information that can be derived from the behavior of a wave. In ultrasonic flaw detection, the generally accepted lower limit of detection for a small flaw is one-half wavelength. Anything smaller than that will be invisible. In ultrasonic thickness gaging, the theoretical minimum measurable thickness one wavelength.
Modes of Propagation: Sound waves in solids can exist in various modes of propagation that are defined by the type of motion involved. Longitudinal waves and shear waves are the most common modes employed in ultrasonic flaw detection. Surface waves and plate waves are also used on occasion.
- A longitudinal or compressional wave is characterized by particle motion in the same direction as wave propagation, as from a piston source. Audible sound exists as longitudinal waves.
- A shear or transverse wave is characterized by particle motion perpendicular to the direction of wave propagation.
- A surface or Rayleigh wave has an elliptical particle motion and it travels across the surface of a material, penetrating to a depth of approximately one wavelength.
- A plate or Lamb wave is a complex mode of vibration in thin plates where material thickness is less than one wavelength and the wave fills the entire cross-section of the medium.
Sound waves may be converted from one form to another. Most commonly, shear waves are generated in a test material by introducing longitudinal waves at a selected angle. This is discussion further under Angle Beam Testing in Section 4.
Variables Limiting Transmission of Sound Waves: The distance that a wave of a given frequency and energy level will travel depends on the material through which it is traveling. As a general rule, materials that are hard and homogeneous will transmit sound waves more efficiently than those that are soft and heterogeneous or granular. Three factors govern the distance a sound wave will travel in a given medium: beam spreading, attenuation, and scattering. As the beam travels, the leading edge becomes wider, the energy associated with the wave is spread over a larger area, and eventually the energy dissipates. Attenuation is energy loss associated with sound transmission through a medium, essentially the degree to which energy is absorbed as the wave front moves forward. Scattering is random reflection of sound energy from grain boundaries and similar microstructure. As frequency goes down, beam spreading increases but the effects of attenuation and scattering are reduced. For a given application, transducer frequency should be selected to optimize these variables.
Reflection at a Boundary: When sound energy traveling through a material encounters a boundary with another material, a portion of the energy will be reflected back and a portion will be transmitted through. The amount of energy reflected, or reflection coefficient, is related to the relative acoustic impedance of the two materials. Acoustic impedance in turn is a material property defined as density multiplied by the speed of sound in a given material. For any two materials, the reflection coefficient as a percentage of incident energy pressure may be calculated through the formula
2. Ultrasonic Transducers
In the broadest sense, a transducer is a device that converts energy from one form to another. Ultrasonic transducers convert electrical energy into high frequency sound energy and vice versa.
Cross section of typical contact transducer
Typical transducers for ultrasonic flaw detection utilize an active element made of a piezoelectric ceramic, composite, or polymer. When this element is excited by a high voltage electrical pulse, it vibrates across a specific spectrum of frequencies and generates a burst of sound waves. When it is vibrated by an incoming sound wave, it generates an electrical pulse. The front surface of the element is usually covered by a wear plate that protects it from damage, and the back surface is bonded to backing material that mechanically dampens vibrations once the sound generation process is complete. Because sound energy at ultrasonic frequencies does not travel efficiently through gasses, a thin layer of coupling liquid or gel is normally used between the transducer and the test piece.
There are five types of ultrasonic transducers commonly used in flaw detection applications:
- Contact Transducers -- As the name implies, contact transducers are used in direct contact with the test piece. They introduce sound energy perpendicular to the surface, and are typically used for locating voids, porosity, and cracks or delaminations parallel to the outside surface of a part, as well as for measuring thickness.
- Angle Beam Transducers -- Angle beam transducers are used in conjunction with plastic or epoxy wedges (angle beams) to introduce shear waves or longitudinal waves into a test piece at a designated angle with respect to the surface. They are commonly used in weld inspection.
- Delay Line Transducers - Delay line transducers incorporate a short plastic waveguide or delay line between the active element and the test piece. They are used to improve near surface resolution and also in high temperature testing, where the delay line protects the active element from thermal damage.
- Immersion Transducers - Immersion transducers are designed to couple sound energy into the test piece through a water column or water bath. They are used in automated scanning applications and also in situations where a sharply focused beam is needed to improve flaw resolution.
- Dual Element Transducers - Dual element transducers utilize separate transmitter and receiver elements in a single assembly. They are often used in applications involving rough surfaces, coarse grained materials, detection of pitting or porosity, and they offer good high temperature tolerance as well.
Further details on the advantages of various transducer types, as well as the range of frequencies and diameters offered, may be found in the transducer section of our web site.
3. Ultrasonic Flaw Detectors
Modern ultrasonic flaw detectors such as the EPOCH series are small, portable, microprocessor-based instruments suitable for both shop and field use. They generate and display an ultrasonic waveform that is interpreted by a trained operator, often with the aid of analysis software, to locate and categorize flaws in test pieces. They will typically include an ultrasonic pulser/receiver, hardware and software for signal capture and analysis, a waveform display, and a data logging module. While some analog-based flaw detectors are still manufactured, most contemporary instruments use digital signal processing for improved stability and precision.
The pulser/receiver section is the ultrasonic front end of the flaw detector. It provides an excitation pulse to drive the transducer, and amplification and filtering for the returning echoes. Pulse amplitude, shape, and damping can be controlled to optimize transducer performance, and receiver gain and bandwidth can be adjusted to optimize signal-to-noise ratios.
Modern flaw detectors typically capture a waveform digitally and then perform various measurement and analysis function on it. A clock or timer will be used to synchronize transducer pulses and provide distance calibration. Signal processing may be as simple as generation of a waveform display that shows signal amplitude versus time on a calibrated scale, or as complex as sophisticated digital processing algorithms that incorporate distance/amplitude correction and trigonometric calculations for angled sound paths. Alarm gates are often employed to monitor signal levels at selected points in the wave train to flag echoes from flaws.
The display may be a CRT, a liquid crystal, or an electroluminescent display. The screen will typically be calibrated in units of depth or distance. Multicolor displays can be used to provide interpretive assistance.
Internal data loggers can be used to record full waveform and setup information associated with each test, if required for documentation purposes, or selected information like echo amplitude, depth or distance readings, or presence or absence of alarm conditions.
4. Procedure
Ultrasonic flaw detection is basically a comparative technique. Using appropriate reference standards along with a knowledge of sound wave propagation and generally accepted test procedures, a trained operator identifies specific echo patterns corresponding to the echo response from good parts and from representative flaws. The echo pattern from an test piece may then be compared to the patterns from these calibration standards to determine its condition.
- Straight Beam Testing -- Straight beam testing utilizing contact, delay line, dual element, or immersion transducers is generally employed to find cracks or delaminations parallel to the surface of the test piece, as well as voids and porosity. It utilizes the basic principle that sound energy traveling through a medium will continue to propagate until it either disperses or reflects off a boundary with another material, such as the air surrounding a far wall or found inside a crack. In this type of test, the operator couples the transducer to the test piece and locates the echo returning from the far wall of the test piece, and then looks for any echoes that arrive ahead of that backwall echo, discounting grain scatter noise if present. An acoustically significant echo that precedes the backwall echo implies the presence of a laminar crack or void. Through further analysis, the depth, size, and shape of the structure producing the reflection can be determined.
Sound energy will travel to the far side of a part, but reflect earlier if a laminar crack or similar discontinuity is presented.
In some specialized cases, testing is performed in a through transmission mode, where sound energy travels between two transducers placed on opposite sides of the test piece. If a large flaw is present in the sound path, the beam will be obstructed and the sound pulse will not reach the receiver.
- Angle Beam Testing - Cracks or other discontinuities perpendicular to the surface of a test piece, or tilted with respect to that surface, are usually invisible with straight beam test techniques because of their orientation with respect to the sound beam. Such defects can occur in welds, in structural metal parts, and many other critical components. To find them, angle beam techniques are used, employing either common angle beam (wedge) transducer assemblies or immersion transducers aligned so as to direct sound energy into the test piece at a selected angle. The use of angle beam testing is especially common in weld inspection.
Typical angle beam assemblies make use of mode conversion and Snell's Law to generate a shear wave at a selected angle (most commonly 30, 45, 60, or 70 degrees) in the test piece. As the angle of an incident longitudinal wave with respect to a surface increases, an increasing portion of the sound energy is converted to a shear wave in the second material, and if the angle is high enough, all of the energy in the second material will be in the form of shear waves. There are two advantages to designing common angle beams to take advantage of this mode conversion phenomenon. First, energy transfer is more efficient at the incident angles that generate shear waves in steel and similar materials. Second, minimum flaw size resolution is improved through the use of shear waves, since at a given frequency, the wavelength of a shear wave is approximately 60% the wavelength of a comparable longitudinal wave.
Typical angle beam assembly
The angled sound beam is highly sensitive to cracks perpendicular to the far surface of the test piece (first leg test) or, after bouncing off the far side, to cracks perpendicular to the coupling surface (second leg test). A variety of specific beam angles and probe positions are used to accommodate different part geometries and flaw types, and these are described in detail in appropriate inspection codes and procedures such as ASTM E-164 and the AWS Structural Welding Code.
Complete list of Olympus Flaw Detection Application notes.˘
Print
American Society for Nondestructive Testing, Nondestructive Testing Handbook, Volume 7, Ultrasonic Testing
ASM International, Metals Handbook, Volume 17, Nondestructive Evaluation and Quality Control
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