AME Unit 3 Q3

Cracks in Metals

A crack in a metal is a fracture or discontinuity that develops within the material. It represents a separation of the metal’s atomic structure, disrupting its continuity.  

Types of Cracks:

• Surface cracks: These are visible on the metal’s surface.   1. Root Cause Analysis of Surface Cracks in Heavy Steel Plates during the Hot Rolling Process – MDPI www.mdpi.com
• Subsurface cracks: These cracks initiate below the surface and can be difficult to detect.
• Through-thickness cracks: These cracks extend completely through the material.

Causes of Cracks:

• Fatigue: Repeated loading and unloading can lead to microscopic cracks that grow over time.   1. AF lab investigating microscopic crack formations, metal fatigue > Air Force > Article Display www.af.mil
• Stress corrosion cracking: A combination of tensile stress and a corrosive environment causes crack initiation and growth.   1. Corrosion Product Film-Induced Stress Facilitates Stress Corrosion Cracking – Nature www.nature.com
• Hydrogen embrittlement: The presence of hydrogen in the metal can lead to crack formation and embrittlement.   1. Hydrogen at cracks | Max-Planck-Institut für Eisenforschung GmbH www.mpie.de
• Corrosion fatigue: The combined action of fatigue and corrosion accelerates crack growth.
• Overloading: Exceeding the metal’s yield strength can cause sudden fracture.
• Manufacturing defects: Inclusions, porosity, or voids in the metal can act as stress concentrators and initiate cracks.

Consequences of Cracks:

• Reduced strength: Cracks weaken the material, increasing the risk of failure.   1. Why do cracks weaken a material? – DoITPoMS www.doitpoms.ac.uk
• Fatigue failure: Cracks can propagate under cyclic loading, leading to catastrophic failure.   1. Prediction of Fatigue Crack Initiation under Variable Amplitude Loading: Literature Review www.mdpi.com
• Corrosion acceleration: Cracks provide sites for corrosion to initiate and progress.
• Leakage: In pressure vessels or pipes, cracks can lead to leakage of fluids.   1. Pressure Vessel Safety: Leak Before Burst – IQS Newsroom blog.iqsdirectory.com

Understanding the causes and types of cracks is essential for preventing failures and ensuring the integrity of metal components.

Crack detection in metals is crucial for ensuring the integrity and safety of structures and components. Various non-destructive testing (NDT) methods are employed to identify cracks without compromising the material.  

Common Crack Detection Methods:

• Visual Inspection: This is the simplest method, but it’s limited to surface cracks. It involves careful examination of the component for any visible signs of cracks, such as surface breaks or discoloration.   1. What’s the Best Method for Detecting Surface Cracks? – NDT-KITS ndt-kits.com2. Visual Inspection: Definition & Process – Operations1 operations1.com
• Dye Penetrant Testing (PT): A liquid penetrant is applied to the surface, penetrating any cracks. A developer is then applied to draw the penetrant to the surface, making cracks visible.   1. Liquid Penetrant Testing – DEKRA www.dekra.com2. What are the Differences Between Developers, and Which Should I Use For Penetrant Inspections? – Magnaflux magnaflux.com
• Magnetic Particle Testing (MT): This method is suitable for ferromagnetic materials. A magnetic field is applied to the component, and magnetic particles are used to reveal cracks as they disrupt the magnetic field.   1. Ultimate Guide to Magnetic Particle Inspection (MPI) – Flyability www.flyability.com2. Basic Knowledge of Magnetic Particle Crack Detection – Karl Deutsch www.karldeutsch.de
• Ultrasonic Testing (UT): High-frequency sound waves are transmitted through the material. Echoes from cracks or defects are analyzed to determine their location and size.
• Radiographic Testing (RT): X-rays or gamma rays are used to create images of the internal structure of the material, revealing cracks and other defects.   1. X-rays | ARPANSA www.arpansa.gov.au
• Eddy Current Testing (ET): This method uses electromagnetic induction to detect changes in electrical conductivity caused by cracks.   1. Eddy Current Testing (ECT) Explained » NDT Method | AME – Asset Management Engineers www.asseteng.com.au
• Acoustic Emission (AE): This technique detects and analyzes elastic waves generated by the release of energy during crack growth.   1. Acoustic Emission Analysis (everything You Need To Know) – Samotics samotics.com

Factors Affecting Crack Detection:

• Crack size and location: Small or internal cracks can be challenging to detect.
• Material properties: The material’s conductivity, magnetic properties, and density influence the choice of testing method.
• Component geometry: Complex shapes can hinder inspection accessibility.
• Operator skill and experience: The accuracy of crack detection depends on the inspector’s expertise.

By combining multiple NDT methods, it is possible to achieve a comprehensive assessment of a component’s integrity.

No, crack detection is not solely limited to metals. While many of the techniques mentioned were developed for metal inspection, they can be adapted or modified to detect flaws in other materials.

Crack Detection in Non-Metals:

• Plastics: Dye penetrant testing, ultrasonic testing, and radiography can be used to detect cracks in plastics.
• Ceramics: Visual inspection, ultrasonic testing, and radiography are common methods.
• Composites: Ultrasonic testing, radiography, and thermography are often used.
• Rubber and Elastomers: Visual inspection, dye penetrant testing, and ultrasonic testing can be applied.

Specific Techniques for Non-Metals:

• Thermography: This method involves detecting temperature variations on the surface of a material. Cracks can often be identified by heat dissipation patterns.   1. What Is Thermography? – UpKeep upkeep.com
• Acoustic Emission: Similar to its use in metals, acoustic emission can detect crack growth in non-metallic materials.

The choice of inspection method depends on the material type, the expected size and location of the crack, and the required sensitivity of the detection.

Four Methods for Non-Destructive Crack Detection

1. Visual Inspection (VI): This is the simplest method, involving a thorough visual examination of the component’s surface for any visible cracks, defects, or anomalies.
2. Dye Penetrant Testing (PT): A liquid penetrant is applied to the surface, penetrating any cracks. A developer is then used to draw the penetrant to the surface, making cracks visible.
3. Magnetic Particle Testing (MT): This method is suitable for ferromagnetic materials. A magnetic field is applied to the component, and iron particles are used to reveal cracks as they disrupt the magnetic field.
4. Ultrasonic Testing (UT): High-frequency sound waves are transmitted through the material. Echoes from cracks or defects are analyzed to determine their location and size.

These methods provide different levels of sensitivity and are often used in combination for comprehensive crack detection.

Additional Non-Destructive Crack Detection Methods

Beyond the commonly used methods, there are several other techniques for non-destructive crack detection:

• Radiographic Testing (RT): Uses X-rays or gamma rays to create images of internal structures, revealing cracks and defects.   1. Industrial Radiography | US EPA www.epa.gov
• Eddy Current Testing (ET): Employs electromagnetic induction to detect changes in electrical conductivity caused by cracks.   1. How Eddy Current Testing works – NDT Group www.ndtgroup.co.uk
• Acoustic Emission (AE): Detects and analyzes elastic waves generated by the release of energy during crack growth.   1. The dependence of acoustic emission behavior on prior fatigue loading history – Frontiers www.frontiersin.org
• Thermography: Uses infrared cameras to detect temperature variations on the surface, indicating potential cracks.   1. Non Destructive Testing for cracks – Format NDT www.formatndt.co.uk
• Leak Testing: While primarily for detecting leaks, it can indirectly identify cracks if they allow fluid or gas to escape.

These methods offer varying levels of sensitivity and are often used in combination for comprehensive crack detection.

Destructive Methods for Crack Detection

While non-destructive testing (NDT) is preferred for evaluating the integrity of components, there are instances where destructive testing is necessary to fully understand the nature and extent of a crack.

Destructive testing involves the deliberate damage or destruction of a component to examine its internal structure.  

1. Ultimate Guide to Destructive Testing Methods – Flyability

Common destructive methods for crack detection include:

• Metallography: This involves cutting, mounting, and polishing a sample to examine its microstructure under a microscope. Cracks, inclusions, and other defects can be observed.   1. Metallography – an Introduction | Science Lab – Leica Microsystems www.leica-microsystems.com
• Tensile Test: While primarily used for determining material properties, tensile tests can reveal the presence of cracks if they initiate during the test.
• Fracture Mechanics Testing: This involves applying controlled loads to a component with a pre-existing crack to study crack growth behavior.

It’s important to note that destructive testing is typically used for research, quality control, or failure analysis purposes and is generally not practical for in-service components.

Visual Inspection for Crack Detection

Visual inspection is the most basic and often the first method employed in non-destructive testing (NDT) to detect cracks and other surface defects. It involves a thorough examination of a component using the naked eye or with the aid of optical instruments like magnifying glasses, microscopes, or borescopes.

Procedure:

1. Preparation: Ensure adequate lighting and a clean inspection surface. Remove any dirt, grease, or other contaminants that might obscure defects.
2. Inspection: Carefully examine the component for any visible cracks, discontinuities, or changes in surface appearance. Pay attention to areas with high stress concentrations, such as corners, holes, and welds.
3. Documentation: Record the findings, including the location, size, and type of defect. Use photographs or sketches to document the observations.

Enhancing Visual Inspection:

• Lighting: Use appropriate lighting conditions, including natural light, artificial light, or specialized lighting techniques like shadowing or raking light.
• Magnification: Employ magnifying glasses, microscopes, or borescopes to examine small details.
• Contrast agents: In some cases, applying contrast agents like dyes or penetrants can enhance visibility of defects.
• Inspection aids: Use mirrors, endoscopes, or other tools to access difficult-to-reach areas.

Limitations:

• Surface defects only: Visual inspection is limited to surface defects and cannot detect internal cracks.
• Subjectivity: The effectiveness of visual inspection depends on the inspector’s experience and skill.
• Accessibility: Some areas of a component may be difficult to inspect visually.

Despite its limitations, visual inspection remains an essential first step in any NDT process. It provides a quick and cost-effective way to identify obvious defects and inform the selection of further inspection methods.

Dye Penetrant Testing (PT) Procedure

Dye penetrant testing (PT) is a non-destructive examination (NDE) method used to detect surface-breaking defects in materials. It relies on the principle of capillary action to draw a colored liquid into a defect.  

Steps Involved:

1. Cleaning: The surface to be inspected must be thoroughly cleaned to remove any dirt, oil, grease, or other contaminants that might block the penetrant from entering defects.   1. Prepare for Penetrant Testing Success with the Right Precleaning – Magnaflux magnaflux.com
2. Penetrant Application: The penetrant, which is a low-surface-tension fluid containing a dye, is applied to the surface. It can be applied by spraying, brushing, or dipping, depending on the component’s geometry. The penetrant is allowed to dwell on the surface for a specific time, allowing it to seep into any surface-breaking defects.   1. Liquid Penetrant Testing – DEKRA www.dekra.com2. Dye Penetrant Inspection (DPI) – Intertek www.intertek.com3. Liquid Penetrant Examination – The National Board of Boiler and Pressure Vessel Inspectors www.nationalboard.org
3. Penetrant Removal: Excess penetrant is removed from the surface using a solvent or water-based emulsifier. This step is crucial to prevent false indications.   1. Understanding Emulsifiers and Knowing When to Use Them – Magnaflux magnaflux.com
4. Developer Application: A white, chalky developer is applied to the surface. The developer acts as a blotter, drawing the penetrant out of the defect and making it visible.   1. Ultimate Guide to Dye Penetrant Inspection (DPI) – Flyability www.flyability.com2. Benefits of Using Developer During Liquid Penetrant Inspection [Case Study] – Magnaflux magnaflux.com
5. Inspection: The surface is examined under suitable lighting conditions. Any indications, such as bleed lines or spots, reveal the presence of defects.   1. Dye Penetrant Inspection: a beginner’s guide – NDT Group www.ndtgroup.co.uk2. The Ultimate Guide to Visible Dye Penetrant Testing – Magnaflux magnaflux.com

Types of Dye Penetrant:

• Visible Dye Penetrants: Used under normal lighting conditions.   1. Ultimate Guide to Dye Penetrant Inspection (DPI) – Flyability www.flyability.com
• Fluorescent Dye Penetrants: Require ultraviolet light for inspection, providing higher sensitivity.   1. The Ultimate Guide to Fluorescent Penetrant Testing [Infographic] magnaflux.eu

Additional Considerations:

• Penetrant Dwell Time: The time allowed for the penetrant to enter defects varies depending on the material and defect size.   1. Acceptance Criteria Liquid Penetrant Testing – OnestopNDT www.onestopndt.com
• Developer Type: Dry or wet developers can be used, each with its advantages and limitations.   1. Developer Forms – Nondestructive Evaluation Techniques : Penetrant www.nde-ed.org
• Inspection Time: The time allowed for inspection depends on the type of penetrant and developer used.   1. Dye penetrant inspection – Wikipedia en.wikipedia.org
• Cleaning: Proper cleaning is essential for accurate results.   1. Ultimate Guide to Dye Penetrant Inspection (DPI) – Flyability www.flyability.com

By following these steps and considering the specific characteristics of the material and component, dye penetrant testing can effectively detect surface-breaking defects.

Magnetic Particle Testing (MPT) Procedure

Magnetic Particle Testing (MPT) is a non-destructive testing (NDT) method used to detect surface and subsurface cracks in ferromagnetic materials.

Procedure:

1. Cleaning: The surface of the component to be inspected must be clean and dry to ensure accurate results.
2. Magnetization: A magnetic field is induced in the component. This can be done through several methods:
   • Circular magnetization: Passing an electric current through the component to create a circular magnetic field.
   • Longitudinal magnetization: Passing an electric current through a conductor placed through a hole in the component to create a longitudinal magnetic field.
   • Magnetic particle application: While the magnetic field is still on, fine ferromagnetic particles (dry powder or suspended in a liquid) are applied to the surface of the component.
3. Indication: If a crack is present, it disrupts the magnetic field, creating a leakage field. Magnetic particles are attracted to this leakage field, forming a visible indication of the crack.
4. Inspection: The component is carefully examined for indications, which can be observed under normal or ultraviolet light depending on the type of magnetic particles used.
5. Demagnetization: After inspection, the component should be demagnetized to prevent interference with other equipment or processes.

Types of Magnetic Particle Testing:

• Dry particle method: Magnetic particles are applied in powder form.
• Wet particle method: Magnetic particles are suspended in a liquid carrier.
• Fluorescent particle method: Magnetic particles are coated with a fluorescent dye and viewed under ultraviolet light.

By carefully following these steps and using appropriate techniques, magnetic particle testing can effectively detect surface and subsurface cracks in ferromagnetic materials.

Opens in a new windowwww.nationalboard.org

Ultrasonic Testing (UT) Procedure

Ultrasonic testing is a non-destructive testing (NDT) method used to detect internal flaws and discontinuities in materials. It utilizes high-frequency sound waves to inspect the internal structure of a component.

Procedure:

1. Coupling Medium: A coupling medium, such as water or gel, is applied between the transducer and the test piece to ensure efficient transmission of ultrasonic waves.
2. Transducer Placement: The ultrasonic transducer is placed on the component’s surface. The transducer emits high-frequency sound waves into the material.
3. Sound Wave Propagation: The ultrasonic waves travel through the material and are reflected back when they encounter a discontinuity, such as a crack or inclusion.
4. Signal Reception: The transducer also acts as a receiver, capturing the reflected sound waves.
5. Data Analysis: The received signals are displayed on a screen, where the technician analyzes the echoes to identify defects. The time taken for the sound waves to travel to the defect and back determines the depth of the flaw.

Types of Ultrasonic Testing:

• Pulse-echo: The most common method, where a single transducer is used for both transmitting and receiving ultrasonic waves.
• Through-transmission: Uses two transducers, one for transmitting and one for receiving, to measure the attenuation of the ultrasonic beam through the material.

Advantages of Ultrasonic Testing:

• Can detect both surface and internal flaws.
• Provides accurate measurements of flaw depth and size.
• Can be used on a variety of materials.