image credit @ Zetec
Science and industry have been guiding tools for human progress since history started first getting comprehensively recorded. From the basic tools employed during the stone age to complement hunting and gathering to the intricate machines and manufacturing techniques that evolved during the Industrial Revolution in the late 1700s and early 1800s, an astounding increase in the scale of the machinery is observed, from tools that would complement manual labour to Mechanical systems not only replacing human involvement but with a much higher Output yield of finished goods previously thought possible in all sectors of industry.
The need for finished products and services attributed to the increasing standard of living available due to technological advances brought about during the Industrial Revolution required Elaborate machinery and manufacturing plants along with correspondingly complex infrastructure to accommodate the machinery leading to the growth of mechanized factory systems. Naturally, as the payload of volatile fuels and the size of the machinery augmented, system failure and occupational accidents would lead to catastrophe for both human and economic capital in an unprecedented manner.
The year of 1854 for Hartford, the capital city of the U.S. state of Connecticut, proved to be fatal to the lives of 21 workers and left 50 injured, amongst the 300 employed at Railroad-Locomotive Factory located at Fales and Grey Car works. The fatalities were attributed to an explosion from a newly installed 50-HorsePower Boiler with an operation time of under a month.
In Hartford, Connecticut, The Boiler feeding the machinery at Fales and Grey Car Works Exploded on 2nd March 1854, with several casualties. (Image credits: Connecticut Historical Society)
Also Read, Non-destructive Testing vs Destructive Testing
The investigators concluded that the explosion was caused due to an overload of steam in the boiler because of the negligence of the personnel in charge. In Nearly a decade, Hartford officials made Boiler inspections and rigorous training for Engineers allocated mandatory. The inspections at that time relied on Visual examinations of the machinery without invading the integrity or functionality of its operation in the factory which ensured safety without any obstruction during production and served to be precursors to the modern-day Non-Destructive Inspection techniques of visual examination. The Non-Destructive Testing (NDT Testing) techniques and methodologies in their most contemporary form, are discussed in the successive sections.
What is NDT?
Inspection and Analysis of Engineering Materials and Structures imply an imperative need for a thorough examination of all the properties without invading the structural integrity of the materials as the conventional destructive techniques i.e., Harness testing, and tensile testing to name a few, would require the destruction of test specimens leading to obstruction of functionality in Factory or In-situ settings.
Non-Destructive Testing (NDT/ NDT testing) Techniques or Methodologies allow the investigator to carry out examinations without invading the integrity of the engineering specimen under observation while providing an elaborate view of the surface and structural discontinuities and obstructions. The personnel carrying out these methodologies require specialized NDT Training as they involve handling delicate equipment and subjective interpretation of the NDT inspection/NDT testing results.
The NDE Inspection (Non-destructive Evaluation) methodologies can be used on a myriad of materials to evaluate a wide variety of flaws, Ranging from Superficial to sub-strata level of detection, that could lead to premature structural failures and if left unchecked, catastrophe.
The most common form of Contemporary NDT Inspection and testing methodologies are as follows:
Technique
Capabilities
Limitations
Visual Testing/Inspection (VT)
Detection of macroscopic surface flaws
Difficulty in the detection of small flaws and subsurface flaws.
Microscopy
Detection of small surface flaws
Cannot be used for larger surfaces and subsurface flaws
Radiography Testing (RT)
Detection of subsurface flaws
Requires Radiation Protection; Detectability is accurate for defects larger than 2% of the thickness of the structure; No detections of subsurface flaws; not for porous materials
Dye penetrant Testing (PT)
Detection of surface flaws
No detection of subsurface flaws; not for porous materials
Ultrasonic Testing (UT)
Detection of subsurface flaws
The material must be a good conductor of sound
Magnetic Particle Testing (MT)
Detection of surface / near surface and layer flaws
Limited subsurface capability; Can only be used for Ferro-Magnetic materials
Eddy Current/ Electromagnetic Testing (ET)
Detection of surface and near-surface flaws
Interpretation is difficult in specific applications; Can only be used for metals
Acoustic emission Testing (AE)
Complete structural analysis
Interpretation is difficult in specific applications; Equipment is expensive
Various NDT Techniques along with an overview of their capabilities and limitations (Table credits: itu.edu)
The Non-destructive testing techniques are used during the fabrication and deployment processes of various engineering structures, to ensure the integrity and reliability of the established structures. The Non-Destructive inspection methodologies are employed for in-situ inspections of Engineering structures currently being used in ongoing processes which result in a decrease in costs of production as well as ensure stringent quality control in industrial environments.
THE DIFFERENCE BETWEEN NDT AND DESTRUCTIVE TESTING
Destructive testing of materials, machinery, and structures results in permanent damage to the test subject, whereas in non-destructive testing, the specimen can be used in operation after, and in some methodologies, during the testing process without hindrance. Destructive Testing processes include bending tests, tensile tests, compression tests, corrosion, hardness tests, etc.
The primary differences between destructive testing and NDT testing methods are as follows:
- Destructive testing aids in analyzing the material properties (physical and chemical) of the test subjects.
- Destructive testing is beneficial during the planning and conceptualization of the product, whereas Non-destructive testing helps monitor the state of the subject under operation and recognize early signs of defects to avoid failure.
- Destructive testing is time-consuming and wastes resources, whereas non-destructive testing is quicker and can save resources in the long run.
NDT TESTING METHODS
The methods mentioned in the previous sections offer various NDE inspection capabilities to the investigator. Some of the advantages and limitations of commonly used non-destructive testing methods are as follows:
Visual Inspection (VT):
This non-destructive testing methodology uses sight to establish an inference of the state of the object under test. Additional equipment like scales, microscopes, borescopes, etc. can be used for better-quality test results.
Visual Testing/Inspection Non-Destructive testing setup (Image credits: IJRESM Volume-2, Issue-2, Feb 2019)
Advantages of Visual Inspection:
- Does not require complex equipment and testing can be carried out rapidly.
- Does not require an elaborate skill set.
- Cost-effective
Disadvantages of Visual Inspection:
- Adequate experience and understanding of the test subject to infer sufficient data.
- It can only detect surface defects.
- It is a superficial testing method and operators may misjudge a defect as they cannot perceive the test subject beyond its surface.
Liquid Penetrant Testing (PT):
In this method, a dye or liquid indicator is applied on the surface of a subject under test, which is wiped off after a predetermined dwelling period. The dye penetrates the defects and deformities and enhances the visibility of the defects. Surface cracks and porosities can be efficiently detected using this method and it is mostly used in assessing the quality of welds.
Liquid Penetrant Testing Setup (Image Credit: Insight NDT)
Advantages of Liquid Penetrant Testing:
- This method is extremely simplistic and is easy to train operators to perform.
- The equipment is uncomplicated and affordable.
Disadvantages of Liquid Penetrant Testing:
- This process poses challenges for automation as it has multiple steps.
- Requires a specific frequency of white light or ultraviolet light to perceive the defects marked by the dye.
- The surface is required to be meticulously cleaned of dirt, debris, or grime.
Magnetic Particle Testing (MT):
This method uses magnetic flux induced on the test subject, where points of defects and deformities will display flux leakage. Magnetic particles such as iron filings and powder are sprinkled on the surface, which gather around these points of flux leakage and provide a visual indication of defects. This method can be used for the detection of cracks and defects on the surface of a test subject. The object under test is required to be ferrous in nature, which includes materials like nickel, iron, cobalt, etc.
Magnetic particle testing for detection of a crack in the specimen (Image credits: Insight NDT)
Advantages of Magnetic Particle Testing:
- The test process is not time-consuming.
- The process can be modified to be automated.
- No preparation of the surface of the test subject is required.
- The magnetizing probes are affordable.
Disadvantages of Magnetic Particle Testing:
- Components need to be demagnetized after testing to ensure smooth operation.
- Testing of the subject twice is recommended to ensure correct results.
- It is hard to detect diagonal defects using this method.
Ultrasonic Testing (UT):
This method uses ultrasonic waves induced on the surface of an object under test to obtain data on the structural integrity and presence of defects. This is one of the most widely used testing mechanisms in industries and large-scale manufacturing units.
This non-destructive testing method uses transducers in a probe that is held in contact with the workpiece to initiate the incidence of the ultrasonic waves on the test subject. The ultrasonic waves, when in contact with the defect, change their behaviour and that is reflected into the receiver, which gives the operator data on the defects and deformities.
Ultrasonic testing using a normal probe (Left) and an angle probe (right) (Image credits: Insight NDT)
Advantages of Ultrasonic Testing:
- This method provides deep penetration of ultrasonic waves, hence providing greater accuracy on resulting data.
- The testing apparatus is portable and lightweight.
- This method can be used to measure the thickness of a test subject.
- Access to only one side of the test subject is required to conduct Ultrasonic Testing.
- Test process is faster compared to many other NDT methods.
Also Read, Advanced Ultrasonic Testing Methods
Disadvantages of Ultrasonic Testing:
- A medium for coupling is required to conduct the test.
- Test apparatus can be expensive.
- Operators need to be experienced and skilled to perform this test.
- The presence of paint and rust can hinder the test results.
- It is difficult to test small-sized parts for defects.
Eddy Current Inspection (ET):
This method uses a coil carrying alternating current placed next to the object under testing, creating a variable magnetic field that generates eddy currents in the test subject. This method relies heavily on magnetism and the principle of electromagnetic induction.
Eddy current testing helps detect cracks, porosities, discontinuities (such as voids and inclusions), and pitting. It can also be used to measure the thickness of a material.
Eddy's current Inspection Setup with different winding configurations for crack detection (Image credits: Insight NDT)
Advantages of Eddy Current Inspection:
- This testing method is not time-consuming.
- Almost no surface preparation is required.
- This process is easy to automate.
- Operators do not need to be highly skilled.
- This is a no-contact process.
Disadvantages of Eddy Current Inspection:
- The sensitivity of the test results is highly dependent on the size of the eddy current probe.
- Complex geometries cannot be analyzed using this process.
Further Reading, Top 7 Eddy Current Testing Companies You Must Know
- The testing process can only be used to detect shallow defects as it does not penetrate deeply.
Radiographic Inspection (RT):
This method uses radiographic waves like X-rays and Gamma rays that are highly penetrating in nature to get an in-depth analysis of the state of the test subject. Unlike other testing methods, the accuracy of testing increases with the increase of thickness or density of the test material.
This process can be extremely hazardous to the operators in the long run because of its penetrative nature and elaborate safety measures need to be taken to ensure the safety of the workers. Radiographic inspection can be used to detect cracks, hidden defects, porosity, voids, inclusions, and weld defects.
Radiography used in crack detection (Image credits: Insight NDT)
Advantages of Radiographic Inspection:
- Radiographic testing equipment is portable.
- This testing process gives permanent results if films are used to record data, and this can be digitally recorded for future reference.
- It can be used to assess larger surface areas simultaneously.
Disadvantages of Radiographic Inspection:
- Access to opposing sides of the test subject is imperative.
- Gamma radiation is extremely penetrative and dangerous and special measures should be taken to store the source of the radiation.
- Operators need to be highly trained and experienced.
Also Read, Types Of Radiography Testing
APPLICATIONS OF NDT
Non-destructive testing (NDT Testing) is a flexible process and is widely used in many fields, like aerospace, manufacturing, petrochemical, automotive, and construction industries. Some common applications of non-destructive testing are:
Automotive Industry:
To ensure quality and uniformity of mass-produced vehicle components, to assure the operability of large-sized vehicles, to check if vehicles can continue operation after being under load for extensive periods, etc.
Aerospace industry:
Non-destructive testing methods are used in space operations and manufacturing equipment for quality control and safety checks.
Manufacturing:
Mass-produced products and components can be efficiently tested for quality and performance in the production line using NDT techniques.
Medicine:
Medical equipment can be quality-checked using NDT techniques.
Power generation:
Large turbines, welds, and machinery in power generation industries can be checked for defects to ensure unobstructed operation.
Petrochemical industries:
Pipelines, Tanks, and drums carrying hazardous material need to be checked for cracks, leaks, and defects to ensure perfect sealing and load-bearing abilities, and to avoid hazardous material-related mishaps.
ADVANTAGES AND DISADVANTAGES OF NDT
Non-destructive testing is an extremely viable option for most modern industries as it enables economical processes and production. Some of the advantages of Non-destructive testing are as follows:
- Non-destructive testing equipment is often portable, accessible, and easy to handle and operate.
- Data handling in NDT processes is convenient and processes can easily undergo multiple iterations.
- Testing processes are not time-consuming and give accurate results.
- The components can resume normal operation post-testing.
- Most non-destructive processes are easy to operate.
- Non-destructive testing is economical in the long term as it encourages the early detection of defects followed by reworking or restoration.
Like many other technologies, non-destructive testing methods come with a set of limitations, some of which are:
- The initial investment for most equipment is very high.
- Some defects may go undetected if their size or shape does not meet acceptance criteria.
- Some test methods involve the use of electricity or penetrative electromagnetic waves that can be hazardous to operators.
- Some tests can only be performed on conductive or metallic materials.
- Certain testing methods cannot be performed on test subjects with complex geometries.
HOW DRONES CAN HELP WITH NDT
Unmanned aerial vehicles were first developed and used during the First World War. The missions carried out by these UAVs were often cumbersome, hazardous, or unhygienic for human operatives.
Contemporary Unmanned Ariel Dynamic Remotely operated navigation equipment (UAV DRONE) used in NDE Inspection (Image credits: NDT.net)
Drone technology has gained massive momentum since then. The term DRONE itself is an abbreviation for Dynamic Remotely Operated Navigation Equipment. Multiple categories of drones with specialized features have been built, which makes them a viable option for multiple applications. Some of these categories of drones are as follows:
UAV (Unmanned Aerial Vehicles)
- HTOL (Horizontal Take off and Landing)
- VTOL (Vertical Take off and Landing)
- Tilt-rotor
- Tilt-wing
- Tilt-body
- Ducted fan
- Helicopter
- Help wing
- Others
Micro UAV
- HTOL
- VTOL
- Tilt-body
- Tilt-wing
- Ducted fan
- Flapping wing
- Ornithopter
- Helicopter
SD (Smart dust)
PAV (Passenger Air Vehicle)
- Flapping wing
- Quadrotor
- Mono copter
- Bio PAV
MAV (Micro Air Vehicle)
- Fixed wing
- Flapping wing
- VTOL
- Tilt-rotor
- Ducted fan
- Helicopter
- Orni copter
- Live Bio MAV
- Fixed/Flapping wing.
- Rotary wing
- Mono copter
- Twin copter
- Tri copter
- Quadcopter
- Penta copter
- Hexa copter
- Octocopter
- Deca copter
- Dodeca copter
NAV
- Fixed wing
- Flapping wing
- Helicopter
- Mono copter
- Quadrotor
- Hexa copter
- Bio NAV
- Taxidermy
- Live
These technologies have trickled into the domain of non-destructive testing. Several defect and deformity analyses need to be conducted on tall and inaccessible structures, which may be dangerous or even fatal to human beings depending on the environmental and ambient conditions.
Drones used for NDE Inspection of a Storage Tank (Image credits: NDT.net)
Ultrasonic Testing apparatus have been combined with drone technology to perform non-destructive testing on such structures. Drones provide the benefit of being elevated to high altitudes and can hover at various points of the structure for extended periods.
This aerial access to structures and mechanisms helps reduce the waste of time and human resources and the risks associated with them. The petroleum industry, offshore machinery, telecom, and construction industries can highly benefit from such mechanisms.
Ultrasonic testing drones can be used to perform defect inspection on the thickness of the test subjects and can also perform visual testing of the structures. These drones have transducer arms installed which can be kept in contact with the test structure.
This probe can be used to examine the thickness of the material and is not obstructed by any paint or coatings. The probe is also not limited by the angle of the test subject and can be adjusted accordingly to maintain physical contact.
The flight path and procedure should be thoroughly studied to avoid mid-air interception. These drones perform routine analysis. In case of non-conformity to the intended standards and quality, the mechanism is renovated or replaced accordingly. This ensures the unobstructed operation and safety of the process.
These drone movements can be automated in cordoned-off aerial zones and drones can perform routine analyses on pre-charted flight paths, with the inferred data being uploaded to company data systems without any human interference or human resource wastage in regions where human involvement can be difficult or hazardous.
Potential applications of drones in NDT testing also include the following:
Building Inspection
- Energy efficiency
- Crack detection
- Geometric inspection of buildings
Bridge Inspection
- Crack and Stain detection
- Detection of degradation and deterioration
Road Inspection
- Sink hole detection.
- Crack detection
Pipe Inspection
- Water Leak detection
- Inspection of buried pipes
- Detection of thickness loss
Transmission Line Inspection
- Detection of polluted insulators
- Detection of overhead power lines
- Inspection of steel towers
- Inspection of power lines
- Pole detection and counting
Wind Turbine Inspection
- Surface damage detection on wind turbine blades
- Crack detection
Photo Voltaic Power System Inspection
- Quality Assurance
- Detection of failed solar panels
CONCLUSION
Non-Destructive Testing and NDT inspection techniques and methodologies have furthered the reach of humankind beyond the constraints of the physical senses for a comprehensive view of Various Engineering and industrial structures previously considered mythological.
Conventional destructive techniques would completely obstruct the functionality the structures are involved in while providing only a fraction of the insight that Science and Technology have guided Humanity to encapsulate with NDE and NDT techniques. NDT testing provides detailed properties of the tools and structures that make extensive Global Industrial Landscape meet the ever-growing demand of a prospering people.
Human Exploration has evolved through the years, helping us access the deepest high-pressure regions of the oceans, Inhabit the Coldest polar regions, and even transgress the boundaries of space. These advancements relay an imperative need for technology and artificial intelligence to substitute the hazardous roles thus created, NDT testing and NDE inspection play a major role in enabling these advancements as they help in the maintenance and upkeep of the structures and intricate machinery enabling Humanity to evolve further as a species.
