Destructive vs Non-destructive Testing: What's the difference?

Table of Content

• What is Non-destructive Testing?
• Need for Testing
• What is Destructive Testing?
• Methods of Destructive Testing
• Methods of Non-destructive Testing
• Difference in Applications of Destructive and Non-destructive Testing
• Why is Non-Destructive Testing considered better than Destructive Testing
• Conclusion

What is Non-destructive Testing?

Metal and metal forging has been prevalent since before 6000 BC. The metal smiths, since those primitive days of metal processing, developed various methods to estimate the temperature of the metal in the furnace to gauge if it was ready for its intended use (hammering, casting, heat treatment, etc.) 

The visual indicator, in this case, was the color of the heated/superheated metal, which gave the metal smith an idea of its temperature and properties. The Visual Testing color indication chart for a steel alloy with a 0.40 carbon percentage is shown in the image below:

Modern-day techniques allow the usage of a no-contact infrared-based measuring device, called a pyrometer, for the aforementioned purpose. Non-destructive Testing is a method for qualitatively analyzing structures, materials, products, and/or machinery. This Non-destructive Evaluation technique provides the opportunity to assess the status of structures, machinery, and materials without causing any damage or inhibiting their purpose.

The Types of Non-destructive Testing available in today’s day and age are as follows:

• Visual Inspection
• Magnetic Particle Inspection
• Dye Penetrant Inspection
• Ultrasonic Testing
• Phased Array Ultrasonic Testing
• Leak Testing
• Radiographic Testing
• Acoustic Emission Testing

Non-destructive testing permits the evaluation of the test subjects (materials, products, or structures) during manufacturing, usage, and after failure.

This makes it a viable testing option for the Automation Industry, construction, petrochemical, aerospace, mining, and electrical power industries.

Need for Testing

The NDT Evolution of Technology and the availability of new resources, materials, and production methodologies pose further challenges in ensuring their safety, integrity, and feasibility. Material Testing furnishes the option to eliminate trial and error and squandering of time, material, resources, and labor. 

The following are the reasons why material testing is imperative:

• It helps to gauge the test subject’s adherence to industry standards.
• Material testing assists in the testing of products during their manufacturing process.
• It aids in checking the viability of prototypes, NDT Products, or materials for their intended use.
• It can also be used for quality and uniformity checks on manufacturing NDT Equipment, processes, and products.

What is Destructive Testing?

Destructive testing analyzes the behavior of a material, structure, or product until the point of breakdown or failure. Certain quantifiable properties like toughness, hardness, elongation of material, and tensile strength are undetectable by NDT Methods, as applying load is an indispensable step.

The Destructive Testing process involves constant load application until the subjects undergo deformation or failure. The test subjects cannot be utilized after the destructive testing procedure has been carried out on them and need to be scrapped or reworked after thorough analysis.

Mass production units often use Destructive Testing methods to obtain quantifiable data on the performance of their products under real-life environmental stresses and usage. Products like gaming controllers, brakes, keyboards, and even cars are intentionally utilized to the point of destruction to get a real-world idea of their longevity and efficiency.

Crash testing of vehicles is vital in understanding the safety of the passenger and goods carried, if any, in the event of an accident, fire, or untoward mishaps that are likely during their operation. Destructive Testing in these cases may cause a waste of resources. Still, they provide a realistic understanding of the performance of vehicle safety features, like airbags, and potential risks that may expedite further damage and help minimize any harm caused.

A variety of destructive techniques are available for the evaluation of structures, materials, and products, some of which are:

• Tensile Testing
• Bending Test
• Compression Test
• Hardness Testing
• Creep Analysis
• Charpy Impact Test
• Fatigue Test
• Ductility Test or Cupping Test

Destructive tests can be used to replicate failure during operation and curb catastrophic accidents, recall products, and provide accurate, measured data on the performance and longevity of test subjects.

Methods of Destructive Testing

The methods used in Destructive Testing are varied and provide rich data, albeit at the cost of the tested subject. The methods are as follows:

1. Tensile Test

This Tensile Testing is conducted by tugging the sample to analyze its behavior under tension. The plastic deformation, reduction in diameter, and point of failure are noted; hence, the strength properties of the material can be calculated.

2. Compression Test

The Compression Test subject's behavior is studied under this methodology by exposing it to a crushing load (e.g., Load under a universal testing machine) until failure to gauge the subject's compressive stress and strain properties. This testing is carried out along various axes of the subject and yields data like yield strength, compressive strength, and point of yield.

3. Bending Test

Also called a Flexural Test, this analysis method uses the technique of loading the portions of the subject, causing it to bend, and provides data on the ductility of the subject. The bending causes compressive stress on the inner side of the curve created, while tensile stress is created on the outer side of the curve created in the subject. This test provides the bending elastic modulus, flexural strength, and strain.

4. Hardness Test

This test involves introducing a bit of a specific dimension and gradually increasing the pre-set load on the surface of the test subject (e.g., Matsuzawa Hardness Testing machine). The size and depth of the indentation formed are recorded. The hardness data is empirical; hence it varies for different testing processes (e.g., Brinell Hardness number, Knoop Hardness, Shore Hardness, Rockwell Hardness Test, and Vickers Hardness Test). This test proves the resistance of the subject to permanent deformation.

5. Torsion Test

This test involves inducing a twist in the material to gauge its resistance to deformation caused by the twisting force. This test provides data on the shear modulus, rupture modulus, and yield strength of torsion. Vibrations and operational motion often cause twisting forces in components, and it’s often vital to gauge the effects of such forces on a subject.

Methods of Non-destructive Testing

Non-destructive Testing is a versatile and cost-efficient process and provides a variety of methods of analysis.

Some of those methodologies are:

1. Visual Inspection

This Visual Inspection Method uses the human eye to gauge the presence of surface defects and deformities. These microscopic defects may include cracks in concrete, tears in material, improper Welds, formation of craters, chipping, etc. of material. Visual adherence to industry standards, ill-fitting components, dents, and delamination of subjects are some of the many observations that can be made using visual testing by a trained inspector.

2. Microscopic Testing

This Non-destructive Testing Method is used to detect subsurface flaws by observing the surface under 6x magnification or more. This helps detect minute flaws that may cause spot Corrosion or have underlying defects that may cause the failure.

3. Radiographic Inspection

The Radiographic Method of non-destructive testing uses X-rays to impinge on a metal's surface. The sensitivity of this method is around 2% of the metal thickness. Cracks and voids in the test subject are hence made evident. Gamma Rays are also used for this testing method; however, the process may be hazardous due to the penetration power of said rays that may cause DNA alterations and tissue damage to humans.

4. Dye Penetrant Inspection

This method, called Liquid Penetrant Testing, uses a dye applied to a non-porous metal, which gets drawn into cracks, voids, and other surface and sub-surface defects because of capillary action. The dye is then observed under white light of a specific frequency or ultraviolet light.

5. Ultrasonic Inspection

In this NDT Method, sound waves of a specific frequency impinge on the surface of a test subject using transducers housed in a probe. The receiving unit in the apparatus reads the incoming signals after the induced waves interact with deformities and defects and are converted to data that can be recorded and interpreted easily.

6. Magnetic Particle Inspection

In this procedure, the magnetic powder of iron fillings and magnetic material is applied to a magnetized material. The powder acts per the laws of magnetism and forms patterns in the shape of magnetic field lines. The cracks, voids, or any disparities on the surface and within the sub-surface defects disrupt the field line patterns formed by the iron fillings and powder mixture. This method is cost-effective, accessible, and works efficiently for all ferromagnetic materials.

7. Eddy Current Testing

This process of Non-destructive Testing uses the principles of induction of electricity. Like a transformer, a secondary conductor is bought into the vicinity of a conductor carrying an Alternating Current, creating a magnetic field; the secondary conductor develops an electrical field due to conduction. This current can be detected by a receiver which provides data on the cracks, defects, and deformities present in the subject.

8. Acoustic Emission Testing

This Non-destructive Testing Method detects the acoustic waves emitted from the test subject under structural deformation using piezoelectric sensors and ceramic probes for good sound conductance. This method can consistently monitor vital structures and detect major defects for safety measures.

Difference in Applications of Destructive and Non-destructive Testing

Multiple industries and environments require a thorough analysis of products, materials, and structures until failure to gauge their feasibility in real-world usage.

Destructive Testing is found to be most appropriate to obtain such data and is used in the following applications:

• Destructive testing is used to conduct seismic analysis of buildings, roads, and structures to analyze the structure's response to various inevitable environmental and tectonic events. An earthquake simulator conducts the study called a shake table.
• Automotive structural integrity and functioning are tested in crash tests, which are conducted for land and air vehicles. This testing also checks if the vehicle meets industry and environmental safety standards.
• Digital programs and software are also tested under destructive testing, wherein the software is pushed to its limits regarding security, validity, and the load it can handle.
• Vehicle load testing is conducted apart from crash testing in the Automotive Industry, where the vehicle's payload is tested to its limit during operation to evaluate its maximum potential.
• Manufacturing industries conduct destructive testing on products before their release in markets to quantify their longevity specification and test conformity to industry standards by utilizing them to failure and conducting random checks on products in the production line.
• Material Testing is carried out in composites, metallurgy, and material science to test the properties of various materials, alloys, and composites to analyze their properties.
• Destructive testing is also conducted to drive a material or product to failure to assess weaknesses in design and properties to create contingency plans or take corrective measures.
• Destructive testing is also essential in testing various safety equipment and gear. Industry requirements must be followed to justify the usage of such equipment in industries successfully.

NDT Methods, on the other hand, can be used during the service of the test subjects without hindering their operation and creating bottlenecks in ongoing processes.

This makes it versatile and viable for a large variety of applications and industries, which include the following applications:

• In the Oil and Gas Industry, non-destructive techniques gauge carrier pipes for defects, welding errors, leaks, and corrosion. Operational factors may cause changes in the dimensioning of machinery and pipes used, and Non-destructive Testing helps analyze and record these changes to approximate the quality of continued operation.
• Production of various automotive accessories and components requires thorough quality checks before assembly or release into markets to ensure adherence to design requirements and to assess their conformity to industry standards.
• Large buildings, roads, and bridges utilize tons of raw material, time, and resources, and potential failure may cause threats to human and environmental safety. Regular Non-destructive Testing is carried out on such structures in the construction industry to avoid catastrophes.
• The Nuclear Power Industry utilizes non-destructive testing to check fuel rods and protect structures' integrity to avoid human proximity to hazardous environments and substances.
• Measurement tools are tested using Non-destructive Testing Techniques during manufacturing to scrutinize them for conformity to metrology standards and measurement accuracy.
• Non-destructive testing techniques are used to analyze spacecraft and other Aerospace Industry equipment during and before usage to avoid failure that could have adverse impacts.
• The marine industry uses Non-destructive Testing Methods to check boats and ships' structural robustness and functionality without hindering their operation on high seas and while docked.

Why is Non-destructive Testing considered better than Destructive Testing

Non-destructive Testing is often considered superior to destructive testing in various situations due to several key differences between destructive testing and non-destructive testing

1. Preservation of Assets

NDT allows the tested components or materials to remain intact and functional after inspection. This is particularly valuable for expensive or irreplaceable items like aircraft components, historical artifacts, or critical infrastructure, where preserving the asset's functionality is essential.

2. Cost-Effectiveness

NDT can be more cost-effective in the long run because it doesn't require creating multiple test specimens or replacing tested items, as is often the case with Destructive Testing. The ability to inspect items without causing damage can lead to significant cost savings in industries like aerospace, where components are expensive to manufacture.

3. Safety

NDT Methods are inherently safer because they don't involve subjecting materials or components to potentially destructive forces. In contrast, DT can be hazardous, especially when testing high-stress components that might fail catastrophically.

4. Continuous Monitoring

NDT Techniques can be used for continuous monitoring and routine inspections, allowing for the early detection of defects or damage. This proactive approach can prevent accidents and unexpected failures, enhancing safety and reliability.

5. Non-Invasiveness

NDT Methods are non-invasive, meaning they don't alter the physical properties of the tested item. This is crucial when assessing the structural integrity of critical components, as even minor alterations can affect their performance.

6. Time Efficiency

NDT is often faster than Destructive Testing because it doesn't involve the time-consuming processes of creating and testing multiple specimens. This efficiency is particularly beneficial in industries with tight production schedules.

7. Accessibility

NDT can be applied in situ, meaning it can be performed on-site or in the field without the need to transport materials or components to a specialized testing facility. This accessibility is advantageous for industries like construction and infrastructure maintenance.

8. Reduced Environmental Impact

Destructive Testing often generates waste materials from the tested specimens, which may require disposal or recycling. NDT, being non-destructive, reduces the environmental impact associated with waste generation.

9. Continuous Inspection

Some NDT methods, such as Ultrasonic Testing and radiography, provide real-time or near-real-time results, enabling continuous inspection and immediate feedback, which is critical for quality control in manufacturing processes.

10. Versatility

NDT encompasses a wide range of techniques suitable for various materials and applications. This versatility allows NDT to be applied in diverse industries, from aerospace and automotive to healthcare and infrastructure.

Key Takeaways

• It's important to note that the choice between NDT and Destructive Testing depends on the specific requirements of the application based on the differences.
• While Non-destructive Testing offers many advantages, there are situations where DT is necessary, such as when gathering precise material properties or understanding the behavior of materials under extreme conditions.
• In many cases, a combination of both NDT and DT may be used to ensure comprehensive testing and assessment. For more details, check out OnestopNDT. 

References

• Production Machining
• Metengr
• Admet
• MTS
• MSP METROLOGY
• Kristofer Dressler
• URS Inspection
• NDE.NET
• MME Group
• Iircengg
• EveryeNG
• NDT Technologies
• TWI Global