What Are Destructive and Nondestructive Testing?

What is destructive testing?

Destructive testing evaluates materials and components by subjecting them to extreme conditions that lead to “plastic deformation,” i.e., failure, damage, or even destruction. Common destructive testing methods include tension, compression, impact, and hardness tests. Here are the most common DT procedures:

Common types of destructive testing

• Corrosion testing subjects materials to various environments to determine the amount of erosion (loss of material) over time. These environments include salt and freshwater, soil, bacteria, and microorganisms.

• Fatigue testing applies various tension and compression cyclic loading combinations to determine the number of load cycles an object can withstand until structural failure occurs.

• Impact testing simulates sudden loading conditions by subjecting a material to a large force for a short time, providing insights into its toughness and resistance to dynamic loads. The most common methods of performing destructive impact testing include the Charpy impact test and the Izod impact test.

• Tensile testing involves subjecting a material to a controlled tension until it fractures. This determines the material's tensile strength, yield strength, elongation, and other properties.

• Compression testing assesses the material's ability to withstand compressive loads, providing insights into its strength and behavior under crushing forces. In the example below, a large-scale Baldwin compression machine applies more than 3.1 million pounds (>1.4 million kilograms) to a concrete pylon until it fails.

• Hardness testing measures a material's resistance to indentation or scratching. Common methods include Brinell, Rockwell, and Vickers hardness tests, providing insights into the material's wear resistance, machinability, and overall hardness properties.

• Residual stress effects can remain in a structure even after its causes have been removed. They are measured by drilling a hole in the structure and removing the material. Hole deformation is analyzed optically or via a strain gauge to determine the nature and extent of the structure's residual stress.

• Torsion testing applies a twisting force to determine the shearing of a structure before it deforms.

• Bend testing assesses a material's elasticity and flexibility by bending a sample to a specified angle or curvature until it fractures or exhibits visible defects. It is particularly useful for evaluating the material's ability to deform without breaking under bending loads.

What is non-destructive testing?

Nondestructive testing encompasses various techniques to evaluate materials, components, and structures. Unlike destructive testing methods, NDT inspection techniques allow examination and analysis without damaging the test article.

Common types of nondestructive testing

• Ultrasonic testing detects internal flaws and assesses material thickness by sending high-frequency sound waves through a material. The waves travel through the material until they encounter a boundary or defect, and the reflected energy indicates the flaw's location. It's commonly used in the aerospace, automotive, and manufacturing industries for quality control of metals, composites, and plastic structural components.

• Magnetic particle testing detects surface and near-surface flaws in ferromagnetic materials. The test article is magnetized, and magnetic particles are applied to the surface, usually as a dry powder or a wet suspension. If defects such as cracks or discontinuities are present, the magnetic particles will be attracted to and accumulate around these areas, becoming easily visible in proper lighting. MPT is highly effective for detecting defects in steel, iron, and nickel alloys. It is widely used in manufacturing, aerospace, automotive, and oil and gas for quality control and inspection of components like welds, castings, and forgings.

• Eddy current testing is used for flaw detection and coating thickness measurements. A harmless magnetic field is applied inductively to a test article, and the field's deviations can be measured and analyzed.

• Acoustic emission testing detects ultrasonic stress waves produced by test articles when subjected to pressure, load, or temperature. The rate and amplitude of these emissions indicate a test article’s structural integrity.

• Visual inspection is the direct observation of a test article to detect defects, surface cracks, corrosion, weld discontinuities, and other visible flaws. It can be performed by the naked eye, magnifying glasses, mirrors, or borescopes. While limited to surface defects, it is valuable for routine quality control, maintenance checks, and initial assessments of component integrity in numerous industries.

• Dye penetrant testing (liquid penetrant testing) reveals surface defects in non-porous materials like metals, plastics, and ceramics. A penetrant is applied via immersion, spraying, or brushing. When a developer is applied, defects like cracks or fractures become visible. This method is cost-effective and widely used in railway, aerospace, and automotive industries.

What are the pros and cons of destructive and nondestructive testing?

The primary advantage of DT is that it provides definitive results regarding a material's properties and behavior under stress. By subjecting the material to failure, engineers can accurately assess the material's maximum load capacity, fracture toughness, and other critical parameters essential for design and manufacturing processes. The main drawback of DT is that the test article is destroyed. This is impractical in some applications.

The primary advantage of NDT is that it does not destroy the test article, allowing for repeated testing or continued use after inspection. This saves money when test articles are expensive. NDT is particularly important in applications where even minor defects can have catastrophic consequences. However, NDT is less effective at identifying internal flaws or weaknesses, particularly in complex structures or materials like cast iron, concrete, fiber-reinforced composite, or certain coatings.

Applications of destructive and nondestructive testing

In the aerospace sector, DT is commonly used to evaluate the structural integrity of critical components such as wings, control surfaces, fuselage sections, and engine components. Manufacturers subject these components to extreme conditions to ensure their compliance with stringent safety standards.

The car crash test is the best-known destructive test in the automotive sector, but it is also applied to trucks, trains, motorcycles, and other vehicles.

Sports equipment, notably helmets, and protective gear, is tested to ensure it can dissipate impacts and protect the wearer. 

In the automotive sector, NDT methods such as ultrasonic and radiographic testing are utilized to inspect welds, engine components, and chassis structures for hidden defects that could compromise vehicle safety and performance.

In the construction sector, NDT is indispensable for assessing the integrity of concrete structures, detecting flaws or voids in reinforced concrete elements, and evaluating the quality of welds in steel structures. Similarly, NDT is employed in the oil and gas sector to inspect pipelines, storage tanks, and pressure vessels for corrosion, weld defects, and material degradation, ensuring critical infrastructure's safe and reliable operation.

Conclusion

Destructive and Nondestructive testing methodologies are indispensable in evaluating, inspecting, and ensuring the quality of materials, components, and structures across many industries. While destructive testing provides definitive insights into a material's properties through deliberate damage, Nondestructive testing allows for thorough inspection without compromising the integrity of the tested objects. 

By understanding both testing methods' principles, advantages, and limitations, engineers, researchers, and quality assurance professionals can make informed decisions to meet the stringent quality and safety standards required in today's dynamic and demanding industries.