Non-Destructive Testing in Railways: A Comprehensive Overview

Due to the critical role of railways in transportation over land, ensuring their safety and operation is a priority. Railway specialists formerly destroyed the rail infrastructure to inspect it. The process of destroying, inspecting, repairing, and reconstructing typically involved considerable costs and took a lot of time, during which the railways were out of service. This is why an innovative method has emerged: inspecting and assessing the rail infrastructure without any destruction.

Non-destructive testing (NDT) is a method of evaluating railway infrastructure in depth. This method is widely used in maintenance and repair processes, using technology to identify flaws and ensure their safety and operation without requiring the destruction of the railway. This blog post will discuss what non-destructive testing is, how it works, what types it includes, and any other details.

Understanding Non-Destructive Testing

The reliability and safety of railway infrastructure can directly impact its operation. There are two methods of inspection used to ensure the railway system’s reliability, safety, and operation: Destructive Testing (DT) and Non-Destructive Testing (NDT). Destructive testing was previously used to inspect the railway system in depth by damaging its infrastructure. Since this method incurred significant costs and was time-consuming, it was alternated with the nondestructive testing method.

Non-destructive testing (NDT) is a method in the railway industry for assessing rail materials, components, or systems without damage. In other words, non-destructive testing refers to inspecting the railway system in depth using technology to prevent the intensive cost of damaging rail infrastructure and this time-consuming process. Non-destructive testing utilizes high-frequency sound waves, acoustic impulses, or gamma rays to efficiently identify flaws that are not recognizable to the naked eye.

Nondestructive inspection is an integrated part of railway maintenance and repair. By inspecting different components of the railway system, non-destructive testing can detect cracks, wear, weld discontinuities, or any other defects that can disrupt its operation and safety. However, based on the type of inspection and operation, nondestructive testing is performed using various methods.

Types of Non-Destructive Testing Methods

Due to the variety of railway components and materials used in this system, non-destructive testing is categorized into various methods based on how they inspect and what they inspect specifically. Here are the most common and primary types of nondestructive testing methods used in the railway industry:

Visual Testing (VT)

Rolling stock traffic incurs surface defects on railway systems over time, such as cracks, fatigue, squats, short-pitch corrugation, corrosion, etc. These surface defects can be directly identified with bare eyes, which is referred to as visual testing (VT). However, this method is unsafe, unreliable, and time-consuming. Automatic visual testing (AVT) is an innovative type of visual testing in which digital cameras, image processing devices, and other sophisticated systems are used to inspect surface defects of railways more accurately and fast.

Ultrasonic Testing (UT)

Ultrasonic testing (UT) transmits ultrasonic high-frequency waves (typically 2.25 MHz) into the different railway components to detect both interior and surface defects. These sound waves or pulses are reflected back to a receiving transducer or the same transducer that transmits the energy initially. The ultrasonic waves travel at a specific speed (typically 5900 m/s or 19,356 ft/s for longitudinal waves), and the time at which the reflected signal is received can be referred to as a distance in the rail materials.

Magnetic Testing (MT)

Magnetic testing (MT) refers to using a magnetic field to detect surface or slightly subsurface flaws in ferromagnetic materials.  This method is typically performed in two types: Magnetic Particle Testing (MPT) and Magnetic Flux Leakage (MFL). In magnetic particle testing, the current is directly applied to the rail components by a strong magnetic field, and perturbations in the current flow or the ferrous particles indicate defects or discontinuities in those rail components. In magnetic flux leakage, on the other hand, the rail components are indirectly magnetized by a strong magnetic field. The defect or discontinuity in rail components causes changes in magnetic flux lines, their directions, and flux leakage.

Eddy Current Testing (ECT)

Eddy current testing (ECT) utilizes electromagnetic induction to inspect the rail tracks. This method places a coil with an alternating current (AC) near the rail track. The alternating current in the coil generates a changing magnetic field, which interacts with the conductive metal, inducing circular eddy currents within the rail. These currents generate their own magnetic field, opposing the original one. If there is a crack, corrosion, or wear on the rail track’s surface or just below, the eddy currents are disrupted, allowing inspectors to detect potential defects.

Radiography Testing (RT)

Radiography testing (RT) is another nondestructive inspection in which X-rays or gamma rays are used to inspect the railway system. Before testing, the component must be isolated and cleaned to remove any external materials that could interfere with the imaging process. Moreover, warning signs and safety barriers must be used to prevent radiation exposure to operators and nearby personnel. Radiation from an X-ray generator or a radioactive isotope is directed through the railway component onto a film or digital detector, capturing an image of its internal structure. Variations in the image reveal defects such as cracks, voids, or inconsistencies within the material.

Acoustic Emission Testing (AET)

Acoustic emission testing (AET) refers to a non-destructive testing method used to detect structural defects in railway systems by capturing acoustic emissions. Sensors are placed on the surface of the railway components to monitor the high-frequency waves generated by the release of energy from defects such as cracks. The data collected is analyzed to determine the location and severity of these defects accurately.

Thermographic Testing (TT)

Thermographic testing (TT), also known as Infrared testing (IT), refers to inspecting railway systems using thermal cameras. Defects like cracks, fatigue, or internal voids change the thermal energy emitted from the railway components. By providing infrared images, thermal cameras can specifically capture heat signatures that vary from normal areas, detecting defects in different components of the railway system. Moreover, this method can be used to predict potential defects before they occur.

Penetrating Testing (PT)

Penetrating testing (PT), also known as dye penetrating testing (DPT) or liquid penetrating testing (LPT), is a nondestructive inspection method in which a liquid dye is applied or sprayed on a specific component of the railway system and penetrates cracks or flaws. The failures of railway components are visualized properly using UV light, a developer, or even the bare eye (in some cases). Therefore, this method can be efficiently used to detect surface or slightly subsurface defects in railway systems.

Destructive Vs Nondestructive Testing: Pros and Cons

Railway specialists and engineers utilize two methods to inspect the railway system in maintenance and repair processes: destructive testing and non-destructive testing. Destructive testing involves damaging the railway system's infrastructure to inspect different components, while nondestructive testing refers to inspecting the railway system and detecting defects without any damage or failure in its infrastructure.

Each method offers some benefits and challenges. Here are the most considerable advantages and disadvantages of both destructive and non-destructive testing in the railway industry:

Destructive Testing: Benefits and Challenges

Damaging the structure of the railway components provides comprehensive and precise data for the inspection. Subjecting the materials used in railway components to failure properly assesses their behavior under stress and factors like maximum load capacity. In addition, interior defects or those not visible from the surface can be detected. However, this method makes the inspected component unusable, or in cases where it is a critical component, the railway system will be out of service until it is repaired. Moreover, destructive testing is a time-consuming process and incurs a significant cost.

Non-Destructive Testing: Benefits and Challenges

Nondestructive testing uses technological methods to offer a cost-effective railway inspection that takes noticeably less time. It allows in-depth monitoring with methods such as ultrasonic testing, eddy current testing, magnetic testing, and so on that detect both surface and interior defects. In cases where surface defects are detected by the naked eye, typically, no specific equipment is needed, but the other methods require both sophisticated equipment and expert personnel to analyze the results. However, some of the failures in railway components may not be detected.

What Tools and Equipment Used in Non-destructive Testing?

Non-destructive testing (NDT) in railway systems involves a variety of tools and equipment designed to inspect components without causing damage. These tools are selected based on factors such as the type of material, the location of defects, and the required level of accuracy. Common categories of NDT tools include ultrasonic testing devices, which use sound waves to detect internal flaws; magnetic particles and eddy current testers, which identify surface and subsurface defects in metal components; and radiographic testing systems, which use X-rays or gamma rays to examine internal structures. However, there are some efficient tools for other methods, such as thermographic cameras and acoustic emission sensors, that help detect stress, cracks, or overheating issues.

Conclusion

In summary, non-destructive testing is one of the most crucial methods in the maintenance and repair processes in the railway industry. It inspects the different rail components to detect current or potential defects without any damage or failure in their infrastructure, which plays a significant role in ensuring the railway’s safety, reliability, and operation. In this article, we proceed with nondestructive testing in detail and introduce its various types of methods that will offer a comprehensive overview of how it works.