Recent Non-Destructive Testing Developments for Steel Structures

Article updated on 17 August 2021

The construction industry is responsible for consuming around 50% of the steel produced globally, given its strength and ability to be shaped into a myriad of different building components such as beams, channels, columns, and sections that support high loads without excessive sagging. It is essential that these building components and other prestressed steel constructions, for example, cables and aerospace components, are inspected post-production and receive long-term health and condition monitoring when in-situ.

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Awareness of stress changes is essential and without ongoing inspection, these can lead to undesirable stress gradients that lead to crack propagation, premature wear, and the threat of dangerous component failure.

In the context of steel structures, the evolution of non-destructive testing (NDT) has largely been driven by a need for ensuring manufacturing quality control, improved efficiency, and greater safety. Here, some of the traditional NDT methods still being deployed effectively are covered while the advantages of using more recently developed, advanced focusing techniques are highlighted.

Micromagnetic Method-3MA

A property known as yield strength is the most important material characteristic of steel because it determines its load-bearing capacity; steel quality classifications tend to be defined by it. The magnetic properties of differing steel types are established and assigned to their yield strength and measured, traditionally, in a tensile testing regime. Determining yield strengths in beams within a building, for example, can allow the risk of collapse to be ascertained.

The application of micromagnetic methods for NDT of steel is well established, with a versatile technique called Multi-parameter Micromagnetic Microstructure and stress Analyzer or 3MA.

Developed at the Fraunhofer IZFP and using a combination of four micro-magnetic methods, 3MA allows the predictions of yield strengths while quantitatively determining other mechanical properties such as hardness and residual stress with a high degree of accuracy. Different material depths can be sampled simultaneously and factors such as batch variation minimized because of the different interaction mechanisms between the combined methods. A broad range of probe heads has been developed for 3MA over the years, resulting in large inspection systems for continuous testing of semi-finished products such as strip steel.

Dynamic testing parameters such as fatigue and creep can also be investigated by 3MA in this, by now, industrial standard.

Enhancing the sophistication and taking 3MA to the next level concerns its implementation into closed-loop control systems where a feedback signal is generated between output and input, correcting measurement errors and improving accuracy.

The next-generation 3MA devices will use this type of mechanism in combination with a test based on a phenomenon called dynamic magnetostriction (DM).

Dynamic Magnetostriction is where the magnetic state of a material’s dimensions is changed. Applying a magnetic field causes magnetoelastic interactions and deformations when the volume of the distorted magnetic domains remains constant. This induces strain, changing the magnetic state, and vice versa. This reciprocal effect can be exploited in determining what is crystalline and what is polycrystalline, for example, in metallic samples.

Phased Array (PA) Ultrasonics Testing

With its high penetrating power and sensitivity, Ultrasonic Testing (UT) has been extensively used in steel structure NDT to date, not least because of its ability to detect both deep, internal flaws but also, due to its sensitivity and identification of extremely small defects.

A transmitter sends ultrasound through one surface and a separate receiver detects the amount of sound that has reached it on another surface after traveling through the material. Typically, pulse waves of ultrasounds ranging from 0.1-15- 50 MHz are transmitted and imperfections are revealed by reducing the amount of sound detected.

With the escalating geometrical complexity of new steel components, however, standard UT has been reaching its operational limits. A restricted ability to maneuver because of its fixed angles precludes UT in locating, for example, defects within tight angles and awkward configurations.

New industrial NDT Instruments that permit more flexible scanning and deeper probing are needed and likely to be realized by a combination of advanced focusing and ultrasonic imaging techniques such as Phased Array (PA) ultrasonics, an advanced method of ultrasonic testing.

Rather than physically scanning in a fixed direction through an area of interest using a conventional probe, PA imaging uses a beam composed of multiple elements, each of which can be pulsed individually using computer-calculated timing.

The beam can be focused and swept electronically without moving the probe. “Array” refers to the multiple elements, while “phased” refers to the timing. The PA probe comprises several small ultrasonic transducers which can be pulsed independently.

Patterns of constructive interference and the resulting data used to construct images depicting slices through a steel sample are created. Operating at one million data points per frame, PA probes allow multiple angles in a single testing phase, delivering greater detection reach and revealing more flaws. Deployment in detecting corrosion and investigating the quality of welds are among PA’s most prevalent uses.

NDT Standards of the Future