Systematic inspection of welded joints

Weld seam testing examines both the external geometry and the internal structure of welded joints. The method determines whether a weld seam meets the defined quality criteria. Critical defects are reliably detected in the process.

The inspection covers various aspects of the joint. Surface features such as cracks, pores or uneven seam shapes can be detected by visual methods. Hidden defects inside the weld seam, on the other hand, require special techniques of non-destructive materials testing.

Different materials react differently to the welding process. Steel, aluminium, copper and plastic each place their own demands on the test. The heat input affects the material properties around the weld and can cause stresses there.

The goal of all test methods is to identify defects that impair the strength or service life of the joint. Regular inspections ensure consistent compliance with norms and safety standards. The test thus protects not only the component itself, but also people and the environment.

Fields of use and areas of application

Weld seam tests are indispensable in numerous industries. Wherever welded joints are exposed to high mechanical loads, inspection is used. The range extends from the construction industry to medical technology.

In the construction industry, tests ensure the stability of steel structures. Bridges, high-rise buildings and industrial halls rely on flawless weld seams. A single defect could have serious consequences here.

The automotive industry inspects body and chassis components for the highest quality. Every welded joint must withstand crash loads and survive years of vibration. Modern vehicles contain hundreds of tested weld points.

Tests in aviation are particularly critical. Safety-relevant components undergo strict inspection procedures before they are put into use. There is no room for compromise here when it comes to weld seam quality.

In the energy sector, pressure vessels and pipelines are subject to extreme conditions. High pressures, temperatures and aggressive media demand flawless joints. Regular inspection prevents leaks and dangerous failures.

Medical technology also relies on precisely tested welded joints. Implants and medical devices must meet the highest quality standards. Here it is a matter of human lives.

Welding defects can considerably reduce load-bearing capacity. In the worst case, they lead to material failure with serious consequential damage. That is why thorough testing is an investment in safety and longevity.

How sound waves track down defects

Ultrasonic testing works with high-frequency sound waves that are inaudible to humans. A special test instrument sends these waves through a probe directly into the material. The waves spread evenly as long as the material is homogeneous.

As soon as the sound waves hit an irregularity, something interesting happens. They are reflected at this point and return as an echo. The test instrument captures this echo and evaluates it.

The depth of the defect can be calculated from the transit time of the signal. The strength of the echo reveals something about the size and type of the defect. In this way, inspectors can precisely locate pores, cracks or lack of fusion.

There are various techniques for ultrasonic testing:

Angle-beam testing is particularly well suited to welded joints. The waves penetrate the seam at an angle and reliably detect planar defects as well. Lack of fusion running parallel to the seam surface thus becomes more visible.

The advantages of ultrasonic testing are obvious. The method is fast, mobile and cost-efficient. Mobile devices enable tests directly on site – even on large structures such as pipelines or steel bridges.
The method reacts very sensitively to many types of defects, especially to planar flaws – however, detectability depends on the defect orientation and the test parameters. Even tiny irregularities in the material generate measurable echoes. However, trained personnel are needed for the correct interpretation of the signals.
Rough surfaces can make testing more difficult. The sound waves can then not be optimally coupled into the material. Complex geometries also sometimes pose a challenge.

Looking inside with radiation

Radiographic testing uses a completely different physical approach. Here X-rays or gamma rays are used that penetrate the material. On the other side, the transmitted radiation is captured by a film or digital detector.

The resulting image shows the internal structure of the weld seam like a kind of X-ray. Volume defects with lower density let more radiation through and appear darker; dense inclusions, on the other hand, can cause brighter indications.

X-ray testing offers a particular advantage: it creates permanent documentation. These images can be archived and assessed by several experts if required. Years later, they can be evaluated again.

Volume defects such as pores, slag inclusions or insufficient penetration are clearly recognisable. The method makes these irregularities clearly visible. However, planar defects such as thin cracks have a disadvantage.

If they are unfavourably oriented to the direction of the radiation, they may remain undetected. Radiographic testing also requires access to both sides of the weld seam. This is not always possible.

The method also brings challenges:

Despite these limitations, X-ray testing remains indispensable in critical areas. In pressure vessel construction, petrochemistry and aerospace it is standard. The reliable and well-documented quality control justifies the effort.

Both methods complement each other ideally in practice. Ultrasound scores with speed and flexibility. Radiographic testing, on the other hand, delivers vivid images that can be used over the long term.

The eye inspects too: visual inspection as the first step

Visual inspection forms the basis of every weld seam inspection. An experienced inspector already recognises many quality characteristics and potential weak points with the naked eye. This method is fast, cost-efficient and requires no elaborate equipment.

During visual inspection, the specialist examines the weld seam systematically. Are the seam transitions even? Do cracks, pores or spatter appear on the surface? Does the seam geometry look harmonious, or do irregularities catch the eye?

For precise measurements, special weld seam gauges are used. These mechanical measuring tools enable the exact determination of important parameters. The throat thickness of a fillet weld, the seam width or the reinforcement can be reliably recorded with them.

Modern aids considerably extend the possibilities of visual inspection. Magnifiers enlarge details, endoscopes reach hard-to-access spots, and mirrors enable views into hidden areas. For initial quality checks during production, this method is indispensable.

The limits, however, are clearly obvious. Visual inspection captures only surface defects. What lies hidden inside the weld seam remains invisible. In addition, the reliability depends heavily on the experience and attentiveness of the inspector.

Despite these limitations, visual inspection is prescribed as a mandatory test in almost all relevant norms. It forms the first step before more elaborate methods of non-destructive materials testing are used.

Making the finest cracks visible: penetrant testing

Penetrant testing tracks down even the finest hairline cracks that the human eye would overlook. This method uses the capillary action of special liquids to make surface defects visible. For materials that are not magnetic, this method represents a valuable alternative.

The test process follows a clear scheme in several steps. First, the weld seam is thoroughly cleaned and degreased. Only in this way can the pores and cracks really be open and accessible.

Then the inspector applies a liquid penetrant – usually a bright red or fluorescent liquid. This has an extremely low surface tension and creeps through capillary forces even into microscopically small cracks. After a dwell time of a few minutes, the excess penetrant is carefully removed from the surface.

In the next step, the developer comes into play. This white, absorbent powder or suspension draws the penetrant remaining in the defects back to the surface. There, high-contrast indications form: red or fluorescent lines mark exactly the position and shape of the cracks.

The great advantage of this method lies in its material independence. It works on all non-porous materials – metals, ceramics, glass and certain plastics. Aluminium, stainless steel or titanium can be tested excellently with it, even if they are not ferromagnetic.

Penetrant testing achieves an impressive sensitivity. Cracks in the micrometre range are reliably detected. However, the method is time-consuming, as several process steps must be carefully completed.

One important limitation that beginners should know: only open surface defects are captured. Defects lying just below the surface or internal volume defects remain invisible. In aviation and power plant construction, this method is nevertheless part of the standard.