Is Visual Inspection an Effective NDT Method?

Is visual inspection effective? Overall, the visual inspection method on its own cannot produce the same quality results as other nondestructive testing (NDT) methods. For one, it cannot detect certain minute flaws regardless of how keen an eye or how much experience an analyst has. Even the use of visual aids and tools is sometimes insufficient—and even minor defects may develop into critical problems if overlooked for too long. Where visual inspection can play a role is for monitoring existing imperfections or as an initial tool for a very basic inspection.

Visual inspections are inexpensive and require less training than other testing methods. However, the discipline simply cannot compare to the advanced flaw detection and comprehensive scanning capabilities of advanced NDT equipment. Eddy current testing (ECT) and ultrasonic testing (UT), in particular, can provide levels of convenience, flexibility, and efficiency with which visual inspections simply cannot compete.

Eddy Current vs. Visual Inspection

Visual inspection (VI) is the oldest and most common nondestructive testing method, but it doesn’t offer the same cutting-edge technology and specialized techniques as eddy current testing. ECT is among the most commonly used NDT options for a wide swath of applications due to its ability to detect surface and near-surface flaws quickly and accurately without interfering with the design form of the tested item.

Visual inspection experts maintain that visual aids such as those listed below can enhance VI testing quality:

• Magnifiers
• Telescopes
• Microscopes
• Fiber-optic instruments
• Endoscopes

However, none of these devices can detect subsurface indications and smaller flaws. If inspecting a painted welding sheet, for instance, the visual method would call for stripping away the paint before the inspection begins. ECT, on the other hand, can penetrate through surface barriers and reach the desired testing area without probe-to-surface contact.

No matter how advanced visual inspection equipment may be, it cannot compare to eddy current testing because flaw assessment on a deeper level is impossible with VI. Visual inspection is primarily used to detect larger defects that can be seen with the naked eye—not smaller underlying defects that could compromise the integrity of the tested asset. With ECT, analysts can detect incredibly minute flaws without hassle, allowing managers to address the deviations early before they develop into a greater danger.

ECT can detect flaws in the form of:

• Cracking
• Corrosion
• Pitting
• Thickness variations
• Material degradations

These defects could be in places where a VI instrument couldn’t reach them. Visual inspection is an incomplete form of testing that cannot paint a full picture of an asset’s flaws, and even though analysts may use other NDT methods in conjunction with VI, it remains an ineffective standalone method.

With eddy current testing, analysts can get faster results via enhanced signal quality and handheld portability. With a handheld ECT instrument, analysts can cover wider areas with a single pass. Moreover, the use of surface array allows an ECT instrument to conform to unconventional shapes and further outline the profile of many flaw types. Such a feat is unattainable during a visual inspection, as VI does not incorporate enhanced probing or advanced strategies that can detect additional defects. And since the VI method relies heavily on the human eye, it’s all too easy for analysts to miss surface indications, especially minuscule cracking barely visible to the naked eye. 

Ultrasonic vs. Visual Inspection

Ultrasonic testing is another method that can pinpoint aberrations that visual inspection may easily miss. Using various UT instruments, analysts can easily and accurately probe large infrastructures, such as oil pipelines or large welding sheets. With the visual inspection method, on the other hand, analysts depend on having direct physical and visual access to the entirety of the surface being inspected—which may not always be possible when dealing with larger test objects.

UT can be used to inspect larger assets in two key ways:

• Corrosion Scanning: Scanning devices can detect corrosive anomalies and other indications, in addition to wall thinning stemming from corrosive substances. It can also adapt to curved surfaces and detect abnormalities below the surface level. VI cannot conform to unconventional shapes with the same efficiency as the scanning method, nor can it determine wall thickness without the aid of another nondestructive testing method.   
• Phased Array: Phased array Ultrasonic Testing (PAUT) is an enhanced UT technique that provides custom beam shapes and multiple scanning angles. PAUT allows analysts to probe subsurface flaws on a deeper level than visual inspection or even eddy current testing. VI also lacks the flexibility that PAUT provides. 

In particular, corrosion scanning with PAUT instrumentation surpasses visual inspection by adapting to curved and irregular surfaces with ease. This is possible through encoded wheels that follow the scanner position in real-time, ensuring complete coverage of the testing radius. Additionally, it can provide analysts with a 3D image layout of the tested area, with the scanner allowing inspectors to identify areas that haven’t been covered. Once again, visual inspections can match neither the efficiency nor the accuracy of advanced NDT methods like UT.

As for phased array, PAUT comes with the same benefits as a corrosion scanning. If probing a solid object, for example, a PAUT instrument could detect small cracking in the center of that object. This would be impossible using VI since it can only detect deviations at the surface level. And since PAUT comes with multiple probing angles in a single scan, analysts no longer have to change the wedges or probes frequently, further streamlining the inspection process.

Is Visual Inspection Effective When Combined With Other Methods?

Visual inspection may be successful when used in conjunction with other techniques, but it should not be used on its own as a means for a complete inspection on critical assets. Analysts can significantly increase probability of detection (POD) in most cases by using an eddy current and/or ultrasonic testing solution that reduces inspection times and extracts impeccable data.

Even when dealing with larger, visible flaws, visual inspection on its own is not as effective as more advanced nondestructive testing methods due to the larger margin for human error. It’s all too easy to overlook even noteworthy flaws when depending solely on the human eye to collect data, and factors such as whether the analyst is sick, tired, or distracted may further compromise data quality. Technologies such as those used in eddy current and ultrasonic testing equipment mitigate this margin for error while offering deeper probing capabilities, data analysis support, and more comprehensive results.

Visual inspection may be less expensive and more accessible for monitoring existing flaws and for basic inspection checks, but these attributes are no substitute for high-quality data and quick, efficient testing processes required for complete critical asset inspections.

Zetec is a leading provider of eddy current and ultrasonic testing solutions that can help analysts easily detect hidden and subsurface flaws that would be overlooked by a typical visual inspection. Contact us today for more information!