There are several advantages to using liquid penetrant testing (LPT) for surface defect detection, experts say. LPT is versatile, portable, simple, and sensitive, and can be used on a wide range of materials and even very complex parts.

The liquid penetrant testing process is excellent at finding surface discontinuities such as surface defects, porosity, lack of fusion on a surface, or surface-breaking cracks, said Cheri Stockhausen, product applications manager, Magnaflux.

“Liquid penetrant testing can be used to test large volumes of parts with lower start-up costs relative to some other NDT methods,” Stockhausen said. “Additionally, LPT can be used on parts of different shapes, sizes, and geometries while being compatible with almost all metal types.”

The first and most prominent advantage of LPT is its ability to detect surface breaking discontinuities on material, said Paul Lang, executive director of global certification & accreditation for the American Society of Nondestructive Testing (ASNT).

LPT is not suited for subsurface or internal discontinuities that do not connect to the surface, however, said Lang.

Training, Preparation and Consistency

Liquid penetrant testing is known for being relatively easy to perform, and it is usually one of the first methods for which quality technicians obtain certification, Lang said. Still, it requires skilled technicians to perform and interpret results accurately and consistently.

“Even though it’s one operation it has multiple steps and small variations during penetrant application, removal of excess surface penetrant, application of developer, or examination can produce large differences in inspection results,” he said.

The LPT process is highly dependent on the nondestructive technician’s knowledge, skill, and experience, Lang explained. As such, training and certification “are a must,” he said.

The most important part of an inspection comes before the actual inspection is conducted, said Stockhausen. Technicians must understand and plan for the unique requirements of each inspection.

“Additionally, having the proper materials and calibrated equipment in good working order is a necessary part of the inspection process and without it, consistency and quality cannot be ensured,” Stockhausen said.

Preparing the surface before inspection is key, said Greg Weaver, president, Weaver NDT.

“Any surface contamination, such as dirt, oil, grease, or metal smearing will negate the liquid penetration. If the material is not properly prepped, the inspection will be invalid.”

Proper procedure adherence is also important, and that’s why technicians benefit from training and certification, Weaver said. “As LPT is a stepwise process, these steps must be completed in the proper sequence and with the proper timing,” he explained.

Accessibility and material compatibility are also important, Lang said.

“LPT is not suitable for just any material, such as highly porous and chemically reactive materials,” he explained. Not only does the material surface need to be clean, properly using, storing, and cleaning up chemicals may come with its own learning curve.

New Advancements

Liquid penetrant testing is seeing innovation, such as digital imaging, low-VOC (volatile organic compound) penetrants, which reduce environmental impact and improve worker safety; penetrants with enhanced sensitivity to detect smaller and more subtle defects; penetrant inspection (FPI) systems that utilize fluorescent dyes and UV light to enhance defect detection sensitivity, and others.

Lang called LPT “one of the oldest and most widely used nondestructive testing methods,” as its basic application and process has remained the same for decades. “But over that time, variations have been developed in the method which the industry calls techniques within the method,” Lang explained.

Penetrant Makeup Evolves

Changes in the penetrant makeup have also changed over time. Today’s penetrants are carefully formulated to produce the level of sensitivity desired by the technician and examiner / inspector to achieve the proper inspection results, Lang said.

“Visible penetrants contain a red dye that is easily seen against a white developer background. Fluorescent penetrants contain a dye or multiple dyes that fluoresce when exposed to ultraviolet light,” Lang said. “The industry classifies penetrants based on detectability of the indication the technician is looking to evaluate. The strength of the penetrant has five classifications ranging from ultra-low sensitivity to ultra high sensitivity.”

Improved Methods

Techniques have evolved as well. The three primary methods technicians use are: water washable, where penetrant is removed from the part by rinsing with water; post-emulsifiable, where oil penetrant interacts with emulsifier to remove or lift excess penetrant; and solvent removal, which requires the use of a solvent to remove the penetrant from the part.

However, they have improved over time to “give a well trained and certified NDT professional greater flexibility, increased accuracy, and reliability … whether in the field or in the shop,” Lang said. For example:

• Water Washable: Improvements in the formulation of water-washable penetrants have made them easier to remove with water, reducing the risk of over-washing and losing indications. Enhanced dye formulations have increased the visibility of indications against the white developer background.
• Post-Emulsifiable: Advances in emulsifier technology have improved professionals’ abilities to control the interaction between the oil-based penetrant and the emulsifier. This has led to more precise removal of excess penetrant, which improves the accuracy of the indications without compromising sensitivity.
• Solvent Removal: The development of more effective and less hazardous solvents has made the process safer and more efficient. Improved solvent formulations help in thoroughly removing the penetrant without damaging the part, thereby increasing reliability in the inspection results.

Automation’s Role

Additionally, experts are seeing penetrant systems with varying levels of automation — such as full process conveyors and Automatic Defect Recognition (ADR).

Automation has provided “a uniform and consistent process, which has in turn allowed for more reliable inspection sensitivity,” Weaver said.

Advanced LPT technologies such as portable devices, automated systems, and digital imaging software enable faster inspection speeds compared to traditional methods, says David Sanders, NDT Level III inspector at Disneyland Resort. Wireless connectivity and IoT integration make real-time monitoring and data analysis possible. Inspectors can access inspection data remotely, monitor inspection progress, and analyze results in real-time, which helps them make decisions more quickly and quickly respond to potential issues.

Weaver agrees. “Digital imaging has enhanced interpretation as well as the possibility for remote viewing, which has completely changed documentation and traceability efforts," he said.

Additionally, integrated LPT software platforms streamline data management, analysis, and reporting processes. Inspectors can use these tools to easily capture, store, and analyze inspection data, generate comprehensive inspection reports, and track inspection history for compliance and quality assurance purposes, Sanders said.

Adapting to the Labor Shortage with Automation

Because finding qualified NDT personnel to perform inspections is becoming increasingly difficult, shifting a portion of the inspection work to automated processes can allow qualified NDT personnel to focus on other tasks and optimize how inspectors work, Stockhausen said.

Shifting some inspection tasks to automated processes can help in the following ways:

• Efficient Use of Human Resources: Automated processes can handle routine and repetitive tasks, allowing qualified NDT personnel to focus on more complex and critical aspects of inspections that require their expertise and decision-making skills.
• Increased Productivity: Automation can speed up the inspection process, increasing the overall productivity of the inspection team. This means that more inspections can be completed in a shorter amount of time.
• Consistency and Reliability: Automated systems can perform inspections with a high level of consistency and reliability, reducing the risk of human error and ensuring that inspections are performed to the same standard every time.
• Optimized Workflows: By integrating automated processes, organizations can optimize workflow for NDT inspections. This allows inspectors to manage their time more effectively and focus on tasks that add the most value, such as interpreting results and making decisions based on findings.

Liquid penetrant testing is a tried-and-true nondestructive testing method that excels at finding surface defects in various materials and complex parts. It’s cost-effective, versatile, and subtly evolving.