Liquid Penetrant Testing

Liquid penetrant testing

Liquid penetrant testing is widely used to detect surface breaking flaws in both ferrous and non-ferrous parts. It can be used both on site or in a laboratory application with either colour contrast or fluorescent techniques.In the laboratory it is particularly suitable for the inspection of batch quantities of machined parts and is a cost effective and efficient method of testing for surface breaking discontinuities.

Liquid penetrant inspection (LPI) is one of the most widely used nondestructive evaluation (NDE) methods. Its popularity can be attributed to two main factors, its relative ease of use and its flexibility. LPI can be used to inspect almost any material provided that its surface is not extremely rough or porous.

Materials that are commonly inspected using LPI include, Metals (aluminum, copper, steel, titanium, etc.), Glass, Ceramics, Many ceramic materials, Rubber, Plastics, Purpose of Penetrant Testing.

Some purposes of penetrant testing :

Liquid penetrant testing
  • Detect surface discontinuities
  • Examine many types and shapes of nonporous material
  • Detect defects during and after manufacture
  • Minimize failures through early detection
  • Detect various types of conditions

Properties of good Penetrant :

To perform well, a penetrant must possess following important characteristics :

  • Spread easily over the surface of the material being inspected to provide complete and even coverage.
  • Be drawn into surface breaking defects by capillary action.
  • Remain in the defect but remove easily from the surface of the part.
  • Remain fluid so it can be drawn back to the surface of the part through the drying and developing steps.
  • Be highly visible or fluoresce brightly to produce easy to see indications.
  • Must not be harmful to the material being tested or the inspector.

Penetrants removable method:

The liquid penetrant processes are further sub-divided according to the method of washing of the specimen.

  • Water-washable,
  • Post-emulsifiable (An emulsifier is added to the excess penetrant on surface of the specimen to make it water-washable).
  • Solvent removable (The excess penetrant is needed to be dissolved in a solvent to remove it from the test specimen surface).

Steps of a Liquid Penetrant Inspection :

  • Surface Preparation
  • Penetrant Application
  • Penetrant Dwell
  • Excess Penetrant Removal
  • Developer Application
  • Indication Development
  • Inspection
  • Clean Surface

Penetrant Materials

Penetrant materials come in two basic types. These types are listed below:

  • Fluorescent Penetrants
  • Visible Penetrants


The role of the developer is to pull the trapped penetrant material out of defects and spread it out on the surface of the part so it can be seen by an inspector. Developers perform is to create a white background so there is a greater degree of contrast between the indication and the surrounding background.

Developers are classified into six standard forms. These forms are

  • Form a - Dry Powder
  • Form b - Water Soluble
  • Form c - Water Suspendable
  • Form d - Nonaqueous Type 1 Fluorescent (Solvent Based)
  • Form e - Nonaqueous Type 2 Visible Dye (Solvent Based)
  • Form f - Special Applications

The developer classifications are based on the method that the developer is applied. The developer can be applied as a dry powder, or dissolved or suspended in a liquid carrier. Each of the developer forms has advantages and disadvantages.


  • The method has high sensitivity to small surface discontinuities.
  • The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected.
  • Inexpensive, sensitive, versatile, portable, easy to leans
  • Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.
  • Parts with complex geometric shapes are routinely inspected.
  • Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.
  • Aerosol spray cans make penetrant materials very portable.
  • Penetrant materials and associated equipment are relatively inexpensive.


  • Only surface breaking defects can be detected.
  • Only materials with a relatively nonporous surface can be inspected.
  • Pre cleaning is critical since contaminants can mask defects.
  • Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI.
  • The inspector must have direct access to the surface being inspected.
  • Surface finish and roughness can affect inspection sensitivity.
  • Multiple process operations must be performed and controlled.
  • Post cleaning of acceptable parts or materials is required.
  • Chemical handling and proper disposal is required.

Download Sample Report and Procedure of Liquid Penetrant Testing

Liquid penetrant testing NDT Sample Procedure
Liquid penetrant testing NDT Sample Test Report Format

This course covers the principles of Liquid Penetrant Testing and prepares a candidate to :

  • Setup and calibrate equipment
  • Interpret and Evaluate Results with respect to Applicable Codes, Standards and Specifications
  • Familiar with the scope and limitations of the Methods
  • Write test reports

Responsibilities of ASNT Level II Liquid Penetrant Testing (LPT) Personnel

  • A certified ASNT Level II Liquid Penetrant Testing (LPT) personnel is qualified to perform Liquid Penetrant Testing (LPT) according to the recommended procedure
  • A certified ASNT Level II Liquid Penetrant Testing (LPT) personnel isqualified to select Liquid Penetrant Testing (LPT) among various penetrant removable methods to be used
  • A certified ASNT Level II Liquid Penetrant Testing (LPT) personnel is qualified to Calibrate the equipment and carry out Penetrant test
  • A certified ASNT Level II Liquid Penetrant Testing (LPT) personnel is qualified to Interpret and evaluate Liquid Penetrant Testing (LPT) result according to applicable standards.
  • A certified ASNT Level II Liquid Penetrant Testing (LPT) personnel is qualified to Prepare Liquid Penetrant Testing (LPT) report for acceptance/rejection Criteria
  • A certified ASNT Level II Liquid Penetrant Testing (LPT) personnel is qualified to Know about the merits and demerits of Liquid Penetrant Testing (LPT) and other common NDT methods

Personnel Certification: ASNT SNT-TC-1A

Module 1: Manufacturing Discontinuities

Types of Discontinuities: Inherent, Processing and Service Casting Discontinuities: Hot Tear, Cold Shut, Porosity, Shrinkage.

  • Primary Processing Discontinuities including discontinuities in Rolling, Forging, Drawing, Extruding.
  • Secondary Processing Discontinuities including discontinuities in Grinding, Heat Treating, Machining, Welding, Plating.
  • Service Discontinuities: Erosion, Wear, Fatigue, Corrosion, Creep, Hydrogen Attack

Module 2: Principles

  • Purpose of Liquid Penetrant Testing
  • Physical Principles
  • Wetting Characteristics and Contact Angle
  • Wetting Ability
  • Force of Cohesion and Surface Tension
  • Capillary Action
  • Viscosity
  • Application of Penetrant: Dwell Time
  • Reversed Capillary Action
  • Visibility of Indication
  • Categories of Test Processes
  • Types of Liquid: Type I and Type II
  • Methods of Removal of excess Penetrant including Water Washable, Emulsifiers and Solvent Removable.
  • Sensitivity Levels – ½, 1, 2, 3, 4
  • Selection of Process
  • Limitations of Penetrant Testing

Module 3: Basic Steps

This Module covers the basic steps involved in the following processes :

  • Method A – Water Washable
  • Method B – Lipophilic Emulsifier
  • Method C – Solvent Removable
  • Method D – Hydrophilic Emulsifier

Module 4: PRE and Post Cleaning

  • Choice of Cleaning Method
  • Different Cleaning Methods including Detergent, Solvent, Alkaline, Steam, Ultrasonic cleaning, Vapor Degreasing.

Module 5: Application of Penetrants & Developers

  • Different ways of applying penetrants
  • Standard Temperature Limits
  • Dwell time
  • Drying
  • Drying Parameters
  • Drying Time Limits
  • Application of Developers
  • Types of Developers
  • Developing Time
  • Fluorescent Inspection
  • Black Light, Black Light Warm Up Time,
  • Visual Adaptation
  • Post Cleaning

Module 6: Interpretation

  • Interpretation of Test Results
  • Flow Chart for Interpretation
  • Types of Indications like True, False, Relevant and Non Relevant indications
  • Categories of Indications: - Rounded and Linear
  • Evaluation of Indications
  • ASTM E-433 Reference Photographs of Indications types

Module 7: Codes & StandardS (Specific Training)

Interpretation including Relevant, False, Non-relevant indications

  • ASME Section V, Article 6
  • ASME Section VIII, Appendix 8 (Accept/Reject Criteria) Standards
  • ASTM E-165
  • ASTM E –1417
  • Other codes and standards can be discussed at the request of the students. Please make such requests at time of registration.