Non-Destructive Testing Overview

Non-Destructive Testing (NDT) Overview

• NDT is used to inspect materials without destroying them
• This is important for reusable parts that are valuable or components that need to be reused after testing
• Testing is done for quality control purposes on components that cannot be harmed during testing, such as a fetus in a mother's uterus.

NDT Methods

• Visual Testing: Uses naked eyes or aided devices like magnifying glasses or borescopes for surface defect detection, and reliability depends on the inspector
• Hydrostatic Testing: Uses water (or helium in high sensitivity tests) at 1.5-2 times the working pressure to check components for leaks. Leaks are detected by pressure changes or seepage
• Dye Penetrant Testing: Used for finding surface flaws in homogeneous materials. A dye is introduced to surface openings, excess dye is removed, and internal flaws show up as dye penetration
• Magnetic Particle Testing: Employed to find surface or subsurface flaws only in ferromagnetic materials. The specimen is magnetized, particles are applied, and flaws attract the particles, forming visible indications
• Radiographic Testing: Uses X-rays (or gamma rays) to detect internal defects in materials. Images are made based on how materials attenuate radiation;
• Ultrasonic Testing: A technique used for inspecting internal defects and measuring material thicknesses. Ultrasound waves are used to check for flaws that show as echoes.
• Eddy Current Testing: Detects surface and subsurface flaws in electrically conductive materials, by detecting changes in the magnetic field.

Visual Testing Specifics

• Based on visual inspection of surfaces (e.g, using tools, mirrors, mirrors, fiberscopes, etc.).
• Optical devices like magnifying glasses, and borescopes may be used for more detailed observation.

Hydrostatic Testing Specifics

• Components are pressurized with water at 1.5-2 times the working pressure. Seeps or pressure changes indicate leaks.

Liquid Penetrant Testing

• Detects surface flaws in homogeneous materials.
• A dye is used, and excess dye is removed, internal flaws show up as dye.
• Visible dyes are red hues, observed under white light
• Fluorescent dyes are viewed under UV or black light

Liquid Penetrant Testing - Removal of Excess Penetrant

• Three ways to remove excess penetrants: Water Washable, Post-emulsified or Solvent removable

Liquid Penetrant Testing - 6 Basic Processing Steps

• Pre-cleaning, Penetrant application, Excess penetrant removal, Developer application, Inspection and interpretation, Post-cleaning.

Liquid Penetrant Testing - Methods of Penetrant Application

• By spraying, immersion or dipping and brushing. For localized inspections or thicker pieces, a thin layer application is preferred.

Liquid Penetrant Testing - Removal of Excess Penetrant - Specifics

• Water pressure should not be too high
• Spray nozzles should not be too closed to allow proper penetrant flow to surface crevices.
• Removal using solvent-dampened cloth is preferable and wiping direction should only be in one direction.

Types of Developer for Liquid Penetrant Testing

• Dry developer is a powder, non-aqueous wet developer is a suspension or solution in a liquid. Wet developer is dissolved in liquid or solution.

Liquid Penetrant Testing - Inspection and Interpretation

• Inspecting the test surface under normal white light or black light
• Identifying and recording type of indication
• Assessing the indications' relevance

Types of Penetrant Relevant Indication

• Fluorescent penetrant shows cracks as fluorescent lines
• Visible penetrant shows cracks as glowing spots

Magnetic Particle Testing - Specifics

• Finds surface or subsurface flaws only in ferromagnetic materials (metals with iron, nickel, or cobalt)
• Materials are magnetised with strong magnetic fields, causing magnetic lines that run through the component
• Fine iron particles coated with a dye are applied, particles accumulate on the flaws due to disrupted flux path or leakage field. Indications are visible under proper lighting

Magnetic Particle Testing - Defect Orientation

• Cracks between 90° and 45° show correctly
• Cracks parallel to the magnetic field lines may not be detected

Magnetic Particle Testing - Magnetisation Techniques

• Employing permanent magnets or electromagnets in contact with the workpiece or passing electric current through it directly or passing electric current through a coil of wire around the part.
• The field direction is relevant for defect detection.

Magnetic Particle Testing - Types of Magnetic Particles

• Dry magnetic particles (powder): Used for visible inspection. May be coloured to highlight indications more clearly.
• Wet magnetic particles (liquid suspension): Generally more sensitive than dry particles, use liquids for better mobility enabling more sensitivity

Magnetic Particle Testing - Limitations

• Only works well for ferromagnetic materials.
• Direction of the defect relative to the magnetic field is significant.
• Current magnetisation may result in marks on the item being inspected.

Magnetic Particle Testing - Advantages

• Sensitive for detecting both surface and subsurface defects
• Simple and inexpensive method
• Visible indication on surface
• Unaffected by surface contaminants.

Magnetic Particle Testing - Post-cleaning

• Residual magnetism can hinder subsequent processes.
• Demagnetization is necessary to minimize its impact.

Magnetic Particle Testing - Demagnetization

• Demagnetization units can reduce magnetism
• A gauss meter is used to check for satisfactory demagnetization (below 2 Gauss).