Visual Inspection ROBIN JAMES Assistant Professor Providence College

Visual Inspection ROBIN JAMES Assistant Professor Providence College of Engineering, Chengannur 9446791838 robinjames 35@gmail. com

Visual Inspection Visual inspection is by far the most common nondestructive testing (NDT) technique. “Visual inspection is defined as the process of using the unaided eye, alone or in conjunction with various aids, as the sensing mechanism from which judgments may be made about the condition of a unit to be inspected. ”

• When attempting to determine the soundness of any part or specimen for its intended application, visual inspection is normally the first step in the examination process. • Generally, almost any specimen can be visually examined to determine the accuracy of its fabrication. • For example, visual inspection can be used to determine whether the part was fabricated to the correct size, whether the part is complete, or whether all of the parts have been appropriately incorporated into the device.

Basic Principle • Visual Inspection involves illumination of the test specimen with light, usually in the visible region. • The specimen is then examined with eye or by light sensitive devices such as photocells. • The surface of the specimen is adequately cleaned before being inspected.

Human Eye • The human eye is one of mankind’s most fascinating tools. • It has greater precision and accuracy than many of the most sophisticated cameras. • It has unique focusing capabilities and has the ability to work in conjunction with the human brain so that it can be trained to find specific details or characteristics in a part or test piece. • It has the ability to differentiate and distinguish between colors and hues as well. • The eye can perform accurate inspections to detect size,

Inspection Requirements • Requirements for visual inspection typically depends on 1. vision of the inspector 2. the amount of light falling on the specimen, 3. whether the area being inspected is in anyway obstructed from view

Defects which can be detected by Unaided Visual Inspection • General Condition of the component • Presence or absence of oxide film or corrosive product on the surface • Presence or absence of cracks, orientation of the cracks • Potential sources of mechanical weakness such as sharp notches or misalignment etc

• Mechanical and/or optical aids may be necessary to perform visual testing. • Mechanical aids include: Measuring rules and tapes, calipers and micrometers, squares and angle measuring devices, thread, pitch and thickness gages, level gages, and plumb lines.

• Optical aids include 1. Mirrors (especially small, angled mirrors) 2. Magnifying glasses, eye loupes, multilens magnifiers, measuring magnifiers 3. Microscopes (optical and electron) 4. Optical flats (for surface flatness measurement) 5. Borescopes and fiber optic borescopes 6. Optical comparators 7. Photographic records 8. Closed circuit television (CCTV) systems (alone and coupled to borescopes/microscopes) 9. Machine vision systems 10. Positioning and transport systems (often used with CCTV systems)

Optical Aids

• The use of optical instruments in visual inspection is recommended to ▫ 1. Magnify detects that cannot be detected by unaided eye. ▫ 2. Permits visual checks of areas not accesible to unaided eye.

Microscope • A microscope is an instrument used to see objects that are too small for the naked eye. • An optical microscope is a combination of lenses used to magnify the image of small object. • The object is placed close to the lens to obtain as high magnification as possible. • The distance from lens to object is adjusted untill the object is at depth of field of lens and is in focus.

• Magnification of single lens is determined by the equation M = 10/f where f is the focal length of the lens. The focal length of an optical system is a measure of how strongly the system converges or diverges light. If focal length is 5 ll then it’s a 2 X microscope. Field of view is the area seen through the magnifier.

• Field of view is the area seen through the magnifier. • Selection of magnifier with proper field of view is important. • The proper procedure is to use first a low power magnifier, marking questionable areas and then examine the suspected areas in detail with high powered magnifier.

Depth of Field • Depth of Field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image.

Image of an ANT obtained from Scanning Electron Microscope

Borescope • A borescope is an optical device consisting of a rigid or flexible tube with an eyepiece on one end, an objective lens on the other, linked together by a relay optical system in between. • Light source located infront or ahead of the object lens provides illumination for the part being examined.

• As the length of the borescope increases the image become less bright because of loss of light. • Borescopes are available in 2. 5 to 19 mm diameter and few meters in length. • In most borescopes the observed visual area is approximately 25 mm in diameter at 25 mm distance from the object.

Endoscope • Endoscope is much like borescope except that it has superior optical system and a high intensity light source. • A unique feature of endoscope is that objects are constantly in focus from about 4 mm to infinity. • The no focussing feature of endoscope makes it much easier to use than a borescope which needs to be focused at inspecting area. • Endoscopes are available in diameters down to 1. 7 mm and in lengths from 100 to 1500 mm.

Endoscope

Flexiscope • Flexible fibre optic borescopes permits manipulation of instrument aroung corners and through passages with several directional changes. • These devices are designed to create sharp and clear images of the part and interior surfaces that are normally impossible to inspect. • The working lengths are normally from 60 to 365 cm with diameters from 3 to 12. 5 mm.

Holography • Holography is the name given to the method of obtaining an accurate three dimensional image of an object. • Holography is used for the NDT of surfaces of highly complicated and precision components without the disadvantage of having to use a high power microscope.

Telescope • Telescope is used to obtain magnified images of objects at a considerable distance from the eye. • It essentially consists of two lenses called the objective and eye piece. • Telescope is normally used in conjuction with periscope for viewing a concealed surface. • CCTV camera is also used for this purpose.

Periscope