Module 8: Non-Destructive Testing

1. physical/mechanical deterioration

2. chemical deterioration

2 main deterioration methods for reinforced concrete

physical/mechanical deterioration

brought about by activities like abrasion/erosion, continuous overloading, or fatigue due to the mechanical loads, freezing and thawing

chemical deterioration

commonly due to the alkali-aggregate reaction, sulfate attack microbiological-induced attack and the corrosion of the rebar

nondestructive testing

non-invasive procedure that allows engineers/operators to quantitatively measure some characteristics of an object without interfering or altering integrity of material or suitability for service

1. structural members

2. moist/damp areas exposed to marine environment

3. locations with vibrations

locations usually inspected for damage

1. cracks

2. deflection/movement

3. discoloration

4. efflorescence

what is usually looked for before testing

visual and optical testing

most commonly used NDT method; no specialized operator

visual and optical testing

involves the visual observation of a surface for physical damage, cracks, corrosion, or any other form of surface discontinuities

direct viewing

viewing of an object in the operator’s immediate presence

remote viewing

viewing of an object not in the operator’s immediate presence

hand-held lenses

equipment for visual inspection; available from 1.5x up to 10x magnification; better designs will remove chromatic aberration effects

chromatic aberration

color fringing at the edges of the image

microscopes

has magnification up to 2000x

polarization

in microscope; useful for studying most materials with directional optical properties

phase contrast

in microscope; for inspecting transparent materials with discontinuities that are only faintly seen in normal microscope

interference microscope

in microscope; for extremely precise measurements of surface contour and other characteristics

rigid borescopes

for inspecting inside of tubes or pipes; can have various angles of view

endoscope

similar to rigid borescope but flexible due to use of fiber optics for both light guide and image guide

ultrasonic testing

uses sound frequencies to detect discontinuities in a material

500 kHz to 25 MHz

range of sound frequencies used for ultrasonic testing

1. pulser/receiver

2. transducer

3. display devices

components of a typical ultrasonic testing system

pulser/receiver

for UT; electronic device that can produce high voltage electrical pulse

transducer

for UT; generates high frequency ultrasonic energy operating based on the piezoelectricity technology using quartz, lithium sulfate, or various ceramics; driven by pulser/receiver

couplants

used to transmit ultrasonic waves from transducer to test piece; typically water, oil, glycerin, grease

acoustic emission test

involves stressing a part of the structure and monitoring acoustic response through material; during loading, if there are changes, friction and sliding will be source of acoustic signals; effective for long-term surveillance of load-bearing structures

acoustic impact technique

involves introduction of acoustic waves into an object through tapping and analyzing the resulting sounds to detect discontinuities and flaws; results depend on geometry and mass of members

radiography

uses x-ray and gamma rays to detect voids and position of stressing ducts; if a void or crack is present, more x-ray will pass that area into the film, so more exposure than area without discontinuity

eddy current testing

uses EM induction; utilizes coils where alternate current is passed through; changes in material can be detected by changes in coil’s impedance; limited by material’s conductivity, permeability, and set-up geometry

magnetic particle testing

limited to ferromagnetic materials or some of their alloys, as they can be magnetized to a level that will allow magnetic particle inspection

1. electromagnetic yoke

2. kerosene-iron filling suspension

apparatus needed for magnetic particle testing

magnetic particle testing

iron filling suspension is poured onto surface before magnet is switched on; magnetic flux will be broken to form new north and south pole around each edge of discontinuity, attracting the iron particle to edges of crack; resulting cluster of iron will make it easier to locate actual discontinuity

dye penetrant testing

uses fluorescent penetrant liquid applied to surface and allowed to seep into surface defect; excess liquid removed before developer powder applied so that penetrant can be seen; UV light can be used to increase test sensitivity