May 18, 2009
Infrastructure is defined as the basic physical systems that serve a community's population, such as roads, bridges, utilities, water, and sewage. These systems are essential for enabling productivity in the economy, so it is crucial to ensure they are fit for use. Nondestructive testing (NDT) techniques determine whether a component has reached its useful service life and needs repair or replacement.
In-service load conditions produce incremental movement in materials and structural members. Whenever a force is applied to a member, it becomes stressed. The stresses cause strains, or movements, which are explained by the material properties. In-service conditions cause stress, strain, distortion, fatigue, and corrosion, which can manifest as discontinuities or defects.
A discontinuity is an interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. A discontinuity is not necessarily a defect. A defect is discontinuity that either by nature or by accumulated effect renders a part unable to meet minimum applicable acceptance standards or specifications. In other words, the part is rejected.
NDT locates discontinuities or defects in critical areas without causing structural damage.
Two fundamental categories of discontinuities that are located by NDT are surface and subsurface discontinuities.
Surface discontinuities, including cracks, porosity, slag inclusions, excess reinforcement, inconsistent weld bead profile, undercut, and blisters, usually can be identified by visual testing. To enhance surface VT, magnetic particle testing or liquid penetrant testing is employed.
Visual Testing (VT). VT is the most basic, cost-effective NDT method. It should take place prior to, during, and after welding. Many standards require it before other methods because there is no point in submitting an obviously bad weld to sophisticated inspection techniques. Welding codes always state that welds subject to nondestructive examination shall have been found acceptable by visual examination. VT requires good eyesight in the technician and sufficient light, a weld size gauge, a magnifying glass, and a 6-in. metal ruler.
Magnetic Particle Testing (MT). Magnetic particle testing detects surface and near-surface defects in ferromagnetic materials only. Defects in magnetized materials will distort the magnetic field, causing a leakage field. When fine ferromagnetic particles are applied to the surface, they concentrate at the defect by getting caught in the leakage field.
Liquid Penetrant Testing (PT). PT detects surface-breaking defects in any nonporous material. A liquid penetrant is applied to the surface and is drawn into defects by capillary action. Once a preset dwell time has passed and excess penetrant removed, a developer is applied to draw out the penetrant from the defect. Visual inspection is then performed.
PT also reveals surface cracks and pinholes that are not visible to the naked eye. It is used to locate leaks in welds and can be applied with austenitic steels and nonferrous materials on which magnetic particle inspection would be useless.
The NDT technician may use VT, MT, or PT during the welding operation to assist the welder in revealing discontinuities (for example, performing MT on the weld after backgouging to ensure sound weld metal has been obtained before welding the second side).
Subsurface discontinuities include underbead cracks, wormhole porosity, lack of fusion, slag inclusions, voids, and laminations. Radiographic testing (RT) or ultrasonic testing (UT) help to locate these volumetric discontinuities which are not visible on the surface.
Ultrasonic Testing (UT). UT detects discontinuities within the internal structure of welds. The advantage of this testing method is its ability to help establish internal integrity without destroying the welded component.UT is used on ferrous and nonferrous materials and is suited for testing thick sections accessible from one side only. It can also locate fine linear or planar defects. UT uses mechanical vibrations similar to sound waves but at a higher frequency. A beam of ultrasonic energy is directed into the object. The beam travels through the object with insignificant energy loss, until it is intercepted and reflected back to the UT instrument screen by a discontinuity.
UT generally uses the contact pulse reflection technique, in which a transducer converts electrical energy into mechanical energy. The transducer is excited by a high-frequency voltage that causes a piezoelectric crystal to vibrate mechanically. The crystal probe becomes the source of ultrasonic mechanical vibration. These vibrations are transmitted into the item through a couplant fluid. When the ultrasonic wave pulse strikes a discontinuity in the test item, it is reflected back to its point of origin. Thus, the energy returns to the transducer, which also serves as a receiver for the reflected energy.
The detection, location, and evaluation of discontinuities become possible because the velocity of sound through a material is considered constant, making distance measurement possible, and the relative amplitude of a reflected pulse is proportional to the size of the reflector.
One of the most useful characteristics of UT is its ability to determine the exact position of a discontinuity in a weld. NDT technicians performing the tests are required to be qualified as a level II in accordance with ASNT standards. This method requires a high level of operator competence and depends on establishing and applying test procedures.
Radiographic Testing (RT). RT uses X-rays produced by an X-ray tube or gamma rays produced by a radioactive isotope to test welds that can be accessed from both sides. Though it is slow and expensive, it detects porosity, inclusions, cracks, and voids inside the metal or welds.
In this method, penetrating radiation is passed through a solid object onto photographic film, creating an image of the item's internal structure. The amount of energy absorbed by the object depends on its thickness and density. Energy not absorbed by the object causes exposure of the radiographic film. These areas will be dark when the film is developed. Therefore, areas of the object where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film. Low-density inclusions, such as slag, will appear as dark areas on the film, while high-density inclusions, such as tungsten, will appear as light areas.
All discontinuities are detected by viewing the weld shape and variations in the density of the processed film. This permanent film record of weld quality is easy to interpret by personnel who are properly trained. Only qualified personnel should conduct radiography and radiographic interpretation because X-ray and gamma radiation can be hazardous.
During bridge fabrication, the American Welding Society (AWS) has outlined the percentage of welds that must be tested (D1.5, "Bridge Welding Cod—2008, Section 6.7"). For existing bridges, in-service engineering examinations follow NDT guidelines provided for new construction in the bridge code.
The code states that NDT (RT or UT) in addition to VT is to be performed to comply with the code by the following frequency requirements:
One hundred percent of each joint subject to calculated tension or reversal of stress, except on welds in vertical butt joints in beams or girder webs as follows:
1/6 of the web depth beginning at the point or point of maximum tension.
25 percent of the remainder of the web depth needs to be tested.
If unacceptable discontinuities are found in spot RT or UT, the entire length shall be tested. The requirements for RT and UT shall apply equally to shop and field welds. MT of fillet welds and partial-penetration groove welds joining primary components of main members shall be tested.
When inspecting infrastructure, the certified welding inspector needs to know the original requirements of the component being examined. For instance, the inspector has to make certain that the size, length, and location of all in-service welds conform to the requirements of the code and original detail drawings, and that unspecified welds have not been added without approval of the engineer.The expected quality of every in-service component is that every significant feature continues to meet the original designer's intent. NDT examinations provide reliable confirmation of fitness-for-service quality in accordance with the intent of the original designer.
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