Laser inspection

A schematic diagram of the polymer precipitation process is shown in Figure 8. The hot polymer solution is cast onto a water-cooled chill roU, which cools the solution, causing the polymer to precipitate. The precipitated film is passed through an extraction tank containing methanol, ethanol or 2-propanol to remove the solvent. Finally, the membrane is dried, sent to a laser inspection station, trimmed, and roUed up. The process shown in Figure 8... 

The AeroSizer, manufactured by Amherst Process Instmments Inc. (Hadley, Massachusetts), is equipped with a special device called the AeroDisperser for ensuring efficient dispersal of the powders to be inspected. The disperser and the measurement instmment are shown schematically in Figure 13. The aerosol particles to be characterized are sucked into the inspection zone which operates at a partial vacuum. As the air leaves the nozzle at near sonic velocities, the particles in the stream are accelerated across an inspection zone where they cross two laser beams. The time of flight between the two laser beams is used to deduce the size of the particles. The instmment is caUbrated with latex particles of known size. A stream of clean air confines the aerosol stream to the measurement zone. This technique is known as hydrodynamic focusing. A computer correlation estabUshes which peak in the second laser inspection matches the initiation of action from the first laser beam. The equipment can measure particles at a rate of 10,000/s. The output from the AeroSizer can either be displayed as a number count or a volume percentage count. 

Laser inspection of the sample surface can also yield information on surface flaws such as the surface roughness and defects. The laser beam incident on the surface of the sample is scattered, and the scattered light intensity decreases when a crack is present on the surface. The experimental arrangement for scanning the surface of a sample is shown in Figure 2.7. A laser-based profilometer may also be used in the measurement of pit depths. 

The eombination in a compact system of an infrared sensor and a laser as excitation source is called a photothermal camera. The surface heating is aehieved by the absorption of the focused beam of a laser. This localisation of the heating permits a three-dimensional heat diffusion in the sample to be examined. The infrared (IR) emission of the surface in the neighbourhood of the heating spot is measured by an infrared detector. A full surface inspection is possible through a video scanning of the excitation and detection spots on the piece to test (figure 1). 

The efficiency of gas turbines is limited by the maximum allowable turbine inlet temperature (TIT). The TIT may be increased by cooling of the blades and vanes of the high pressure turbine. Cooling channels can be casted into the components or may be drilled afterwards. Non-conventional processes like EDM, ECD or Laser are used for drilling. Radiographic examination of the drilled components is part of the inspection procedure. Traditional X-Ray film technique has been used. The consumable costs, the waste disposal and the limited capacity of the two film units lead to the decision to investigate the alternative of Real-Time X-Ray. 

Speckle shearing interferometry, or shearography, is a full field optical inspection teclmique that may be used for the nondestructive detection of surface and, sometimes, subsurface defects. Whilst being more sensitive in the detection of surface defects, it may also be considered for pipe inspection and the monitoring of internal conoslon. In contrast, laser ultrasound and other forms of ultrasound, are point by point measurement techniques, so that scanning facilities and significant data processing is required before information on local defects is extracted from any examination of extensive areas [1 - 3]. 

Laser-based profilometry is now being applied to a wide variety of both NDT and Quality Control gauging applications. In the world of NDT, the primary interest is in the details associated with surface topography or deformation of a particular component. Laser-based profilometry systems are commonly used to inspect surfaces for defects such as pitting, corrosion, deformation and cracking. Quality control gauges are used for absolute measurement of dimensions, such as the diameter and thickness of a given part. 

One of the earliest NDT applications for laser-based profilometry systems was for the inspection of marine boiler tubes. Under funding from the U.S. Navy", several systems were developed that were capable of rapidly and accurately mapping the inside surface of 25 mm and 50 mm diameter boiler tubing. Features such as internal pitting and corrosion can be located and quantitatively assessed using computer-graphic analysis. The system employs an... 

A particularly insidious failure mechanism that is commonly found in carbon-steel tubing is under-deposit corrosion. In many cases, corrosion products fomi a scab that can mask the presence of the pitting, making it difficult to quantitatively assess using conventional NDT methods. However, by combining proper cleaning procedures with laser-based inspection methods, the internal surface of the tubing can be accurately characterized and the presence of under-deposit corrosion can be confirmed and quantified. 

Laser-based profilometry systems have also been applied for nondestructive testing and measurement of both smooth-bore and rifled gun tubes. Working through Small Business Innovation Research program, the U.S. Army has developed laser-based profilometry systems for the inspection of the 120mm cannon used on the MI-Al Abrams main battle tank. Systems have also been built to measure the erosion of 25 mm and 155 mm rifled gun tubes. 

The laser-based gun tube inspection systems are intended to augment and, perhaps eventually, replace the labor-intensive, time consuming and subjective visual inspection methods that are currently employed by these facilities. 

Another application of laser-based profilometry is the inspection of rocket and missile components. The U.S. Air Force has funded work to develop a non-contact laser-based profilometer for the inside surface of solid rocket motors. Over time, these devices are subject to slumping and cracking, which could potentially render the rocket motor ineffective and hazardous. When fully implemented, this system will provide a meaningful screening method for evaluating the condition of aging rocket motors. 

Other examples of government and military applications of laser-based profilometry include the evaluation of rocket thruster nozzles to locate and measure flame erosion remote inspection of hypervelocity test track and the measurement of sludge deposits on tube internal surfaces. 

Lalumandier, S. and Cowling, T., Integrated Laser Profilometry/Ultrasonic Inspection System for Furnace Tubes ASNT Fall Conference, Pittsburg PA, 1997. 

Doyle, J. L., and Bondurant, P. D. (1991) Development of an Automated, Laser-Based Gun Tube Inspection System, QUEST Technical Report No. 542, December. 

The laser beam is focussed on the sample surface using a microscope objective, and the signal is taken on axis through the same objective. An optical digital microscope allows for the visual inspection of the sample during the measurement the sample is mounted on a three-axis motorised stage, controlled by a personal computer, for adjustment of the focusing position. 

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