Designed to provide accurate extension measurements from less than 1% to greater than 1000% strain, this versatile optical scanning device performs well for sample testing with an environmental chamber at elevated or even very low test temperatures, as well as at normal temperatures and environments. The United Laser Extensometer is unique in its ability to measure a broad range of materials including ultra-high modulus carbon composites, ceramics, steel and non-ferrous metals, paper and wood products as well as most varieties of elastomers.
LASER EXTENSOMETER THEORY OF OPERATION
A visible-light laser diode transmits a low-power laser beam to a rotating mirror that scans the sample material along the testing axis. A pair of retro-reflective gage targets affixed to the sample prior to the test returns the laser light to the rotating mirror and then to a laser receiver. The system measures the scan time between targets and continuously calculates the varying distance between targets trigonometrically.
Features & Benefits
OPTICAL EXTENSOMETRY: IT’S HERE TO STAY!
by Tom Settimi, Senior Scientist at United Calibration Corp, Huntington Beach, California
After several years since first appearing in testing laboratories, optical extensometry techniques have made important inroads in the materials testing community. In this article, the requirements which led to development of the current technology are summarized and the choices available to users are examined.
Testing laboratories and departments have long sought reliable strain measurement methods which avoid actual physical contact with the test specimen. In some cases, this requirement is due to an environmental limitation of the measurement device. For example, mechanical strain gage extensometers may not accurately measure or even survive at temperatures above 350 Deg F. Non-contact requirements may also be imposed by test specimen characteristics. In the testing of composite fiber strands or delicate films and adhesives, imperceptible damage from contact with knife edges or the influence of extensometer weight may significantly impact measurement results.
In recent years the testing community has been offered a number of new optical-, video- and laser-based devices hoping to satisfy the need for non-contact strain measurement. The technical approach employed by these devices varies widely; from video image processing of gage marks to measuring lateral displacement at two gage locations using phase measurement of scattered laser light. One method uses speckle metrology to measure strain at a single specimen location. With respect to sample preparation, most methods require the application of gage marks, targets or flags. One system uses drawn gage lines on the specimen while another requires no marks or flags at all. The performance claims are similarly varied, and as one would expect, so is the pricing.
The Laser Extensometer
Development of the Laser Extensometer began in the early 1980’s at United, combining the operational theory of optical scanners and opto-mechanical design of laser radar units. The earliest models were quite large and less convenient to use because they employed available non-visible (infra-red) laser diodes.
The current United Model EXT-62-LOE Laser Extensometer uses a two-facet rotating mirror assembly and visible light laser diode transmitter to generate a top-to-bottom scan line along the axis of a test specimen as shown in Figure 1.
The scan line intercepts a pair of retro-reflective targets which have been applied to the specimen prior to the test and which define the initial gage length. When the transmitted beam encounters a retro-target, energy is returned along the very same optical path, first back to the rotating mirror assembly and then to a detector mounted coaxially with the laser diode transmitter. The video pulses generated at the detector have an amplitude proportional to the energy reflected by the gage marks and a duration proportional to the width of the gage marks. A video processor samples the maximum height of each pulse and triggers the timing system when pulse amplitude decays to 50% of the peak. Thus the gage marks are defined by the bottom edge of each retro-target. The mirror rotation time required to scan the laser beam from the top of the scan at +40 degrees to the first retro-target is accurately recorded with a crystal controlled counter and defines Angle A to the first target. A second counter determines Angle B, the measured angle between the two retro-targets. Now knowing the two angles A and B and the distance from the rotating mirror to the specimen, the actual % Extension or Gage Elongation may be calculated with high precision in real time.
Laser Extensometer Performance
The United Laser Extensometer is a highly versatile extension measuring device for ambient as well as elevated temperature testing and is capable of accurate measurement from less than 1% to more than 1,000% extension. With input power conditioning provided by the High Modulus Unit (now included as standard equipment) and an initial gage length of 2 inches or greater, the Laser Extensometer meets or exceeds ASTM E-83 Class B1 requirements which call for a maximum error of +/- 0.5% of strain when actual strain exceeds 4% and a maximum (fixed) error of +/- 100 microstrain below 4% actual strain.
Extensometer resolution depends upon test distance as well as the data rate. For example, at a test distance of 9.5″, the resolution at 30 measurements per second is .0001″ ( 2.5 micron). At one measurement per second, the resolution improves to .000003″ ( .08 micron). The data rate is adjusted by providing the programmable counter board with coded instructions on how many successive scans to combine to create one extension data point reading. The maximum data rate is 60 measurements per second.
Laser Extensometer Advantages
In an actual testing situation, the user will find the Laser Extensometer exhibits the following useful characteristics:
Safety. The United laser extensometer emits visible laser light at power levels comparable to the average supermarket barcode scanner.
Versatility. It is able to accurately measure a wide variety of materials including high modulus composites, metals, plastics and elastomers. With a suitable environmental chamber that provides a window for external scanning, high temperature ( 700+ Deg F) and low temperature testing present no special difficulties. As with other non-contact methods, testing through to specimen break without having to remove a mechanical extensometer is a definite advantage.
Easy Setup and Use. Because the scan line is visible to the user, the Laser Extensometer is easy to aim; there are no special viewing lenses required. If a high elongation test is to be performed or an extra long gage length is needed, the unit is simply moved back, further from the specimen. There is no need maintain a suite of interchangeable lenses to accommodate different test setups.
High Accuracy. The system achieves a high level of performance measurable by established and accepted ASTM standards. The potential buyer should be wary of systems with vaguely worded performance specifications, especially those which avoid the widely accepted ASTM E-83 standard for extensometers. In addition, the automatic self-calibration feature assures accurate strain measurement at ambient temperature as well as for testing through the viewing port of an environmental chamber. A very small refractive error (typically 0.1% of the measured strain value) associated with scanning through the window of an environmental chamber is far less than the user can expect to experience from the barrel or pincushion distortion common to the imaging lenses used in video camera based extensometer systems.
Value. The United Laser Extensometer design started with a blank piece of paper. It does not rely on an integration of expensive components adapted to the task of extension measurement. With a price of under $ 10,000 the system is a clear example of more performance at far less cost to the user.
Application. When used with United computerized testing machines, the extensometer digital counter board is simply added to the testing machine computer. United testing system software is pre-configured to accept digital strain data from the United Laser Extensometer. When used with non-United testing machines, a Serial and Analog Module, available at moderate additional cost, is used to send strain data to either (1) the testing machine computer serial port or (2) an analog strain channel input on the load frame.