Laser scanners use a laser to scan the surface of an object to derive various information, such as surface profile, distance, and even more. Dimensional measurements are captured in one of two ways: either by laser triangulation or by laser displacement.
In both methods, the sensor emits a laser at an object, which then reflects the laser beam back to the receiving element to pick up and interpret. Laser triangulation relies on the angle of the reflected light to calculate distance by applying the triangulation principle on angles formed by the laser, the spot on the object, and the receiver.
A laser displacement sensor, on the other hand, measures the distance to the target by detecting the time it takes for a laser beam to reflect from the target&#;s surface. This non-contact laser measurement method is also known as the time-of-flight method, and it&#;s most used in long-range industrial applications.
Laser triangulation is also used in industrial settings, particularly automation, robotics, and quality control processes.
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The material and environmental conditions control the dimensional stability of solid bodies. Strains at break should be precisely recorded from a low percentage in the case of metals and around 100% in the case of elastomers and plastics.
Contact extension measurement up to break on flexible and highly elastic specimens like wires or synthetic ropes can pose a major problem. At specimen break, the damaged ends of the specimen whiplash and often hit the specimen&#;s parts that are still in the grips. Such an effect is induced by elastic resilience. Subsequently, the specimen ends could wrap around the arms of the sensor and impair them. The use of non-contact measurement systems is recommended to achieve safe and precise measurements when testing high elastic, high extension, and contact sensitive materials.
These systems can exactly determine the changes in length, without coming into contact with the specimen at normal temperatures and also at fluctuating temperatures for tests in temperature chambers. Non-contact measurement systems provide a high degree of operational safety for specimen whiplashing at break, thus discharging high mechanical energy, or which unravel at break, for instance, hemp and wire ropes, fiber-reinforced plastics, and elastomers.
The laserXtens stands out from the series of contact-free extensometers as it is a unique development. Since it works without measurement marks, it has absolutely no effect on the specimen.
A technology only offered by Zwick: Non-contact extensometers according to the laser speckle principle.
The option strain distribution is used for determining localized strains, which are subsequently available as channels in testXpert®. In addition, the automatic recognition and evaluation of around 16 measurement marks are possible. The beginning gage length can also be balanced to automatically follow the necking-in in real time (according to ISO -1, annex H).
Through the optional Test Re-Run module, an image series can be recorded at the time of a test to be utilized for subsequent recalculation of the strain that has a varied initial gage length (if a number of markings are present). This can be particularly beneficial if local strains have to be assessed at varied locations in component testing, or alternatively, if the necking of the specimen in a regular tensile test took place beyond the original initial gage length. If testXpert® II is used to record the test, then the recalculated strain can be naturally synchronized later with the other measured values. The testXpert® II version 3.4 provides this option.
videoXtens offers high-resolution, contact-free measurement of extension, both in the compression and tensile direction, on different types of metal, plastic, foils, composites, and rubber. It can also be used for determining R-values according to ISO and ISO , proof strength (offset yield) in tensile tests according to ISO -1, and transverse strain.
The digitized image of the specimen, which is processed in real time, is generated by a full-view camera. The gage marks are automatically detected and the displacement of the marks from one frame to another is transformed to an extension value and conveyed to the measurement and control electronics.
videoXtens&#;Non-contact measurement in a temperature-controlled fluid bath
The camera&#;s field of view decides the resolution of the videoXtens. While smaller image translates to better resolution, the measuring range also tends to be smaller. The Array variant of the videoXtens provides a flexible solution when an application needs a huge measurement displacement integrated with extremely high resolution. Here, a single large field of view is formed by combining the overlapping fields of view from two or more cameras. Markings exiting one camera&#;s field of view are automatically forwarded to the subsequent camera&#;s field of view, and so on.
Varied combinations are thus possible, for instance, a pair of cameras in a single videoXtens standard housing, combined with a total field of view, is suitable for the majority of metal tensile tests that have an initial gage length of around 100 mm. A measuring head containing three cameras is provided in an enlarged videoXtens housing. It is especially appropriate for testing wires, structural steel, heavy plates, and a number of plastics. Cameras can also be combined, each in a standard videoXtens housing, to create an array with an extremely large total field of view.
Extensometers available from the laserXtens system series can be used for measuring deformation or strain on many different materials. The use of the latest laser speckle technology eliminates the need to attach marks and prevents contact with the specimen at the time of the test.
This allows the laserXtens systems to operate in various applications:
Flexible yet easy to operate, the laserXtens systems provide the perfect solution for quality control applications and, at the same time, offer key technological advantages to organizations involved in research and development.
The laserXtens systems include measuring heads comprising of a laser light source and digital cameras. When the specimen is illuminated with the laser light, a speckle pattern is produced on the test specimen&#;s surface.
The speckle pattern can be considered as a &#;digital fingerprint&#; or &#;virtual measuring mark&#; on the specimen&#;s surface and is tracked with the two full-frame digital cameras. This virtual measuring mark is tracked by the laserXtens software in consecutive images captured at the time of the test, and this process is known as speckle tracking.
When a load is applied to the specimen through the testing machine, the speckle pattern shifts and the laserXtens software iteratively tracks the speckle pattern from one image to another in real time and establishes the strain in the specimen.
A speckle pattern is generated on the surface of the specimen with the laser light
Each camera image is shown in an analysis window on the PC and yellow cross hair lines define the gage length. For the optional measurement of transverse strain, the transverse measurement systems can be set up using the available crosshairs.
It is possible to mount the laserXtens at varied distances to the specimen, thus making it appropriate for use with temperature chambers, for instance. When mounted close to the specimen, the laserXtens HP becomes suitable for strain-controlled tests in accordance with ISO -1 method A1closed loop.
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Two modes of operation are included in the software algorithm. As the virtual marks shift inside the camera&#;s field of view, the software automatically shifts the analysis window. In this mode, an elongation of typically 40 mm can be determined. When the analysis window arrives at the edge of the field of view, there is an automatic switch to the second measuring mode.
The material flow inside the analysis windows is now determined and the strain value is calculated accordingly. Based on material and specimen&#;s behavior, this mode results in highly accurate measurements (class 1), even being not according to the standard.
After the extensometer is securely and mechanically linked to the testing machine, it follows the cross-head at half the test speed. This guarantees that the laserXtens remains invariably in the center of the test area, and consequently, the effective measuring range for speckle tracking is augmented.
Figure 1. Equipped with green laser light and high-temperature tunnels the laserXtens can also be used for high temperature testing up to °C.
The single-camera measuring systems are specifically suitable for testing small to micro specimens. Alternatively, the laserXtens can also work with just a single camera head. Both analysis windows, in this configuration, are set within the one camera image and the gage length is indicated by the distance between the analysis windows. Therefore, the gage length is restricted to the size of the camera&#;s field of view.
Figure 2. laserXtens Compact
The laserXtens Compact is compatible with all Allround-Line floor and table-top testing machines, while the laserXtens Compact HP has been developed for mounting on the Precision Line Vario.
The laserXtens Array measuring head includes four fixed high-resolution cameras, unlike the regular laserXtens, on which a pair of digital cameras placed on motorized slides can be applied to set varied initial gage lengths.
Figure 3. laserXtens Array with four cameras
A single, large image is formed by combining the overlapping fields of view of the four cameras. Here also, the two virtual gage marks are followed at the time of the loading process (speckle tracking). When a gage mark meets the edge of the field of view of one camera, it is forwarded to the field of view of the neighboring camera. This technique considerably expands the measuring range.
Figure 4. Complete specimen image assembled from four images
The system moves to flow mode only when one of the gage marks arrives at the edge of the total field of view. In flow mode, the material flow below the evaluation window is measured to determine the calculated value. In addition, based on the material and/or the specimen deformation, excellent results (that is, accuracy grade 1) are achieved with this non-standard technique.
lightXtens® is an optical extensometer and is suitable for providing accurate and reliable measurements in tensile tests on highly elastic, highly ductile, and touch-sensitive materials like latex, elastomers, as well as all types of foil.
It is appropriate for all specimens exhibiting whiplash or high energy at break and is thus likely to affect mechanical, contact-measuring systems. This is usually the case with ropes, belts, and steel litz wire, for instance.
It offers accurate, non-contact strain measurement, even over long temperature ranges in temperature chambers. lightXtens provides an attractive alternative to video or laser-based extensometers, thanks to its user-friendly operation and robustness in test conditions.
Figure 5. lightXtens
Figure 6. lightXtens identifies gage marks automatically
This information has been sourced, reviewed and adapted from materials provided by ZwickRoell GmbH Co. KG.
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