Advanced optical High-Speed testing: Investigations of materials and components utilizing High-Speed cameras
Keywords: High-Speed Tensile test, crash, drop test, cell phone, machine tool, process optimization
Optical 3D measuring systems for movement and deformation analysis like ARAMIS and PONTOS are becoming more and more accepted in material and component testing for several years now. Due to their large number of measuring points and the full-field results, they provide a better understanding of the material and component behavior and of the material failure mechanisms.
When determining material characteristics, optical measuring techniques provide a larger validity range, particularly with respect to plastic deformation as they measure the plasticization locally. Thus, parameters can be determined that take the local plasticization into account. Therefore, in sheet metal forming, flow curves from tensile and bulge tests, Forming Limit Curves and much more are determined.
Higher demands on crash safety in the automotive industry and on the dynamic behavior of other products result in a large demand for more convincing experimental methods. With this respect, the non-contact optical 3D measuring technology has a particular advantage as no sensor technology is exposed to the high speeds or accelerations.
To examine such fast processes, CMOS cameras are used which provide a sufficient frame rate to record the test, which normally takes just a few milliseconds, with enough images. In addition, to capture three-dimensional data, two cameras are used which, above all, need to be operated synchronously for achieving precise 3D results (Fig. 1). Generally, this requirement can be met by modern and identical cameras from manufacturers like Photron, Vision Research, Redlake, NAC, etc.
Such cameras typically achieve frame rates of 1 to 3 kHz at an image size of approx. 1 M pixels. For a reduced image resolution, higher frame rates of up to 50 kHz or more may be achieved.
The fracture behavior of many materials depends on the rate of loading. To provide strain rate dependent values, ARAMIS is used with high-speed cameras in a high-speed tensile test. Fig. 2 shows a corresponding tensile test with a plastic specimen, and the cameras achieved a frame rate of 45,000 images per second. The entire test up to the moment of breaking took 1.3 ms and thus was captured in about 60 stages. The resulting strain rate at the failure location is 1500 per second.
Fig. 3 shows the path of motions of the saw blade drive of a pendulum jigsaw. The movement of the saw blade in sawing direction (pendular stroke) and the motion of the guide roller is displayed using the PONTOS system. For this purpose, the reference point markers are applied to the movable but also to the fixed parts (e.g. the housing) of the jigsaw. The rigid body movement like oscillations and the forward movement of the saw are determined based on the movement of the housing and are compensated by the motions of the movable parts. Thus, the movement of components relative to other components like the housing can be determined. A sawing cycle takes 20 ms with 200 images being recorded. As of approx. 14 ms, a lift off of the saw blade from the guide roller can be observed.
Fig. 4 shows the major strain of a circular plate which was broken through with a bolt. The frame rate is 30,000 images per second and the breaking occurs after approx. 1.5 ms and at a major strain of 7%. From the displacements perpendicular to the component's surface, a deformation velocity of 5 m/s can be determined.
Dropping down a mobile phone is something almost everybody experienced already. Intently, we look for possible damages resulting from the fall. Sensitive parts like the displays growing in size or the electronics need to be protected against such cases by the housing or by flexible suspensions.
PONTOS not only provides for measuring and displaying the deformations resulting from the impact visually and qualitatively but also quantitatively.
Fig. 5 shows the drop-down test of a mobile phone. The 3D displacements of the display relative to the housing can be determined based on reference point markers applied to the phone. The maximum displacement of the display is 0.55 mm at a height of drop of 1m. The deformations of the housing or other relative movements at the moment of impact can also be measured.
For decades, high-speed cameras have been established when carrying out crash tests in the automotive industry. However, only in recent years the conditions for real 3D measurements were established due to the synchronous image capturing of the digital cameras. PONTOS is a fast and easy to operate system and can flexibly be introduced to crash tasks. Its software provides various evaluation possibilities.
Fig. 6 shows the test car and a mobile deformable barrier for a not standardized side impact test. The pre-calibrated PONTOS sensor is positioned directly behind the test car, Fig. 6. We will look at the deformations of the inner side of the front passenger door and of the B-pillar. The seats were removed from the car to achieve an optical access to the components. As the entire vehicle performs a movement to the side (towards the measuring system), this movement is measured by means of reference points on the driver's side. In the following, it is subtracted from the movement of the points on the front passenger door and the B-pillar. Thus, for these parts, we only get the deformation due to the impact and not the entire movement of the car. The B-pillar deforms at an impact velocity of 50 km/h up to 400 mm into the passenger area, see Fig. 7. The test takes 100 ms and is analyzed with 50 images. The compact design of the PONTOS system with integrated lighting provides for a flexible use in crash tasks. Setup and preparation time was less than 30 minutes. In a similar way, PONTOS may be introduced to further crash tasks.