Image analysis with ultra-high-speed camera and two dimensional dynamic numerical analysis are applied to study the rapid unstable growth of wing crack under the uniaxial compression. Growing wing crack terminates and restarts its unstable rapid growth in some cases. Such a termination and restart behavior of the growing crack is studied through the experiment and numerical analysis in this work. First, rectangle transparent specimen, including the initial crack inclined to the compressive axis, is subjected to the uniaxial compression till the wing cracks start unstable rapid growth from both ends of the initial crack. Images of growing cracks and those of stress distribution, visualized as the photo-elastic fringe pattern, are captured by the high speed camera with the frame rate of 500k frames per second. The behavior of growing crack and the change in the stress field due to the crack growth are discussed through the captured images. Next, two dimensional dynamic numerical analysis is carried out. PDS-FEM (Particle Discretization Scheme), which allows the discontinuity of the displacement in the continuous analytical domain, is combined with the central difference time integration scheme to simulate the rapid unstable growth of the wing crack dynamically. The accuracy of the proposed simulation is discussed through the comparison with the images, captured by the experiment.
An ultra-high speed camera of 1Mfps was applied to visualize the crack propagation. Change of stress field around the
propagating crack tip was captured as a change of the fringe pattern by means of the photo-elastic imaging technique.
Newly developed video trigger system is employed to detect the occurrence of the crack propagation as a trigger in the
experiment. The trigger successfully perceived the initiation of the crack propagation stably. Also its response time was
fast enough even for the image capturing with 1Mfps. As a result, it is revealed that the elastic wave, propagating in the
continuous body, has a significant effect on the velocity and kinking behavior of the propagating crack.
An image sensor for an ultra-high-speed video camera was developed. The maximum frame rate, the pixel count and the number of consecutive frames are 1,000,000 fps, 720 x 410 (= 295,200) pixels, and 144 frames. A micro lens array will be attached on the chip, which increases the fill factor to about 50%. In addition to the ultra-high-speed image capturing operation to store image signals in the in-situ storage area adjacent to each pixel, standard parallel readout operation at 1,000 fps for full frame readout is also introduced with sixteen readout taps, for which the image signals are transferred to and stored in a storage device with a large capacity equipped outside the sensor. The aspect ratio of the frame is about 16 : 9, which is equal to that of the HDTV format. Therefore, a video camera with four sensors of the ISIS-V4, which are arranged to form the Bayer’s color filter array, realizes an ultra-high-speed video camera of a semi-HDTV format.
Presented in this paper is an outline of the R and D activities on high-speed video cameras, which have been done in Kinki University since more than ten years ago, and are currently proceeded as an international cooperative project with University of Applied Sciences Osnabruck and other organizations. Extensive marketing researches have been done, (1) on user's requirements on high-speed multi-framing and video cameras by questionnaires and hearings, and (2) on current availability of the cameras of this sort by search of journals and websites. Both of them support necessity of development of a high-speed video camera of more than 1 million fps. A video camera of 4,500 fps with parallel readout was developed in 1991. A video camera with triple sensors was developed in 1996. The sensor is the same one as developed for the previous camera. The frame rate is 50 million fps for triple-framing and 4,500 fps for triple-light-wave framing, including color image capturing. Idea on a video camera of 1 million fps with an ISIS, In-situ Storage Image Sensor, was proposed in 1993 at first, and has been continuously improved. A test sensor was developed in early 2000, and successfully captured images at 62,500 fps. Currently, design of a prototype ISIS is going on, and, hopefully, will be fabricated in near future. Epoch-making cameras in history of development of high-speed video cameras by other persons are also briefly reviewed.
The ISIS, In-situ Storage Image Sensor, may achieve the frame rate higher than 1,000,000 pps. Technical targets in development of the ISIS are listed up. A layout of the ISIS is presented, which covers the major targets, by employing slanted CCD storage and amplified CMOS readout. The layout has two different sets of orthogonal axis systems: one is mechanical and the other functional. Photodiodes, CCD registers and all the gates are designed parallel to the mechanical axis systems. The squares on which pixels are placed form the functional axis system. The axis systems are inclined to each other. To reproduce a moving image, at least fifty consecutive images are necessary for ten-second replay at 5 pps. The inclined design inlays the straight CCD storage registers for more than fifty images in the photo- receptive area of the sensor. The amplified CMOS readout circuits built in all the pixels eliminate line defects in reproduced images, which are inherent to CCD image sensors. FPN (Fixed Pattern Noise) introduced by the individual amplification is easily suppressed by digital post image processing, which is commonly employed in scientific and engineering applications. The yield rate is significantly improved by the elimination of the line defects.