International Sales ContactsEducation and Research
| Project: | Digital image monitoring and correlation of fatigue crack
growth using high resolution telecentric optic |
| Customer: | University of Freiberg |
| Task: |
The Institute of Materials Engineering at the University
of Freiberg is a well known and respected researcher in
the area of multi-axial material fatigue testing and
associated fracture mechanics. Interest in this area is
driven by the need to understand the behaviour of construction
materials such as steel and other metals when
placed under very high stress loads. Areas of particular
interest in this field concern the fracture mechanics of
such materials.
Typically, research and testing is performed by large, biaxial cruciform testing machines, such as those manufactured by the company Instron. Cruciform testing machinery usually incorporates 4 precision controlled hydraulic rams mounted in-plane and perpendicular to each another. Such machines are able to apply load capacities ranging from 1 kN to 250 kN in highly controlled manner. A number of analysis techniques have been used in the past to capture the sample distortion data generated by such machines. Historically, techniques such as ‘displacement vectoring’ are used, whereby speckle features (eg paint spots) are applied to the surface prior to testing. This enables researchers to then analyse the vector displacement of each speckle in relation to its starting position after testing, and to report on the deformation performance. In recent years, and inline with a need to gain a far deeper understanding of such material behaviour, particularly its fracture characteristics, a need has emerged for additional optical techniques to be employed. One such technique is the utilisation of ‘Digital Image Correlation’ or ‘DIC’. This refers to an optical method that employs tracking and image registration techniques based on real, high resolution imagery captured during live testing. The use of DIC techniques enables highly accurate, very high resolution 2D and 3D measurements of deformations, displacement, and fracture characteristics from digital images captured by a special camera and optic during live testing. However, in order to capture useful, very high resolution imagery during testing, it is critical that an optical system is used which is free from distortion or optical aberration, and which has a sample illumination system capable of optimally illuminating a variety of samples. Furthermore, it is critical that the optical resolution of such a system is as high as possible and able to capture very large fields of view, whilst also being portable, to enable usage with other testing machines in a laboratory. The task was to develop a portable, very high resolution optical image capture system to acquiring extremely high resolution digital imagery from a wide variety of stress and fracture testing machinery, especially biaxial cruciform machinery during sample loading.  Start, mid and end phase of a test (top to bottom), details of the original imagery with a resolution of 4059px x 20823 px with 9.32 Mb each |
| Solution: | In order to meet the current and future requirements of the University of Freiburg, Opto
developed a very high resolution, ultra-low distortion ‘telecentric’ optical system designed
specifically for this purpose. The optical system has a working distance of 385mm enabling
the system to be positioned a safe distance from the sample area, whilst also enabling
an operator to gain access to the sample area without moving the optical system. A
very large sample field of view of a diameter of 120mm is provided by the optical system
enabling large areas to be analysed in single shots, with no need for mosaic image stitching.
Futhermore, distortion is virtually zero with image field curvature across this area
being less than 20µm.
A number of different illumination options were conceived, with the optimum lighting configuration comprising a large, fully programmable customised 24-LED ringlight integrated into the front of the telecentric lens. This enabled a number of different illumination programs to be stored according to the sample type. In order to offer portability but maximum stability, the entire system was mounted onto Opto’s unique rolling floorstand chassis. This highly robust chassis incorporates soft roller wheels enabling maximum manoeuvrability of the system, but ultimate stability after locking in position with the integrated braking system. Fine tune target positioning of the optic is then possible via a series of ultra high accuracy micropositioners. A coarse height adjust is provided, enabling the optic to be raised between 1.3 and 1.7m from ground level. An onboard laptop table is also integrated into the main column of the stand to reduce lengthy cables and to enable the operator to remain close to the instrument. Image capture is provided by an integrated 100 Megapixel Pentacon Scan 6000 linescan camera. This ultra high performance and highly unique camera consists of a scanning CCD sensor line consisting of 3 parallel sensors for red, green, and blue channels ensuring a very high dynamic range, and ultra low image noise level. Captured imagery can then be easily exported into any number of DIC software packages. |
| Further development: | Future versions of this system will increase resolution even further, with increases also in
field of view.
 Illustrating front of telecentric optic fitted with high power LED illumination array and programmable illumination zone switches (left) and illuminated sample area (right) |
| Download: | Download PDF (548KB) |
