Analysis of crack propagation in metallic and composite material system

Kurzfassung

A spin off company of an Italian Institute for the Physics of Matter has developed a system for shooting and analysing the images detected during fatigue and static tests on metallic and composite specimen, subject to standard test method for measurement of crack growth rates.The system is able to detect the crack propagation with a resolution of about 10-4m.The partners sought are industries, especially in mechanical and aeronautical sectors, for commercial agreement with technical assistance.

Details

The system is composed by software and hardware designed to acquire and process image during fatigue and static tests on metallic and composite specimen, subject to standard test method for measurement of crack growth rates.
The system is a single product made of:

- Electronic of synchronization with the instrumentation for the application of stress.
- System of Image Acquisition is made of two high resolution camera (sensor CCD with a resolution of 5Mpx 2588x1958)
- LED lamps illumination system synchronized with the motion of the specimen in fatigue tests with variable stress.
- Software designed for the purpose of calibrate the cameras, acquire and process images to obtain the necessary information for measures, according to the standard.
Focus system has a focal length of 35mm, providing at 70cm a spatial resolution of 0.07 mm/px. It is possible to integrate a focus system able to frame a more detailed region of the specimen for an analysis of greater precision.

The system records not only the images but also information on electronic instrumentation at the moment of the acquisition, in particular it records the time, the number of cycles counted on the system of sync with the acquiring system and the medium stress applied to the plate during data acquisition.
The system of processing is based on an algorithm for the recognition of the images that allows to solve the problem of the movement. In fact, the system recognizes in each image some points of reference set in calibration phase. The measuring points are always scaled in function of these points of reference in order to minimise the problem due to the oscillation and deformation of the plate during the test.

The measured data can be displayed with the system of post-processing, that provides a section of viewing and editing of the data collected to correct any anomalies. An integrated module of statistical analysis of the images allows to automatically evaluate the crack growth and to make an automatic evaluation of ASTM (American Section of the International Association for Testing Materials) standard diagrams (Rif. ASTM E647 e ASTM E561) necessary for carrying out the measures in accordance with the standard.

Innovative Aspects:
- The system is automated and able to capture the crack growth with high resolution of about 10-4 m.

Current system called extensometers have a resolution of about 10-2 m.
- The system provides accurate measurement and clear characterization of testing material, as established in ASTM (American Society for Testing and Materials) standard method for measurement of Fatigue Crack Growth Rates.