Non-contact crack opening displacement measuring device and method for bending fracture test

Abstract

The invention belongs to the field of material performance test, and especially relates to a non-contact crack opening displacement measuring device and method for a bending fracture test. The non-contact crack opening displacement measuring device comprises a mechanical experimental machine; a bending test tooling including an upper pressing head and two lower pressing heads, and a sample is clamped between the upper pressing head and the lower pressing heads; a digital image acquisition system including an industrial camera, an imaging lens, an annular LED illuminating light source arrangedat the front end of the imaging lens, a camera support system and a reflecting mirror, wherein the reflecting mirror is arranged at the lower part of the sample at an inclined angle of 45 degrees; thereflecting mirror reflects light generated by the light source to a fracture surface of the sample, and meanwhile, a surface image of the fractured sample is reflected into the imaging lens; and a control system for displaying an image of the industrial camera in real time, and calculating the real-time crack opening displacement. According to the non-contact crack opening displacement measuringdevice for the bending fracture test provided by the invention, an optical imaging system is used for collecting images around a crack opening of the sample, and a digital image correlation algorithmis used for calculating the real-time crack opening displacement, so that non-contact measurement can be realized, and the measurement device does not have any influence on the sample.

Description

technical field [0001] The invention belongs to the field of material performance testing, in particular to a non-contact crack opening displacement measuring device and method for bending fracture testing. Background technique [0002] Single-sided notched specimen bending fracture testing, such as three-point bending or four-point bending, is a common method for evaluating the fracture toughness of materials. For brittle materials, when the sample meets the conditions of linear elastic fracture mechanics, the three-point bending test generally refers to the national standard GB / T 4161-2007 or the American ASTM E399-17 standard to directly obtain the critical stress intensity factor KIC for material fracture. For tough materials, the national standard GB / T 21143-2014 or the American ASTM E1820-15 standard is generally referred to, and the critical J integral or critical crack tip opening displacement based on elastic-plastic fracture mechanics is adopted. [0003] Regardle...

AI Technical Summary

Problems solved by technology

However, there are many deficiencies in the clip-type extensometer: (1) the gauge length is fixed, the measuring range is limited, and it is difficult to measure a wide range of opening displacement; (2) it is in direct contact with the sample and exerts a large It is used for particularly brittle or soft materials; (3) before and after the sample test, the extensometer needs to be installed and removed, which is complicated to operate; (4) when the material breaks suddenly, the extensometer is easy to drop and mechanically damaged; (5) limited Applicable temperature range, not compatible with high temperature test

This will inevitably complicate the design of the environmental chamber, increase the difficulty of heat insulation and low-temperature dielectric loss, and may introduce test errors

When carrying out fracture tests in different temperature environments (high temperature, room temperature and low temperature), it is necessary to replace the corresponding extensometer, which makes the test process cumbersome and increases the purchase cost of the equipment

In addition, if the fracture test needs to be carried out in a corrosive medium environment, the test conditions will be more complicated, and the corrosive medium may damage the extensometer

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