Carbon fiber reinforced plastic (CFRP) composite has high specific strength and high specific stiffness, and has excellent performance in corrosion resistance, fatigue resistance, high and low temperature resistance, etc., so improving the amount of composite material is one of the main development directions to reduce the structure quality of civil aircraft.
Porosity is one of the main types of defects in composite materials, which will change the volume fraction, elastic constant and microstructure of materials, weaken the adhesion between fiber and resin matrix, cause stress concentration, induce microcracks and even expand component layers around the pores, and severely weaken the mechanical properties of composite materials.
The pore size magnitude, dispersion distribution and micro morphology are quite different from the conventional macro defects such as delamination, fat rich, inclusion and porosity, as shown in Figure 1. For example, the size magnitude is usually 10-100 μ m, far less than the conventional macro defects; dispersive distribution characteristics in the material; when the porosity is less than 1.5%, it is mostly ellipsoid, when the porosity is more than 1.5%, it is mostly flat; when a large number of pores gather, it even forms stratification. Therefore, ultrasonic testing of composite porosity not only needs to determine the defect properties of the suspected defects, but also needs to quantify the size of the defect clusters and the porosity evaluation.
Fig. 1 micro morphology of stratified, discrete pores and pores
(a) Interlayer stratification; (b) interlayer pores; (c) discrete pores; (d) pores in layers
The characteristics of the pores in composite materials determine that the testing method should be different from the conventional macro defects, that is, the ultrasonic testing method used for the conventional macro defects requires that the information such as the size and depth of the defects can be quantitatively tested, while the pore defects usually do not need to quantitatively test the size or location of a single pore, or from the point of view of the influence of pores on the mechanical properties, The evaluation of the total content of pore defects is more significant than the over detection of single pore.
Porosity is the percentage of pore volume in the whole composite material, which is one of the important indexes to evaluate the structural properties of composite materials. Ultrasonic testing is the most important testing technology to evaluate the content of pores in composite materials. Its main idea is based on the influence of pores on ultrasonic signal, to obtain ultrasonic characteristic parameters, to establish the relationship formula between characteristic parameters and porosity, and then to realize the numerical characterization of porosity. The ultrasonic characteristic parameters affected by porosity include: ultrasonic attenuation, acoustic impedance, sound velocity, signal spectrum, nonlinear coefficient, etc. the detection methods and characterization forms of porosity are various. Among them, the ultrasonic detection method based on ultrasonic attenuation to establish quantitative evaluation detection is the one with high sensitivity, easy implementation, low cost, and the most widely used engineering carbon fiber composite porosity evaluation Method, Fig. 2 is the porosity attenuation curve based on ultrasonic reflection method and ultrasonic penetration method.
Figure 2 porosity attenuation curve of ultrasonic reflection method and ultrasonic penetration method
In this work, the ultrasonic detection method of pore defects is studied from two aspects of qualitative detection and quantitative evaluation. At the same time, the ultrasonic reflection method and ultrasonic penetration method are used to carry out ultrasonic A-scan detection (as shown in Fig. 3 and Fig. 4) and ultrasonic C-scan imaging detection (as shown in Fig. 5) on the reference block of carbon fiber reinforced resin matrix composite with porosity coverage of 0.0% ~ 3.0%. The detection resolution of pore defects is analyzed The results show that the ultrasonic reflection method has higher detection sensitivity to the pore defects, more detailed division ability to the numerical evaluation of the pore rate difference, and information hidden in the pore details The characteristic waveforms in A-scan provide more criteria for the identification of pore defects; the ultrasonic penetration method has higher acoustic penetration ability, which is suitable for the pore detection and evaluation of composite materials with high porosity or large thickness or high attenuation.
Figure 3 typical A-scan test results of ultrasonic reflection method
Figure 4 typical A-scan test results of ultrasonic penetration method
Fig. 5 C-scan test results of porosity reference block
Porosity /%: (1) 0-0.5; (2) 0.5-1.0; (3) 1.0-1.5; (4) 1.5-2.0; (5) 2.0-2.5; (6) 2.5-3.0
(a) Based on ultrasonic reflection method; (b) based on ultrasonic penetration method
Author: Shi Junwei, engineer, testing and Evaluation Research Office of AVIC composite materials Co., Ltd., mainly engaged in the research and application of composite materials and welding nondestructive testing technology,
E-mail:aaron_sjw@163.com
Source:
Comparative analysis of ultrasonic reflection method and penetration method in detecting composite porosity
Shi Junwei, Liu Songping, Xun Guoli
2020, 40(2): 89 -99
doi: 10.11868/j.issn.1005-5053.2019.000044
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