Non-destructive Testing Of 3D Braided Composites Based On SQUID

Three-dimensional braided composites have become important structural materials in aviation, aerospace, automotive, medical, sports and other fields. However, there are no specialized technologies and standards for the detection of three-dimensional braided composites currently.In this paper, a three-channel high-Tc SQUID NDT model SMM-7 from American T Technologies, Inc. has been used to carry out detection experiments. It utilizes high-Tc dc SQUID micro magnetometer mode HTM-8. The magnetic field sensitivity of SQUID is better than 30×10-15T(?). Its spatial resolution is about 1mm. Tested samples are woven from carbon fiber (3K) T300B by four-way 1×1 four-step method. The fibers are implanted in a matrix of epoxy-matrix (TDE-86).First this paper describes the background and significance of works. Basing on magnetic flux quantum and Josephson Effect, the paper explains the principles of SQUID system and core components, such as sensor and flux lock-in amplifier.Then mathematical derivation and modeling are carried out. Magnetic dipole model and multi-dual model are described. The simulation of latter model agree with the experiment result well. A sheet model of eddy current distribution for a circular defect is created. Model simulation is agreement with experiment result. Applying phase rotation method, signal collecting capability of the system is increased significantly.In the course of the experiments, four-probe technique is used to measure composites resistance and conductivity. The experiment results show that the direction and thickness of carbon fiber play a key role to evaluate resistance. The SQUID system is used to test magnetic field penetration depth of the specimen. The experiment result shows that the system can distinguish thickness of composite based on electromagnetic properties. Then a three-layer artificial neural network is used to locate defects. Because normalization process causes a distortion of the magnetic field signal slope, the ANN system cannot properly distinguish different energy damage for the same thickness samples. Flux magnetic detecting method gets positions and sizes of defects correctly. Images similar with other traditional technique are obtained. Considering detection depth and sensitivity, SQUID has obvious advantages. Finally a normal non-defective specimen with different internal weaving methods is tested by SQUID NDE system. Even without any defects in specimen, SQUID system can still distinguish different internal structure caused by different weaving methods in the three-dimensional braided composites.Compared with eddy current, ultrasonic and other traditional NDE techniques, SQUID-based nondestructive testing technology is much more advanced and suitable for three-dimensional composite materials.