1Instrumentation Technology and Economy Institute ( Institution) 397A Guanganmenwai St., Beijing, China
2Beijing Institute of Technology ( Institution) No.5 Zhongguancun South St., Beijing, China
Nano-Micro Conference, 2017, 1, 01076
Published Online: 30 November 2017 (Article)
Terahertz nondestructive detection system is to test the image of the carbon fibre composite material. We use the continuous back wave oscillator (BWO) to pump the terahertz radiation, which the frequency is range from 210-250 GHz. By the reflection type detection system, the terahertz probe focus moves from the surface of the concealed carbon fibre composite material. In order to get real-time scanning the material image,the material is located by the 2D translation machine,which is connected to the computer.The data is collected by the phase-locked amplifier and calculated by the Matlab using the least square method(LSM). The interior structure, such as cracks,inclusions, empty and bubbles, can be detected by the images nearly 1mm resolution,which is a more convenient and simple method of nondestructive detection of the carbon fiber composite material.
The composite material is made from different materials,which is produced by physical or chemical mechanism in the macroscopic.It is largely depended on the performance of various materials,which can make it better than the original composition. For this reason,it have played an important role in many fields,such as aerospace,automobile,shipbuilding,construction and other products.As the biggest aircraft,the carbon fibre composite is used to cut down the weight of 50% less than before.So it has been in a new fashion to lightweight,energy saving,safety development and green manufacturing on changing the traditional materials.But the carbon fibre composite material is difficult to the cutting processing,the blade wear in the interaction process of fiber is still very serious.In addition, cause it has very different material properties, low bond strength between the layers,such as fiber pull out, torn between the layers, voids and the other serious problems influence the quality of the product.So how to detect the quality and nondestructive have been the key to restrict its further application.
Continuous terahertz wave have found the unique capacity to penetrate composite material and identify defects such as cracks,inclusions,voids and other [1-7]. Many years, there have been much research work done on the terahertz detecting cause of its importance character as high transmittance, safety for human beings and for other organisms. The ability of imaging the defects in aerospace composite materials, acquired during operation, by THz time-domain spectrometer (TDS) was researched in . As the TDS system always needs a stable and complex optical roots,our work shows the ability of non-destructive detection in composite materials, reinforced by carbon fiber, by using continuous terahertz wave imaging system.The experimental is easy to build up and results show that the resolution is better than 1mm, and the information to collect the image of the measured sample can be obtained by two-dimensional (2D) scanning system.
Principe of the methods
With the complex structure of fibre carbon composite materials, there is a bunch of characteristic parameters that have to be focused on: fibre volume content, porosity, inclusions,voids,regions of insufficient resin infiltration and others. Continuous terahertz wave imaging system is a non-destructive and contactless inspection technique that is based on the propagation and reflection of optical waves. All the imaging system is based on source,our work is used the continuous back wave oscillator (BWO) as the source of terahertz radiation.It can easily operate the frequency and range from 160 to 2100 GHz.The output power has the relationship with the frequency,which is given by:
The Ux is the voltage diffusion in the BWO instruction and the frequency is the ƒ.As the continuous terahertz wave incidents to the surface of the composite material, it will be directly reflected by two roots, one of the terahertz wave reflected from the front surface of the material and the other one reflected from the material attached to the high reflective surface.From the difference for the reflective of the thickness and the surface of the sample, the intensity of the continuous terahertz wave is not the same.For the BWO is a continuous terahertz source of fixed frequency output, obtained the intensity by the detector,we use point to point scanning imaging method to record the intensity and put the relevant information into computer,which can image the composition of the sample,such as the shape and the size.
Figure 1 presented the reflected imaging system from the sample of the composite material.The BWO with an output power in 0.4mW is build up as the terahertz wave source.We choose the output frequency is from 206.2 to 260 GHz, which is extremely coherent, polarized, and monochromatic (△ν/ν ≈ 10−5).The continuous terahertz wave generated by the BWO is modulated by a chopper. The wave exiting from BWO is divided into two parts by a high-resistivity(R≧10000Ω) silicon wafer ,with 0.5mm thick, acting as a wave splitter. One of the split beams directly incidents into the focusing lens with collecting the wave light focusing onto the sample of composite material.The focusing lens always consist with two lens,which are collimator lens.With the attenuated reflection,the polyethylene lens can get the wave focus well to the sample. The length of optical root should be short and high transmittance.In this lens group,we choose the polyethy lens,which are well performance in terahertz wave focusing.The other split beam incidents into the detector,which converts the intensity of the terahertz into voltage signal as the input signal for the lock-in amplifier, and reference frequency for the lock-in amplifier is the same as the chopper. By this way the noise of the signal is filtered , we can obtained the intensity of the surface of the sample.After,with the acquisition unit, the data is transferred into the computer. Finally we can calculate each point by matlab and imaging the sample.
Figure 1 Schematic of the terahertz reflected imaging system.
As depicted in Figure 2 is the imaging system we set up in the lab. We put one piece of paper before the sample in order to achieve the non-destructive of the material.Due to the simple system,it can easy to connect all the instrument without the external interference.
Figure 2 Schematic view of the experimental non-destructive system.
Compared with the traditional transmission optical non-destructive system, the terahertz reflection non-destructive imaging system is a little different to set up, firstly, the terahertz is invisible, and the reflected direction is always changed, so the collection is easy to be affected by the surface of the composite material orientation, component space distribution and other factors. Secondly, the reflected wave consists of lens reflect wave diffuse wave and scattered wave. And the roughness, the size of the focusing and the signal to noise ratio of the sample can also influence the composition of the reflected wave and the data processing method.
For the continuous terahertz wave generated from the BWO, the optical focusing should adjust to the plate of the sample,which is highest energy point of the sample.We use the point-to-point scanning system to detect the sample,so the wave spot from the polyethy lens is changed by the focal. The Figure 3 is the measured result of energy density distribution in the plane view,which is the sample position located, acquired by using a 2D terahertz camera.The terahertz camera is Ophir-Spiricon, made from America.In this case, the optical roots can be ready to detect the sample of the composite material.
Figure 3 2D energy spot of the continuous terahertz wave.
In order to verify the experimentally non-destructive system of modeling, Before the sample of carbon fibre composite material(as shown in Figure 4),we put a piece of paper to set up the detection system.The sample is like square with 5×4×0.5cm of the LWH size.With place the sample at the focal point of the focusing lens,the reflect wave can reach into the detector.After the data processing in the computer, the detection image of the sample is obtained with the resolution within 1 mm.The resolution depends on the output of the BWO frequency and the distance of the X-Y plane.But if the resolution is higher,the time of the scanning image would be longer.And the theoretical focal spot diameter at 206.2GHz is 0.93 mm by the relationship of NA. Figure 5 have shown the image result of the composite material with the Matlab. It can easily present the profiles of the sample’s surface with non-destructive the material. Therefore, the system can be used as a non-contact scanning image system for detecting various composite materials.
Figure 4 The sample of fibre cabin composite material.
Figure 5 The image result of scanning the sample
In this paper, the method of a reflection terahertz wave non-destructive system is demonstrated to image the carbon fibre composite material. By the pyroelectric detector to get the signal to phase-lock amplifier, it converts the optical intensity to electric signal,which we can test the sample. Compared with other image systems, the reflection terahertz imaging system has more practical application value than the transmission imaging system. it can apply on nondestructive, non-contact and extraction of material surface profile.The resolution of the measurement is higher than 1mm. But in some practical situations, the target object needs higher resolution with fast way, such as 3D imaging, recognition and tracking. In these cases, the detection imaging system will be replace as new detector,but the means of reflection detection would never be replace.Meanwhile, the terahertz wave can be a powerful and easy way for imaging and detecting the composite materials.
This work was supported by a grant from Ministry of Science and Technology (No.2016YFF0202702).Thanks to Prof.Yuejin Zhao,Dr.Hui yuan and Dr.Xiaohu Guo,giving the advice of the detection system.
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