The studies on glass-fiber reinforced composites, due to their growing popularity and high diversity of industrial applications, are becoming an increasingly popular branch of the nondestructive testing. Mentioned composites are used, among other applications, in wind turbine blades and are exposed to various kinds of damages. The equipment reliability requirements force the development of accurate methods of their health monitoring. In this paper we present the study of composite samples with impact damages, using three methods: terahertz time domain inspection, active thermography with convective excitation, and active thermography with microwave excitation. The results of discrete Fourier transform of obtained time sequences of signals will be presented as well as some image processing of resulting amplitude and phase images. Proposed experimental methods combined with harmonic analysis are efficient tool of defects detection and allowed to detect flaws in examined specimens. Reader may find it interesting that in spite of differences in nature of applied experimental methods, one technique of signal processing (harmonic analysis) gave adequate and comparable results in each case.
Polymer composite materials, because of their high strength to weight ratio and corrosion resistance, are more and more intensively used in various industries. One of the most important applications are hulls of ships and aircrafts, piping systems of liquid fuels, and wind turbine blades. All the mentioned structures are exposed to various environmental conditions, also mechanical impacts. The impact damages result in delaminations and significant weakening of the composite structure strength thus their detection is important issue. For this reason, impact damages should be evaluated using an appropriate technique. The ultrasonic testing, radiography, and shearography are common methods of composite materials nondestructive evaluation [
In the second part of this paper, the basics of active thermography with convection heating excitation and with microwave excitation will be presented. The pulse-phase thermography with discrete Fourier transform of thermogram sequences [
Samples utilized in our experiments are presented in Figure
Photos of utilized impact damages in various glass-fibre-reinforced composite materials. Left: impact source side, right: opposite side.
Pulsed terahertz NDT system based on the Tray-4000 spectroscope of Picometrix and its simplified scheme is presented in Figure
Photo (a) and simplified scheme (b) of pulsed terahertz measuring system.
The photoconductive antenna (PCA) based transmitter and receiver heads work in a reflection mode (as shown in Figure noncontact measurement in reflection and transmission arrangement, nonionizing nature, inner structure and spectral information is obtainable, fraction of millimeter resolution.
Main disadvantages of terahertz technique are low power of THz emitters, low speed of examination (need of raster scanning), restriction to nonconductive materials (because of high frequency and skin effect).
The impact damaged samples were examined using pulsed terahertz technique in reflection arrangement as it was shown in Figure
Results of S1 sample THz inspection: (a) raw B-scan signal, (b) frequency response of damaged and healthy material, (c) spatial distribution of measured waveform’s magnitude in case of selected frequency, (d) distribution of phase.
Results of S2 sample THz inspection: (a) raw B-scan signal, (b) frequency response of damaged and healthy material, (c) spatial distribution of measured waveform’s magnitude in case of selected frequency, (d) distribution of phase.
Results of S3 sample THz inspection: (a) raw B-scan signal, (b) frequency response of damaged and healthy material, (c) spatial distribution of measured waveform’s magnitude in case of selected frequency, (d) distribution of phase.
Results of S4 sample THz inspection: (a) raw B-scan signal, (b) frequency response of damaged and healthy material, (c) spatial distribution of measured waveform’s magnitude in case of selected frequency, (d) distribution of phase.
Results of S5 sample THz inspection: (a) raw B-scan signal, (b) frequency response of damaged and healthy material, (c) spatial distribution of measured waveform’s magnitude in case of selected frequency, (d) distribution of phase.
We propose harmonic analysis of terahertz signals in order to obtain an information about impact damage position. Before this, all measured signals were median filtered in time domain and processed by Fourier transform. Frequency responses of damaged and healthy materials are presented and compared in Figures
In case of frequencies higher than 0.9 THz, because of very low SNR, signal contains no applicable information about defect (Figures
Composite samples with impact damages were investigated using active infrared thermography. As the excitation, we propose two energy sources: convective heat flow from inductively heated steel plate (contact method) and microwave heating (contactless method). In the convective heating, the composite sample is placed on the inductively heated steel plate (Figure
The method’s scheme. (a) active thermography with convective excitation, (b) active thermography with microwave excitation.
The active thermography with microwave excitation is the contactless method. In this technique, examined sample is heated by high power microwaves (500 W, working at the frequency 2.45 GHz). Heating phase is observed by the properly secured thermovision camera. A schematic drawing of the method is shown in Figure
Experimental setup for active thermography with microwave excitation.
Since in both techniques the heating phase can be observed, the pulsed phase thermography (PPT) could be applied. This method combines the experimental procedure used in pulsed thermography (PT), with signal analysis used in modulated thermography (MT) [
The analysis of obtained sequence is based on discrete Fourier transform (DFT), which allows to evaluate the output as the combination of phase and amplitude. The procedure scheme is presented in Figure
The discrete Fourier transform for thermogram sequence scheme.
The well-known Fourier Transform of each pixel in the thermogram sequence may be written as follows [
In case of convection heating, all five samples were tested, using the same heating time and recording frequency. The observation time was set to 55 second, and recording frequency was 9 Hz, which allowed us to obtain 495 thermograms in one sequence. For every sample, the same procedure of signal processing was used: the DFT of thermograms sequence was performed, chosen amplitude images and phaseograms were then processed using median or standard deviation filter, to enhance the contrast between the background and defect.
The results (chosen phaseograms, and image processing of selected amplitude images and phaseograms) are shown in Figures
Obtained phaseograms for sample S1 (a) 0.018 Hz, (b) 0.036 Hz, (c) 0.054 Hz, (d) 0.072 Hz.
Image processing of chosen amplitude images and phaseograms for sample S1. (a) Amplitude image enhanced with median and standard deviation filtering, (b) phase enhanced with median filtering.
Obtained phaseograms for sample S2. (a) 0.018 Hz, (b) 0.036 Hz, (c) 0.054 Hz, (d) 0.072 Hz.
Image processing of chosen amplitude images and phaseograms for sample S2. (a) Amplitude image enhanced with median and standard deviation filtering, (b) phase enhanced with median filtering.
Obtained phaseograms for sample S3. (a) 0.018 Hz, (b) 0.036 Hz, (c) 0.054 Hz, (d) 0.072 Hz.
Image processing of chosen amplitude images and phaseograms for sample S3. (a) Amplitude image enhanced with median and standard deviation filtering, (b) phase enhanced with median filtering.
Obtained phaseograms for sample S4. (a) 0.018 Hz, (b) 0.036 Hz, (c) 0.054 Hz, (d) 0.072 Hz.
Image processing of chosen amplitude images and phaseograms for sample S4. (a) Amplitude image enhanced with median and standard deviation filtering, (b) phase enhanced with median filtering.
Obtained phaseograms for sample S5. (a) 0.018 Hz, (b) 0.036 Hz, (c) 0.054 Hz, (d) 0.072 Hz.
Image processing of chosen amplitude images and phaseograms for sample S5. (a) Amplitude image enhanced with median and standard deviation filtering, (b) phase enhanced with median filtering.
It can be notice that best results for phase images of samples S2–S5 (Figures
Microwave-enhanced infrared thermography is a relatively new NDT method. Using microwaves as the energy source gives a possibility of volumetric heating of the material, which can significantly speed up the heat process. Moreover, this method is contactless. Unfortunately, the high power microwaves, needed to obtain visible temperature differences between the defect and the background, may cause damage to the thermovision camera. Therefore, additional protective housing is needed in this case. Special metallic mesh, used as camera lens protection, increases the noise level in obtained thermograms. The image processing of thermogram sequence is then more demanding and time consuming.
In case of microwave heating the observation time was set to 100 seconds, and recording frequency was set to 15 Hz, which allowed to obtain 1500 thermograms in one sequence. After the DFT procedure, chosen amplitude images were enhanced using procedure based on multiply filtering. Obtained results (only for samples S2 to S5) are promising (Figures
Obtained results for sample S2. (a) Chosen initial amplitude image, (b) amplitude image after several filtering procedures.
Obtained results for sample S3. (a) Chosen initial
Obtained results for sample S4. (a) Chosen initial amplitude image, (b) amplitude image after several filtering procedures.
Obtained results for sample S5. (a) Chosen initial amplitude image, (b) amplitude image after several filtering procedures.
The image processing of available amplitude images was based on median filtering and contrast enhancement. Obtained results give the defect approximate location, but the information about flaws’ size is not contained in resulting images.
Due to high noise level in the output thermograms, the research on improving the methodology of measurement should be continued. The time of heating extension in order to increase the temperature contrast between the background and defect as well as the sequence of thermograms recording frequency increment is, therefore, considered.
Both utilized methods of glass-fiber-reinforced composites examination enable detection of impact caused defects. The pulsed THz technique offers very wide and unique (compared to other common methods) abilities of inspection: high resolution, no need to use any additional coupling medium, availability of spectroscopic information, and finally a defect depth information is also provided. Simple harmonic analysis is sufficient tool in detection of damages caused by mechanical impacts in case of various kinds of materials.
Active infrared thermography is a fast (sometimes it allows real time monitoring of structures) and giving tangible results method. The convection excitation allows obtaining information about the location and size of the defect. The harmonic analysis of obtained thermograms’ sequences for specimens S1–S5 (representing different types of composite material which may be found in practical usage) proved to be sufficient to obtain reliable results. In each case, the defect itself is clearly visible, moreover, additional analysis using standard devaition filtration, allowed to visualize delamination arose in the vicinity of damage. The application of this method in practice, however, due to the fact that it requires contact with the heat source, can be sometimes difficult. The microwave excitation, on the other hand, is a contactless method. However, it requires additional thermovision camera protection, which causes significant increment of obtained thermograms noise level. Therefore, the received thermogram sequence image processing is much more difficult. The harmonic analysis in this case was connected with additional signal processing involving trend removal based on median filtering. Nevertheless, obtained results allow only to an approximate localization of the defect. The further development of the active infrared thermography with microwave excitation method, however, is highly warranted because of the ease of its industrial application, high speed, and the ability to simultaneous study of materials’ large surfaces.
Each sample was examined using two different methods (i.e., THz imaging and active thermography with two energy sources), but the signal processing technique was chosen to be the same for data obtained with both utilized methods. Reader may notice that harmonic analysis allowed to detect the flaws in examined samples, and obtained results may be used to quantitative and qualitative evaluation of materials.
This work was supported in part by European Commission project HEMOW: Health Monitoring of Offshore Wind Farms (reference: FP7-PEOPLE-2010-IRSES-GA-269202).