The 14th International Conference of the Slovenian Society for Non-Destructive Testing »Application of Contemporary Non-Destructive Testing in Engineering«

September 4-6, 2017, Bernardin, Slovenia

ULTRASONIC C-SCAN TESTING OF EPOXY/GLASS FIBER COMPOSITE

Zoran Bergant1, Joseph Janez2 and Janez Grum3

1Faculty of mechanical engineering, Aškerčeva 6, SI-1000 Ljubljana, zoran.bergant@fs.uni-lj.si

2Mistras Group, SA, 27 rue Magellan 94370 Sucy en Brie, France, jjanez@mistrasgroup.eu

3Faculty of mechanical engineering, Aškerčeva 6, SI-1000 Ljubljana, janez.grum@fs.uni-lj.si

ABSTRACT Pulse-echo ultrasonic C-scan method was used for examination of various processing defects in a composite plate. The glass fibre reinforced epoxy matrix composite laminate [0]6 was prepared using vacuum infusion (VI). The defects were embedded into test plate varying in shape, volume and depth. During the specimen preparation with VI, the air was introduced through small non-sealed spot in vacuum membrane. On the selected location, the PVC, aluminium foils and aluminium chips were inserted between layers. The immersion ultrasonic system with water couplant was used to study the defects in plate with 4D C-scan method. The C-scan images show the detection and location of PVC foils and aluminium chips were successful. Also, the porosity can be clearly detected for individual layers. The thin, 0.04 mm aluminium foils were not detected with this method. Key words: Ultrasound examination, C-scan, GFRP, epoxy, composite, vacuum infusion.

1. Introduction Composite materials consist of two or more elements, one of which, the fibre is dispersed in a continuous matrix phase [1]. The two elements work together to produce material properties that are different to the properties of the elements of their own. Composites offer the designer a combination of properties not available in traditional materials. The production process for polymer matrix composites has the potential to introduce variety of processing defects. Fibre misalignment can occur when fibres are laid up. Fibres in the same layer may be misaligned relative to each other or misalignment between layers. Foreign inclusions such as dirt and debris may contaminate the matrix or act as a local stress concentration in the finished product. These defects can lead to delamination either during the production process or later on when the component is in service. The use of composite materials is constantly increasing in aviation, aerospace, energy (wind turbines) where reliable inspection methods must be applied frequently for safe operation. There are many detection principles which are continuously evolving. Among those, ultrasonic and thermographic methods are the most sucessfull and widely used [1]. Composite materials are highly anisotropic with a high amount of boundaries which lead to high noise levels which limits

41

Mor

e in

fo a

bout

this

art

icle

: ht

tp://

ww

w.n

dt.n

et/?

id=

2254

7

the detection accuracy. With improved transducers and signal processing methods, modern C-scan methods have become very effective for composite testing. C-scan analysis provide information of the location and size of the inpurities, fibre layup-plan and misalignment, delaminations, impact damage, fiber fatigue damage, water filled cores, etc. [2-3]. There are also new ultrasonic methods evolving with guided ultrasonic waves [4]. Hasiotis et al [5] studied different C-scan techniques, focusing on two materials glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CRFP). In general, it’s is more difficult to detect defects in glass fibre reinforced composite due to the high reflectivity of glass fibres. Vacuum infusion (VI) is a widely used technique for production of large composite parts [6]. Typical mistake in vacuum infusion is the non-sealed vacuum membrane with a large impact on quality and in most cases, a waste of component and many hours of labour work. The aim of our research is to study and detect the various artificial defects in glass fibre reinforced polymer composite using 4D C-scan technique. 2. Theoretical background Ultrasonic testing using pulze-echo method is conducted using a ultrasonic transducer, which is transmitter and receiver at the same time. Transmitter is a piezoelectric crystal, which generates ultrasonic waves. Transmitter wave travels and reflects from the back-wall or from the defect. Any ultrasonic instrument typically records two fundamental parameters of an echo: how large it is (amplitude), and where it occurs in time with respect to a zero point (pulse transit time). Transit time in turn is usually correlated to reflector depth or distance, based on the sound velocity of the test material. The most basic presentation of ultrasonic waveform data is in the form of an A-scan, or waveform display, in which echo amplitude and transit time are plotted on a simple grid with the vertical axis representing amplitude and the horizontal axis representing time. The principle of testing is shown in Fig. 1.

Fig. 1: Pulse-echo method

For succesful detection of defects, the wavelength of ultrasonic waves should be higher than ½ of the wavelength of waves. The wavelength, , is defined by:

][mf

v ,

where v is the velocity and f is the frequency. Thus, for detection of smaller defects, the higher frequency sensor must be used. Contrary, using the higher frequency sensor, the noise level increases. The C-scan consists of many assembled A-scans. In the composite ultrasonic testing, high attenuation of waves makes the detection difficult in higher depths. Attuneation is a combined influence of scatterring and absorbtion. Scattering occurs when waves are reflected in different direction. The waves are also being absorbed in the materials and the sound energy is

42

being transformed into the other forms (heat). The decrease of amplitude because of attenuation can be represented as:

zeAA 0 ,

where A0 is wave amplitude, A reduced amplitude, and is the damping coefficienet in z-direction of sound propagation.

3. Experimental setup

3.1 Specimen preparation The 6-ply laminate was constructed using plain weave glass fabrics (280 g/m2) and epoxy-resin Renlam LY 5138/HY5130 (Huntsmann). Defects were inserted before infusion of the resin. The defects are in a form of round foils, as shown in Fig. 2. In between the plies, the following defects were inserted: Al-foil 4, 6, 8, 10, 12 mm, thickness 0.04 mm PVC foil 4, 6, 8, 10, 12 mm, thickness 0.1 mm First row was inserted between first and second ply, second row between second and third ply and continued until the last row was inserted between fifth and sixth ply. Vacuum infusion was conducted using 100 kPa of pressure, at room temperature with classical layup of peel ply and infusion mesh. The final thickness of plate was 1.9 mm. During infusion, the vacuum foil was damaged using a sharp pin to introduce air porosity.

Fig 2: Vacuum infusion (a) preparation of 6-ply laminate with embedded defect, (b) after vacuum infusion

3.2 Ultrasonic testing C-scan set-up The ULTRAPAC immersion ultrasonic testing system consists of an UPK-T24 5-axis motorized immersion tank with 5 motorized axes controlled and a computer composed by a SMC-8 motion control board and an IPR-AD1210 ultrasonic acquisition board, Fig. 3. Acquisition board includes the high voltage stage and the signal conditioning and digitalization.

43

Fig. 3: ULTRAPAC system (on the left); GRPC plate in the immersion tank (on the right) The system is used to perform automated ultrasonic inspection of the GFRP plate by immersion pulse-echo method. The aim of this inspection is to optimize the settings for GFRP plate inspection in order to provide the maximum possible information about the indications of defect in the GFRP specimen. The C-scan experimental set up is shown in Fig. 4. Testing parameters are given in table 3.

Fig. 4: Experimental setup of GRPC plate C-scan (water couplant) In preliminary tests, different transducers were tested on specimen. To obtain an optimal cartography of the plate in to minimize the ultrasonic attenuation, a transducer of 10 Mhz, 3/8”, focused at 2” was used. The water couplant was used to maintain consistent coupling while moving and manipulating the transducer, which was set to face orthogonally the sample surface at a distance of 2” (50mm). The entire specimen was inspected from both sides, moving the transducer with 5-axis motorized arm. Tabela 1: Parametri testiranja s C-Scan metodo

Scanning length [mm] Scanning resolution [mm] Scanning speed [mm/s] X-axis 240 0,5 120 Y-axis 220 0,5 120

Frequency

Voltage

Damping

Pulser 10 MHz 400V 2000

Filter Gain

Receiver 8-15MHz 20 dB

44

On the A-Scan, echos are analyzed by means of time analysis gates, which provide the maximum amplitude [%] and the associated time of flight [µs] on a time section of waveform. The A-Scan graph in figure 5 represents the voltage received from the transducer according to the material thickness where the time is converted in mm through the calibration method. The signal values ranges from positive to negative value of 1 V and -1 V, respectively. Analyzed amplitudes used for C-Scan are absolute voltage value expressed in %. When performing the conventional C-scanning, 3 gates are used to analyze the frontwall echo (Gate 1), echo coming back from the material thickness (Gate 2) and the backwall echo (Gate 3). In our study, the single gate on the whole signal from the plate was used which allows to reveal the presence of defects and their depth more efficiently. This method produces 4D C-scan cartographies. Single gate (Gate 4) was divided into 14 slices. Each slice corresponds to narrow depth interval. All slices have equal width, which correspond to thickness of laminate (2,174mm/14 = 0.155mm). The UTwin software algorithm locates and records the signal peak amplitude within the sliced part of the waveform. Those two methods are presented in figure 5 and 6.

Fig. 5: A-Scan graph with Analysis Gate (on the left); Gate settings (on the right)

The sound velocity in GRPC material was calibrated by measuring the back wall echo time-of-flight and the sample thickness. The time of flight, which can be measured on the whole signal is equal to zero at the beginning of the front wall echo. This calibration gave a velocity of 3000 m/s for longitudinal waves along the sample thickness. 4. Analysis of the results

4.1 Foils The round PVC-foils with thickness of 0.1 mm were detectable. Fig. 7 shows C-scan image of amplitudes at 13th slice of 24th at the mould side (smooth) where all foils in a row were embedded between 3rd and 4th ply. Not all foils are visible on 13th slice, therefore a slice before and after have to be examined too.

45

Fig. 7: C-scan for 24 slice gate, 13th slice display

Fig. 8 shows the analysis from rough side of the plate (peel ply). From this set of images, foils at different depth were found by surveying through slices. In this side, the pattern of twill-weave fabrics can be observed. This is a powerful tool for analyzing the fiber misalignments.

5th slice of 24 slice gate

6th slice of 24 slice gate

8th slice of 24 slice gate

Fig. 8: C-scan for 24 slice gate, 5th slice display

46

In composite materials, the gel-coat material frequently hides the defects in a form of inclusions and porosity. The Fig. 9 below is a result of a higher resolution scanning, where small surface defects between fiber tows and inclusion fiber can be observed.

The ultrasonic inspection of the specimens

Fig. 9: Image of analyzed detail (Scan Resolution of 0,1mm), slice 2 of 48 slice gate The dark blue areas are the spots where the amplitude is lowest. This corresponds to the porosity “clouds” in composite. The C-scan image was obtained from near-back wall echo at 13th of 14th slices. Thus, from the Fig. 10 it can be seen that the porosity clouds that are visible through a transparent glass-composite laminate, are also observed in C-scan image.

Fig. 10: Image of porosity “clouds” 5. Conclusions The 4D C-scan method where the signal is divided on many slicer is a powerful tool to obtain different C-scan images that reveal the defects in composites. The optimal selection of number of intervals has to be selected in order to maximize the visibility of the defect on the C-scans.

47

The detection of a defect is limited by its thickness and by the wavelength of the testing ultrasonic wave. The detection of 0.1 mm thick PVC foils is possible with examination of C-scan images through slices. The detection reliability is reduced at higher depths. Ultrasonic method is also useful to detect fiber misalignment. The porosity clouds can be detected using C-image of slice, very close to the back-wall. 6. Acknowledgement

The authors gratefully acknowledge the assistance of Mistras Group, Division EPA. 7. References [1] A. Kapadia, Non Destructive Testing of Composite Materials, National Composite

Network, TWI [2] M.R.P.Dasarath Rao, Review of non-destructive evaluation techniques for FRP composite

structural components, Doctoral thesis, West Virginia, USA, 2007. [3] R. Grimberg, P. Barsanescu, I Cortu, S. Leitoiu, A.Savin: Correlation between structures

and phase transition and ultrasound characteristics at fiber reinforced plastics composites, 10th International Conference of the Slovenian Society for Non-destructive Testing, pp. 59-65, 2009.

[4] B. B. Djordjevic: Ultrasonic characterization of advanced composite materials, 10th International Conference of the Slovenian Society for Non-destructive Testing, pp. 47-57, 2009.

[5] T. Hasiotis, E. Badogiannis, N. G. Tsouvalis: Application of Ultrasonic C-Scan Techniques for Tracing Defects in Laminated Composite Materials, Journal of Mechanical Engineering 57(2011), 3, 192-203, 2009.

[6] A. Ragondet: Experimental Characterisation of the vacuum infusion process, doctoral thesis, University of Nottingham, 2005

48