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ScienceDirect Procedia Engineering 88 (2014) 194 – 199
International Symposium on Dynamic Response and Failure of Composite Materials, DRaF2014
Non destructive techniques for the impact damage investigation on carbon fibre laminates V. Antonuccia, M. R. Ricciardia, F. Caputob, A. Langellac, V. Loprestoc, V. Pagliarulod*, A. Roccod, C. Toscanoe, P. Ferrarod, A. Ricciob a
CNR Research National Council, IMCB, Inst. of Comp. and Biom. Mat., P.E. Fermi, Portici (NA), Italy Dpt. of Aerospace and Mechanical Engineering, Second University of Naples, via Roma, 29, Aversa (CE), Italy c Dpt. of Materials and Production Engineering, University of Naples “Federico II”, P.le Tecchio, 80, Naples, Italy d CNR Research National Council, INO, Istituto Nazionale di Ottica, Via Campi Flegrei 34, 80078 Pozzuoli (NA) Italy b
e
CIRA Italian Aerospace Research Centre, Via Maiorise sn, 81043 Capua (CE) Italy
Abstract Ultra sound technique (US), thermography and holography were adopted to investigate the delamination in carbon fibre laminates after low velocity impact loads. The scope was to investigate the ability of new tools in giving information about internal damage on composites. The carbon laminates were fabricated by a new vacuum assisted technology, labelled as “pulsed infusion". The efficiency of this innovative fabrication technology was studied too, comparing the delamination extensions with the measurements from literature on impacted autoclave cured laminates. © 2014 The The Authors. Authors. Published Published by by Elsevier ElsevierLtd. Ltd.This is an open access article under the CC BY-NC-ND license © 2014 Peer-review under responsibility of the Organizing Committee of DRaF2014. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of DRaF2014 Keywords: non destructive evaluation; carbon fibre laminates; indentation; delamination; pulsed infusion.
1. Introduction The non-homogeneous and anisotropic nature of composites causes the reduction of the integrity of impacted structural components even if there is no visual damage. Composites can fail in a wide variety of damage modes like indentations, inter-laminar fractures (delamination) caused by inter-laminar stresses, fibres cracks and matrix cracks.
* Corresponding author. Tel.: +39-081-8675337/38; fax: +39-081-8675118; E-mail address:
[email protected]
1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of DRaF2014 doi:10.1016/j.proeng.2014.11.144
V. Antonucci et al. / Procedia Engineering 88 (2014) 194 – 199
It is so very important to understand where and when the damage starts and propagates [1] and the complex interaction between failure modes. A lot of efforts were already done [2] at the aim to investigate about the influence of the large number of parameters involved in the dynamic phenomena. In this work, a large experimental low velocity impact campaign has been carried out on carbon laminates. The laminates made by carbon fibres in epoxy matrix RTM6, have been manufactured by a new vacuum-assisted technology, "Pulsed Infusion". At the aim to obtain the whole load displacement curve, low velocity impact tests were carried out, up to complete penetration, allowing the penetration energy measurement as the area under the complete load curve. Different energy values were, then, evidenced in correspondence of load drops or changing in slope on the load curve, and successively measured for the indentation tests. The indentation depths were measured by a confocal microscope, and, together with the penetration energies, were used to validate existing semi empirical models for the prediction of the impact energy and the residual strength. The largely used ultra sound technique (US) was adopted to investigate about the delamination after the low velocity impact tests. The results were compared with data obtained by thermographic and holographic analysis on the same specimens. The scope was to investigate the ability of these less used techniques in giving right information about non visual impact damage within the composites. The efficiency of a new fabrication technology was studied, too, comparing the delamination extensions with the measurements from literature on impacted specimens, autoclave cured. 2. Materials and experimental set up Composite laminates, 400x400 mm, made by carbon fibres in polymeric matrix RTM6, have been manufactured by "Pulsed infusion" [3]. Unidirectional layer of dry fibres were overlapped with the stacking sequence [(0)/(90)/(+45)/(-45)]s, resulting in a nominal thicknesses of 2.5 mm. Impact tests have been carried out on a Ceast Fractovis MK4 drop weight machine. The specimens were centrally impacted by a cylindrical instrumented impactor with hemispherical nose, 19.8 mm in diameter. Rectangular specimens, 100x150 mm, were cut by a diamond saw and clamped using a clamping device suggested by the EN6038 standard. The force-displacement curves obtained by the penetration tests were recorded by DAS16000 acquisition program. The penetration energy was measured as the area under the complete curve whereas the variable energy values, 6J, 10J and 13J, for the indentation tests, where obtained in correspondence of the characteristic points clearly evidencing changing in material behaviour like in presence of damages. The indentation depth was measured by a confocal microscope Leica DCM3D [4]. Olympus OmniScan® SX Phased array automated data acquisition system was used to non destructively investigate the damage by the Ultra Sound tool. The Pulse Echo technique has been used. The C-scan inspection was here adopted providing a plane view of the specimen parallel to the scan pattern of the transducer. It allows to visualize and measure the in plane damages like delamination. The impact damage on and inside the coupons was, then, analysed via NDT (Non Destructive Testing), by using Lock-In Thermography [5, 6, 7]. The used set-up is made up of an infrared camera SC5000 MW InSb by FLIR®, four 1KW halogen lamps and their related driving system, and a pc with a dedicated software for the data acquisition and processing. In the following, an example of interesting thermal phase maps, is shown (Fig. 1). Each group of coupons were inspected by framing their front and backside, obtained with a lighting frequency of 0.05Hz. In the central coupon, impacted at 13J of energy, the halo of the benchmark on the rear side (coin) can be observed since evidenced by a red arrow. The impact areas are highlighted by blue arrows.
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6J
13J
10J
6J
a)
13J
10J
b)
Fig. 1: Phase maps of the coupons impacted with energies of 6J, 13J, and 10 J (front side (a)), back side (b)).
The damages were evaluated also by optical interferometric technique: the Electronic Speckle Pattern Interferometry (ESPI), that allows the measurement of displacements, strains and cracks on rough surfaces with high sensitivity [8,9]. This technique, based on the analysis of interferograms recorded by a digital camera, enables the measurement in real time of displacements in different points simultaneously, without direct contact. By this technique, it is possible to reveal hidden defects with high precision. The results were compared with data obtained by US and thermal technique. 3. Damage investigations In [10], data from literature obtained by low velocity impact tests on different CFRP laminates, were used to assess an exponential law equation, valid for a quite large class of laminates, for the prediction of the impact energy and the residual strength from simple indentation measurements. The indentation values obtained here were used in [11] to validate the existing law: good agreement was obtained confirming the quite general applicability of the semiempirical law, being scarcely affected by the fibre type and orientations, matrix type and fabrication method. Also the power law previously found [12] for the prediction of Up, necessary to predict the impact energy from a simple indentation measurement, was validated by the data found in the present research. This highlights the efficiency of the new pulsed infusion technique in the laminates fabrication. By comparing the images of the damage inspection obtained by the three different non destructive techniques [13], it was possible to note their ability in investigating the delamination. In Table 1, the average values of the maximum lengths and widths, dmax and dmin, of the delamination, measured on all the impacted specimens at the selected impact energies by the NDE used in this work, are reported. Good agreement was found between US, thermography and ESPI, in particular at high energy. Table 1. Comparison between the results, in terms of length of the delamination, obtained by the different NDE used in the work.
6J 10J 13J
dmax (mm) dmin (mm) dmax (mm) dmin (mm) dmax (mm) dmin (mm)
Thermography 14 7 29 11 41 18
Holography 11 7 26 12 43 14
Ultrasonic C-Scan 20 14 33 17 42 20
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The maximum and the minimum extensions of the delamination, length dmax and dmin, measured by ESPI on the back side of the specimens analysed in the present research, were plotted in Fig. 2 against the impact energy. On the same graph, data from literature obtained on glass fibre laminates similar in thickness, were added for comparison purpose: a general linear trend was observed and similar values were found comparing dmin and dmax for 6J and 10J whereas a higher extension of the delamination was observed in correspondence of 13J of impact energy.
50
d (mm)
40
30
20
10
Glass t=2 and 3mm Carbon dmax Carbon dmin
0 0
20
40
60
80
U (J)
Fig. 2. Maximum and minimum extensions of the delamination against impact energy.
The delaminated area, A, was also measured by the different technologies adopted in the present work. In Table 2, all the measurements were reported for comparison purpose. Table 2. Results, in terms of delaminated area, obtained by the different NDE used in the work.
6J
A(mm2)
Thermography 37
Holography 60
Ultrasonic C-Scan
10J
A(mm2)
181
244
314
13J
A(mm2)
430
472
414
153
It is possible to note the very little differences between the different methods of damage investigations. The good result is more clear in Fig. 3, where the results obtained by ESPI, Termography and US and showed in Table 2, are reported and compared with results from literature about glass fibre laminates having similar thickness.
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Fig. 3. Delaminated area, A, against impact energy, U.
As it is simple to note, also in this case a good agreement was obtained as general trend. However, the comparison with data from literature [14] obtained on carbon fibre laminates is here not reported since very lower delaminations were found in the present work. The phenomena is not explained yet since more experimental data are necessary to confirm the assertion but it should be a good result considering the innovative fabrication technology adopted. 4. Conclusions In the present research, low velocity impact tests at complete penetration and at different energy values, were performed on carbon fibre laminates obtained by a new fabrication technology named "Pulsed Infusion". The internal damage was investigated by three different non-destructive techniques: Ultra Sound, Thermography and by an Electronic Speckle Pattern Interferometry (ESPI) that is an optical interferometric technique. The results in terms of maximum length of the delamination and delaminated area were compared: a good agreement was found between data obtained by thermography, US and ESPI. All the techniques clearly evidenced the lobe shape of the delamination allowing simple experimental measurements. The results were, then, compared also with data from literature and the previous found linear trends of the damage as a function of the impact energy, were here confirmed denoting a good efficiency of the innovative fabrication technology and the different damage investigations. Acknowledgement The authors gratefully acknowledge the Italian University and Research Ministry Council (MIUR), under the National Research Program (PRIN) 2009 - prot. 2009KR5PKJ, for the financial support provided to this research. References [1] S. Liu, Z. Kutlu, and F. K. Chang, Matrix cracking and delamination propagation in laminated composites subjected to transversely concentrated loading. Journal of Composite Materials, 27 (5), (1993) 436-470. [2] G. Schoeppner, S. Abrate, Delamination threshold loads for low velocity impact on composite laminates, Compos. Part A, 31 (2000) 903– 915. [3] M. R. Ricciardi, V. Antonucci, M. Durante, M. Giordano, L. Nele, G. Starace, A. Langella, A new cost-saving vacuum infusion process for fiber-reinforced composites: Pulsed infusion, J. of Compos. Mat., 48(11) (2014) 1365-1373
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