Italian Association of Aeronautics and Astronautics XXII Conference Napoli, 9-12 September 2013
ELECTRONIC SPECKLE PATTERN INTERFEROMETRY (ESPI) FOR IMPACT DAMAGES EVALUATION ON CFP V. Pagliarulo1*, V. Lopresto2, M. Matrecano1, V. Antonucci3, A. Rocco1, M. R. Ricciardi3 and P. Ferraro1 1 CNR, Istituto Nazionale di Ottica, 80078 – Pozzuoli (Na) 2 Università Federico II, DICMAPI, 80124 – Napoli 3 CNR, Istituto per i Materiali Compositi e Biomedici, 80055 – Portici (Na) *
[email protected]
ABSTRACT In this work, it is presented the application of the Electronic Speckle Pattern Interferometry (ESPI) application in the analysis of composites, used in the aeronautic field. Through realtime surface illumination by visible laser (i.e. 532 nm), the ESPI technique allows the noncontact, non-destructive detection of micro-deformations, micro-cracks, residual stress and delamination. The method is based on recording the surface field differential displacement, due to thermal or mechanical strains. A CCD camera records the whole field deformation into images to be processed. By this technique, it is possible to reveal hidden defects and to evaluate the effective delamination area due, for example, to impact damages. In this study, ESPI technique has been used to evaluate the effective delamination area of damaged EpoxyCarbon Fibers Composites that have been manufactured by Pulsed Infusion. Keywords: CFP, impact damage, ESPI 1
INTRODUCTION
A fundamental requirement of a primary aeronautical structure made with carbon fiber reinforced plastics (CFRP), is its capability to bear the applied loads when a realistic distribution of barely visible impact damages (BVID) is present. The definition of “barely visible” for an impact damage is not well defined. The parameter most used to quantify this property is the indentation, i.e. the dent depth resulting from the accidental impact of the material surface with an object travelling at low, medium, or high velocity [1]. The tool universally used to test an aeronautical component for a BVID, is an impactor: the geometry of the head destined to impact the wanted BVID is fixed while the energy, necessary to induce the specified damage level, is experimentally found by trial-and-error and the dent depth corresponding to a BVID is assigned using a specimens representative of the actual situation; finally, the information gathered is employed to induce the desired damages in the real component to be tested. In general, the dent depth is strongly dependent on many parameters (i.e. thickness, constraint conditions, tup geometry, impact speed) and can be not easily visible.
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In this work, the use of a non-destructive and non-contact optical testing technique to reveal hidden and barely visible impact damages is shown. The ESPI [2] allows to estimate the effective damaged area as well as the delaminated one by the measure of the specimen deformation out of plane due to thermal or mechanical strains. CFRP specimens, with the same characteristics but damaged with different impact energy levels, are analyzed and the obtained results discussed. 2
EXPERIMENTAL
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MATERIALS
In conventional vacuum infusion processes (VIP, VARTM), the impregnation of the dry fiber reinforcement with a thermoset liquid resin occurs by vacuum application. In this study, Epoxy-Carbon Fibers Composites have been manufactured by Pulsed Infusion, an Italian patented innovative vacuum assisted infusion process [3]. This process involves three steps: lay-up of a fiber preform, vacuum application and fiber impregnation with a thermoset resin, and finally cure of the resin. The reinforcement (carbon or glass fabric) is placed on a onesided rigid mould and a formable vacuum bag material. The resin is injected through one or more inlet gates, depending on the geometry. Vacuum is exerted through one or more vents in order to remove the air from the fiber preform and force the resin through the fibers for impregnation. A resin distribution medium is posed over the reinforcement for several reasons: to promote resin flow, to allow complete wet-out of the preform and to eliminate voids and dry spots in the finished part. In the case of Pulsed Infusion, two vacuum bags are used and the resin distribution medium is eliminated. The lower vacuum bag determines a lower chamber, where the resin infusion occurs almost like in the VARTM process. The lower vacuum bag is stacked on the dry fiber reinforcement without a resin distribution medium. A suitably designed pressure distributor is positioned on the lower vacuum bag and under the second upper vacuum bag, creating an upper chamber. Through different vacuum pressure application in the two chambers and the timely control of the pressure difference between the chambers, the resin flow is pulsed and assisted both in the plane and through the thickness of the reinforcement. In particular, the composite panels have been realized by using a one-component commercial aeronautical resin (RTM6 from Hexcel) and eight dry unidirectional carbon plies having a quasi isotropic stacking sequence [0°, 90°, +45°,-45°]S and sized 100mm X 150mm and 2.5mm thick. 2.2
OPTICAL SETUP
A typical system for recording speckle interferometry measurements is illustrated in Figure 1 [4]:
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Figure 1: Components layout for the ESPI system.
The laser beam is split into a reference beam and an object beam by means of a beam splitter, which enables control of the relation between reference and object. A speckled image of the object surface (result of the interference between the wavefronts of the two beams) is recorded through a CCD camera. When the object is deformed by an external perturbation, the wavefront reflected from it is slightly changed, while the wavefront coming from the reference beam is unchanged. Thus, the CCD camera records a new speckle pattern and the subtraction of the speckle patterns registered for the deformed and non-deformed states provides the correlation fringes. When the subtraction is computer-aided, the technique is called electronic speckle pattern interferometry. The correlation fringes are digitally treated for noise removal and contrast enhancement allowing to obtain the phase contrast maps, from which it is possible to measure the displacement field with high accuracy, depends on the wavelength. In this work, the light source is a 532nm solid-state laser with an output power of 50mW. The specimens are mounted on a sample holder, the strain is induced by a heating gun and the temperature is monitored by a thermal camera to assure a temperature variation of about 1°C. 3
RESULTS AND DISCUSSION
Three equivalent CFRP plate, with the characteristics described above, have been damaged with different impact energy, in particular E= 6, 10 and 13 J respectively, loaded at the center by a hemispherical steel indentor (diameter 12.7mm).
Figure 2: Correlation fringes (6J impact energy)
The Figure 2 shows the correlation fringes due to a heating process that has raised the specimen temperature of about 1°C. This external stress is applied to all plate. Where a delamination phenomenon due to a damage is present, the out of plane deformation, induced by the thermal heating, is amplified. Through suitable image analysis technologies, the fringes discontinuities are utilized to show the impact zone (i.e. the zone where the deformation has a different value). A numerical elaboration allows to plot a 3D profile (Figure 3) and from that,
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to identify and measure the effective delamination area also when it is not clearly visible by visual inspection.
Figure 3: 3D elaboration of figure 2
From this elaboration, the delamination area has been estimated in 157,56mm2. The second sample to be examined is impacted with energy E= 10J. The Figure 4 shows the correlation fringes while the Figure 5 the numerical elaboration:
Figure 4: Correlation fringes (10J impact energy).
Figure 5: 3D elaboration of figure 4
In this case, the delamination area results in 182,52mm2. The last examined plate has been damaged with a higher energy E= 13J. In that case, the plate was visibly damaged and the discontinuity of the correlation fringes appears clearly (Figure 6):
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Figure 6: Correlation fringes (13J impact energy)
The 3D reconstruction shows, as expected, a larger delamination area, which is calculated to be 436.80 mm2.
Figure 7: 3D elaboration of figure 6
From a preliminary analysis of the obtained results, it is possible to note the correspondence between the impact energy and the delamination (i.e. damaged) area. In particular, at 13J impact energy, it seems that some threshold has been exceeded because the increase in the delaminated area appears much bigger respect to lower energy impacts. 4
CONCLUSION
In conclusion, through the ESPI technique, applied on damaged Epoxy-Carbon Fibers Composites, and the most recent and sought image analysis techniques, we are able to assess accurately the delamination damage. The obtained results are in good agreement with the literature impact model data. This confirms the reliability of the proposed technique and opens the way to new and interesting application fields. REFERENCES [1] G. Caprino and V. Lopresto, The significance of indentation in the inspection of carbon fiber reinforced plastic panels damaged by low-velocity impact”, Comp. Science and Technol., 60, pp. 1003-1012 (2000). [2] L. Farge et. Al Damage characterization of a cross-ply carbon fiber/epoxy laminate by an optical measurement of the displacement field Comps. Sci. Technol. 70, 94-101 (2010).
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[3] V. Antonucci M.R. Ricciardi, Pulsed Infusion: a new liquid moulding process, Jec. Comp. Mag. 77, 63-64 (2012). [4] K. Stetson, Mathematical refocusing of images in electronic holography App. Optics 48, No. 19/1 (2009).
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