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Evaluating impact damage in CFRP using fibre optic sensors
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 1235–1242 www.elsevier.com/locate/compscitech
Evaluating impact damage in CFRP using fibre optic sensors A.R. Chambers a
a,*
, M.C. Mowlem b, L. Dokos
a
School of Engineering Sciences, University of Southampton, Highfield, Southampton SO17 1BJ, UK b NOC, University of Southampton, Southampton SO17 1BJ, UK Received 13 February 2006; received in revised form 24 May 2006; accepted 31 May 2006 Available online 24 July 2006
Abstract Damage in unidirectional carbon/fibre composite resulting from both low and high velocity/energy impacts was evaluated using embedded fibre Bragg grating (FBG) sensors, C-scan and microscopic analysis. It was found that the FBG sensors located 10 mm from the impact site could detect residual strains from a 0.33 J (1.3 m s1) impact which was not detectable by C-scan or visual inspection. The measured residual strain increased with impact energy and damage changed from matrix cracking to severe delaminations. High velocity impacts (225 m s1, 11 J) resulted in test panel perforation and delaminations. FBG sensors located within a distance of 2–3 the damage radius detected residual strain from the impact. With an array of embedded sensors it is believed that it will be possible to identify the site of both low and high velocity energy impacts and predict the damage from the response of the adjacent sensors providing the sensors are located sufficiently close to the impact site. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Carbon fibres; B. Impact behaviour; A. Smart materials
1. Introduction Composite materials as used in space vehicles, aircraft, modern vehicles and light weight structure are susceptible to impact damage. Low velocity impacts which may occur during manufacture, maintenance and by careless handling [1] are considered to be dangerous for a composite structure because the damage caused tends to be created on the back face or within the laminate and hence is difficult to detect [2]. Although small in area, such damage in the form of matrix cracking and delaminations can have a significant adverse effect upon the mechanical properties of the laminate [3,4]. High velocity impacts such as those caused by the impact of space debris and micrometeroids can result in damage ranging from minor to catastrophic depending on the size of the impacting particle. Even minor damage (especially if accumulated from impacts by smaller particles which are more numerous) can affect *
Corresponding author. Tel.: +44 2380592164; fax: +44 2380593016. E-mail address: [email protected] (A.R. Chambers).
0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.05.023
the surface properties of the space vehicle and hence its performance. The methods currently used to detect damage such as Cscan require the structure to be taken out of service and often disassembled which is both uneconomical and difficult/impossible to implement [5]. For this reason fibreoptic sensors which can be embedded into a structure and provide real time in situ measurements of condition are an interesting and promising alternative. Bragg grating sensors (FBG) have to date shown the most potential for low impact damage detection having the advantage of giving both static and dynamic strain responses. The static response is related to the magnitude of damage and the frequencies contained in the dynamic signal may be related to specific damage mechanisms. The early efforts employing embedded fibre optic sensors could only detect severe damage and did not have the sensitivity to identify barely visible impact damage (BVID) as might be associated with low velocity impacts [6–8]. The research of Chang and Sikris [9] was more successful and showed that impacts as low as 0.56 J could be
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detected. Debonding of the in-line fibre etalon sensors (ILFE) sensors was a problem with this technique. The optical-fibre based impact detection system developed by Doyle et al. [10] was capable of detecting 5 J impacts in filament wound tubes. Employing reinforcing fibre light guides (RFLG) increased the sensitivity allowing 2 J impacts to be detected. Bocherens et al. [11] used an optical time domain reflectometer in their research which is based on the hypothesis that an impact is able to produce a permanent local bend in the structure and thus an optical fibre embedded in that structure. More recently, Tsutsui [12,13] and Tsuda [14] have used fibre optic sensors and FBGs to detect impact damage initiation and location in composite laminates and stiffened panels and Kuang et al. [15] has used strain measurements to detect damage in fibre metal laminates. Although transverse cracking under tensile loading [16,17] and fatigue [18] have been successfully detected, there have been few attempts to use such sensors to quantify and identify impact damage mechanisms in composite materials. The aim of this research was to evaluate the potential of using Bragg grating fibre-optic sensor (FBG) for detecting and quantifying low and high velocity impact damage and identifying the damage mechanisms. To achieve this aim, u.d. carbon fibre laminates with embedded sensors were subjected to low velocity (energy) impacts from a drop tower test facility. High velocity impacts were achieved by firing 4.5 mm diameter air rifle pellets at the composite target at a velocity of 225 m s1. Damage was assessed by C-scan and destructive microscopic analysis and used in the interpretation of FBG static residual strain impact response. Wavelet transform analysis (WTA) was used on the dynamic signal in order to establish whether it contained frequencies which could be attributed to damage mechanisms. 2. Experimental procedure 2.1. Principle of fibre Bragg grating sensors
fibre core. A Bragg grating sensor can be used to measure the internal strain of the host by measuring the shift in wavelength of the reflective wave peak, which is directly related to the axial strain in the fibre. The shift in the wavelength of the device, which can be observed in a reflected or transmitted spectrum, is caused by the perturbation of the grating. The strain response arises due to both the physical elongation of the sensor (and corresponding change in the grating pitch) and the change in fibre index due to photo elastic effects [19]. The equation that governs the shift of the peak reflected wavelength under strain is often expressed as: DkB =kB ¼ jDe
where j is the strain gauge factor [15]. A typical FBG response to an impact showing the wavelength shift is given in Fig. 1. 2.2. FBG interrogation system The interrogation system employs an acousto-optic tuneable filter (AOTF) and uses wavelength division multiplexing. An EDFA is used as a light source and the control of the AOTF drive frequency is achieved via a high stability digital frequency synthesiser. Drift compensating reference gratings are interrogated within the interrogation cycle of the sensor array. Bragg wavelength can be determined utilising one of two possible algorithms, i.e a pseudo-edge filter algorithm or a centroid algorithm. The edge filter algorithm sets the centre wavelength of the AOTF passband such that it interacts with a single Bragg grating. Any change in Bragg grating centre wavelength then causes modulation in optical power. The centroid algorithm samples optical power on either side of a Bragg grating peak and is insensitive to intensity fluctuations. A schematic of the interrogation system is shown p in Fig. 2. The resolution of the system is 0.18 le/ Hz and, with appropriate referencing and oversampling an accuracy of <1 le is achieved.
Gratings are simple intrinsic sensing elements which can be photo-inscribed into an optical fibre. The have an inherent self referencing capability and are easily multiplexed in a serial fashion along a single fibre. The principle of measurement by an embedded FBG is based on the following two assumptions: (a) The optical fibre axial strain is equal to that of the host in the optical fibre direction. (b) There is no transverse strain transmitted from the host to the optical fibre [20]. The centre wavelength of the grating is determined by the Bragg equation: kB ¼ 2n1 D
ð1Þ
where kB is the Bragg wavelength, D is the period of this index modulation and n1 is the average refractive index of the
ð2Þ
Fig. 1. Typical impact FBG spectrum.
A.R. Chambers et al. / Composites Science and Technology 67 (2007) 1235–1242
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20 mm from the central x-axis (transverse axis) of the test panels. 2.4. Impact tests
Fig. 2. Interrogation system.
2.3. Materials The laminates used in the low velocity impact tests were manufactured from u.d. carbon fibre/epoxy prepreg. The panels 200 mm · 90 mm · 3 mm comprised 20 plies. The construction of panels was: rear face – [0/902/0/90/0/FBG/0/902/02/902/02/90/0/902/0] – front (impact) face. The optical fibre was embedded parallel to the fibres between plies 6 and 7 to minimise perturbation within the laminate. This position was selected taking into account the work of other researchers [21]. The Bragg grating sensor wavelength was 1545 nm and sensing length was 3 mm. The panels were prepared on a specially designed frame which allowed the optical fibre to be accurately positioned during the construction of the laminate. This consisted of a frame with integral optical fibre and a screw driven riser platform on which a ply stack (with the appropriate stacking sequence could be placed). The ply stack could then be raised until it touched the optical fibre. Strain relief sleeves were then placed over the optical fibre such the would enter the finished composite to a depth of approximately 10 mm. Further plies were then added to encapsulate the fibre and finish the ply stack prior to curing using an autoclave for 2 h at 120C. The 0.5 mm thick panel for the high velocity impact tests was manufactured from 4 plies of u.d. carbon/epoxy prepreg. The test panel contained five parallel optic fibre lines located 5 mm apart in the centre of the test panel. Each line contained eight gratings located 1, 2, 3, 4, 6, 8, 12 and
For the low velocity impact tests a driven dart type impact rig based on ASTM D 30292 was designed and manufactured. A weight falls from a predetermined height and strikes the test specimen which is supported in the horizontal plane. The four edges of the specimen are securely clamped between flat rectangular frames with a circular test window. A hemispherical nose impactor is guided to fall in the centre of the test window and the bouncing impactor is caught in order to prevent rebounds. Prior to testing, the rig was calibrated in order to establish impact energy and velocity for any given drop height. The impact energies selected for this research were 0.33, 1.67 and 2.99 J. The corresponding impact velocities were 1.3, 3.1 and 4.1 m s1, respectively. For the high velocity impact tests a rig was designed to accurately control the location and incidence angle of a 4.5 mm diameter lead air rifle pellet fired at an accurately measured velocity of 225 m s1. The test panel containing the five parallel fibre-optic lines was located such that the lines were 10, 15, 20, 25 and 30 mm from the impact. The position of each FBG in relation to the impact site is shown in Fig. 3. 2.5. Damage assessment Damage resulting from impacts was assessed using a commercial ultrasonic C-Scan. C-scan is especially effective in detecting planar defects such as delaminations or dry areas within a composite laminate. After C-scan, the specimens were sectioned through the centre of the impact and metallographically prepared prior to an optical microscope examination.
Fig. 3. FBG sensor locations.
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3. Results and discussion 3.1. Damage assessment – low velocity impact The samples impacted at energies 0.33, 1.67 and 2.99 J were inspected using ultrasonic C-scan. The damage, where apparent, typically comprised of two distinct regions; a damaged circular central zone (CDA) which was located directly beneath the impact and a less heavily damaged area radiating out from the perimeter of the CDA. The CDA is a direct consequence of the impact whereas the TDA damaged area is the result of the propagation of damage initiating in the CDA. The CDA and the total damaged area (TDA) were measured form the C-scans and the results presented in Fig. 4. The C-scan failed to identify any damage from the 0.33 J impact; an energy which was clearly below the C-scan resolution and that required for visible damage. At 1.67 J, the damage was apparent with a small measured CDA and a larger secondary area. At 2.99 J, both the CDA and TDA had further increased. In order to identify the nature of the damage, the samples were sectioned and metallographically prepared prior to a microscopic examination. With a 0.33 J impact, the only evidence of the impact was a small depression at the point of impact and minor deformation/fibre bending (indentation) on the tensile side of the laminate (Fig. 5). There was no evidence of matrix cracking or fibre breaking. The 1.67 J impact resulted in significant matrix cracking and delaminations. This form of damage is typically found in impacted laminates and is commonly known as top hat damage (Fig. 6). The damaged increased at 2.99 J with the development of both the transverse matrix cracks and an increase in the number and the length of the delaminations (Fig. 7). A further difference between the 1.67 J impact and the 2.99 J impact was that at 2.99 J both fibre breakage and cracks associated with bending were also observed (Fig. 8).
Fig. 5. Damage from a 0.33 J impact.
Fig. 6. ‘Top hat’ damage from 1.67 J impact
180 160
TDA
Damaged Area (mm2)
140 120 100
Fig. 7. Transverse cracks and delaminations from 2.99 J impact.
80 60 40
CDA
20 0 0
0.5
1
1.5
2
2.5
3
3.5
Impact Energy (J)
Fig. 4. C-scan damage measurement (a) central damage area (CDA) and (b) total damage area (TDA).
A concern of embedding optical fibres within a composite laminate is whether the presence of the fibres affects the properties or damage mechanisms of the laminate. Fig. 9 shows a section through a damaged region on a panel containing the optical fibre lying perpendicular to the ply direction. Even in this worst case scenario where the fibre is causing a perturbation and a resin rich pocket, there is no evidence of the fibre acting as initiators for either cracking or delamination.
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Fig. 10. FBG residual strain.
Fig. 8. Fibre fracture and bending cracks from 2.99 J.
Fig. 9. Section through damaged area containing optical fibre.
3.2. Fibre-optic sensor investigation – residual strain Two FBGs, 20 mm apart, were positioned with the intention that each would be 10 mm from the centre of the impact. The average residual strain results form these gratings is shown in Fig. 10. It should be noted that the FBGs were located below the centre line (between plies 6 and 7) and hence experienced tensile loading. All measured strains were therefore tensile and can be related to the deformation and damage caused by the impact. The 0.33 J impact resulted in a small, 3 le, residual strain which increased to 25 le at 1.67 J. A further increase in the impact energy to 2.99 J caused a large increase in the residual strain to 605 le. The drift in the signal from the 2.99 J impact was the result of the large wavelength shift associated with this strain causing the centre wavelength to move outside the sample points used to calculate its centroid and
hence the ability of the interrogation system to ‘lock on’. The residual strain readings from the 2 FBGs were within 5%. The differences can probably be attributed to differences in distance to the impact site. The research has demonstrated that fibre-optic sensors located 10 mm from the impact site can detect residual strain at impact energies as low as 0.33 J. This compares with 0.56 J which is the lowest value reported in the literature [9]. At 0.33 J, no damage in the form of cracking or delamination was detected either by visual inspection, Cscan or by microscopic analysis. This demonstrates that FBGs are capable of detecting deformation which occurs prior to matrix cracking or fracture. Thus FBGs are clearly capable of detecting the BVID (barely visible impact damage) which is concern to many of the users of composite materials. Of course in practice, the energy of impact which can be detected will depend on the construction and properties of the laminate and the distance of the FBG from the impact site. The 1.67 J impact was easily detected by the C-scan and microscopic examination with the latter identifying matrix cracking and delaminations. The C-scan showed a centrally damaged region directly associated with the impact and a second region which from the microscopic examination can be attributed to delaminations initiating form the impact site. For this impact, the FBGs recorded only a relatively small increase in strain (22 le). This may be because the FBGs (located 10 mm from the impact site) were too far from the extremity of the damaged area which from the C-scan was estimated to be 6 mm from the impact. At the location of the sensor, the damage comprised matrix cracking and fibre breakage. The 2.99 J impact resulted in a significantly larger residual strain. In this case the damaged region was much larger and the location of the FBGs corresponded to the edge of the damaged region as detected by C-scan. At the sensor location the damage comprised extensive transverse cracking and severe delaminations. Although the research has shown that the FBGs can detect low level damage, the damage can neither be easily
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quantified or damage mechanism detected. The strain measured by one sensor is a local strain, which will be a function of the distance of that sensor from the impact site. From the results it is suggested that to detect matrix cracking and fibre breakage with certainty, the sensor would need to be positioned within a distance no further than 2–3· the measured damage radius. The only way that the impact energy can be quantified would be through embedding an array of sensors and using triangulation to locate the impact site and estimating the impact energy using a prior calibration of the material. If the maximum impact energy could be quantified, it may be possible to predict the nature of the damage if the material’s response to impact had been calibrated through research such as this.
pling frequency of 1 MHz have reported that frequencies associated with matrix cracking, fibre breakage and delaminations are found in the 7–300 kHz range [22]. Thus, if FBGs are to be used to detect damage mechanisms then an interrogation system which can operate at high sampling rates will be required. 3.4. High velocity impact
A typical transient strain response from an impact is identified on Fig. 12. Wavelet transform analysis (WTA) was used to examine the frequency content of the transient dynamic response. It was hoped that WTA could identify frequencies in the interrogation system signal which could be related to the damage mechanisms such as matrix cracking and delaminations. The only frequency identified in the transient by WTA was at 15 Hz and this was found in all the samples. As the damage varied from insignificant (0.33 J) to severe delaminations (2.99 J), it can be concluded that this low frequency was associated with the test rig/method rather than damage mechanism. The fact that it was not possible to isolate frequencies for the dynamic signal that can indicate specific damage mechanisms is not surprising. For reasons associated with the interrogation system, the maximum sampling frequency was 1000 Hz and as defined by the Nyquist theorem the frequency window availability for investigation is 500 Hz. Thus the analysis can only investigate signal features, discontinuities and breaking points up to 500 Hz. From the literature, researchers using acoustic sensors with a sam-
Typical impact damage resulting from impact by a 4.5 mm diameter lead pellet fired at a velocity of 225 m s1 (11.9 J) is shown in Fig. 11. The aim was to replicate morphologies and mechanisms caused by hypervelocity (>5 km s1) impact which is common in spacecraft applications. The damage, which is in the form of a hole with delaminations running in the fibre direction, is similar to that reported for hypervelocity (>5 km s1) impacts from 1 mm particles of the same density. The diameters of both the holes (2.5–3· pellet diameter) and areas of delamination (2–3· hole diameter) are in close agreement with those predicted by Christiansen’s equations [23] for hypervelocity impacts and the experimentally achieved values reported by Tennyson and Lamontagne [24]. This inspires confidence in the use of this test methodology as a hypervelocity analogue. Real time interrogation of a single grating during the impact event produced a transient strain response which shows as a step increase at the point of impact (Fig. 12). The relatively low sampling rate prevented any conclusions concerning the impact event being drawn. High speed digital photography clearly captured the formation and progression of a circular acoustic wavefront with its origin at the impact site. Post impact vibration caused the opening and closure of an impact induced delamination. Fig. 13 shows the closure of such a delamination as a function of time after impact. The response of the interrogation system was not sufficiently fast to gain any information regarding the impact induced acoustic wave. Fig. 14 shows a strain contour map derived from the measurements of the 40 FBGs. The strain measurements
Fig. 11. High velocity impact damage.
Fig. 12. Real time FBG strain response.
3.3. Dynamic response
A.R. Chambers et al. / Composites Science and Technology 67 (2007) 1235–1242
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Fig. 13. Opening and closing of crack after impact: (a) 5.36 ms; (b) 5.7 ms; (c) 6.25 ms.
Fig. 14. Strain contour map from high velocity impact.
are plotted as a function of distance from the impact site in both the fibre and transverse directions. The main features to be noted from Fig. 14 are: • Initially residual strain increases with distance from the impact site reaching peak values at approximately 20 mm in the fibre direction and 11 mm in the transverse direction before decaying to zero. The initial increase from a comparatively low value in the heavily damaged region close to the impact site can be attributed to the damage in the form of severe delaminations and transverse cracking causing a decoupling of the FBGs and a loss of strain transfer. • The contour map shows a directionality of the damage with measureable strains being detected to a greater distance from the impact site parallel to the fibres compared with transverse to the fibres. This is consistent with the damage pattern shown in Fig. 11.
• Residual strains were measured up a distance of two to three times that of the limit of visual cracking and delamination. These lower strains are a measure of non-visible matrix cracking and fibre breakage.
4. Conclusions The ability of FBGs to detect impacts from residual strains has been demonstrated for both low and high velocity impacts. The value of the residual strain is dependent on the location of the FBG in relation to the impact site and hence in order to detect the impact site an array of sensors would be required. To detect an impact, the FBG should be placed within 2–3 radius of the damaged area. The research has shown that different impact damage mechanisms are associated with different impact energies and hence if a panel contained an array of closely spaced
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sensors it should be possible to locate the impact site, predict the maximum impact energy and hence predict the area of damage. It may be possible to predict the damage mechanisms from a strain contour map if the response of the composite to impact had been calibrated. In the case of high velocity impacts, detection of an impact event requires sensors to be located from the impact at a distance no more than 3· the radius of the damage from the impact. For hypervelocity impacts from micrometeoroid particles this has practical implications. Analysis of the dynamic signal using wavelets revealed discrete low frequencies in the signal from low velocity impacts. However these were not associated with impact damage. The sampling speed of the FBG interrogation system was too slow to identify what are believed to be high frequency responses. References [1] Bert C. Recent advances in dynamics of composite structures. Compos Struct IV, Damage Assess Mater Eval 1987(2):1–17. [2] Cantwell W, Morton J. Detection of impact damage in CFRP laminates. Compos Struct 1987;3:241–57. [3] Pintado P, Vogler T, Morton J. Impact damage development in thick composite laminates. Compos Eng 1991;1:195–210. [4] Kim C, Jun E. Measurement of impact delamination by de-ply technique. Exp Technol 1993:26–8. [5] Lin M, Chang F. Composite structures with built-in diagnostics. Mater Today 1984;22:674–92. [6] Hale KF. An optical fibre crack-detection and monitoring system. Smart Mater Struct 1992;1:156–61. [7] Crane R, Macander A, Gagoric J. Fibre optics for a damage detection system for fibre reinforced plastic composite structures. Rev Progr Quant Non-Destruct Eval 1982;2B:1419–30. [8] Hofer B. Fibre optic damage detection system in composite structures. Composites 1987;18(4):309–16.
[9] Chang C, Sikris J. Impact induced damage of laminated graphite/ epoxy composites monitored using embedded in-line fibre etalon optic sensors. J Intell Mater Struct 1997;8(10):829–41. [10] Doyle C et al. In-situ process and condition monitoring of advanced fibre-reinforced composite materials using optical fibre sensors. Smart Mater Struct 1998;7:145–58. [11] Bocherens E et al. Damage detection in a radome sandwich material with embedded fibre optic sensors. Smart Mater Struct 2000;9:310–5. [12] Tsutsui H, Kawamata A, Sanda T, Takeda N. Detection of impact damage of stiffened composite panels using embedded small-diameter optical fibres. Smart Mater Struct 2004;13:1284–90. [13] Tsutsui H, Kawamata A, Sanda T, Takeda N. Proc SPIE 2000;3986: 112–20. [14] Tsuda H, Noboyuki T, Urabe K, Takatsubo J. Impact damage detection in CFRP using fiber Bragg gratings. Smart Mater Struct 2004;13:719–24. [15] Kuang K, Kenny R, Whelan M, Cantwell W, Chalker P. Residual strain measurement and impact response of optical fibre Bragg grating sensors in fibre metal laminates. Smart Mater Struct 2001;10:338–46. [16] Okabe Y, Mizutani T, Yashiro S, Takeda N. Detection of microscopic damages in composite laminates. Compos Sci Technol 2002;62:951–8. [17] Okabe Y, Yashiro S, Kosaka T, Takeda N. Detection of transverse cracks in CFRP composites using embedded fiber Bragg grating sensors. Smart Mater Struct 2000;9:832–8. [18] Takeda N. Characterisation of microscopic damage in composite laminates and real time monitoring by embedded optical fibre sensors. Int J Fatigue 2001;9:281–9. [19] Kersey D et al. Fibre grating sensors. J Lightwave Technol 1997; 15(8):1442–63. [20] Tang L, Tao X, Choy C. Effectiveness and optimisation of fibre Bragg grating sensor as embedded strain sensor. 1999;8:154–60. [21] Cantwell W et al. Post impact fatigue performance of carbon fibre laminates with woven and non-woven layers. Composites 1983;3: 301–5. [22] Qi G. Wavelet-based AE characterisation of composite materials. Composites Sci Technol 1997;57:389–403. [23] Christiansen E. Investigation of hypervelocity impact damage to space station truss tubes. Int J Impact Eng 1990;10(1–4):125–33. [24] Tennyson R, Lamontagne C. Hypervelocity impact damage to composites. Composites A 2000;31A(8):785–94.
Evaluating impact damage in CFRP using fibre optic sensors A.R. Chambers a
a,*
, M.C. Mowlem b, L. Dokos
a
School of Engineering Sciences, University of Southampton, Highfield, Southampton SO17 1BJ, UK b NOC, University of Southampton, Southampton SO17 1BJ, UK Received 13 February 2006; received in revised form 24 May 2006; accepted 31 May 2006 Available online 24 July 2006
Abstract Damage in unidirectional carbon/fibre composite resulting from both low and high velocity/energy impacts was evaluated using embedded fibre Bragg grating (FBG) sensors, C-scan and microscopic analysis. It was found that the FBG sensors located 10 mm from the impact site could detect residual strains from a 0.33 J (1.3 m s1) impact which was not detectable by C-scan or visual inspection. The measured residual strain increased with impact energy and damage changed from matrix cracking to severe delaminations. High velocity impacts (225 m s1, 11 J) resulted in test panel perforation and delaminations. FBG sensors located within a distance of 2–3 the damage radius detected residual strain from the impact. With an array of embedded sensors it is believed that it will be possible to identify the site of both low and high velocity energy impacts and predict the damage from the response of the adjacent sensors providing the sensors are located sufficiently close to the impact site. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Carbon fibres; B. Impact behaviour; A. Smart materials
1. Introduction Composite materials as used in space vehicles, aircraft, modern vehicles and light weight structure are susceptible to impact damage. Low velocity impacts which may occur during manufacture, maintenance and by careless handling [1] are considered to be dangerous for a composite structure because the damage caused tends to be created on the back face or within the laminate and hence is difficult to detect [2]. Although small in area, such damage in the form of matrix cracking and delaminations can have a significant adverse effect upon the mechanical properties of the laminate [3,4]. High velocity impacts such as those caused by the impact of space debris and micrometeroids can result in damage ranging from minor to catastrophic depending on the size of the impacting particle. Even minor damage (especially if accumulated from impacts by smaller particles which are more numerous) can affect *
Corresponding author. Tel.: +44 2380592164; fax: +44 2380593016. E-mail address: [email protected] (A.R. Chambers).
0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.05.023
the surface properties of the space vehicle and hence its performance. The methods currently used to detect damage such as Cscan require the structure to be taken out of service and often disassembled which is both uneconomical and difficult/impossible to implement [5]. For this reason fibreoptic sensors which can be embedded into a structure and provide real time in situ measurements of condition are an interesting and promising alternative. Bragg grating sensors (FBG) have to date shown the most potential for low impact damage detection having the advantage of giving both static and dynamic strain responses. The static response is related to the magnitude of damage and the frequencies contained in the dynamic signal may be related to specific damage mechanisms. The early efforts employing embedded fibre optic sensors could only detect severe damage and did not have the sensitivity to identify barely visible impact damage (BVID) as might be associated with low velocity impacts [6–8]. The research of Chang and Sikris [9] was more successful and showed that impacts as low as 0.56 J could be
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A.R. Chambers et al. / Composites Science and Technology 67 (2007) 1235–1242
detected. Debonding of the in-line fibre etalon sensors (ILFE) sensors was a problem with this technique. The optical-fibre based impact detection system developed by Doyle et al. [10] was capable of detecting 5 J impacts in filament wound tubes. Employing reinforcing fibre light guides (RFLG) increased the sensitivity allowing 2 J impacts to be detected. Bocherens et al. [11] used an optical time domain reflectometer in their research which is based on the hypothesis that an impact is able to produce a permanent local bend in the structure and thus an optical fibre embedded in that structure. More recently, Tsutsui [12,13] and Tsuda [14] have used fibre optic sensors and FBGs to detect impact damage initiation and location in composite laminates and stiffened panels and Kuang et al. [15] has used strain measurements to detect damage in fibre metal laminates. Although transverse cracking under tensile loading [16,17] and fatigue [18] have been successfully detected, there have been few attempts to use such sensors to quantify and identify impact damage mechanisms in composite materials. The aim of this research was to evaluate the potential of using Bragg grating fibre-optic sensor (FBG) for detecting and quantifying low and high velocity impact damage and identifying the damage mechanisms. To achieve this aim, u.d. carbon fibre laminates with embedded sensors were subjected to low velocity (energy) impacts from a drop tower test facility. High velocity impacts were achieved by firing 4.5 mm diameter air rifle pellets at the composite target at a velocity of 225 m s1. Damage was assessed by C-scan and destructive microscopic analysis and used in the interpretation of FBG static residual strain impact response. Wavelet transform analysis (WTA) was used on the dynamic signal in order to establish whether it contained frequencies which could be attributed to damage mechanisms. 2. Experimental procedure 2.1. Principle of fibre Bragg grating sensors
fibre core. A Bragg grating sensor can be used to measure the internal strain of the host by measuring the shift in wavelength of the reflective wave peak, which is directly related to the axial strain in the fibre. The shift in the wavelength of the device, which can be observed in a reflected or transmitted spectrum, is caused by the perturbation of the grating. The strain response arises due to both the physical elongation of the sensor (and corresponding change in the grating pitch) and the change in fibre index due to photo elastic effects [19]. The equation that governs the shift of the peak reflected wavelength under strain is often expressed as: DkB =kB ¼ jDe
where j is the strain gauge factor [15]. A typical FBG response to an impact showing the wavelength shift is given in Fig. 1. 2.2. FBG interrogation system The interrogation system employs an acousto-optic tuneable filter (AOTF) and uses wavelength division multiplexing. An EDFA is used as a light source and the control of the AOTF drive frequency is achieved via a high stability digital frequency synthesiser. Drift compensating reference gratings are interrogated within the interrogation cycle of the sensor array. Bragg wavelength can be determined utilising one of two possible algorithms, i.e a pseudo-edge filter algorithm or a centroid algorithm. The edge filter algorithm sets the centre wavelength of the AOTF passband such that it interacts with a single Bragg grating. Any change in Bragg grating centre wavelength then causes modulation in optical power. The centroid algorithm samples optical power on either side of a Bragg grating peak and is insensitive to intensity fluctuations. A schematic of the interrogation system is shown p in Fig. 2. The resolution of the system is 0.18 le/ Hz and, with appropriate referencing and oversampling an accuracy of <1 le is achieved.
Gratings are simple intrinsic sensing elements which can be photo-inscribed into an optical fibre. The have an inherent self referencing capability and are easily multiplexed in a serial fashion along a single fibre. The principle of measurement by an embedded FBG is based on the following two assumptions: (a) The optical fibre axial strain is equal to that of the host in the optical fibre direction. (b) There is no transverse strain transmitted from the host to the optical fibre [20]. The centre wavelength of the grating is determined by the Bragg equation: kB ¼ 2n1 D
ð1Þ
where kB is the Bragg wavelength, D is the period of this index modulation and n1 is the average refractive index of the
ð2Þ
Fig. 1. Typical impact FBG spectrum.
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20 mm from the central x-axis (transverse axis) of the test panels. 2.4. Impact tests
Fig. 2. Interrogation system.
2.3. Materials The laminates used in the low velocity impact tests were manufactured from u.d. carbon fibre/epoxy prepreg. The panels 200 mm · 90 mm · 3 mm comprised 20 plies. The construction of panels was: rear face – [0/902/0/90/0/FBG/0/902/02/902/02/90/0/902/0] – front (impact) face. The optical fibre was embedded parallel to the fibres between plies 6 and 7 to minimise perturbation within the laminate. This position was selected taking into account the work of other researchers [21]. The Bragg grating sensor wavelength was 1545 nm and sensing length was 3 mm. The panels were prepared on a specially designed frame which allowed the optical fibre to be accurately positioned during the construction of the laminate. This consisted of a frame with integral optical fibre and a screw driven riser platform on which a ply stack (with the appropriate stacking sequence could be placed). The ply stack could then be raised until it touched the optical fibre. Strain relief sleeves were then placed over the optical fibre such the would enter the finished composite to a depth of approximately 10 mm. Further plies were then added to encapsulate the fibre and finish the ply stack prior to curing using an autoclave for 2 h at 120C. The 0.5 mm thick panel for the high velocity impact tests was manufactured from 4 plies of u.d. carbon/epoxy prepreg. The test panel contained five parallel optic fibre lines located 5 mm apart in the centre of the test panel. Each line contained eight gratings located 1, 2, 3, 4, 6, 8, 12 and
For the low velocity impact tests a driven dart type impact rig based on ASTM D 30292 was designed and manufactured. A weight falls from a predetermined height and strikes the test specimen which is supported in the horizontal plane. The four edges of the specimen are securely clamped between flat rectangular frames with a circular test window. A hemispherical nose impactor is guided to fall in the centre of the test window and the bouncing impactor is caught in order to prevent rebounds. Prior to testing, the rig was calibrated in order to establish impact energy and velocity for any given drop height. The impact energies selected for this research were 0.33, 1.67 and 2.99 J. The corresponding impact velocities were 1.3, 3.1 and 4.1 m s1, respectively. For the high velocity impact tests a rig was designed to accurately control the location and incidence angle of a 4.5 mm diameter lead air rifle pellet fired at an accurately measured velocity of 225 m s1. The test panel containing the five parallel fibre-optic lines was located such that the lines were 10, 15, 20, 25 and 30 mm from the impact. The position of each FBG in relation to the impact site is shown in Fig. 3. 2.5. Damage assessment Damage resulting from impacts was assessed using a commercial ultrasonic C-Scan. C-scan is especially effective in detecting planar defects such as delaminations or dry areas within a composite laminate. After C-scan, the specimens were sectioned through the centre of the impact and metallographically prepared prior to an optical microscope examination.
Fig. 3. FBG sensor locations.
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3. Results and discussion 3.1. Damage assessment – low velocity impact The samples impacted at energies 0.33, 1.67 and 2.99 J were inspected using ultrasonic C-scan. The damage, where apparent, typically comprised of two distinct regions; a damaged circular central zone (CDA) which was located directly beneath the impact and a less heavily damaged area radiating out from the perimeter of the CDA. The CDA is a direct consequence of the impact whereas the TDA damaged area is the result of the propagation of damage initiating in the CDA. The CDA and the total damaged area (TDA) were measured form the C-scans and the results presented in Fig. 4. The C-scan failed to identify any damage from the 0.33 J impact; an energy which was clearly below the C-scan resolution and that required for visible damage. At 1.67 J, the damage was apparent with a small measured CDA and a larger secondary area. At 2.99 J, both the CDA and TDA had further increased. In order to identify the nature of the damage, the samples were sectioned and metallographically prepared prior to a microscopic examination. With a 0.33 J impact, the only evidence of the impact was a small depression at the point of impact and minor deformation/fibre bending (indentation) on the tensile side of the laminate (Fig. 5). There was no evidence of matrix cracking or fibre breaking. The 1.67 J impact resulted in significant matrix cracking and delaminations. This form of damage is typically found in impacted laminates and is commonly known as top hat damage (Fig. 6). The damaged increased at 2.99 J with the development of both the transverse matrix cracks and an increase in the number and the length of the delaminations (Fig. 7). A further difference between the 1.67 J impact and the 2.99 J impact was that at 2.99 J both fibre breakage and cracks associated with bending were also observed (Fig. 8).
Fig. 5. Damage from a 0.33 J impact.
Fig. 6. ‘Top hat’ damage from 1.67 J impact
180 160
TDA
Damaged Area (mm2)
140 120 100
Fig. 7. Transverse cracks and delaminations from 2.99 J impact.
80 60 40
CDA
20 0 0
0.5
1
1.5
2
2.5
3
3.5
Impact Energy (J)
Fig. 4. C-scan damage measurement (a) central damage area (CDA) and (b) total damage area (TDA).
A concern of embedding optical fibres within a composite laminate is whether the presence of the fibres affects the properties or damage mechanisms of the laminate. Fig. 9 shows a section through a damaged region on a panel containing the optical fibre lying perpendicular to the ply direction. Even in this worst case scenario where the fibre is causing a perturbation and a resin rich pocket, there is no evidence of the fibre acting as initiators for either cracking or delamination.
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Fig. 10. FBG residual strain.
Fig. 8. Fibre fracture and bending cracks from 2.99 J.
Fig. 9. Section through damaged area containing optical fibre.
3.2. Fibre-optic sensor investigation – residual strain Two FBGs, 20 mm apart, were positioned with the intention that each would be 10 mm from the centre of the impact. The average residual strain results form these gratings is shown in Fig. 10. It should be noted that the FBGs were located below the centre line (between plies 6 and 7) and hence experienced tensile loading. All measured strains were therefore tensile and can be related to the deformation and damage caused by the impact. The 0.33 J impact resulted in a small, 3 le, residual strain which increased to 25 le at 1.67 J. A further increase in the impact energy to 2.99 J caused a large increase in the residual strain to 605 le. The drift in the signal from the 2.99 J impact was the result of the large wavelength shift associated with this strain causing the centre wavelength to move outside the sample points used to calculate its centroid and
hence the ability of the interrogation system to ‘lock on’. The residual strain readings from the 2 FBGs were within 5%. The differences can probably be attributed to differences in distance to the impact site. The research has demonstrated that fibre-optic sensors located 10 mm from the impact site can detect residual strain at impact energies as low as 0.33 J. This compares with 0.56 J which is the lowest value reported in the literature [9]. At 0.33 J, no damage in the form of cracking or delamination was detected either by visual inspection, Cscan or by microscopic analysis. This demonstrates that FBGs are capable of detecting deformation which occurs prior to matrix cracking or fracture. Thus FBGs are clearly capable of detecting the BVID (barely visible impact damage) which is concern to many of the users of composite materials. Of course in practice, the energy of impact which can be detected will depend on the construction and properties of the laminate and the distance of the FBG from the impact site. The 1.67 J impact was easily detected by the C-scan and microscopic examination with the latter identifying matrix cracking and delaminations. The C-scan showed a centrally damaged region directly associated with the impact and a second region which from the microscopic examination can be attributed to delaminations initiating form the impact site. For this impact, the FBGs recorded only a relatively small increase in strain (22 le). This may be because the FBGs (located 10 mm from the impact site) were too far from the extremity of the damaged area which from the C-scan was estimated to be 6 mm from the impact. At the location of the sensor, the damage comprised matrix cracking and fibre breakage. The 2.99 J impact resulted in a significantly larger residual strain. In this case the damaged region was much larger and the location of the FBGs corresponded to the edge of the damaged region as detected by C-scan. At the sensor location the damage comprised extensive transverse cracking and severe delaminations. Although the research has shown that the FBGs can detect low level damage, the damage can neither be easily
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quantified or damage mechanism detected. The strain measured by one sensor is a local strain, which will be a function of the distance of that sensor from the impact site. From the results it is suggested that to detect matrix cracking and fibre breakage with certainty, the sensor would need to be positioned within a distance no further than 2–3· the measured damage radius. The only way that the impact energy can be quantified would be through embedding an array of sensors and using triangulation to locate the impact site and estimating the impact energy using a prior calibration of the material. If the maximum impact energy could be quantified, it may be possible to predict the nature of the damage if the material’s response to impact had been calibrated through research such as this.
pling frequency of 1 MHz have reported that frequencies associated with matrix cracking, fibre breakage and delaminations are found in the 7–300 kHz range [22]. Thus, if FBGs are to be used to detect damage mechanisms then an interrogation system which can operate at high sampling rates will be required. 3.4. High velocity impact
A typical transient strain response from an impact is identified on Fig. 12. Wavelet transform analysis (WTA) was used to examine the frequency content of the transient dynamic response. It was hoped that WTA could identify frequencies in the interrogation system signal which could be related to the damage mechanisms such as matrix cracking and delaminations. The only frequency identified in the transient by WTA was at 15 Hz and this was found in all the samples. As the damage varied from insignificant (0.33 J) to severe delaminations (2.99 J), it can be concluded that this low frequency was associated with the test rig/method rather than damage mechanism. The fact that it was not possible to isolate frequencies for the dynamic signal that can indicate specific damage mechanisms is not surprising. For reasons associated with the interrogation system, the maximum sampling frequency was 1000 Hz and as defined by the Nyquist theorem the frequency window availability for investigation is 500 Hz. Thus the analysis can only investigate signal features, discontinuities and breaking points up to 500 Hz. From the literature, researchers using acoustic sensors with a sam-
Typical impact damage resulting from impact by a 4.5 mm diameter lead pellet fired at a velocity of 225 m s1 (11.9 J) is shown in Fig. 11. The aim was to replicate morphologies and mechanisms caused by hypervelocity (>5 km s1) impact which is common in spacecraft applications. The damage, which is in the form of a hole with delaminations running in the fibre direction, is similar to that reported for hypervelocity (>5 km s1) impacts from 1 mm particles of the same density. The diameters of both the holes (2.5–3· pellet diameter) and areas of delamination (2–3· hole diameter) are in close agreement with those predicted by Christiansen’s equations [23] for hypervelocity impacts and the experimentally achieved values reported by Tennyson and Lamontagne [24]. This inspires confidence in the use of this test methodology as a hypervelocity analogue. Real time interrogation of a single grating during the impact event produced a transient strain response which shows as a step increase at the point of impact (Fig. 12). The relatively low sampling rate prevented any conclusions concerning the impact event being drawn. High speed digital photography clearly captured the formation and progression of a circular acoustic wavefront with its origin at the impact site. Post impact vibration caused the opening and closure of an impact induced delamination. Fig. 13 shows the closure of such a delamination as a function of time after impact. The response of the interrogation system was not sufficiently fast to gain any information regarding the impact induced acoustic wave. Fig. 14 shows a strain contour map derived from the measurements of the 40 FBGs. The strain measurements
Fig. 11. High velocity impact damage.
Fig. 12. Real time FBG strain response.
3.3. Dynamic response
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Fig. 13. Opening and closing of crack after impact: (a) 5.36 ms; (b) 5.7 ms; (c) 6.25 ms.
Fig. 14. Strain contour map from high velocity impact.
are plotted as a function of distance from the impact site in both the fibre and transverse directions. The main features to be noted from Fig. 14 are: • Initially residual strain increases with distance from the impact site reaching peak values at approximately 20 mm in the fibre direction and 11 mm in the transverse direction before decaying to zero. The initial increase from a comparatively low value in the heavily damaged region close to the impact site can be attributed to the damage in the form of severe delaminations and transverse cracking causing a decoupling of the FBGs and a loss of strain transfer. • The contour map shows a directionality of the damage with measureable strains being detected to a greater distance from the impact site parallel to the fibres compared with transverse to the fibres. This is consistent with the damage pattern shown in Fig. 11.
• Residual strains were measured up a distance of two to three times that of the limit of visual cracking and delamination. These lower strains are a measure of non-visible matrix cracking and fibre breakage.
4. Conclusions The ability of FBGs to detect impacts from residual strains has been demonstrated for both low and high velocity impacts. The value of the residual strain is dependent on the location of the FBG in relation to the impact site and hence in order to detect the impact site an array of sensors would be required. To detect an impact, the FBG should be placed within 2–3 radius of the damaged area. The research has shown that different impact damage mechanisms are associated with different impact energies and hence if a panel contained an array of closely spaced
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sensors it should be possible to locate the impact site, predict the maximum impact energy and hence predict the area of damage. It may be possible to predict the damage mechanisms from a strain contour map if the response of the composite to impact had been calibrated. In the case of high velocity impacts, detection of an impact event requires sensors to be located from the impact at a distance no more than 3· the radius of the damage from the impact. For hypervelocity impacts from micrometeoroid particles this has practical implications. Analysis of the dynamic signal using wavelets revealed discrete low frequencies in the signal from low velocity impacts. However these were not associated with impact damage. The sampling speed of the FBG interrogation system was too slow to identify what are believed to be high frequency responses. References [1] Bert C. Recent advances in dynamics of composite structures. Compos Struct IV, Damage Assess Mater Eval 1987(2):1–17. [2] Cantwell W, Morton J. Detection of impact damage in CFRP laminates. Compos Struct 1987;3:241–57. [3] Pintado P, Vogler T, Morton J. Impact damage development in thick composite laminates. Compos Eng 1991;1:195–210. [4] Kim C, Jun E. Measurement of impact delamination by de-ply technique. Exp Technol 1993:26–8. [5] Lin M, Chang F. Composite structures with built-in diagnostics. Mater Today 1984;22:674–92. [6] Hale KF. An optical fibre crack-detection and monitoring system. Smart Mater Struct 1992;1:156–61. [7] Crane R, Macander A, Gagoric J. Fibre optics for a damage detection system for fibre reinforced plastic composite structures. Rev Progr Quant Non-Destruct Eval 1982;2B:1419–30. [8] Hofer B. Fibre optic damage detection system in composite structures. Composites 1987;18(4):309–16.
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