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Mapping of the fluid distribution in impregnated reinforcement textiles using Magnetic Resonance Imaging: Methods and issues
Composites: Part A 42 (2011) 265–273
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Mapping of the fluid distribution in impregnated reinforcement textiles using Magnetic Resonance Imaging: Methods and issues Andreas Endruweit a,⇑, Paul Glover b, Kay Head b, Andrew C. Long a a b
Faculty of Engineering – Division of Materials, Mechanics & Structures, University of Nottingham, University Park, Nottingham NG7 2RD, UK Sir Peter Mansfield Magnetic Resonance Centre, School of Physics & Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
a r t i c l e
i n f o
Article history: Received 13 September 2010 Received in revised form 15 November 2010 Accepted 20 November 2010 Available online 25 November 2010 Keywords: A. Fabrics/textiles E. Resin transfer moulding (RTM) B. Porosity Magnetic Resonance Imaging (MRI)
a b s t r a c t Magnetic Resonance Imaging techniques were found to be suitable for mapping the fluid distribution in impregnated textile fabrics. They allowed the state of impregnation of E-glass and carbon fibre fabrics of different architectures with a test fluid (engine oil) to be identified and dry spots to be detected. An UltraShort Echo-Time technique was found to be relatively robust to signal loss caused by relaxation processes, which are related to the dispersion of the fluid in the fabric and local discontinuities in magnetic susceptibility. On a 3 T scanner, 3D images of specimens with dimensions of 140 mm 90 mm 4.7 mm at an isotropic resolution of 0.5 mm were acquired at scan times of 21 min. However, radio-frequency eddy currents may be induced if the fabrics are conductive and result in partial cancellation of the signal, rendering it undetectable inside the sample. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The structure of reinforcement fabrics for polymer composites affects the composite properties directly, but also determines their impregnation with a liquid resin system. This is critical for the quality of the finished component, in particular if Liquid Composite Moulding (LCM) processes are employed for manufacture. Darcy’s law [1] states that the fluid flow during impregnation depends on the applied pressure gradient, the fluid viscosity and the textile permeability, which is determined by the fibre arrangement in the reinforcement structure. Fluid flow and textile permeability can be described at multiple scales, reflecting the structure of the reinforcement: Component shape and geometrical features: macro-scale (m). Geometrical arrangement of fibre bundles and inter-bundle voids in the textile unit cell: meso-scale (mm). Distribution of filaments and inter-filament voids in the fibre bundles: micro-scale (lm). To characterise the resin flow in reinforcement fabrics, visual flow monitoring using transparent injection tools [2,3] and permeability measurement at the macro-scale, based on tracking of the flow front propagation [4], are frequently adopted procedures. However, measurement of the macro-scale permeability implies a homogenisation of local fabric properties and does not allow identi⇑ Corresponding author. Tel.: +44 (0)115 95 14037; fax: +44 (0)115 9513800. E-mail address: [email protected] (A. Endruweit). 1359-835X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2010.11.012
fication of meso- and micro-scale effects. The injected fluid normally flows through inter-bundle voids at a different velocity than through inter-filament voids (due to the difference in void dimensions, i.e. different local permeabilities), resulting in an uneven flow front at the meso-scale [5]. This may cause formation of resin-free dry spots [6] and thus result in a reduction in the matrix-dominated mechanical properties, in particular shear strength, flexural strength and compressive strength, of the finished composite. Since visualisation of fabric impregnation at the meso- and micro-scale is generally not possible using conventional means, identification of dry spots is in practice based on analysis of moulded and cured specimens. This is typically done by microscopy, although more advanced methods (e.g., ultrasound techniques [7], Optical Coherence Tomography [8] or micro-Computed Tomography [6]) have been applied with some success. For flow front tracking during the impregnation process and for monitoring of dry spot formation, Magnetic Resonance Imaging (MRI) is potentially suitable. The basic principles of MRI are discussed in detail in a variety of textbooks (e.g. by Callaghan [9]) and tutorials (e.g. by Hornak [10]). Here, only a brief overview can be given: in MRI, ensembles of atomic nuclei are exposed to a magnetic field. The nuclear spins precess around the field direction. For the example of protons (hydrogen nuclei), they can take two possible orientations, corresponding to two energy states. If the difference energy, which depends on the strength of the applied magnetic field, is supplied in the form of a pulse of electromagnetic radiation at the right frequency, the orientation of the spins tilts. This phenomenon is referred to as Nuclear Magnetic Resonance (NMR). The precessing spins are also aligned in phase
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with the oscillating electromagnetic field. Once the electromagnetic irradiation stops, the spins keep precessing, and the system emits radiation at the resonance frequency while returning to its equilibrium state. This decay of the excited state (‘‘longitudinal’’ or ‘‘spin–lattice’’ relaxation with time constant T1) is described in terms of the net magnetisation Mz of the spin ensemble along the direction of the applied magnetic field (in z-direction),
M z ðtÞ ¼ M z0 ð1 expðt=T 1 ÞÞ;
ð1Þ
where t is the time after excitation, and Mz0 is the equilibrium value of the longitudinal magnetisation. Eq. (1) assumes that the net magnetisation is tilted by 90° into the x–y plane. Dephasing of the precessing spins in the ensemble (‘‘transverse’’ or ‘‘spin–spin’’ relaxation with time constant T2) is described in terms of the net magnetisation Mxy perpendicular to the applied field direction (in x–y plane),
M xy ðtÞ ¼ Mxy0 expðt=T 2 Þ;
ð2Þ
where Mxy0 is the initial value of the transverse magnetisation immediately after excitation. Both relaxation times, T1 and T2, are determined by the chemical structure of the substance containing the emitting nuclei and of the surrounding medium. For imaging, the signal corresponding to the oscillating xy-component of magnetisation is sampled. Since, according to Eq. (2), it decays exponentially with time, the acquired signal is determined by the time interval between excitation and sampling, in MRI also referred to as echo time (TE). It is transformed into an intensity value at the given frequency, which depends on the concentration of emitting nuclei and the characteristic relaxation times. Since the signal frequency is proportional to the magnetic field strength, application of gradient fields allows spatial information on its origin to be encoded in the signal. A spatial image of the local concentration of nuclei can be generated. A variety of MRI techniques, based on different sequences of excitation of the nuclei and sampling of the emitted signal, have been developed for different applications. By weighting the signal characteristics (in terms of T1 and T2) captured in the images, specific information on the emitting substance can be highlighted. MRI techniques have proved particularly useful in diagnostic medical imaging of soft tissue, but have also been applied successfully to the investigation of fluid content [11,12] and flow processes [13,14] in non-biological porous media. For the latter, an example of high practical relevance is flow in industrial processing technologies [15]. The permeability of porous media has been determined from Darcy’s law based on one-dimensional (1D) flow front tracking along the flow direction [13] or on measurement of 1D pressure profiles of a flowing signal-emitting (compressible) gas [16]. Mapping of flow velocity fields based on dynamic techniques has been reported [17]. Regarding investigation of the static or dynamic fluid distribution in fully or partially wetted fabrics as a special class of porous media, MRI is applicable to opaque textiles, e.g. from carbon fibre, since it does not depend on optical signals. However, in terms of image resolution, MRI cannot compete with optical techniques. While the achievable image resolution, which is in the order of 0.1 mm for typical MRI procedures, does not allow distinction of filaments and inter-filament voids (lm), it is sufficient for visualisation of the impregnation of textile structures at the unit cell level and distinction of the fluid content in fibre bundles and inter-bundle voids. Several applications are documented in the literature: one-dimensional profiles (through-thickness) of the fluid content in a nylon cut-pile carpet have been measured based on the local signal intensity, and a drying process has been monitored via their change with time [11]. The pressure-driven through-thickness ingress of a liquid into a heterogeneous sample of non-woven polypropylene fibre layers has been monitored quantitatively based on 1D profiles of the fluid
contents [18]. For investigation of the structure of a plain weave nylon fabric, static quantification of the fluid content (2D or 3D) in the saturated fabric allowed mapping of inter-bundle voids and determination of their dimensions [19]. The flow through aligned filaments transverse to their axes has been characterised by 2D visualisation of the radial flow front propagation during impregnation (repeated imaging at constant time intervals) of large-diameter glass fibre bundles submersed in a test fluid, and the effect of capillary forces and air entrapment on the flow has been studied [20]. In scaled-up simulated fibre arrays (fully immersed Perspex cylinders with diameters of several mm), the flow velocity field transverse to the cylinder axes has been visualised in 2D, and the difference in flow velocity between intra- and inter-bundle domains has been quantified [21,22]. The velocity of through-thickness flow in a saturated fabric (coarse plain weave, fibre material not specified) has been measured at the meso-scale by 2D particle tracing within the bulk fluid [23]. Here, the ultimate aim is tracking of the flow front propagation at the meso-scale during in-plane impregnation of actual reinforcement fabrics (glass or carbon fibre, different architectures) using 3D MRI techniques. This would allow the typical impregnation behaviour of specific fabrics to be characterised and complement macro-scale permeability data acquired by more conventional means. As a first step, this study aims to identify the potential and issues of MRI for mapping the fluid distribution in impregnated textile fabrics and to establish a robust imaging procedure. In particular, the influence of fabric parameters such as filament material and diameter, fibre volume fraction and fibre orientation on the imaging process and the achievable image quality is discussed.
2. Experimental set-up Imaging experiments were carried out in a clinical whole-body scanner with a magnetic field strength of 3 T (Philips Achieva), corresponding to a signal frequency of 128 MHz (for protons). Increased field strength compared to most clinical MRI scanners with typically between 0.5 T and 1.5 T, allows imaging either at enhanced signal-to-noise-ratio, higher resolution or shorter scan time. An 8-channel head coil designed for human brain imaging was used to receive the radio-frequency signal emitted from the test fluid in the samples. Any part of the tool, in which the fabric specimens are impregnated, needs to meet the minimum requirement of magnetic compatibility of the first kind [24], i.e. when exposed to the magnetic field of the scanner, (attractive) magnetic forces must be small enough not to cause any potentially hazardous movement. In addition, the tool needs to be non-conductive to avoid shielding of the radio-frequency signal. For this study, an impregnation tool was made consisting of two 25 mm thick Perspex plates and a 4.7 mm thick spacer frame, creating a cavity with dimensions 140 mm 90 mm 4.7 mm (length width height). The cavity is large enough to accommodate macro-scale fabric specimens with multiple unit cells along the weft and warp direction. The outer dimensions of the tool allow it to be placed inside the receiver coil. A flow-driving pressure gradient for in-plane impregnation can be applied along the longitudinal axis of the cavity via an injection gate and a vent in the top plate of the tool. Nuts and bolts holding the tool together, and tubes and fittings for fluid supply and venting are made from nylon. Because of the relatively low modulus of Perspex compared to metals, reaction forces exerted by fabric specimens compressed between the plates may result in tool deflections. These may affect the local porosity across the tool dimensions and thus the fabric impregnation behaviour. Measurement of the total tool thickness with and without fabric suggested that, for the specimens studied here, tool deflections are
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not significant (less than 0.1 mm at a cavity height of 4.7 mm, i.e. less than 2%). The fluid distribution was investigated for different examples of glass fibre and carbon fibre reinforcements, which are listed in Table 1. Engine oil (Trent Oil HDX30) was used as a test fluid. Since its viscosity (g = 0.28 Pa s at T = 20 °C) is in the same order of magnitude as that of liquid epoxy or polyester resin systems, the oil is expected to exhibit similar flow characteristics (low Reynolds number, viscous forces are dominant) during impregnation. In addition, the proton concentration in oil is high, and relaxation times for bulk fluid are relatively long compared to typical echo times, resulting in a strong MR signal. 3. Specific issues 3.1. Fluid dispersion In composite components, the global fibre volume fraction is frequently between 40% and 60%. The local fibre volume fraction in the fibre bundles is determined by the global fibre volume fraction and the fabric architecture. It has a theoretical maximum of 91% in case of perfect hexagonal packing of the cylindrical filaments, but in practice its maximum is more likely between 60% and 70%, implying that the fluid concentration in fully impregnated fibre bundles is between 30% and 40%. The MR signal intensity is proportional to the proton density, i.e. fluid concentration. However, the fluid in the fibre bundles is dispersed in microscopic inter-filament voids, where, depending on the molecular structure of the fluid, it may be bound to the filament surfaces. For the general case of fluids in porous media, the signal relaxation time, i.e. the time constant for decay to 1/e of the initial intensity, is typically significantly shorter than in bulk fluid. This effect is sometimes attributed to the presence of paramagnetic sites present on the pore surfaces (in natural heterogeneous materials, e.g. rocks) [25], but is also related to constraints on the molecular mobility near the surfaces. It depends on the ratio between pore surface area and volume [25]. This dependence implies that the relaxation time of a test fluid in a fibre bundle decreases with decreasing filament diameter at constant fibre volume fraction and with increasing fibre volume fraction at constant filament diameter. Since diameters are typically 7 lm for carbon filaments and up to around 20 lm for glass filaments, the reduction of the relaxation time at comparable fibre volume fractions is expected to be more significant in carbon than in glass fibre bundles (assuming that material compositions are controlled and no paramagnetic sites are present in fibres). In addition, reinforcement fabrics are typically treated with sizing such as silane (on glass fibre) or polyurethane or epoxy (on carbon fibre), to improve bonding between the filaments and the surrounding polymer matrix in a composite. The sizing may also enhance bonding of the test fluid to the filament surfaces, thus potentially adding to the effect of reduced relaxation time. For fabrics wetted with water, Leisen and Beckham [11] found effects like this to be significant. They discussed how at given echo time, reduced relaxation time can result in a non-linear relationship between fluid concentration and sampled signal intensity. In combination with relatively low initial signal intensity, it may even become impossible to detect any signal at all (Fig. 1). For oil as a test fluid, the bulk relaxation time is inversely proportional to the viscosity [26] (for the oil used here, the relevant transverse bulk relaxation time, T2, is in the order of 10 ms). The reduction of relaxation times in porous media is generally less significant for oil than for water [27], since, due to the polarity of the molecules, water, unlike oil, forms hydrogen bonds with functional groups on pore (or filament) surfaces.
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3.2. Susceptibility effects Discontinuities in the magnetic susceptibility v between different materials exposed to a magnetic field cause field disturbances, which are generally proportional to the difference in susceptibility, Dv. Field disturbances depend on the geometry of the disturbing object and decay with increasing distance from the material interface. Their occurrence affects the local distribution of signal frequencies, which deviates from the assumed linear function of position encoded in the applied magnetic field gradients. This can result in image distortion. In addition, accelerated dephasing of spins, i.e. reduced transverse relaxation time (T 2 ), results in reduced signal intensity at given echo time (Fig. 1). Since field disturbances are also proportional to the strength of the applied magnetic field, the drawback of scanning at high field is the enhanced sensitivity of the relaxation behaviour to susceptibility effects. Regarding the effect of the geometry on field disturbances, analytical solutions exist for special cases. Here, the case of long cylinders [24] is of specific interest, since it describes the behaviour of filaments in a fibrous structure. Outside a cylinder, no field disturbance occurs if the cylinder axis is oriented parallel to the field direction. For orientation perpendicular to the field direction, the disturbance is proportional to Dv and the cylinder cross-sectional area. Inside the cylinder, a uniform field disturbance occurs, which is twice as strong for orientation parallel to the field direction as for perpendicular orientation. Independent of the geometry, Schenck [24] suggested that, as a rule of thumb, effects on the image quality in standard MRI techniques for medical imaging are negligible if |Dv| < 3 106, noticeable but small if |Dv| < 10 106, and significant, but in most cases still acceptable, if |Dv| < 200 106. In published studies on imaging of textile impregnation, susceptibility effects have been minimised by using combinations of test fluid and fibres with small |Dv|, e.g. water or water-based fluids (with susceptibilities generally similar to that of water, v = 9.05 106 [24]) and Perspex [21,22] or nylon [19] (Table 2). Most studies deal with saturated fabrics, thus eliminating the discontinuity along the moving boundary between the propagating fluid and air or vacuum [19,21–23]. Studying unsaturated flow in aligned glass fibres, Neacsu et al. [20] tried to minimise the effect of field disturbances by orienting the fibres along the field direction. Since this study aims to characterise the flow through actual reinforcement fabrics, |Dv| and the fibre geometry are predetermined and cannot be adapted to minimise problems in the imaging process. Estimates for the magnetic susceptibility of all components of the experimental set-up, based on published data [28–31], are listed in Table 2. While most materials show isotropic behaviour, the diamagnetic susceptibility (v < 0) of carbon fibres is transverse isotropic (absolute values are higher perpendicular to the fibre axis than parallel to the axis). The influence of fibre orientation and fibre type has been discussed in the literature [31–35]. Absolute susceptibility values and ratios of anisotropy quoted in these sources differ significantly. These inconsistencies are caused by differences in the properties of the specific fibre types investigated, and probably also by different definitions of v [24] and inconsistent measurement methods. Generally, high modulus carbon fibres show stronger anisotropy in v than high strength fibres. This effect is illustrated by the observations by Scott and Fischbach [31], who studied the magnetic properties of 19 different fibre types. They found that the ratio of anisotropy of the susceptibility values perpendicular and parallel to the fibre axes varies approximately linearly with the axial Young’s moduli E0 of the fibres. The limit values for the ratio were approximately 1 for E0 70 GPa and approximately 22 for E0 700 GPa. Examples for the results they presented are given in Table 2.
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Table 1 Details on fabrics used in this study; axial Young’s modulus, E0, tensile strength, rt, filament diameter, d, filament count, cf, and superficial density, S0, are given where appropriate. Description
Details
S0/g/m2
Random discontinuous carbon fibre preform
STS40 fibres, E0 = 240 GPa, rt = 4.00 GPa, d = 7 lm, cf = 24 K, chop length 58 mm; 6% by weight thermoset powder binder Weft direction: d = 13 lm, 600 tex; polyester fixation thread; warp direction: d = 17 lm, linear fibre bundle density 1200 tex STS40 fibres, E0 = 240 GPa, rt = 4.00 GPa, d = 7 lm, cf = 24 K; thermoplastic coated glass fixation thread Both fabric directions: d = 17 lm, linear density 2400 tex Warp direction (6 layers): HTS40 fibre, E0 = 240 GPa, rt = 4.30 GPa, d = 7 lm, cf = 12 K; weft direction (7 layers): HTA40, E0 = 238 GPa, rt = 3.95 GPa, d = 7 lm, cf = 2 6 K; binder: HTA40, E0 = 238 GPa, rt = 3.95 GPa, d = 7 lm, cf = 1 K 0° Direction: HTS40 fibre, E0 = 240 GPa, rt = 4.30 GPa, d = 7 lm, cf = 12 K; bias directions: HTA40, E0 = 238 GPa, rt = 3.95 GPa, d = 7 lm, cf = 6 K
1400
0°/90° Stitch-bonded non-crimp E-glass fibre fabric UD carbon fibre fabric Plain weave E-glass fibre fabric 3D woven carbon fibre fabric (orthogonal weave)
Triaxially braided 0°/±60° carbon fibre fabric
For different commercial glasses, isotropic diamagnetic susceptibilities are quoted as v = 11 106 [28] and v = 13.88 106 [24], and for SiO2 as v = 16.3 106 [24]. Reinforcement fibres in composite materials are typically from E-glass, which contains trace amounts of iron oxide (Ramachandran and Balasubramanian [30] quote a Fe2O3 content of 0.3% by weight for an E-glass). The iron oxide content dominates the magnetic properties, and from conversion of the given experimental data [30], an approximate value v = 20 106 can be estimated for the effective paramagnetic susceptibility (v > 0). The values in Table 2 suggest that, in this study, discontinuities in susceptibility are most significant at interfaces between oil and certain types of carbon fibres, when oriented perpendicular to the field direction, or E-glass fibres. 3.3. Eddy currents Eddy currents are induced in objects from electrically conductive material exposed to an electromagnetic field. They create a second electromagnetic field, (partially) cancelling the applied field. In a conductive medium, the amplitude of an electromagnetic wave is attenuated to 1/e of its original value at the ‘‘skin depth’’ below the surface. Limited penetration into the medium effectively results in shielding of the electromagnetic radiation, the efficiency of which at a given radiation frequency depends on the material conductivity, the orientation and volume of the object, and number and size of apertures in the object. The effect on the image quality in MRI was discussed by Camacho et al. [36]. For a conductive loop from copper, eddy currents induced by the radio-frequency field, but not by switching of the gradient fields, were found to significantly reduce the signal intensity. Here, similar effects may occur for specimens from carbon fibre, which has a conductivity typically 2–3 orders of magnitude smaller than that of metals [24]. The architecture of the carbon fibre fabrics determines if closed conductive loops are formed and eddy currents are induced. For the carbon fibre materials used in this study (Table 1), the conductivity was characterised qualitatively in a series of experiments. A random discontinuous carbon fibre preform was found to be non-conductive. It is thought that the applied thermoset powder binder [37], which prevents the preform from disintegrating, insulates the individual chopped fibre segments electrically. Because of its specific architecture, a uni-directional (UD) carbon fibre fabric was found to be conductive along the fibre axes, but not perpendicular to the axes. A 3D woven carbon fibre fabric and a triaxial carbon fibre braid were found to be conductive in any direction in-plane and through-thickness. This implies that there is electrical contact between the yarns, and that the sizing applied to the filament surfaces has no insulating effect. While it is not quite clear to what extent the MR signal emitted from within
800 450 912 4775
493
a conductive porous medium may be affected, some effect of radiofrequency shielding is expected to be observed for the latter two fabrics. To give an upper estimate for this effect, the skin depth in a hypothetical homogeneous conductor with a resistivity of 1.6 105 X m (as commercial grade carbon fibre [38]) is approximated as 0.2 mm at a frequency of 128 MHz. The skin depth is expected to increase (mm) with yarn spacing, i.e. aperture size, in fabrics from fibres with the same resistivity. 4. Imaging experiments 4.1. General considerations A major challenge specific to this study is finding the optimum imaging protocol and parameters. Because of the relaxation effects discussed in Sections 3.1 and 3.2, the echo time is critical for signal detection. As illustrated schematically in Fig. 1, reduction of TE allows signal loss due to T2 and T 2 relaxation effects to be minimised. An additional challenge is the multi-scale nature of the investigated problem. Distinguishing the fluid concentrations in fibre bundles and inter-bundle voids, while mapping the macro-scale fluid distribution, requires high resolution at a large field of view. In addition, three-dimensional imaging is required to allow identification of potential through-thickness effects. Choosing a large image matrix size is expected to result in prolonged scan times. As a rough estimate, typical scan times are in the order of several minutes for most 3D MRI techniques, but depend strongly on the specific protocol. Since, at a later stage, the fabric impregnation behaviour is to be studied based on multiple scans at different injection times, the scan time needs to be kept as short as possible. In most studies dealing with the impregnation of fabrics [19– 22], imaging is based on Spin-Echo (SE) techniques frequently employed in medical imaging. While these techniques are typically relatively robust to susceptibility effects, Leisen and Beckham [11] found that, for the example of carpet impregnated with water, they allow only semi-quantitative imaging of fluid concentrations and cannot detect concentrations below approximately 10%. As discussed in Section 3.1, the signal relaxation time, and thus the strength of the acquired signal at a given echo time (typically >10 ms for SE), decrease when water is bound to the fibres. A decrease in fluid concentration, i.e. in the ratio between free and bound water in the carpet, results in non-linear signal reduction. A drawback of SE sequences is that scan times, which are not an issue for 1D or 2D imaging [11,18,20], may become long for 3D imaging at the required resolution (up to approximately 1 h). For accurate quantification of the fluid concentration in textiles, Single-Point Imaging (SPI) was suggested as an alternative [11]. In SPI, the signal is sampled immediately after excitation (i.e. the concept of echo time does not apply), making the procedure
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1.0
Vf = 0 % (bulk fluid)
0.9
relative signal intensity
0.8
decay depends on fluid properties only, time constant T2
0.7 0.6 0.5 0.4
Vf = 50 % (fluid in fibrous medium)
0.3 0.2 0.1 0.0
time constant T2* 0 TEs
2
initial intensity (t = 0) is proportional to fluid concentration
decay accelerated by • fluid dispersion • susceptibility effects 4
TEl
6
8
10
time / a.u.
Fig. 1. Schematic illustration of relaxation effects described in Sections 3.1 and 3.2: MR signal intensity as a function of time after excitation, parameter fibre volume fraction Vf. Comparison of intensity at different echo times, TEs and TEl, indicates that time interval between excitation and sampling is critical for signal detection.
Table 2 Approximate values for the magnetic susceptibilities v of the components in the imaging experiments, derived from published data (PAN: Polyacrylonitrile). Material
v/106
Vacuum Air [28] Perspex [28] Nylon [29] E-glass [30] Carbon fibre, PAN-based, High strength (E0 = 240 GPa) [31] Carbon fibre, PAN-based, High modulus (E0 = 380 GPa) [31] Mineral oil [24]
0 0.36 9.70 9 20 3.3 (axial) 9.5 (radial) 15 (axial) 214 (radial) 8.80
insensitive to relaxation effects. However, the scan time is proportional to the number of image points and would be in the order of hours for 3D imaging at high resolution, which is not considered acceptable here. 4.2. Results and discussion For the purpose of this study, FLASH-type (Fast Low-Angle Shot [39]) imaging sequences were employed with some success. These techniques generally allow a good compromise between scan time and image resolution to be achieved. Compared to SE imaging, the scan time is typically significantly shorter, but the imaging procedure is less robust to susceptibility effects. This is mitigated to some extent by short echo times, typically in the order of a few milliseconds (here, echo times were between 3 ms and 4 ms). For illustration of the achieved image quality and the trade-off between resolution and scan time for a specific imaging sequence, Fig. 2 shows slices (view along vertical axis) through the Perspex tool containing a random discontinuous carbon fibre preform [37] (Table 1; 3 layers, fibre volume fraction Vf 51%) and a 0°/ 90° stitch-bonded non-crimp E-glass fibre fabric (Table 1; 6 layers, Vf 39%) with the weft direction oriented along the tool axis, both impregnated with the test fluid. One slice is from a 3D scan with 256 256 14 voxels (along longitudinal axis cross axis vertical axis) at an isotropic resolution of 0.8 mm, acquired in 237 s, the other one is from a scan with 360 352 22 voxels at an isotropic resolution of 0.5 mm, acquired in 433 s. The greyscales indi-
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cate the local fluid concentration in each voxel (bright: high; dark: low). Completely dark voxels suggest absence of fluid. Comparison of the slices with a corresponding photograph indicates plausibility of the acquired image data. Features of the fibrous structures visualised in the photograph can be clearly identified. The gap left in the tool at the injection gate (bottom of images) acts as a manifold allowing the fluid to distribute across the tool cross-section before impregnating the fabric. Thus, pure in-plane flow can be achieved. In the glass fibre fabric, fibre bundles (low fluid concentration: dark grey) and inter-bundle voids (high fluid concentration: bright grey) can be distinguished. The (almost) regular structure can be identified. In the carbon fibre preform, the random distribution of the fibre bundles results in irregular variations in the fluid concentration. Signal intensity fluctuations caused by susceptibility effects around the edge of the specimen are considered acceptable. To allow small-scale defects to be identified clearly, all subsequent scans were acquired at a 0.5 mm isotropic resolution. To illustrate the combined influence of the anisotropic geometry effect and anisotropic susceptibility discussed in Section 3.2, Fig. 3 shows slices through the tool containing impregnated UD carbon fibre fabric (Table 1; 9 layers, Vf 49%), oriented parallel and perpendicular to the field direction in the scanner. The actual susceptibility values of the specific type of carbon fibre are unknown. The slices in the figure are from scans acquired at parallel and perpendicular orientation of the tool longitudinal axis relative to the magnetic field direction. The image quality is high, where the fibres are oriented parallel to the field direction. Fibre bundles and inter-bundle voids can be clearly distinguished, and the fixation thread can be identified. Where the fibres are oriented perpendicular to the field direction, the average signal intensity is reduced by approximately 35%, and the image appears more blurred, although the main features can still be identified. Quantitative evaluation of intensity profiles indicates that the distances between intensity maxima, corresponding to the fibre spacing, are (3.63 ± 0.50) mm and (3.25 ± 0.98) mm if the fibres are oriented parallel and perpendicular to the field direction. Both values are consistent with the theoretical value of 3.64 mm given by the fabric manufacturer. However, if the fibres are oriented perpendicular to the field direction, the uncertainty is higher, since the signal-tonoise ratio in the images is reduced. While no significant geometrical image distortion occurs, micro-scale non-uniformity within the yarns causes field disturbances across the voxel dimensions, which depend on the fibre orientation and also on the local fibre volume fraction. This results in signal loss as described above. Due to the orientation of the tool, the right image in Fig. 3 also shows some effect of inhomogeneity in the radio-frequency field, which is ignored here. A general observation is that the impregnation of the fabric in this specific specimen is poor. Multiple dry spots are clearly visible. These were not visible to the naked eye (viewing though the transparent Perspex tool), and thus demonstrate the potential of MRI to identify defect formation in a laminate. It is to be noted that effects related to the discontinuity in susceptibility between test fluid and residual gas in the dry spots (Table 2) affect the apparent size of dry spots in FLASH scans, which is larger than their actual size. The difference between apparent and actual size increases with decreasing size of dry spots, which makes quantification of the volume difficult, but helps identification of small defects. The reason for occurrence of the defects in this specimen is that at the injection gate, no gap acting as a manifold was left in the cavity. Thus, impregnating flow has a through-thickness component. This makes complete impregnation more difficult, since the throughthickness permeability of fibre bundles is typically significantly smaller than the in-plane permeability. The anisotropic geometry effect at isotropic susceptibility is illustrated in Fig. 4A, which shows a slice through an impregnated
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Fig. 2. Injection tool with fully impregnated random discontinuous carbon fibre preform and 0°/90° stitch-bonded E-glass fibre fabric; left: photograph; centre: FLASH, slice with isotropic resolution 0.8 mm; right: FLASH, slice with isotropic resolution 0.5 mm.
Fig. 3. Injection tool with fully impregnated uni-directional carbon fibre fabric, oriented parallel and perpendicular to tool longitudinal axis; left: tool longitudinal axis parallel to magnetic field direction; right: tool longitudinal axis perpendicular to magnetic field direction; FLASH, slices with 0.5 mm isotropic resolution.
plain weave E-glass fibre fabric (Table 1; 6 layers, Vf 45%) with the warp direction oriented along the tool axis. While the architecture of the fabric can clearly be identified, and the fluid content in the warp yarns results in a detectable signal, signal loss obscures the impregnation state of the weft yarns, which are oriented perpendicular to the field direction. The effect is more severe than for the UD carbon fibres in Fig. 3 at similar fibre volume fraction. Since the influence of fluid dispersion on the signal relaxation is expected to be smaller (Section 3.1), this suggests that the product of |Dv| relative to oil and filament cross-sectional area is higher for the specific type of E-glass than for the STS40 carbon fibre. Localised signal loss as in Fig. 4A can be mistaken for dry spots and thus result in false positives in defect detection. In cases where FLASH-type imaging sequences fail to detect a signal, Ultra-Short Echo-Time (UTE) imaging [40] with typical echo times of less than 1 ms (here, echo times were 0.2 ms) is suitable to minimise signal loss due to low fluid concentration and susceptibility effects. As an example, Fig. 4B shows a slice through a specimen of the same specifications as in Fig. 4A. A 3D UTE scan with 336 336 17 voxels at an isotropic resolution of 0.5 mm was acquired in 21 min. This is still considered acceptable for imaging of
impregnation processes, if it can be combined with intermittent injection techniques (assuming that effects of capillary flow during the scan time are negligible, which will be discussed in the second part to this study). Compared to Fig. 4A, signal loss is reduced. Unlike in the FLASH-type scan, the greyscale values in the weft yarns are significantly higher than in the background, indicating the presence of fluid. Because of the specifics of the UTE imaging sequence, backfolding artefacts (from the fluid in the tubes) appear near the injection gate and vent. Since these do not affect the main area of interest, they are simply ignored here. However, in some cases, mapping of the fluid concentration may still be impossible. For a 3D woven carbon fibre fabric (Table 1; 1 layer, Vf 58%) with the warp direction oriented along the tool axis, signal was detected only from the specimen surface, where V-shaped gaps between the yarns and the tool surface [41] result in locally increased fluid concentration. No signal was detected from within the specimen, although micrographic analysis of moulded specimens (with resin) at identical fibre volume fraction indicated that the fibre bundles were impregnated [41]. Since this fabric was found to show some electrical conductivity (see Section 3.3), it is suspected that signal reduction due to induced eddy currents
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Fig. 4. Injection tool with fully impregnated plain weave E-glass fibre fabric; slices with 0.5 mm isotropic resolution. (A) FLASH; (B) UTE. ((A) and (B) show different specimens of the same type.)
intensity at the minimum near the middle layer is approximately 26% of the highest intensity observed near the surfaces. The symmetrical shape of the profile cannot be explained by relaxation effects, which are expected to result in random intensity variations through the specimen thickness (Fig. 6B), since they reflect the effect of variations in fibre packing density due to (random) nesting between adjacent layers. The systematic variation in signal intensity suggests that the effect of radio-frequency shielding is dominant here.
5. Conclusions
Fig. 5. Injection tool with fully impregnated 0°/±60° triaxially braided carbon fibre fabric; UTE, slice with 0.5 mm isotropic resolution (tone balance adjusted for clarity).
occurs in addition to relaxation effects, thus effectively rendering the signal undetectable. Similarly, no signal was detected from within an impregnated specimen of a triaxially braided 0°/±60° carbon fibre fabric (Table 1; 8 layers, Vf 48%) with the 0° direction oriented along the tool axis. Comparison with the results for the uni-directional fabric at similar fibre volume fraction (Vf 49%), for which a clear signal was detected at longer echo time even at unfavourable fibre orientation (Fig. 3), suggests that the influence of the braid conductivity (see Section 3.3) on the signal is significant. However, for a specimen of the same braid at 7 layers, i.e. Vf 41%, the fluid content could be visualised (Fig. 5), indicating that the issue of signal loss is strongly sensitive to Vf. Fig. 6A shows a through-thickness profile of the recorded signal intensity taken at the centre of the specimen in Fig. 5. The
The problem of mapping the fluid content in impregnated reinforcement textiles using MRI techniques is dominated by signal relaxation effects, which result in local signal loss. They are related to the dispersion of the test fluid in the fabric and to discontinuities in magnetic susceptibility, in particular at the interfaces between fluid and fibres. Their severity depends on material and diameter of the filaments, fibre volume fraction and fibre orientation. Employing imaging techniques with short echo times allows the influence of signal relaxation effects on the image quality to be minimised. For 3D imaging of macroscopic specimens with dimensions of 140 mm 90 mm 4.7 mm at an isotropic resolution of 0.5 mm, FLASH and UTE imaging sequences were implemented on a 3 T whole-body MRI scanner. Typical scan times were in the order of 7 min and 21 min, respectively. For examples of E-glass and carbon fibre fabrics of different architecture and engine oil as a test fluid, the obtained images allowed the impregnation state at the meso-scale to be identified qualitatively and dry spots to be detected. The UTE sequence was found to be more useful than the FLASH sequence for this application, since the acquired images are less affected by signal loss. However, in addition to relaxation effects, radio-frequency eddy currents may be induced if the fabrics are conductive and result in partial cancellation of the signal. Depending on reinforcement type and fibre volume fraction, this effect may render the signal undetectable inside the specimen. A second part to this study will deal with application of the imaging procedure established here to tracking of the flow front propagation at the meso-scale during in-plane impregnation of reinforcement fabrics. The aim is to characterise the typical
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impregnation behaviour of specific fabrics, which would complement macro-scale permeability data acquired by more conventional means. Acknowledgements This work was funded by the Engineering and Physical Science Research Council (EPSRC) via the Nottingham Innovative Manufacturing Research Centre (EP/E001904/1) and the Platform Grant (EP/ F02911X/1). The scanning experiments were approved by the Magnetic Resonance Strategy Board, University of Nottingham. Technical support was provided by Dr. Matthew Clemence, Philips Healthcare, UK. References [1] Darcy H. Les fontaines publiques de la ville de dijon. Paris: Victor Dalmont; 1856. [2] Luo Y, Verpoest I, Hoes K, Vanheule M, Sol H, Cardon A. Permeability measurement of textile reinforcements with several test fluids. Compos Part A Appl Sci Manuf 2001;32(10):1497–504. [3] Lundström TS, Gebart BR, Sandlund E. In-plane permeability measurements on fiber reinforcements by the multi-cavity parallel flow technique. Polym Compos 1999;20(1):146–54. [4] Parnas RS, Salem AJ. A comparison of the unidirectional and radial in-plane flow of fluids through woven composite reinforcements. Polym Compos 1993;14(5):383–94.
[5] de Parseval Y, Pillai KM, Advani SG. A simple model for the variation of permeability due to partial saturation in dual scale porous media. Transport Porous Med 1997;27(3):243–64. [6] Schell JSU, Deleglise M, Binetruy C, Krawczak P, Ermanni P. Numerical prediction and experimental characterisation of meso-scale-voids in liquid composite moulding. Compos Part A Appl Sci Manuf 2007;38(12):2460–70. [7] Stone D, Clarke B. Ultrasonic-attenuation as a measure of void content in carbon-fiber reinforced-plastics. Non-Destruct Test 1975;8(3):137–45. [8] Dunkers JP, Parnas RS, Zimba CG, Peterson RC, Flynn KM, Fujimoto JG, et al. Optical coherence tomography of glass reinforced polymer composites. Compos Part A Appl Sci Manuf 1999;30(2):139–45. [9] Callaghan PT. Principles of nuclear magnetic resonance microscopy. Oxford: Oxford University Press; 1993. [10] Hornak JP. The basics of MRI. 1996–2010.
A. Endruweit et al. / Composites: Part A 42 (2011) 265–273 [18] Bencsik M, Adriaensen H, Brewer SA, McHale G. Quantitative NMR monitoring of liquid ingress into repellent heterogeneous layered fabrics. J Magn Reson 2008;193(1):32–6. [19] Leisen J, Beckham HW. Void structure in textiles by nuclear magnetic resonance, part I. Imaging of imbibed fluids and image analysis by calculation of fluid density autocorrelation functions. J Text Inst 2008;99(3): 243–51. [20] Neacsu V, Leisen J, Beckham HW, Advani SG. Use of magnetic resonance imaging to visualize impregnation across aligned cylinders due to capillary forces. Exp Fluids 2007;42(3):425–40. [21] Mantle MD, Bijeljic B, Sederman AJ, Gladden LF. MRI velocimetry and lattice boltzmann simulations of viscous flow of a newtonian liquid through a dual porosity fibre array. Magn Reson Imaging 2001;19(3/4):527–9. [22] Bijeljic B, Mantle MD, Sederman AJ, Gladden LF, Papathanasiou TD. Slow flow across macroscopically semi-circular fibre lattices and a free-flow region of variable width – visualisation by magnetic resonance imaging. Chem Eng Sci 2004;59(10):2089–103. [23] Leisen J, Farber P, Farber K, Dressen S, Gräbel J. NTC Project F04-GT05: MicroFlow in Textiles. National Textile Center Annual Report; 2004. [24] Schenck JF. The role of magnetic susceptibility in magnetic resonance imaging: MRI magnetic compatibility of the first and second kinds. Med Phys 1996;23(6):815–50. [25] Cohen MH, Mendelson KS. Nuclear magnetic relaxation and the internal geometry of sedimentary rocks. J Appl Phys 1982;53(2):1127–35. B. [26] Bryan J, Mirotchnik K, Kantzas A. Viscosity determination of heavy oil and bitumen using NMR relaxometry. J Can Pet Technol 2003;42(7):29–34. [27] Kleinberg RL, Kenyon WE, Mitra PP. Mechanism of NMR relaxation of fluids in rock. J Magn Reson Ser A 1994;108(2):206–14. [28] Bakker CJG, de Roos R. Concerning the preparation and use of substances with a magnetic susceptibility equal to the magnetic susceptibility of air. Magn Reson Med 2006;56(5):1107–13.
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[29] Tanimoto Y, Fujiwara M, Sueda M, Inoue K, Akita M. Magnetic levitation of plastic chips: applications for magnetic susceptibility measurement and magnetic separation. Jpn J Appl Phys 2005;44(9A):6801–3. [30] Ramachandran BE, Balasubramanian N. Coordination of iron in E-glass. J Mater Sci Lett 1986;5(10):1073–4. [31] Scott CB, Fischbach DB. Diamagnetic studies on as-processed carbon fibers. J Appl Phys 1976;47(12):5329–35. [32] Arajs S, Moyer CA, Kote G, Kalnins IL. Diamagnetic susceptibility of carbon fibre reinforced epoxy resin composites. J Mater Sci 1978;13(9):2061–3. [33] McClure JW, Hickman BB. Analysis of magnetic susceptibility of carbon fibers. Carbon 1982;20(5):373–8. [34] Arkhipov AA, Rodin YP. Anisotropy of magnetic susceptibility of carbon fiber-base polymer composite materials. Sov J Nondestr Test+ 1991;27(9): 635–8. [35] Arkhipov AA, Rodin YP. Possibilities of nondestructive testing the properties of components of carbon composites by analyzing their magnetic susceptibility. Mekhanika Kompozitnykh Materialov 1992;1:119–21. [36] Camacho CR, Plewes DB, Henkelman RM. Nonsusceptibility artifacts due to metallic objects in MR imaging. J Magn Reson Imaging 1995;5(1):75–88. [37] Endruweit A, Harper LT, Turner TA, Warrior NA, Long AC. Random discontinuous carbon fiber preforms: experimental permeability characterization and local modelling. Polym Compos 2009;31(4):569–80. [38] Delivery programme and characteristics for Tenax HTA/HTS filament yarn. Wuppertal, Germany: Toho Tenax Europe GmbH; 2008. [39] Haase A, Frahm J, Mathaei D, Haenicke W, Merboldt KD. FLASH imaging. Rapid imaging using low flip-angle pulses. J Magn Reson 1986;67(2):256–66. [40] Rahmer J, Börnert P, Groen J, Bos C. Three-dimensional radial ultrashort echotime imaging with T2 adapted sampling. Magn Reson Med 2006;55(5): 1075–82. [41] Endruweit A, Long AC. Analysis of compressibility and permeability of selected 3D woven reinforcements. J Compos Mater 2010;44(24):2833–62.
Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Mapping of the fluid distribution in impregnated reinforcement textiles using Magnetic Resonance Imaging: Methods and issues Andreas Endruweit a,⇑, Paul Glover b, Kay Head b, Andrew C. Long a a b
Faculty of Engineering – Division of Materials, Mechanics & Structures, University of Nottingham, University Park, Nottingham NG7 2RD, UK Sir Peter Mansfield Magnetic Resonance Centre, School of Physics & Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
a r t i c l e
i n f o
Article history: Received 13 September 2010 Received in revised form 15 November 2010 Accepted 20 November 2010 Available online 25 November 2010 Keywords: A. Fabrics/textiles E. Resin transfer moulding (RTM) B. Porosity Magnetic Resonance Imaging (MRI)
a b s t r a c t Magnetic Resonance Imaging techniques were found to be suitable for mapping the fluid distribution in impregnated textile fabrics. They allowed the state of impregnation of E-glass and carbon fibre fabrics of different architectures with a test fluid (engine oil) to be identified and dry spots to be detected. An UltraShort Echo-Time technique was found to be relatively robust to signal loss caused by relaxation processes, which are related to the dispersion of the fluid in the fabric and local discontinuities in magnetic susceptibility. On a 3 T scanner, 3D images of specimens with dimensions of 140 mm 90 mm 4.7 mm at an isotropic resolution of 0.5 mm were acquired at scan times of 21 min. However, radio-frequency eddy currents may be induced if the fabrics are conductive and result in partial cancellation of the signal, rendering it undetectable inside the sample. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The structure of reinforcement fabrics for polymer composites affects the composite properties directly, but also determines their impregnation with a liquid resin system. This is critical for the quality of the finished component, in particular if Liquid Composite Moulding (LCM) processes are employed for manufacture. Darcy’s law [1] states that the fluid flow during impregnation depends on the applied pressure gradient, the fluid viscosity and the textile permeability, which is determined by the fibre arrangement in the reinforcement structure. Fluid flow and textile permeability can be described at multiple scales, reflecting the structure of the reinforcement: Component shape and geometrical features: macro-scale (m). Geometrical arrangement of fibre bundles and inter-bundle voids in the textile unit cell: meso-scale (mm). Distribution of filaments and inter-filament voids in the fibre bundles: micro-scale (lm). To characterise the resin flow in reinforcement fabrics, visual flow monitoring using transparent injection tools [2,3] and permeability measurement at the macro-scale, based on tracking of the flow front propagation [4], are frequently adopted procedures. However, measurement of the macro-scale permeability implies a homogenisation of local fabric properties and does not allow identi⇑ Corresponding author. Tel.: +44 (0)115 95 14037; fax: +44 (0)115 9513800. E-mail address: [email protected] (A. Endruweit). 1359-835X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2010.11.012
fication of meso- and micro-scale effects. The injected fluid normally flows through inter-bundle voids at a different velocity than through inter-filament voids (due to the difference in void dimensions, i.e. different local permeabilities), resulting in an uneven flow front at the meso-scale [5]. This may cause formation of resin-free dry spots [6] and thus result in a reduction in the matrix-dominated mechanical properties, in particular shear strength, flexural strength and compressive strength, of the finished composite. Since visualisation of fabric impregnation at the meso- and micro-scale is generally not possible using conventional means, identification of dry spots is in practice based on analysis of moulded and cured specimens. This is typically done by microscopy, although more advanced methods (e.g., ultrasound techniques [7], Optical Coherence Tomography [8] or micro-Computed Tomography [6]) have been applied with some success. For flow front tracking during the impregnation process and for monitoring of dry spot formation, Magnetic Resonance Imaging (MRI) is potentially suitable. The basic principles of MRI are discussed in detail in a variety of textbooks (e.g. by Callaghan [9]) and tutorials (e.g. by Hornak [10]). Here, only a brief overview can be given: in MRI, ensembles of atomic nuclei are exposed to a magnetic field. The nuclear spins precess around the field direction. For the example of protons (hydrogen nuclei), they can take two possible orientations, corresponding to two energy states. If the difference energy, which depends on the strength of the applied magnetic field, is supplied in the form of a pulse of electromagnetic radiation at the right frequency, the orientation of the spins tilts. This phenomenon is referred to as Nuclear Magnetic Resonance (NMR). The precessing spins are also aligned in phase
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with the oscillating electromagnetic field. Once the electromagnetic irradiation stops, the spins keep precessing, and the system emits radiation at the resonance frequency while returning to its equilibrium state. This decay of the excited state (‘‘longitudinal’’ or ‘‘spin–lattice’’ relaxation with time constant T1) is described in terms of the net magnetisation Mz of the spin ensemble along the direction of the applied magnetic field (in z-direction),
M z ðtÞ ¼ M z0 ð1 expðt=T 1 ÞÞ;
ð1Þ
where t is the time after excitation, and Mz0 is the equilibrium value of the longitudinal magnetisation. Eq. (1) assumes that the net magnetisation is tilted by 90° into the x–y plane. Dephasing of the precessing spins in the ensemble (‘‘transverse’’ or ‘‘spin–spin’’ relaxation with time constant T2) is described in terms of the net magnetisation Mxy perpendicular to the applied field direction (in x–y plane),
M xy ðtÞ ¼ Mxy0 expðt=T 2 Þ;
ð2Þ
where Mxy0 is the initial value of the transverse magnetisation immediately after excitation. Both relaxation times, T1 and T2, are determined by the chemical structure of the substance containing the emitting nuclei and of the surrounding medium. For imaging, the signal corresponding to the oscillating xy-component of magnetisation is sampled. Since, according to Eq. (2), it decays exponentially with time, the acquired signal is determined by the time interval between excitation and sampling, in MRI also referred to as echo time (TE). It is transformed into an intensity value at the given frequency, which depends on the concentration of emitting nuclei and the characteristic relaxation times. Since the signal frequency is proportional to the magnetic field strength, application of gradient fields allows spatial information on its origin to be encoded in the signal. A spatial image of the local concentration of nuclei can be generated. A variety of MRI techniques, based on different sequences of excitation of the nuclei and sampling of the emitted signal, have been developed for different applications. By weighting the signal characteristics (in terms of T1 and T2) captured in the images, specific information on the emitting substance can be highlighted. MRI techniques have proved particularly useful in diagnostic medical imaging of soft tissue, but have also been applied successfully to the investigation of fluid content [11,12] and flow processes [13,14] in non-biological porous media. For the latter, an example of high practical relevance is flow in industrial processing technologies [15]. The permeability of porous media has been determined from Darcy’s law based on one-dimensional (1D) flow front tracking along the flow direction [13] or on measurement of 1D pressure profiles of a flowing signal-emitting (compressible) gas [16]. Mapping of flow velocity fields based on dynamic techniques has been reported [17]. Regarding investigation of the static or dynamic fluid distribution in fully or partially wetted fabrics as a special class of porous media, MRI is applicable to opaque textiles, e.g. from carbon fibre, since it does not depend on optical signals. However, in terms of image resolution, MRI cannot compete with optical techniques. While the achievable image resolution, which is in the order of 0.1 mm for typical MRI procedures, does not allow distinction of filaments and inter-filament voids (lm), it is sufficient for visualisation of the impregnation of textile structures at the unit cell level and distinction of the fluid content in fibre bundles and inter-bundle voids. Several applications are documented in the literature: one-dimensional profiles (through-thickness) of the fluid content in a nylon cut-pile carpet have been measured based on the local signal intensity, and a drying process has been monitored via their change with time [11]. The pressure-driven through-thickness ingress of a liquid into a heterogeneous sample of non-woven polypropylene fibre layers has been monitored quantitatively based on 1D profiles of the fluid
contents [18]. For investigation of the structure of a plain weave nylon fabric, static quantification of the fluid content (2D or 3D) in the saturated fabric allowed mapping of inter-bundle voids and determination of their dimensions [19]. The flow through aligned filaments transverse to their axes has been characterised by 2D visualisation of the radial flow front propagation during impregnation (repeated imaging at constant time intervals) of large-diameter glass fibre bundles submersed in a test fluid, and the effect of capillary forces and air entrapment on the flow has been studied [20]. In scaled-up simulated fibre arrays (fully immersed Perspex cylinders with diameters of several mm), the flow velocity field transverse to the cylinder axes has been visualised in 2D, and the difference in flow velocity between intra- and inter-bundle domains has been quantified [21,22]. The velocity of through-thickness flow in a saturated fabric (coarse plain weave, fibre material not specified) has been measured at the meso-scale by 2D particle tracing within the bulk fluid [23]. Here, the ultimate aim is tracking of the flow front propagation at the meso-scale during in-plane impregnation of actual reinforcement fabrics (glass or carbon fibre, different architectures) using 3D MRI techniques. This would allow the typical impregnation behaviour of specific fabrics to be characterised and complement macro-scale permeability data acquired by more conventional means. As a first step, this study aims to identify the potential and issues of MRI for mapping the fluid distribution in impregnated textile fabrics and to establish a robust imaging procedure. In particular, the influence of fabric parameters such as filament material and diameter, fibre volume fraction and fibre orientation on the imaging process and the achievable image quality is discussed.
2. Experimental set-up Imaging experiments were carried out in a clinical whole-body scanner with a magnetic field strength of 3 T (Philips Achieva), corresponding to a signal frequency of 128 MHz (for protons). Increased field strength compared to most clinical MRI scanners with typically between 0.5 T and 1.5 T, allows imaging either at enhanced signal-to-noise-ratio, higher resolution or shorter scan time. An 8-channel head coil designed for human brain imaging was used to receive the radio-frequency signal emitted from the test fluid in the samples. Any part of the tool, in which the fabric specimens are impregnated, needs to meet the minimum requirement of magnetic compatibility of the first kind [24], i.e. when exposed to the magnetic field of the scanner, (attractive) magnetic forces must be small enough not to cause any potentially hazardous movement. In addition, the tool needs to be non-conductive to avoid shielding of the radio-frequency signal. For this study, an impregnation tool was made consisting of two 25 mm thick Perspex plates and a 4.7 mm thick spacer frame, creating a cavity with dimensions 140 mm 90 mm 4.7 mm (length width height). The cavity is large enough to accommodate macro-scale fabric specimens with multiple unit cells along the weft and warp direction. The outer dimensions of the tool allow it to be placed inside the receiver coil. A flow-driving pressure gradient for in-plane impregnation can be applied along the longitudinal axis of the cavity via an injection gate and a vent in the top plate of the tool. Nuts and bolts holding the tool together, and tubes and fittings for fluid supply and venting are made from nylon. Because of the relatively low modulus of Perspex compared to metals, reaction forces exerted by fabric specimens compressed between the plates may result in tool deflections. These may affect the local porosity across the tool dimensions and thus the fabric impregnation behaviour. Measurement of the total tool thickness with and without fabric suggested that, for the specimens studied here, tool deflections are
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not significant (less than 0.1 mm at a cavity height of 4.7 mm, i.e. less than 2%). The fluid distribution was investigated for different examples of glass fibre and carbon fibre reinforcements, which are listed in Table 1. Engine oil (Trent Oil HDX30) was used as a test fluid. Since its viscosity (g = 0.28 Pa s at T = 20 °C) is in the same order of magnitude as that of liquid epoxy or polyester resin systems, the oil is expected to exhibit similar flow characteristics (low Reynolds number, viscous forces are dominant) during impregnation. In addition, the proton concentration in oil is high, and relaxation times for bulk fluid are relatively long compared to typical echo times, resulting in a strong MR signal. 3. Specific issues 3.1. Fluid dispersion In composite components, the global fibre volume fraction is frequently between 40% and 60%. The local fibre volume fraction in the fibre bundles is determined by the global fibre volume fraction and the fabric architecture. It has a theoretical maximum of 91% in case of perfect hexagonal packing of the cylindrical filaments, but in practice its maximum is more likely between 60% and 70%, implying that the fluid concentration in fully impregnated fibre bundles is between 30% and 40%. The MR signal intensity is proportional to the proton density, i.e. fluid concentration. However, the fluid in the fibre bundles is dispersed in microscopic inter-filament voids, where, depending on the molecular structure of the fluid, it may be bound to the filament surfaces. For the general case of fluids in porous media, the signal relaxation time, i.e. the time constant for decay to 1/e of the initial intensity, is typically significantly shorter than in bulk fluid. This effect is sometimes attributed to the presence of paramagnetic sites present on the pore surfaces (in natural heterogeneous materials, e.g. rocks) [25], but is also related to constraints on the molecular mobility near the surfaces. It depends on the ratio between pore surface area and volume [25]. This dependence implies that the relaxation time of a test fluid in a fibre bundle decreases with decreasing filament diameter at constant fibre volume fraction and with increasing fibre volume fraction at constant filament diameter. Since diameters are typically 7 lm for carbon filaments and up to around 20 lm for glass filaments, the reduction of the relaxation time at comparable fibre volume fractions is expected to be more significant in carbon than in glass fibre bundles (assuming that material compositions are controlled and no paramagnetic sites are present in fibres). In addition, reinforcement fabrics are typically treated with sizing such as silane (on glass fibre) or polyurethane or epoxy (on carbon fibre), to improve bonding between the filaments and the surrounding polymer matrix in a composite. The sizing may also enhance bonding of the test fluid to the filament surfaces, thus potentially adding to the effect of reduced relaxation time. For fabrics wetted with water, Leisen and Beckham [11] found effects like this to be significant. They discussed how at given echo time, reduced relaxation time can result in a non-linear relationship between fluid concentration and sampled signal intensity. In combination with relatively low initial signal intensity, it may even become impossible to detect any signal at all (Fig. 1). For oil as a test fluid, the bulk relaxation time is inversely proportional to the viscosity [26] (for the oil used here, the relevant transverse bulk relaxation time, T2, is in the order of 10 ms). The reduction of relaxation times in porous media is generally less significant for oil than for water [27], since, due to the polarity of the molecules, water, unlike oil, forms hydrogen bonds with functional groups on pore (or filament) surfaces.
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3.2. Susceptibility effects Discontinuities in the magnetic susceptibility v between different materials exposed to a magnetic field cause field disturbances, which are generally proportional to the difference in susceptibility, Dv. Field disturbances depend on the geometry of the disturbing object and decay with increasing distance from the material interface. Their occurrence affects the local distribution of signal frequencies, which deviates from the assumed linear function of position encoded in the applied magnetic field gradients. This can result in image distortion. In addition, accelerated dephasing of spins, i.e. reduced transverse relaxation time (T 2 ), results in reduced signal intensity at given echo time (Fig. 1). Since field disturbances are also proportional to the strength of the applied magnetic field, the drawback of scanning at high field is the enhanced sensitivity of the relaxation behaviour to susceptibility effects. Regarding the effect of the geometry on field disturbances, analytical solutions exist for special cases. Here, the case of long cylinders [24] is of specific interest, since it describes the behaviour of filaments in a fibrous structure. Outside a cylinder, no field disturbance occurs if the cylinder axis is oriented parallel to the field direction. For orientation perpendicular to the field direction, the disturbance is proportional to Dv and the cylinder cross-sectional area. Inside the cylinder, a uniform field disturbance occurs, which is twice as strong for orientation parallel to the field direction as for perpendicular orientation. Independent of the geometry, Schenck [24] suggested that, as a rule of thumb, effects on the image quality in standard MRI techniques for medical imaging are negligible if |Dv| < 3 106, noticeable but small if |Dv| < 10 106, and significant, but in most cases still acceptable, if |Dv| < 200 106. In published studies on imaging of textile impregnation, susceptibility effects have been minimised by using combinations of test fluid and fibres with small |Dv|, e.g. water or water-based fluids (with susceptibilities generally similar to that of water, v = 9.05 106 [24]) and Perspex [21,22] or nylon [19] (Table 2). Most studies deal with saturated fabrics, thus eliminating the discontinuity along the moving boundary between the propagating fluid and air or vacuum [19,21–23]. Studying unsaturated flow in aligned glass fibres, Neacsu et al. [20] tried to minimise the effect of field disturbances by orienting the fibres along the field direction. Since this study aims to characterise the flow through actual reinforcement fabrics, |Dv| and the fibre geometry are predetermined and cannot be adapted to minimise problems in the imaging process. Estimates for the magnetic susceptibility of all components of the experimental set-up, based on published data [28–31], are listed in Table 2. While most materials show isotropic behaviour, the diamagnetic susceptibility (v < 0) of carbon fibres is transverse isotropic (absolute values are higher perpendicular to the fibre axis than parallel to the axis). The influence of fibre orientation and fibre type has been discussed in the literature [31–35]. Absolute susceptibility values and ratios of anisotropy quoted in these sources differ significantly. These inconsistencies are caused by differences in the properties of the specific fibre types investigated, and probably also by different definitions of v [24] and inconsistent measurement methods. Generally, high modulus carbon fibres show stronger anisotropy in v than high strength fibres. This effect is illustrated by the observations by Scott and Fischbach [31], who studied the magnetic properties of 19 different fibre types. They found that the ratio of anisotropy of the susceptibility values perpendicular and parallel to the fibre axes varies approximately linearly with the axial Young’s moduli E0 of the fibres. The limit values for the ratio were approximately 1 for E0 70 GPa and approximately 22 for E0 700 GPa. Examples for the results they presented are given in Table 2.
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Table 1 Details on fabrics used in this study; axial Young’s modulus, E0, tensile strength, rt, filament diameter, d, filament count, cf, and superficial density, S0, are given where appropriate. Description
Details
S0/g/m2
Random discontinuous carbon fibre preform
STS40 fibres, E0 = 240 GPa, rt = 4.00 GPa, d = 7 lm, cf = 24 K, chop length 58 mm; 6% by weight thermoset powder binder Weft direction: d = 13 lm, 600 tex; polyester fixation thread; warp direction: d = 17 lm, linear fibre bundle density 1200 tex STS40 fibres, E0 = 240 GPa, rt = 4.00 GPa, d = 7 lm, cf = 24 K; thermoplastic coated glass fixation thread Both fabric directions: d = 17 lm, linear density 2400 tex Warp direction (6 layers): HTS40 fibre, E0 = 240 GPa, rt = 4.30 GPa, d = 7 lm, cf = 12 K; weft direction (7 layers): HTA40, E0 = 238 GPa, rt = 3.95 GPa, d = 7 lm, cf = 2 6 K; binder: HTA40, E0 = 238 GPa, rt = 3.95 GPa, d = 7 lm, cf = 1 K 0° Direction: HTS40 fibre, E0 = 240 GPa, rt = 4.30 GPa, d = 7 lm, cf = 12 K; bias directions: HTA40, E0 = 238 GPa, rt = 3.95 GPa, d = 7 lm, cf = 6 K
1400
0°/90° Stitch-bonded non-crimp E-glass fibre fabric UD carbon fibre fabric Plain weave E-glass fibre fabric 3D woven carbon fibre fabric (orthogonal weave)
Triaxially braided 0°/±60° carbon fibre fabric
For different commercial glasses, isotropic diamagnetic susceptibilities are quoted as v = 11 106 [28] and v = 13.88 106 [24], and for SiO2 as v = 16.3 106 [24]. Reinforcement fibres in composite materials are typically from E-glass, which contains trace amounts of iron oxide (Ramachandran and Balasubramanian [30] quote a Fe2O3 content of 0.3% by weight for an E-glass). The iron oxide content dominates the magnetic properties, and from conversion of the given experimental data [30], an approximate value v = 20 106 can be estimated for the effective paramagnetic susceptibility (v > 0). The values in Table 2 suggest that, in this study, discontinuities in susceptibility are most significant at interfaces between oil and certain types of carbon fibres, when oriented perpendicular to the field direction, or E-glass fibres. 3.3. Eddy currents Eddy currents are induced in objects from electrically conductive material exposed to an electromagnetic field. They create a second electromagnetic field, (partially) cancelling the applied field. In a conductive medium, the amplitude of an electromagnetic wave is attenuated to 1/e of its original value at the ‘‘skin depth’’ below the surface. Limited penetration into the medium effectively results in shielding of the electromagnetic radiation, the efficiency of which at a given radiation frequency depends on the material conductivity, the orientation and volume of the object, and number and size of apertures in the object. The effect on the image quality in MRI was discussed by Camacho et al. [36]. For a conductive loop from copper, eddy currents induced by the radio-frequency field, but not by switching of the gradient fields, were found to significantly reduce the signal intensity. Here, similar effects may occur for specimens from carbon fibre, which has a conductivity typically 2–3 orders of magnitude smaller than that of metals [24]. The architecture of the carbon fibre fabrics determines if closed conductive loops are formed and eddy currents are induced. For the carbon fibre materials used in this study (Table 1), the conductivity was characterised qualitatively in a series of experiments. A random discontinuous carbon fibre preform was found to be non-conductive. It is thought that the applied thermoset powder binder [37], which prevents the preform from disintegrating, insulates the individual chopped fibre segments electrically. Because of its specific architecture, a uni-directional (UD) carbon fibre fabric was found to be conductive along the fibre axes, but not perpendicular to the axes. A 3D woven carbon fibre fabric and a triaxial carbon fibre braid were found to be conductive in any direction in-plane and through-thickness. This implies that there is electrical contact between the yarns, and that the sizing applied to the filament surfaces has no insulating effect. While it is not quite clear to what extent the MR signal emitted from within
800 450 912 4775
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a conductive porous medium may be affected, some effect of radiofrequency shielding is expected to be observed for the latter two fabrics. To give an upper estimate for this effect, the skin depth in a hypothetical homogeneous conductor with a resistivity of 1.6 105 X m (as commercial grade carbon fibre [38]) is approximated as 0.2 mm at a frequency of 128 MHz. The skin depth is expected to increase (mm) with yarn spacing, i.e. aperture size, in fabrics from fibres with the same resistivity. 4. Imaging experiments 4.1. General considerations A major challenge specific to this study is finding the optimum imaging protocol and parameters. Because of the relaxation effects discussed in Sections 3.1 and 3.2, the echo time is critical for signal detection. As illustrated schematically in Fig. 1, reduction of TE allows signal loss due to T2 and T 2 relaxation effects to be minimised. An additional challenge is the multi-scale nature of the investigated problem. Distinguishing the fluid concentrations in fibre bundles and inter-bundle voids, while mapping the macro-scale fluid distribution, requires high resolution at a large field of view. In addition, three-dimensional imaging is required to allow identification of potential through-thickness effects. Choosing a large image matrix size is expected to result in prolonged scan times. As a rough estimate, typical scan times are in the order of several minutes for most 3D MRI techniques, but depend strongly on the specific protocol. Since, at a later stage, the fabric impregnation behaviour is to be studied based on multiple scans at different injection times, the scan time needs to be kept as short as possible. In most studies dealing with the impregnation of fabrics [19– 22], imaging is based on Spin-Echo (SE) techniques frequently employed in medical imaging. While these techniques are typically relatively robust to susceptibility effects, Leisen and Beckham [11] found that, for the example of carpet impregnated with water, they allow only semi-quantitative imaging of fluid concentrations and cannot detect concentrations below approximately 10%. As discussed in Section 3.1, the signal relaxation time, and thus the strength of the acquired signal at a given echo time (typically >10 ms for SE), decrease when water is bound to the fibres. A decrease in fluid concentration, i.e. in the ratio between free and bound water in the carpet, results in non-linear signal reduction. A drawback of SE sequences is that scan times, which are not an issue for 1D or 2D imaging [11,18,20], may become long for 3D imaging at the required resolution (up to approximately 1 h). For accurate quantification of the fluid concentration in textiles, Single-Point Imaging (SPI) was suggested as an alternative [11]. In SPI, the signal is sampled immediately after excitation (i.e. the concept of echo time does not apply), making the procedure
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TEl
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Fig. 1. Schematic illustration of relaxation effects described in Sections 3.1 and 3.2: MR signal intensity as a function of time after excitation, parameter fibre volume fraction Vf. Comparison of intensity at different echo times, TEs and TEl, indicates that time interval between excitation and sampling is critical for signal detection.
Table 2 Approximate values for the magnetic susceptibilities v of the components in the imaging experiments, derived from published data (PAN: Polyacrylonitrile). Material
v/106
Vacuum Air [28] Perspex [28] Nylon [29] E-glass [30] Carbon fibre, PAN-based, High strength (E0 = 240 GPa) [31] Carbon fibre, PAN-based, High modulus (E0 = 380 GPa) [31] Mineral oil [24]
0 0.36 9.70 9 20 3.3 (axial) 9.5 (radial) 15 (axial) 214 (radial) 8.80
insensitive to relaxation effects. However, the scan time is proportional to the number of image points and would be in the order of hours for 3D imaging at high resolution, which is not considered acceptable here. 4.2. Results and discussion For the purpose of this study, FLASH-type (Fast Low-Angle Shot [39]) imaging sequences were employed with some success. These techniques generally allow a good compromise between scan time and image resolution to be achieved. Compared to SE imaging, the scan time is typically significantly shorter, but the imaging procedure is less robust to susceptibility effects. This is mitigated to some extent by short echo times, typically in the order of a few milliseconds (here, echo times were between 3 ms and 4 ms). For illustration of the achieved image quality and the trade-off between resolution and scan time for a specific imaging sequence, Fig. 2 shows slices (view along vertical axis) through the Perspex tool containing a random discontinuous carbon fibre preform [37] (Table 1; 3 layers, fibre volume fraction Vf 51%) and a 0°/ 90° stitch-bonded non-crimp E-glass fibre fabric (Table 1; 6 layers, Vf 39%) with the weft direction oriented along the tool axis, both impregnated with the test fluid. One slice is from a 3D scan with 256 256 14 voxels (along longitudinal axis cross axis vertical axis) at an isotropic resolution of 0.8 mm, acquired in 237 s, the other one is from a scan with 360 352 22 voxels at an isotropic resolution of 0.5 mm, acquired in 433 s. The greyscales indi-
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cate the local fluid concentration in each voxel (bright: high; dark: low). Completely dark voxels suggest absence of fluid. Comparison of the slices with a corresponding photograph indicates plausibility of the acquired image data. Features of the fibrous structures visualised in the photograph can be clearly identified. The gap left in the tool at the injection gate (bottom of images) acts as a manifold allowing the fluid to distribute across the tool cross-section before impregnating the fabric. Thus, pure in-plane flow can be achieved. In the glass fibre fabric, fibre bundles (low fluid concentration: dark grey) and inter-bundle voids (high fluid concentration: bright grey) can be distinguished. The (almost) regular structure can be identified. In the carbon fibre preform, the random distribution of the fibre bundles results in irregular variations in the fluid concentration. Signal intensity fluctuations caused by susceptibility effects around the edge of the specimen are considered acceptable. To allow small-scale defects to be identified clearly, all subsequent scans were acquired at a 0.5 mm isotropic resolution. To illustrate the combined influence of the anisotropic geometry effect and anisotropic susceptibility discussed in Section 3.2, Fig. 3 shows slices through the tool containing impregnated UD carbon fibre fabric (Table 1; 9 layers, Vf 49%), oriented parallel and perpendicular to the field direction in the scanner. The actual susceptibility values of the specific type of carbon fibre are unknown. The slices in the figure are from scans acquired at parallel and perpendicular orientation of the tool longitudinal axis relative to the magnetic field direction. The image quality is high, where the fibres are oriented parallel to the field direction. Fibre bundles and inter-bundle voids can be clearly distinguished, and the fixation thread can be identified. Where the fibres are oriented perpendicular to the field direction, the average signal intensity is reduced by approximately 35%, and the image appears more blurred, although the main features can still be identified. Quantitative evaluation of intensity profiles indicates that the distances between intensity maxima, corresponding to the fibre spacing, are (3.63 ± 0.50) mm and (3.25 ± 0.98) mm if the fibres are oriented parallel and perpendicular to the field direction. Both values are consistent with the theoretical value of 3.64 mm given by the fabric manufacturer. However, if the fibres are oriented perpendicular to the field direction, the uncertainty is higher, since the signal-tonoise ratio in the images is reduced. While no significant geometrical image distortion occurs, micro-scale non-uniformity within the yarns causes field disturbances across the voxel dimensions, which depend on the fibre orientation and also on the local fibre volume fraction. This results in signal loss as described above. Due to the orientation of the tool, the right image in Fig. 3 also shows some effect of inhomogeneity in the radio-frequency field, which is ignored here. A general observation is that the impregnation of the fabric in this specific specimen is poor. Multiple dry spots are clearly visible. These were not visible to the naked eye (viewing though the transparent Perspex tool), and thus demonstrate the potential of MRI to identify defect formation in a laminate. It is to be noted that effects related to the discontinuity in susceptibility between test fluid and residual gas in the dry spots (Table 2) affect the apparent size of dry spots in FLASH scans, which is larger than their actual size. The difference between apparent and actual size increases with decreasing size of dry spots, which makes quantification of the volume difficult, but helps identification of small defects. The reason for occurrence of the defects in this specimen is that at the injection gate, no gap acting as a manifold was left in the cavity. Thus, impregnating flow has a through-thickness component. This makes complete impregnation more difficult, since the throughthickness permeability of fibre bundles is typically significantly smaller than the in-plane permeability. The anisotropic geometry effect at isotropic susceptibility is illustrated in Fig. 4A, which shows a slice through an impregnated
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Fig. 2. Injection tool with fully impregnated random discontinuous carbon fibre preform and 0°/90° stitch-bonded E-glass fibre fabric; left: photograph; centre: FLASH, slice with isotropic resolution 0.8 mm; right: FLASH, slice with isotropic resolution 0.5 mm.
Fig. 3. Injection tool with fully impregnated uni-directional carbon fibre fabric, oriented parallel and perpendicular to tool longitudinal axis; left: tool longitudinal axis parallel to magnetic field direction; right: tool longitudinal axis perpendicular to magnetic field direction; FLASH, slices with 0.5 mm isotropic resolution.
plain weave E-glass fibre fabric (Table 1; 6 layers, Vf 45%) with the warp direction oriented along the tool axis. While the architecture of the fabric can clearly be identified, and the fluid content in the warp yarns results in a detectable signal, signal loss obscures the impregnation state of the weft yarns, which are oriented perpendicular to the field direction. The effect is more severe than for the UD carbon fibres in Fig. 3 at similar fibre volume fraction. Since the influence of fluid dispersion on the signal relaxation is expected to be smaller (Section 3.1), this suggests that the product of |Dv| relative to oil and filament cross-sectional area is higher for the specific type of E-glass than for the STS40 carbon fibre. Localised signal loss as in Fig. 4A can be mistaken for dry spots and thus result in false positives in defect detection. In cases where FLASH-type imaging sequences fail to detect a signal, Ultra-Short Echo-Time (UTE) imaging [40] with typical echo times of less than 1 ms (here, echo times were 0.2 ms) is suitable to minimise signal loss due to low fluid concentration and susceptibility effects. As an example, Fig. 4B shows a slice through a specimen of the same specifications as in Fig. 4A. A 3D UTE scan with 336 336 17 voxels at an isotropic resolution of 0.5 mm was acquired in 21 min. This is still considered acceptable for imaging of
impregnation processes, if it can be combined with intermittent injection techniques (assuming that effects of capillary flow during the scan time are negligible, which will be discussed in the second part to this study). Compared to Fig. 4A, signal loss is reduced. Unlike in the FLASH-type scan, the greyscale values in the weft yarns are significantly higher than in the background, indicating the presence of fluid. Because of the specifics of the UTE imaging sequence, backfolding artefacts (from the fluid in the tubes) appear near the injection gate and vent. Since these do not affect the main area of interest, they are simply ignored here. However, in some cases, mapping of the fluid concentration may still be impossible. For a 3D woven carbon fibre fabric (Table 1; 1 layer, Vf 58%) with the warp direction oriented along the tool axis, signal was detected only from the specimen surface, where V-shaped gaps between the yarns and the tool surface [41] result in locally increased fluid concentration. No signal was detected from within the specimen, although micrographic analysis of moulded specimens (with resin) at identical fibre volume fraction indicated that the fibre bundles were impregnated [41]. Since this fabric was found to show some electrical conductivity (see Section 3.3), it is suspected that signal reduction due to induced eddy currents
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Fig. 4. Injection tool with fully impregnated plain weave E-glass fibre fabric; slices with 0.5 mm isotropic resolution. (A) FLASH; (B) UTE. ((A) and (B) show different specimens of the same type.)
intensity at the minimum near the middle layer is approximately 26% of the highest intensity observed near the surfaces. The symmetrical shape of the profile cannot be explained by relaxation effects, which are expected to result in random intensity variations through the specimen thickness (Fig. 6B), since they reflect the effect of variations in fibre packing density due to (random) nesting between adjacent layers. The systematic variation in signal intensity suggests that the effect of radio-frequency shielding is dominant here.
5. Conclusions
Fig. 5. Injection tool with fully impregnated 0°/±60° triaxially braided carbon fibre fabric; UTE, slice with 0.5 mm isotropic resolution (tone balance adjusted for clarity).
occurs in addition to relaxation effects, thus effectively rendering the signal undetectable. Similarly, no signal was detected from within an impregnated specimen of a triaxially braided 0°/±60° carbon fibre fabric (Table 1; 8 layers, Vf 48%) with the 0° direction oriented along the tool axis. Comparison with the results for the uni-directional fabric at similar fibre volume fraction (Vf 49%), for which a clear signal was detected at longer echo time even at unfavourable fibre orientation (Fig. 3), suggests that the influence of the braid conductivity (see Section 3.3) on the signal is significant. However, for a specimen of the same braid at 7 layers, i.e. Vf 41%, the fluid content could be visualised (Fig. 5), indicating that the issue of signal loss is strongly sensitive to Vf. Fig. 6A shows a through-thickness profile of the recorded signal intensity taken at the centre of the specimen in Fig. 5. The
The problem of mapping the fluid content in impregnated reinforcement textiles using MRI techniques is dominated by signal relaxation effects, which result in local signal loss. They are related to the dispersion of the test fluid in the fabric and to discontinuities in magnetic susceptibility, in particular at the interfaces between fluid and fibres. Their severity depends on material and diameter of the filaments, fibre volume fraction and fibre orientation. Employing imaging techniques with short echo times allows the influence of signal relaxation effects on the image quality to be minimised. For 3D imaging of macroscopic specimens with dimensions of 140 mm 90 mm 4.7 mm at an isotropic resolution of 0.5 mm, FLASH and UTE imaging sequences were implemented on a 3 T whole-body MRI scanner. Typical scan times were in the order of 7 min and 21 min, respectively. For examples of E-glass and carbon fibre fabrics of different architecture and engine oil as a test fluid, the obtained images allowed the impregnation state at the meso-scale to be identified qualitatively and dry spots to be detected. The UTE sequence was found to be more useful than the FLASH sequence for this application, since the acquired images are less affected by signal loss. However, in addition to relaxation effects, radio-frequency eddy currents may be induced if the fabrics are conductive and result in partial cancellation of the signal. Depending on reinforcement type and fibre volume fraction, this effect may render the signal undetectable inside the specimen. A second part to this study will deal with application of the imaging procedure established here to tracking of the flow front propagation at the meso-scale during in-plane impregnation of reinforcement fabrics. The aim is to characterise the typical
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impregnation behaviour of specific fabrics, which would complement macro-scale permeability data acquired by more conventional means. Acknowledgements This work was funded by the Engineering and Physical Science Research Council (EPSRC) via the Nottingham Innovative Manufacturing Research Centre (EP/E001904/1) and the Platform Grant (EP/ F02911X/1). The scanning experiments were approved by the Magnetic Resonance Strategy Board, University of Nottingham. Technical support was provided by Dr. Matthew Clemence, Philips Healthcare, UK. References [1] Darcy H. Les fontaines publiques de la ville de dijon. Paris: Victor Dalmont; 1856. [2] Luo Y, Verpoest I, Hoes K, Vanheule M, Sol H, Cardon A. Permeability measurement of textile reinforcements with several test fluids. Compos Part A Appl Sci Manuf 2001;32(10):1497–504. [3] Lundström TS, Gebart BR, Sandlund E. In-plane permeability measurements on fiber reinforcements by the multi-cavity parallel flow technique. Polym Compos 1999;20(1):146–54. [4] Parnas RS, Salem AJ. A comparison of the unidirectional and radial in-plane flow of fluids through woven composite reinforcements. Polym Compos 1993;14(5):383–94.
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