THREE DIMENSIONAL MICROSTRUCTURE OF POLYMERIC COMPOSITE MATERIALS - PDF document

18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS THREE DIMENSIONAL MICROSTRUCTURE OF POLYMERIC COMPOSITE MATERIALS USED IN SANDWICH STRUCTURES USING DUAL MODALITY FROM COMBINED HIGH RESOLUTION X-RAY AND NEUTRON TOMOGRAPHY F. Kim 1 , D. Penumadu 1 , A. Siriruk 1 , J. Bunn 1 , and N. Kardjilov 2 1 Civil and Environmental Engineering, University of Tennessee, Knoxville, USA, 2 Helmholtz Center Berlin for Materials and Energy, Berlin, Germany * Corresponding author (fkim@utk.edu) Keywords : Carbon Fiber, Composite Material, Neutron Tomography, X-ray Tomography Abstract mechanics based modeling of constitutive material Use of dual modality to study polymeric composite properties of facing, foam core, and interfacial materials with high resolution X-ray and neutron fracture of sandwich structures due to harsh sea tomography is demonstrated for the first time in this environment is currently based on the observation of study. X-ray and neutron tomography techniques macroscopic laboratory data, and authors are provide an ability to visualize and quantitatively evaluating micro-tomography data to further describe the microstructure in three dimensions (3- enhance the related basic research. Radiation (X-ray D) non-invasively. 3-D image based registration is and neutron) based imaging can provide important performed to combine the two modalities that information about the state of microstructure of the account for varied resolutions and contrast. Authors composite sample in terms of the distribution of are studying the effect of degradation of carbon various phases corresponding to fibers, resin, fiber vinyl ester based composites due to sea water manufacturing defects, and the underlying damage exposure and the damage diagnostic techniques and failure mechanisms when coupled with in situ from tomography, and are finding beneficial use to mechanical testing systems. Coupling of powerful understand the complex coupling of microstructure- tomography based imaging systems with proven and mechanical property relationships. Quantitative well calibrated ultrasound based diagnostic information such as local versus global variation of techniques have significant future potential in such fiber and resin volume fraction, analysis of voids or studies. cracks, and anisotropy of attenuation properties are In order to visualize the interior of a sample, being evaluated in 3-D for use in studying these imaging technology has been applied in the present complex materials used for marine ship sandwich paper to study carbon fiber and vinyl ester based polymeric composite facing materials. Until structures. 1 Introduction recently, microstructure for composite materials is Carbon fiber reinforced polymer composite obtained by destructive techniques such as thin materials are used as structural materials in the sectioning of impregnated and damaged samples, construction, automotive, and aerospace industries subsequent careful polishing, obtaining two for their high specific strength and stiffness, dimensional images from optimal microscopy corrosion and fatigue resistance and ease of handling followed by digital image analysis. This method is and fabrication [1]. Carbon fiber composite not only time consuming, but also prone to have materials are used as the facing materials for errors such as non-parallel sectioning, fiber shape sandwich structures, and sandwich structures have distortion during the sample preparation, and been investigated for possible use in naval mismatch of fiber between adjacent dissected application. The environmental degradation due to surfaces [4]. X-ray tomography method has been long term exposure of sea water effects on the used as an alternative. Lab based microtomography composite facing material is a significant research (micro-CT) machines are readily available and problem which is actively being evaluated [2, 3].The provides resolutions in the order of a micrometer or observed degradation and related mathematical less. The highest resolution is achievable at

synchrotron imaging facilities [5] using following established procedures [13]. Coupon monochromatic X-rays. X-ray tomography samples obtained from the face sheets were then techniques have been applied to study damage from used for this study. One batch of samples was kept mechanical and thermal stresses and UV radiation dry (allowed to age) and another batch was soaked for polymeric composites [6-8]. Synchrotron by immersing the samples in a bath of sea water at tomography imaging experiments coupled with 40 ˚ C for at least 6 months prior to testing. This conceptual in-situ mechanical tests on carbon fiber duration was sufficient to reach equilibrium sea laminated epoxy composite materials were recently water uptake based on periodic weight gain reported with impressive spatial resolution of measurements. Generally the equilibrium state for visualizing individual fibers for damage analysis and sea water diffusion was reached upon immersion shows promise [9-12]. within approximately three months, with a weight For the first time, authors performed simultaneous gain of about 0.4%. Two samples with the X-ray and neutron tomography experiments on wet dimension of 25.4 mm × 50.8 mm × 2.8 mm from and dry carbon fiber polymeric composite specimens large sheets were prepared by getting a coarse using unique imaging resources at Helmholtz Center sample using a diamond saw and sample with final Berlin for Materials and Energy (HZB), Berlin, dimensions using surface grinder with diamond Germany. The objective of this paper is to present a blade. These dry and aged sample and wet (soaked) new concept of studying polymeric composite sample were transported to imaging facility in sealed bags for tomography experiments. materials using combined use of X-ray and neutron tomography techniques to exploit the different 2.2 Neutron Tomography and X-ray contrast possible from two types of radiation. The Tomography contrast of neutron and X-ray tomography data is compared, and spatial registration of sample in 3-D X-ray tomography was performed using a laboratory microfocus X-ray machine. Neutron tomography using the two modalities is also demonstrated. The methods developed in the research will be extended was performed at a dedicated neutron imaging beam line at HZB called the cold neutron radiography and to future in-situ loading experiments to study the tomography (CONRAD) with cold neutron spectrum failure mechanism of wet and dry carbon fiber composite materials to understand the fundamental coming from a 10 MW reactor using the high resolution setup with CCD camera and a 10 μ m stress-strain behavior and damage evolution under extreme environments. thick gadox scintillator [14, 15]. The imaging parameters are provided in Table. 1. Both systems 2 Sample Description and Experimental provided relatively large field of view (FOV) and Procedure spatial resolution adequate for engineering applications. 2.1 Sample Preparation High spatial resolution is required to visualize the The carbon fiber composite material in this study fine features of the sample. However, increasing the was made of carbon stitch bonded fabric designated spatial resolution inevitably reduces the achievable as LT650-C10-R2VE supplied by the Devold AMT FOV in general. The neutron imaging detector used AS, Sweden. This was an equibiaxial fabric in this research can achieve up to 13.5 μ m/pixel produced using Toray’s T700 12k carbon fiber tow resolutions with 27.6 mm × 27.6 mm FOV. The with a vinyl ester compatible sizing. The individual FOV and spatial resolution can be changed by carbon fiber diameter is 7 μ m. The resin matrix used varying the working distance and magnification of was Dow Chemical’s DERAKANE 510A-40, a the lens. Even the highest resolution (13.5 μ m/pixel) brominated vinyl ester, formulated for the Vacuum would still be too coarse to visualize the individual Assisted Resin Transfer Molding (VARTM) fiber (7 μ m dia.). As a result, the FOV was adjusted process. The bromination imparts a fire-resistant to visualize the entire sample area. In case of property to the composite. Carbon fiber microfocus X-ray tomography system, the reinforced/vinyl ester laminated composites magnification was adjusted to visualize the top half consisting of [90/0] 2s cross-stitched lay-ups are used of the sample with 13.2 μ m/pixel resolution. For in this study. Test materials were fabricated many engineering applications and simulations, it is