Michael W. Czabaj

Determination of the mode I, mode II, and mixed-mode I–II delamination toughness of a graphite/polyimide composite at room and elevated temperatures:

A study is described in which the mode I, mode II, and mixed-mode delamination toughnesses of a woven fabric graphite/polyimide composite are determined at room temperature and at 300℃. It is shown that, with some modifications, commonly used delamination toughness test methods can be extended for use into this relatively high-temperature regime. At room temperature, the mode II toughness was found to be significantly higher than that obtained in mode I. Relative to their fracture toughnesses at room temperature, the mode I and mixed-mode toughnesses increased at elevated temperature, while the mode II toughness decreased. Thus, at 300℃, the mode I toughness was larger than that obtained for mode II. The temperature dependence in all three modes was found to be consistent with what is predicted based on micromechanical considerations for the effects of temperature on the resin. In addition, the local tow orientations influenced the fracture path and produced toughening mechanisms that were both temperature and mode mix dependent.

Automated 3D digital reconstruction of fiber reinforced polymer composites

A 3D reconstruction system is described that generates digital reconstructions of an aerospace-grade graphite fiber reinforced composite in an accurate and highly automated way using high resolution X-ray computed tomography (CT) imagery. The 3D reconstruction system is based on algorithms from the multi-target tracking literature including a tracker using a global nearest neighbor data association architecture and Kalman filter, along with algorithms for track stitching and smoothing. Additionally, two new algorithms are developed to ensure that none of the virtual fibers in the 3D reconstruction overlap each other. First, is an algorithm for selecting among multiple possible tracks in the case of significant overlap. Second, is an algorithm that solves for an optimal, in the maximum a posteriori sense, adjustment for the virtual fiber locations and radii such that none of the virtual fibers overlap. The 3D reconstruction system is evaluated with X-ray CT data for an AS4/3501-6 graphite/epoxy composite. A representative sub-volume containing 629 fibers across 427 CT images, is used for evaluation. Additional algorithms are developed to manually correct any minor errors in the automatically generated 3D reconstruction leading to a fully accurate 3D reconstruction. In addition to being highly automated, the 3D reconstruction system is readily extensible to larger test specimens and other fiber arrangements.

2D Microscale Observations of Interlaminar Transverse Tensile Fracture in Carbon/Epoxy Composites

This study presents a new experiment to image and analyze the evolution of transverse tensile fracture in tape-laminate carbon/epoxy composites at the microscale. To this end, a miniature double cantilever beam specimen is developed to produce stable transverse tensile fracture and various amounts of crack turning through a stack of 90° plies. The specimen is wedge-loaded with a custom micromechanical tester, while the crack growth is optically monitored at both the micro- and macro-scale. Post-test, microscale images are analyzed to determine the transverse crack path relative to the applied loading, and to directly measure local strains using 2D digital image correlation. The experimental data sets obtained in this study enable direct validation of finite element simulations of transverse tensile cracking in fibrous composites at the microscale and provide impetus for their further model development.

Mechanical Characterization of Open Cell Aluminum Foams Using X-ray Computed Tomography

Open-cell aluminum foams show excellent potential for use in a variety of applications. In order to accelerate the use of aluminum foams in engineering industry, it is important to accurately understand the relationship between the manufacturing processes and the resulting foam properties. Two tests are developed to support development of finite element (FE) models that will be used to design foam geometries optimized to meet specific design criteria. A bulk crush test is created for small foam blocks which uses in situ X-ray computed tomography (CT) imaging. A tensile experiment is developed to test individual foam ligaments, which measures the aluminum’s mechanical behavior and will be used to improve the simulation’s accuracy beyond what is possible using published bulk material properties of aluminum.

Imaging the Life-Cycle of CMCs Using High-Resolution X-Ray Computed Tomography

In this study, the entire life cycle of a ceramic matrix composite (CMC) manufactured using polymer infiltration and pyrolysis (PIP) is imaged using high-resolution X-ray micro-computed tomography (μCT). The entire PIP process is imaged ex situ to capture the evolution of voids and shrinkage cracks during laminate densification. After manufacturing, two specimens are extracted from the CMC laminate, and subsequently tested to failure in flexure and tension at 1000°C. Gray-scale image segmentation methods are used to quantify the evolution of porosity within the microstructure during PIP processing. X-ray μCT image results from in situ testing are qualitatively examined to quantify presence of individual fiber breaks, fiber pull-outs, matrix cracking, fiber-matrix decohesion, to name a few. These results are used to motivate development of new algorithms for segmentation of microstructural features from X-ray μCT data sets.

Effect of Processing Parameters on Interlayer Fracture Toughness of Fused Filament Fabrication Thermoplastic Materials

Additive manufacturing by fused filament fabrication (FFF) is a promising method for rapid manufacturing of complex components for a wide variety of applications. FFF is often limited to non-structural and non-load bearing applications due to insufficient strength and stiffness of the end-material. This is particularly true in the direction of layer deposition, due to poor adhesion between FFF layers. Processing parameters such as extrusion temperature and print speed have been shown to have significant effect on the mechanical performance of FFF components, but these studies have often neglected interlayer properties. This work develops and experimental approach for quantifying the relationship between processing parameters and interlayer fracture toughness of FFF specimens. The processing parameters considered include extrusion and bed temperatures, extrusion speed, raster spacing, and cooling-fan speed. FFF test blocks were fabricated to identify which parameters would best optimize interlayer fracture toughness. To measure interlayer fracture toughness, unidirectional ABS double cantilever beam specimens were fabricated according to the parametric test matrix with guidance from the test block results. In situ full-field thermography was used to record the specimen thermal history during fabrication. X-ray computed tomography was used to determine the internal void resulting from varying the raster spacing. Finally, optical and SEM fractography was used to perform post mortem categorization of specimen fracture surfaces. The fracture toughness data measured in this study is used to develop an approach for rapid optimization of interlayer properties of FFF components.