Ronnie Bolick

Effect of Electrospun Fibers on the Interlaminar Properties of Woven Composites

Failure by delamination of composite laminates due to low velocity impact damages is critical because of the subsurface nature of delamination. Traditional methods such as stitching and Zpinning, while improving interlaminar properties in woven composites, lead to a reduction of the inplane properties. To alleviate these problems, use of Tetra Ethyl Orthosilicate (TEOS) nano fibers manufactured using electrospinning technique in fiber Glass-Epon composite laminates is investigated for their potential to improve the interlaminar properties. Electrospun coated fiber glass woven mats are impregnated with epoxy resin using Heated-Vaccum Assisted Resin Transfer Moulding (H-VARTM) process. The interlaminar properties of the nano engineered hybrid composites obtained using ASTM Double Cantilever Beam (DCB) tests and short beam shear tests are compared with those without the presence of electrospun fiber layers, to study their influence. The short beam shear tests revealed a 20% improvement due to presence of TEOS interlaminar electrospun nanofibers. It is also noteworthy that fibers cured at different temperature levels had variation in performance as observed in MSBS test results.

Innovative Method for Improving Composite Fatigue Life Estimations

[ABST RACT ] App ro ximately 70% of structural failures are due to fatigue. The fatigue mechanism in composites is much more complex than that of metals. The damage may be in one or more forms, such as a failure in a fiber -matrix interface, matrix cracking, delami nation (s), and /or fiber breakage. Matrix cracking and delamination both reduce stored energy and stiffness. Damage from these mechanisms can be detected very early in the fatigue life of a composite. This damage produces a reduction of elastic properties s uch as stiffness. There is always a correlation between damage and stiffness reduction. Fatigue in multidirectional laminates is a well understood phenomenon. It may be said that the damage process of multi -directional laminates has been clarified, at lea st qualitatively, if not quantitatively. Even though fatigue behavior is similar in woven composites, it is incorrect to apply the same concepts of the damage mechanism of multi -directional laminates to woven composites. This is because each ply in woven c omposites is itself bi -directionally reinforced by fiber bundles in the different directions. Recently, the use of woven composites for aerospace and other industrial applications has grown exponentially. These composites are typically manufactured using the low cost vacuum assisted resin transfer molding process (VARTM). With the large variations in fiber volume fraction, weave angle, thickness variations , an accurate estimation of the fatigue life prediction curve become s a challenging task. It is common knowledge that the ultimate strength of composite materials is dependent on the fiber volume fraction, thickness of the laminate and the warp weft angle. Most of the fatigue analyses today are based upon the average ultimate tensile strengths conducted p rior to fatigue testing . Variations in the se ultimate strengths of the composites can lead to extensive scatter in the fatigue data. This paper presents an innovative technique for the accurate prediction of the fatigue life in woven and braided composites . The technique uses a mu lti -variant analysis in conjunction with experimental strength data. This procedure first predicts an accurate tensile/compressive strength of a specimen and then uses these results to develop the fatigue life setup criteria . Resul ts indicate that this technique produces minimal scatter in the fatigue data, accurately predicts the tensile/compressive strengths as a function of the weave angle, the fiber volume fraction and the thickness of the specimen. This paper illustrates the ap plication of this newly developed technique in the estimation of the fatigue life in woven and braided composites.

Performance Evaluation of Notched Biaxial Braided Composites

Braided composites have good properties in mutually orthogonal directions, more balanced properties than traditional tape laminates, and have potentially better fatigue and impact resistance due to the interlacing. Another benefit is reduced manufacturing cost by reducing part count. Because of these potential benefits braided composites are being considered for various applications ranging from primary/secondary structures for aerospace structures [1]. These material systems are gaining popularity, in particular for the small business jets, where FAA requires take off weights of 12,500 lb. or less. The new process, Vacuum Assisted Resin Transfer Molding (VARTM), is low cost, affordable and suitable for high volume manufacturing environment. Recently the aircraft industry has been successful in manufacturing wing flaps, using carbon fiber braids and epoxy resin and the VARTM process. To utilize these VARTM manufactured braided materials to the fullest advantage (and hence to avoid underutilization), it is necessary to understand their behavior under different loading and environmental conditions. This will reduce uncertainty and hence reduce the factor of safety in the design. It is well known fact that the strength of the composite structure reduces because of discontinuities and abrupt change in the cross-section. Accurate knowledge of strength and failure mechanism of notched and unnotched composites is very important for design of composite structures. This research addresses the behavior of notched braided composites under static tensile loading.Copyright

Performance Evaluation of Unstitched, Stitched and Z-Pinned Textile Composites Under Static Loading

Advances in conventional tape laminates and textile composites provide aircraft manufacturers important technology, but the industry lacks the confidence to use these composites to manufacture wing and fuselage structures due to high cost and low damage tolerance. In order to overcome the high cost and to improve the damage tolerance of composites, researchers have developed new through-the-thickness reinforcement techniques, such as stitching through the thickness. This reinforcement technique can be used to join the skin, stiffeners, ribs and spars to form an integral structure. The structures are typically more damage tolerant, contain fewer fasteners and are less expensive to manufacture than conventional composite or metallic structures. Furthermore, stitching reduces the manufacturing time and labor compared to drilling holes for fasteners, and may eliminate the problems of fatigue and/or corrosion from galvanic reactions with metal fasteners. Woven composites with through the thickness reinforcements such as stitching have good properties not only in mutually orthogonal directions but also in the transverse direction and more balanced properties than traditional tape laminates. They are also expected to have better fatigue and impact resistance due to the interlacing. Another benefit is reduced manufacturing cost by reducing part count. Because of these potential benefits, these composites are being considered for various applications including primary/secondary components for aerospace structures. The objective of this effort is to develop experimental tools for comparing the performance of these composites reinforced by stitching to unstitched composites. Identification of damage mechanisms and forces available to grow damage is essential for identifying the primary parameters that determine performance. Accurate determination of the driving forces will require extensive manufacturing and experimentation. However, once the reinforcement techniques are well understood, it is anticipated that simplified experiments can be developed that could be used routinely by designers to evaluate the effects of the reinforcements on damage tolerance. This paper specifically addresses the performance evaluation of stitched low cost manufactured composites subjected to static loading. Static tension and compression testing was conducted to determine the Ultimate Tensile and Compressive Strengths, Young’s Moduli and Poisson’s Ratio. Two different stitch patterns or stitch densities were used for comparison. The first density was five rows of stitching per inch of width, with eight stitches per inch over the entire length. The second density was three rows of stitching per inch of width, with four stitches per inch over the entire length.Copyright

Effect of Interleaved Electrospun Nanofiber on Flexural Properties of Fiber Glass Composites

Electrospinning is regarded as one of the most efficient processes to generate one-dimensional nano structures. The electrospinning process is simple and provides consistent mass production of nanofibers. The scalability of the electrospinning process has an excellent potential to fulfill the high volume requirements of nanofibers in the infrastructure applications. The present work emphasizes the use of interleaved electrospun nanofibers in fiber glass composite beams. The Flexural behavior of a simply supported beam under a centrally concentrated loading is studied. Flexural properties of a fiber glass composite beam with interleaved electrospun nanofibers are compared with a fiber glass composite beam without electrospun nanofibers. The material configuration of the composite beams is: woven E-glass fabric prepregs with a low temperature molding resin. In addition, interleaved between the plies are TEOS (Tetra Ethyl Orthosilicate) electrospun nanofibers. The nanofibers were produced by developing optimized operating process parameters and a stabilized sintering temperature cycle to ensure consistency in the fiber morphology and pore structure. The successful integration of the electrospun nanofibers within the prepreg layers was obtained by pre-impregnation with a B-staged resin film and de-bulking to remove excessive resin prior to vacuum bagging. A series of mechanical Flexure tests were performed per the ASTM D7264 standard specification. Micrographs were obtained to study the progressive deformation and damage mechanics due to flexural loading in the specimens and clearly illustrate the differences in the failure mechanism with and without the electrospun interface layers.Copyright