Steven L. Donaldson

Strength of Composite Angle Brackets with Multiple Geometries and Nanofiber-Enhanced Resins

The purpose of this study was to investigate the change in strength and failure mode of composite angle brackets due to changes in both the angle bend radius and laminate thickness, as well as the addition of vapor-grown carbon nanofiber to the epoxy resin matrix. The brackets explored within this study each contained a 90° bend and was subjected to four-point bend loading. Such angle brackets exhibit weakness around the radius due to the excessive through-the-thickness tensile stresses which can lead to delamination. Composite brackets of 8- and 16-plies were examined, with bend radii of 3.175 and 6.35 mm. The composites consisted of Hexcel AS4 carbon fiber, five-harness satin weave, and Epon 862/Epikure W epoxy resin. Specimens were fabricated with and without ASI PR-24 vapor-grown carbon nanofiber in the epoxy matrix. A servo-hydraulic load frame was used to perform a four-point-bend test as per American Society for Testing and Materials International D6415 for measuring the curved beam strength (CBS) of a fiber-reinforced polymer matrix composite. Despite a threefold difference in failure load and CBS, data reduction using both closed-form and finite element modeling resulted in a nearly single critical value of radial peel stress at initial failure of 30—32 MPa for all specimens. The mechanical load response (large load drop vs. ‘stick-slip’ after initial failure) and optical microscopy results are explored in detail.

Experimental and finite element evaluations of debonding in composite sandwich structure with core thickness variations

An important failure mode in sandwich structures is the debonding between the core and facesheet, which can destroy the load capacity of the structure. This work addressed the critical interfacial modes and studied the effects of thickness variation of the core material. The single cantilever beam geometry is utilized for conducting experiments after optimizing the thicknesses of the core and facesheet by minimizing the difference in the bending stiffness matrix between the upper facesheet and the lower facesheet/core combination. Two different core material thicknesses were tested. The experimental results showed that the critical energy release rate could be influenced by core thickness variations. Furthermore, the cohesive zone method and elastic–plastic core material model in conjunction with fracture criteria were used to model the entire structure failure response. The validation results predicted load–extension curves in agreement with actual tests for both single cantilever beam geometry specimens. The model also had the ability to predict the crack initiation in the core materials which occurred under the interface zone as in the actual test. In addition, the mixed-mode ratios through the interface area were analyzed as function of crack length to assess its influence on both single cantilever beam thickness specimens.

Biomimetic Composite Structural T-joints

Biological structural fixed joints exhibit unique attributes, including highly optimized fiber paths which minimize stress concentrations. In addition, since the joints consist of continuous, uncut fiber architectures, the joints enable the organism to transport information and chemicals from one part of the body to the other. To the contrary, sections of man-made composite material structures are often joined using bolted or bonded joints, which involve low strength and high stress concentrations. These methods are also expensive to achieve. Additional functions such as fluid transport, electrical signal delivery, and thermal conductivity across the joints typically require parasitic tubes, wires, and attachment clips. By using the biomimetic methods, we seek to overcome the limitations which are present in the conventional methods. In the present work, biomimetic co-cured composite sandwich T-joints were constructed using unidirectional glass fiber, epoxy resin, and structural foam. The joints were fabricated using the wet lay-up vacuum bag resin infusion method. Foam sandwich T-joints with multiple continuous fiber architectures and sandwich foam thickness were prepared. The designs were tested in quasi-static bending using a mechanical load frame. The significant weight savings using the biomimetic approaches is discussed, as well as a comparison of failure modes versus architecture is described.

Experimental and Finite Element Investigations of Damage Resistance in Biomimetic Composite Sandwich T-Joints

Composite sandwich structural joints, such as T-joints, are used in many different composite applications to transfers the load orthogonally between two sandwich elements. However, these joints connecting the sections can represent the weakest link in sandwich composite structures due to the lack of reinforcement in the out-of-plane direction. Therefore, this paper presents a new methodology for the design and analysis of composite sandwich T-joints using new biomimetic fabrication methods. The fabricated idea comes from biological fixed joints as an evolutionary alteration processes of trunk-branches of trees. It offers unique attributes to optimize the continuous fiber paths for minimum stress concentrations and multi-sandwich layers to increase the bending stiffness and strength. The focus is on how the biomimetic technique can improve sandwich T-joint structures by increasing their strength and load carrying capability without adding a significant weight penalty. The major attention is to investigate the comprehensive failure modes in the joint numerically and verified by experiments. Investigations were conducted on three different designs of biomimetic composite sandwich T-joints under tension and bending loads. The results show significant improvements to the ultimate load up to 68% in the case of bending load and 40% in the case of pull-off load in the biomimetic sandwich T-joints compared to the reference conventional T-joint design. The final failure was significantly deferred in both load status. The FE models provided important insights into the core failure and delamination of multi-interface biomimetic T-joints.

Interlaminar fracture toughness and electromagnetic interference shielding of hybrid-stitched carbon fiber composites

In the current study, the production of multifunctional hybrid-stitched composites with improved interlaminar fracture toughness and electromagnetic interference shielding effectiveness is reported. Unidirectional carbon fiber-epoxy composite laminates stitched with Kevlar, nylon, hybrid stitched with both Kevlar and nylon and unstitched were prepared using resin infusion process. Representative specimens from unstitched and stitched composites were tested using rectangular waveguide and Mode I double cantilever beam tests. The Mode I experimental results showed that composite stitched with Kevlar exhibited the highest crack initiation interlaminar fracture toughness (GIC-initiation), whereas composite stitched with nylon exhibited the highest maximum crack propagation interlaminar fracture toughness (GIC-maximum). The four-hybrid stitching patterns exhibited higher GIC-initiation than the unstitched and stitched with nylon composites and lower than stitched with Kevlar composite, whereas they had higher GIC-maximum than the unstitched and stitched with Kevlar composites, although lower than stitched with nylon composite. The electromagnetic shielding effectiveness experimental results showed that stitched composites exhibited improved shielding effectiveness compared to unstitched composites. For example, composite stitched with nylon had highest shielding effectiveness value of 52.17 dB compared by the composite stitched with Kevlar which had 40.6 dB. The four hybrid-stitched composites exhibited similar shielding effectiveness with an average value of 32.75 dB compared to the unstitched composite shielding effectiveness of 22.84 dB. The experimental results comply with the initial goal of this study to manufacture multifunctional hybrid stitching composites with combined properties between Kevlar and nylon-stitched composites.

Comparison of methods for the characterization of voids in glass fiber composites

Voids are a concern in composite materials, as they may have a negative effect on the mechanical properties of the laminates. Voids may develop especially in low cost or off-optimum process conditions. In this study, samples of glass reinforced epoxy laminates with void volume fractions in the 0.5–7% range were successfully obtained by varying the vacuum in the hand layup vacuum bagging manufacturing process. Void content was experimentally characterized using four different methods: ultrasonic scanning, epoxy burn off, serial sectioning, and X-ray computed tomography. The goal of this paper was to determine how the methods compared with respect to each other at quantifying void content. The specimens were taken from nearby locations in the same panels, so a true comparison of the methods could be obtained. The results showed, for the specific material and manufacturing conditions used, that the four different techniques can quantify voids content but with a large variation in the accuracy. X-ray computed tomography was the most successful technique to characterize voids, followed by serial sectioning. Ultrasonic scanning and epoxy burn off were not recommended techniques to characterize voids for laminates manufactured with these materials and process conditions. However, epoxy burn off was a successful technique to calculate fiber and resin weight fraction.

Sensitivity Study of Modified Micromechanics Relations for Composite Materials

Simple modified rule of mixture relations to determine the elastic properties of a composite ply are taken from the literature and studied. Fiber properties are back calculated for AS1 fibers. Plots are given to emphasize the sensitivity of the ply proper ties to perturbations in each of the constituent material properties. Particular attention is paid to ply transverse modulus.