Search

Ongoing research in additive manufacturing towards structural and industrial application has led to the use of commingled roving as a manufacturing feedstock for printing high fiber volume fraction composites. The prospects of using this technology for high performance applications necessitates the need for a comprehensive experimental investigation into the effects of processing parameters on the quality of an additively manufactured composite printed from commingled roving feedstock. In this work, transverse flexure and void fraction matrix pyrolysis testing are both performed to evaluate composite quality. The transverse flexure test is a testing approach that evaluates the quality of the interfacial fiber-matrix bond while the void fraction test estimates the void content in the printed composite. A full observational study consisting of 27 different test combinations is done to investigate the effects of three different process parameters namely, temperature, pressure, and print speed across three different levels. Composite samples were made from commingled roving of E-glass and amorphous PET using an in-house built continuous fiber composite digital manufacturing system. Least squares regression analysis is performed to study the main, interaction and quadratic effects of process parameters. A statistical regression model having an R2 adjusted value of 80.1% is generated from the transverse flexure study, which is used to explain main and interaction effects and also predict performance. Response surface plots are also generated and are used to optimize process parameters which can subsequently be of help in scaling up composite manufacturing. Results show that all three process parameters are highly statistically significant at the 0.01 level of significance. Pressure * Temperature and Pressure * Printspeed are significant interaction terms. Pressure plays a weightier role when print speed is increased or temperature is decreased as it closes more voids that would ordinarily have been introduced because of drop in polymer melt viscosity. Micrographic analysis is also performed. [ABSTRACT FROM AUTHOR]

With the advent of bonded composites in today's aircraft, there is a need to verify the structural integrity of the bonded joints that comprise their structure. To produce adequate joint integrity, strict process control is required during bonding operations. The latest non-destructive joint inspection techniques cannot quantify the strength of the bond and only indicate the presence of disbonds or delaminations. Expensive and timely proof-load testing of the joints is required to demonstrate structural performance. This work focuses on experimentally evaluating joint-health monitoring with piezoelectric sensors exposed to repeated loadings until failure. Single-lap-shear composite joints are structurally tested while using sensor electromechanical impedance response as a health indicator. Based on these experiments, validation of this novel method is achieved through detailed evaluation of the sensor impedance response characteristics during loading, which enable initial and prognostic joint health assessments. The experimental results indicate that the embedded piezoelectric sensors are able to measure the sensor impedance radial and thickness resonance response changes prior to joint failure, without sacrificing the joints' structural performance. [ABSTRACT FROM AUTHOR]

Grid stiffened continuous fiber reinforced composite panels are an attractive option for creating lightweight structures due to the tailorability for various applications and the resulting high specific properties. However, the panel stiffeners and stiffener intersections result in high tooling complexity and correspondingly high cost of implementation. These factors have limited the impact of such structures in the composites industry. Previous research has demonstrated the ability to produce high quality, high aspect ratio beams, representative of individual grid stiffeners, using E-glass/PET comingled tow via direct digital manufacturing. Further, prior preliminary efforts have demonstrated the potential to use the same approach to manufacture grid intersections that have continuous fiber in both directions. To expand on the previous efforts in grid stiffeners produced by direct digital manufacture with radically reduced tooling requirements, this effort compares two methods of providing positioning and consolidation, nozzle vs. roller. Both processes are based on a commingled yarn feedstock. The extrusion through a nozzle has been shown to enable grid intersection control through local variations in applied consolidation and serves as the baseline process. However, this approach requires a continuous placement path to create the complete grid stiffened panel as no mechanism for cutting and restarting has been implemented. Alternatively, a newly developed placement head incorporating cut and refeed, mounted to a 6-axis robot, offers the potential of improved path placement efficiency. The two techniques are used to produce similar grid composite stiffeners to evaluate the effectiveness of producing the grid intersections. Rate of deposition of the two end effectors are compared and the quality of the associated grid stiffeners, and intersections, are determined through measurement of geometry, fiber volume fraction and void fraction. [ABSTRACT FROM AUTHOR]

Composite bonded structure is a prevalent portion of today's aircraft structure. Adequate bond integrity is a critical aspect of fabrication and service, especially since many of today's structural bonds are critical for flight safety. Over the last decade, non-destructive bond evaluation techniques have improved but still cannot detect a structurally weak bond that exhibits full adherend/adhesive contact. The result is that expensive and time-consuming structural proof testing continues to be required to verify bond integrity. The objective of this work is to investigate the feasibility of bondline integrity monitoring using piezoelectric sensors, embedded at different locations within the composite joint, and to assess the benefits of monitoring the thickness mode in addition to the radial mode. Experiments and analyses are performed on single lap shear composite joints, with and without embedded sensors, subjected to incrementally increasing tensile loads. The results indicate that the embedded piezoelectric sensors measure a change in the resonance in both the radial and thickness mode during incremental loading and that the thickness resonance shows enhanced sensitivity to impending failure. Thus, it is demonstrated that monitoring both modes of the piezoelectric sensor provides addition details for prognostic performance evaluation. [ABSTRACT FROM AUTHOR]

As an alternative to traditional composite sandwich panels, grid stiffened laminates using continuous fiber in the stiffeners have the potential to demonstrate high out-of-plane stiffness to weight and allow new opportunities related to high performance design. However, only limited results have been presented on the production of high aspect ratio grid stiffeners, and the majority of the previous efforts have required a significant investment in tooling. Thus, while promising, the traditional methods of producing composite grid stiffened structures require complex tooling to develop the geometry, position, and orientation of the stiffening ribs. This study demonstrates the use of direct digital manufacture to produce continuous fiber reinforced high aspect ratio composite grid stiffeners, from glass fiber/thermoplastic commingled tow feedstock, with radically reduced tooling requirements. Stiffener specimens with a height to width aspect ratio of 6:1 were produced by consolidating 80 layers of commingled tow onto a flat tooling plate using an automated continuous fiber extrusion system. The method of placement and consolidation is described, and process parameters are shown to enable the production of high-quality specimens without complex tooling. The quality through the height of printed stiffeners was evaluated by the experimental measurement of fiber volume fraction and void content and by optical microscopy. The results of this study suggest that the application of direct digital manufacture to the placement and consolidation of commingled tow for the fabrication of high aspect ratio grid stiffeners, without the need for tooling, can readily achieve fiber volume fractions greater than 50% and void fractions as low as 5%. [ABSTRACT FROM AUTHOR]

Sandwich panels are commonly used as structure, based on fiber reinforced composites, with the goal of high flexural stiffness and low mass. It is most common to separate two high performance composite facesheets with a low-density core, generally in the form of a foam or honeycomb. A recent concept has been to replace these traditional core materials with fiber reinforced truss-like structures, with the goal of further reducing mass. A system is described that can radically reduce the amount of tooling required for truss core sandwich panel manufacture. This system, which is a digital manufacturing platform for the extrusion of continuous fiber reinforced commingled glass fiber/PET tow, was developed to demonstrate the rigidization of composites both on, and off, a tool surface. Navtruss core panels were successfully manufactured using this digital manufacturing platform, without conventional tooling, and the resulting through thickness compression moduli and panel shear moduli were within 14.6% and 23% of the values baseline compression molded specimens. Thus, the results suggest that, with further development, complex truss core structures with performance approaching that of compression molded panels can be manufactured with radically reduced tooling requirements from high volume fraction, continuous fiber reinforced thermoplastic matrix composites. [ABSTRACT FROM AUTHOR]

The rail transportation sector is currently seeking to decrease greenhouse gas emissions by incorporating composite materials that can reduce the mass of vehicles. During early adoption of composites in the rail transportation industry, these materials have predominantly been applied to simple design geometries and lightly loaded structures, have been optimized only through modification of composite thickness and composite layer shape, and have only been constrained with respect to a single mechanical performance metric. This study investigates the use of finite element analysis software in the simulation of fiber-reinforced composite materials applied to, and optimized for, a complex and heavily loaded rail vehicle anchor bracket. The research assesses the applicability of optimization methodologies to a complex and heavily loaded structure and advances established practices by constraining the solution with respect to multiple design requirements: manufacturing, compliance, and failure criterion. The optimization process successfully developed a composite structure with a predicted mass reduction of 33% compared to an existing steel design, and simultaneously met compliance, manufacturing, and failure criteria constraints. [ABSTRACT FROM AUTHOR]

During the development of digitally manufactured, commingled tow continuous fiber reinforced composites, consolidation force was controlled using a controlled spring force that yielded a repeatable tow width. However, the use of the extruder face to consolidate the material requires that the extruder remain perpendicular to the placement surface throughout the process. When considering more complex tool contours including sloped surfaces, more than three axes of motion are necessary to maintain the perpendicularity of the extruder tip to the surface. In this effort, a five-axis system is developed and used to demonstrate the ability to consolidate over complex contours. In addition, the nozzle face temperatures required for good consolidation and wetout result in poor tow path fidelity when complex paths are introduced. The implementation of an automated, computer-controlled localized cooling system enables both good wetout and consolidation while also enabling more accurate changes in tow path due to improvements in local tow tack. With the development of the five-axis system it is also shown that the tow width can be adjusted by rotating the existing placement nozzle to angles not equal to 90°. Thus, through a combination of controlled localized cooling and real-time control of the nozzle angle, a possible approach to control of tow width, independent of the tow placement angle and radius of curvature during tow steering, is described. [ABSTRACT FROM AUTHOR]

The present article has reported the effects of several nanofiller’s aspect ratio, length and interfacial strength on Mode-I fracture toughness (KIC) of geopolymer as the matrix of continuous fibre reinforced composites. These nanofillers have been chosen based on the variations in the surface chemistry and nature of interfacial bonding with geopolymer, which include Carbon, Alumina and Silicon carbide. Geopolymer matrix was subjected to the addition of single volume fraction, 2% of each type of nanofiller with two aspect ratios, designated as nanoparticles and nanofibers. Notched beam flexure tests (SEVNB) of neat and each nanofiller reinforced samples suggest that, while baselineKICof neat geopolymer improved with heat treatment, nanofibers with high interfacial bond strength showed maximum capability in further improvingKIC. Among those nanofibers, 2 vol% Silicon Carbide Whisker (SCW) showed the largest improvement inKICof geopolymer, which is ~164%. After heat treatment at 650 °C, SCW reinforcement was also found to be effective, with only ~28% lower than the reinforcing performance at 250 °C, while the performance of Alumina Nanofiber reinforced geopolymer notably reduced. SEM and EDS analysis suggested that the inhomogeneity in neat geopolymer and length of nanofibers control the reinforcing capability as well as crack propagation resistance of geopolymer. For instance, minimum length of nanofillers to toughen this geopolymer at 250 °C was required as ~2 μm. The results further suggested that the sample failure occurred due to the dominance of tensile failure of nanofibers over the interfacial separation. [ABSTRACT FROM AUTHOR]

Resin transfer molding of an inorganic polymer binder was successfully demonstrated in the preparation of ceramic fiber reinforced engine exhaust valves. Unfortunately, in the preliminary processing trials, the resulting composite valves were too brittle for in-engine evaluation. To address this limited toughness, the effectiveness of a modified fiber-matrix interface is investigated through the use of carbon as a model material fiber coating. After sequential heat treatments composites molded from uncoated and carbon-coated fibers are compared using room temperature 3-point bend testing. Carbon-coated Nextel fiber reinforced geopolymer composites demonstrated a 50% improvement in strength, versus that of the uncoated fiber reinforced composites, after the 250 °C postcure. [ABSTRACT FROM AUTHOR]

Direct digital manufacturing of continuous fiber-reinforced thermoplastics exhibits the potential to relieve many of the constraints placed on the current design and manufacture of composite structures. At present, the additive manufacturing of continuous fiber-reinforced thermoplastics is demonstrated to varying extents; however, a comprehensive investigation of manufacturing defects and the quality of additively manufactured high fiber volume fraction continuous fiber-reinforced thermoplastic composites is limited. Considering the preliminary nature of the additive manufacturing of continuous fiber-reinforced thermoplastics, composites processed in this manner are typically subject to various manufacturing defects, including excessive void content in the thermoplastic matrix. Generally, quality evaluation of processed composites in the literature is limited to test methods that are largely influenced by the properties of the continuous fiber reinforcement, and as such, defects in the thermoplastic matrix are usually less impactful on the results and are often overlooked. Hardware to facilitate the direct digital manufacturing of continuous fiber-reinforced thermoplastic matrix composites was developed, and specimens were successfully processed with intentionally varied void content. The quality of the additively manufactured specimens was then evaluated in terms of the measured maximum storage modulus, maximum loss modulus, damping factor and the glass transition temperature by means of dynamic mechanical analysis (DMA). DMA allows for thermomechanical (i.e., highly matrix sensitive) evaluation of the composite specimens, specifically in terms of the measured elastic storage modulus, viscous loss modulus, damping factor and the glass transition temperature. Within the tested range of void contents from roughly 4–10%, evaluation by DMA resulted in a distinct reduction in the maximum measured storage modulus, maximum loss modulus and glass transition temperature with increasing void content, while the damping factor increased. Thus, the results of this work, which focused on the effect of void content on DMA measured properties, have demonstrated that DMA exhibits multi-faceted sensitivity to the presence of voids in the additively manufactured continuous fiber-reinforced thermoplastic specimens. [ABSTRACT FROM AUTHOR]

This study proposes a novel methodology to combine topology optimization and ply draping simulation to partition composite structures, improve structural performance, select materials, and enable more accurate representations of cost- and weight-efficient manufacturable designs. The proposed methodology is applied to a structure as a case study to verify that the methodology is effective. One design concept is created by subjecting the structure to a kinematic ply draping simulation to inform the partitioning of the structure, improve drapability and performance, and reduce structural defects. A second design concept is created that assumes that plies are draped over the entire structural geometry, forming an integral design. The two design concepts' topologies are subsequently optimized to specify ideal material and ply geometries to minimize mass and reduce costs. The results indicate that the partitioned structure has a 19% lower mass and 15% lower material costs than the integral design. The two designs produced with the new methodology are also compared against two control designs created to emulate previously published methodologies that have not incorporated ply draping simulations. This demonstrates that neglecting the effects of ply draping produces topology optimization solutions that under-predict the mass of a structure by 26% and costs by 38%. [ABSTRACT FROM AUTHOR]

The addition of vertically oriented shear spikes (fiberglass reinforced polymer rods) was shown to increase the effective stiffness of the stringers of a full-scale timber bridge chord specimen. Results found from the flexural load testing of a full-scale timber bridge chord laboratory specimen are presented. Reinforcement was provided with 19 mm diameter shear spikes bonded to the wood by an epoxy resin. The bridge chord specimen was intentionally damaged to simulate degradation. Shear spikes were then installed from the top of the member into predrilled holes to provide horizontal shear resistance and to improve the flexural effective stiffness. Results from the testing showed that with the insertion of five sets of shear spikes the average flexural effective stiffness recovered in the four stringers of the chord was 91.6%. [ABSTRACT FROM AUTHOR]

Understanding viscoelastic properties of composite materials is essential for the design and analysis of many advanced structures. However, experimental viscoelastic characterization of anisotropic materials can be complicated because of the number of independent parameters to be evaluated. Recently, an approach leading to the 3-D viscoelastic characterization of transversely isotropic materials using a reduced number of measured parameters has been developed. The model reduces the number of independent parameters to describe the viscoelastic behavior of transversely isotropic materials, which greatly simplifies the experimental procedures. Based on this recently developed model, the present article evaluates time and temperature effects on the viscoelastic properties of a fiber reinforced lamina. The experimental investigation is conducted on sub-scale specimens loaded in flexure, using Dynamic Mechanical Analysis (DMA) equipment. Ultimately, the approach presented in this work will allow the construction of master curves for the independent viscoelastic parameters that characterize a transversely isotropic material. Therefore, the experimental technique presented in this work provides a means for the study of viscoelastic properties of fiber reinforced composites, and constitutes a valuable contribution to the understanding of time and temperature dependence of these mechanical properties. [ABSTRACT FROM AUTHOR]