A process for prestaging thermosetting towpreg

A method for prestaging thermosetting towpreg consisting of passing the material through a tunnel oven was developed to address the difficulties inherent in making thick-walled thermosetting composite parts: aging, moisture uptake, and low resin viscosity during lay-up; and residual stresses, warping, thermal degradation, resin gradients, and excessive void nucleation during the final cure. AS-4 carbon fiber in a 3501-6 epoxy matrix was used to test the continuous prestaging process. With prestaging, significant reductions in towpreg bulk, voids, and total enthalpy were achieved. Cure shrinkage also was achieved. The final towpreg product had good drape and greatly reduced tack. These changes addressed many of the aforementioned difficulties. A relation between residence time at 200°C (the process temperature) versus percent degree of cure (%DOC) was developed. Manufacturing characteristics such as drape, tack, and bulk were related to %DOC. A processing window of 15 to 22 %DOC yielded towpreg with optimal characteristics. Prestaged towpreg would be well suited to manufacturing with advanced fiber placement (AFP) type equipment. Initial trials in industry have been promising.     

Mechanical properties of parts manufactured from prestaged thermosetting towpreg

A prestaging process consisting of passing thermoset towpreg through a tunnel oven and roller apparatus was developed to address the difficulties inherent in making thick-walled thermosetting composite parts: aging, moisture uptake, and low resin viscosity during lay-up; and residual stresses, warpage, thermal degradation, and excessive void nucleation during the final cure. Unidirectional laminates were fabricated from prestaged and as-received AS4/3501-6 towpregs to understand the relationship between the parts' mechanical properties and the amount of prestaging (degree of cure) of the towpregs. In this manner, the feasibility of manufacturing with prestaged thermosetting towpreg was confirmed. Ultrasonic nondestructive evaluation showed that the parts manufactured from prestaged towpreg were uniform and relatively low in voids as compared to parts manufactured with as-received towpreg. This was confirmed with scanning electron microscopy. Using three-point bending tests, a drop in mechanical properties was found in parts manufactured from towpreg prestaged beyond 19% degree of cure. However, the parts were well consolidated and had predictable mechanical properties up to approximately 40% degree of cure. The equivalent limits on degree of cure from a companion processing study are between 15% and 22%.     

On-line consolidation mechanisms in thermoplastic filament winding

The on-line consolidation process, using a single roller, in thermoplastic filament winding (tape laying) has been investigated. A one-dimensional, semi-empirical, on-line consolidation process model is proposed that describes the deformation and flow phenomena. The model is used to predict the applied load necessary to achieve a desired fiber volume content of filament wound thermoplastic composite rings consolidated at various process conditions. Composite rings are fabricated from 50 layers of 12K carbon fiber tow APC-2 tape (6 mm wide) on a 178 mm diameter mandrel. It was found that the winding speed had statistically significant effects on the void content, the fiber volume fraction, and the final thickness; whereas the applied load had a statistically significant effect on the final thickness. These results show the effect of shear thinning on the viscoelastic response of the material, facilitating faster processing speeds. The results of the experiments compared well to the consolidation model's predictions.     

Processing parameters for consolidating PEEK/carbon fiber (APC-2) composites

This consolidation of unidirectional APC-2 was studied experimentally to determine the effects of processing parameters (pressure, temperature, and number of layers) on resin flow as measured by the Kozeny constant and the bulk modulus. The Kozeny constant obtained was between 0.0205 and 0.107. It was found to depend on the consolidation pressure, temperature, number of layers, and their interactions at a 99% confidence level. The value of the bulk modulus was found to be between 1.21 MPa (175 psi) and 8.58 MPa (1244 psi). It also was found to depend on the temperature, pressure, number of plies, and their interactions at a 99% confidence level. These results show the critical effects of fiber orientation on resin flow during processing.     

On-line consolidation of thermoplastic towpreg composites in filament winding

A study of the use of thermoplastic powder coated towpregs in filament winding was conducted. A suitable technique for manufacturing parts with adequate on-line consolidation was identified through an experimental investigation that compared several techniques. This process utilized a single compaction roller on-line consolidation head with hot air guns to obtain consolidation at the lay-down point on the mandrel. The addition of a heated pultrusion die between the creel system and the filament winder was found to produce the best process. Nylon 11/E-glass towpreg was used in this study. The effect of winding speed and compaction force on the quality of the resulting parts was determined in order to optimize the process. Only the speed was found to have a significant effect, with better quality parts being made at higher speed. Observations were also made on parts manufactured with polypropylene and E-glass towpreg, as well as on parts wound with helical patterns.     

Influence of Different Woven Geometry and Ply Effect in Woven Thermoplastic Composite Behaviour-Part 2

Abstract

Works on woven composite both thermoset and thermoplastic are numerous, however in most instances they involve the use of preimpregnated fabric. It is apparent that woven thermoplastic system has significant potential due to the combined properties such as better damage tolerence, recyclability, easy processing, storage, etc. Here work on woven thermoplastic composites based on Continuous Fiber Impregnated towpreg (COFIT) tape rather than conventional approach is reported. The influences of different woven geometry and ply effect were investigated. Correlations between different woven geometry and weave characteristics were also noted. In general, the woven composite properties are influenced by many process variables such as type of towpreg, woven geometry, number of plies etc.     

The effect of sizing on composite properties for materials used in the shuttle filament wound case

An epoxy resin and a sized carbon fiber have been used to produce a light-weight filament wound case for the Space Shuttle. The sizing facilitates fiber handling during winding but may affect the amount of resin absorbed by the fiber during impregnation and the final mechanical properties of the composite. Naval Ordnance Lab rings were wound to study the effect of the sizing content on the resin absorption by the fiber bundles, the final tensile properties of the composite, and the type of failure observed at burst. The resin content of the rings studied was between 20 to 40 percent, and the sizing content, 0 to 1.6 percent by weight. Results showed that the sizing content was a critical parameter which determined the amount of resin absorbed by the fibers. The final tensile strength was dependent on the amount of sizing present. The tensile strength decreased by as much, as 60 percent when a low resin and high sizing content were present. The type of failure at burst was a function of resin content rather than size content.     

Influence of geometrical features of electrospun nylon 6,6 interleave on the CFRP laminates mechanical properties

Twenty plies, woven carbon fiber/epoxy matrix, virgin, and Nylon 6,6 electrospun nanofiber interleaved composite laminate beams are tested under Modes I and II fracture mechanic to assess the effect of nanointerleave on the global behavior of the laminate. Aim of the work is to study the effect of geometrical features of the nanoreinforce on the mechanical response of a laminate subjected to fracture mechanic loads. Nanofiber orientation, nanolayer thickness, and nanofiber diameter have been considered. From the experiments, mechanical parameters have been extracted and used to compare the configurations. The work highlights the importance of an accurate choice of the nanointerleave and showed that thick interleaves may weaken the interface if a high number of voids is left after the curing process, the smaller is the fiber diameter the higher is the energy absorbing capability of the specimens, and that fiber orientation has different effect depending on the loading mode. POLYM. COMPOS., 35:137–150, 2014. © 2013 Society of Plastics Engineers     

A machine system for the rapid production of composite structures

A rapid manufacturing process for thermosetting materials was developed to produce composite structures in a manner similar to rapid prototyping. The machine system builds the part layer by layer with continuous curing and consolidation, making traditional curing methods unnecessary. Each part layer, which has been prestaged to provide a desired degree of cure, moves under a heat gun and a roller for curing and consolidation. The heat gun initiates the cure in the part layer, partially curing it and making it tacky enough for the next layer to stick to it when it goes through the consolidation process. The cure continues to advance as each layer is placed. Testing was performed by fabricating three‐layer parts and performing tests on single strips of towpreg to determine how temperature and time affect the degree of cure. The material was IM7 carbon fiber/977‐3 epoxy towpreg, prestaged in a tunnel oven to an approximate 30% degree of cure. Hot air temperatures of 150 – 380°C (302 – 716°F), roller forces of 236 – 472 N (53 – 106 Ibf), table speeds of 13 – 38 mm/sec (0.5 – 1.5 in/sec), and times of exposure to hot air of 5, 10, and 20 seconds were used. The results showed that the machine system can continuously cure and consolidate parts. The cure was advanced to any level up to 100% degree of cure with an air temperature of 380°C. Parts were produced with layers well stuck together. Within the limitations of the experiments, temperature was determined to be the most significant factor affecting degree of cure. With the correct processing conditions, the machine system has the capability to produce well‐consolidated, high quality parts without using traditional curing methods. The design of the machine system allows for the addition of components for the fabrication of arbitrarily shaped parts and for different curing methods.     

Resin transfer molding with powder-coated preforms

Incomplete fiber wetting in a resin transfer molded composite may result in poor surface finish, high void content, and reduced mechanical properties. This work studied the use of tows that are precoated with a powdered version of the liquid molding resin (towpregs). The goal is to see if such preforms improve the final part properties because of better fiber wetting. Hercules 12K AS-4 fibers and PR500 (liquid) and PS500 (Powder) resins (3M) were used to make fabrics from towpregs containing 50 wt% total resin (liquid and powder combined). The powder fractions were 0, 13, 21, 50 wt%. Samples were resin transfer molded from preforms made from the towpreg fabrics. Results showed that samples molded with powder-coated preforms had improved surface finishes and reduced void contents (1.4 vs. 5%), but that the mechanical properties were not improved (transverse moduli of ∼ 7.8 Gpa and axial moduli of ∼ 100 Gpa), probably because of defects inherent in the hand-woven towpreg fabric that was used.     

Isotactic polypropylene hollow microfibers prepared by CO2 laser‐thinning

An isotactic polypropylene hollow microfiber was continuously produced by using a carbon dioxide (CO₂) laser‐thinning method. To prepare the hollow microfiber continuously, the apparatus used for the thinning of the solid fiber was improved so that the laser can circularly irradiate to the hollow fiber. Original hollow fiber with an outside diameter (OD) of 450 μm and an internal diameter (ID) of 250 μm was spun by using a melt spinning machine with a specially designed spinneret to produce the hollow fiber. An as‐spun hollow fiber was laser‐heated under various conditions, and the OD and the ID decreased with increasing the winding speed. For example, when the hollow microfiber obtained by irradiating the CO₂ laser to the original hollow fiber supplied at 0.30 m min^?1 was wound up at 800 m min^?1, the obtained hollow microfiber had an OD of 6.3 μm and an ID of 2.2 μm. The draw ratio calculated from the supplying and the winding speeds was 2667‐fold. The hollow microfibers obtained under various conditions had the hollowness in the range of 20–30%. The wide‐angle X‐ray diffraction patterns of the hollow microfibers showed the existence of the highly oriented crystallites. Further, the OD and ID decreased, and the hollowness increased by drawing hollow microfiber obtained with the laser‐thinning. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2600–2607, 2006     

Statistical properties of interparticle/void distance for amorphous plastics toughening

Statistical properties of interparticle/void distance (ID) for various particle/void and dispersion types are studied in relation with toughening of plastics using computer‐generated three‐dimensional models. Particle/void size groups adopted were either of constant diameter or of log‐normal distribution. Particles/voids were dispersed at uniform‐random or flocculated with multiple clusters. It was found that IDs are (a) of approximately Gaussian distribution for particles/voids of either a constant diameter or a log‐normal distribution, when they are dispersed at uniform‐random, but (b) not of Gaussian distribution for particle/void sizes of bimodal log‐normal distribution, nor for flocculated log‐normal distribution of particles/voids dispersed with multiple clusters. It was also found that the degree of ID uniformity for a single group of log‐normally sized particles/voids is not sensitive to standard deviation of particle/void size. Mixing effect on ID properties using two groups of log‐normally distributed particles/voids with similar mean particle/void diameters was simulated. It was found that, when a significant amount (36 vol %) of particles/voids of a small mean and standard deviation of ID was mixed with a group of particles/voids of a large mean and standard deviation of ID, mean and standard deviation of ID for the mixture were not substantially lower than those of the group of particles/voids of the large mean and standard deviation of ID. It was also found that the degree of ID uniformity for the mixture of the two groups was lower than those of individual groups, indicating that the mixing has deleterious effect on toughening. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4256–4262, 2006     

Effect of winding tension and cure schedule on residual stresses in radially-thick fiber composite rings

In filament winding, both the tension applied to the fiber strands and the subsequent cure schedule are important manufacturing variables that produce residual stresses and strains. A “radial cut method” was used to determine the circumferential and radial residual stress profiles in S2-glass fiber/epoxy and E/XA-A carbon fiber/epoxy circumferentially-wound composite rings. Of particular interest in this study were radially-thick composite rings in which the effect of cure schedules as well as constant and variable winding tension profiles on the radial residual stress profiles were evaluated. Several design-oriented relationships for estimating the maximum radial residual stress were developed.     

Mechanical properties of carbon fiber/epoxy composites: Effects of number of plies,fiber contents,and angle‐ply layers

Multi‐axial multi‐ply fabric (MMF) composites are becoming increasingly popular as reinforcing materials in high‐performance composites due to their high mechanical properties. This work aimed to study the effects of three variable parameters including fiber contents, numbers of plies, and layer orientations on the mechanical properties of MMF composites. Unidirectional carbon fibers and a two‐part epoxy resin were employed to produce the composite laminates using the manual lay‐up process. It was found that the mechanical properties of composites made with 5‐ply were slightly greater than 3‐ply composites. However, there was no highly significant difference between them. Generally, the angle‐ply of the composites showed the greatest effect on the mechanical properties compared with number of plies and layer orientations. The significant improvements in mechanical properties of the composites were further supported using scanning electron microscopy (SEM). Morphologies of the tensile fracture surfaces of composites revealed that the presence of fiber pulled out results in the creation of voids between the fibers and matrix polymer. This causes the mechanical properties of the composites to be reduced. Finally, the enhancement of mechanical properties of composites clearly confirmed that angle‐ply layer (0°,?35°,0°,+35°,0°) had the most significant reinforcing effect among other parameters evaluated. POLYM. ENG. SCI., 54:2676–2682, 2014. © 2013 Society of Plastics Engineers     

Effect of Thickness of the Intergranular Film on Fracture in Si3N4

Molecular dynamics computer simulations were used to study the fracture behavior of silica intergranular films (IGFs) between silicon nitride crystals as a function of film thickness. Results showed a significant increase in fracture stress with decreasing IGF thickness. IGFs that are 2 nm thick fracture similarly to bulk silica glass, while the 1 nm IGF fractured at a much higher value. The simulations show bond rupture and rearrangement of siloxane rings that coalescence to form larger rings, or "voids." The delineating difference in the systems is the significantly larger concentration of six-membered rings in the 1 nm IGF in comparison to the 2 nm IGF or bulk silica glass. The Si–O bond is more stable in six-membered rings than other ring sizes. Rupture is more catastrophic in the 1 nm IGF than the other systems where stress decays more slowly with strain.     

Experimental investigation of flow through multi-layered preforms

This research investigates resin flow phenomena through preforms composed of multiple layers of reinforcement material. Two complete studies were performed: the first investigates the effect of varying the order of lay-up of a fixed number of plies, and the second studies the impact of varying the thickness of the individual layers of a thick preform. In each case, a set of compression experiments were first performed to characterize the preforms so that in-situ layer thicknesses can be calculated. Next, flow experiments were performed upon the preforms to determine the permeability and flow front behavior. In the first study, it was found that the weighted average scheme provides a reasonable estimate for the effective permeabilities of the preforms, with errors ranging from 14.2% to 23.8%. In the second study, however, the weighted average scheme was found to provide only a rudimentary estimate of the permeability, with an error that was seen to increase in magnitude during the course of each experiment. In addition, computer simulations were performed for the second study, which show the possibility of deriving transverse permeability values without direct measurement, although significant errors in inlet pressures were once again evident.     

Multilayered epoxy/glass fiber felt composites with excellently acoustical and thermal insulation properties

In this article, a series of epoxy composites consisting of multilayered ultra-fine glass fiber felts (GFFs) were produced by a hand lay-up process. The incorporation of GFFs greatly enhances the sound-absorption and sound-insulation properties of epoxy composites. It can be mainly attributed to great numbers of voids introduced into the matrix and the increasing interfacial area between glass fiber and epoxy resin, which is confirmed by scanning electron microscopy results. Furthermore, the thermal insulation performance of epoxy/glass fiber felt (EP/GFF) composites is continuously improved with the growing GFF layer, and meanwhile EP/GFF composites exhibit the satisfactory mechanical property. Such novel EP/GFF composites can serve as promising structural, heat-insulated, and soundproof materials in many multifunctional systems including buildings, aircrafts, constructions, vehicles, etc. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46935.     

Fiber coiling during bladder molding of thermoplastic composites

An investigation is presented into fiber movement during the consolidation of cylindrical, filament-wound preforms when pressure is applied internally (bladder molding). It is necessary to characterize the fiber movement to predict the final fiber orientation of successful parts as well as to predict which parts will not be successful. Unsuccessful parts have fibers that clump together in thick coils at the interior surface of a part, resulting in an uneven wall thickness and poor mechanical properties. Cylindrical preforms (∼7.5 cm in diameter) made from glass fiber (20 μm in diameter, ∼50 vol%) coated with polypropylene powder (powder-coated towpregs) were filament-wound in a range of angles, interweaving patterns, and diameters. Trellising, a mode of fiber movement in which the fibers rotate as if they are pinned at the interweaving points, occurs first as the preforms expand. If the preforms “trellis” as much as possible without reaching the mold wall, shearing between adjacent fibers must occur to complete the expansion. Coiling occurs when a large amount of shearing is necessary to provide the final expansion needed to bring the preform to the mold diameter. A dimensionless “coiling” number is presented that predicts coiling.     

Monitoring the reaction progress of a high‐performance phenylethynyl‐terminated poly(etherlmide). Part II: Advancement of glass transition temperature

The cure schedule for carbon fiber‐reinforced, phenylethynyl‐terminated Ultem™ (GE Plastics) composites was studied in an attempt to optimize the resultant glass transition temperature, T_g. Reaction progress and possible matrix degradation were monitored via the T_g. On the basis of previous research, matrix degradation induced T_g reduction was expected for increases in cure time or temperature beyond approximately 70 minutes at 350°C. Using the central composite design (CCD) of experiment technique, composite panels, neat resin, and polymer powder‐coated tow (towpreg) were cured following various cure schedules to allow for the measurement of the glass transition temperatures resulting fronm cure time and temperature variations. The towpreg and neat resin specimens were cured in a differential scanning calorimeter. The glass transition temperatures of all specimens were measured via differential scanning calorimetry; the composite glass transition temperatures were also measured with dynamic mechanical thermal analysis. The composite panels and towpreg specimens showed similar trends in T_g response to cure schedule variations. Composite and towpreg glass transition temperatures increased to a plateau with increasing cure time and temperature, whereas, the neat resin showed an optimal T_g followed by T_g reduction with increasing cure time and temperature. The optimal neat resin T_g occurred within a cure time and temperature significantly below that required to maximize the composite and towpreg glass transition temperatures.     

基于非测地线理论五坐标缠绕的实验研究

本文分析了4坐标缠绕对纤维缠绕构件机械性能和质量的影响,提出摆角坐标B轴的意义,并针对椭球压力容器这种典型的回转体芯模,研究了具体的各坐标机器路径。为了解决4坐标纤维缠绕过程中纤维在封头处的扭曲、堆积现象,进行了5坐标纤维缠绕实验研究。通过4坐标和5坐标的一个标准缠绕线型,分析和对比纱线的变化情况。实验表明,5坐标缠绕较好地解决了封头处纤维堆积问题。