Fabrication, characterization and mechanical testing of carbon fiber sandwich composites with nanoparticle included polyurethane adhesives

In honeycomb core and composite face sheet sandwich panels, it is essential to understand the bonding characteristics of adhesive in relevance with its properties to observe synergistic effects of reinforcing nanoparticles such as multi-walled carbon nanotubes (MWCNTs). This study investigates the effects of MWCNT inclusion on polyurethane (PU) adhesive, which directly affects sandwich structures' structural and mechanical performance. MWCNTs are added to PU adhesive up to 0.2%, and their RAMAN spectroscopic analysis, Fourier transform infrared spectroscopy (FT-IR), thermo-gravimetric analyses (TGA) and differential-scanning calorimetry analyses (DSC) are evaluated. Aluminum honeycomb carbon-fiber-reinforced composite (CFRC) sandwich panels are fabricated using an out-of-autoclave manufacturing process. Carbon-fiber prepreg is used for top/bottom face sheets. Mechanical strength of face/core bonding evaluated as a function of MWCNT addition and core cell sizes. Manufactured sandwich composite structures are investigated for flat-wise tensile strength and three-point bending strength. Results show that MWCNT reinforcement to PU adhesive and lower cell size increases bending and flat-wise tensile resistances.

Ultrasonic Evaluation of Porosity in Out-of-Autoclave Carbon Fiber–Reinforced Polymer Composite Material

Ultrasonic longitudinal wave propagation is studied in out-of-autoclave (OoA) carbon fiber–reinforced polymer composite material with varying levels of porosity contents. A combination of cure pressures and a solvent is used to produce specimens with void contents in the range of 0% to 22%. Ultrasonic measurements are made in through-transmission mode, and the data is processed to study various aspects of wave interaction with porosity in OoA specimens. The specimens with a wide range of void contents have enabled the study of broader trends of ultrasonic center frequency, wave velocity, and attenuation with respect to porosity. Results show ultrasonic center frequency and wave velocity are decreased linearly as the void content increases. The relationship of ultrasonic wave attenuation can be approximated by a logarithmic relationship when considering the full range of void content studied. Strength measurements of specimens with varying void contents are made using the flatwise tensile (FWT) test. It is observed that the strength rapidly decreases with increasing porosity. Correlations made between FWT strength, ultrasonic wave velocity, and attenuation are best described by logarithmic relationships. The data shows a potential for inferring strength knockdowns due to the presence of porosity based on ultrasonic measurements.

Carbon Fiber Reinforced Polymers

The current demand for lightweight and high-performance structures leads to increasing applications of carbon fiber reinforced polymers, which is also made possible by novel production methods, automation with repeatable quality, the reduced cost of carbon fibers, out of autoclave processes such as resin transfer molding and resin infusion technologies, the re-use of waste fibers, development in preform technology, high-performance, fast-curing resins, etc [...]

Powder Epoxy for One-Shot Cure, Out-of-Autoclave Applications: Lap Shear Strength and Z-Pinning Study

Large composite structures manufactured out-of-autoclave require the assembly and bonding of multiple parts. A one-shot cure manufacturing method is demonstrated using powder epoxy. Lap shear plates were manufactured from powder epoxy and glass fiber-reinforced plastic with four different bonding cases were assessed: secondary bonding using standard adhesive film, secondary bonding using powder epoxy, co-curing, and co-curing plus a novel Z-pinning method. This work investigates the lap shear strength of the four cases in accordance with ISO 4587:2003. Damage mechanisms and fracture behavior were explored using digital image correlation (DIC) and scanning electron microscopy (SEM), respectively. VTFA400 adhesive had a load at break 24.8% lower than secondary bonding using powder epoxy. Co-curing increased the load at break by 7.8% compared to powder epoxy secondary bonding, with the co-cured and pinned joint resulting in a 45.4% increase. In the co-cured and co-cured plus pinned cases, DIC indicated premature failure due to resin spew. SEM indicated shear failure of resin areas and a large amount of fiber pullout in both these cases, with pinning delaying fracture phenomena resulting in increased lap joint strength. This highlights the potential of powder epoxy for the co-curing of large composite structures out-of-autoclave.

Conical Thermoplastic Composite Anisogrid Lattice Structure by Innovative Out-of-Autoclave Molding Process

A new manufacturing process for thermoplastic (TP) composite parts has been used to produce conical anisogrid composite lattice structure (ACLS). An out-of-autoclave (OOA) process has been prototyped by using the compaction exerted by a heat-shrink tube after its exposition to heat in oven. Narrow thermoplastic prepreg tapes have been wounded on a metallic conical patterned mold at room temperature; then, the conical structure has been inserted in the heat-shrink tube and heated. TP unidirectional prepreg tapes have been used with polypropylene matrix and glass fibers. After molding, the TP ACLS has been tested under axial and transverse compression. Conical adapters were used in the transverse loading condition to allow uniform application of the load. Density measurement has been also performed to assess the quality of the OOA process. Results of this study show that TP ACLS with complex shape may be produced with OOA solutions without affecting mechanical performance. In fact, porosity levels of the consolidate ACLS are comparable with the initial prepreg despite of the absence of vacuum during molding. Moreover, high compressive stiffness was measured along both directions without observing damages, buckling or cracks in multiple tests. In the future, this kind of technology could be used for larger ACLSs by substituting the heat-shrink tube with a narrow tape to be wound as well after lamination.

A Review on Fabrication of Thermoset Prepreg Composites using Out-of-Autoclave Technology

Autoclave is the technology that has been extensively used to manufacture high-grade performance composite parts for aerospace applications. This technology has been limited to aerospace industries only, primarily due to its high cost in manufacturing parts. The researchers then considered an alternative approach “Out-of-autoclave” (OOA) process, aiming at cost and time optimization. Non-autoclave methods such as OOA cure processes have been developed lately. The OOA process has a high potential for a drastic cost reduction in the manufacturing of composite aerospace structures. It processes parts that have a quality similar to that of parts cured using the autoclave technology. Specially designed OOA prepregs are available in the market for OOA processing, some of which are certified for aerospace manufacturing. This review paper briefly focuses on OOA prepregs and OOA processes that are used for aerospace components manufacturing. Future areas of development in the aerospace sector based on cost optimization and faster cycle times are also discussed in this paper.