Voids, the most studied type of manufacturing defects, form very often in processing of fiber-reinforced composites. Due to their considerable influence on physical and thermomechanical properties ...

https://journals.sagepub.com/doi/pdf/10.1177/0021998318772152

Voids in fiber-reinforced polymer composites: A review on their formation, characteristics, and effects on mechanical performance:

3D printed continuous CF/PA6 composites: Effect of microscopic voids on mechanical performance

Several consolidation cycles were performed to study the impact of processing parameters on void content, ultrasonic NDT and mechanical performance of CF/PEEK composites. A series of [+45/−45]4S laminates were manufactured by out-of-autoclave technologies including vacuum bag only, hot-press, and automatic lay-up with in-situ consolidation. A range of porosity between 0 and 19% of void content was assessed by varying the processing temperature from 340 to 500 °C and pressure from 0.25 to 1.5 MPa. Void content, shape and location were characterized by using C-scan, matrix digestion and 2D microscopy. Mechanical characterization including in-plane shear and interlaminar shear testing was performed. The results show a critical void content where properties start to decrease at a given drop-off rate. The results suggest that each processing technology should have its own quality control criteria. The proposed methodology is useful for engineering applications of thermoplastic composites and the creation of NDT acceptance criteria.

Effect of processing parameters and void content on mechanical properties and NDI of thermoplastic composites

The ability of a modern near infra-red laser tape placement system to produce high-quality laminates is investigated by performing short beam strength tests on samples manufactured at different process temperatures from 400 °C to 600 °C at placement rates of 100 mm/s and 400 mm/s. The temperature history in tape placement is highly dynamic and the correlation between the process control temperature, laser power and the consolidation temperature is not well understood. The complete temperature history was therefore estimated with a previously developed optical-thermal model and validated using long wave infra-red imaging. Short beam strengths equivalent to conventional manufacturing methods were found for placement rates of 400 mm/s. Failure modes of the samples were elucidated by scanning electron microscopy of the fracture surfaces. Signs of degradation were observed on samples prepared with a 600 °C process temperature at 100 mm/s, however none was evidenced at 400 mm/s for the same process temperature.

The effect of processing temperature and placement rate on the short beam strength of carbon fibre-PEEK manufactured using a laser tape placement process

Poly (ether ether ketone) (PEEK) is a high-performance engineering thermoplastic polymer with potential for use in a variety of metal replacement applications due to its high strength to weight ratio. This combination of properties makes it an ideal material for use in the production of bespoke replacement parts for out-of-earth manufacturing purposes, in particular on the International Space Station (ISS). Additive manufacturing (AM) may be employed for the production of these parts, as it has enabled new fabrication pathways for articles with complex design considerations. However, AM of PEEK via fused filament fabrication (FFF) encounters significant challenges, mostly stemming from the semi crystalline nature of PEEK and its associated high melting temperature. This makes PEEK highly susceptible to changes in processing conditions which leads to a large reported variation in the literature on the final performance of PEEK. This has limited the adaption of FFF printing of PEEK in space applications where quality assurance and reproducibility are paramount. In recent years, several research studies have examined the effect of printing parameters on the performance of the 3D-printed PEEK parts. The aim of the current review is to provide comprehensive information in relation to the process-structure-property relationships in FFF 3D-printing of PEEK to provide a clear baseline to the research community and assesses its potential for space applications, including out-of-earth manufacturing.

Fused Filament Fabrication of PEEK: A Review of Process-Structure-Property Relationships

Obtaining autoclave-level mechanical properties using in-situ consolidation of thermoplastic composites by Automated Tape Placement (ATP) is challenging. However, relatively recent availability of high quality ATP grade pre-preg material and tape heads equipped with more efficient heat sources (e.g. lasers) offers an opportunity to achieve improved mechanical properties and deposition rates. In the present study, carbon fibre–PEEK laminates, manufactured by laser-assisted ATP (LATP) and autoclave, are compared. Analysis of the through-thickness temperature distribution during LATP processing using thermocouples indicates that LATP cooling rates are extremely rapid and suggests full through-thickness melting of the pre-preg tape may not occur. Inadequate crystallinity, in conjunction with voids, compromised mechanical properties compared to autoclaved laminates but was beneficial in terms of the toughness of LATP laminates. Optimisation of pre-preg properties and processing parameters is required to realise the full potential of the LATP process in terms of mechanical properties, energy requirements, cost and deposition rates.

Mechanical characterisation of carbon fibre–PEEK manufactured by laser-assisted automated-tape-placement and autoclave

Laser-assisted Automated Tape Placement (LATP) in-situ consolidation of thermoplastic composites has significant potential to produce repeatable and accurate components more efficiently than conventional processing methods. In addition, LATP can process Variable Angle Tow (VAT) laminates, that allow modification of load paths within the laminate to result in more favourable stress distributions and improved laminate performance. Previous studies have reported dry fibre placement and thermoset pre-preg tape placement processed VAT laminates. This work investigates the processing parameters required to produce VAT laminates from carbon fibre (CF)/polyether ether ketone (PEEK) tapes using LATP. Testing was carried out to examine the effect of lay-down speed and steering radii on defect frequency and bond strength. Measurements showed that the width and thickness of the CF/PEEK tapes changed as a result of steering. In addition, bond strength, determined using a novel wedge peel rig for steered tape, was found to be a function of lay-down speed.

A study of the influence of processing parameters on steering of carbon Fibre/PEEK tapes using laser-assisted tape placement

A comparative study is presented on the fracture toughness of carbon fiber/PEEK composites manufactured by autoclave and laser-assisted automated tape placement (LATP). Formation of a good inter-laminar bond is always a concern in ATP due to the short time available for intimate contact development and polymer healing, yet our double cantilever beam (DCB) tests reveal 60- 80% higher Mode I fracture toughness for the LATP processed specimens than for the autoclave processed specimens. This magnitude of difference was unexpected, so specimens were further examined via differential scanning calorimetry, dynamic mechanical analysis, nano-indentation, and scanning electron microscopy. The results indicate that the LATP process has been very effective in heating and consolidating the surface of plies, creating an excellent bond. However, it has been less effective in processing the interior of plies, where a low crystallinity and poor fiber-matrix bonding are evident. The higher fracture toughness of the LATP processed specimens is also not solely due to a better bond, but is partially due to significant plastic deformation in the interior of plies during the DCB test. The findings indicate there is still considerable scope for optimizing the laser-assisted ATP process, before the optimum balance between strength and toughness is achieved at favorable lay-down speeds. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41643.

Fracture toughness of carbon fiber/polyether ether ketone composites manufactured by autoclave and laser-assisted automated tape placement

Mechanical characterisation of carbon fibre–PEEK manufactured by laser-assisted automated-tape-placement and autoclave Part A Applied science and manufacturing

David Jones

Papers

Mechanical characterisation of carbon fibre–PEEK manufactured by laser-assisted automated-tape-placement and autoclave

A study of the influence of processing parameters on steering of carbon Fibre/PEEK tapes using laser-assisted tape placement

Fracture toughness of carbon fiber/polyether ether ketone composites manufactured by autoclave and laser-assisted automated tape placement

Mechanical characterisation of carbon fibre–PEEK manufactured by laser-assisted automated-tape-placement and autoclave Part A Applied science and manufacturing

Concurrent design and manufacture of a thermoplastic composite stiffener