Fibre-reinforced polymer composites have many useful properties, but their big drawback is they are typically complex and expensive to manufacture. In recent years, three-dimensional (3D) printing of composites has been successfully demonstrated using thermoplastic polymers and discontinuous fillers, but the resulting 3D-printed composites often have poor mechanical properties and low service temperature due to the limitations of the constituent properties. Consequently, 3D printing of composites using continuous carbon fibres and thermosetting polymers is expected to offer exceptional mechanical properties and thermal stability as well as featured design flexibility, low cost, reliability, and repeatability. However, no additive manufacturing technique has ever been reported to process continuous carbon fibres and thermosetting polymers for direct 3D printing of the finished composite.

Now, a team of engineers from the University of Delaware has developed a 3D printing technology that enables low-cost, flexible production of items made of fibre-reinforced polymer composites using continuous carbon fibres and thermosetting polymers. Their results were recently published in the journal Matter.

This is believed to be the first time anyone has achieved such 3D printing of continuous carbon fibre and thermosetting composite

Continuous carbon fibres and thermosetting resins are very important to make strong and lightweight composites, and they are widely used in many applications, such as aerospace, automotive, and sports products,” said Kun (Kelvin) Fu, “3D printing could reduce labour and tooling cost, and fabricate composite in a more energy-efficient, rapid, and reliable way with minimum defects.

The team developed an approach called localised in-plane thermal assisted (LITA) 3D printing, which allows the user to control the thickness and degree of curing of liquid polymer that solidifies into the desired shape.

In LITA 3D printing, the researchers carefully manipulate the temperature of the carbon fibers, aiding the flow of liquid polymers into channels between the carbon fibers. Then, the polymers are cured, solidifying into a three-dimensional structure. No post-curing is needed in LITA 3D printing, which could save a large amount of energy compared to the conventionally fabricated composites requiring tens of hours of post-curing.

The team developed a robotic system that includes a unique printing head and automated robot arm. This customized 3D printer allows the group to print a variety of shapes and structures.

LITA 3D printing could provide many industries with a rapid, energy-efficient method to make composite components in a variety of shapes using a variety of combinations of polymers and fibers.

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TuFF was patented in June 2020 with 32 claims. According to the U.S. Patent Office, the claim(s) within a patent application clearly define the invention, its scope and what aspects are legally enforceable.

TuFF represents a paradigm shift in composites design and opens the door for composites to replace metals in a variety of applications in the automotive, aerospace, infrastructure, electronics industries and more. Many common products, from kitchen appliances to smartphones and more, are now made with stamped sheet metal, and manufacturers might someday use TuFF instead.

TuFF is a low cost, can be made quickly, and is recyclable. Instead of expecting the metal manufacturers to redesign metal parts like aeroplanes, we decided to create a new material that can be designed and processed like metals using their existing manufacturing equipment – while still providing 40-70% weight savings

Jack Gillespie, director of CCM

While transforming existing industries, TuFF could enable the development of new products, such as flying cars, said John Tierney, senior scientist at CCM. “For urban air mobility, you need aerospace performance at automotive rates, which is exactly what TuFF provides,” he said.

Researchers at CCM started working on TuFF in 2016 when they received a $14.9 million, three-year cooperative agreement from the Defense Advanced Research Projects Agency (DARPA) for the Tailorable Feedstock and Forming (TuFF) Program. The objective of the TuFF program was to develop new composite materials with properties equivalent to previously used materials and develop a single-step manufacturing process that enables the use of the advanced materials for small parts weighing less than 20 pounds at costs competitive with aluminium. The project also included CCM faculty alumni collaborators at Clemson, Drexel and Virginia Tech universities.

About four decades ago, scientists theorised that if they could align these short carbon fibres precisely, they could make composites with desirable properties, but no one achieved this feat in practice until now. It took a few years, but after trying several different alignment mechanisms, the team at CCM figured out how to bring everything in line. The process can now use any type of fibre (or combinations) with nearly all polymers (thermoplastics and thermosets).

The Composites Centre has established a semi-automated pilot plant incorporating new control systems and inline sensors for quality control. TuFF product forms range from 20-inch wide rolls, tailored blanks for forming parts and narrow and steerable tapes for additive manufacturing processes. The team has demonstrated the feasibility and scalability of novel technologies developed through this program and are looking to supply TuFF material to designated industry partners for evaluation, prototype development and scale-up.

Researchers are now conducting additional experiments, including modelling and simulation, to further understand the behaviour of TuFF so that they can tailor it for more applications.

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The three day course titled Advanced Manufacturing Composites will run from August 12–14th and is co-hosted by the Delaware Valley Industrial Resource Centre. On the 12–13th Stefan Kercher, head of the Materials and Process Technology Laboratory at BMW Group Munich, will present a two-day course, “Composites at BMW”, while the final day is devoted to hands-on laboratory experiences for students.

The composites at BMW course will detail BMW’s project i program and the background behind it including the theoretical background of the materials used in composites at BMW, coupon-level testing and crash concepts and energy absorption of composite structures.

Topics to be covered on the third day include chemical and mechanical materials characterisation, thermoforming of thermoplastics and liquid moulding processing. The event is open to UD students and current members of CCM’s Industrial Consortium.

Founded in 1974, CCM conducts basic and applied research, educates scientists and engineers, and develops and transitions technology. Since 1985, CCM has been designated a centre of excellence through seven programs.

The centre has some 250 affiliated personnel, more than $12 million in annual expenditures, and over 2,000 alumni worldwide. More than 3,500 companies have benefited from affiliation with CCM over the past three decades.

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Chou followed his adviser’s suggestion and went on to become a pioneer in advanced composites, working over the years with a wide variety of materials and processes. Almost five decades later, he is still on the hunt for innovations that will make advanced composites more affordable, reliable and functional.

His latest breakthrough builds on work he did in the 1980s and ’90s on textile structural composites. This technology applies braiding, weaving, knitting and stitching techniques to produce 3D reinforcements, which are then combined with a binder, or matrix, to make complex shapes.

While textile structural composites offer such advantages as structural integrity, damage tolerance and cost-effectiveness, some fundamental technological barriers remain in their manufacture, which can lead to inconsistencies in performance.

Now Chou, Pierre S. du Pont Chair of Engineering at the University of Delaware, is part of an international team of researchers that is examining the feasibility of using additive manufacturing to produce 3D preforms.

Their work is documented in a paper published in the web version of Materials Today on May 23.

Additive manufacturing, also broadly known as “rapid prototyping” and “freeform fabrication,” is a process in which an object is built up layer by layer from a computerised model. The technique enables direct fabrication of complex-shaped objects without tooling and machining, and it eliminates the need to join a number of single parts into a single complex one.

In traditional processes, complex parts are usually built by assembling separate simple parts, which can lead to premature structural failure at material joints.

Another advantage of this technology is that material composition can be changed at specified locations within a part at the processing stage, enabling various functions and graded properties to be incorporated directly during manufacturing.

The process also shortens lead time and makes small-lot-size customisation — even a run of just a single part — economical.

Finally, in additive manufacturing, the material is placed just where it is needed, and the residual material can often be readily recycled or reused, reducing material waste.

“All of these features make additive manufacturing an attractive option for composite materials development,” Chou says.

The paper reviews the state of the art within the scope of composites development and discusses challenges facing the broad adoption of additive manufacturing for directionally reinforced composites processing.

Those challenges include the need for new CAD tools and engineering standards, difficulties in process monitoring, and limitations in part size, printing accuracy, layer thickness, and surface smoothness.

Despite these limitations, Chou sees great potential in additive manufacturing of fibre-reinforced preforms, which, he says, are especially desirable for composite parts in aerospace and biomedical applications.

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Both light and strong, carbon nanotubes (CNTs) are known as a revolutionary material with excellent mechanical, electrical and thermal properties. Continuous CNT fibres are one-dimensional assemblies of CNTs that show potential to retain the superb properties of individual CNTs on a macroscopic scale. They belong to a new class of nano-structured materials with potential applications in electronics, sensing and conducting wires.

Motivated by their high electrical conductivity and ability to kink without cracking, Tsu-Wei Chou, Pierre S. du Pont Chair of Engineering, and his research team used CNT fibres to fabricate a stretchable conductor. The result was a CNT fibre/PDMS flexible composite that can be subjected to repeated stretching-and-releasing cycles up to a prestrain level of 40 percent with little variation in electrical resistance.

According to the paper’s lead author Mei Zu, a visiting student from Tongji University in Shanghai, China, these findings demonstrate the potential of these flexible CNT fibres to be used as reinforcements for ultra-light weight multifunctional composites.

Under Chou’s guidance, Zu spent two years exploring the electrical and mechanical behaviour of CNT-based fibres and composites as a doctoral student in UD’s Department of Mechanical Engineering and at the Centre for Composite Materials.

This research was supported in part by the U.S. Air Force Office of Scientific Research and the National Research Foundation of Korea through the Global Research Laboratory program.

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Cork, known for its use in such low-tech applications as wine bottle stoppers and bulletin boards, now shows promise as the core material in composite sandwich structures for use in high-tech automotive, aircraft and energy applications.

A research team at the University of Delaware is investigating this natural material as an environmentally friendly solution for quiet sandwich composites. They recently published a paper on the work in Scientific Reports, an online, open-access research publication from the publishers of Nature that covers all areas of the natural sciences.

Jonghwan Suhr, assistant professor in the Department of Mechanical Engineering and an affiliated faculty member in the Center for Composite Materials says;

Cork is a natural product with intriguing properties it’s energy absorbing, tough, lightweight and impact resistant, and it has excellent vibrational and acoustic damping properties. Its unique cellular arrangement also results in good thermal properties, and it’s impermeable to moisture.

Suhr was adviser to the lead author on the paper, James Sargianis, who completed his master’s degree in mechanical engineering at UD in May. The third member of the team was Hyung-ick Kim, a postdoctoral researcher at CCM who is an expert in mechanical characterization of advanced materials.

Sargianis’s graduate research focused on exploring natural material-based sandwich composites with enhanced noise mitigation. Cork turned out to be one of the most promising alternatives to traditional sandwich structures.

Suhr explains that composite sandwich structures — typically made from synthetic foam cores or honeycomb materials bonded to carbon-epoxy face sheets — are commonly used in aerospace applications because they offer high bending stiffness and are very lightweight. However, he says, they are also good at radiating noise, which is not a desirable feature in an airplane. The current solution is to line the interior with four to six inches of glass fabric, but this increases weight and reduces space inside the cabin.

Enter cork as the new core for the sandwich. In the recently reported study, the researchers compared sandwich structures made from a natural cork agglomerate core with those using a core made from a high-quality synthetic foam called Rohacell. Carbon-epoxy was used as the face sheet material with both cores.

We achieved a 250 percent improvement in damping performance using the cork-based materials, with no sacrifice in mechanical properties, further, cork radiates little to no noise and is inexpensive. It’s also sustainable and environmentally friendly because there are no carbon emissions associated with its production.

Since the paper was published in May, the team has been approached by Portugal-based Amorim, a world leader in the production of thermal and acoustic insulation materials based on natural raw cork.

Suhr sees the potential for application of cork-based sandwich structures in not only aircraft cabins but also car engine mounts, launch vehicle fairings, and wind turbine blades.

In the next phase of the project, the team will investigate the low-velocity impact of these materials.

Sargianis, who also earned his bachelor’s degree at UD, in 2010, has accepted a position with the Naval Air Systems Command (NAVAIR) in Lakehurst, N.J. He took first-place honors at the 2012 SAMPE National Student Research Symposium in May for his work on natural material based-sandwich composites.

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