Getting It Right with Composites

Bruce Morey

"Composite design and analysis is a highly integrated activity,” said Chris Gear, Chief Technology Officer & Senior Technical Fellow for GKN Aerospace. He noted that how composite material is placed, how it moves, how it cures, and the quality and conformance of the product are all interrelated. All of these factors are considered in the final release of the data for manufacturing, according to him, aided by design-for-manufacturing (DFM).

Complicating manufacturing optimization is the very nature of advanced composites, requiring a unique design process, unlike isotropic, homogenous metal. Controlling fiber orientation and number of layers of fiber embedded in a plastic matrix is vital for its performance. Initial CAD definitions that specify the outer and/or inner mold lines of the part require further definition of material type, fiber orientation, stackup order, balance, symmetry, drop-offs, splices, and darts.

“DFM is a very important aspect on any composite design, where the manufacturing process and materials used will drive the final design solution and are key to meeting our internal requirements on weight, costs, and robustness of product,” said Gear.

He explained that in the early stages of a design, GKN will use their own or a customer’s design methods for composites within GKN’s own CAE toolset. This is to ensure they characterize and simulate how the material will lay down into GKN’s double curvature tools, identifying “hot spots where extra care is needed in manufacturing and pinpoint where we need to validate an area that is beyond the limitations of our existing methods,” he said.

Composites Design, Composite Constraints

John O’Connor, Director, Product and Market Strategy for Siemens PLM, provider of the Fibersim tool for design with composites, noted that there are three areas where engineers can improve production rates for composites. One is to improve at the point of production itself, with faster machines or better tools. The second is asking how to modify a design for faster manufacturing.

“The third step is the furthest upstream and that is how to optimize the design for both its purpose, for example least weight and maximum strength, while incorporating manufacturing constraints to produce it as quickly as possible,” he said. An important element in this design process, according to O’Connor, is to incorporate in the process the automated tool used to make the part, for example automated fiber placement (AFP) versus automated tape laying (ATL).

Optimizing material also reduces weight. That was a goal of the new Multiply design feature in their latest Fibersim release. Unlike traditional ply-, zone-, or grid-based methods, the engineer places independent reinforcement regions on top of other regions, eliminating tedious zone or grid redefinition. With this Multi-ply approach, the design is updatable between geometry and associated ply definitions, eliminating rework.

“Multi-ply makes it easier and quicker to define a design, maintaining communication between analysis and redesign,” he said.

According to O'Connor, the Multi-ply function was developed through working with Siemens’ automotive customers. “The traditional zone- or grid-based design approaches were more than automotive needed. But once our aerospace customers saw [this feature], they knew they could use it to their advantage.” He predicts more automotive to aerospace spillover as the industry continues to emphasize rate.

“We need to ensure there is no disconnect between the design engineer, the manufacturing engineer, and the shop floor,” said Rani Richardson, Composites Consultant for Dassault Systèmes, providers of a full suite of Product Lifecycle Management (PLM) software as well as the Composite Workbench set of tools for designing and analyzing composite structures.

She agrees that when it comes to helping aerospace increase production rates, lessons learned from automotive will be a powerful tool. “One example of that is our new CATIA Composites Braiding Designer tool,” she said, developed with a major European automotive OEM.

“With this, we simulate the actual braiding machine,” including parameters like mandrel speed, carrier rotation, and orientation. “We can do this all in the design phase before we pass it to CAE simulation. We are designing properly right from the start rather than having to go through that iteration loop,” she said. While developed for the automotive market, it provides a useful tool to aerospace users as well.

In fact, there are plenty of synergy opportunities as composites and advanced composites become more popular in many applications. For example, Richardson expects government funding of institutes such as the Institute for Advanced Composites Manufacturing Innovation (IACMI), of which Dassault Systèmes is a charter member, to also advance tools for better design for manufacture.

“Industries such as automotive, wind energy, or compressed gas storage have the same goal [as aerospace] – develop tools for building quality, robust composite parts faster and cheaper,” she said. The materials and resins may be a little different, and certainly crashworthiness means different things between autos and airplanes, but the basic tools will be the same.

An especially interesting new development in CAE simulation is Dassault Systèmes 2014 acquisition of Accelrys, now known as the Biovia brand within Dassault Systèmes. This software models molecular formation of resins and the resin curing cycle through chemical kinetics simulation. Optimizing the chemistry through design of the plastics used to bind composites could mean stronger materials, and faster curing cycles and manufacturing efficiencies.

“That brings a whole new element to our design for manufacturing that we are starting to incorporate,” said Richardson. “We can predict delamination or lack of chemical bonding that will affect the lifecycle performance.”

Machines and Design

Richardson also noted that, with the increased emphasis in aerospace on DFM that a number of machine tool builders are working more closely with software providers like Dassault Systèmes. Current partners include Fives, Ingersoll Machine Tool, Mtorres, and Coriolis. This is important because how a machine operates is best incorporated in the design for maximum manufacturing efficiency.

The final product of a design process involves using an advanced composites machine, such as an ATL or AFP, to make the part. Fives makes a number of such devices and provides software – the Advanced Composites Environment Suite – that takes input such as CAD models and ply contours from the CATIA Composites Designer or Siemens Fibersim and produces machine instructions that are used to build the part.

“The engineer designing the part needs to know something about how the machine will make the part,” said Robert Harper, Director, Technical Sales, Fives Cincinnati. Parameters include material width, minimum steering radius for that width, material thickness, and the number of layers the machine can place. “They need to know these and limitations, such as minimum coarse length in an AFP and minimum cut length of the material, so when the engineer creates the design the machine is capable of creating that part. They need to know the machine’s capabilities in localized contours as well.”

at the designer has access to that in the CATIA Composites Workbench. While having access to such data is useful, educating design engineers directly is just as important. “Making parts using advanced composites is fairly new, especially compared with the 100 years of experience in metal cutting.”

He said that they supply data to companies like Dassault Systèmes, such as minimum tow length, so thCoriolis Software also provides software packages that specialize in composites design and offline programming solutions for various machines. The parent, Coriolis Composites, specializes in building AFP based on 6-axis robots for manufacturing composite parts. To program their own robots, they needed to develop software that could produce an optimized design for the system and produce a program off-line for the robot itself. The now independent Coriolis Software extended their capabilities to generalized CNC composite machines.

The output from the company’s software is a design of the part optimized for manufacturing and a machine program that produces that part. They use FEM modeling to ensure the final model meets strength requirements. They offer a package integrated into CATIA Composites Designer, or a standalone package that can import data from either CATIA Composites Designer or Siemens Fibersim.

“The objective of our software is to fill the ply contours with material strips in the most efficient way,” said Olivier Munaux, Software Manager, Coriolis Software. “An enriched data model serves the basis for running fast simulations at an early stage in the design process, giving engineers the opportunity to get feedback from the 'as built' as soon as possible.”

This is a multi-objective optimization problem when accounting for all of the design drivers including cost, weight, and cycle time. Coriolis employs a genetic algorithm as an optimization engine, embedded in a framework to automate the process. Munaux believes his customers want built-in tools that are easy to use, that incorporate requirements and geometry, and compute a solution that is the best compromise between all of the competing requirements.

“The aircraft industry recognizes both the benefits and the need [of simulation optimization] as aircraft production rates have increased,” said GKN's Gear. He believes the challenge relates to overreliance on testing to validate solutions as opposed to using the full potential of simulation techniques available today. “As more automation of manufacturing is being brought into our factories we need better methods to simulate and define our products in shorter lead times.”

Using a DFM approach is helping GKN establish how to do this more effectively. “[It] is assisting us in gaining a comprehensive understanding of our products before we enter full scale production," he said. "More importantly, DFM has significantly reduced non-conformances and lowered waste in our manufacturing processes.”