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Unknown Qualities OPEN ACCESS

Because Composites Don’t Behave Like Metals they can Present Particular Challenges to Designers. Tools are Evolving to Work with these Complex Materials.

[+] Author Notes

Jean Thilmany is an associate editor of Mechanical Engineering magazine.

Mechanical Engineering 135(08), 44-49 (Aug 01, 2013) (6 pages) Paper No: ME-13-AUG2; doi: 10.1115/1.2013-AUG-2

This article reviews various tools that are evolving to help engineers work with complex composite materials. Specialized composite analysts are needed to help find the best material for a particular use and to determine if it can be manufactured with the chosen material and in a particular shape. The Falcon Heavy spaceflight system is planned to launch on a SpaceX-designed rocket engine. As composite materials are lighter than metal, SpaceX engineers realized that composites could improve the strength-to-weight ratio of its materials. In order to help engineers choose the best composite for the design, the software system contains a large library of material models that designers can use to explore a composite material and determine how it might behave under a variety of circumstances. Predicting how the cracks will affect the part long-term still cannot be done within software packages and must be prototyped.

An aircraft manufacturer came to HCL Technologies, an engineering services firm in Bangalore, India, for help in designing a lighter-weight vertical stabilizer for the tail assembly of a commercial passenger aircraft. The manufacturer wanted to reduce weight by using composite materials instead of metal. The manufacturer was asking HCL's engineers to manage some challenges in design and analysis, because composite structures can be far more complex than metals. The manufacturer was not alone in having to deal with that kind of complexity.

To increase fuel efficiency and reduce structural weight the automotive, aeronautical, and other industries are increasingly turning from metal to composite materials, said Vinay Dayal, associate professor of aerospace engineering at Iowa State University.

Dayal has worked with composites for the past 25 years on projects for the U.S. Air Force and Navy and for NASA. He's currently helping a team that includes members from Iowa State find the best way to manufacture composite blades for offshore wind turbines.

For engineers, designing and analyzing composite structures will never be as straightforward as doing so with metal parts. Composite materials are engineered. They’re formed by placing fiber material into a mold cavity or onto a mold surface and then solidified using polymers.

By definition, the constituent materials of a composite remain separate and distinct in the final structure.

Metals behave in the same way no matter where a force is applied; their properties are known and can be coded into design and analysis software. The same isn’t true for composites, where material properties vary widely, Dayal said. A look at what constitutes a composite material points out why this is so.

Many composites are composed of a resin solution mixed with reinforcing materials, often fiber. Fiber-reinforced composites are either continuous-fiber-reinforced or short-fiber-reinforced. The former is a layered structure, and the latter is available in a variety of forms, Dayal said.

Advanced systems like cars and aircraft usually incorporate composites comprising heat-resistant, strong, synthetic fiber such as carbon fiber in an epoxy resin.

Workbench Ansys

Fibersim Siemens PLM

Grahic Jump LocationWorkbench AnsysFibersim Siemens PLM

Composites made with glass and Kevlar fibers are on the market with a wide variety of fiber weaves, Dayal added.

And composite parts are more often bound together with adhesive today, as the variety, strength, and understanding of adhesives has improved.

So how's an engineer to choose the right composite material for a part when the variety to choose from seems endless?

“The variables are so many you cannot pinpoint what is the best,” Dayal said. “Fiber, laminate, interfaces, the preferable adhesive, manufacturing method, how the epoxy flows in the mold: you have to look at all of these. If you put all these variables together you’ll never be able to mathematically optimize them.

“So you have to start limiting those variables and then say, ‘Let me look for these things under these particular constraints’ and find what works that way,” he said.

This means design isn’t simply done in a CAD package. According to Dayal, it's an ongoing process that marries design with input from analysis and manufacturing.

Engineering software has come a long way in easing composite design and analysis challenges, Dayal said.

Several analysis programs exist that are specific to composites, including Workbench from Ansys, Nastran from MSC Software, Abaqus from Dassault Systèmes’ Simulia, and Fibersim from Siemens PLM. Dayal said the quality of all composite analysis programs has improved in the time he's been working with composite materials.

Specialized composite analysts are needed to help find the best material for a particular use and to determine if it can be manufactured with the chosen material and in a particular shape.

“Metals have been around much longer so people know how to analyze them and know more or less how the metal is going to behave,” said Prabhakar Vallury, director of business development for e-Xstream Engineering.

His company makes Digimat software, which simulates composite behavior by providing analysis software with material properties about many types of composites. MSC Software, a maker of engineering software in Santa Ana, Calif., purchased e-Xstream Engineering earlier this year.

Abaqus dassault systèmes’ simulia

Nastran MSC Software

Grahic Jump LocationAbaqus dassault systèmes’ simuliaNastran MSC Software

“One of the attractive things about metal is that if you take a rectangle of steel or copper or aluminum and pull on it from one direction with a machine, it will stretch a certain amount,” Vallury said. “Pull on it from another direction it will stretch the same amount.”

“If one piece of metal can hold up to a certain load or force you’re good to go because which side the load comes from doesn’t matter,” he said.

In composite materials, the angle of the fibers contained within the resin makes a difference in how the part behaves. Pull in one direction—say the same direction in which the fibers run—and the material holds to the force. But pull in another direction—against, rather than with the fibers, say—and the material may fall apart, Vallury said.

The Digimat software allows designers to find the best composite without the need to change part design, he said.

Pull in the Same Direction in which the Fibers Run: The Material Holds to the Force.

Pull Against, Rather than with the Fibers: The Material may Fall Apart.

In Composite Materials, The Angle of the Fibers Contained within the Resin makes a Difference in how the Part Behaves.

“Many times, changing the shape of the part is like a death sentence in industries like the automotive industry because the whole assembly has to change and the die has to change,” he said. “Engineering change is the least desired option.”

Engineering Software has come a Long way in Easing Composite Design and Analysis Challenges.

According to Dayal, growth in the variety of composites has happened in tandem with the growing use of composites in advanced products.

For instance, when Space Exploration Technologies Corp., SpaceX for short, was looking to enhance the performance of its Falcon Heavy rocket, currently under development, engineers adopted composite materials as part of the rocket system's makeup.

The Falcon Heavy spaceflight system is planned to launch on a SpaceX-designed rocket engine. Because composite materials are lighter than metal, SpaceX engineers realized that composites could improve the strength-to-weight ratio of its materials, said Kirk Matthes, design manager for SpaceX in Hawthorne, Calif.

The company now uses a software package, Fibersim, from Siemens PLM to help create designs, to help analyze designs, and to update manufacturing files, Matthes said.

For HCL Technologies, changing the vertical stabilizer from aluminum, which resists impact well, to composites was challenging because composites are more brittle than metal and more vulnerable to impact, said P. Ganthimathinathan, senior project manager in the HCL engineering and research and development services department.

“We looked at a variety of material options for the vertical stabilizer, including all-composite and composite-metal hybrid versions,” he said.

In order to be flight certified by the U.S. Federal Aviation Administration and the European Aviation Safety Agency, commercial aircraft must be able to withstand a collision with a four-pound bird anywhere on its structure at cruise speeds and still be able to continue to fly and land safely, Ganthimathinathan said.

“Each design iteration required its own bird-strike analysis to see if it met the appropriate safety regulations,” he added.

The HCL research and development team used the Abaqus system to analyze its vertical stabilizer, said Anwar Ibrahim, an HCL project manager.

“Bird strike involves complex contact behavior, especially after impact when fragments of the damaged structure and the bird hit other parts of the aircraft,” Ibrahim said.

To help engineers choose the best composite for the design, the software system contains a large library of material models that designers can use to explore a composite material and how it might behave under a variety of circumstances, he said.

They can see, for example, how a particular composite fiber would buckle under a load.

While the goal of the stabilizer analysis was to explore lightweight designs, the study started by verifying a model of a metal stabilizer to establish a baseline, Ganthimathinathan said.

“Earlier analyses have focused on metal aerostructures, so we used aluminum to figure out the best way to approach the problem,” he added.

fyi

HCL TECHNOLOGIES LTD. hcltech.in

HEADQUARTERS: Bangalore, India.

FOUNDED: 1976.

EMPLOYEES: 84,400.

HCL Technologies offers information technology and engineering services to industries including financial services, manufacturing, consumer services, public services, and health care. The company has offices in 26 countries and global revenues of $4.5 billion.

The engineering team analyzed a series of composite and hybrid stabilizer designs. For each iteration the number of plies and material composition of the layup were changed, and a bird-strike analysis was performed. For the structure to pass, the leading edge of the stabilizer needed to withstand impact, as modeled virtually, he said.

While a 30-ply all-composite design of made of glass and fiber survived the impact of the four-pound bird model, a similar 20-ply design failed. To increase the strength of the lighter 20-ply design, engineers replaced the outermost composite ply with a single aluminum layer 0.9 mm thick.

fyi

Space Exploration Technologies Corp. SpaceX.com

HEADQUARTERS: Hawthorne, Calif.

FOUNDED: 2002

EMPLOYEES: More than 3,000.

SpaceX is a private company owned by management and employees, with minority investments from Founders Fund, Draper Fisher Jurvetson, and Valor Equity Partners. The company designs, manufactures, and launches Falcon launch vehicles and the Dragon spacecraft. SpaceX operates in California, Texas, the District of Columbia, and Florida.

When the model bird flew into this hybrid design, the outer metal layer was damaged though the underlying composite layers weren’t severely damaged, Ganthimathinathan said.

The Falcon 9 Rocket's First Stage is Test Fired at SpaceX's Rocket Development Facility in McGregor, Texas.

PHOTO: SpaceX

Grahic Jump LocationThe Falcon 9 Rocket's First Stage is Test Fired at SpaceX's Rocket Development Facility in McGregor, Texas.PHOTO: SpaceX

Physical testing of the successful hybrid design for flight certification is pending.

Despite this forward momentum in software created to aid composite design and simulation, there are still challenges, Dayal said.

For instance, engineers have a hard time modeling curved composites, though they’ve developed workarounds based on flat models.

Also, microscopic cracks begin developing early in composite parts, which modeling and analysis software doesn’t account for. Predicting how the cracks will affect the part long-term still can’t be done within software packages and must be prototyped, Dayal said.

Predicting the fatigue life of a composite material also can’t be done well in today's analysis packages, he added. Fatigue develops in a laminate layup—composed of many layers of material.

“If you start testing every laminate for fatigue you will never make anything,” Dayal said. The fatigue life depends on the laminate ply orientation which can have infinite combinations.

“People really don’t know what the fatigue life of a composite part would be,” he added. “That puts additional pressure on following and monitoring your parts.”

But with their promise of lighter weights composites aren’t going away. And neither are the software applications that analyze composite structures. There are planes and cars that depend on them.

Copyright © 2013 by ASME
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