Simulation tools can help take the weight – and worry – out of the design and manufacture of carbon fiber composite bike frames and parts.
Fashion models don’t worry about their weight as much as professional cyclists. In the elite levels of cycling, lightweighting takes on a religious significance. Every gram on a serious rider and their bike is blessed in its removal or absence. Racers go crazy: dieting, drilling holes in their bikes,throwing away water bottles during a ride… anything in their desperation to get an edge on the competition.
Bike companies that make high-end production bikes cater to that obsession to lose weight. The largest part of the bike, the frame, gets a lot of attention. A bike frame may appear simple to the casual cyclist. But if your last use of a bike was to deliver newspapers, getting into a bike shop and picking up a modern racing bike will immediately inform you that the game has changed. At less than a kilogram (2.2 lbs), you can hold some bike frames on the fingers of an outstretched hand.
Now, get used to paying more for less. These superlight bikes are not cheap. The most expensive ones in the bike shop rival the least expensive cars at the dealership. But when every gram translates into seconds off the clock—or staying with younger, fitter riders on a Saturday group ride—it’s “damn the cost and full speed ahead”.
Chances are the lightest and most expensive bikes will have a carbon fiber frame. Originally used as filaments in lightbulbs in 1860, it was not till a hundred years later that carbon fiber’s high strength to weight was found useful in British aircraft. It is now also a favorite with automotive engineers trying to meet fuel efficiency demands. Carbon fiber composites can be found in various high-end sporting goods, like tennis rackets, golf clubs and bikes.
But in the race to make bikes lighter and lighter, the walls of the frame tubes are getting paper thin. “We can make them even lighter,” said one bike designer, “but I wouldn’t be able to sleep at night.” The international body that governs all the big races, the UCI, imposes a lower limit of 6.8kg (15 lbs) for a full bike, knowing bike racers will put themselves—and other riders—at risk with bike frames that could too easily collapse.
How to make the lightest frames and still get some sleep? Enter simulation software.
Composites Hard to Analyze and Manufacture
Analysis of composite materials is hardly straightforward. By comparison, metal, with uniform, identical (“isotropic”) properties in all directions, is a piece of cake. A metal part can be analyzed with single, simple material properties, like one modulus of elasticity good for the whole part. A carbon fiber part, on the other hand, will not only have two materials, it demands the analyst pay attention to the orientation of the anisotropic material (the fibers) , which can vary within each ply. Carbon fibers have to stay somewhat attached to the material in which they are embedded, AKA the “matrix”, usually an epoxy. Carbon fibers will resist forces parallel to their orientation quite nicely, having a strength higher than strong metal alloys. It is the job of the matrix to keep the fibers in their place. Out-of-plane forces are resisted solely by the matrix material, which is considerably weaker than carbon fiber. Composite parts also lack the toughness of metal parts, a property material scientists refer to as “energy release rate.” Composites are prone to abrasion.
The method of making a shape using carbon fiber is like no other type of manufacturing process. Carbon fiber bike frames usually start with a “lay-up,” where the layers of “prepreg”, cut to shape, are draped one on top of the other, varying fiber direction, sometimes over a mandrel. The whole mess is put into a mold; then the mold is put in an oven to cure.
Analyze This
Bertram Stier, an avid mountain biker, is also a research engineer at Collier Research, a smallish CAE vendor (20 or so employees) that makes HyperSizer, simulation software for composite fiber material. Stier wants to transition from the aluminum alloy bike frames (he owns three mountain bikes) to bicycles with lighter weight composite frames—but not until his software assures him they are safe and until they get less expensive.
Stier has a masters degrees in structural mechanics and light weight design and is on his way to a PhD in multi-scale analysis of fiber reinforced composites at Germany’s RWTH Aachen University. He presented on “Facing the Challenges of Computational Composite Bike Frame Optimization” at the Cyclitech event, organized by JEC and SPE, held last December in Newport Beach, CA.
Collier’s HyperSizer is a structural analysis program that helps in the analysis and optimization of composite material parts. In the case of bike frames, it can adjust the thickness of the frame to react to stress distributions, thickening the walls where stress is highest and thinning elsewhere to save weight. The program will show the number of layers of carbon-fiber-impregnated sheets, or plies, needed to affect the thickness. HyperSizer can add plies “internally,” so it does not change the look of bike frame. This result can be given to the shop so the plies can be applied.
Used in military and aerospace applications, HyperSizer has been made simpler to use and is offered in an Express version. With the software, bike designers and engineers in other sporting goods companies, automotive components makers or small- and mid-size manufacturers in general can use the application without needing the advanced degrees or dedication of fully committed analysts common to large firms, such as in the aerospace and automotive industries.
But don’t get Stier started on how a bike frame would actually be hard compared to an aerospace part. “Although, on many levels, an aircraft wing is certainly more complex than a bike frame, there are some details in a bike—such as highly doubly curved surfaces and joint sections—that are actually more complex than in an airplane,” he says.
Testing, Testing
Before a new design goes into production, it should be fatigue tested. Fatigue can tear a carbon fiber frame apart. Also, cyclists crash, drop their bikes and, in what is the most embarrassing accident ever, ram it into the garage while it is mounted to the car roof rack [Ed note: Author not admitting to having done this]. A new frame design will undergo one torture test after another, simulating as many mishaps as possible, with each failure potentially returning the design back to the designer.
This trial and error, often lengthy, rubs Stier the wrong way. He claims his company’s software takes much of the guesswork out. HyperSizer can evaluate many design iterations and settle on the most promising ones. The designer can essentially craft a final game plan for the craftsman to execute, with confidence that it will sail through the torture tests being planned for it. Proper analysis tools can greatly shorten the amount of testing needed, says Stier.
“Virtual design with the latest software tools can shorten product development time dramatically,” he says. “An optimization tool like HyperSizer Express takes your manufacturing process into account and provides you with several options that prevent you from a result that is not damage tolerant. You can define a variety of layup and design rules that will result in the most damage-tolerant design that uses the least amount of expensive composite material.”
Optimization
HyperSizer Pro and HyperSizer Express, like any structural analysis program, will run an analysis to calculate forces and stresses. Then, the optimization kicks in to add material (sheets of prepreg cut to a pattern) where the higher stress calls for it. HyperSizer is smart enough to add material on the inside (important when designers will not tolerate anyone messing with how the part looks) or the outside (if the internal volume is to be fixed) or preserve the midplane, adding thickness equally inside and out.
Collier Research, founded by Craig Collier and headquartered outside of Washington DC, claims to be the first software spinoff from NASA—predating even NASTRAN. While small compared to CAE giants such as ANSYS and MSC Software, with a staff of about 20, it may have the highest ratio of engineers to employees of any CAE company, with 19 out of 20 being degreed engineers, many with advanced degrees and PE status.
Altair HyperWorks
Also at Cyclitech was Altair’s vice president of additive manufacturing and composites, Dr. Robert Yancey, who was co-presenting “Composite Manufacturing/Design Defects: What is Observed, Avoiding Them.” Yancey has worked for Altair since 1999 and now lives in Washington state. He Has an MS in engineering mechanics from Virginia Tech and a PhD in materials engineering from the University of Dayton.
While bike frames represent a niche market for composites, Altair has made it a focus with strong tools for composite analysis, according to Yancey. OptiStruct, Altair’s structural optimization software, can help determine the number of plies, the shape of each ply and their orientation.
Altair’s OptiStruct can use topology, topographic and free-form optimization to help create shape concepts. Then, using proprietary technology, OptiStruct can include manufacturing constraints such as ply drop-off or build up ramp rates.
Altair counts as a success what they did for Robot Bike Co., a UK mountain bike manufacturer that makes a combination carbon composite/titanium frame customized to each of its customers (see our previous coverage here). Customization is achieved by feeding rider parameters, height, position, etc., to get the frame size. The frame is scaled up or down by cutting carbon composite tubes to the right length. The titanium lugs that hold the tube together are 3D printed and designed with topology optimization in OptiStruct.
Close-up of a Specialized time trial bike, used in the Tour de France, shows brake calipers integrated into the fork for aerodynamics. (Image courtesy of Specialized).
Specialized, one of the world’s biggest bike manufacturers, supplies teams in the Tour de France with its most advanced road bike, the S-Works. The S-Works uses carbon fiber extensively in the frame and other components, such as the seat post, handle bars, fork and wheel rims. Frame tubes are anything but tubular as the carbon fiber is shaped to be thicker at the joints, thin as possible from the front, sculpted to just clear the wheels, and include a host of other details that, combined, make the S-Works the most aero-styled bike made in quantity. It is the malleability of carbon fiber composites that make this possible. Not only can you put material where it is needed for strength and leave it out where it is not, you can design for superior aerodynamics.
Specialized used HyperWorks for its aerodynamics and uses OptiStruct for optimization of the carbon fiber layup and structural analysis.
Siemens PLM Software’s Fibersim and Simcenter
Enterprise design and engineering heavyweight Siemens PLM Software offers Fibersim and Simcenter 3D for use on carbon fiber bike frames. Fibersim can be used for design and manufacturability and Simcenter 3D, part of Siemens’ Simcenter portfolio, for simulation. With the wide-spread use of both applications in automotive and aerospace industries, one can be confident that they are robust. While these tools feature good integration with NX, Siemens’ flagship of design, they also integrate with Dassault Systèmes’ CATIA and PTC’s Creo. However, there seems to be no direct integration between Fibersim and SOLIDWORKS.
To provide details from Siemens were Leigh Hudson, who rides his Jamis Commuter 2.6 miles to and from work, weather permitting, and Julien Simon, also a cyclist. Hudson comes to Siemens with the acquisition of Vistagy, the originator of Fibersim. Fibersim integrates with many MCAD products and will let the bike frame designer choose materials, orient the plies and help plan the manufacturing processes. Users can change variables and see the effect this will have on manufacturing the frame.
“Simcenter 3D delivers a unified, scalable, open and extensible environment for 3D CAE with connections to design, 1D simulation, test, and data management,” say the guys from Siemens . “It is a general purpose simulation software with advanced capabilities in multiple domains, as well as dedicated capabilities for the simulation of composites.”
Simcenter 3D uses “best-in-class solvers” to compute results of laminate composite structures that would certainly apply to bike parts. Simcenter 3D’s solvers can do more, such as first ply failure analysis and beyond what is required of two wheels on the ground, like progressive damage simulation, springback prediction, thermo-mechanical simulation and acoustics.
Simcenter 3D is a simulation platform, Siemens answer to any type of structural, thermal and fluid, acoustics and motion simulation. Composite analysis for bike frames? “Yes, why not?” says Simon, who is product manager for Simcenter 3D composites. Supported solvers include Siemens’ NX Nastran and LMS Samtech Samcef, which can analyze for short- and long-fiber composites, optimize composite layups and suggest techniques to minimize damage due to local failure, delamination and crack propagation. In addition, these products can perform manufacturing simulation of the curing of thermoset material. They will correlate tests with analysis and update the FE model as needed. Users can declare zones in the mesh or on the plies, or a combination of the two. And bi-directional links exist between Simcenter 3D and Fibersim.
Fibersim has the unique ability to not only define the composite product completely, but also to either simulate its manufacturing or to assess it.
“The simulation is based on the selected material, geometry and manufacturing process,” says Leigh Hudson. “But uncertainty in material behavior during manufacturing often results in overbuilt designs, increasing cost and time to market.”
Fibersim’s intended user is a design engineer, says Hudson. “If you use CAD, you fill find it complementary.”
Simcenter 3D is probably best suited for dedicated analysts, though it does let you create templates and “guided workflows” for the designer or design engineer.
ANSYS
Leading CAE vendor ANSYS offers specialized simulation tools for composites that can be applied to the design of bike frames and parts.
Bikes, though a niche compared to automotive and aerospace applications, are nevertheless an important niche to ANSYS—and to Richard Mitchell, product manager and avid bike rider. Transplanted to western Pennsylvania from his home country, Richard rides an aluminum-frame bike or British origin for fun and commuting, but clearly has an interest in carbon fiber.
For the analysis of composites, ANSYS Composite PrepPost (short for pre- and post-processing) simulates how the material will act under loading, taking into account fabrics, laminates or sublaminates (combination of laminates and fabrics), along with properties such as thickness and fiber orientation. plies, etc. Material properties can be manually input or imported from a materials library, such as ESAComp.
ANSYS goes one step further and analyzes for the distortion that can take place during the curing of the composite bike parts, a process that normally relies on trial and error. The heating and cooling required for a thermoset will cause chemical changes that produce shrinkage. The shrinkage changes not just size, but angles. Called “spring-back,” this causes components not to fit and frames that curve away from a straight line, etc. The industry has coped with making a first pass with a mold fabricated to design specification, then to machine the mold to correct for the spring-back.
LMAT, a UK-consulting firm, has developed ANSYS Composite Curing Solution (ACSS) to predict the spring-back to eliminate the trial and error process, according to Mitchell. To do so, ACSS gets down to the level of molecules of the polymer matrix, usually an epoxy. The polymer, when heated, starts bonding and, in the process, slimming down. Not only does this cause distortion of the part, but it can lead to cracks in the part. Both PrepPost and ACCS work are integrated under ANSYS Workbench.
ANSYS has been used for Scott bikes. Scott Sports SA, a Swiss company that claims to be one of the first to use carbon fiber. The company sells half a million bikes per year, according to Benoit Grelier, Head of Bike Engineering, in an interview by MSM. Scott’s high-end bicycles are prized by bike enthusiasts. “We manufacture the lightest frames on the market for mass production,” adds Grelier.
Scott engineers chose ANSYS first for simulation of carbon fiber composites because ANSYS also covers many other types of simulation, such as plastics, aerodynamics and dynamics. “We can increase simulation skills without scattering across different solutions,” says Grelier.
Also using ANSYS for the development of the “super-light” e-bike is Audi. KTM Technologies, a German consulting firm that specializes in composites, used ANSYS to trim the e-bike frame down to 1.6 Kg.
Trek Bikes
Trek Bicycles, the largest “specialty” bike manufacturer, has standardized on Dassault Systèmes software, using CATIA for design of its carbon fiber bikes and Simulia for the analysis. Trek has been making carbon fiber bikes since 1989, but it was their use in the Tour de France by the US Postal team that elevated the Trek bike into stardom. Though the team’s leader, Lance Armstrong, has fallen from grace, left the sport and been scrubbed from Trek’s history, Trek carries on with versions of the Madone, a superlight, high-end bike, made with the company’s patented OCLV process.
Trek’s engineers used to patch together separate applications for design and analysis, using neutral files to shuttle data, a process that led to as many as 6 different file formats, according to Mark Wilke, chief process engineer for Trek, in a Dassault Systèmes case history. But having all apps under one roof has made the process seamless, he says.
“The ply layout is developed in CATIA and CATIA Composites Design (CPD) by creating sequence charts, material tables and lay-up books,” explains Wilke. “The finite element model is prepared within CATIA Advanced Meshing Tools. Simulayt Composite Modeler provides bidirectional and seamless integration of the CATIA Composites model into the SIMULIA FEA software.”
Jing Bi is a technical consultant at Dassault Systemes (Image supplied by Jing Bi)
Jing Bi, who joined Dassault Systèmes in 2012 after she got her PhD from University North Carolina at Charlotte, was able to get us up to date on Dassault Systèmes’ 3DEXPERIENCE platform, of which SIMULIA is a part, and how they could be used for carbon fiber bike design and analysis.
The 3DEXPERIENCE Platform and SIMULIA products are general purpose simulation products, but provide composites solutions in many key areas that are important to consider when it is composite bike frames that are being designed.
“First of all, the design and manufacture of composites relies on the accurate simulation of composites and that means we must consider in detail the manufacturing procedures used,” says Bi. The carbon fiber part can be made from a variety of procedures: draped, braided, wound, molded or additively manufactured. “Each procedure is intrinsically linked to how it will behave in service. Tools in 3DEXPERIENCE Platform are available to simulate ‘as-manufactured’ components as well as the manufacturing processes themselves,” she adds.
Simulation of the behaviour of a material microstructure provides essential information in order to predict the corresponding behaviours at the meso and component scales. Dassault Systèmes SIMULIA can handle multiscale simulations, starting from homogenization at microstructure scale to manufacture and in-service simulations at component and assembly scale, according to Bi.
Multiscale material modeling is addressed with two methods: the Abaqus Micromechanics FE-RVE plugin and mean field homogenization. Abaqus is also used as the global solver as it supports not only DS’s own modelers but external modelers such as AlphaSTAR and MultiMechanics.
Figure 16. Delamination modeling of a double cantilever structure with shell elements and surface based cohesive contact. (Image courtesy of Dassault Systèmes.)
SIMULIA offers a range of fracture and failure modelling approaches. “[These predict] when and how failure will occur in composite parts. Component failure is a critical aspect of composite design,” says Bi, who, along with her colleagues, continues to promote the usage of advanced fracture and failure models in the industry.
Technologies are available in Abaqus for intra-laminar damage with continuum damage models; inter-laminar delamination modeling with VCCT and cohesive element/contact; composites crash modeling with CZone; built-in damage models for unidirectional fiber and woven fabric; standard and physically based failure measures; crack propagation using XFEM; advanced post-processing and high performance visualization, according to Bi.
Working in the 3DEXPERIENCE Platform allows engineers to capture key stages of a typical design cycle of laminar composites. This includes preliminary design, detailed design, validation, manufacturability, manufacturing process and tooling optimization.
Another tool available in the Dassault Systèmes family is SOLIDWORKS Plastics, which can simulate the injection molding process of chopped fiber composites. Dassault Systèmes Abaqus can be used for simulating the additive manufacturing process of fiber reinforced composites.
Finite elements for composites support both 2D and 3D stress states such as plane stress elements (which Abaqus designates as S4, S3, S4R, S3R), continuum shell (SC6, SC6R, SC8, SC8R), plane stress (CPS family), membrane (M3D family) elements and 3D stress-displacement continuum elements (C3D4, C3D6, C3D8R, C3D10M) with exceptions based on individual features. For example, XFEM supports family of 3D stress-displacement continuum elements. Inputs are usually characterized material properties of composite laminar or properties of individual constituents, volume fractions, inclusion shapes and distribution. Outputs are stress, strains, failure and damage predictions.
Software Resources
Abaqus – general purpose FEA software by Dassault Systèmes
ANSYS – simulation software of many kinds, including composites
CATIA Composites Design (CPD)
Composite Modeler for Abaqus/CAE – add on for Abaqus, by Dassault Systèmes
Composite PrepPost – pre- and post-processing of composite materials, by ANSYS
Dassault Systèmes – design, simulation, PLM software
Fibersim – specialized software for simulation of fiber reinforced material, by Siemens
HyperSizer – composite material analysis and optimization, by Collier Research
HyperWorks – simulation and optimization, including composites simulation, by Altair
Multiscale Designer –simulation of multiscale material models, including continuous, woven, and/or chopped fiber composites, by Altair
OptiStruct – structural optimization, by Altair
Simcenter – simulation portfolio by Siemens PLM Software
SIMULIA – full simulation package by Dassault Systemes