Structural+Design

=Introduction= toc The objective of the structural design process is to engineer elements into the human-powered vehicle (HPV) which help it resist loads encountered in normal operation, handling, and in the event of a crash. Resisting loads implies protecting the occupant, as well as preserving the integrity and shape of the aerodynamic shell and mechanical components.

Crucial considerations throughout the structural design process include (but are not limited to):
 * Light weight: a lighter structure allows the occupant to stop-start with less expenditure of energy;
 * Form Factor: the structure must not interfere with the occupant's capability to control and power the vehicle;
 * Mechanical Integration: the structure much provide hard points for the attachment of the drivetrain, wheels, seat, and any other elements;
 * Stiffness: the structure must resist deformation in order to preserve the alignment and position of other components and enhance the occupant's feeling of security and stability;
 * Strength: the structure must resist loads imposed upon it to preserve the safety of the rider and operational integrity (within reason, as to be discussed);
 * Manufacturability: the structure must lend itself to manufacturing techniques that fit within the team's expertise, timeline, and budget;
 * Durability: the structure must stand up to long term use and racing given variable road conditions, inclement weather, novice riders, and some crashing.

This section will address the structural design and manufacturing process, in the following steps:
 * 1) Configuration Design: the arrangement and location of structural elements;
 * 2) Modelling and Analysis: 3D CAD modelling of the structure and stress/deformation analysis using computational methods;
 * 3) Tooling: design and manufacture of the molds and jigs required to fabricate the structure;
 * 4) Fabrication: the manufacture of the structure itself, using the aforementioned tooling and according to the design determined during modelling and analysis//.//

This design guide currently treats only monocoque-type structures, as have been built by the U of T HPVDT for the past several years. Monocoque means "single shell", and is a structural design methodology in which the vehicle's exterior skin/shell structure contains the primary load-bearing elements, as well as providing the overall shape. Initial design tradeoff in 2009 determined that this is likely the most weight-effective and robust means of constructing an HPV, versus shell-on-frame for example (where a lightweight and very delicate shell is attached at a few points to a more solid load-bearing structure).

=1. Configuration Design= The configuration design is the first step in the structural design process, and also the most open-ended. Configuration design is the selection of structural (i.e. load bearing) elements, arrangements, and rough locations.These elements carry loads between rider contact points, the seat, drivetrain attachment points, and wheels. They may be lightly-loaded, in the case of aerodynamic forces exerting pressure on the thin outer shell, or highly-loaded, in the case of the primary roll cage during a side-on collision or crash.

1.1 Structural Elements and Nomenclature
In a monocoque structure, the load-bearing structural elements are all integrated and co-bonded with the outer shell of the vehicle. A description of each of the usual elements of a monocoque HPV structure follows. In the geometry of the vehicle, the relations front/back, left/right, and top/bottom are used and should be self-explanatory. Where "longitudinal" and "lateral" are used, longitudinal refers to the front-to-back direction of the vehicle and lateral refers to the side-to-side direction. A "station" is delineated by a plane which is oriented laterally to the vehicle (at a certain distance from the front) and perpendicularly to the ground. "Circumferential" or "Hoop" directions refer to the direction in the circumference/perimeter of the shell given when a cross-section is taken at a station plane.

The following are structural elements commonly encountered in an HPV:
 * Shell: this is the lightly-loaded exterior surface of the vehicle, and is typically required to locate/stabilize the other co-bonded primary structural elements, bear aerodynamic pressure forces, and impart torsional stiffness to the overall structure of the vehicle;
 * Rib: this is a lateral or hoop-direction member, typically used to transfer loads across the width of the shell or from one side to the other. Ribs typically have a fineness ratio (i.e. long dimension/short dimension) greater than 3, and a thickness ratio (i.e. thickness/short dimension) greater than 0.2;
 * Longeron (also stringer, gunwale): this is a longitudinal member typically used to transfer loads from front to back between the drivetrain, wheels, and rider contact points. Like ribs, longerons typically have a fineness ratio greater than 3, and a thickness ratio greater than 0.2. "Stringer" is used interchangeably with longeron, and "gunwale" (pronounced "gunnal") is a term borrowed from ship building, referring to the longitudinal member immediately below and framing the top opening in a shell (especially in ACE, Bluenose);
 * Panel: this is a large-area reinforcement, typically a thin sandwich panel (to be explained) used to tie together ribs and longerons where a single point of connection is not sufficient, or to strengthen/stiffen a large area of shell where substantial handling or impact loads are expected. Panels typically have a fineness ratio less than 3, and a thickness ratio less than 0.01;
 * Rollcage (or ring): this is a circumferential element similar to a rib, but typically encompassing an entire hoop or cross-section of the shell. A rollcage is always present enclosing the rider and providing protection in the case of an ASME HPVC-certified vehicle, and additional rollcages (or rings) may be present to reinforce and protect the rider at other stations of the shell or to tie together top and bottom loading locations (in the case of a drivetrain's top and bottom attachment points).

A typical structure will be symmetric about the centerline-plane of the vehicle, with multiple ribs, longerons, at least one rollcage, and potentially panels. The location and number of these elements will be determined by the drivetrain design and mechanical/wheel attachment points, shell design and intended contact points in a crash, and rider points of contact. Primary structural elements are only necessary where loads are transmitted from one entity (i.e. the occupant's center of mass) to another (i.e. the ground).

1.2 Composite Structures
A "composite" is a material that is non-homogeneous and comprised of multiple distinct elements. This is usually a woven reinforcement (for example fiberglass, carbon fiber, or Kevlar) and a matrix material to tie the load-bearing fibers of the reinforcement together (for example epoxy or polyester resin). Composite structures give the ability to tailor the orientation, number, and type of reinforcements at every location for an optimal bearing of the imposed loads. Typical composite structures are as follows:
 * Laminate: a laminate is a thin, low-curvature plate-like structure (with the thickness very small compared to the other two dimensions), comprised only of layers of woven reinforcement. A laminate is not typically used to resist large forces applied normal to its face (because the second moment of area of a laminate is so small), but again may be appropriate for resisting small forces such as aerodynamic pressure;
 * Sandwich: a sandwich is a structure comprised of two laminate faces bonded to either side of a lightweight (and otherwise weak) core material. The combination of the face sheets placed far from the sandwich's centroid yield a far higher second moment of area, and thus make a sandwich a very weight-effective structure for resisting surface-normal or bending loads;
 * Shell: a shell in this context refers to a thin-walled hollow tube of triangular, rectangular, or rounded cross-section. A shell is another method of turning a laminate into a shape with a higher second moment of area, but a shell avoids the use of a core material. This sort of structure is typically more difficult to manufacture due to the requirement that the core material be removable.

A few important notes about designing and planning a composite layup (these details may not need to be present in a 3D model used for design/analysis):
 * Tapered Structural Transitions: stress concentrations build in corners and sharp transitions from one structural element to another. Rib profiles should therefore be trapezoidal (rather than rectangular) in cross-section for example, and care should be taken to avoid sharp corners where structural members intersect;
 * Incorporating Sandwich Structures in a Laminate: For the best adhesion of co-bonded structures and stress-transfer, it is best to encapsulate reinforcements inside laminate plies where a shell and rib intersect. For example, a rib would have unidirectional reinforcements bonded equally to both faces during layup, and that rib/uni member would be nested inside the plies (e.g. 2 on the inside, 2 on the outside) of the overall shell structure.
 * Impact Resistance and Fatigue: Much of the structures manufactured by the U of T HPVDT are designed to deal with crash and impact loads. Typically these are designed for a worst-case load, which would likely permanently damage the structure. However, often there are loads which are substantially less than an "Ultimate Design Load" which are experienced repeatedly, such as when a bike falls over in the course of rider training. These smaller yet more frequent loads can fail a structure just as surely as the Ultimate Design Load, given enough time. Care should therefore be taken to design in impact-loaded points of contact which are over-designed for every-day riding, and also to select materials which are known to have superior impact-performance.

1.3 Material Selection (see below)
Composite material selection is subject to the same selection pressures as any other materials selection process. However, in many cases can give the designer more freedom and therefore become more difficult than typical design with metal, for example. Composite materials are orthotropic, that is they have different material properties depending on the alignment of the material and the loads imposed. Typically, composites have their best properties in the direction of the fibers of the advanced reinforcement (e.g. a woven laminate will have excellent properties in the plane of the laminate, typically in the two directions of the fiber weave, and poor properties in the direction perpendicular to the laminate, where the matrix properties dominate). Material selection and orientation therefore concerns both the selection of the matrix material and the woven reinforcement.

For example, the elastic properties of a composite are typically determined by the "Rule of Mixtures", based on the volume fraction (Vf) of the multiple components. The properties of both the matrix and reinforcement are taken into account, but often the properties of the cured laminate are dominated by those of the reinforcement. For example, in determining the stiffness of a composite:

Stiffness_composite = (Stiffness_reinforcement)*(Vf_reinforcement) + (Stiffness_matrix)*(Vf_matrix)

For an introduction to the various materials frequently used by the team, see **Materials** (To be expanded).

1.3.1 Matrix
As mentioned previously, the matrix is typically a resin used to bond the fibers of the reinforcement together and carry load from one fiber to another. A few matrix materials are epoxy, polyester resin, and vinylester resin. Epoxy typically has the best strength/stiffness/stability of these materials, as well as having the fewest volatiles, the most versatility in layup options, and the best compatibility with the range of reinforcements available. Although it is slightly more expensive, in the quantities and frequency (i.e. prototyping and non-commercial) basis used by this team that is not a substantial concern. Epoxy is typically available in two-part chemistries which "cure" or harden in a chemical process, with a resin base (Part A) and hardener additive (Part B). With most epoxy systems, a variety of hardeners are available with either varying cure times (and hence working life) or for example transparency of the cured resin.

A range of epoxies are available for a number of applications, often depending on the processing technique to be used (see description of processing techniques below). Two of the most common used by U of T are:
 * Laminating Epoxy: This is a medium-viscosity epoxy used generally for wet-lay and vacuum-bagging. Laminating epoxy is good for general-purpose layup and has excellent all-around properties and handling characteristics.
 * Infusion Epoxy: This is a very low-viscosity epoxy used for resin infusion. "Toughened" infusion epoxies are available and may have improved impact strength, but these are often comparable to laminating epoxies.

The most common epoxy used by U of T is PTM & W's Aeropoxy PR2032 resin, with a range of hardeners available yielding 30 minute (PH3630) to 4 hour (PH3670) working life.

1.3.2 Reinforcement
The reinforcement is the most critical and variable component when designing a composite laminate. The reinforcement is typically much stronger and stiffer than the matrix. That being said, a huge range of reinforcements with varying material properties are available. Reinforcements are typically available in three forms:
 * Mat: randomly-oriented short fibers, fairly low performance in all directions of a laminate;
 * Cloth: woven continuous fibers woven in two or more axes, good performance in the fiber directions;
 * Unidirectional: fibers entirely oriented in one direction, with excellent performance in the fiber direction but matrix-dominated properties in the other directions

The choice of reinforcing material is determined by considerations including desired stiffness, strength, toughness (or impact resistance) and cost. The varied requirements of each structural component (e.g. flexure in the drivetrain, impact resistance in the shell, weight in the seat) may yield varied material choices for each structural component, or a generally high-performing material could be chosen for simplicity. The costs associated with prototyping on the scale of an HPV usually are not prohibitive, and this concern becomes secondary. This material selection chart is qualitative and adapted from Composites Canada's catalog:


 * || **Best** ||  ||   || **Worst** ||
 * **Cost** || E-Glass || S-Glass || Kevlar || Carbon ||
 * **Weight (Density)** || Kevlar || Carbon || S-Glass || E-Glass ||
 * **Stiffness** || Carbon || S-Glass || Kevlar || E-Glass ||
 * **Impact Resistance** || Kevlar || S-Glass || E-Glass || Carbon ||

Often the specific properties (i.e. value/weight) of a given laminate are the determining factor in laminate design for human-powered vehicles, as structural efficiency and not ultimate structural performance is the true metric being evaluated. For example, Steel is much stiffer than carbon, but on a per-weight basis carbon has much higher performance. Typically the U of T HPVDT has used carbon fiber as the reinforcement of choice, both in woven and unidirectional reinforcements. Kevlar has been used before (typically as a rider-protection ply on the inside of the composite shell, as carbon has been known to fracture and splinter on impact), but due to first-hand experience of the benign failure of carbon and the added difficulty in working with Kevlar (very hard to cut), this has been largely abandoned.

1.3.3 Sandwich Core Materials
As mentioned previously, sandwich structures are a very important part of effective composite structural design. However, as the objective of a core material is to be as light as possible (and substantially less strong than the surrounding laminate), care must be taken in making an appropriate material selection here for optimal weight savings with sufficient strength.

Core materials are usually either a structural foam (variety of constituent materials available), end-grain Balsa wood, or Nomex honeycomb. Structural foam typically has the highest specific stiffness and impact strength, and the greatest variation in densities (hence the desired strength is highly tailorable). Structural foams can also be bought in highly-flexible and easy to form varieties, making them ideal for high-curvature structural panels (however this can come at the expense of strength and stiffness). End-grain balsa is economical, but subject to degredation and sometimes difficult to form (most often used in boat hulls and wind-turbine blades). Nomex honeycomb is an aramid-paper cell structure that is very light and stiff, but generally has poor impact resistance and can be difficult to manufacture into a structure.

U of T HPVDT has most often used structural foam in various varieties, but pre-fabricated Nomex honeycomb and carbon sandwich panels have been used in many cases for convenience. The structural foam used has either been very high-strength and toughness (Corecell, in ACE), high flexibility (Divinycell F low-density, in Bluenose), or somewhere between (Divinycell F medium-density, in Vortex).

1.3.4 Material Properties Determination
Because composite materials are orthotropic (have different material properties in multiple axes) it is important to define the frame of reference when doing composite analysis. The "material axes" are in a frame of reference attached to a single composite ply, usually with one axis in a fiber direction (the _11 axis) and the other two orthogonal to that. The "structural axes" are in a frame of reference fixed to the structure being analyzed, typically with one axis (the _x axis) aligned in the long-direction of the structure and again the other two orthogonal to that.

The material axes are those in which the properties of a composite are usually measured and defined. This is because given knowledge of the reinforcement and matrix constituent properties (e.g. the compressive strength of carbon fiber and epoxy), the rule of mixtures can be used to approximate several of the properties of the composite with little first-hand experimentation. The most important of these are tensile strength, tensile modulus, and density. Also importantly, the rule of mixtures can be used to approximately scale known test values from one composite to another (e.g. 50% Carbon/50% Epoxy to 60% Carbon/40% Epoxy).

That being said, some composite properties are not easily determined from those of the two components. For example, compressive strength and stiffness, shear strength and stiffness, and toughness should be determined via testing, as these properties are derived from a less-straightforward interaction of the constituent materials. As before though, properties obtained for similar materials can act as an excellent design guide (epoxy manufactures may have testing data online of their products reinforced with various materials, and reinforcement manufacturers may have testing data online of their materials with a generic epoxy), and can be scaled with the rule of mixtures as a rough estimate.

Material properties in the material axes can be used directly for design when using a finite-element package (such as Solidworks' COSMOS, where property transformation is done within the FEA) and when the fiber is oriented in the direction of the major stresses imposed on a structure. However, when the material and structural axes do not conveniently align, an algebraic transformation must be carried out in order to use these properties for design. The foundation of such orthotropic transformations will not be treated here, but can be found in several resources or in print. The codes in "Composite Material Transformations" in the Design Programs directory of the HPVDT shared documents are designed for this purpose.

It is important to note that composite failure modes are always highly dependent on the geometry of the structure being analyzed. Therefore failure testing on representative structures and proof-loading of finished parts (i.e. loading to the ultimate design load, with a Factor of Safety of 1) should be done whenever possible. This provides valuable feedback into the design process and also ensures that no major errors are made which would cause unnecessary danger to the occupant of an HPV.

1.4 Considerations for Configuration Design
Much understanding of configuration design comes with experience, iteration, and learning what to look for. This is a well-considered (but not necessarily exhaustive) list of subsystems and concerns to bear in mind:
 * Driver Protection System Locations: The ASME HPVC requires that a rollcage fully-enclose the occupant of the HPV for protection. Thus the location of the rider determines the placement of one of the most significant structural components to some degree (i.e. the rollcage must roughly enclose the rider's body and center of mass);
 * Impact/Contact Locations: In addition to the loads required by ASME rules, crash loads will be applied to the structure at various points. Depending on the crash case (e.g. wobble and rollover), these will be determined by a combination of the wheelbase and outer shell design (for example to determine the first point of impact on the shell). The design of the shell will also determine whether crash loads will be borne on a single point of impact (in which case a rib at that location is desired) or whether the shell will roll onto and then around an isolated but larger area (in which case a larger sandwich panel may be desirable);
 * Drivetrain Loads and Attachment Points: The internal drivetrain structure must resist the pedalling loads imposed by the occupant through the structure, and also bear the loads of the front wheel (in the case of all U of T bikes to-date). Therefore hardpoints must be built into the members of the vehicle's primary structure where appropriate.
 * Mount Points for Other Hardware, Components: Other components which bear or transition loads to the primary structure include the seat, wheel-dropouts, integrated brake mounts, etc. For example, the rear wheel dropout would not be bonded directly to the thin laminate of the aerodynamic shell only;
 * Door Opening and Canopy Location: Large openings in a structure create gaps in the otherwise most efficient load path or reduce the stiffness of the overall structure. To this end, frame members or ribs are often placed along the edges of large openings;
 * Landing Gear: Landing gear requires hardpoints for attachment as the gear bears part of the gravity load of the bike, as well as impact loads from rolling on rough surfaces. In addition the landing gear must be deployed via either small (less concerning) or large (and often extremely inconvenient) openings, depending on the mechanism.
 * Handling Loads: Handling loads are often difficult to predict (how much force will someone squeeze the shell with when the bike is lifted?). However, these can often be designed from experience with similar structures in previous bikes. Reinforcement around large openings is typical, frame members are necessary on areas the pilot is expected to grasp when entering/exiting the bike, etc. Some areas must be preserved as off-limits during handling (e.g. large unsupported panels) in order to forgo strengthening the entire shell beyond a reasonable level.

1.5.1 ACE (2009-2010)
ACE's primary structure was contained entirely in the bottom shell (or bathtub structure), with the top shell (opening along the horizontal plane intersecting the nose) only stiff enough to support aerodynamic loads. The bathtub primary structure elements included gunwales running along the edge, a reinforcing panel bearing the weight of the occupant and the rear drivetrain attachment point, ribs bearing the load of the two forward drivetrain attachment points to the gunwales and around the front wheel opening, the rollcage (with bottom half-ring integrated into the bathtub and top half-ring bonded afterwards), and the rear "stays" connecting the rear dropouts to the rollcage and gunwales. A sandwich panel was bonded in the rollcage to strengthen the overall ring structure and act as a "shear wall", but this resulted in the whole rollcage being overbuilt. ACE we designed for the "Speed Class" at the time, and thus no allowances for landing gear were made.

ACE had several concerns from a rider protection and usability standpoint. For one, the lack of structure in the top shell made it very flexible and at times left the rider feeling exposed (and depending on the thin top shell for protection). The front drivetrain structure design (aluminum tubing with effectively two attachment points) was very flexible and prone to deflection-induced chain drops.

1.5.2 Vortex (2010-2011)
Vortex was designed with a more closed shell and smaller door opening rather than the top/bottom split (for improved aerodynamics), resulting in a more rigid overall structure. Near the nose of the bike was a front ring, which was connected at the top to form one drivetrain attachment point, and bonded at the bottom to a horizontal sandwich panel which gave the bottom-forward and bottom-rearward drivetrain attachment points (3 overall). A longeron went towards the back of the bike from the ring, connecting with the rollcage around the pilot (behind the door opening). One short rib was used to tie the longeron into the back end of the horizontal sandwich panel, behind the front wheel opening. The drivetrain structure was formed from laminated honeycomb sandwich panels, which was extremely heavy. Behind the rollcage, a "chainstay" and "seatstay", oriented as on an upright bicycle rear-wheel support, tied the rear dropouts into the back edge of the rollcage. The rollcage incorporated a Nomex sandwich panel shear wall as with ACE before. The door cutout was also framed with a lower-strength sandwich member to bear handling loads from the pilot, as well as to act as a forward rollcage to a degree. This member ended up taking a large portion of the crash loads imposed on the shell. The landing gear attachment was made via bonded threaded inserts into the rear face of the shear wall, and the hole for the landing gear's extension-style deployment was arranged to pass behind the rollcage and under the chainstay, thus missing any major structural components.

Vortex has proved to be an extremely resilient structure. The closed-shell and front door frame have been excellent for increased overall stiffness and rider protection. Unfortunately, the triangle formed by the front ring, front door frame, and longeron provide a solid and consistent wear patch during rollovers, causing excessive abrasion on this patch. The use of sandwich panel also on this bike may have been excessive and heavy, and should be avoided in the future.

1.5.3 Bluenose (2011-2012)
Bluenose incorporated large sandwich panels in the side surface of the shell, from the front drivetrain attachment point back to the rollcage. This was to facilitate the structure rolling around during a crash, rather than wearing on only a few high-points as occurred with Vortex's ribs. The front drivetrain attachment was formed by a thick sandwich panel connected to the side panels. The drivetrain structure itself was formed as a long hollow shell structure. In addition to the rollcage in the usual location, a small secondary rollcage formed the canopy front frame (the opening meant for pilot entry was very small). The large front opening meant for the windshield was framed by a thick gunwale/longeron. The chainstay and seatstay were located as with Vortex, and again the landing gear openings were in the same location (behind the rollcage, below the chainstays). Bluenose also had two aluminum tube crossbars, one in front of the canopy at the level of the gunwale (for the rear drivetrain attachment point) and one behind the pilot's shoulders (for the top seat attachment, as well as to stabilize the two sides of the rollcage).

The large side panels were not as resilient as had been hoped. A large number of crashes during rider training, as well as a weaker sandwich core meant to confer more formability during manufacturing, has resulted in a large failed patch of the sandwich panel where the structural rigidity is diminished. This is an extremely extensive repair to carry out. Also, the front aluminum tube broke free at one point, illustrating the difficulty of bonding aluminum to composites.

=2. Modelling and Analysis= Summary

2.1 Design Process and Modelling
-Modelling process, design cycle (parallelizing) (model elements to be considered later) -may need to be done in several methods for different considerations (exact model with finite thicknesses, for tooling generation, or surface based for FEA) -review all considerations before embarking on a strategy -Surface modelling handbook -Solidworks tutorials

2.2 Load Cases
-applied (ASME, other)

2.3 Structural Analysis in Solidworks
Intro, summary giving background on solidworks, potential future work in finding other package -FEA tools available, used, considerations (surface model, overlaps) -FEA in solidworks (suspect solution, validation required, consider another solution? HyperSizer) -optimization in the future -potential for empirically-based design

2.3.1 Model Generation
(surface model, with full overlaps of each member to its connecting elements)

2.3.2 Material Definition
-for each element, directionality, Material Library

2.3.6 Review Results
-stress concentrations, overall stresses

2.3.7 Revise and Repeat
=3. Tooling Methods for Composites= Summary -composite processing addressed here

3.1 Preliminary Considerations
-depends on timeline, robustness of aero, material processing method, costs

3.2 Survey of Tooling Production Methods
-,advantages (tooling for hardware integration also)

3.3.3 Bluenose (2011-2012)
=4. Fabrication and Processing Methods for Composites= Summary

4.1 Preliminary Considerations
-cost -time -experience -complexity

4.2 Survey of Processing Methods
-available (prepreg, resin infusion, wet-lay, vacuum bagging) -multi-step processes to integrate structural elements