Introduction

Designers are, as they say, spoiled for choice. In fact, the sheer number of materials available can be a bit daunting. Materials that are stiffness-driven, or dominated by the load in one direction are generally better candidates for carbon fiber composites. Structures that are loaded in two or more directions at the same time are probably better candidates for metal. Generally. Probably.

“You can’t really always know beforehand what material system is best for your application,” admits Craig Collier of Collier Research. “It has a lot to do with shape, and the loading of the structure.”

Your preconceived notions, he says, may be wrong: “We’ve seen that composites do bring down weight in many applications, but not in all applications.” He offers as an example his firm’s case study of the Orion spacecraft heat shield (images below) for the NASA Engineering Safety Council. When Collier’s HyperSizer software was used to explore the different material types, titanium turned out to be substantially lighter than carbon graphite composites.

See Relevant News Article:    Digital Engineering – Lightweight material champions

Conceptual Design

During conceptual phase of design explore alternative configurations and architectural layouts.

There are three fundamental approaches to generate variant conceptual structural designs.

1st is to repeat the process of generating new CAD surfaces to reflect the geometry of your structure; and from these surfaces, mesh a FEM and apply updated external loads.

This is implemented by importing the new FEM into HyperX .

Though this is referred to as a ‘fresh’ FEM import, the database’s materials, design properties and analysis properties can quickly be reapplied to the new mesh.

The 2nd is to reuse your existing FEM by morphing the mesh and keeping the external applied loads the same.

This is implemented by reimporting the modified FEM into HyperX. HyperX setup stays intact.

A typical use case is an airframe wing trade study of different wing shape designs that vary the chord to camber with an overall increase or decrease in wetted areas. Though these variants have different geometry, the geometry changes can be handled by lofting the existing nodal grids to the new surface. The morphed mesh maintains the original property definitions and is handled with either of our FEM reimport capabilities.

See page FEM Workflow: FEM reimport.

The 3rd is to not use FEA but rather calculate internal forces closed form.

This is usually implemented with spreadsheets that pass data using the HyperX API.

In early launch vehicle studies the rocket’s diameter, length, and placement and sizes of tanks for LOX, Hydrogen, other fuels are traded, as well as lengths of interstages, intertanks, etc. Additionally the internal ullage pressure of the fuel is traded against the weight savings of buckling stable tank walls vs extra weight of turbo pumps.

These multitude of possibilities can be defined in a spreadsheet.  Simple closed form equations calculate internal loads such as hoop (Ny) transverse tension from internal pressure as a function of diameter, axial compression from launch F=MA.  Being able to call HyperX from the spreadsheet is enabled via scripts using the software’s API.

As a conceptual design example, the NASA Engineering Safety Council (NESC) was tasked to design alternative crew module architectures. While maintaining structural safety, light weight is also paramount.

Three different OML shapes were evaluated. Since the driving load case was internal pressure, the mesh was morphed into different shapes without having to recompute external applied loads. The objective was to determine the shape most efficient at supporting the internal load paths. The 2nd approach described above was appropriate and HyperSizer (HyperX) was able to quickly and accurately quantify the weights and structural integrity of the three variants.

Preliminary Design

During preliminary phase of design evaluate the performance of different materials and panel & joint concepts.

Composite

Metal Machined Grid Stiffened

After conceptual design, there remains a multitude of more design decisions to make. A design decision should not be made independently as its choice will affect the performance of other design possibilities.  A factorial combination of your chosen possibilities needs to be first established, and then using HyperX you are able to execute complete project evaluations of each unique combination rapidly without having to remesh your FEM.  Performing these types of trades below is quickly and conveniently accomplished in the preliminary design phase.

Composite Material Trades

• Tape or Fabric
• Thermoset or thermoplastic or resin infusion
• Intermediate modulus or high modulus

Metal Material Trades

• Aluminum or Titanium or high strength steel
• Thermal environment vs temperature dependent material properties

Panel Trades

• Uniaxial stiffened open Tee shape or closed section hat/omega shape
  • • Height and all cross-sectional dimensions of the shape
• Composite solid Laminate or honeycomb core sandwich
  • • Ply layup
• Metal integrally machined grid stiffened orthogrid or isogrid
  • • The spacing pattern and heights of the webs, pocket size of the skin, gauge thicknesses

Joint Trades

• Single, double, or triple shear joints
• If mechanically fastened: Fastener types, diameters, spacings, rows
• If adhesively bonded: Overlap or stepped, tapers, adhesive material and thickness, bond line lengths

Structural Layout Trades

• Wing rib spacing and fuselage frame spacing
• If stiffened panel: stiffener spacing
• If sandwich panel: core ramp and edge band

Manufacturing Trades

• Large unitized parts or smaller joined parts
• Integrally machined plate or riveted sheet metal
• Autoclave or out of autoclave

Laminate Producibility Trades

• Ply angle % limits to quantify weight vs producibility. See page: Producibility – Laminate Family Design Study
• Laminate family alternatives to quantify weight vs producibility.
• See page: Producibility – Laminate Family Design Study

Though the performance of each factorial combination should be quantified, there may be too many to consider. Thus, a funneling down process may have to be followed to limit the count to a reasonable number of evaluations. If so, in general, design decisions most likely would be made in the order of trades identified above.

Detailed Design

During detailed design phase establish the optimum dimensions, thicknesses, and laminates of each part of your structure. 

Trade studies are performed for insight into how much customization of a part should be done. More variability permits weight savings, but also additional challenges analyzing and producing.

For composite, at this point in the design, some dimensions are locked down such as stiffener spacing of composite panels as the tooling is a long lead schedule item. Though the ply material is usually decided, other composite variables such as the laminate stackings are still available for improving.

For machined metal parts, the material and grid stiffening pattern such as orthogrid vs isogrid have typically been decided. But the individual gauge thicknesses of the skin and webs are still being refined to achieve weight savings while meeting the local load gradients of the part.

Commercial wing box rib. Each skin pocket and web flange may be optimized to different thicknesses during detailed sizing. 

Dashboard

Upload HyperX results to a database hosted in either the cloud or on company servers. Users and project managers can view the results using typical web browsers from a laptop or smart phone.

As engineers continue to search for lightweight structures, the most innovative will emerge from robust trade study optimization that tackles every variable, generates a number of alternative designs, and identifies substantial weight savings. The role of the dashboard is to preserve this generated data and to make the weight trends available to all.

Benefit

Your company’s users can upload HyperX results from their local database to a shared database hosted in either the cloud or on company servers.

Users and project managers can view the results using typical web browsers from a laptop or smart phone.

Interactive 2D curve plots allow browser to select any general combination of data types. Trendlines give immediate insight into design studies. These company specific and proprietary computed results enable decision making for your current project, and are invaluable for future project planning.

In Action

The dashboard is an active archive of your company’s valuable analysis and sizing results that is assessable immediately from any device by click of a button. It’s sort of like a ‘Google search’ of your own secured data, but provided in a meaningful trendline. 

Similar to Amazon and Best Buy websites, powerful and intuitive filters allow user to drill down in GBs of data to isolate results of interest.

You can identify data such as

• • On the K4 wing upper surface, what are the tape and fabric materials.
  • Weight difference between thermoset, thermoplastic, and resin infused wing box on the model x5 aircraft.

You can define trendlines such as

• • Total count of different fastener types and sizes used on the 2009 model X fuselage, the 2015 model Y fuselage, and the 2021 model Z fuselage.
  • On the K4 wing upper surface, what were the laminate ply angle 0/45/90 precents and ply counts per spanwise station. Create plots with and without including small damage compression after impact and with and without including large notch damage two bay crack criteria.

User can export the data from the dashboard to an Excel spreadsheet. Data can only be imported to the dashboard from HyperX as it secures all data credentials and classification tags.