A case study in applying RDM to Automotive BIW Design · RDM®’s Application in BIW Design •Classical BIW Topology Optimisation can be hard to interpret. •It also creates structures

A Case Study in Applying RDM® to Automotive BIW Design

Oliver Tomlin

October 2013

Contents

• The Reinforcement Derivation Method (RDM®)

• The Automotive Body Design problem

• Interpretation and Example results

• Other Applications?

What is RDM®? • It combines the Topology method with an incredibly

efficient way of creating links to an existing structure.

• It is Genesis’ ability to deal with very small volume fractions that allows this method to succeed.

• It came from Martin’s head as a light bulb moment! Once we had calmed him down and got him to speak coherently again all became clear!

• In the first few weeks it was called “Magic Custard” with reference to the fact that all results seemed to be yellow!

The Reinforcement Derivation Method (RDM®)

• RDM® defines the additional reinforcements and load-paths to support a pre-existing structure in the relevant locations to achieve specified targets.

• A significant advantage is the use with a pre-existing structure and load-cases and the simplicity of creating it.

Top-hat Beam with RDM placed to maximise stiffness for a UDL.

RDM®’s Application in BIW Design

• Classical BIW Topology Optimisation can be hard to interpret.

• It also creates structures that are not specific to a manufacturing process, when an Automotive BIW is generally defined by its annual volumes.

• Can only ever be applied before a platform/chassis is defined.

BIW Topology Optimisation Result

Why No Blank Sheet?

• Every structural engineer would like to believe that the layout of the BIW is entirely driven by the efficiencies of load management.

• But having packaged the occupants, the powertrain, some doors and styled the vehicle there is little space for the structure to sit!

• Generally the body structure fills the gaps left by the more important items that make a vehicle function.

Importance of First Mode

• Assessment of fully free natural frequency of the untrimmed BIW:

– This analysis can precede release of all trim, closures and finer details

– Ensures efficient usage of mass (a low frequency is either heavy or soft).

– A good high frequency produces a vehicle that is • Quieter

• Feels of higher quality

• Has better handling

• Retain these characteristics for its lifetime.

– Combined with a static torsion test the BIW can be

developed with “honesty” in each update.

Where To Start?

• A body design is released which reflects the package limitations and manufacturing choices.

• Often sections are carry over from previous vehicles

• It needs to meet the modal and static torsion targets – how do we develop it rapidly to deliver these targets?

The Example Fiesta FE Model

• 2002 Ford Fiesta used for our study, previously validated to a number of tests.

• Mass 405kg (inc bolt on parts)

2002 Ford Fiesta courtesy BAARG

Item

First Mode (Hz)

Static Torsion (kNm/deg)

Test Vehicle (5dr)

32.8 10.6

FE Model (3dr)

27.8 11.8

RDM®

• An FE mesh of the vehicle is our starting point.

• The RDM® region is automatically created in Design Studio over the top at user-defined limits.

• Genesis carries out an optimisation for multiple load-cases, removing superfluous material and only leaving the most important parts.

Interpreting the Results

• Joining (Spot-welds) – No additional mass

– Minimal cost increase

• The RDM® result is ideal for identifying areas of weakness and demonstrates the most effective solution.

• However, the results need to be interpreted into feasible changes which will satisfy manufacturing and practicality constraints. 3 options apply:

• Geometry Modifications – Minimal mass addition

– No extra tools or processing

• Additional Components – Largest stiffness

improvements

Joining (Spot-welds)

• The smaller local RDM® “nuggets” are represented as spot-welds. A total of 10 were created which increased the body stiffness by 1% for no mass change.

Geometry Updates

• Extension of Body Side inner and reinforcement to improve connection with rear boot aperture.

– 1.7kg (0.4%) mass increase

– 1.2Hz (5%) increase in first mode.

Geometry Updates

• Geometry change in lower corner of boot aperture to improve connection of inner back panel to longitudinal



Geometry Updates

• Shape change of back panel around latch cutout and edge of spare wheel clearance.



Geometry Updates

• Geometry change in upper corner of boot aperture to reduce offset in flow of load-paths.



Additional Component

• Additional connection between rear suspension turret top and back panel.



Ford’s Update

2002 Fiesta 2008 Fiesta

• 2008 information only found after RDM study carried out.

RDM® Performance Improvements • 10 spot welds, 4 geometry changes and 1 extra panel.

– First Mode increased from 28Hz to 40Hz (43%)

– Static Torsion increased from 11.8kNm/deg to 19.8kNm/deg (68%)

– At a cost of 7.5kg (1.8%)

– These are quite clearly efficient performance improvements as the static stiffness has increased more than modal performance.

• Quite literally all in a day (or two’s) work!

• Final question – can we reduce the mass penalty and maintain the

same performance?

Thic

knes

s C

han

ge (

mm

)

Gauge Optimisation

Increase Only

Increase & Decrease

No Thickness Change

BIW

First Torsion Mode (Hz) Mass (kg) Torsional Stiffness (kNm/deg)

Baseline 27.8 405 11.8

Modifications from RDM 39.9 412 19.8

Gauge Optimised 40.0 383 19.8

Overview

• The Reinforcement Derivation Method (RDM®)

• Interpretation and Example results

• Other Applications?

• The Automotive Body Design problem

What Else Can RDM Be Used For?

• Plastic trim parts

• Gearbox casings

• Bulkhead designs

• Stiffness for mounting energy absorbing components.

• Non-automotive applications

• Definition of design spaces around existing designs for classical topology.

• Any Questions?

A case study in applying RDM to Automotive BIW Design · RDM®’s Application in BIW Design •Classical BIW Topology Optimisation can be hard to interpret. •It also creates structures

Documents