Analysis of Shape and Load-Displacement Response of Convertible Car Hoods.
A multidisciplinary project involving civil and mechanical engineering, optical measurement, and material
testing. Poster presentation in the House of Commons, “Britain’s Younger Engineers”, 4th December 2000
University of Warwick Research Team:
Civil Engineering: Dr W.Lewis (project leader), L.W.Choong), S.Shire.
Mechanical Engineering: Dr A.Lucey, Dr C.Shaw, J.Knight.
Optical Engineering: Prof. P.Bryanston-Cross.
Industrial Collaborator: Jaguar Cars, A.Thomas, Dr. M. Richardson.
Project Objectives:
Development of a computer-aided design methodology
is required to complement/ eliminate expensive
prototyping methods, with the emphasis on:
•Production of an appropriate cutting pattern to avoid
differential stressing of fabric, which affects the
durability of the hood.
•Prediction of the effect of aerodynamic loading on the
shape of hood and stress pattern, particularly for the
newly designed hoods.
•Inclusion of:
-tolerances in the frame/ padding manufacture of the
composite hood.
-uncertainties associated with material properties.
Structural Analysis:
Single layer hoods, ‘rag-tops’.
•The novel design methodology uses the principle
of a minimal surface to calculate the shape.
•The calculated shape is consistent with a constant
tension field (as found in natural structures such
as soap films) -a desirable feature from the point
of view of durability.
Double layer ‘composite’ hoods (with padding between
the inner and outer layers of fabric).
•Shape is pre-determined by designers. Stress field is found
by computation involving iterative methods and making
use of non-linear material properties.
•Anisotropic material properties are to be taken
into account to provide a cutting pattern for the
fabric.
•Load-displacement response to aerodynamic loading is
found by fluid-structure interaction involving coupling
between the structural solver and fluid dynamic simulations.
•Successful experimental work on small-scale models was
carried out to test the methodology and involved optical
measurement.
•Resultant forces on the supporting structures, i.e. the
padding and the hood frame, are provided.
•Work on the cutting pattern completes the progress.
A soap film will naturally form a minimal surface.
Such surfaces use a minimum amount of material
and have a constant value of surface tension.
Materials testing:
Results are shown for 5 uniaxial tests and 4 biaxial tests in warp and weft directions respectively
•The graphs show highly anisotropic and non-linear fabric properties which needed to be established from tests.
•Properties vary significantly from one sample to another.
•The knowledge of material properties is essential in determining the best cutting pattern for the hood fabric.
Uniaxial tests (weft)
60
35
50
30
40
true
30
head
20
Stress (kN/m)
Stress (kN/m)
Uniaxial tests (warp)
10
25
20
true
15
head
10
5
0
0
0
4
8
12
16
20
24
0
5
10
15
25
30
Combined biaxial test results -weft
Combined biaxial test results -warp
18
55
50
45
40
35
30
25
20
15
10
5
0
y = 0.874x - 0.3756
16
y = 1.8644x + 16.132
14
Stress (kN/m)
Stress (kN/m)
20
Strain (%)
Strain (%)
y = 3.6212x + 9.393
12
10
y = 2.1666x - 15.338
8
6
4
y = 8.0382x + 0.0257
y = 0.2579x - 0.2186
2
0
0
2
4
6
8
10
True Strain (%)
12
14
16
18
0
2
4
6
8
10
True Strain (%)
12
14
16
18
1200
Fluid Dynamics
Modelling:
1000
•Predicts aerodynamic
loading on the hood.
front
•Pressures generated
from 3D modelling are
used to determine the
forces acting on the hood
and displacements.
800
600
400
Initial prediction of pressure field in the
centre-line plane for turbulent flow over
the passenger compartment of the car.
200
0
0
500
1000
1500
side
Computed contours of pressure over the hood
Optical measurement
-flat roof model:
•Used to test the methodology
adopted, using a small scale
physical model in a wind tunnel.
•Measurement of the
displacement of the membrane
was made at several wind speeds
using Infra-red proximity probes.
•Plots show the initial shape of
the membrane (with zero
pretension), and its deformed
shape at a wind speed of 9.57m/s.
Tensions in fabric under wind loading:
warp-weft alignment
1100
-6 0 0
1000
-4 0 0
-2 0 0
x(mm)
z(mm)
900
800
0
200
700
400
600
600
500
200
400
600
800
1000
y (m m )
1200
1400
200
1600
Displacement of hood fabric under aerodynamic loading:
Contour plot of hood displacement under wind loading
18
600
16
14
400
12
front of hood
200
10
8
0
6
-200
4
2
-400
0
-600
-2
200
400
600
800
1000
1200
1400
1600
400
600
800
1000
y (m m )
1200
1400
1600