Page 601 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 8. 1 Design and material utilization

to four times the static wheel load. Predicted life based are considered, before moving to the spaceframe concept
on specimen test data could be twice that obtained from and finally the inevitable hybrid configurations. In this
service load data. Calculation of the damage contribution context hybrid means mixed material content and
of the individual events counted in the rainflow analysis introduces the ‘user friendly’ advantages of polymers
can be compared with conventional cyclic fatigue data to (low impact resistance and styling freedom) combined
obtain the necessary factoring. In cases where complete with the lightweight advantage of aluminium plus the
load reversal does not take place and the load alternates practicality and safety connotations associated with steel.
between two stress values, a different (lower) limiting The latter has always been a strong argument of the
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stress is valid. The largest stress amplitude which alter- anti-CAFE lobby in the USA who contend that the
nates about a given mean stress, which can be withstood benefits in fuel economy accompanying hybrid lighter-
‘infinitely’, is called the fatigue limit. The greatest en- weight bodies are achieved at the expense of vehicle
durable stress amplitude can be determined from a fa- safety and claim to have accident statistics to prove this.
tigue limit diagram, (e), for any minimum or mean stress.
Stress range R is the algebraic difference between the
maximum and minimum values of the stress. Mean stress 18.1.7.1 The unitary aluminium body
M is defined such that limiting stresses are Mþ= R=2.
Fatigue limit in reverse bending is generally about 25% The development of the all aluminium body is now more
lower than in reversed tension and compression, due, it is associated with the A8 and A2 spaceframe type of vehicle
said, to the stress gradient – and in reverse torsion it is (described later) which constitutes a different type of
about 0.55 times the tensile fatigue limit. Frequency of concept, and the need for a fundamentally different type
stress reversal also influences fatigue limit – becoming of design may become more obvious if the production of
higher with increased frequency. An empirical formula an aluminium body is considered with conventional pro-
due to Gerber can be used in the case of steels to estimate duction technology. Although some use was made of
the maximum stress during each cycle at the fatigue limit aluminium prior to 1900 for the Durkopp developed
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as R/2 þ (s u 2– nRs u ) 1/2 where s u is the ultimate tensile sports car and later reference was made to the Pierce
stress and n is a material constant ¼ 1.5 for mild and 2.0 Arrow body (1909) which incorporated rear end panel,
for high tensile steel. This formula can be used to show roof, firewall and doors in cast aluminium, the Dyna
the maximum cyclic stress s for mild steel increasing Panhard was probably the first aluminium bodied car to be
from one-third ultimate stress under reversed loading to mass produced in Europe in any numbers. The Honda
0.61 for repeated loading. A rearrangement and simpli- NSX sports car represents the most recent conventional
fication of the formula by Goodman results in the linear body built within the context of modern manufacturing
relation R ¼ (s u /n)[1– M/s u ] where M ¼ s – R/2. The and proves that although some equipment modifications
view in (e) also shows the relative curves in either were necessary assembly was possible in moderate
a Goodman or Gerber diagram frequently used in fatigue numbers.
analysis. If values of R and s u are found by fatigue tests
then the fatigue limits under other conditions can be 18.1.7.1.1 The Honda NSX
found from these diagrams. Following a consideration of specific strength, specific
Where a structural element is loaded for a series of rigidity and equivalent rigidity compared with sheet steel
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cycles n1, n2. at different stress levels, with corre- and SMC the decision was made to manufacture the
sponding fatigue life at each level N1, N2. cycles, BIW in aluminium to reduce the weight by about 140 kg.
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failure can be expected at n/N ¼ 1 according to The rigidity of a car such as the NSX is critical to
Miner’s law. Experiments have shown this factor to vary maintain steering stability, and to help improve this the
from 0.6 to 1.5 with higher values obtained for sequences sills were produced as extrusions with variable side wall
of increasing loads. thickness. The comparison is shown in Fig. 18.1-22.
To satisfy different requirements for strength, form-
ability, weldability and coating, detailed preparatory
18.1.7 Alternative body architecture background studies showed that different alloys should
be used for different panel applications and these are
Before examples of more adventurous modern designs indicated in Fig. 18.1-23.
are presented, certain vehicles are now highlighted which It was found that wrinkling and shape control were the
illustrate further interesting steps in body and materials main problems on forming, attributed to lower modulus
development. Having commenced with essentially steel which resulted in more springback (compared to steel)
bodies of unitary design, and it must be remembered that and also the lower ‘r’ value. Twice the overcrowning
these still constitute the vast majority of volume cars allowance was required than for steel in the forming of
produced, ‘conventionally built’ aluminium structures door outer panels. Together with proportionally lower

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