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4.1 Introduction
The main types of structure used in automobile construction are:
Monocoque or Integral - the load is carried by all the members except the doors and hatches.
Semi-Integral - there is a separate chassis (often a 'ladder' type - two main longitudinal
sections connected with cross members) but the rest of the structure
carries part of the load. Examples of this can be found in many medium van designs. The stiffness - in torsion -
of this type of design can be further improved by including a cruciform component.
Punt - a 'floor' is some sort of box section with sufficient stiffness so open sports bodywork can be fitted.
More specialist types include:
Space Frame - a triangulated structure of small tubes means that the tubes act primarily in
compression or tension, though welded construction inevitably means some bending and torsion restraint is
present. This type of design has been popular for sports and racing cars.
Torque tube backbone frame - as a closed box section has a much greater torsional stiffness than an
open box section, this has been exploited by sports car manufacturers (Lotus). At the front and rear splayed
beams are added to carry the engine and suspension and a low floor level can easily be provided.
4.2 Design Criteria
To provide the degree of comfort now demanded requires soft, (independent - at least at the front) suspension
with long travel. For the suspension to function properly (and to permit good fit and operation of doors)
it requires that the connections between the suspension mounting points (and the whole bodyshell) be stiff.
The torsional stiffness of twist between the front and rear axles is critical and the bending deflection is
also important. Typical values (ref. S1) are that the minimum torsional stiffness is 4500lbf/deg (6100 Nm/deg)
but 5000 - 5500 lbf/deg (6500 - 7500 Nm/deg) is preferable. Mid span bending deflection should not exceed 0.05 in
(1.27 mm) for a 1500lb (680 kg) mid span load.
The primary design criteria is stiffness rather then strength.
Optimum material thickness or gauge selection is a compromise between the factors listed below:
| <- Thinner |
Thicker -> |
| lower weight |
higher weight |
| lower cost |
higher cost |
| poorer dent resistance |
better dent resistance |
| lower stiffness |
higher stiffness |
lower buckling resistance |
better buckling reasistance |
More expensive materials may be justifiable if they provide better strength or stiffnerss for a given (or even a reduced)
thickness or give less costly manufacturing (processing AND assembly). In recent years there has been a lot of research
into alternative materials and their processing in an effort to reduce weight and costs while maintaining or improving vehicle strength
and stiffness. A key part of this has been the increasing sophistication of analysis tools available to designers,
particularly finite element analysis (FEA) which is enabling better analysis of structures and processes, particularly
sheet metal operations.
An additional consideration is the need for whatever sheet material is chosen to be able to display a
high quality finish. In the case of steel, this may involve using a particular surface finish on the rolls in the
rolling mill.
4.3 Design Methods
i) For the simplest designs using a ladder chassis, the distribution of loads and then stresses and deflections
along the chassis, can be determined by elementary strength of materials methods.
ii) An intermediate analysis method was proposed in 1964 by Pawlowski is called 'Simple Structural Surfaces' (SSS).
This method can be used to determine the loads on and then the stresses in the main structural elements of an integral
structure. A SSS, which is plane, can carry loads (tension, compression, bending and shear) in its plane, but not loads normal to
its plane or out of plane bending. The procedure is to devise a simple representation of a structure using a small number
of flat sheets (and beams). Sheets can represent the floor, roof, bulkheads, side panels etc. The loads transmitted
between each SSS can be calculated and then the stress in each and their deflections can be found. Increasing the
number of SSS gives better accuracy.
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iii) Finite element analysis (FEA) is a (substantially automated) computer based method of dividing a structure
into a large number of simple elements, the stiffness of each is found and the overall stiffness of the vehicle
can be determined by 'assembling' the results for all the finite elements used to model the system. Modern
powerful computers enable good models to be developed containing as many as 100,000 elements and nodes which provide
a high level of accuracy for many types of load case and can even give a reasonable prediction of accident damage.
4.4 Typical Structures
Most structures in automobiles can be considered to be thin sheet panels reinforced by 'beams'. These 'beams'
may be locally thicker pieces of material (eg 'tailor welded blanks') folded seams or formed sections. Flat panels
are avoided as they lead to excessive drumming, where nominally flat areas are required, eg for floors and truck
beds, they are normally reinforced by pressing shallow recesses into them (car carpeting means they are
not noticed by passengers). Suspension and engine may be mounted onto channel fabricated from folded sheet. Typically
mild steel pressings are made from 0.65 mm thickness sheet, where higher strength steels are being employed, thinner
gauges are used.
Although spot welding is very widely used for economy while providing adequate strength for steel body shells, some vulnerable parts (such
as front wings and bumpers) are commonly bolted on to facilitate repair. To save weight a few cars have used moulded plastic
for the bonnet and tailgate, where strength is not important.
While steel is still the most common material, for small production runs, fibreglass body work on a steel chassis
of some type has been popular. In recent years, in an effort to reduce weight, a lot of work has been done on aluminium
for body and / or chassis and Audi now have 2 models (A8 and A2) that make extensive use of aluminium. This has involved
a lot of work on developing adhesive joining techniques.
4.5 Developments in Steel Sheet.
To counter competitive materials and offering weight reduction etc, steel producers have been been developing new
grades and processes. Zinc coating - often as hot dip galvanizing - has been used for many years to protect automobile
pressings, particularly those used for under body application. Zinc may be combined with other metals, eg nickel, to
confer additional benefits and also incorporated into organic coatings to give a good base for a paint finish. Thin organic
coatings may be used as lubricants during forming, if they are thin (1 micron) they will have negligible effect on spot
welding, thicker coatings will reduce weldability.
Pre-painted deep - drawing steel in weldable and non-weldable forms has been developed by the Dutch Hoogevens Group.
These would be particularly helpful with closed box sections which can not be reached satisfactorily by primer.
High Strength Low Alloy (HSLA) or microalloyed steels. A wide range of these steels is available offering
yield strengths between 200 and 1000 MPa (mainly 300 - 450 MPa), according to alloy content and processing. These steels contain less than
0.1% each of elements such as niobium, vanadium and titanium to promote grain refinement or precipitation hardening.
Increasingly used in automotive, offshore, pressure vessels and pipeline applications, typically with a tensile
strength of about 500 MPa. For some automotive applications, these steels do not offer sufficient cold formability where
stretch forming is required. Dual phase steels contain a microstructure of ferrite plus islands of austenite-martensite
which is obtained by quenching from between the A1 and A3 temperatures. This structure gives a more gradual
yielding with a high work hardening rate and large elongation at fracture (rather than a sharp yield point). After some
forming, the yield strength will typically be about 350 MPa, which is similar to HSLA steels.
Laser Welded Tailored Blanks - using laser butt welding to make up a single blank from an appropriate patchwork of different
grades and thicknesses of sheet steel prior to pressing was developed by Audi in the 1980's. This approach is proving to be a highly
efficient method of providing material and strength where needed. The process is now quite widely used, by Ford, BMW, Rover and
Volkswagen.
Applications for this technique include door blanks, pillars, floors and frame members.
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