Automotive Technology - MECH 330 - Notes

1. Introduction

These notes are intended to give an outline of the material to be covered in this module which aims to give students a good understanding of how light weight high strength automotive structures and components are designed and produced by examining in detail the specialist materials, manufacturing methods and analysis techniques that are used. The topics to be covered are:

The Assignments for this module are here

1.1 Loads
Automobiles are subjected to a full range of loads including shear and bending (due to the vehicle mass and loads being distributed) torsion (caused by irregular road, putting 1 wheel on a curb) lateral loading (caused by cornering) and fore and aft loading (caused by acceleration and decelleration). In practice these occur in combination together with dynamic / transient effects.
Allowable Stress - typically an automobile structure will be designed so that under the worst envisaged dynamic load conditions there will still be a factor of safety based on the yield strength of 1.5, ref A1. This is usually adequate for guarding against fatigue failure, but fatigue calculations are still used where stress concentration effects are likely to be significant.
In practice, vehicle design is normally governed by stiffness requirements rather than maximum permitted stress levels.

2.1 Casting

Casting of metals is a very long established manufacturing method and many variations have been developed over the years to cater for new materials and products. There are a number of areas that are of specific interest to automotive engineers including:

Some recent developments are described in ref. C4.

One of the benefits of cast irons is their ability to suppress vibration due to comparatively high internal damping capacity (ref G1), brought about partly by the micro-structure.

Material Relative damping capacity
Grey iron, coarse flake 100-500
Grey iron, fine flake 20-100
Ductile iron 5-20
Pure iron 5
Eutectoid steel 4
Aluminium 0.4

2.2 Cast Irons for Crankshafts

Crankshafts suffer cyclic loading so the fatigue endurance strength of the material is critical to their performance. For cast irons a key factor determining the fatigue performance is the distribution of the graphite in the structure. In grey or flake irons, the graphite is distributed as long thin flakes, many of which will be exposed on any machined surface. These provide large numbers of in-built surface defects from which fatigue cracks can grow. Hence the fatigue endurance strength of such irons is quite low. The presence of these flakes also means that such irons are relatively insensitive to notches.

Fatigue testing of ferrous materials indicates a stress level below which no failure occurs, however many load cycles are applied, this is called the fatigue endurance strength or endurance limit. Normally the number of cycles to reach this limit is between 106 and 107 and if a test piece lasts this long it is assumed that it will last indefinitely. However some failures of cast iron have occurred after more than 107 load cycles and hence. Consequently fatigue testing carried out by BCIRA is done on the basis of 2 x 107 cycles (ref. C1).

The ratio of fatigue endurance strength/UTS for cast iron in reversed bending is between 0.33 and 0.47 (ref. C1). Alloying additions and heat treatment which raise the UTS will normally not improve this ratio. Fillet rolling / surface rolling does normally improve the fatigue performance.

Iron with the graphite in nodular form is ductile and can readily be cast by including elements that promote agglomeration of the graphite, additions of magnesium and cerium are frequently used, this was discovered in the 1940's. For many applications nodular irons are superior to other types of iron and can be processed to have mechanical properties similar to those of low and medium carbon steels (UTS: 370 - 800 MPa, 0.2% proof stress: 230 - 460 MPa, ref. C1). This has resulted in considerable increase in the production and use of nodular irons for items such as crank shafts for car engines where reasonable strength combined with the capability for economical large scale production is somewhat more important than obtaining the maximum possible strength.

Compacted Graphite Iron (CGI) - developed in the 1970's - has a structure intermediate between those of grey flake iron and nodular ductile iron. CGI can be made consistently by the Bruhl Oxycast route or the SinterCast method. This material was developed to provide an iron that did not need extensive alloying but would be stronger than grey iron while being easier to machine than nodular iron. CGI's typically have a 35% greater stiffness and 75% higher tensile strength than grey iron and a higher fatigue strength than aluminium at automobile engine operating temperatures. Tensile strengths between 250 and 450 MPa, ref. C2, are available (yield strengths 175 to 315 MPa). The first likely large scale automotive application, due 2003, for this material will be the cylinder blocks of the diesel engine being developed by PSA (Peugeot Citroen Group) and Ford to power their luxury cars. Using this material means that wall thicknesses need only be half those with flake cast iron. The block will have a weight of only 60 kg, but be stronger and stiffer. Much development work has concentrated on machining (high power is needed and clamping must be done carefully to avoid distortion while machining) and this can now be done satisfactorily.

2.3 Austempered Nodular Cast Irons

Austempering as a treatment for alloy steels was discovered in the 1930s. The main advantages are that although the structure formed (bainite - acicular ferrite with carbide needles) is not as hard as martensite (about 50 Rc compared to about 60 Rc) which forms during a conventional quenching process, the structure is much tougher and because the cooling is interrupted, there is less distortion. In irons the high silicon content suppresses the precipitation of the carbide phase and a lamellar structure of acicular ferrite and high carbon austenite is formed.

With unalloyed irons, very rapid quenching from the autenitising temperature - even of small section thicknesses - is necessary to to avoid the precipitation of ferrite or pearlite. By incorporating alloying elements that lengthen the ferrite and pearlite transformation times, slower cooling rates, such as can be obtained by salt quenching or forced air cooling, can be used. The most effective alloying addition is molybdenum, but additionally copper and / or nickel may also be added as may limited ammounts of manganese. The material retains the nodular graphite distribution of ductile iron, but the matrix is acicular ferrite in a high carbon austenite. UTS values of 800 to 950 MPa with elongations of 8 - 10% are readily obtained.
The fatigue endurance strength of austempered low alloy irons is about 0.35 times the UTS for UTS values below 800 MPa, for higher UTS values the rate of increase of fatigue strength is less.
A lot of detailed information can be found in reference C3.

The advantageous mechanical properties combined with the ability to cast parts means that these irons are replacing plain carbon and low alloy steels for many components.

References
A1: 'An Introduction to Modern Vehicle Design', Ed. J Happian-Smith, Butterworth-Heinemann, 2001.
A2: 'Light Metals for the Automotive Industry', SAE SP-1683, 2002.
A3: 'Designing and Achieving Lightweight Vehicles', SAE SP-1684, 2002.
A4: 'First and Second International Symposia on Superplasticity and Superplastic Forming Technology', Ed. D G Sanders and D C Dunard, ASM, 2003, ISBN: 0-87170-758-6.
C1: 'Cast Iron, Physical and Engineering Properties' by H T Angus, Butterworths, 2nd ed. 1976.
C2: 'Marks' Standard Handbook for Mechanical Engineers', 10th Ed., McGraw-Hill, 1996.
C3: 'High Strength Austempered Ductile Cast Iron', by E Dorazil, Ellis Horwood, 1991.
C4: 'Casting Solutions for the Automotive Industry', SP-1504, SAE, 2000.
G1: 'Properties and Selection of Irons, Steels and High Performance Alloys', ASM Metals Handbook, 10th Ed. 1990.
G2: 'Metals Handbook', ASM, 2nd, Desk ed., 1998.
G3: 'Innovations in Sheet Steel and Bar Products', SP-1764, SAE 2003.

Meaning of reference numbers prefix letters:

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David J Grieve, 4th June 2004.