25-04-2014, 03:24 PM
Mechanics of Sheet Metal Forming
Introduction
Modern continuous rolling mills produce large quantities of thin sheet metal at low cost.
A substantial fraction of all metals are produced as thin hot-rolled strip or cold-rolled
sheet; this is then formed in secondary processes into automobiles, domestic appliances,
building products, aircraft, food and drink cans and a host of other familiar products. Sheet
metals parts have the advantage that the material has a high elastic modulus and high yield
strength so that the parts produced can be stiff and have a good strength-to-weight ratio.
A large number of techniques are used to make sheet metal parts. This book is concerned
mainly with the basic mechanics that underlie all of these methods, rather than with a
detailed description of the overall processes, but it is useful at this stage to review briefly
the most common sheet forming techniques.
Common forming processes
Blanking and piercing. As sheet is usually delivered in large coils, the first operation
is to cut the blanks that will be fed into the presses; subsequently there may be further
blanking to trim off excess material and pierce holes. The basic cutting process is shown
in Figure I.1. When examined in detail, it is seen that blanking is a complicated process of
plastic shearing and fracture and that the material at the edge is likely to become hardened
locally. These effects may cause difficulty in subsequent operations and information on
tooling design to reduce problems can be found in the appropriate texts.
Application to design
The objective in studying the basic mechanics of sheet metal forming is to apply this to
part and tool design and the diagnosis of plant problems. It is important to appreciate
that analysis is only one part of the design process. The first step in design is always to
determine what is required of the part or process, i.e. its function. Determining how to
achieve this comes later. When the function is described completely and in quantitative
terms, the designer can then address the ‘how’. This is typically an iterative process in
which the designer makes some decision and then determines the consequences. A good
designer will have a feeling for the consequences before any calculations are made and this
ability is derived from an understanding of the basic principles governing each operation.
Once the decision is made, simple and approximate calculations are usually sufficient to
justify the decision. There will be a point when an extensive and detailed analysis is needed
to confirm and prove the design, but this book is aimed at the initial but important stage
of the process, namely being able to understand the mechanics of sheet forming processes
and then analysing these in a quick and approximate manner.
Material properties
The most important criteria in selecting a material are related to the function of the
part – qualities such as strength, density, stiffness and corrosion resistance. For sheet mate-
rial, the ability to be shaped in a given process, often called its formability, should also be
considered. To assess formability, we must be able to describe the behaviour of the sheet
in a precise way and express properties in a mathematical form; we also need to know
how properties can be derived from mechanical tests. As far as possible, each property
should be expressed in a fundamental form that is independent of the test used to measure
it. The information can then be used in a more general way in the models of various metal
forming processes that are introduced in subsequent chapters.
In sheet metal forming, there are two regimes of interest – elastic and plastic
deformation. Forming a sheet to some shape obviously involves permanent ‘plastic’ flow
and the strains in the sheet could be quite large. Whenever there is a stress on a sheet
element, there will also be some elastic strain. This will be small, typically less than one
part in one thousand. It is often neglected, but it can have an important effect, for example
when a panel is removed from a die and the forming forces are unloaded giving rise to
elastic shape changes, or ‘springback’.
Tensile test
For historical reasons and because the test is easy to perform, many familiar material
properties are based on measurements made in the tensile test. Some are specific to the
test and cannot be used mathematically in the study of forming processes, while others
are fundamental properties of more general application. As many of the specific, or non-
fundamental tensile test properties are widely used, they will be described at this stage and
some description given of their effect on processes, even though this can only be done in
a qualitative fashion.