Page 29 - Mechanical Behavior of Materials

30 Chapter 1 Introduction

more durable items require less frequent replacement, thus reducing the environmental impact of
manufacturing new items, including pollution, greenhouse gas emissions, energy use, depletion of
natural resources, and disposal and recycling needs.

1.3.1 Iterative and Stepwise Nature of Design

A flow chart showing some of the steps necessary to complete a mechanical design is shown in
Fig. 1.12. The logic loops shown by arrows indicate that the design process is fundamentally
iterative in nature. In other words, there is a strong element of trial and error where an initial design
is done and then analyzed, tested, and subjected to trial production. Changes may be made at any
stage of the process to satisfy requirements not previously considered or problems just discovered.
Changes may in turn require further analysis or testing. All of this must be done while observing
constraints on time and cost.
Each step involves a synthesis process in which all of the various concerns and requirements
are considered together. Compromises between conflicting requirements are usually necessary, and
continual effort is needed to maintain simplicity, practicability, and economy. For example, the
cargo weight limit of an aircraft cannot be made too large without causing unacceptable limits on
the weight of fuel that can be carried, and therefore also on flight distance. Prior individual or
organizational experience may have important influences on the design. Also, certain design codes
and standards may be used as an aid, and sometimes they are required by law. These are generally
developed and published by either professional societies or governmental units, and one of their
main purposes is to assure safety and durability. An example is the Bridge Design Specifications
published by the American Association of State Highway and Transportation Officials.
One difficult and sometimes tricky step in design is estimation of the applied loads (forces or
combinations of forces). Even rough estimates are often difficult to make, especially for vibratory
loads resulting from such sources as road roughness or air turbulence. It is sometimes possible to
use measurements from a similar item that is already in service, but this is clearly impossible if the
item being designed is unique. Once at least rough estimates (or assumptions) are made of the loads,
then stresses in components can be calculated.
The initial design is often made on the basis of avoiding stresses that exceed the yield strength
of the material. Then the design is checked by more refined analysis, and changes are made as
necessary to avoid more subtle modes of material failure, such as fatigue, brittle fracture, and creep.
The geometric shape or size may be changed to lower the magnitude or alter the distribution of
stresses and strains to avoid one of these problems, or the material may be changed to one more
suitable to resist a particular failure mode.

1.3.2 Safety Factors

In making design decisions that involve safety and durability, the concept of a safety factor is often
used. The safety factor in stress is the ratio of the stress that causes failure to the stress expected to
occur in the actual service of the component. That is,

stress causing failure
X 1 = (1.1)
stress in service