Page 121 - Mechanical Behavior of Materials

122 Chapter 4 Mechanical Testing: Tension Test and Other Basic Tests

upon a test specimen. Note that the only active motion is that of the actuator rod and piston
combination. Hence, the stroke of this actuator replaces the crosshead motion in the older types
of testing machines.
The closed-loop servohydraulic concept is the basis of the most advanced test systems in use
today. Integrated electronic circuitry has increased the sophistication of these systems. Also, digital
computer control and monitoring of such test systems has steadily developed since its introduction
around 1965.
Sensors for measuring forces and displacements by means of electrical signals are important
features of testing machines. Linear variable differential transformers (LVDTs) were used in
this manner relatively early for measuring displacements, which in turn give strains in test
specimens. Wire strain gages were developed in 1937, and the wire elements were replaced
by thin foil elements, starting around 1952. Strain gages change their resistance when the
material to which they are bonded is deformed, and this change can be converted to an electrical
voltage that is proportional to the strain. They can be used to construct load cells for measuring
applied force and extensometers for measuring displacements on test specimens. The Instron
and closed-loop servohydraulic testing machines require electrical signals from such sensors.
Strain gages are the type of transducer primarily used at present, but LVDTs are also often
employed.
Besides the general-purpose test equipment just described, various types of special-purpose test
equipment are also available. Some of these will be discussed in later chapters as appropriate.

4.1.2 Standard Test Methods
The results of materials tests are used for a variety of purposes. One important use is to obtain values
of materials properties, such as the strength in tension, for use in engineering design. Another use is
quality control of material that is produced, such as plates of steel or batches of concrete, to be sure
that they meet established requirements.
Such application of measured values of materials properties requires that everyone who makes
these measurements does so in a consistent way. Otherwise, users and producers of materials will
not agree as to standards of quality, and much confusion and inefficiency could occur. Perhaps even
more important, the safety and reliability of engineered items requires that materials properties be
well-defined quantities.
Therefore, materials producers and users and other involved parties, such as practicing
engineers, governmental agencies, and research organizations, have worked together to develop
standard test methods. This activity is often organized by professional societies, with the American
Society for Testing and Materials (ASTM International) being the most active organization in this
area in the United States. Many of the major industrial nations have similar organizations, such as
the British Standards Institution (BSI). The International Organization for Standardization (ISO)
coordinates and publishes standards on a worldwide basis, and the European Union (EU) publishes
European Standards that are generally consistent with those of ISO.
A wide variety of standard methods have been developed for various materials tests, including
all of the basic types discussed in this chapter and other, more specialized, tests considered in later
chapters. The Annual Book of ASTM Standards is published yearly and consists of more than 80
volumes, a number of which include standards for mechanical tests. The details of the test methods
differ, depending on the general class of material involved, such as metals, concrete, plastics, rubber,