Raymond F.      Mignogna,P.E.

A monthly series devoted to considerations of basic technical topics related to metallurgy and statistics. 

TENSILE TESTING* MACHINES

Tensile testing machines, many of which can be used to perform compression and bending tests (and therefore are also sometimes referred to as Universal Testing Machines, or UTM's), are today manufactured by several companies and come in a number of configurations ranging from small table top models to multistory units designed to test large diameter steel rods (rebar) and other massive specimens.

The basic function of the machine is to apply loads to a specimen in order to produce a stress-strain curve for the material being tested.  There are two ways of applying a load to a specimen.  The first of these is done by using an electromechanical machine based upon a variable speed electric motor which drives a gear reduction system which then drives a number of screws that move the crosshead up or down, thereby applying either a tensile or compressive load to the sample.  The speed of the crosshead is changed by changing the motor speed, the gear ratios, or both.  The second type of machine uses a hydraulic piston to move the crosshead, the loading rate being controlled by a pressure-compensated needle valve (generally set by the operator) or an electrically operated servo valve for more precise control.  ASTM E-4 discusses methods used to verify the accuracy of forces or applied loads to tensile testing machinery, among others.

Electromechanical machines have a greater range of test speeds and crosshead displacements, while hydraulic units can generate higher loads at lower cost.

An important, but often overlooked consideration is that of maintaining a constant strain rate (which is not necessarily the same as constant crosshead speed) during the test.  Work done by a number of researchers (including myself) over the years has shown that, for many materials, there can be considerable variation in both yield and tensile strength values depending upon the speed (strain rate) at which the test is conducted.  However, in most cases, this effect occurs over large orders of magnitude and in practical application, maintaining a constant crosshead speed is sufficient to avoid difficulties.  

There are several methods for mounting and gripping the specimen within the machine.  All of them rely on maintaining a reduced cross section within the specimen so that failure will occur within that region.  However, it is also important to properly mount and align the sample, as significant differences in test results can occur if these tasks are not done properly.  While there are formalized methods to verify that alignment meets certain minimum requirements, method 3 of ASTM E-102 simply requires that an appropriate visual and dimensional inspection be made of the test setup and specimen mounting prior to conducting the test.

When conducting tests at elevated (or reduced) temperatures, it's generally necessary to have an environmental chamber mounted within the machine so that the specimen can be heated or cooled to the test temperature.  It's important to pay close attention to heating and cooling rates when doing this, as the test can be invalidated if the sample is not uniformly at the correct temperature when the test is conducted.

The last major equipment item that's usually involved in a tensile test is an extensometer, which is a device that directly measures the extension of the sample as the load is being applied to it.  Most modern units have an electronic signal capability that's fed directly back into the machine, so that deflection can be plotted internally as the load is being applied.  Again, there are several typesof extensometers, and ASTM E-83 discusses their calibration and use.

Further information on tensile test machinery is available from the various equipment manufacturers.**

*An excellent reference work on the entire subject is "Tensile Testing", 2nd Edition, J.R. Davis, Ed., ASM Int'l. publ., 2004

**Google "tensile testing machines" for access to their web sites.