MATERIALS ENGINEERING CONCEPTS - Materials for Civil and Construction Engineers - page 32

C

FIGURE 1.23 Correction of toe region in stress-strain

A B

Strain

curve.

possible trends of means and ranges in statistical control charts (Burati and
Hughes 1990). Figure 1.22(a) shows sustained sudden shift in the mean. This
could indicate a change of a material supplier during the project. A gradual
change in the mean, as illustrated in Figure 1.22(b), could indicate a progres-
sive change brought on by machine wear. An irregular shift in the mean, as
shown in Figure 1.22(c), may indicate that the operator is making continuous,
but unnecessary, adjustments to the process settings. Figure 1.22(d) shows a
sudden change in range, which could also indicate a change of a material sup-
plier during the project. Figure 1.22(e) shows a gradual increase in the range,
which may indicate machine wear. Finally, Figure 1.22(f) shows an irregular
shift in both mean and range, which indicates a flawed process.

1.6.4 Experimental Error
When specimens are tested in the laboratory, inaccuracy could occur due to
machine or human errors. For example, Figure 1.23 shows a stress-strain
curve in which a toe region (AC) that does not represent a property of the
material exists. This toe region is an artifact caused by taking up slack and
alignment or seating of the specimen. In order to obtain correct values of
such parameters as modulus, strain, and offset yield point, this artifact must
be compensated for in order to give the corrected zero point on the strain
axis. This is accomplished by extending the linear portion of the curve back-
ward until it meets the strain axis at point B. In this case, point B is the cor-
rected zero strain point from which all strains must be measured. In the case
of a material that does not exhibit any linear region, a similar correction can
be made by constructing a tangent to the maximum slope at the inflection
point and extending it until it meets the strain axis.

1.7 Laboratory Measuring Devices

Laboratory tests measure material properties. Frequently, specimens are
made of the material in question and tested in the laboratory to measure