CHAPTER 16
EXPERIMENTAL STRESS ANALYSIS
Introduction
We live today in a complex world of manmade structures and machines. We work in
buildings which may be many storeys high and travel in cars and ships, trains and planes; we
build huge bridges and concrete dams and send mammoth rockets into space. Such is our
confidence in the modern engineer that we take these manmade structures for granted. We
assume that the bridge will not collapse under the weight of the car and that the wings will not
fall away from the aircraft. We are confident that the engineer has assessed the stresses within
these structures and has built in sufficient strength to meet all eventualities.
This attitude of mind is a tribute to the competence and reliability of the modern engineer.
However, the commonly held belief that the engineer has been able to calculate mathemati-
cally the stresses within the complex structures is generally ill-founded. When he is dealing
with familiar design problems and following conventional practice, the engineer draws on
past experience in assessing the strength that must be built into a structure. A competent civil
engineer, for example, has little difficulty in selecting the size of steel girder that he needs to
support a wall. When he departs from conventional practice, however, and is called upon to
design unfamiliar structures or to use new materials or techniques, the engineer can no longer
depend upon past experience. The mathematical analysis of the stresses in complex
components may not, in some cases, be a practical proposition owing to the high cost of
computer time involved. If the engineer has no other way of assessing stresses except by
recourse to the nearest standard shape and hence analytical solution available, he builds in
greater strength than he judges to be necessary (i.e. he incorporates a factor of safety) in the
hope of ensuring that the component will not fail in practice. Inevitably, this means
unnecessary weight, size and cost, not only in the component itse