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Thermophysical Properties of Solid Materials Project Section 1A - Cooperatice Thermal Expansion Measurements

The Structures and Materials Panel of the Advisory Group for Aerospace Research
and Development (AGARD) comprises scientists, engineers and technical administrators
from government, universities and industry, who are concerned with the advancement of
aerospace research and development and with the provision of data necessary for the
design and fabrication of the vehicles and equipment which NATO requires. The Panel
provides a mechanism for discussion, the exchange of information and for conducting co-
operative theoretical and experimental studies in selected areas.

The development of aerospace vehicles and equipment requires solid materials which
can withstand increasing temperatures and loads. Typically, gas turbine rotors have to be
made in larger sizes and to operate at higher peripheral speeds. Components such as these,
and many others, are subjected to thermal distributions under conditions of constraint with
consequent deformation and the generation of thermal stresses. Therrnophysical properties,
primarily heat capacity, specific heat, thermal conductivity, diffusivity and thermal expansion
and mechanical properties such as modulus of elasticity, Poisson’s ratio and yield point, are
involved in these phenomena. All these properties vary with temperature in a non-linear
manner.

Thermal stresses, which usually reach a maximum during transient heating or cooling,
are superimposed on stresses resulting from external loads, inertia forces and on residual
stresses. Varying operational conditions involve cyclic variations of temperature and thermal
stress with the possible consequence of low cycle fatigue. This can be as critical a factor as
creep behaviour under steady elevated temperature.

The mathematical treatment of thermal stresses is complex and, because of the many
non-linear relationships involved, iterative methods of solution have to be employed. Further,
because reliable data on the variation of the several properties with temperature are not
available, average data have to be used with consequent loss of precision in the theoretical
predictions. Under these circumstances, reliable results can only be obtained by resort to
expensive and time consuming tests of structural components and assemblies.

The obvious need for reliable data stimulated the Structures and Materials Panel to form
a Working Group on Thermophysical Properties in early 1966. At first attention was given
to the state of knowledge in the appropriate fundamental and applied sciences as well as to
the rapidly developing experimental techniques for temperatures up to 3000°C. In March
1967 Prof. Dr Erich Fitzer reported on the state of the art. As a result, the Panel decided
to institute a co-operative programme among interested scientists, in several NATO countries,
for the purpose of checking their experimental equipment and procedures by using well
defined samples of very pure metals, ceramics, graphites and some engineering materials.