cranfield university mahadi abd murad an integrated structural ...

5.4 Methodology of Thermal Expansion Experiments
In this thermal expansion experiment, the new technique used two well-matched
rectangular strain gauge rosettes in which the Self-Temperature-Compensation (S-T-C)
number 06 and the resistance of 350 Ohm were first bonded to the reference composite
material, referred to as Experiment 1; the second gauge to the test composite material
(i.e. the test rig) is referred to as Experiment 2. However, both have the same
mechanical properties and thickness. The objective of Experiment 1 was to determine
the thermal output of the strain gauge bonded on to the stress-free specimen and
Experiment 2 was to determine the CTE of the composite repair material on the stress
constrained specimen. In Experiment 2, it also included determination of the CTE of the
steel pipe test rig. The experimental result was later compared to the known CTE value
of the TWF composite and steel pipe. The objective was to study how these two
materials behaved in terms of thermal expansion and also to prove that this approach
was acceptable in determining the CTE in any engineering material.
In Experiment 1, the TWF cloth used as a reference material was cut into 20 x 20
cm square and then impregnated with the matrix epoxy; the process of preparing this
composite was similar to the process of preparing the test material except that this final
cured reference material was on a flat surface rather than a circumferential shape and is
referred to as the test material. The approach to determine the CTE and the thermal
output in the anisotropic TWF composite materials using rectangular strain gauges was
referred to procedural guidelines provided by Vishay Micro Measurements (2007) and
their technique seems to be a more practical approach than the above laboratory
methods, based on simplicity and cost. However, not all the procedures in the guidelines
were complied with because they used two stress-free condition materials; one is treated
as the reference material in which the CTE is known already and the second is called a
specimen material where the CTE is going to be determined. Their technique is further
discussed in the following section.
In this approach, the nature of this experimental test material involved several
other constraints such as different dimensions, geometry, environments etc. and several
assumptions were made. For example, the previous verification has been made
171