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modelling purposes. It is not only the relative ease of the manufacturing process but also
the high flexibility concerning a desired formulation, i.e. creating models providing certain
characteristics. Typical thermoset cast polymers like phenol-formaldehyde and epoxy resins
are nowadays relieved by systems subject to radical or cationic polymerization processes
initiated by UV or by some kind of ionizing radiation, e.g. electron or gamma radiation.
Despite some problems concerning dimensional stability due to shrinking and as a subsequent
effect the generation of internal stresses did not effectively prevent these types of
resins and/or monomers from gaining ground in this field of application. But it is not only the
steric polymerization or the socalled stereolithography these systems are recommended for
(1,2) but also for obtaining answers in that part of experimental mechanics where the
anticipated characteristic of the material is the balance of mechanical requirements and a significant
postcure response.
Polymers suggested to serve as material for photoelastic investigations must exhibit a list of
special requirements, they have to be fairly transparent, usually their glass transition
temperature should be beyond ambient temperatures, they should provide acceptable
mechanical data like Young's modulus etc. but the most desirable characteristics are a
photoelastic sensitivity, matching the required elastic modulus and a rather low degree of
creep or relaxation. A preferred uncomplicated and rather easy approach to manufacturing
and reproducibility of models need not be emphasized particularly.
The photoelastic effect is the instantaneous generation of birefringence in solid/glassy
polymers being due to the orientation of structural units, permanent or induced dipoles,
produced by the application of a stress to the material. Usually this stress should only come
into effect during the investigations and not casually during the curing process. However the
method of simulating three dimensional deformations and analysing the stresses within the
observed parts or load bearing structural components cannot be used as a reliable tool
unless the relationships between the stress-optical behaviour and the other variables of the
system are thoroughly established. Thus an experimental procedure including a standardi-
zed sample manufacturing process has to be created to provide reliable values of the
material properties, stress-optical coefficient and optical creep behaviour.
