Abdominal aortic aneurysm (AAA) sac size is a major clinical problem. The maximum sac diameter is the major determining factor for treatment. There is a direct correlation between the wall stress and rupture sites for AAA with maximum wall stress not being dependent on the maximum diameter of the vessel but on the morphology of vessel. The reflective photoelastic method was developed to experimentally evaluate the stresses and strains in a model AAA. The epoxy resin material was used as the self‐supporting structure and had similar mechanical properties to the aneurysmal aorta. For both static and dynamic fluid testing the high‐stressed regions were located proximally and distally to the aneurysm. This correlates to reported in vivo rupture sites. There was a low‐stress region at the location of greatest diameter at the centre of the aneurysm. The direction of the maximum principal stress was found to be in the circumferential direction. The maximum stress for a high blood pressure of 22.7 kPa (170 mmHg) was 35% greater than that for a normal blood pressure of 16.0 kPa (120 mmHg). The photoelastic method is a powerful and innovative method of analysing stresses in AAA models, producing results, which are visual and easy to interpret.

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A major disadvantage in strain gauge work on plastics is the effect of strain gauge reinforcement, which induces signifcant errors in the quantitative evaluation of strain derived from the plastic. The parameters influencing reinforcement are well documented and procedures have been developed that permit the identification and correction of this effect in certain situations. The principal causes, effects and corrective procedures are reviewed and where appropriate compared with results obtained by the authors of this article.

Experimental and finite element analyses have attributed considerable significance to the role of the prosthetic collar in load transfer to the proximal femur; however, some clinical evidence suggests that collar/calcar contact is unnecessary and detrimental. This study investigates the effects of collared versus collarless prostheses on cement mantle stresses in a model analysis of the proximal femur.Three-dimensional embedded strain gauging is used in a three-times full-size model of an implanted left femur. Tensor stresses in the cement mantle are derived for a collarless ExeterrM stem, a small and a large medial-collared stem and a full-collared prosthesis. Results show that the collar causes an increase in proximal-medial compressive longitudinal stresses, with other proximal stresses significantly reduced. Subsidence of the stem in the cement mantle is prevented, inducing unfavourable pivoting of the prosthesis about the calcar into varus. The collar type produces only minor additional effects in the mechanism of load transfer.The analysis suggests that collars may inhibit the prosthesis from attaining long-term secondary stability, by preventing the stem subsidence evident with cemented collarless implants. Derived stresses also illustrate the considerable variation in the mechanism of load transfer between collared and collarless prostheses in both the proximal and distal regions. prostheses designs: namely, a collarless Exeter stem, a small and a large medial-collared stem and a fullcollared prosthesis.

The embedding of three‐dimensional strain rosettes embedded into epoxy models provides an experimental technique for analysing complex structures; however, this technique has been known to produce data that were difficult to explain in terms of their physical significance. To gain a greater insight into the behaviour of a three‐dimensional strain rosette used in this way, a three‐dimensional strain rosette was embedded into each of two separate prismatic bars of square cross‐section and subjected to fundamental tests of compression and torsion in standard commercial testing machines. In initial tests on a bar containing a three‐dimensional strain rosette (Bar A) the data derived from the individual gauges sometimes departed from the theoretical values by more that 30 μe. After critical evaluation of the procedures used for making and testing Bar A, further tests were carried out on Bar B, which led to a reduction in the difference between theoretical and experimental data to 14 μe, acceptable for most practical purposes. The use of square plugs containing three‐dimensional strain rosettes which are embedded into square cavities in the model, and the measurement of the actual direction cosines of the gauges on the square plug prior to embedment is a distinct advantage over the use of cylindrical plugs. In addition, the use of testing machines with a fixed base as opposed to a floating lower platen is recommended.