1979-02-01

INVESTIGATIONS OF ENGINE COMPONENTS BY MEANS OF PHOTOELASTICITY 790859

Use is made of computer programs and photoelasticity within the scope of pressure and thermal stress analysis when developing and testing components for internal combustion engines, e.g. pistons.
Calculation, using finite elements, is described in a number of papers, but comparatively few publications today deal with the possibilities opened up by photoelasticity. The aim of this paper is therefore to indicate the potential applications of photoelasticity in the stress and deformation analysis of engine components with reference to a few examples. In view of the fact that MAHLE GmbH makes extensive use of both engine-component calculation methods employing 2D and 3D finite element computer programs and the opportunities presented by photoelasticity in stress and deformation analysis, this paper is justified in presenting the photoelasticity method with all its merits and demerits and in comparing it with computer calculation using finite elements.
One of the most important objectives of engineers designing and testing engine parts has been to determine the stresses and deformations which occur under operating conditions. A number of strength formulas make it possible, even in the case of fairly complex components, to estimate the stresses which they can withstand without being damaged.
However, engine parts exposed to complex mechanical and thermal loads cannot be accurately analysed with ordinary strength formulas. A typical example is the piston of a combustion engine. The forces acting on it from outside, i.e. the combustion pressure on its crown and the counteraction of the piston pin in the pin holes, rapidly alternating stresses with sometimes high crown temperatures, soon make it clear that such a complex engine part cannot be calculated using simple conventional methods. For many years, therefore, apart from temperature measurements, excellent aids in designing pistons have been stress measurements using strain gages, the demonstration of stress concentrations by the brittle lacquer and photostress methods, and fatigue investigations on resonance in hydraulic pulsators. Over the past 10 years photoelastic investigations of models and computer calculations based on the finite-elements method have been increasingly employed as additional off-the-engine means of determining stresses in, and deformations of, pistons.
In the development of engine components part of the testing work was increasingly carried out in the laboratory.

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