Photoelastic Investigation Using New STL-Resins

Stereolithography is not only ideal to study the function and the design of a simple or complex component, but also for stress and strain analysis by means of photoelasticity. The basis for using Stereolithograpy components is the biremngent property of the photoPOIYffiers, which has been discovered in 1991 in the Lab ofPhotoelasticity and Holography [5]. Therefore, a few acrylate and epoxy resins develoPed by Ciba-Geigy were calibrated and compared with the most commonly used resin, Araldite B (manufactured by Ciba-Geigy, too). The experience shows that static and dynamic photoelastic investigations by using the new STL-resins are possible. The time saving for photoelastic investigations amounts to values about 10 months and the cost saving is equivalent to 90010. Introduction Since the begffining ofthis century, photoelastic investigations have been realized to get a statement about the stress and strain distribution within a component. Concerning photoelasticity, one event ofthe century was the introduction ofphenol resins in 1931 and epoxy resins in 1950. Unfortunately, photoelasticity fell into oblivion through the years due to the introduction of FEA By introducting Stereolithography the main problem of photoelasticity, how to get a transparent model of the prototype, vanished. The biremngent properties of the STL-resins were discovered in the Lab ofPhotoelasticity, Holography and Shearography at the University of Kassel in 1991. Recently, a few companies, especially from the automotive and aerospace industries in the USA, UK and France rediscovered photoelasticity as a suitable method for stress and strain analysis. Mainly, STL-resins based on acrylates and epoxies have been investigated, which were developed by Ciba-Geigy and distributed by 3D Systems. Moreover, a few STL-components were investigated, made from resins of other manufacturers, such as DuPont and Allied Signal and were built on 3D Systems, EOS and Fockele & Schwarze machines. On principle, the most suitable resins to realize photoelastic investigations are the new Ciba-Geigy ePQXY resins (XB 5170 and XB 5180), treated by using the drawing style ACES, but especially the DuPont resins SOMOS 3100 and SOMOS 3110 are suitable, too. The main problem using these DuPont resins are the unsuited drawing styles to realize photoelastic investigations. Most ofthe drawing styles to treat the DuPont resins are leaving cavities within the cured part in order to get fast and accurate STL-components. Unfortunately, on the mechanical point ofview these components are punched discs respectively plates. The most important disadvantage of the STL-components made from the Allied Signal resins "Exactomer" is the poor transparency ofthose parts, especially ifthe components are warmed up to the glass transition temperature. Afterwards, the color ofthe STL-components is nearly brown. Thus, it will be very difficult to examine the isochromatic mnge order. Apart from that, the advantage of the STLcomponents made from "Exactomer" are the high quality in size deviation and surface finishing. Because ofthe above mentioned disadvantages we use Ciba-Geigy resins and the drawing style ACES. Fundamentals of Photoelasticity The determination ofstresses and strains within a component by means ofphotoelastic testing is based on the temporary biremngence ofthe loaded transparent materials, mainly plastics. The biremngence ofthese materials can be shown in polarized white or monochromatic and non-coherent light. Biremngence separates a beam of polarized light into two beams of light oscillating perpendicularly to each other. If loaded solids are biremngent, the directions ofthe two beams correspond to the directions ofthe principal 204 stresses. However, the speed ofthese two beam components is different, which leads to a path difference or relative phase difference. As a result, the difference ofthe principal stresses can be shown as lines ofthe same color, also called isochromatics. An experimental setup for performing photoelastic investigations is shown in Figure 1. It is a very simple and low-cost assembly where the light source can be a common electric light bulb. A lightwave going through the polarization filter becomes linearly polarized. photoelostic signal to be measured light source observer detect zero "black is observed"