An acrylic (PMMA) window is clamped in a thick low carbon steel frame at  . The temperature falls to  , putting the window under tension because the thermal expansion coefficient of PMMA is larger than that of steel. If the window was stress-free at 20°C, what stress does it carry at −20°C? Use the result that the bi-axial stress caused by a bi-axial strain difference is

where E is Young’s modulus for PMMA and Poisson’s ratio  . You will find data for expansion coefficients in Table A6 and for moduli in Table A3, Appendix A. Use mean values.

Data From Appendix Table A3

Data From Table A6

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T=20C.