This is a preprint copy of a conference paper publ. in: Proc. of 11-th Int. Conf. on Mechanics and Technology of Composite Materials, Sofia, 2006, pp. 241 - 246. CREEP-RUPTURE STRENGTH OF EPOXY COMPOSITE R. Krastev*, G. Zachariev*, J. Minster**, J. Hristova* *Central Laboratory of Physico-Chemical Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 1, 1113 Sofia, Bulgaria, e-mail: r_krastev@netbg.com ; **Institute of Theoretical and Applied Mechanics, Czech Academy of Sciences, Prosecka str., 190 00 Prague, Czech Republic, e-mail: minster@itam.cas.cz . 1. INTRODUCTION It is known that plastic products have limited time of application. This is a result of their damage under the both mechanical stress and ageing. The latter is a natural deterioration of their mechanical properties in time. The dependence of creep-rupture strength (CRS) characterizes the long-term mechanical behavior of a material. It could be determined experimentally, but this is a time-consuming and expensive task. Two methods for the assessment of CRS on the basis of short- term tests are known: a method for “strip prediction” of the CRS area [1] and a method for “fork prediction” of the CRS area [2]. The aim of the present work is to verify these express methods. 2. METHODS FOR PREDICTING THE CREEP-RUPTURE STRENGTH Data needed for predicting the CRS of a material under study are found via short-term mechanical tests on beams fabricated from that material. Tests till fracture are performed applying several constant velocities ] , 1 [ , n i v i  of the loading head. Index “BK” refers to short-term tests. Due to the difference between the dimensions of the tested beams, the corresponding conditional maximal edge stress is used instead of the respective force BK F , according to a known formula for bending: b BK BK W F M ) (   (1) Here ) ( BK F M is the maximal bending moment and b W is the resisting moment at bending. The term “conditional” shows the distinction between the actual edge stresses and those calculated via eq. (1). This difference is a result of the occurrence of non-elastic deformations. Factors BK BK / ~ * e K   are determined from each stress-strain (    ) test diagram - see fig.1a. Deformations BK е and BK ~  are found accounting for the cross points between both tangents to the line const  BK  . The first one osculates to the diagram origin and the second one is collinear with line OB . Factor K* represents material ability to deform non-elastically under short-term tests, from the loading start to the moment of fracture.