A Method For Evaluating Long-Term Strength Of Polymer Composites
Andrzej Bełzowski Politechnika Wrocławska, Instytut Materiałoznawstwa i Mechaniki Technicznej, ul. Wybrzeże Wyspiańskiego 27, 50-370 Wrocław
Annals 1 No. 1, 2001 pages 38-41
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abstract Degradation processes in polymer composites (PC) induced by load and various physical/chemical effects result in decreasing mechanical properties. At present the long-term strength of PC pipework is determined according to the ASTM D 2992 standard. The test consisting in long-term loading of a large lot (at least 18) of big samples is expensive, difficult to perform and lasts up to two years. That is why a simpler and cheaper method is highly desirable. The presented method of evaluating the long-term strength of PC is based on the presumption that mechanical degradation induced by dynamic or prolonged static loading may be looked at through changes in the mean strength value of a material and its dispersion (Fig. 1). A greater degree of strength scatter due to fatigue degradation may lead to reduced residual strength (Fig. 2). The autor’s research makes him state that a higher strength scatter affects the safety factor in degree that is by one order of magnitude higher than the influence of decreasing mean strength. Another noteworthy phenomenon affecting structural integrity of pipework is the size effect (SE) predicted by the Weibull theory (formula (1)). The SE manifests itself by material strength values becoming lower with increasing volume of a structure and increasing strength value dispersion. Rough calculations by the present author who used data from cyclic 3-point bending of UD composite samples showed that a similarly loaded actual structural member with mass 103 higher could undergo a 26.4% decrease in strength after 105 cycles. This value may be considered as a sum of the following contributions: 1.5% - a decrease in mean strength, 17% - a decrease due a initial SE, 7.9% - a decrease due to the degradation-related SE (increased scatter). For 3.1×105 cycles the above contributions are as follows: 38% = 2.7% + 17% + 18.3%. The results are presented in Fig. 3. They reveal the dependence of material strength values on characteristics of scatter. The proposed method for evaluating long-term strength of PC may be presented as a sequence of three steps: 1. Analysis of service conditions of a structure and stress variation at critical points to design tests that would approximate most closely the actual degradation process. 2. Determination of the tensile stress (Rm) from as many as 8÷10 specimens and determination of the residual degradation curve ΔR(1) along with the corresponding standard deviation value. 3. Determination of the strength decrease due to ES (ΔR(2)) and to increased scatter (ΔR(3), see formula (1)) and determination of the final strength Rk = Rm − (ΔR(1) + ΔR(2) + ΔR(3)). The method is faster, cheaper and easier to use than that conforming to the ASTM D 2992 recommendations. The significance of size effects requires that the use of the method should be restricted to comparative assessments.