Metoda oceny wytrzymałości długotrwałej kompozytów polimerowych

• Autorzy: Bełzowski A.

Warianty tytułu

EN

A method for evaluating long-term strength of polymer composites

• Języki publikacji: PL

Abstrakty

PL

Wykorzystywanie kompozytów polimerowych w budowie rurociągów wiąże się z koniecznością oceny ich własności mechanicznych w całym okresie planowanej eksploatacji. Metody określania wytrzymałości długotrwałej elementów rurociągów stosowane obecnie są kosztowne, trudne technicznie i czasochłonne. Przedstawiona koncepcja metody oceny kompozytów pod względem ich przydatności do pracy przy długotrwałych obciążeniach, oparta na badaniach wytrzymałości resztkowej materiału oraz szacunkach uwzględniających efekt skali, jest potencjalnie szybsza, tańsza oraz wolna od problemów związanych z dużymi rozmiarami próbek.

EN

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 10 3 higher could undergo a 26.4% decrease in strength after 10 5 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 3.1xl0 5 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 AR(1) along with the corresponding standard deviation value. 3. Determination of the strength decrease due to ES (AR(2)) and to increased scatter (AR(3)), see formula (1)) and determination of the final strength Rk = Rm- (AR(1) + AR(2) + AR(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.

Słowa kluczowe

PL

kompozyty; kompozyty polimerowe; rurociąg; wytrzymałość

EN

composites; polymer composites; strength; mechanical properties

• Wydawca: Wydawnictwo Politechniki Częstochowskiej
• Czasopismo: Kompozyty
• Rocznik: 2001
• Tom: R. 1, nr 1
• Strony: 38--41
• Opis fizyczny: Bibliogr. 9 poz., tab., wykr.

Twórcy

autor

Bełzowski A.

• Politechnika Wrocławska, Instytut Materiałoznawstwa i Mechaniki Technicznej, ul. Wybrzeże Wyspiańskiego 27, 50-370 Wrocław

Bibliografia

• [1] Pabiot J. i in., Influence of the fibre/matrix interface on the long-term behaviour of filament wound epoxy/ glass tubing systems, Composite Polymers 1991, 4, 5, 299-315.
• [2] ASTM D 2992 (1991), Standard Practice for Obtaining Hydrostatic or Pressure Design Basis for „Fiberglass” (Glass-Fiber-Reinforced Thermosetting-Resin) Pipe and Fittings.
• [3] Bełzowski A., Krawczak P., Pabiot J., Zagadnienie wytrzymałości długotrwałej rur z kompozytów polimerowych, Dozór Techniczny 2000, 2(188), 31-36.
• [4] Marques Torres A., De Brito Martins F., Comparaison of methodologies for prediction of long-term properties of GRP pipes, Conf. Proc. Progress in Durability Analysis of Composite Systems, eds Reifsnider & Cardon Balkema, Rotterdam 1998, 289-294.
• [5] Beaumont P.W.R., The failure of fibre composites: an overviev, J. Str. Anal. 1989, 24, 4, 189-205.
• [6] Bełzowski A., Badanie degradacji mechanicznej oraz ocena stopnia uszkodzenia polimerowych kompozytów włóknistych, II International Congress of Technical Diagnostics, Warszawa 2000, 10.
• [7] Andersons J., Korsgaard J., Residual strength of GRP at high cycle fatigue, ICCM11 1997, SII-135-144.
• [8] Zweben C., Is there a size effect in composites? Composites 1994, 25, 6, 451-454.
• [9] Wisnom M.R., Size effects in the testing of fibre-composite materials, Comp. Sci. Technol. 1999, 59, 1937-1957.