Fatigue strength of laminate pipes (GRP) for mining industry
Kazimierz Wilczak*, Jacek Mamos** *Politechnika Śląska, Gliwice, Główny Instytut Górnictwa, pl. Gwarków 1, 40-166 Katowice **Główny Instytut Górnictwa, pl. Gwarków 1, 40-166 Katowice
Annals 4 No. 10, 2004 pages 194-199
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abstract The publication discusses test results of glass-polyester pipes submitted to a cyclically variable internal pressure. The results have been presented in the form of Wohler’s charts showing fatigue strength of spirally wound pipes made from glass roving and different types of polyester resins such as slow-burning and anti-electrostatic resins approved in the mining industry. Tests have been run on pipes of internal diameter of 90 mm and wall thickness of 4 mm, made from: 1) internal protective layer, rich in resin, made from surface glass mat of 30 g/m2 and construction mat of 300 g/m2, 0.5 mm thick in total, 2) load-bearing (construction) layer, made of three spirally wound roving layers, approximately 3.5 mm thick in total. The angle between roving fibers and pipe’s longitudinal axis equaled 54o45’ and was optimal for pipe’s internal pressure load ratio. Figure 1 shows placement of roving fibers in the construction layer. Fatigue strength tests have been conducted with rectangular cyclic variable internal pressure, as presented in Figure 2. The cycle parameters have been listed in Table 3. Tests have been carried out on the equipment presented in Figure 3, which shows also the actual pattern of pressure impulses. The actual impulses in the samples have been registered by tensometric manometers and an oscillograph. Tests results have been presented in Tables 1, 4, 5, Figures 4, 5 and summarized in Figure 6. Tests results have been shown graphically as Wohler’s charts and ratios σ1max = f(N), where N represents a number of cycles before breakage occurs. Tests have been conducted at 5 to 6 levels of load pressure σ1max. The levels have been determined in such a way as to obtain a number of cycles (N) equivalent to pressure changes of 106, which is commonly recognized as the maximum long-term fatigue strength. The charts show linear ratio y = f(x) which is equal to σ1 = f(N), separately for lower and upper levels of σ1max and determine the trend lines. The most important interdependence is at the lower level of pressure, which shows pipe’s behavior when a big number of fatigue cycles approaches pressures which determine fatigue strength. Key words: plastics, composites, pipes, fatigue strength