دانلود رایگان مقاله تحلیل اثر قوس الکتریکی ضربه ناشی از نقص شکست

ABSTRACT

A temporary threadbar used for precast segmental construction broke during the posttensioning stage before reaching the service load. Failure analysis showed that the premature failure of the bar was due to the presence of an arc strike. The arc strike effects on the material and threadbar integrity are considered.

4. Mechanical analysis

There are two experimental results which enable that the linear elastic fracture mechanics is the appropriate tool to perform mechanical analysis of the threadbar failure. First, because of the brittle aspect of the fracture surfaces of both the threadbar broke in service and that corresponding to the sample of test #7 (Fig. 8) are nearly identical. Second, because of the load-elongation curve of the test #7 revealed that the rupture took place in the elastic region of the curve. From the broken bar analysis, it is understood that the fracture was triggered by a semielliptical crack with semi-major and semi-minor axes of 4.5 mm and 2.5 mm lengths, respectively. The stress intensity factor for a cylindrical bar with a semielliptical surface crack was calculated using the following expression [11]: KI ¼ 4P pD2 ffiffiffiffiffiffi pa p XXCij a D i a b j (2) where P is the applied load, D is the bar diameter, Cij are the interpolation coefficients (given in Table 2 of Ref. [11]), and a and b are the lengths of the semi-minor and semi-major axes. Because the linear elastic fracture mechanics is applicable, the fracture load is obtained using the following fracture criterion: KIKIc (3) Considering the following data: D = 26.5 mm; a = 2.5 mm; b = 4.5 mm and KIc = 70 MPa m1/2, the calculated fracture load was 467 kN, which is slightly higher than the design load of 435 kN. This load shows a good correlation with the design load, considering any experimental scatter and the fact that the calculated is only 7% higher than the design load. However, there are two factors which could cause further reduction in the service load of a threaded bar with an AS. First, the fracture toughness value to be considered is not that of the massive material but that of the heat affected material ahead the current crack tip. Second, due to the dynamic character of the initiation of the AS inducted fracture, the bar failure is not controlled by the static fracture toughness (KIc) but the crack-arrest toughness (KIa) which is significantly lower [12].