Tham khảo tài liệu 'failure analysis case studies ii episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 93 reinforcing buttresses the occurrence of several crack arrest lines on the fracture surface indicated that the crack extended after the initial fracture either while fluid was still issuing from the tank or at a later stage when the crack was investigated by the firemen and others at the scene of the accident. Simple hand pressure on the side panel revealed the fracture surface the act of doing so putting extra stress on the long crack which already existed. . Pressure conditions in the failure tank The exact stress on the lower tank panel where fracture occurred can be calculated relatively easily since the volume of the contents is known and the height of the top level of the contents is also known within reasonably close limits. Together with a knowledge of the density of the caustic soda the hydrostatic pressure acting on the panel at the failure locus can be determined from the simple formula pressure p hpg 1 where h is the height from the origin of the fracture to the top level of the liquid when fully loaded just prior to the accident p is the density of caustic soda at ambient temperature and g is the gravitational constant m s-2 . The thickness of the panel at the origin of fracture is about 12 mm and the ratio to the radius of the tank of about m is a figure well in excess of the figure of about 10 normally regarded as the threshold ratio between thick and thin walled pressure vessels 4 . The failed tank can therefore be regarded as a thin-walled vessel subjected to simple hydrostatic pressure. Neglecting for the moment the self weight of the vessel and any creep in the plastic wall the situation is that described by Roark 4 in his Table 28 case Id . There is only one important stress acting in the wall the hoop stress ƠH acting around the circumference of the wall. There is no stress acting in a vertical direction. It is therefore a force tending to extend the circumference in tension and hence acting on the vertical welds. This .
