检查焊接钢结构的疲劳破坏
疲劳裂纹生长
起重机结构设计时所采用的许用应力水平，考虑到此种现象。容许应力水平采用 200 万个应力波动循环设定，并应采用数据带的下限安全系数。一些检测结果（约5%中）在许用应力以下破坏。这是统计学现象，也就是说，在实际的起重机结构中有可能发生裂纹增长现象。
世界上至今还没有这样的公认的起重机规范，说明在某一低水平的应力不会导致裂纹增长（随机的）。裂纹最终会逐渐变大，直至可用无损探伤方法，即磁粉探伤（MT）、超声波探伤（UT） 或“染色-渗透”法探伤（DPT）。一些裂纹在出现破坏前可用外观检查。
在很多情况下，即使有裂纹发生，结构装置也不会免受严重的损坏，只要将起重机立即停止并修理。
需要进行定期检查，检测裂纹是否有很快增大，以预防起重机出现严重的结构故障。
如果发现裂纹，应及时修理!
一般建议结构应恢复到原始的状况。如果不可行，则通过切割或打磨，改变其几何尺寸是比较经济的。
有时会发现裂纹比预计的增长快很多。这可能是由于起重机的过载操作造成的。如果发现这种情况，应检查一下操作情况，看看是否有过载发生，采取补救措施，以免以后操作中出现过载。裂纹可能由于实际和试验的几何尺寸之间的差别造成的，在这种情况下，修正几何尺寸应该是比较经济的方法。
Inspection for fatigue cracks growth in welded steel structures
Fatigue crack growth.
The allowable stress levels used in the design of the crane structures takes this phenomenon
into account. Allowable stress levels are determined by analyzing laboratory tests of typical
details subjected to two million cycles of stress fluctuation and by applying a factor of safety
to the lower limit of the test data scatter band. In a few laboratory tests (about 5 %), failure
occurs at a stress level below the allowable. This is a result of the statistical nature of the
phenomenon and means fatigue crack growth will occur in all real crane structures.
There are no recognized container crane specifications used in the world today that have
stressesat such a lowlevelthat fatigue crackgrowth willnotbe expectedto occuron arandom
basis. Eventually, the fatigue cracks will become large enough to be detectable by means of
nondestructive testing methods, i.e. MT, UT or DPT. Some cracks are detectable by visual
inspection before fracture increase.
Even if the fatigue cracks occur, in many cases the remaining elements of the structure will
prevent seriousdamage so long asthe crane isimmediatelytaken out of service and repaired.
Periodic inspection is required to detect fatigue cracks that have growth to a significant size
in order to avoid serious failure in the crane structure.
When the crack is found, repair is necessary!
Restoring the structure to it’soriginal condition is usuallyrecommended. Sometimesthis isnot
practical, and a change in geometry by cutting and grinding means is more economical.
Sometimes fatigue cracks are found with a frequency significantly greater than would be
expected considering the stress levels used in the design. This may be caused by overload
in the crane during operation. If such of cracks are found, a review of the operation should be
made to see if overload is occurring, and remedial measures should be developed to avoid
future overloading. The cracking may be due to differences in geometry between the actual
detail and the tested detail. In this case, it may be economical to modify the geometry.