檢查焊接鋼結(jié)構(gòu)的疲勞破壞

疲勞裂紋生長

起重機結(jié)構(gòu)設(shè)計時所采用的許用應(yīng)力水平，考慮到此種現(xiàn)象。容許應(yīng)力水平采用 200 萬個應(yīng)力波動循環(huán)設(shè)定，并應(yīng)采用數(shù)據(jù)帶的下限安全系數(shù)。一些檢測結(jié)果（約5%中）在許用應(yīng)力以下破壞。這是統(tǒng)計學現(xiàn)象，也就是說，在實際的起重機結(jié)構(gòu)中有可能發(fā)生裂紋增長現(xiàn)象。

世界上至今還沒有這樣的公認的起重機規(guī)范，說明在某一低水平的應(yīng)力不會導(dǎo)致裂紋增長（隨機的）。裂紋最終會逐漸變大，直至可用無損探傷方法，即磁粉探傷（MT）、超聲波探傷（UT） 或“染色-滲透”法探傷（DPT）。一些裂紋在出現(xiàn)破壞前可用外觀檢查。

在很多情況下，即使有裂紋發(fā)生，結(jié)構(gòu)裝置也不會免受嚴重的損壞，只要將起重機立即停止并修理。
需要進行定期檢查，檢測裂紋是否有很快增大，以預(yù)防起重機出現(xiàn)嚴重的結(jié)構(gòu)故障。

如果發(fā)現(xiàn)裂紋，應(yīng)及時修理!

一般建議結(jié)構(gòu)應(yīng)恢復(fù)到原始的狀況。如果不可行，則通過切割或打磨，改變其幾何尺寸是比較經(jīng)濟的。

有時會發(fā)現(xiàn)裂紋比預(yù)計的增長快很多。這可能是由于起重機的過載操作造成的。如果發(fā)現(xiàn)這種情況，應(yīng)檢查一下操作情況，看看是否有過載發(fā)生，采取補救措施，以免以后操作中出現(xiàn)過載。裂紋可能由于實際和試驗的幾何尺寸之間的差別造成的，在這種情況下，修正幾何尺寸應(yīng)該是比較經(jīng)濟的方法。
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.