机械类毕业设计外文翻译--龙门式起重机金属材料的疲劳强度预测-材料科学(编辑修改稿)

机械类毕业设计外文翻译--龙门式起重机金属材料的疲劳强度预测-材料科学(编辑修改稿)内容摘要：

oint between spreader andgrab takes place, the tension of the ropes happens 1 s after switching the electric drive on, the clearance in the joint taking up. The revolutions of the electric motors reach the nominal value in O.} s. The detachment of a load from the base, from the moment of switching electric motors on to the moment of full pull in the ropes takes s, the tensions in ropes increasing smoothly to maximum. The stresses in the metalwork of the bridge and supports grow up to maximum values in 12 s and oscillate about an average within %. When a rigid load is lifted, the accelerated velocity of loading in the rope hanger and metalwork is practically the same as in case of fast hoisting of a log pack. The metalwork oscillations are character ized by two harmonic processes with periods and 2 s, which have been obtained from spectral analysis. The worst case of loading ensues from summation of loading amplitudes so that the maximum excess of dynamic loading above static can be 1314%.Braking a load, when it is lowered, induces significant oscillation of stress in the metalwork, which can be }r7% of static loading. Moving over rail joints of 3} mm height misalignment induces only insignificant stresses. In operation, there are possible cases w hen loads originating from various types of loading bine. The greatest load is the case when the maximum loads from braking of a load when lowering coincide with braking of the trolley with poorly adjusted brakes. 4. Fatigue loading analysis Strain measurement at test points, disposed as shown in Figs 4 and 5, was carried out during the work of the crane and a representative number of stress oscillograms was obtained. Since a mon operation cycle duration of the crane has a sufficient scatter with average value } , to reduce these oscillograms uniformly a filtration was implemented to these signals, and all repeated values, . while the construction was not subjected to dynamic loading and only static loading occurred, were rejected. Three characteristic stress oscillograms (gauge 11) are shown in Fig. 6 where the interior sequence of loading for an operation cycle is visible. At first, stresses increase to maximum values when a load is hoisted. After that a load is transferred to the necess ary location and stresses oscillate due to the irregular crane movement on rails and over rail joints resulting mostly in skew loads. The lowering of the load causes the decrease of loading and forms half of a basic loading cycle. . Analysis of loading process amplitudes Two terms now should be separated: loading cycle and loading block. The first denotes one distinct oscillation of stresses (closed loop), and the second is for the set of loading cycles during an operation cycle. The rain flow cycle counting method given in Ref. [2] was taken advantage of to carry out the fatigue hysteretic loop analysis for the three weakest elements: (1) angle of the bottom chord(gauge 11), (2) Ibeam of the top chord (gauge 17), (3) angle of the support (gauge 8). Statistical evaluation of sample cycle amplitudes by means of the Waybill distribution for these elements has given estimated parameters fisted in Table 4. It should be noted that the histograms of cycle amplitude with nonzero averages were reduced afterwards to equivalent histograms with zero averages. . Numbers of loading cycles During the rain flow cycle counting procedure, the calculation of number of loading cycles for the loading block was also carried out. While processing the oscillograms of one type, a sample number of loading cycles for one block is obtained consisting of integers with minimum and maximum observed values: 24 and 46. The random number of loading cycles vibe can be described by the Poisson distribution with parameter  =34. Average numbers of loading blocks via months were obtained earlier, so it is possible to find the appropriate characteristics not only for loading blocks per month, but also for the total number of loading cycles per month or year if the central limit theorem is taken advantage of. Firstly, it is known from probability theory that the addition of k independent Poisson variables gives also a random variable with the Poisson distribution with parameter k},. On the other hand, the Poisson distribution can be well approximated by the normal distribution with average}, and variation },. Secondly, the central limit theorem, roughly speaking, states that the distribution of a large number of terms, independent of the initial distribution asymptotically tends to normal. If the initial distribution of each independent term has a normal distribution, then the average and standard deviation of the total number of loading cycles for one year are equal to 423,096 and 650 accordingly. The values of k are taken as constant averages from Table 3. 5. Stress concentration factors and element endurance The elements of the crane are jointed by semiautomatic gas welding without preliminary edge preparation and consequent machining. For the inspected elements 1 and 3 having circumfere。