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CERTAIN LAWS OF FATIGUE DGES IN WELDED CRANE GIRDERS A. B. PatrikeevUDC 624.014.25.004.6 Massive damage to crane runway girders, recently observed in shops at various plants, is attracting broad attention from specialists.In connection with this, questions regarding the improvementof the girders and developing methods of designing them have been the subject of a large number of investigations. However, there has yet been no significant increase in girder reliability, and the discussion continues on such important matters as the mechanism of formation of cracks and methods of their prevention. Together with fundamental problems caused by the specifics of the stress state of the sections in which cracks are forming and growing (nonuniform biaxial compression, reorientation of the principal sections with each loading cycle, etc.), the developmentof methods of predictingfatigue and improving the design of the beams is being delayed by the fact that insufficientattention is being given to the features of the damage they incur in service. Examined below are the most common types of damage in welded crane runway girders and certain laws governing their formation, which have been established by investigating designs in existing shops and studying specimens obtained from these beams. Figure 1 schematically depicts the main types of fatigue cracks which occur in the upper sections of runway girders. The data in Table i, obtained independently of each other, show that the main form of girder fracture is cracking of the welds connectingthe web with the top flange. About 80% of all of the types of damage are accounted for by such cracks. These cracks were also cited as the main reason for the failure of girders at plants in the Federal Republic of Germany i, Poland 2, and England 3. To study the conditions of formation and character of propagation of the cracks, batches (groups) of specimens were obtained from dismantled girders from different shops. The girders differed in the dimensions of their cross sections, the magnitudes of the service loads, and the fracture features. Examination of metallographic sections established that all of the specimens of the first and second groups in the web-flange joints had unfused gaps from 1.5 to 6.5 mm wide, with cracks (Fig. 2) developing from the ends of most of these gaps (in i0 of 17 cases and 23 of 28 cases, respectively). The branching of these cracks, features of which are discussed below, in the flange welds sometimes resulted in secondary cracks not directly connected with the unfused areas (Fig. 2c). The flange welds in the specimens of the third group hadunfused areas extending through the entire thickness of the web, except for certain specimens with a small unfused area extending up to 1.5 mm. Cracks did not form in the welds in these specimens and were not observed in the girders from which the specimens were obtained. The specimens of all of the groups had clear signs of mushroom-shaped plastic strains (Figs. 2a, 7a), the mechanism of formation of which was examined in 4. Although the sections on which cracks grew in this case had undergone plastic deformation, the fracture was reminiscent of high-cycle fatigue. This conclusion is based on the fact that the number Fig. 1. Main types of cracks in the upper sections of the girders: WDI) crack in the flange weld; WD2) crack in the weld connecting the top flange and the web; WD3) crack in the weld connectingthe top flange and the stiffening rib; WI, W2, W3) cracks in the web (variety); E) crack in the end of the girder; A) region of initiation of cracks of type W3. Fig.2. Macroscopic metallographic specimens of sections of contact between the web and the top flange. of load cycles to the appearance of the first cracks was at least (0.7- 1.0).106 5, 6 and that the specimen parts divided by a crack were immediately adjacent to each other over the entire fracture surface. The width of the crack opening ranged from 0.001-0.004 mm near the tip to 0.015- 0.03 mm at sections far from the tip. The finding from the inspectionsthat main cracks grow in the specimens in the flange welds exclusively from the ends of unfused areas allows us to conclude that such nonfusions are evidently the only source of this most common type of fracture. This conclusion received strong support from study of the girders in existing shops, the results of which are shom in Fig. 3. Each strip in this figure correspondsto a group of girders of the same type in one shop (bay). The length and position of the strip relative to the x axis corresponds to the range of measured values of the legs of the right triangles of the welds K, expressed in fractions of the thickness of the web we. It follows from Fig. 3 that only welds of small cross section experience fracture. Girders with welds of sufficientlygreat cross section do not fracture, even though they may be in long (12-17 yrs) and intensive use. The presence of nonfusions in the girders of the first type is confirmed by the fact that, with the typical depths of fusion and the form of the welds made by automatic submergedarcwelding under typical conditions 7, the cross sections of these welds are inadequate to ensure complete fusion. In fact, for these welding conditions, the depth of fusion S TABLE i. Relationship between Number of Cracks in Flange Welds and Webs of Girders Notes. 1.) No. I gives results of inspections made mainly by the author; No. 2 generalizes data obtained in TsNIIPSK, UkrPSK, and the E. O. Paton Institute ofElectricWelding. 2) The numerator shows the absolute number of cracks, while the denominator shows the number in terms of percentage. is less than 0.56K, according to 7. Taking S0.56we as an approximate criterion for ensuring complete fusion, we find that this inequality is satisfied en K/6we0.89. It is apparent from Fig. 3 that this condition is not met for girders with cracked welds. Apart from the commonness, cracks in flange welds are characterized by the following features. 1. They occur earlier than fatigue damages of other types. The appreciably (by a factor of three to five) shorter life of the welds is due to the sharp stress concentrator, wtlich is the end of the nonfusion. 2. As a result of their branching, these cracks often reach the web. In this case, cracks often do not originate independently in the web 8. Being the source of yet another type of girder fracture, cracks in flanges are thus actually responsible for most of the damages, as indicated in Table i. Without going into several interestingfeatures of the macro- and microstructure of the fractures,which are the subject of a special investigation, we would like to comment on the following characteristic detail, concerning one of the forms of crack branching discovered. Certain fracture surfaces have sections with irregularly located folds (ridges), the sharp crests of which serve as a source of secondary cracks (Fig. 4). Propagating over internal sections of the welds, these cracks reach the web (Fig. 5) or grow in welds parallel to the main cracks (Fig. 2c). Branching is seen only in girders subjected to local stresses of a fairly high level 8. The specimens of the second group correspondto such girders. As has been repeatedlynoted, the cracks are distributed evenly along the girders I,4, 9, except for the end sections, where the most damage is usually seen 3, 4, 6, i0, ii. The reasonsfor the formationof cracks primarily(sometimesexclusively)in the bearing Fig. 3. Fracture of welds of different thicknesses (in materials from girders in existing shops): a)no cracks in welds; b) welds with fatigue cracks. Fig. 4. Macrostructure of the fracture surface through the weld (the strip inthe middle part of the specimen is the unfused area). Fig. 5. Transitionalsectionof crack from weld to web as a result of branching. sections of the girders are not clear. The opinion has been advanced that this is due to the mismatching of the height of the bearing sections of adjacent girders due to imprecise construction, so that one of the ends is subjected to greater loads 3, i0. Another point of view is that the bearing sections are more prone to incur damage because the combination of the components of the stress state with respect to the conditions for resisting fatigue are less favorable than for sections farther from the support ii. The first hypothesis is not consistentwith data from studies showing that cracks frequently occur simultaneously at the ends of two girders on a given support. It is also significant that no similar pattern was found in an analysis of the distributionof longitudinal cracks in the equal-leg angles of riveted girders, suggestingthat the common instances of preferentialcrack formation in the ends of welded girders cannot be explained either by the features of the loading conditions or by the stress state of these sections. Study of damaged elements and specimens obtained from them showed that preprocessing defects of two types act as sources of fracture. The first such flaw occurs when the flange welds are made by an automatic welder without the use of lead-out straps and are ended i00-400 mm before the ends. These sections are welded at subsequent girder processingstages by manual or semiautomatic welding (Fig. 6a). The defective character of such sections is due to the presence of a butt between welds (or a place where the weld begins again after it was interrupted) and shallow fusion depth from the manual or semiautomatic welding (Fig. 2d). Defects of the second type are related to the joining of three mutually perpendicular welds in the end of the girder. Under the existing practice, these welds are made separately. Thus, defects at the point of their contact are unavoidable, and they will give rise to cracks (Fig. 6b). The study data shows that the end sections with defects of the first type are characterized by low endurance (3-4 yrs), while end sections with defects of the second type often crack after 10-15 yrs of intensive use. The most common of the variety of fatigue cracks encountered in the webs and shown in Fig. 1 are types W2 and W3 4, 9, I0, which occur in the plane of the intermediate stiffening ribs (Fig. 7). The zones in which these cracks are nucleated differ appreciably in their design, but both types of cracks are seen with equal frequency.In a number of cases, such cracks are seen simultaneously in the vicinity of one rib. According to our observations, the appearance of cracks near the intermediateribs precedes the fracture of the welds connecting the ends of these ribs to the top flange (Fig.7). Here, to form a crack, it is sufficient that the weld fracture only at one of two ribs located in the same section on opposite sides of the web (Fig. 1). Fig. 6. Cracks in girder ends: a) growth of crack from section of juncture of welds made by automatic (A) and manual or semiautomatic M(SA) welding; S. S. P.) site of beginning of weld after its interruption (from：“stop-start position”); b) growth of crack from section of juncture of differently oriented welds. Fig. 7. Cracks of types W2 (a) and W3 (b). (The arrow denotes the fractured weld - crack of type WD3 in Fig. I.) The stress state of web sections near the intermediatestiffening ribs under the influence of concentrated loads was studied in 13, 14. Much attention is being given to the fatigue resistance of these sections in the USA, where W3-type cracks represent the main form of damage to crane runway girders 9, 15. The design features of the girders used in American plants are such that the ribs have a small (h20 mm) opening - Fig. 1 - and their ends are not welded, but rather only fitted against the top flange. Certain conclusions reached in the above-cited works have proven to be contradictory. According to the test data of Yu. I. Kudishin 13, the vertical compressive stresses near the top of a web in the plane of ribs attached to a flange with an opening h =50 mm are twice as great as in the case of ribs without openings. A considerable fraction of these stresses were due in the first case to bending of the girder out of its own plane as a load was transmitted through the rail, which was displaced from the axis of the web by the amount e =15 mm. Together ith this, according to 15, no bending stresses were created in the web section within the boundaries of the opening with either central (e =0) or off-center (e =38 mm) loading of models when the ends of the ribs were attached to the top flange. The high stresses in the webs of the models with ribs which were only driven against the flange were caused by angular and linear displacements of the flange relative to the ribs within the limits of the play which the driving of the ribs failed to completely eliminate. It should be noted that when the flange of the rail is wide enough to overlap to openings in the ribs, there is no good explanation for the appearance of significant bending stresses in the web if the ribs are attached to the flange. Analysis of the stress state by the finite-elements method for the case of central loading showed that the stresses in the web decrease with an increase in the height of the opening h and as a result of attachment of the ribs to the top flange 9. Fatigue tests of models did not show large openings to have any particular advantages,but did confirm the expediency of attaching the ribs to the flange. As a result of welding of the ribs, the stresses in the web were reduced by a factor of 5.6 and the endurance with respect to crack formation increased from 0.9.106 to 5.5.106 cycles 15. It is interesting that the appearance of a crack in the web preceded fracture of the welds connecting the ribs to the flange, which is in agreement with the above-noted data from studying girders in existing shops. A significant reduction in stresses in the web as a result of welding of the ribs to the flange was noted in 14. Here the author observed a high stress concentration at the end of the vertical weld attaching the rib to the web. No such stress concentration was establishedin 9 in a finite- elements study employing a sparse lattice. Nevertheless, the finding in 14 is important, since it characterizes the stress state in the immediate vicinity of the nucleation site for type-W3 cracks. The above empirical results and the data on the condition of girders in existing shops permit us to conclude that welding intermediateribs to the top flange is undoubtedly necessary to prevent cracks to types W2 and W3. Since the welds of these elements fracture over time in service, the recommendation that the welds be made with complete fusion 9, 15 merits attention. As concerns the openings in the ribs, it is difficult to make supportable conclusioDsdue to the inadequacy and ambiguity of the available data. It can only be said that the size of the opening is not important, although extreme cases (an excessivelylarge opening or the complete lack of an opening) are unacceptable. Thus, the use of ribs with openings with h 250 mm (as was proposed in 9) makes the upper sections of the ribs less stable. On the other hand, when there is no opening (more accurately, when it is large enough only to permit passage of the flange welds of girders), cracks may form in the vicinity of the junction of several differently oriented welds - similar to the cracks which occur in the end sections of girders. LITERATURE CITED I. G. Maas, “New investigations of the fatigue strength of crane runways,“ Chernye Met.,No. 19, 23-29 (1971). 2. Ya. Augustin and E. Shledzevskii, Failures of Steel Structures Russian translation,Stroizdat, Moscow (1978). 3. A. G. Senior and T. R. Gurney, “The design and service life of the upper part of welded crane girders,“ Struct. Eng., 43, No, i0, 301-312 (1963). 4. A. B. Patrikeev, “Fracture mechanism of the upper sections of steel crane runway girders,“ Prom. Stroit., No. 5, 38-43 (1971). 5. A. B. Patrikee