Keywords: contact problem, boundary non-linearity, fatigue life, high-strength steel, numerical calculations, strain-life method.

High level of material exploitation is often requested in optimization of crane structures. One way to achieve this goal is by crane design modification. Other possibility is to reduce the cranes structural member cross-sections by using high strength steel. This results in lower material and production costs and appropriate operational safety of the crane structures, which have to be designed to eliminate every possibility of critical damage or even failure. Clear design guidelines are needed in both cases to ensure that fatal fatigue failure of the crane structure is avoided. The possibility of predicting operational strength of cranes, which are exposed to variable amplitude loading in service, is therefore of a significant importance.

This paper deals with the problem of service life evaluation of counterweight bar bolted connection by means of computational analysis and experimental testing [3,4,5]. The bolted joint fatigue is critical for a number of reasons: high values, sensitivity of high strength steel to notch effects and the quality of surface treatment (grinding, machined etc.), eccentricity and clearance of the joint inducing secondary bending etc.

Computational analysis was done in two steps. First, a stress field in the bar end connection is determined by solving the contact problem between the bolt, connection plates and the bar by means of the finite element analysis [1]. The computational analysis provides the stress concentration points in the bar end, which are also the most critical failure points. Second, the fatigue analysis has been performed, where the local strain-life method ( ) has been applied [2]. This method is based on ration determination between the specific deformation and the number of loading cycles
[6,7,8].

Experimental fatigue tests of high strength steel bars were carried out in a specially constructed hydraulic pulsation machine. Complete description of experimental testing of presented problem is given in [3,4]. The main drawbacks of experimental testing are high costs and time consumption, although this approach is the most reliable way of component verification that is subject to high stresses and a large number of load cycles.

Computational analyses and experimental observations have shown that the connection hole in the bar end is the most critical location for crack initiation and final failure (Figure 1). Comparison of results is given in Table 1. Average number of loading cycles to failure determined with experiments is approximately loading cycles. A significant difference of results can be attributed to the fact that bar specimens used in experiments was produced with thermal cutting process, which resulted in initial surface damage. Although additional surface grinding was applied, this apparently was not sufficient to alleviate sensitivity of high strength steel to notches and other material imperfections (for instance micro-cracks, inclusions etc.).

1

Abaqus Version 6.4-PR11. Online Documentation, Generated: 2003.

2

Fe-safe Works Version 5. User's Manual, Generated: 2003.

3

J. Kramberger, I. Potrc, G. Bombek, J. Flašker, "Fatigue assessment of high strength steel beams for crawler track cranes", Gép, 54 (10/11), 87-90, 2003.

4

J. Kramberger, M. Šraml, I. Potrc, "Experimental Investigation of Fatigue Strength of High Strength Steel Bars", MATRIB 04, Vela Luka, Croatia, 2004.

5

M. Šraml, J. Kramberger, M. Horvat, I. Potrc, "Numerical calculation of fatigue life of bar connection at crane structures", MATRIB 04, Vela Luka, Croatia, 2004.

6

S. Suresh, "Fatigue of materials", Cambridge University Press (Second Edition), Cambridge, 1998.

7

E. Zahavi, V. Torbilo, "Fatigue design", CRC Press, New York, 1996.

8

N. Dowling, "Mechanical behaviour of materials", 2nd edition, Prentice Hall, 1998.

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