Methods for Solving a Stress Behaviour of Welded Joints under Repeated Loads

Mechanics, Materials Science & Engineering, May 2016

ISSN 2412-5954

Methods for Solving a Stress Behaviour of Welded Joints under Repeated Loads 1, a

1, b

1

Department of Aviation Technical Studies, Technical University of Kosice, Faculty of Aeronautics, Slovakia

a

karol.semrad@tuke.sk

b

jozef.cernan@tuke.sk DOI 10.13140/RG.2.1.5113.9440

Keywords: finite element analysis (FEA), factor of Safety for fatigue stresses (FSF), factor of safety for static stresses (FS), the stress ratio (R), the experimental maximum stress at R=0 (S0), slope of experimental curve (m), allowable maximum stress (f), equivalent stress (feq), bending stress (fb), shear stress (fs), yield strength (fy).

ABSTRACT. The article processes issue of strength of cyclically loaded welded joints with a focus on fillet welds. The y and from articles by Lehigh University and the University of Illinois in USA. The practical application of the solution is presented for crane car body to crawler connection.

Introduction. The paper deals with the design of structural members subjected to repeated loads. The design of fillet welded connections is included and emphasized. The practical application is presented for crane car body to crawler connection.

Fatigue strength of fillet welded connections. The fatigue strength of fillet welds depends on the type of connection in which the weld is used; for example, the fatigue strength of a fillet welded lap joint is much lower than the fatigue strength of fillet welded flange to web connections in fabricated beams. Hence, data must be obtained for specific types of fillet welded connections rather than for fillet welds in general.

The fatigue strength of fillet welded flange to web connections in a fabricated member is less than the fatigue strength of the base metal from which the member is fabricated, and cannot be increased significantly by using larger fillet welds. Fatigue data presently available from continuously fillet welded tee specimens loaded axially at the centroid of the tee cross section so that axial stresses but no shear stresses are developed, indicate that such longitudinal flange to web fillet welded connections have fatigue strength equal to, or greater than, transversely groove welded joints with the weld reinforcement in place. This comparison is for welds made in the same steel subjected to equal fatigue lives. Therefore, it is conservative to design longitudinal fillet welded joints subjected to repeated normal stresses only (axial or bending stresses without shear stresses) by the same formulas used to design transversely groove welded joints with the weld reinforcement in place [1].

The available fatigue data on flange to web fillet welds under combined stresses indicate that it is reasonable to design such welds subjected to combined bending and shear stress for the following equivalent stress, feq:

(1) where fb and fs - are the bending and shear stress present, respectively (feq - should be given the same algebraic sign as fb). The maximum and minimum values of fb and fs caused by given loading may be MMSE Journal. Open Access www.mmse.xyz

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Mechanics, Materials Science & Engineering, May 2016

ISSN 2412-5954

used to compute the maximum and minimum values of feq , respectively. The maximum and minimum values of feq , in turn, may then be used to obtain the fatigue life from the appropriate fatigue chart for groove-welded plates. Conversely, Equation (1) can be used in conjunction with allowable stress formulas to determine allowable weld sizes. In many practical applications the effect of shear on the fatigue strength of fabricated beams is small enough to be neglected [2].

Application of fatigue data to design. To efficiently design a structure to resist fatigue, each individual detail should be checked for the stress conditions that exist at that detail. For example, in designing a rolled beam with a splice in the region of low stress, the fatigue life of the splice under the low stress would be checked, and the fatigue life of the beam itself would be checked at the location of maximum stress.

To utilize the experimental fatigue charts in the design of structures, however, it is usually necessary to apply a factor of safety to compensate for: 1.

Scatter among the fatigue data;

2.

And uncertainties in the loading.

The choice of the magnitude and method of application of the factor of safety for a specific application can best be made by the designer. However, a smaller factor of safety is usually justified for fatigue stresses than for non-repetitive stresses because of the minor effect of a few overloads on fatigue life, and the decreased likelihood of the number of stress cycles occurring at the design stress magnitude [3].

One convenient method of applying the factor of safety is to multiply either the maximum design stress or both the maximum and minimum design stresses by the factor and to use the resulting stresses in the mean value fatigue charts to determine expected life. Another method is to derive allowable stress formulas by dividing the stresses in the fatigue charts by the factor of safety. Again, this latter method can be done in several different ways. Both the maximum and minimum stresses corresponding to any point on a life line of an experimental fatigue chart can be divided by the factor of safety to obtain a point on a life line of an allowable stress chart as shown in Fig. 1. Alternatively, the experimental maximum stress and not the minimum stress can be divided by the factor of safety to obtain the allowable stress chart that might be appropriate for some applications in which the minimum stress results from a known dead load. For another alternative, the experimental stress range the difference between the maximum and minimum stresses can be divided by a factor of safety to obtain an allowable stress range [4].

Fig.1. Fatigue chart for design

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Mechanics, Materials Science & Engineering, May 2016

ISSN 2412-5954

The allowable design stresses shown in the illustrative fatigue chart (Fig. 1), may be expressed as the smaller of the following equations: ,

In the equations, f S0

(2)

is the allowable maximum stress;

is the experimental maximum stress at R=0;

m is the slope of the experimental fatigue curve; R

is the stress ratio (algebraic ratio of minimum to maximum stress);

FSF is the factor of safety for fatigue stresses; FS

is the factor of safety for static stresses.

Equations (2) give allowable stresses in as-received material and transversely groove-welded joints for the high-strength and high-strength low-alloy steels, and the structural carbon steels. The fatigue strength of groove-welds with the reinforcement properly removed is about the same as that of asreceived material. Allowable stresses for fillet welded joints of various types can be computed from the formulas for allowable stresses for transverse groove welds by applying the suggested reduction factors to the suggested allowable stress values given for transverse groove-welded connections. For allowable stresses in longitudinally fillet-welded flange-to-web connections subjected to fluctuating combined normal and shear stresses, Equation (1) can be used with the allowable formulas for transverse groove welds [4].

Design example of the crane carbody to crawler connection. The practical application of the solution is presented for crane car body to crawler connection (Fig. 2). We made 18 stress analyses by finite element method for 18 load cases for 2 series of the cranes with and without load on the hook and directly in line with center of car body, directly over front corner and directly over side of the crane.

From these analyses we found maximum and minimum equivalent stress in the interesting area [5, 6, 7].

Fig. 2. Pro/Engineer CAD model

Fig. 3. Pro/Mechanica FEA results

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Mechanics, Materials Science & Engineering, May 2016

ISSN 2412-5954

Summary. The Stress von Mises (equivalent stress) in the interesting area from FEA is about 2,5 times higher than the maximum allowable stress. The value is about 25000psi. We need the value about 10000psi. From 25000psi to 10000psi is 2,5 times less.

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Mechanics, Materials Science & Engineering, May 2016

ISSN 2412-5954

Fig. 4. Design of the crane carbody connection lugs and shape of the section References ., 2013. 350 p. ISBN 978-80-8086-223-7.

-80-8086-137-7.

IUM, 2010, 1160 p. ISBN 978-80-214-2629-0.

[4] Design for Repeated Load. Articles by Lehigh University and the University of Illinois, USA. [5] Pro/ENGINEER & Pro/MECHANICA. User guide, Parametric Technology Corporation, Waltham, 1997.

[6] CADTRAIN-COACH for Pro/ENGINEER. COACH e-Learning Solutions, Irvine, USA, Available from: <http://www.cadtrain.com/> [7] Pro/ENGINEER Wildfire 3 Help Page. Cambridge University Engineering Department, Cambridge UK, Available from: http://www.eng.cam.ac.uk/DesignOffice/cad/

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