ESDU 86013+Amendment (A)-1987 钢制焊接接头疲劳强度数据项使用指南

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86013

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GUIDE TO THE USE OF DATA ITEMS ON THE FATIGUE STRENGTH OF WELDED

JOINTS IN STEELS

1. INTRODUCTION

This Data Item provides an introduction and guide to the use of existing Data Items concerned with the

fatigue strength of welded joints. Included in this guide are some details concerning design of welded joints

and principal causes of fatigue failure, together with the reasons for the relatively small influence that parent

material strength has on the fatigue strength of welded joints. The fatigue strengths in terms of stress range

at cycles of typical welded joints range from 170 MN/m2 ( lbf/in2) for a good quality

transverse butt weld down to 55 MN/m2 ( lbf/in2) for a partial penetration cruciform butt weld.

These values are much lower than the fatigue strength of as-rolled carbon steel unwelded plate which,

expressed as stress range for endurance of cycles, is in the region of 250 MN/m2 (

lbf/in2).

2. NOTATION

Both SI and British units are quoted, but any coherent system of units may be used.

crack length (for a crack free to extend at one end only) m in

coefficient in simple crack growth rate law defined by

elastic stress concentration factor

stress intensity factor range N/m3/2 lbf/in3/2

moment applied to beam section N m lbf in

number of load cycles

characteristic of slope of crack growth rate curve

applied load N lbf

nominal stress in absence of crack N/m2lbf/in2

maximum value of S in fatigue cycle N/m2lbf/in2

minimum value of S in fatigue cycle N/m2lbf/in2

nominal stress range in absence of crack N/m2lbf/in2

weld leg length m in

210

×25 103

810

210

×36 103

------- C∆K()

3/2

()

--------------------in

in3/2

()

lbf

--------------------

∆K

RSmin/Smax

Smax

Smin

∆S

Issued July 1986

With Amendment A, September 1987

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86013

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2.1 Code Designations

In most design codes, welded joints are classified according to type and class, where each type exhibits

identifiable features that are known to affect fatigue strength. the fatigue performance of different types

can then be compared in terms of allowable stress versus number of cycles to failure data and a particular

class designated accordingly. In British Standards the class designations range from A to W representing

progressively decreasing fatigue strengths. The complete range of type and class designations is given in

Table 17, Reference 5.

3. WELDED JOINT BEHAVIOUR AND FACTORS AFFECTING FATIGUE STRENGTH

The fatigue strength (at long endurances) of a welded joint between similar steels depends upon a number

of factors, in particular the geometry of the joint. Other factors include the particular joint design with

regard to the final shape of the weld overfill and its configuration relative to the load, welding process,

weld defects (particularly if crack-like and normal to the load), and post-weld treatments. The material of

the weld is formed at the time of welding and is a mixture of the welding rod material and the parent material.

It is therefore inhomogeneous and possesses properties that are difficult to quantify. Additionally the parent

plate material local to the weld in the area known as the ‘heat affected zone’ experiences large variations

in temperature during the welding process which can affect material properties, and so lead to a degradation

of fatigue properties. In welded joints it is found that tiny intrusion defects associated with the weld and

invisible to the naked eye are always present in addition to any other visible defects, and these act as

additional initiation regions for fatigue cracks. The intrusion defects are present in welds that would

normally be regarded as “sound” and free from significant defects. It may be noted that the fatigue strength

of a welded structure is dependent upon the fatigue strength of its individual joints and has little to do with

the fatigue strength of the plate material as will be discussed later.

3.1 Methods of Improving Fatigue Strength

For a given design of joint, fatigue crack initiation occurs due to a combination of the varying stress, the

residual stresses, and the stress concentration present at the weld toe acting in conjunction with intrusion,

or any other, defects present. For both as-welded and stress relieved joints, improvement of the weld overfill

profile by grinding or other forms of machining provides an opportunity to reduce stress concentrations

and hence improve the fatigue strength. This process would also tend to remove small surface cracks present

at the weld toe and hence retard fatigue crack initiation. Other processes such as peening or the provision

of a tungsten-inert-gas weld run at the weld toe can also improve fatigue strength.

3.2 Influence of Parent Material Type

For the range of weldable steels the fatigue strength of the parent plate has little influence upon the fatigue

strength at long endurances of any welded joint within that parent plate. This can be appreciated by

considering the fracture mechanics aspects of welded joint fatigue failure. In the case of fatigue cracking

of plain homogeneous material, it has been observed by Paris (see Reference 1) that the increment of fatigue

crack extension per cycle is proportional to a power of stress intensity factor range under cyclic loading

nominal thickness of loaded plate m in

size of scaled component used as a reference m in

geometrical factor in stress intensity factor

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86013

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(see Data Item No. 80036*), that is,

, (3.1)

where . (3.2)

Experimental work has shown that this is indeed the case for many materials, provided that the maximum

applied K does not approach the material fracture toughness, and also that the range is above a

minimum threshold level. It has also been shown that the constants C and n vary little for all weldable steels

and most steel weld metals. For welded details it has been found that the inherent tiny intrusion defects

associated with a weld act as initiation regions for fatigue cracking, and virtually all of the fatigue life is

occupied by propagation of the crack from an initial to a final size. This implies that once a crack has

initiated at a weld it will propagate either into the weld or into the parent plate at a rate that is insensitive

to the strength of the parent plate material.

3.2.1 Implications of using a higher strength steel

Substitution of a higher strength steel in place of one of lower strength in welded construction to

accommodate increased loading is only advantageous if that increased loading is essentially static or

deadweight in nature. In welded joints, if the applied stress range is increased in proportion to the increased

static material strength the result is an increased and hence a much greater rate of fatigue crack

propagation and shorter fatigue life. Thus the use of high strength steels in welded construction often does

not improve the fatigue performance, since if used at higher stress ranges they will fail earlier than low

strength steels operating at a lower stress range.

3.3 Size Effect

It is important to appreciate that increasing the absolute size of a welded joint can lead to a significant

reduction in fatigue life at a given stress level†. Available experimental data suggest that the fatigue strength

of a welded joint decreases with increased plate thickness. A specific example of size effect for which data

are available can be seen in Data Item No. 78023‡, Figure 1. This shows toe failure data for transverse fillet

welded joints under bending loading for various plate thicknesses. However, the vast majority of tests for

fatigue strength of welds have been conducted on specimens of limited size, having thicknesses typically

in the range of 8 to 20 mm and having restricted length. These do not show clearly the effects of absolute

size. Therefore an over-estimate of fatigue strength can be made if the results obtained from these specimens

are assumed to be representative of much larger and thicker section components.

It is often difficult to obtain unambiguous experimental evidence of the influence of plate thickness on the

fatigue strength of welded joints. This is because in increasing the plate thickness a number of other relevant

variables are often affected, including the detailed joint geometry and the number of weld passes necessary

to make the joint, which increases the risk of significant crack-like defects. A higher level of residual stresses

is also associated with a larger plate thickness and this in turn increases the likelihood of fabrication cracks

and crack propagation in service.

The theoretical reasons for lower crack propagation threshold and higher subsequent crack propagation

rate in larger sized joints can be seen by adopting a fracture mechanics viewpoint. Consider two

geometrically similar components containing idealised weld shaped features, one being proportionally

larger than the other, as shown in Sketch 3.1.

*Data Item No. 80036 “Introduction to the use of linear elastic fracture mechanics in estimating fatigue crack growth rates and residual

strength of components”.

†Data Item No. 87016 “Effect of size on fatigue strength of steel components”.

‡Data Item No. 78023 “Fatigue strength of transverse fillet welded joints and attachments in steels under bending loading”.

------- C∆K()

∆K∆Sπa()

1/2 α

∆K()

∆K

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标签： #接头 #钢 #焊接

摘要：

本文档《ESDU 86013+Amendment (A)-1987 钢制焊接接头疲劳强度数据项使用指南》提供了针对钢制焊接接头疲劳强度评估的关键数据项及其使用方法，适用于结构设计、疲劳寿命预测及工程安全审查场景。该标准基于大量实验数据与行业实践，详细说明了在不同应力状态、接头类型及环境条件下，如何准确选取和应用疲劳强度数据，帮助工程师规避因焊接缺陷、应力集中或热影响区弱化导致的失效风险。文档特别引用了1987年发布的修订版内容，整合了早期版本的补充修正，确保数据项在对接、角接、搭接等常见焊接形式

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ESDU 86013+Amendment (A)-1987 钢制焊接接头疲劳强度数据项使用指南

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