Talat Lecture 2406: Fatigue An Fracture In Aluminium Structures: Proposal For A National Application Document

TALAT Lecture 2406

An Update of TALAT Chapter 2400

Fatigue and Fracture in Aluminium Structures Proposal for a National Application Document (Update from the TAS project) TAS

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Leonardo da Vinci program Training in Aluminium Alloy Structural Design

5 pages, 1 Figure Prepared by Prof. Dimitris Kosteas Technische Universität München, Germany

Date of Issue: 1999  EAA - European Aluminium Association

2406. Proposal for a National Application Document (NAD) Contents 2406. Proposal for a National Application Document (NAD) ............................. 2 2406.01 Introduction ........................................................................................................2 2406.02 Justification and Necessity of the Task.............................................................2 2406.03 Revisions in the Text.............................................................................................3 2406.04 List of Figures .......................................................................................................5

The following statements are taken from the work report of the ad hoc Task Group ENV 1999-2 submitted to the European Aluminium Association, Brussels. The EAA brought this proposal to the attention of national standards bodies.

2406.01

Introduction

After several consultations of some of the members of the CEN/TC250/SC9 committee regarding the most appropriate format in practice for design purposes after Part 2/Fatigue of the Eurocode ENV 1999 document, edition May 1998 (as finalized on the plenary meeting in London in September 1997), and following an agreement within the European Aluminium Association an ad hoc group of experts composed of members of Alusuisse and the Technische Universität München (TUM) - since the larger and significant body of experimental evidence and respective evaluations (especially tests on full-size welded beams, i.e. component tests) has been undertaken at these two institutions in the past years - produced the mentioned NAD proposal. These investigations utilized also the original evaluations for the ERAAS Fatigue Design document (1992).

2406.02

Justification and Necessity of the Task

Within the above context of ERAAS 1992 rules a closer adherence to cases with existing actual test results with components (beams) is achieved - this is not to say that the attempt to provide some extrapolation or interpolation in the ENV 1999-2 rules for a larger number of cases has not been based on sound engineering know-how and judgement, but it is still a fact that some of these assumptions have not been adequately verified by testing. A rather high quality level may be assumed for the products and the welders manufacturing structural parts for characteristic aluminium applications. A quality classification referring to the specific manufacturing and necessary or recommended inspection characteristics will help distinguish more effectively between the different structural details.

TALAT 2406

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An adaptation to European quality standards (and welder training/acceptance standards) is considered essential - these would be more readily conceivable and acceptable by welders and engineers. A distinction between internal and geometric (external) imperfections as well as additional requirements allows a practice-oriented quality distinction of structural details. Within this revision cross-reference is thus provided to the DIN EN 30042 (ISO 10042) document on guidance on quality levels for imperfections. Nevertheless it is being explicitly noted that a better harmonization of national/international specifications on quality classification and, especially, on the statement of common limit values for imperfections, is urgently needed and should be made one of the primary tasks in drafting the rules for „Execution of Structures“.

2406.03 Revisions in the Text There were two main tasks to be carried out. The first was the adaptation of structural details and their respective fatigue design values as far as possible in relation with the ERAAS document and the adjustment to the general system of parameter reference in the schematic of the ENV 1999-2. The second was the relation of the structural details thus accounted for to quality specifications as given in the DIN EN 30042 document. In the course of these changes and amendments the whole document ENV 1999-2 has been checked and changes and remarks or explanations are given in tabular form. Only some significant points are reproduced here as follows: • Following the prerequisites of the fatigue design rules and in the case of fatigue loaded structures executed by qualified and certified manufacturers the fatigue design values in Tables 5.1.1* to 5.1.5* and 5.2.1* to 5.2.5* may be used - these are the tables given in the NAD proposal. • Tables 5.1.1, 5.1.2a, 5.1.2b and 5.1.3 and Tables 5.2.1, 5.2.2a, 5.2.2b and 5.2.3 of ENV 1999 Part 2 with design S-N curves shall be substituted accordingly by Tables 5.1.1* to 5.1.5* and Tables 5.2.1* to 5.2.5*. Cross references to these tables shall be changed accordingly. • The rules of chapter 6 „Quality Requirements“ and of Annex D, here especially Tables D.1 and D.2, as well as until further evidence can be supplied of clause D.2 „Castings“ of ENV 1999 Part 2 are devoid of application.- Instead the rules and proposed corrections after chapter 6* are introduced. The new text in this chapter 6 is as follows: Quality requirements are defined in EN 30042. For fatigue design purposes with welded components it is appropriate to distinguish between geometric (external) and internal imperfections or defects at the weld. This leads to the respective quality classification format in Tables 5.1.1* to 5.1.5*.

TALAT 2406

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It has also been necessary to introduce an additional quality classification feature, which is not included in the current version of EN 30042. This concerns longitudinally loaded welds only, and accounts for the regularity (smoothness) of the weld surface profile in the direction of the weld, i.e. the respective discontinuity (notch) shall not be higher than 0,1t (t: plate thickness) or exhibit a slope not steeper than 1:4. Inspection The provisions in specifications for structures under primarily static loading are valid in principle - see ENV 1999-1-1, chapter 6 - as well as the new document „Execution of Structures“ to be drafted by the CEN/TC135/WG11. As far as welded components under fatigue loading are concerned the method, the extent and the time of inspection will depend on a number of parameters, such as the weld type, the extent (degree) to which use is made of the fatigue strength of the weld detail under consideration, its accessibility to inspection, the reliability of the method, the redundancy of the structure, and the consequences of damage, the ability to inspect during service, and the inspection intervals, finally the projected service life of the structure. For these reasons it is appropriate that type, extent and time of inspection shall be determined by agreement between the design engineer, the client and the user of the structure. Agreements, further considerations, and control tests shall be documented by the design engineer and included in the project specifications. The following control criteria relating to quality requirements and inspection shall be observed and may not be reduced unless in specially justified cases: a) In structures or parts thereof with a degree of fatigue strength utilization lower than 30% it is sufficient to observe the rules for quality assurance as these are specified for structures under primarily static loading. b) Welds or parts thereof with the type numbers 3.1, 4.1, 4.2, 4.4, 5.1, and 5.2 (in tables 5.1.3*, 5.1.4*, and 5.1.5*) and with a degree of fatigue strength utilization higher than 80% shall be inspected to 100% visually for geometric (external) imperfections and to 100% with non destructive methods (ultrasonic or X-ray) for internal imperfections. c) In all other weld cases the following provisions, in addition to the ones specified for structures primarily under static loading, shall be observed − visual inspection for 100% of the welds, − a random sample of 10% of the welds under consideration shall be inspected with non destructive methods if the degree of fatigue strength utilization is lower than 50%, − a random sample of 20% of the welds under consideration shall be inspected with non destructive methods if the degree of fatigue strength utilization is higher than 50%, − should a weld or part thereof within the above samples not fulfill any of the specified quality criteria, then the inspection shall be extended to double the original random sample size, − in case of further non-fulfillment of the specified quality criteria all (100% of) welds with a degree of fatigue strength utilization higher than 50% shall be repaired by appropriate and specified procedures and methods, in case of welds with a degree of fatigue strength utilization lower than 50% need not be repaired if the detected imperfection size lies within the range of critical values of the next lower quality classification case.

TALAT 2406

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The inspection extent and procedure as described is demonstrated also in the following Figure 2406.03.01. percentage utilization of fatigue strength 100 % 80 appropr. repair 100 % critical defect ? N D Testing of internal imperfections min 40%

+

50

30

possibly no repair

yes

min 20%

critical defect ? yes

test sample size min 20%

quality criteria as for statically loaded structures

min 10%

100 % visual test sample size / geometric,external

100 % welds 3.1/ 4.1 / 4.2 / 4.4 / 5.1 / 5.2 / 5.3

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100 %

Inspection Inspection Size Size EC9-NAD EC9-NAD // proposal proposal 2-99 2-99

Inspection extent and procedure in ENV 1999-2 / NAD proposal

2406.04 List of Figures Figure Nr. 2406.03.01

TALAT 2406

Figure Title (Overhead) Inspection extent and procedure in ENV 1999-2 / NAD proposal

5

2406.03.01