DVS_1608_2010_E
Intended as replacement for the issue of May 1983 This draft is presented to the public for scrutiny and statement. Additional proposals or proposals for modifications should be submitted to the DVS e.V., Postfach 10 19 65, 40 010 Düsseldorf.
This directive is a revised version of leaflet 1608 of May 1983. The revision was carried out with the aim to implement the DIN EN 15085 (formerly DIN 6700) requirements for the determination of weld seam grades, taking into account the strength and safety requirements for weld joints in the rail vehicles construction. Additional to the nominal stress concept, the notch stress concept was included as evaluation methodology.
Also the procedure for static proof was taken into account.
The validity of this directive must be agreed between the originator and contractor.
The directive was created by representatives of the rail vehicle industry, the DB AG and the Federal Railway Authority, as well as by staff of IMA Materialforschung und Anwendungstechnik GmbH Dresden.
Index:
1 General
2 Area of Validity
3 Welding manufacturing and quality assurance 3.1 Welding planning and construction advice
3.2 Requirements on the manufacturing equipment 3.3 Working tools and devices
3.4 Cleaning and degreasing
3.5 Joint preparation
3.6 Tack welding
3.7 Preheating
3.8 Welding procedures, welding consumables and
shielding gases
3.9 Heat treatment after welding
3.10 Straightening of welded components
3.11 Information regarding welding ban zones
3.12 Maintenance work
3.13 Evaluation of the welding process
4 Design of welded connections
5 Fundamentals of interpretation
5.1 Strengths requirements
5.2 Requirements for strength
5.3 Recommendations for the evaluation of finite-
element-results
5.4 Nominal stress concept
5.5 Notch stress concept
5.6 Frequently used abbreviations and symbols
6 Proof of the static strength
6.1 Welded through butt welds
6.2 Non-through-welded butt welds
6.3 Welded through T-butt joints
6.4 Non-through-welded T-butt joints 6.5 Fillet welds
6.6 Material properties
7 Proof of fatigue strength
7.1Basics
7.1.1 Medium voltage sensitivity of welded components 7.1.2 Measures to increase the fatigue strength of welded
components
7.1.3 Thickness influence of welded components
7.1.4 Evaluation of multiple axis stresses in the base material
7.1.5 Evaluation of multi-axial stress welded components 7.2Fatigue strength according to the nominal stress
concept
7.2.1 Fatigue strength values for the base material after the
nominal stress
7.2.2 Fatigue strength values for welding seams according to
the
nominal stress concept
7.2.3Notes to notch type catalogue in annex C
7.3 Fatigue strength verification according to the notch
stress concept
7.4 Fatigue detection
7.4.1Definition of the S/N curve
7.4.2 Procedure for fatigue detection
8 Assignment of fatigue strength, weld seam grade and
safety need
8.1Fatigue strength design based on nominal voltages 8.2Fatigue strength design according to the notch stress
concept or other directives or regulations
8.3Regulations for the conversion of the weld seam grade
of DIN 6700 to 15085
9 Bibliography
9.1 Technical standards and technical regulations
9.2 References
Annex A:
Material grades
Annex B:
Guidelines and standards with requirements for the fatigue strength verification of welded joints Annex C:
Oscillating fatigue strength values (fatigue strength values for 10 million load cycles) for welded connections of Al- alloys
Annex D:
Example for the application of the Miner elementary method with “Cut off”- rule for the damage accumulation
This publication has been drafted in voluntary work by a group of experienced specialists. It is recommended to regard it as an important source of knowledge. The user must ensure how far the content on each specific case is applicable and if the presented version is still valid. A liability of the Deutschen Verbandes für Schweistechnik e.V. and those, who participated on the development, is excluded.
1 General
This guideline contains information about design and specifications for the design of welded structures made of aluminum alloys as well as a compilation of the rail vehicle construction explicitly welded construction details (notch line cases), which, for security, performance, light weight, economy have proven production and maintenance.
The procedures for the detection of both static strength and the detection of fatigue strength of base material and welded joints are being described.
For the proof of the fatigue strength through the nominal stress, a notch line case catalogue has been compiled. In the catalogue the connection details the weld quality classes according to DIN EN 15085-3 (formerly DIN 6700-3) and the notch line cases or fatigue strengths values are assigned, have arisen by the revision of Directive DVS 1608:1983 and were also part of this guideline. Together with the consideration of different safety needs the requirements of the standard series DIN EN 15085 are fulfilled.
The fatigue strength values of the notch line case catalogue refer to the basis material as well as to the welded component connections in the rail vehicle construction and the here mentioned force loads and manufacturing conditions. The catalog for welding connections in Annex C gives no claim to completeness.
In addition to the nominal stress concept, the notch stress concept is proposed as an evaluation methodology.
For the sufficient fatigue strength SN curves are given for the nominal voltage concept as well as for the notch stress concept. As a result an assessment of the operational stability or a life span assessment through damage accumulation is possible.
The guideline is used for the design engineers and designers to construct a welding joint that is appropriate to the operational demands and supports welding- and inspection engineers to find solutions of quality assurance and production tasks.
2 Area of validity
This guideline applies to design and layout of static strength and fatigue strength of the base material and arc welding connections of the vehicle construction-used aluminum alloys that are listed in the standard series DIN 5513 or DVS leaflet 1623. It must be applied for constructions with a wall thickness of t ≥ 1.5 mm.
The following alloys are recommended for the vehicle construction use:
- Extruded profiles:
EN AW-6005A, EN AW-6082, EN AW-6060, EN AW-6106 - Bands, sheets and plates:
EN AW-5083, EN AW-5454, EN AW-5754, EN AW-6082
- Aluminium castings:
EN AC 21000, EN AC 42000, EN AC 43300, EN AC 51200
- Forged parts:
EN AW-5754, EN AW-5083, EN AW-6005A, EN AW-6082. Alloy EN AW 7020 (AlZn4, 5Mg1) should only be used in the vehicle construction with caution, as this alloy is highly notch-sensitive and investigations showed that the fatigue strength of this alloy is subject to a strong age-related waste. In the case of new projects, the usage of alloy EN AW 7020
in principle is not recommended.
3 Welding manufacturing and quality assurance
For design of welding constructions in the rail vehicle construction the requirements of the standards DIN EN 15085 must be noted.
The construction drawings must be set up considering DIN EN 15085-3 and leaflet DVS 1610.
According to standard DIN EN 15085 for a welding construction in rail vehicle construction, the weldability must be ensured in accordance with ISO/TR 581.
In detail:
- The weldability of the materials is guaranteed if the material meets with the requirements of DIN EN 15085-3, paragraph 6.1.
- The weldability of the welding filler is guaranteed if the welding fillers of the respective material according to DIN EN 15085-4, paragraph 5.3 are selected and qualified.
- The welding safety of the construction is guaranteed if the construction, taking into account the behavior of the material, can withstand the stresses. The requirements of DIN EN 15085-3 and DIN EN 15085-4 must be observed.
- The welding possibility in production is guaranteed, if the construction in compliance with the certification level and the in the operation possible welding processes is manufactured.
In addition to weldability of the construction, the testability of the design (pre-destructive tests must be possible) must be ensured as well according to DIN 27201-6 compatibility maintenance (see also Leaflet DVS 1620).
For the assignment of components and parts to the certification levels the requirements of DIN EN 15085-2, Annex A must be applied. For the classification of welded joints in the weld grades, section 8 of this directive includes simplistic assertions with which the guidelines of DIN EN 15085-3, Table 2, are met. Furthermore, in this section for the determination of the relevant safety needs information is listed that is compliant with the content in annex G of DIN EN 15085-3.
Note:
It is therefore particularly important that with determination of the weld quality class, also the assignment of components and parts to the certification levels take place, according to DIN EN 15085-2, the certification level primarily is dependent on the welding joint grade. Welded structures in the rail vehicle construction according to standard series DIN EN 15085 have to undergo a welding test in accordance to leaflet DVS 1620.
3.1 Welding planning and constructive advice
In design of welded rail vehicles, manufacturability of welding joints together with welding engineers must be assessed.
In difficult constructions welding sequence plans must be set up. (see leaflet DVS 1610)
The welding joint forms and the requirements for the weld bead quality and test expenditure must be defined between the designer, welding engineer and calculation engineer. 3.2 Requirements on the manufacturing equipment
The workshops, in which aluminum is processed, must be separated from those in which dust, gas or vapor occurs, that may affect corrosion resistance of aluminum or welding quality. For technical welding maintenance on supporting automotive body, appropriate clamping jigs must be used.
3.3 Working tools and devices
The working tools must be used exclusively for aluminum alloys or carefully cleaned prior to use (free from residues of other metals).
Devices and clamping tools from alloy steel should be clean and free of corrosion on the supporting and clamping surface or must be provided with suitable intermediate elements e.g. those made of aluminum, stainless steel or synthetic materials. It must be ensured that these are free of copper.
Container and storage shelves must be protected against rust, e.g. galvanized or should be lined with chlorine free paper or dry wood. Cleaning brushes must have stainless chrome nickel bristles that are clean and grease-free.
3.4 Cleaning and degreasing
Before welding, the parts to be welded must be cleaned mechanically or with appropriate chemical contents (e.g. acetone) to remove grease, dusts and coating materials. 3.5 Joint preparation
The weld preparation must be produced preferably by mechanical machining (e.g. milling). For the root elaboration form cutters must be preferred. Using plastic bounded grinding wheels can cause welding joint porosity, if plastic grits remains at the joint flanks. The weld preps are contained in DIN EN ISO 9692-3; recommendations for the weld preps comprise in DIN EN 15085-3.
3.6 Tack welding
Tack welds remained in the seams are carried out, so that the quality requirements regarding the manufactured welding seam are fulfilled. In highly stressed areas tack welds shall be avoided or carefully removed. This issue can be listed in the references optionally. 3.7 Preheating
Depending on the work piece dimensions and the welding process, preheating is required before start of welding. Preheating shall be carried out with lowest possible evenly energy transfer (soft flame with propane/butane gas and heating input spray). Preheating temperature must be observed with temperature devices. The preheating temperature shall not be higher than 150°C.
3.8 Welding procedures, welding consumables and shielding gases
In the rail vehicle construction inert gas welding processes such as TIG-, MIG- welding and their variants are preferable being used. Before the welding start or restart the joint flanks, seam section, tack welds or intermediate layers must be cleaned carefully. End-crater-cracks, fusion lacks and torn tack welds must be carefully elaborated.
During the MIG welding process fusion defects and end-crater-cracks can be avoided at the beginning and end of the seam through approach or run-off plates. The arc at the seam end of the arc is due to the seam, and welding is to finish on the seam. With interruption of the welding work the welding speed must be increased so that the seam outlet butts cuneiform. For example the end- crater can be filled by lowering the welding current.
In principle gas shielded arc welding processes together with other welding processes such as laser welding or friction stir welding are admitted, however, the interpretation of these high-strength welding seams still must be carried out with the permissible welding process values, if during operation measures of maintenance after accidents or reconstructions are carried out using the inert gas welding process. This must already be considered in the basic design. Only appropriate welding consumables are permitted to be used: The filler metal S-AIMg5 or similar filler metals, such as listed in leaflet DVS 1623, shall be preferred. The usage of filler metal S-AISi5 is only be permitted in exceptional cases or after detailed clarification with the welding supervisors. The minimum values for stiffness of aluminum-welding consumables are listed in table 2.
In general inert gases such as argon, helium and helium-argon-mixtures are used according to DIN EN ISO 14175. Assignment recommendations of the filler metals for selected aluminum and aluminum alloys are listed in leaflet DVS 1623, supplement 1.
3.9 Heat treatment after welding
A heat treatment after welding is generally unnecessary. The material characteristics of age-hardening alloys can be improved by artificial aging after welding, e.g. strength and corrosion resistance.
3.10 Straightening of welded components
Welded constructions can be straightened cold or warm; cracks (structural damages) must not occur.
The application of straightening welding seams must be preferred during the warm straightening process. These straightening welding seams must be applied on already existing welding seams or in the immediate web region of profiles. Straightening welding ranges must be assessed with corresponding strength values of the welding seam. Within static analyzing the strength values of the heat affected zone, within fatigue tests the strength values according to the notch types must be consulted.
Flame straightening of aluminum constructions should be avoided, as the heat input in the practice can hardly be controlled. A local increase of temperature to 150°C can already lead to very adverse strengths reductions.
In case the flame straightening is still being applied, the temperatures must be controlled with measuring instruments. Furthermore the flame straightened areas must be structurally assessed according to the reduced strength properties (heat affected zone). (Flame straightening can only be carried out between the permitted straightening zones. )
In flame straightening zones only higher strength values as in the heat affected zone may be applied, if these strength values are proven. The possible straightening zones must be assessed structurally and be reported by the vehicle manufacturer.
For flame straightening leaflet DVS 1614 should be noted.
3.11 Information regarding welding ban zones
Welding ban zones are areas in that without calculation engineer consultation no additional welding seams, as for straightening welding seams, holder, grounding buds are not allowed to be used.It is recommended to define all welding ban zones.
In the welding ban zones the grounding clips must not be used.
The definitions of the welding ban zones can also be reviewed for later maintenance work after accidents or within the scope of maintenance work.
3.12 Maintenance work
For maintenance welding on railway vehicles DIN 27201-6, conditions of railway vehicles – basics and manufacturing technologies – part 6: welding, must be applied.
The maintenance must be structurally assessed by a counting engineer. A maintenance instruction must be compiled that must be released by the welding supervisors. Necessary repairs are usually carried out using MIG- or TIG welding process. It must be ensured that the filler metal being used is the same as for the new production. For the ascertainment of the originally used filler metal the dot test with a caustic soda of 20% can be applied (filler metal S-AISi5 turns grey, filler metal S-AIMg5 or S-AIMg4.5Mn stays light). 3.13 Check after the welding process
The ZfP-assessments must be carried out after the removal of the clamping jigs. During maintenances welding work on railway vehicles the guidelines of DIN 27201-6, annex F (paragraph F5) must be noted.
4 Design of welded connections
For the constructive design of welded connections it must be noted that the following have to apply
-To withstand stresses;
-Suitable for production;
-Appropriate to the material;
-Testability;
-Low warpage or distortation
-Corrosion-protected;
-Economical;
-Possible to repair;
-For automation and mechanization
Appropriate constructing means for example:
-To keep the stiffness cracks low
-To avoid seam accumulations and seam crossings
-Supported formation of non welded through butt-joints, for example box girders.
Production oriented design requires a sufficient accessibility for the welding process. Modeling material-conforming design means that the possibility of material changes, heat affected zones and cold formed scopes receive attention regarding the design of safe welding. Appropriate designing ensures an appropriate accessibility for the test procedure that is being used.
In addition, standard series DIN EN 15085 contained general construction rules must be applied and general in the literature listed information regarding the design of welded joints must be observed.
5 Fundamentals of Interpretation
5.1 Strengths requirements
For in the interpretation of welding joints in the railway vehicle construction different standards with different requirements must be observed. The strength requirements for vehicle bodies are regulated in DIN En 12663 and of bogie frames in DIN EN 13749.
These standards also include requirements on the design of welded joints, but no concrete strength values. According to these standards welded joints must be applied for exceptional stresses in terms of static strength (Yield strength or breaking strength), or significant permanent deformations or during the operation occurring stresses in terms of fatigue strength (fatigue strength or operational stability).
5.2 Requirements for strength
The material characteristics that must be applied for the static fatigue strengths verification must meet the minimum yield or proof stress and the tensile strength of materials under consideration of the material tensile strength in the field of welding seams. With the application of the material characteristics according to paragraph 6.6 the requirements are met.
The strength values for the fatigue concepts must meet the following requirements:
- Survival probability Pü≥ 97.5% (one-sided confidence interval),
- Classification of designs with regard to the notch cases,
- For the fatigue strength verification (fatigue strength verification with constant load amplitude) with a nominal voltage or notch stresses (at standard load cases in accordance with DIN EN 12663, or DIN EN 13749): strength value for 107 cycles,
- For fatigue strength verification: nominal voltage lines, component or notch stress- S/N lines.
With the application of this directive, the aforementioned conditions are met.
The transferability of specimen values onto real welded components is by considering the residual stress influence given through the factor Mσ. Provided that the provisions DIN EN 15085 are met, for aluminum parts rail vehicles, taken in account the residu al stresses, a value of Mσ = 0.15 and Mτ= 0.09 (see section 7.1.1) must be applied.
The procedure for the evaluation of weld seam stresses in terms of static evaluation criteria is handled in section 6. Section 7 contains details for the ascertainment of the fatigue strength verification. The fatigue strength verification in the form of a fatigue limit according to the nominal stress concept is described in section 7.2 and the corresponding fatigue strength values are listed in section 7.2.1 and 7.2.2. The procedure of the notch stress concept application for fatigue strength value is described in section 7.3. The procedure of the fatigue strength verification is described in section 7.4.2 and the corresponding S/N lines are listed in section 7.4.1.
Further guidelines or standards with the suitable fatigue strengths values for the railway constructions are listed in appendix B.
5.3 Recommendations for evaluation of finite-element-results
The determined stresses using the finite element method correspond to structure stresses, as geometrically induced voltage increases or gradient (due to stiffness cracks) are covered by a sufficiently fine element division separation. For a direct comparison between the finite element stress values and the in the stress-strain gauges determined measurements, it is recommended that the evaluation point of the finite element model is placed on the same as the center of the strain gauges. Since, according to ERRI B12 RP17 the strain gauge should be positioned with a distance of 5 mm from weld transition, on the evaluation point in a finite Element shell model shall ideally be positioned in the following distance (see figure 1, DMS-length corresponds to the measuring grid length):
-Butt weld e = 5 (mm) + “DMS-Length“/2
-T- butt seams e = 5 (mm) + “DMS- Length“/2.
Figure 1. Position of the evaluation point – butt weld
Figure 2. Position of the evaluation point – T- butt seams
5.4 Nominal stress concept
The significant nominal stresses result from the internal forces in terms of the connection cross- section, that lies on the most stressed area or on the possible crack area. Using the nominal stress concept, that is based on the stress resultant area, geometric voltage increase, that result from the constructive connection design, must be taken in account if these are not covered by the notch case detail. Local structural stresses from finite element calculations are generally in the range of significant stress gradients shown above. Local structure stresses of finite element calculations are generally higher expelled in the area of significant tension gradients. The evaluation of stresses must therefore always be carried out with a certain distance to the weld seam (see section 5.3).
5.5 Notch stress concept
With the notch stress concept the local stresses of a welding connection is evaluated directly in the weld seam transition and on the seam root. This requires that the seam transition and the seam root are recorded with a suitable notch or
constant radius, that is assigned to the test results derived fatigue resistance. For welded components with a wall thickness t ≥ 5 mm in general a notch radius of r ref =1 mm is applied.
For a wall thickness of t <5 mm, the reference radius is smaller than 1 mm, which according to recent findings in dependence on the wall thickness and the maximum stressed weld seam zone – seam transition or seam root- can vary between r ref = 0.3 mm and r ref = 0.05 mm. Detailed information including the accompanying fatigue resistance can be found in the report books DVS-256.
When preparing the necessary calculation model in application of the finite element method a sufficient grid density must be guaranteed. Accordingly, guidelines are contained in the IIW recommendations and in supplementary documents IIW (for example IIWDoc. XIII-2240-08/XV-1289-08). For reduction of the modeling effort also different software solutions or program systems with simplified procedures regarding the notch stress determination (see paragraph 7.3) exist.
The notch stress concept is only applicable for the fatigue strength verification.
Note of translator: Zul=Permissible
6 Proof of the Static Strength
The proof of the static strength has only been listed for exceptional loading conditions. The static resistance criteria also must be proven for stresses as a result of fatigue loading conditions.
In the case of materials with sufficient ductility (elongation A50≥ 6%) through multi-axial stresses the Von-Mises-equivalent stress is applied for evaluation. In case the crack strain of material A50 is < 6%, the evaluation according to the amount highest principal stress shall be taken in account. The static detection of welded aluminum compounds generally occurs in three stages for:
-Basis material;
-Heat affected zone;
-Welding seam
The stresses in the base material, heat affected zone and welding seam should be valued separately.
Therefore the following listing describes how this must be applied onto the different seam types:
-Fully penetrated butt welds;
-Non-welded butt welds;
-Welded butt joints;
-Non-welded butt joints;
-Fillet welds
During the welding seam proof for the maximum breaking stress the minimal value of the heat affected zone (see table 2) must be applied. For the permissible yield strength of the welding seam material, the yield strength of the heat affected zone must be applied.
For a breaking elongation A50≥ 6% as equivalent stress the von-mises-tension must be applied, for a breaking elongation A50 < 6% as equivalent stress the highest normal voltage must be applied.
For exceptional heavy loads (static or predominantly static loads or loads with low load alternation numbers) the static strength data and safety factors according to DIN EN 12663 or DIN EN 13749 are determined (depending on application).
6.1 Welded through butt welds
With welded through butt welds, the assessment for the full cross-section (sheet thickness or web thickness) according to DIN EN 15085-3 will be used (see Figure 3: The weld seam is
connected over the full thickness).
Figure 3: Welded through butt welds
6.2 Non-through-welded butt welds
In case of non-through-welded butt welds (see Figure 4: the weld is not connected over the full thickness : te < t).
According to DIN EN 15085-3, the load-bearing seam cross section (te) must be used for the evaluation. This effective cross section is reduced in comparison to the connected sheet or web thickness. In the evaluation of stresses
identified through shell elements, either the shell thickness must be reduced locally or the determined voltages must be
scaled up in proportion to the full web thickness.
Figure 4. Non- through- welded butt welds
6.3 Welded through T-butt joints
With welded through T-butt joints according to DIN EN 15085-3 the full cross section (sheet thickness) of the
connected sheet is used for the evaluation. In addition, with the continuous sheet the equivalent stress must be compared with the heat affected zone strength.
6.4 Non-through-welded T-butt joints
With non-through-welded T-butt joints the supporting seam cross section must be used for the evaluation according to DIN EN 15085-3. This effective cross section in comparison to the connected plate- or path thickness reduced. During the evaluation of tensions that must be determined through shell elements, either the local shell thickness must be
reduced or the determined tensions in proportion to the full path thickness must be scaled up; if no full connection was realized.
In addition, with the continuous sheet, the equivalent stress must be compared with the strength of the heat affected zone. 6.5 Fillet welds
With fillet welds the strength of the welding seam in the vested welding cross section (see figure 5, cross-sectional dimension a) as well as the strength of the heat affected zone on the connected sheet- or path cross-section must be
observed according to DIN EN 15085-3.
Figure 5: Fillet welds
In the evaluation of stresses that are determined through the shell elements, the determined stresses must be converted in proportion of the full wall thickness to the vested welding seam cross section. Also the section sizes for the stress analysis can be used.
In addition, for fillet welds the heat affected zones of the connected paths or sheets must be proved. In the event that different wall thickness will be connected, the smallest wall thickness must be used.
6.6 Material Properties
For the determination of the stresses, the following basic parameters for the aluminum alloys used in the rail vehicle construction must be recognized (see table 1).
The material properties are assumed to be isotropic -
Influences arising from the rolling or pressing direction are already considered in the material parameters. The proof for the static weld material strength required tensile strength is given in table 2.
The material properties are assumed to be isotropic - Influences arising from the rolling or pressing direction are already considered in the material parameters. The proof for the static weld material strength required tensile strength is given in table 2.
7 Proof of fatigue strength 7.1 Basics
The verification of the fatigue strength is used for both the nominal stress and for the notch stress concept and is
presented in the form of a fatigue strength verification and a sufficient fatigue strength verification. The provided fatigue strength verification in the standards DIN EN 12663 and DIN EN 13749 must be carried out with fatigue strength parameters for 10 million cycles. The term fatigue in this directive must be
understood in that sense. For tensions as a result of fatigue load cases also the static resistance criteria must be fulfilled. During the verification within the scope of DIN EN 12663 or DIN En 13749 it is advisable to use a consistent, continuous design procedure for the mathematical fatigue strength verification and possibly other necessary sufficient fatigue strength
verification. The relevant notch details will then be evaluated with the same criteria in all proof stages (in accounting, in experiments or operational measurements).
It is assumed that sufficient protection against adverse environmental conditions exists (corrosion protection).
7.1.1 Medium voltage sensitivity of welded components Provided that the provisions of DIN EN 15085 are met, for
welded rail vehicle aluminum alloy joints the following values of the medium voltage selectivity must be applied considering the medium- and residual stress selectivity:
– medium stress selectivity as a result of the normal stress: Mσ = 0.15 (1), – medium stress selectivity as a result of shear stress: M τ = 0.09 (2). If in the area of the welding joint for the entire series a too low residual tension stress or residual compressive stress condition is proven, a bonus factor for the fatigue strength can be initiated. The bonus factor for lower residual stress arises by increasing the medium voltage sensitivity for normal voltages from Mσ to 0.3 and the medium voltage for shear stresses from Mτ to 0.17. In the case of an R-ratio of 0.5 or more, then again the same strength values as described in section 7.2.2 or 7.3. apply.
7.1.2 Measures to increase fatigue strength of welded
components As a measure to improve the fatigue strength by subsequent processing of the welding seam a low on notch effected grinding of the seam transition is recommended: In this case the fatigue strength value must be increased by 30% - this correspondents a bonus factor of fBonus = 1.3.
σ Fatigue, Grinding = fBonus · σFatigue (3) Sources of error such as incomplete fusions or cracks must be
avoided before grinding (according to DIN EN ISO 10 042). The fatigue strength increased by the bonus factor must not be
greater than the fatigue strength of the untroubled heat
affected zone of the adjacent base material (notch type line B). Low on notch effected grinding on the shop drawing the maximum permitted surface r oughness is Rz ≤ 16 μ and if
required also the grinding direction must be given. The surface roughness must be proved before blasting.
Internal stresses caused by hammering, blasting, pre-stretching or targeted thermal treatments must only be taken into
consideration if these are proven. The bonus factor for internal stresses shall only be applied for small local areas or repairing solutions and not as series solutions. The effect of residual
compressive stresses can be depending on the load sequence in operation also be reduced. Corresponding bonus factors must be secured experimentally.
In document IIW or XIII-2200r1-07 a bonus factor for TIG Dressing can be found if certain conditions are met. TIG-Dressing is only permitted locally. A corresponding bonus factor must be proved. Before the TIG-Dressing, a surface crack detection must be proved in any case.
7.1.3 Thickness influence of welded components
For material thickness of 10 < t ≤ 90 mm the fatigue strength values for welded connections must be lowered according to the following equation: For t > 10mm:
σfatigue,t > 10mm = ft · σfatigue (4) respectively
τfatigue,t > 10mm = ft · τfatigue
with a thickness influence factor:
Sheet thickness influence
Figure 6: Sheet thickness influence
The reduction of the strength depending on the sheet thickness applies to normal stresses as well as for shear stresses.
7.1.4 Evaluation of multiple axis stresses in the base material For the base material the normal stresses in a fixed coordinate system (plane stress state: σz = 0) (σx and σy) and the
corresponding shear stress τ must be evaluated. Initially the fatigue strength value must be carried out separately for every
stress component:
a x Utilization due to normal stress in local x-direction a y Utilization due to normal stress in local y-direction a t Utilization due to shear stress
σx,zul , σy,zul a nd τzul are the fatigue strength values In addition, the multi-axial stresses through the utilization degree of the individual components must be combined using
the following formula:
The factor f(φ) considers the phase of the normal stresses to each other and can assume values between -1.0 and +1.0. The evaluation of factor f(φ) is carried out in the following steps: a) For all relevant loading condition combinations is calculated
at all times:
b) Of which the maximum and minimum of all the Xi are determined: Xmax = MAX (Xi) Xmin = MIN (Xi)
c) The factor f (φ) is calculated as a function of Xmin ≥ 0 → f(φ) = -1 Xmax ≤ 0 → f(φ) = +1
or
The shear stress component is always so superimposed, as whether they would occur in phases.
A simple approach to lie on the safe side is to attach the factor f (φ) = +1 in the equation (6).
7.1.5 Evaluation of multi axial stress welded components Basically, all stress components always must be evaluated: For welded components these are the normal stresses
lengthways along and perpendicular to the seam direction (σII and σ ⊥) and the shear stress lengthways along the seam direction (τ).
The fatigue strength verification is initially carried out
separately for each stress component. The requirements of DIN EN 15085 must be considered:
- a ⊥utilization due to normal stress perpendicular to welding seam,
- a II utilization rate due to normal stress parallel to welding seam,
- a τ utilization rate due to shear stress.
In addition, the multi-axial stresses through the utilization rates of the individual components for welded components must be
combined using the following formula:
Factor f(φ) takes the phase of the normal stresses to each other into account and can have values between -1.0 and +1.0. The calculation of factor f(φ) is carried out in following steps: a) For all relevant load condition combinations at any time it
will be calculated:
b.) Of these, the maximum and minimum of all Xi are determined: Xmax = MAX (Xi) Xmin = MIN (Xi)
c.) The factor f (φ) results depending on Xmin ≥ 0 → f(φ) = -1 Xmax ≤ 0 → f(φ) = +1 or
The shear stress component is always superimposed, as whether it would occur in phases.
A simple and lying on the safe side approach Is that is recognized in equation (7) the factor f (φ) with +1.
7.2 Fatigue strength according to the nominal stress concept The following specified oscillation strength values are based on:
- 10 7
load change,
-
a survival probability of Pü ≥ 97.5% (unilateral confidence interval)
and shall apply to the evaluation of components, since the residual stress influence is taken into account.
The boosting circuit caused by the component- and welding seam shape is being taken into account with notch type dependent fatigue strength values. In appendix C the typical welded connections are compiled. The assigned fatigue
strength values are compiled in the Haigh- and MKJ- diagrams, section 7.2.2. These values apply to the sheet thickness area 1.5 ≤ t ≤ 10 mm unless otherwise specified in the tables of appendix C. For the sheet thickness 10 < t ≤ 90 mm section 7.1.3, equation 4 should be noted.
For the fatigue strength verification the normal stresses
longitudinal and perpendicular to the seam di rection (σII and σ
⊥) as well as shear stresses longitudinal to the seam direction (τ) must be observed. (See Section 7.1.5)
At a potential crack initiation on the seam transition nominal voltage in the cross-section is decisive. For non-through-welded connections the fatigue strength verification must be carried out on all potential crack areas (connected cross-section in the seam transition crack area and seam cross section at root crack). Here the arithmetical weld thickness according to DIN EN 15085-3 should be noted.
7.2.1 Fatigue strength values for the base material after the nominal stress
The fatigue strength values for the base material (wrought materials, forged parts and casting materials) are dependent on the tensile strength and elongation of the material as well as the surface roughness. They are still depending on the stress ratio Rσ or Rτ.
The derivation of the fatigue strength values is based on the FKM guideline. The fatigue strength values for the heat affected zone are listed in section 7.2.2.
Fatigue strength values for normal stresses:
The fatigue limit σW, zd for normal stresses is determined depending on the tensile strength Rm in table 4, the safety factor j ges and the design factor as follows:
For the safety factor j ges the following must be applied: -for sheets, profiles and forgings j ges = 1.0
-
for castings
with the breaking elongation A50 according table 4.
The design factor KWK considers materials- related factors of
the surface roughness and is for sheets, profiles and forged
parts KWK = 1.3.
With this factor the typical roughness of sheets, profiles and
forged parts are considered.
For cast materials:
With the roughness factor KR,σ for a medium roughness
Rz=200μ
m
The fatigue strengths for the materials in table 3 are
summarized in table 4. For cast materials these fatigue strength
specifications apply to the cast surface.
The stress ratio Rσ is a result of the relation of the minimal to the maximal normal voltage in the evaluation point of a given orientation.
The fatigue strength value can be calculated according Gl. (9) to Gl (12) in dependence on Rσ, the medium voltage sensibility Mσ = 0.3 and the fatigue strength σW,zd from table 4.
Section I:
Rσ>1 (Oscillating compressive area)
Section II:
?∞≤Rσ≤ 0 (alternate area)
Section III:
0 < Rσ< 0.5 (low range pulsating tensile stresses)
Section IV:
Rσ≥ 0.5 (high range pulsating tensile stresses)
For some selected materials the permitted upper voltage or lower voltage is shown in figure 7 or 8 in the MKJ-diagrams.
In figure 9 the permitted voltage amplitudes in the Haigh-diagram are given for the base material (with expansion in the pressure range based on the FKM guideline).
MKJ-Diagram for normal stresses in the base material
Figure 7. MKJ-diagram base material for σm≥ 0 (tension change- and tension area), medium stress sensitivity Mσ = 0.3.
MKJ-diagram for normal tenses in the base material
Figure 8. MKJ-diagram base material for σm< 0 (pressure swing- and oscillating compressive strength area), medium stress sensitivity Mσ = 0,3.
Haigh- diagram for normal stresses in the base material
Figure 9: Haigh- Diagram for the base material, medium stress sensitivity Mσ = 0.3.
Fatigue strength values for shear stresses:
The fatigue strength τW, s for shear stresses is determined by
multiplying the fatigue strength with the factor 0.65:
The fatigue strength for shear stresses depending on the stress ratio Rτ and the medium voltage sensitivity Mτ = 0.17 is given by the following equations. Contrary to the normal stresses (Gl. 9 to 2) section l is not applicable and section ll has a changed lower limit.
Section II: -1 ≤ Rτ ≤
Section III: 0< Rτ
<0.5
Section IV: Rτ≥
0.5
7.2.2 Fatigue strength values for welding seams according to the nominal stress concept
The fatigue strength values for aluminum weld joints are
independent of the base material alloy. The strength values are dependent on the tension Rσ or Rτ.
Especially for the welding seams with castings it must be noted that the fatigue strength values of welds seams and their heat affected zone is not higher than the fatigue strength values of the subsequent base material. Fatigue strengths for normal stresses:
The tension Rσ results from the ratio of the minimum to the maximum normal stress (Rσ for σII and σ ⊥ to determine separately). The upper and lower voltage is illustrated in MKJ- diagrams, figure 10 or 11 and the voltage amplitude in the Haigh-diagram, figure 13. For weld joints they contain the lines B to F2, whose assignment to the connection detail is specified in Annex C. In these diagrams the distance of the notch type lines corresponds to the factor 1.12. The notch type lines, that are marked with sign “+” or “-” are each around the factor 1.04 higher or lower.
The fatigue strength values for normal stresses in the trouble-free heat affected zone of the base material must be evaluated with curve B.
The fatigue strength can be calculated according Gl. (17) to Gl. (20) depending on Rσ and the notch type line (exponent x from table 5 or table 6): Section I: Rσ
>1
Section II: ? ∞ ≤Rσ≤ 0
Section III: 0 < Rσ
< 0.5
Section IV: Rσ ≥
0.5
M σ must be determined according to 7.1.1.
Table 6. Exponent x in Gl (17) to (20) for keeling curves E1 to
Fatigue strengths for shear stresses
The fatigue strengths of shear stresses are illustrated in figure 12 and the Haigh-diagrams in figure 14. Contrary to the normal stresses (Gl. 17 to 20) section l is not applicable and section II has a changed lower limit. The exponent x must be taken from table 7.
For the fatigue strength values for shear stresses in the trouble-free heat affected zone of the base material curve G must be used.
Section II: ?1 ≤R τ≤ 0
Section III: 0< Rτ<0.5
Section IV: Rτ≥ 0.5
Mτ must be determined according to 7.1.1.
The fatigue strengths of shear stresses apply to :
– Line G for through- welded connections,
– Line H for non- through welded connections or fillet welds.
R for σm > 0
Figure 10. MKJ- diagram for normal stresses in welding connections at σm≥ 0. Medium stress sensitivity Mσ= 0.15.