The documents are available free of charge in PDF format. This document supersedes DNV-RP-C, October Text affected by the main changes in this. Request PDF on ResearchGate | Background for new revision of DNV-RP-C Fatigue Design of Offshore Steel Structures | The DNV-RP-C Fatigue. Failure Modes Considered in DNV-RP-C 1. Fatigue crack growth from the weld toe. 2. Fatigue crack growth from a notch in the base.
|Language:||English, Spanish, Hindi|
|ePub File Size:||16.61 MB|
|PDF File Size:||14.41 MB|
|Distribution:||Free* [*Sign up for free]|
The electronic pdf version of this document found through scretch.info is the officially binding Recommended Practice DNV-RP-C, October The documents are available free of charge in PDF format. DNVGL-RP- Recommended practice, DNVGL-RP-C – Edition April Page 3. DNV GL AS. If any person suffers loss or damage which is proved to have been caused by any negligent act or omission of Det Norske Veritas, then Det.
Weld connections section 9 is implemented. Alternatively, use the Standard context menu: By default, all welds that match following criteria are included into selection but can be changed by pressing. Weld is included in calculations if it is: 1. Straight with the tolerance of 3 degrees; 2. Double-symmetric - weld is formed by 2 planes that are perpendicular to each other with the tolerance of 5 degrees; Preview weld elements that are not verified press. Preview selected weld elements by pressing.
Here frequency is the number of occurrences. Stress ratio: Ratio of minimum to maximum value of the stress in a cycle. Structural discontinuity: geometric discontinuity due to the type of welded joint, usually found in tables of classified structural details. The effects of a structural discontinuity are i concentration of the membrane stress and ii formation of secondary bending stress.
Structural stress: stress in a component, resolved taking into account the effects of a structural discontinuity, and consisting of membrane and shell bending stress components.
Structural stress concentration factor: The ratio of hot spot structural stress to local nominal stress. Variable amplitude loading: type of loading causing irregular stress fluctuation with stress ranges and amplitudes of variable magnitude. Introduction The main principles for fatigue analysis based on fatigue tests are described in this section. The fatigue analysis may be based on nominal S-N curves for plated structures when appropriate.
Reference is made to [3. When performing finite element analysis for design of plated structures it is often found more convenient to extract hot spot stress from the analysis than that of a nominal stress.
Guidance on finite element modelling and hot spot stress derivation is presented in [4. The calculated hot spot stress is then entered a hot spot S-N curve for derivation of cycles to failure. For design of simple tubular joints it is standard practice to use parametric equations for derivation of stress concentration factors to obtain hot spot stress for the actual geometry.
Then this hot spot stress is entered a relevant hot spot stress S-N curve for tubular joints. Results from performed fatigue analyses are presented in Sec.
The basis for the design charts is that long term stress ranges can be described by a two parameter Weibull distribution. The procedure can be used for different design lives, different Design Fatigue Factors and different plate thickness.
The following fatigue cracking failure modes are considered in this document see also Figure - : Fatigue crack growth from the weld toe into the base material. In welded structures fatigue cracking from weld toes into the base material is a frequent failure mode.
The fatigue crack is initiated at small defects or undercuts at the weld toe where the stress is highest due to the weld notch geometry. Fatigue crack growth from the weld root through the fillet weld. Fatigue cracking from root of fillet welds with a crack growth through the weld is a failure mode that can lead to significant consequences.
Use of fillet welds should be avoided in connections where the failure consequences are large due to less reliable NDE of this type of connection compared with a full penetration weld. However, in some welded connections use of fillet welds can hardly be avoided and it is also efficient for fabrication. The specified design procedure in this document is considered to provide reliable connections also for fillet welds. Fatigue crack growth from the weld root into the section under the weld.
Fatigue crack growth from the weld root into the section under the weld is observed during service life of structures in laboratory fatigue testing.
The number of cycles to failure for this failure mode is of a similar magnitude as fatigue cracking from the weld toe in as-welded condition. There is no methodology that can be recommended used to avoid this failure mode except from using alternative types of welds locally. This means that if fatigue life improvement of the weld toe is required, the connection is subjected to high dynamic stress ranges.
Thus, the connection becomes also highly utilised with respect to dynamic loading and it is also required to make improvement for the root. This can be performed using a full penetration weld along some distance of the stiffener nose. Fatigue crack growth from a surface irregularity or notch into the base material. Fatigue cracking in the base material is a failure mode that is of concern in components with high stress cycles.
The specified design procedure in this document is considered to provide reliable connections also with respect to this failure mode.
Recommended practice DNVGL-RP Page 13 a Fatigue crack growth from the weld toe into the base material b Fatigue crack growth from the weld root through the fillet weld c Fatigue crack growth from the weld root into the section under the weld d Fatigue crack growth from a surface irregularity or notch into the base material Figure - Explanation of different fatigue failure modes. Fatigue damage accumulation The fatigue life may be calculated based on the S-N fatigue approach under the assumption of linear cumulative damage Palmgren-Miner rule.
Due consideration should be given to selection of integration method as the position of the integration points may have a significant influence on the calculated fatigue life dependent on integration method. See Sec. Reference is made to commentary section for derivation of fatigue damage calculated from different processes..
General Fatigue analysis may be based on different methodologies depending on what is found most efficient for the considered structural detail.
Different concepts of S-N curves are developed and referred to in the literature and in this RP. It is thus important that the stresses are calculated in agreement with the definition of the stresses to be used together with a particular S-N curve. Hot spot stress S-N curve that is described in [.
Notch stress S-N curve that is not used in the main part of this RP. This approach is foreseen used only in special cases where it is found difficult to reliably assess the fatigue life using other methods. Nominal stress is understood to be a stress in a component that can be derived by classical theory such as beam theory.
In a simple plate specimen with an attachment as shown in Figure 4- the nominal stress is simply the membrane stress that is used for plotting of the S-N data from the fatigue testing. Hot spot stress is understood to be the geometric stress created by the considered detail. The notch stress due to the local weld geometry is excluded from the stress calculation as it is assumed to be accounted for in the corresponding hot spot S-N curve.
The notch stress is defined as the total stress resulting from the geometry of the detail and the non-linear stress field due to the notch at the weld toe.
Derivation of stresses to be used together with the different S-N curves are described in more detail in the following section. The procedure for the fatigue analysis is based on the assumption that it is only necessary to consider the ranges of cyclic stresses in determining the fatigue endurance i.
The selection of S-N curve is dependent on amount and type of inspection during fabrication; ref. The size of defects inherent the S-N data are described in ppendix [D. Plated structures using nominal stress S-N curves The joint classification and corresponding S-N curves takes into account the local stress concentrations created by the joints themselves and by the weld profile. The design stress can therefore be regarded as the nominal stress, adjacent to the weld under consideration.
However, if the joint is situated in a region of Recommended practice DNVGL-RP Page 4 15 stress concentration resulting from the gross shape of the structure, this must be taken into account. The maximum principal stress is considered to be a significant parameter for analysis of fatigue crack growth. When the principal stress direction is different from that of the normal to the weld toe, it becomes conservative to use the principle stress range together with a classification of the connection for stress range normal to the weld toe as shown in Figure This means that the notch at the weld toe no longer significantly influences the fatigue capacity and a higher S-N curve applies for this stress direction.
More guidance on this for use of nominal S-N curves is presented in commentary D. Plated structures using nominal stress S-N curves.
Stress ranges calculated based on von Mises stress can be used for fatigue analysis of notches in base material where initiation of a fatigue crack is a significant part of the fatigue life.. The effect of stress direction relative to the weld toe as shown in Figure -3 and Figure -4 when using finite element analysis and hot spot stress S-N curve is presented in section [4.
The total stress fluctuation i. Reference is made to Table -8 for selection of S-N curve. It is recommended to perform a finite element analysis for assessment of fatigue of these connections.
The bending stress can be analysed using a modelling of the weld with at least second order solid elements over the throat thickness where each element represents a linear stress distribution. The calculated stress components at a position 0.
The range of the maximum principal stress can then be used together with the F3 curve for calculation of fatigue damage. The S-N curve for air environment can be used. Figure -6 Fillet welded doubling plate. Even though the joint may be required to carry wholly compressive stresses and the plate surfaces may be machined to fit, the total stress fluctuation should be considered to be transmitted through the welds for fatigue assessment.
If it is assumed that compressive loading is transferred through contact, it should be verified that the contact will not be lost during the welding. The actual installation condition including maximum construction tolerances should be accounted for.. General The fatigue design is based on use of S-N curves, which are obtained from fatigue tests.
The design S-N curves which follows are based on the mean-minus-two-standard-deviation curves for relevant experimental data. The S-N curves are thus associated with a Failure criterion inherent the S-N curves Most of the S-N data are derived by fatigue testing of small specimens in test laboratories.
For simple test specimens the testing is performed until the specimens have failed. In these specimens there is no possibility for redistribution of stresses during crack growth. This means that most of the fatigue life is associated with growth of a small crack that grows faster as the crack size increases until fracture if the connection otherwise does not show significant redistribution of stress flow during crack growth.
For details with the same calculated damage, the initiation period of a fatigue crack takes longer time for a notch in base material than at a weld toe or weld root. This also means that with a higher fatigue resistance of the base material as compared with welded details, the crack growth will be faster in base material when fatigue cracks are growing as the stress range in the base material can be significantly higher than at the welds if they are designed with the same fatigue utilization.
When this failure criterion is transferred into a crack size in a real structure where some redistribution of stress is more likely, this means that this failure criterion corresponds to a crack size that is somewhat less than the plate thickness. The test specimens with tubular joints are normally of a larger size. These joints also show larger possibility for redistribution of stresses as a crack is growing.
Thus a crack can grow through the thickness and also along a part of the joint before a fracture occur during the testing. The number of cycles at a crack size through the thickness is used when the S-N curves are derived.
Other types of joint, including tube to plate, may fall in one of the 4 classes specified in Table -, Table - and Table -3, depending upon: the geometrical arrangement of the detail the direction of the fluctuating stress relative to the detail the method of fabrication and inspection of the detail.
Each construction detail at which fatigue cracks may potentially develop should, where possible, be placed in its relevant joint class in accordance with criteria given in pp.. It should be noted that, in any welded joint, there are several locations at which fatigue cracks may develop, e. Each location should be classified separately. Reference is made to [D. The fatigue strength of welded joints is to some extent dependent on plate thickness.
This effect is due to the local geometry of the weld toe in relation to thickness of the adjoining plates. See also effect of profiling on thickness effect in [7.
It is also dependent on the stress gradient over the thickness. Reference is made to pp. For tubular joints the reference thickness is 3 mm. In general the thickness exponent is included in the design equation to account for a situation that the actual size of the considered structural component is different in geometry from that the S-N data are based on.
The thickness exponent is considered to account for different size of plate through which a crack will most likely grow. To some extent it also accounts for local geometry at the weld toe. However, it does not account for weld length or length of component different from that tested such as e.
Then the size effect should be carefully considered using probabilistic theory to achieve a reliable design, see ppendix [D. The thickness or size effect is also dependent on width of weld in butt welds and attachment length in cruciform connections. This parameter is denoted L t in Figure For butt welds where the widths L t may be different on the two plate sides, the width L t for the considered hot spot side should be used.
The size effect is lower than predicted from equation. Thus the thickness t in equation. The T curve is shown in Figure Introduction S-N curves are based on experimental data, where traditional welding methods are adopted.
Therefore, the S-N curves may not correctly represent the fatigue life, where the laser welding method and other non-traditional welding methods are applied. Nowadays the high-speed laser weld method is widely used in the industry. Hence the standards might need new S-N curves that correctly represent the fatigue life of laser welded stainless steel materials.
Laser welding is commonly used to assemble small components in the biomedical, electronics and aerospace industry . In these applications weldings require a very small melted area, hence small laser beams less than 1 kW are used.
The Nd:YAG laser beam has been developed further and the maximum heat output is increased to 6 kW . Consequently, it is possible to weld sheet components with a higher thickness. One of the most significant types of laser welding is the keyhole method , which is also used in this study.
With the keyhole method it is possible to weld, in one process, perpendicular plates. Therefore, in production of perpendicular plate assemblies the keyhole welding method permits high production volume and a low manufacturing price.
The possibility to weld a perpendicular assembly in one single process has been requested by the industry for many years. Therefore, the keyhole method became one of the most important welding procedures with the laser technology.
However, when the keyhole welding is used higher stress concentrations are generated in the welded zone and consequently a lower fatigue life can occur. The fatigue behavior of dynamic loaded structures, where the keyhole method is adopted has not been investigated thoroughly in the literature.
Therefore, it is necessary to investigate the fatigue life of T-joints exposed to dynamic loading. Laser welding is more effective compared to traditional welding methods, i. However, the small melted area cools down rapidly and high thermal stresses are introduced .
The thermal stress influences the metallurgy in the welding area and thereby the mechanical properties . Guidelines The present guidelines for fatigue assessment of different types of welded joints according to the design standards are quite general.
For stainless steel the assessment is independent of several important aspects mentioned next. Geometrically, the laser welding also cause a smaller welding compared to traditional welding types.
However, the smaller welding toe implies in most cases higher stress concentrations and accordingly the fatigue life is decreased. Furthermore, rotating bending fatigue tests  with AISI materials have shown that the fatigue life is sensitive to the initial defects in the material. The guidelines suggest that an averaged thickness of the sheets and an averaged size of welding toes are used in the fatigue assessment.
A fatigue calculation based on these averaged values may not provide a reliable estimate of the fatigue life. The size of the welding toe is known to have a greater impact on the fatigue life in a thin sheet application compared to a situation, where a thick sheet is used . The higher sensitivity in thin plates is caused by the stress concentration in the normal plane to the specimen. When the welding toe is large compared to the plate thickness the toe has a considerable influence on the fatigue life  and .
A reliable estimate of the fatigue life requires that the geometry of the welding toe is taken into account in the design guidelines. Material In the design standards the fatigue assessment guidelines are based on structural steel like SJR. These types of steel do not show the same tendency in change of the microstructure under the maximum crack length is decreased.
However, the crack growth rate is also decreased. The induced strains ahead of a fatigue crack tip can activate a transformation of an austenitic structure to welding process compared to stainless steel. Therefore, we do not expect that the fatigue life calculated based on the standards fit the fatigue life observed in the experiments, where thin sheets of stainless steel AISI are tested.
In some types of stainless steel including AISI the microstructure is changed i. This transformation is mainly controlled by plastic strain rates and the temperature . In case of low temperatures a higher amount of martensitic phase is generated . The martensitic phase is more brittle than the austenitic phase.
Therefore, the fracture toughness limit is lower and a martensitic structure . Furthermore, this transformation also changes the volume and the expansion of the material can lead to compressive stresses. Hence deformation induced martensitic transformation increases fatigue resistance significantly and the threshold stress intensity decreases.
The standard S-N curves for welded structural steel do not include the mean stress because of welding introduced residual stresses. In stainless steel the residual stresses coming from the laser welding process are often more significant compared to standard structural steel.
Therefore, the higher compressive stresses in the welding zone can lead to crack propagation resistance . However, the tensile stresses in the welding zone are increased accordingly and crack initiations are more probable.
The Study In this study the fatigue life of welded and non-welded specimens is investigated experimentally. The welded specimens are produced with the keyhole laser welding method. The results from the experiments are compared to the results from the standards, where the S-N curves are based on structural steel and traditional welding methods.
Hence this comparison and possible deviations will reveal if the guidelines for fatigue assessments in the current standards can be safely adopted when stainless steel is considered. The welded specimen is a T-joint welded with the keyhole method by a laser and is shown in Figure 1. The AISI composition is 0. Figure 1. The geometry of the laser welded specimen in AISI Three different types of specimen are used in the fatigue tests.
Two types are non-welded, where one type is cut with a plasma cutter, whereas the other nonwelded type is cut with a milling cutter. The third type is a specimen with a transverse welding with respect to the load direction T-joint. Argon gas is used to prevent oxidation of the steel. The steel surfaces are smooth with a maximum variation of 0.
The welded surfaces have a variation around 0. A standard servohydraulic Instron test machine is used. The test is completed when a final fracture occurs. Two different methods have been used to cut out the non-welded specimens. The different defects from this cutting process are observed and the influence on the fatigue life is established.
The knowledge about these defects can be transformed to a tolerable defect size for the S-N curve. In Figure 2 the non-welded specimen geometry cut by the milling cutter is shown and Figure 3 shows the geometry cut by the plasma cutter. The missing data points do to the limit number of sample is estimated with interpolation between the low and high cycle fatigue life. Results The shape of the welding significantly influences stress concentrations. Thus the shape of the welding is an important factor in the estimation of the fatigue life.
A macro photo of the cross section of the welding is shown Figure 2. The geometry of the modified non-welded specimens cut out with a milling cutter. Figure 3. The geometry of the modified non-welded specimens cut out with a plasma cutter. The welding has clear defects, which introduce high stress concentrations.
The shape is like a wedge in the area where the two parts are welded together. A crack most probably initiates at the end of the wedge shape and the fatigue life will be lower compared to a specimen with a smooth welding. No materials are added during the welding process, which explains these clear defects shown in Figure 4. Figure 4 clearly reveals that the shape of the toe is non-smooth at the corner compared to a welding geometry obtained from a traditional welding method.
In a standard welding procedure this geometry is not acceptable. The results of the fatigue test of the nonwelded specimens are shown in Figure 5. In the low cycle area the fatigue life is nearly identical for the two specimens. In the high cycle fatigue HCF area the specimens cut by the milling cutter tend to show a higher fatigue resistance. The tendency is that the fatigue resistance is low for the specimens cut by plasma compared to the specimens cut by the milling cutter.
However, the difference in fatigue life for the two types of specimens is insignificant and it can be assumed that the fatigue life is identical.
Furthermore, the S-N curve Figure 4. Transverse section view for the keyhole laser welded T-joint. Figure 5. S-N data results of the fatigue life of mill cut and plasma cut non-welded specimens.