122_EvaluatingWeldFatigue

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Image courtesy of National Optical Astronomy Observatory, operated by the Association of Universities for Research in Astronomy, under cooperative agreement with the National Science Foundation.

The COSMOS CompanionEvaluating Weld Fatigue

Volume 122

Sponsored by:

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2© 2006 SolidWorks Corp. Confidential.

What is the COSMOS Companion?

The COSMOS Companion is a series of short subjects to help design engineers build better products with SolidWorks AnalysisVideo presentations and accompanying exercisesA tool for Continuous Learning on your schedulePre-recorded videos are accompanied by a more detailed webcast with Q & A – Download videos and review webcast schedule at:

http://www.cosmosm.com/pages/news/COSMOS_Companion.html

It is not an alternative to instructor-led introductory training – We highly recommend you take a course with your local reseller to

build a solid knowledge base

If you are new to the COSMOS Companion, a few comments on the program are warranted. The COSMOS Companion series was developed in response to the request from many of our users for more detailed information on specific and/or new functionality within the COSMOS products. Additionally, many users have been asking for clarification of common design analysis questions to enable them to make more representative analysis models and make better decisions with the data. What’s more, users have asked for this material to be made available in a variety of formats so they can review it how and when they wish. To address this, each COSMOS Companion topic has been pre-recorded and made available thru the COSMOS Companion homepage as a downloadable or streaming video with audio, as static PDF slides for printing, or as a live webcast enabling attendees to ask questions and engage in additional discussion. We are trying to provide continuous learning on your schedule so you can be as effective and efficient as possible when using COSMOS for design analysis and validation.

It is important to note that this material is not developed as an alternative to instructor led training. We still believe that the best introduction to any of the COSMOS products is in a class led by your reseller’s certified instructor. In this program, we are hoping to build on the lessons learned in your initial training. In fact, we will make the assumption that you have basic knowledge of the interface and workflow from intro training or equivalent experience. We will try not to repeat what was taught in those classes or can be found in the on-line help but to augment that information.

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Agenda

Limitations and Scope of Techniques Presented

Review of Static Weld Sizing Discussion

Challenges in Estimating Weld Durability

Summary of Published Methods

“Hot Spot” Stress Calculations

Summary of Best Practices for Non-Specialists

List of References

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Limitations and Scope

Techniques presented are primarily related to plate weldments– Solid and shell representations will both be discussed

Not a comprehensive “class” on weld fatigue– As will be shown, the current body of research is too broad and there

is too little agreement on the best approach to offer a definitive method

Due to the difficulties in calculating stresses in welds, the fact that experts (and resulting predictions) disagree shouldn’t overly concern us. Focus on Trend and strive for:– Conservative calculations– Consistent methods– Iterative and diligent correlation to test or field failure with method

refinement

It is important to set expectations early when discussing welds and weldments since the size and shape of components that get welded span thin sheets to house-size castings. The techniques presented where in primarily relate to sheet metal or thin plate weldments. If surface or shell elements are a reasonable option for your model, then this discussion will be most applicable. The applicability to solids will be discussed briefly but these aren’t the focus.We’ll be reviewing a few published techniques for evaluating weld fatigue but there is too much out there and too many approaches to claim to be a comprehensive “class”. If this material sparks an interest and you wish to learn more, some references will be provided at the end of the presentation and an Internet search on FEA-based weld fatigue will yield volumes of data. You are encouraged to dig deeper on your own.Finally, as discussed in the previous session on static weld sizing, any FEA-based technique for attempting to assess stresses in welds will be found wanting. There are too many variables and unknowns to ever think you can predict stress in welds in a general sense. Additionally, even among experts on the subject, there is much disagreement on the best or most reliable means for predicting weld response. This alone should help you realize it can’t be as easy as pulling stresses off a color plot. However, once this uncertainty is understood, you can extract valuable information from your COSMOSWorks model to allow more intelligent weld decisions to be made. Key to this is a focus on conservative assumptions and trend studies. Every opportunity to compare analysis results and predictions with known field performance, both good and bad is critical to establishing a process that works for you and your products. Documenting this information as a consistent practice will go along way to designing better and cheaper weldments more quickly.

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Review of Static Weld Sizing

Welds vary from part to part and even within a given weld:– Chemistry: Carbon, Manganese, Hydrogen– Temperature: Weld, Preheat, Cooling– Base Material: Porosity, Composition– Weld Geometry: Penetration, Convexity, Continuity, Grinding– Heat Affected Zone: Uniformity, Property Degradation (Ductility)– Microcracking at perimeter of Weld– Residual Stress after cooling– Part Geometry: Surface Finish, Alignment, Warpage

This slide was presented in the discussion on static weld sizing and highlights the difficulties in making any sort of prediction of weld. The images on this slide speak towards the difficulty of capturing even the geometry of the weld, something designers mistakenly feel comfortable doing. The photograph of a steel chair shows how different two welds adjacent to each other can be. Can you ever assume to know what the loaded stress in this system is at the welds?

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Review of Static Weld Sizing

Can You Ever Know What Stress a Weld Sees?– Only in the most controlled conditions– Rarely from a predictive standpoint– Possibly from a failure analysis approach where sample is available

for measurement and testing

Stress estimation directly from FEA requires an initial estimate of weld size

Welded intersections in FEA models are notorious for singular or unreasonably high stresses

For the reasons stated previously, welds in production bear little or no resemblance to welds in a CAD or FEA model

In controlled systems where the weld geometry is measurable, usually larger, ground or otherwise fashioned welds, and the weldment is heat-treated to normalize residual stresses, an FE-based assessment of weld stress might be possible. In a predictive design environment, parts don’t yet exist so measurement isn’t usually an option. The most efficient use of prediction is to specify the weld size in advance. However, without an initial estimate of weld size, the load carrying area or throat which controls local stress magnitudes doesn’t exist so any stress calculation must be invalid.Finally, physical (& CAD solids) weld geometry is notoriously singular in that sharp corners at the weld toe or shearing/tearing interfaces at the weld heel are unlikely to ever converge on a final stress solution so compromises or guesses on weld stress need to be made. For these and the reasons on the previous slide, actual stress in a production weld is unlikely to ever match the stress calculated in a finite element representation of that weld.

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First and Foremost… What is the stress in a weld?

Different weld types and geometries respond differently to similar stresses

Durability on simple, polished single-piece test samples exhibits wide scatter!

A weldment is an assembly by nature with all the variability that suggests

Notch stress σ = σ + σ + σ ln m b nlp

Nonlinear stress peak

Total stress

Structural stress

0.4 t

While there are reasonable and tested techniques for estimating weld sizes based on weld loading thru the weld throat, as discussed previously, durability calculations do require stress estimates at the toe of the weld which is where fatigue failures typically occur. Due to the variable nature of welds, fatigue strengths in even the most controlled samples and of assemblies in general, any technique that attempts to predict weld durability must account for this data scatter in some repeatable method.

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Underlying limitations to any FE-based weld fatigue method– Actual stresses in real welds are nearly impossible to predict

Residual stressesGeometric variability & notch effectsLikely post-yield stresses local to welds

– Geometric (thus stress) singularities in an FEA model of an actual weld feature make convergence on the computed stress nearly impossible

The “solution” – Coordinated FE modeling methods and test results to draw an “A = B” comparison– Requires mesh construction in accordance with standard– Requires use of “SN” curve correlated with test and FE modeling– Typically conservative since it attempts to capture ‘typical’ response

In addition to the variability in weld stresses themselves, 3 different finite element models of the same weld, using shells, solids, tetrahedrons, hexahedrons, or different mesh densities, will produce three different stress solutions. A case can be made for any of these…which makes the case in my eyes that none of them are right. A method for predicting weld fatigue using FEA must take this into account as well.Most solutions to this problem that have gained acceptance typically involve embracing the variability of the problem and specifying both the FEA modeling method to be used and a statistically correlated SN curve is based on test with actual welded samples. Since they represent an average or worst-case response, they are typically conservative so some further correlation to your actual part performance is warranted.

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Summary of Published Methods

BS7608 - “Code of Practice for Fatigue Design & Assessment of Steel Structures”

IIW-IIS Research – Hobbacher / Niemi– http://www.iiw-iis.org/

ASME Section VIII-2 Pressure Vessel Design

Battelle “Verity” Nodal Force Based Stress Extraction

Some of the more well-known methods for predicting weld fatigue using FEA are listed in this slide. We’ll briefly review the differences and similarities of each.

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BS7608 Weld Classifications

BS7608 (British Welding Standard) classifies welds based on their geometry and loading direction. The standard provides notes and caveats to each scenario and then assigns a classification letter to the weld. The standard has several pages of pictures and classifications to examine and engineers must choose the best fit for their problem. The welds on this page are all Class F with the exception of the second which is a Class F2.

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BS7608 Design S-N Curves

The allowable stress for each class is then determined from this master SN curve for the desired cycles. The testing for this curve was performed to 107 cycles and then linearly extrapolated to 109. Designers are cautioned to put more thought into predictions beyond 107 cycles using this method and some discussion of this is also offered in the spec.

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IIW FAT Curves

IIW (The International Institute of Welding) has also published a series of master SN curves with their own classification scheme. The fatigue strength curve of FAT 100 is used for full penetration butt welds and FAT 80 for fillet and partialpenetration butt welds in shear. More details can be found in the publication by Hobbacher (1996). FAT 80 & FAT 90 seem to be the most commonly referenced for general weld study.

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IIW FAT80 & FAT90 Curves

These well-documented curves are often published with scatter bars to help users understand their limitations. When only the Mean curve is available, it is more difficult to assess factors of safety. A Design curve is often suggested as the most conservative approach taking into account the poorest performing samples and conditions available.

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Lotsberg Study

Lotsberg makes case to use FAT 80/90 curves as generalized conservative guidelines for fatigue design

A paper by Lotsberg (2005) has provided a generalized method based on FAT 80/90 curves that, while acknowledged as conservative, requires fewer classifications and calculations than some other methods and might be more usable by non-specialists. This paper is freely available on the Internet.

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Lotsberg Study

In this paper, several geometries and welding methods were evaluated.

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Lotsberg Study

Variety of geometries and mesh sizes compared to FAT90 curveIn most cases, the results correlated well with expectations or were conservativeBased on 4-noded (quadrilateral) shell elements and 20-noded solid (brick) elements– COSMOSWorks uses

Triangles and Tets

For each of these, different mesh sizes and stress extraction techniques were compared to determine the best all-around weld fatigue prediction method. All the tests were performed with quadrilateral shell or hexahedral solid elements. COSMOSWorks uses triangular shells and tetrahedral solids so some comparison of this data to the mesh available to COSMOSWorks users is warranted but not yet available. However, initial comparisons using the “hot spot” stress extraction method on quadrilateral & triangular shells is promising. More on “hot spot” stress extraction later in the presentation.

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VerityTM Mesh Insensitive Solution

Developed by Battelle Labs

Uses Nodal Forces to calculate stress response in welds– Sound familiar?

Validated in independent testing by major development organizations

Availability and applicability being investigated

The last technique worth mentioning is a proprietary method developed by Battelle Labs and marketed under the trade name Verity. Verity uses nodal forces, not stresses, to estimate the demand on a weld. This is consistent with the discussions in the Companion unit on static weld sizing where it was suggested that nodal loads were mesh insensitive and not subject to convergence issues posed by geometric singularities. The Verity method works with a proprietary master SN curve, as do the other methods suggested, and has received much attention in recent years for its robustness. Again, there is no shortage of information regarding this on the Internet but, unlike the Lotsberg methods, since it is proprietary, it isn’t something COSMOSWorks users can implement on their own without an additional investment.

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Flat Head Pressure Vessel Study

To tie some of these techniques together, a study was performed by the PaulinResearch Group in Houston, TX that compares predictions from these (with the exception of Verity) and others on welds in a pressure vessel. SolidWorks was not involved in this study and the data presented herein is for illustration purposes only. As with anything found on the Internet, you should contact the publishing authors before reading anything deeper into it. The stated purpose of this study was to attempt to find some consistency in the various prediction methods available. They started with the question, “If we build it and test it, do the rules predict it?”

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Flat Head Pressure Vessel Study

Using several methods, some not clearly identified in the paper, and on 3 specimens, failure was noted to be roughly consistent at around 30,000 cycles. Most of the prediction techniques were extremely conservative and some fared better with different assumptions than did others.

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Overview of Published Methods

Applicability for Purpose needs to be evaluated

No current research was found on High Order Tetrahedron or Tria Models

Weld Sizing still needs to be estimated upfront

To summarize, the concept of “Applicability (or Suitability) for Purpose” needs to be applied to any FE-based weld fatigue method. Obviously, the best way to proceed is to find something that works, document it and stick with it. This applies to calculation methods and welding practices. The technology has not yet reached the point where you can confidently state durability for a welded connection based on FE data alone. While few companies have the capacity to perform large scale, statistically valid testing on multiple weld scenarios, most companies who have developed weldments have many parts which have an acceptable service life and a handful that weren’t quite there. Use that data in conjunction with they techniques presented herein to fine-tune a prediction method that works for you. Static sizing is more predictable and should precede any fatigue calculations. Once you’ve determined the minimum weld size for static loading, you can build that into your model and proceed with stress estimation for fatigue but proceed with caution.

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For the previous few slides, we’ve been discussing stresses to compare to master SN curves using various methods but have somewhat glossed over the fact that stresses are nearly impossible to pull off of a COSMOSWorks model due to the problems associated with geometric singularities. This is illustrated in more detail using these images. The notch effect at the toe of the weld will create a high stress which will continue to rise with increased mesh density. For this reason, a number of different “hot spot” stress estimation methods have been developed that attempt to assign a more reasonable, notch-insensitive stress magnitude to the joint based on the stress distribution beyond the weld.Most “hot spot” techniques involve measuring stresses at two points of a specified distance from the weld toe, defined in fractions of the plate thickness, t. In this example, a line is drawn between the stress at t/2 and 3t/2 from the weld toe. The stress where a linear extrapolation of that stress distribution coincides with the edge of the weld toe, or zero-t, is considered the “hot spot” stress for the joint and is used for subsequent fatigue estimates. Remember that SN curves that use this hot spot stress were developed expecting stresses to be calculated using this method, or something similar to it. The SN curves in the IIW or BS7608 specs are not to be used with stresses pulled directly off the stress plots at the toe of the weld or by some other extrapolation method. They are designed to work together to overcome the variability inherent in this problem.

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Nominal Mesh = t/2

To illustrate this method, we’ll use a simple double-sided fillet weld sample and plot stresses along a split line on the base plate as shown in the image. The nominal mesh size chosen was t/2, again based on the base plate thickness. This allows us to locate points, or nodes, in ½ thickness increments on the line of interest.

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“Hot Spot” Stress CalculationsWeld Toe

Weld Toe

The P1 (maximum principal) stresses are plotted along the central split line and the top edge of the plate. The heavy black line superimposed on the graph represents the edge of the weld toe.

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t = 0.75”Nom Mesh Size = t/2Nodes Spaced at t/4

Weld Toe

0.5t

1.5t

“Hot Spot”Stress

~275psi

Using the worst-case plot, along the top edge, a line is drawn between the nodes at t/2 and 3t/2 and the stress at the weld toe is estimated. This can be done quickly and graphically on a printout or, in this case, in PowerPoint. Due to all the reasons stated earlier in this unit, tying to map a more accurate extrapolation is probably not worth the effort, unless you’ve automated a routine and/or its easier for you to work that way. There is plenty of uncertainty built into this process so minor errors due to round-off or graphing shouldn’t impact your design decision.For this example, the stress to compare to the appropriate weld fatigue SN curve would be 275 psi.

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Summary of Best Practices

Investigate these methods for weld fatigue

Don’t simply rely on “raw” stresses

Use test models to determine correlation of published techniques to your welds and geometries

At bare minimum, use FAT90 & “Hot Spot” calculations suggested by Lotsberg

Size mesh to enable “Hot Spot” calculations

Measure “Hot Spot” points from:– Weld Toe in Solid Models– Surface Intersections in Shell Models

Use P1 (Max Principal Stress) results – Unaveraged across parts.

To sum up our discussions, it is important that you take some time to read through available literature on weld fatigue and evaluation methods. Unfortunately, the technology isn’t quite ready for a comprehensive step-by-step procedure to make this anymore clear cut. Most importantly, don’t look at the “raw” stresses on your COSMOSWorks model at weld features and draw conclusions, even trend conclusions, about the weld durability. If you’ve been doing this, stop!Work with the knowledge available to you and document best practices for your products and methods. If you don’t have time to research all of these methods, read the Lotsberg paper and try to utilize these suggestions for a conservative estimate.Size your meshes to enable hot spot stress extrapolation. You may need to model your system more generally, identify the areas of concern and re-mesh for hot spot stress extraction but it will pay off. It is recommended also that you use P1, maximum principal stress, instead of Von Mises stress as a better indicator of the local tensile stresses. Experiment with this as well. Regardless of stress quantity, make sure you’ve disabled averaging across bonded parts so that you are only looking at stresses calculated within the body of interest.

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Presentation Summary

In this COSMOS Companion unit, we reviewed:

Difficulty of predicting fatigue for weldments

Published methods for estimating fatigue response

Comparison of these methods

“Hot Spot” stress calculations

“Best Practices” for initial ‘foray’ into fatigue prediction

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Conclusion

For more information…

Contact your local reseller for more in-depth training or support on techniques in COSMOSWorks to make your weld sizing & fatigue predictions more efficient.

Review the on-line help for a more detailed description of the features discussed

Attend, or better yet, present at a local COSMOS or SolidWorks user group. – See http://www.swugn.org/ for a user group near you

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