Advancing the Art of Structural Fatigue Analysis

In the early 90's, Battelle scientists recognized the need for a reliable stress analysis methodology that would address the known problems encountered in conventional finite element analysis procedures for welded joints or geometric discontinuities within structures. Consequently, Battelle created, developed and validated a new patent pending methodology known as the Verity® mesh-insensitive structural stress methodology. Although initially viewed as "too good to be true" or "too simple to possibly work," Verity® has now been validated by leading industry experts across a broad array of industries.

The revolutionary new Verity® mesh insensitive structural stress method, developed by Battelle uses new patented methods to calculate the fatigue lives of welded joints.

The method can be applied equally well to structural welds in thick plate, seam welds in thin sheets, and spot welds.

It has been developed in collaboration with over a dozen leading engineering companies, members of Battelle’s Joint Industry Project (JIP), and verified by correlating the results of more than 800 sets of welded joint fatigue test data.

Battelle has licensed the Verity® method to Safe Technology to produce a new add-on module in fe-safe™, the state-of-the-art durability analysis suite for Finite Element models. This will be available from 2006.

Verity®

• offers major benefits in fatigue analysis of welded joints
• is mesh insensitive - detailed FEA models are not required
• uses a single S-N curve for all welds - the user does not have to determine a ‘weld classification’
• includes the option of further improving the accuracy of fatigue life predictions by allowing the user to add specific S-N curves for particular types of welds
• is based on nodal forces at the weld - the user does not need to determine nominal stresses at some distance from the weld toe
• provides excellent correlation with test data
• is available as an add-on capability in fe-safe™ – enabling the user to include the effects of complex loading histories, multiaxial fatigue, and other advanced capabilities
• results can be plotted in FEA packages such as ABAQUS, ANSYS and Nastran
• a tool in HyperMesh supporting Verity® in fe-safe™ is available to automatically identify and generate the weld definitions required by Verity®

Cost:

• Potential to optimize material usage.
• Significant reduction in CAE and FEA modeling time.
• Lower development costs by minimizing fatigue testing requirements.
• Ability to inspect and test components against fatigue criteria should lower “classification” or regulatory compliance costs.

Performance:

• Ability to optimize material performance and engineering designs.
• Reduce tendency to over-design to account for uncertainty may result in lighter structures.

Quality:

• “Mesh-Insensitivity” eliminates variability/inconsistency of current procedures.
• Fatigue life predictions that are accurate (reduce failures, warranty claims, etc.)
• Easy to use and automate; compatible with existing FEA software.

• "Other fatigue software vendors claim to have an effective method, but when you look into the details, they're using a lot of subjective 'fudge factors,' not Pingsha" Hari Agrawal, Senior Technical Specialist with Ford Motor Co.
• "Incredibly significant" and compares its impact on engineering to that of mapping the human genome in the scientific world. Tarsem Jutla, Caterpillar
• "It will change the way fatigue analysis is taught at universities and practiced by industry." Pedro Vargas, ChevronTexaco

“The problem with weld joints is that the weld toe is very sharp, and therefore it is not possible to calculate weld toe stresses. The finer you make the mesh, the more the stress goes off to infinity.

The traditional method abandons any attempt to calculate true weld toe stresses, and uses a reference stress some distance from the weld toe. Each type or ‘class’ of weld has a different response to this reference stress and so different S-N curves are required for each type of weld. The ‘classification’ of the weld to select the correct S-N curve is very error-prone. The calculation is sensitive to the distance from the weld toe used for the reference stress, and in complex joints the reference stress can be impossible to determine.

A more recent method is the ‘hot-spot’ stress approach, where two reference points are chosen on the surface of the member close to the weld toe, and a straight-line extrapolation gives a pseudo-stress or hot-spot stress at the weld toe. It was hoped that if this were done correctly, a single S-N curve would apply to all welds. However, the choice of the two reference points has an impact on the accuracy of the method, the scatter in results has been disappointing, and the method is sensitive to mesh density.

The Verity method is a major departure in that it uses nodal forces to determine a ‘structural stress’ at the weld toe. A single S-N curve can then be applied to all types of welded joint to a good level of accuracy. This avoids the ‘weld classification’ problem. Because the stress refers to the weld toe there is no ‘distance’ problem, and the method is insensitive to mesh density and element type. It can also be applied to a much wider range of welds - structural welds, spot welds, etc, all with a single S-N curve.”

• Aerospace Laurels 2004 Honoree, Aviation Week & Space Technology
• AWS 2004 R. D. Thomas Memorial Award
• Recipient of 2003 SAE International Henry Ford II Distinguished Award for Excellence in Automotive Engineering
• SAE FD&E 2003 Weld Challenge: "The Best Prediction"

For further information and technical details please visit the Battelle website.