Scope

Failure Assessment of Cracked Components

Fraunhofer IWM

Introduction

IWM VERB is a PC program for fracture assessment of components containing crack-like defects. The computational basis of the program consists of methods and solutions of elastic and elastic-plastic fracture mechanics, the assessment methodology follows internationally recognized guidelines and documents, such as SINTAP, R6, BS 7910, API 579, German FKM Guideline, FITNET. The application area mainly comprises metallic components subjected to static or cyclic loading, although non-metallic materials can also be considered provided that their behaviour can be described in terms of conventional fracture mechanics concepts.

The name VERB is an acronym of „Versagensbewertung von rissbehafteten Bauteilen" standing for “Failure Assessment of Cracked Components”. Since 1988, the program has continuously being developed at Fraunhofer Institute for Mechanics of Materials IWM, Freiburg.

Note, that VERB is a calculation tool that does not replace user’s expertise. If in doubt, the user should refer to appropriate guidelines or design rules approved by a specific branch of industry.

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What is VERB

The program VERB can be used to assess the influence of crack-like defects on component’s integrity or its remaining lifetime. Defects or flaws involved into consideration may be real or postulated cracks, depending upon particular objectives of the analysis. The program offers calculation tools for static, cyclic and transient loading. Most of calculations can be executed in both single and multiple mode. In the latter case, some input data is considered as varied or scattered parameter. Additionally, the program includes a module for probabilistic caluclations.

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What’s new

VERB 8 has been released as a major revision of the previous Version 7. To facilitate accurate fracture mechanics calculations and improve complex data exchange between the user and the program, the new version has been established using modern software tools, including object-oriented programming and Microsoft .NET Framework technology. Further objectives were to unify the data treatment for various calculation models and create a software frame that can be easily extended to incorporate future developments in the field of fracture mechanics and fitness-for-service assessment, as well as specific user’s needs.


The most important new features of Version 8 are:

An essential restriction for users of Version 7 is that, due to a different input data structure, input files generated in the old program version cannot be read in Version 8.

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Computational basis

Failure assessment under static (single, monotonic) loading is performed in VERB using the FAD approach. The latter is an engineering method of the elastic-plastic fracture mechanics (EPFM) taking account of both the local stress and strain concentration at the crack tip and the development of the plastic deformation in the cracked section. Analysis results are then displayed in a failure assessment diagram showing the location of the assessment point with respect to the assessment (limit) curve.

Crack propagation under cyclic loading is calculated, starting at certain initial crack size, by integrating a material specific crack growth equation. VERB contains various fatigue crack growth equations, including tabular input to avoid the need of analytical data fit. Cyclic loading parameters can be provided as constant amplitude loading, spectrum data, a load history function, as well as a rainflow matrix.

Various applications deal with loading cases including time dependent stress and temperature fields or stochastic non-proportional stress variation in the prospective crack plane. An additional “transient analysis” module is available in VERB which helps treating those cases.

If uncertain or scattering input data are considered in the analysis, VERB allows for proper solutions either through using partial safety factor methodology or by directly running probabilistic calculations. The probabilistic module includes a number of useful add-ons for the statistical data treatment, data approximation using different distribution functions, validation of statistical assumptions, etc.

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Computational models

The computational model is completed by assigning the selected geometry an appropriate stress intensity factor and, if applicable, a limit load solutions. This is done automatically within the standard release of VERB, whereas the user can select between different solution in the expert release. The rationale for using several solutions is that these often have different applicability ranges with respect to the crack and component geometry, as well as different possibilities to describe the stress distribution in the crack plane. Additionally, as some of the solutions implemented in VERB are included in common fracture assessment codes and guidelines, the user is offered a possibility to perform analyses in agreement with respective codes.

Input of an analysis model includes the selection of:

The following structural (component) models are available in VERB:

Loading for the cracked component is defined by one of the following options:

Crack models available in VERB are:

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