Scope

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?

What's new in VERB 8?

Computational basis

Computational models

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:

new graphic user interface that allows for more flexible data input and control;

unique and flexible input of load parameters for all models and calculation types;

enhanced or/and new solutions for the stress intensity factor and the limit load parameter for most of the component and crack geometries;

calculation of critical conditions and reserve factors as an integrated task of failure assessment analysis;

calculation of leak-before-break diagrams for pressurized pipes and vessels;

enhanced calculations of fatigue crack growth, including both the data input and the integration procedures;

improved accuracy and stability of numerical algorithms for most of the calculation types;

extended set of options to control the calculation performance, process and view input data and results, manage data allocation;

probabilistic calculations

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:

Structural model

Loading type

Crack type

Crack orientation

Crack position

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

Plate

Plate with hole

Hollow cylinder

Round bar

Round bar under rotary load

Sphere

Butt welded cylinder and plate with strength mismatch

Standard speciments

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

Load components: these can be tensile force or membrane stress, bending moment or bending stress, internal pressure

One-dimensional stress profile s(x) in the prospective crack plane; here the coordinate axis x points in the thickness direction

Two-dimensional stress profile s(x,y) in the prospective crack plane; here the coordinate axis x points in the thickness direction and the coordinate axis y points in the length direction

Stress profile can be superimposed with load components