Fracture Assessment Software IWM VERB 8.0

Fraunhofer-Institut für Werkstoffmechanik IWM

Program Scope and Features

IWM VERB is a software tool for the assessment of components containing crack-like defects. The computational basis of the program consists of methods and solutions of the elastic and elastic-plastic fracture mechanics, the assessment methodology follows international documents, such as SINTAP, R6, BS 7910, API 579, German FKM Guideline “Fracture Mechanics Proof of Strength for Engineering Components", FITNET.

Since 1988, VERB has being developed at Fraunhofer IWM and applied by various industrial and research companies. The program allows for a fast assessment of cracked components with respect to their brittle failure, crack initiation, stable or unstable ductile crack extension under static loading, as well as crack growth under cyclic loading. The program provides tools for multiple calculations by varying input parameters, the determination of critical conditions – critical crack size, loading or fracture toughness. Assessment goals cover estimates of safety and availability of components, their remaining fatigue lives, leak-before-break analyses, determination of inspection intervals and requirements for non-destructive examination, material selection and qualification.

Application areas include various industrial and service branches, e.g.

  • power generation
  • aerospace
  • pipeline
  • chemical production
  • railway
  • technical expertise
  • insurances
  • education

Analysis procedures and solution methods employed in VERB are being continuously improved and extended to employ the most recent developments of defect assessment methodology. In particular, major changes have been undertaken in Version 8.0 to provide

  • extended selection of component and crack geometries
  • advanced solutions for non-uniform stress distributions in the crack plane
  • flexible input of loading parameters and material properties
  • improved algorithms for calculating fatigue crack growth and critical conditions
  • wide choice of control options
  • graphic tools for data control

The program is released in the standard or expert modus. In the expert modus, the program additionally includes leak-before-break (LBB) calculations for pressurised components, the application of partial safety factor rules, a selection of stress intensity factor and plastic limit load solutions.

Solution Procedures

Assessment at static loading, including the determination of critical conditions, follows the procedure established within R6 and further developed in SINTAP. This is based on methods of the elastic-plastic fracture mechanics and employs the failure assessment diagram (FAD) approach. Various options can be selected to define the FAD assessment curve, and both crack initiation and crack instability can be considered as failure criteria. In the latter case, the crack resistance curve – either as an analytical equation or as a data table – has to be provided. For pressurized components, a leak-before-break diagram can be established.

VERB 8.0 also provides a semi-probabilistic assessment on the basis of partial safety factors following the rules adopted in SINTAP / BS 7910 or API 579.

For components subjected to cyclic loading, the program performs crack growth calculations. The cyclic loading is defined by specifying the upper and lower values of respective load parameters. Both constant and variable amplitude loading can be considered. No load interaction effects are considered in the current version of the program, however, this analysis option is under preparation. Finally, fatigue crack growth calculations can be performed to determine - number of cycles required for crack propagation from the initial to a specified final crack depth, - final crack size for the given initial crack geometry and number of cycles, - conditions for the non-propagation of the initial crack (fatigue proof).

Analytical Basis

VERB 8.0 employs accurate, advanced stress intensity factor solutions for various geometries of cracked components. These are mostly based on the weight function or polynomial influence function techniques and are derived from and validated by numerical calculations. For certain crack geometries, closed form stress intensity factor equations are employed as additional or alternative solutions.

The compendium of the plastic limit load solutions is up-to-date and takes account of most recent developments in the literature as well as some fracture mechanics codes and guidelines.

In the expert modus, the program offers a selection out of several available solutions for the stress intensity factor and the plastic limit load to allow for a flexible variation of geometry and loading parameters, on one hand, and to facilitate a comparison of the results with other codes and programs, on the other hand.

Load Definition

Primary stresses acting over the crack can be defined by providing the following parameters or data series:

  • tensile force or membrane stress,
  • bending moment or bending stress,
  • internal pressure for cylinders and spheres,
  • non-uniform stress distribution over the component wall.

A combination of the above load parameters can also be considered.

Secondary (residual) stresses can be involved in calculations either as uniform (constant) or variable, user defined through-wall stress distributions. Alternatively, secondary stress profiles can be generated using their conservative estimates according to BS 7910 and SINTAP.

Cyclic loading is specified by providing the respective load parameters at maximum (upper load) and minimum (lower load) loading levels. Residual stresses can be additionally included in the analysis, as these affect the R-ratio and, thus, the crack growth rate. In case of variable amplitude cyclic loading, histograms of the mean stress and stress amplitude are requested. An extension of the load input for calculations at variable stress amplitudes is under preparation and will be available in the next program release.

Material Data

Material properties required for static calculations are provided as single parameter values (e.g. yield strength, tensile strength, fracture toughness) or as respective functions (stress-strain curve, crack resistance curve).

Fracture toughness Kmat can also be estimated from the J-integral, crack tip opening displacement or Charpy energy, using correlations developed within SINTAP / FITNET.

The crack growth law at cyclic loading can be defined in an analytical or a tabular form. Several data series or parameter sets corresponding to different stress ratios can be simultaneously specified; the program then applies an interpolation procedure to calculate the crack growth rate for the actual values of stress intensity factor range DK and R-ratio. Some fatigue crack growth equations that explicitly include the stress ration effect (Forman, Erdogan and Ratwani, NASGRO) are available.

Input

Data input is provided via an interactive GUI. Tabulated data – stress-strain and crack resistance curves, fatigue crack growth parameters, stress profiles, load histograms – are simultaneously displayed in diagrams to afford their visualisation and editing.

Input data can be saved in XML format and used or modified in later program sessions.

Output

After calculations, analysis results are displayed in a diagram and as a results table. Depending upon the mode and extent of calculations, different parameter sets can be displayed in the diagram. A protocol including input data, analysis results, as well as the last selected diagram can then be saved as a PDF document. Additionally, a protocol in the ASCII format can be generated. A standard copy and paste function allows for diagrams and related data sets to be transferred to Microsoft® documents or data sheets.

System Requirements

VERB 8.0 runs under Microsoft® Windows® 2000, XP or Vista and requires .NET Framework. The latter, if not installed, is available as a free download at the Microsoft® home page. Free disk space of some 60 MB and 512 MB of RAM (1 GB recommended) are requested.

Maintenance, Updates, Technical Support

All errors or malfunctions detected in VERB will be resolved by Fraunhofer IWM. In such cases, a free update of the program will be sent to the user. Upon essential changes including additional procedures, solutions or features, a new program version can be released and offered to the user as an upgrade.

Questions concerning the technical support or program purchase should be addressed to verb(at)iwm.fraunhofer.de.