XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX''"> Introduction
This chapter on crack growth consists of descriptions of various MSC Fatigue modules. These modules can be accessed from subordinate forms of the MSC Fatigue main form within MSC Fatigue Pre & Post or MSC Patran. With the analysis set to Crack Growth, either the MSC Fatigue module PKSOL(K-solutions), PCRACK (crack growth), or PCPOST (postprocessor) will initiate.
Please familiarize yourself with
Introduction (Ch. 1) before reading this chapter. Please also read
Introduction, 268 in Chapter 5 for more information and
Figure 5‑1 for a schematic of the submittal process for crack growth analysis.
PCRACK is a cycle-by-cycle fatigue crack growth prediction module which calculates the crack extension, da, in each cycle of a sequence of cycles and adds it to the current crack size, a. This process proceeds until the failure condition is reached or a preset crack size is achieved. The driving force for crack growth in each cycle is the effective range of stress intensity, delta K (
). In each cycle, the apparent or applied
is calculated from the stress range, the crack size and the geometry of the component or structure under consideration. This is then modified to effective
to account for the possible occurrence of:
• crack closure
• history effects
• notch effects
• environmental influence
• static fracture mode contribution
Taking these effects into account, the crack propagation life may be modeled accurately and this can only be achieved on a cycle-by-cycle basis.
There are many applications of this approach including: design analysis, pre‑prediction of test programs, defect tolerance assessment, formulation of an inspection related strategy, failure investigation, and decision support. These are discussed in detail in
Fatigue Theory (Ch. 15).
In addition to the crack growth module PCRACK, the K Solution library module, PKSOL is also described. It contains the means to calculate the fracture mechanics stress intensity factor, K, for over 35 geometric cases. The module also generates compliance function tables for use in the MSC Fatigue fracture mechanics prediction module, PCRACK, where K is calculated as a function of crack size, stress history and geometry.
PCPOST allows for viewing of final crack growth information.
All of these modules can be run in stand alone mode also by typing the appropriate symbol at the system prompt (i.e. pksol, pcrack, pcpost).
Terminal Definition
MSC Fatigue runs on a wide range of computers and graphics devices. The parameters used by each graphics device must be defined by using the MENM module, prior to the first use of MSC Fatigue. For details, please see
Module Operations (Ch. 17). This is automatically accomplished when running MSC Fatigue from MSC Patran and is transparent to the user and defaults to the Motif driver.
Analysis Route
To carry out a crack growth analysis from within MSC Fatigue, the following programs are required:
| Enables the generation of a compliance function for the crack geometry under consideration. This is done via the PKSOL module which can be accessed from the Solution Parameters form from MSC Fatigue’s main form or may be run separately by typing pksol at the system prompt. In any case, to submit a job from MSC Fatigue, the compliance function must be selected from the Solution Parameter form. |
| Shell script (necessary for submittal from MSC Fatigue Pre & Post or MSC Patran). When submitting a full analysis this way, the next three programs are run automatically via this script. |
| Translator; creates the fatigue input file, jobname.fes. This file contains all the information necessary to submit to the fatigue preprocessor. |
FATTRANS | New translator that can be used in place of PAT3FAT. |
| Crack growth preprocessor. The difference between this preprocessing and that of the pre-processing done for a crack initiation or an total life analysis is that the cycles are re-ordered into their original sequence after rainflow counting is done. |
| Computes the cycle-by-cycle crack growth life. |
| The results may be reviewed using this module. |
All programs in the MSC Fatigue system may be executed by typing the name of the program or its symbol. These programs may ask questions which are not normally presented to you since they are executed as batch jobs when called from the pre-/post-processing environment. The programs normally used in a typical or basic fatigue analysis are listed below.
1. Data Preparation
| Materials Database Manager and BS5400 Weld Classification Advisor |
| Time History Database Manager and ASCII Time History File Convertor |
| A Peak-Valley Extraction Program for Reducing Lengthy Time Histories |
| A Multi-file Display Program |
| Compliance Function Library/K Solution Generator |
2. Crack Growth Analysis
| Crack Growth Preprocessor (cycle counter/reorderer) |
| Crack Growth Fatigue Analyzer |
| Crack Growth Postprocessor |
3. General Utilities
| FES File ASCII/Binary Convertor |
| Terminal Driver |
| Binary to Binary File Convertor |
Necessary Files
The crack growth analysis requires the definition of a compliance function. Using the stress information selected, it then performs the crack growth calculation, using the crack growth setup forms in MSC Fatigue. For this reason, it is not a good idea to attempt a crack growth analysis outside the pre-/post- processor unless the job has already been defined at least once from within.
To carry out a crack growth calculation, the following files must already be present in the user’s directory.
jobname.fin | This file contains all the analysis parameters that were defined in the main and subordinate MSC Fatigue forms (e.g., loading time history data file names). In addition, the analysis type and job titles are also defined in this file. A full description of this file is contained in The Job Information File (jobname.fin), 336. |
Database | Other pertinent information such as the nodes or elements to use in determining the nominal stress (far field stress) is contained in the MSC Patran database in the form of a group. The component stresses from these locations will be used, superimposed, averaged, and resolved (dependent on various parameter requests) for use as the far field stress in the crack growth equations. |
Results Files | The actual stress results used in the crack growth analysis can be stored in the database or can come from external results files such as MSC Patran results files (.nod, .els) or a MSC Patran FEA (jobname.res) results file. |
filename.ksn | A compliance function file produced by PKSOL. The use of PKSOL is defined in K Solution Library (PKSOL), 508. |
Additional Files | Other files that are necessary to complete a successful crack growth analysis are the time history files (ptime.adb, ptime.tdb and *.dac), and the materials database (nmats.mbd) which is generally held in a central location and not necessary to be located in the user’s local directory. |
After the translator has been run (described in
The Translator (PAT3FAT or FATTRANS), 272) and the fatigue input file (
jobname.fes) has been created, the crack growth preprocessor, PCRACK, is run. A
jobname.tcy file is created which is a scaled, time‑ordered, rainflow cycle count for nominal stresses of the loading time history. The crack growth analysis is performed by the program PCRACK. When run in interactive mode, this program asks for a number of input parameters which are passed in through the
jobname.fin, jobname.fes and
jobname.tcy files when run from the MSC Fatigue menus. A full description of file content is provided in
Description of Files, 336.