1、Chapter 4: Component S-N AnalysisMSC Fatigue QuickStart Guide4Component S-N AnalysisProblem Description 138Geometry 139Set Up the Fatigue Analysis 142Run the Fatigue Analysis 158Review the Results 159Concluding Remarks 164MSC Fatigue QuickStart Guide138Problem DescriptionProblem DescriptionA simple
2、bracket as shown has a design life of 7 years (61,320 hours). Loading occurs at the end of the short section which has been welded and the component is constrained at both ends of the main bar. Because failure is known to occur at the weld, the finite element modeling at the loading point and the st
3、resses found there can be ignored for the purposes of this exercise. The load applied in the model was 900 lbs total. In service, the component experiences loading of 3000 lbs in the direction of the finite element load and 7000 lbs in the reverse direction. This occurs once every 30 minutes. Only a
4、 4% failure rate is allowed.Objective To introduce the concept of a component S-N curve. To learn how to enter materials data into the database manager. To determine if the component achieves its design life. To determine what level of loading can be achieve and what failure rate could be expected -
5、 a sensitivity study. To understand what files are created by an MSC Fatigue analysis.Table 4-1 Chapter 4 Necessary FilesFile P3_HOME/mscfatigue_files/examples/bracket.op2139Chapter 4: Component S-N AnalysisGeometryGeometryInvoke MSC Patran as you did in the previous examples. The geometry can be fo
6、und in the file bracket.op2. The results are from MSC Nastran. Copy the file to your working directory. Open a new database in a clean, empty work directory from the File | New menu. Give the name bracket to the database.Import the Model and ResultsPress the Analysis toggle switch or tab on the main
7、 form. When the form appears set the Action to Access Results, the Object to Read Output2, and the Method to Both (model and results) then press the Select Results File button and select the file bracket.op2 and click Apply. The model will then appear and you are ready to set up a fatigue analysis.V
8、iew the Stress ResultsBefore moving on to the fatigue analysis, press the Results application switch or tab on the main form to view the stress results from the MSC Nastran analysis. Select Stress Tensor, from the listbox and set the MSC Fatigue QuickStart Guide140GeometryQuantity to Maximum Princip
9、al. Click the Apply button and note the areas of high stress mostly around the applied load. This however, is not of concern to us. What we are interested in is the stress at Node 514 of around 2,690 PSI. This will be explained in more detail as we set up the material information.To rotate the model
10、, press the middle mouse button or for a two button mouse, press both at the same time.When you are done, press the Results switch again to close down the Results application form.141Chapter 4: Component S-N AnalysisGeometryMSC Fatigue QuickStart Guide142Set Up the Fatigue AnalysisSet Up the Fatigue
11、 AnalysisTo begin setup for a fatigue analysis, from the Tools pull-down menu in MSC Patran, select MSC Fatigue and then Main Interface. This will bring up the MSC Fatigue main form from which all parameters, loading and materials information, and analysis control are accessed.Once the form is open,
12、 set the Main Form as follows:143Chapter 4: Component S-N AnalysisSet Up the Fatigue AnalysisMSC Fatigue QuickStart Guide144Set Up the Fatigue Analysis1. Analysis: S-N2. Results Loc.: NodeThis simply means that the fatigue lives will be determined at the nodes of the model.3. Nodal Ave.: GlobalAccep
13、t the default which simply means element nodal stresses will be averaged to the nodes. 4. F.E. Results: StressS-N analyses require stresses; you do not have a choice.5. Res. Units: PSIModel dimensions are inches and forces are in Pounds, therefore stress units are PSI.6. Jobname: comp_sn7. Title: Co
14、mponent S-N AnalysisSolution ParametersOpen the Solution Params. form. On this form leave all the defaults except:145Chapter 4: Component S-N AnalysisSet Up the Fatigue AnalysisCertainty of Survival: 96As we learned in the last exercise, the S-N data can have significant scatter associated with it.
15、We are asking MSC Fatigue to calculate a fatigue live with 96% certainty of survival based on the scatter in the S-N data. This corresponds to a 4% failure rate.Click the OK button to continue.Material InformationThe component was tested under constant amplitude, fully-reversed conditions to produce
16、 S-N data. In the previous examples we have used S-N curves that are representative of the material and independent of geometry. They related local stress () to life. Now we have a different situation where the actual component geometry itself as well as the material has been used in tests to create
17、 the S-N curve. This type of S-N curve is called a component S-N curve. These type of curves relate nominal stress (S) to life and are dependent on the geometry of the component. If you change the geometry, the curve will no longer MSC Fatigue QuickStart Guide146Set Up the Fatigue Analysisbe valid.
18、The nominal stress is a location away from the actual failure location. This is usually because it is impossible to place a measurement device such as a strain gauge in the failure location. The stress for the S-N curve was measured using strain gauges at a point one quarter of an inch from the weld
19、 on the main bar and 5 inches from each end of the bar. Node 514 of the model corresponds to this measurement point for the S-N curve. The point of measurement is sometimes referred to as the reference location.For this model we have an S-N curve that needs to be input to PFMAT, the materials databa
20、se manager. Two methods of entering this data will be given.Table 4-2 S-N Data Set for Bracket AssemblyProperties SI ImperialS-N Properties:Stress Range Intercept, SRI1 10,710 MPa 1553 KSIFirst fatigue strength exponent, b1 -0.33333 -0.33333Fatigue transition point (cycles), NC1 1E7 1E7Second fatigu
21、e strength exponent, b2 -0.2 -0.2Standard error of Log (N), SE 0.2 0.2R-Ratio of test, RRAT -1 -1Monotonic Properties:Youngs Modulus, E 205,800 MPa 29, 850 KSIUTS 700 MPa 101.5 KSI147Chapter 4: Component S-N AnalysisSet Up the Fatigue AnalysisManual Entry of Materials DataOpen the Material Info. for
22、m and press the Materials Database Manager button. This will invoke PFMAT. Once the program has started, select Create | data set 1.You will be asked for a password to modify the central database location. If you do not enter a password and simply press the carriage return or the OK button, a copy o
23、f the central materials database will be copied to your local directory where you can then proceed to enter your materials data.Now a series of forms will open requesting data entry. On the first form, Names, enter:1. Primary name: BRACKET_SN2. Anything else you want - not requiredOn the next form,
24、Static Data, enter the generic (monotonic) information:1. UTS: Ultimate Tensile Strength (MPa): 7002. E: Elastic modulus (MPa): 205800Only these two parameters are required to be entered. The next form (E-N data) is for strain data. Skip over this form by clicking the OK button. The next form is for
25、 S-N data. Select Component from the pull-down menu.For the rest of the data, enter the SI values as indicated in Table 4-2. Click the OK button when done. Note: PFMAT always tells you at the top of its main menu whether it is connected to the central database in the MSC Fatigue installation area or
26、 a local database in the current directory, or even some other database that you may have created in another directory.Fracture Mechanics Data is requested next. Just click the OK button to skip over this. Multiaxial data is MSC Fatigue QuickStart Guide148Set Up the Fatigue Analysisrequested next. S
27、kip over this form also by clicking the OK button. The material will be entered into the database. Press or double-click the Graphical Display switch to view the S-N curve.Exit from PFMAT when you are done using the File | Exit and the eXit switch.Hint: We are entering the data here in SI units. All
28、 underlying fatigue calculations are done using SI units. However if you wish to enter and view materials data in Imperial units, set the preference using Preferences | Stress units | PSI. You can save this setting globally, or just locally in your working directory (or not at all) so that each time
29、 you invoke PFMAT it remembers to display values and plots in your units of preference.Note: S-N curves are characterized by a power law and thus appear as straight lines in log-log space. The equation is S=SRI1(N)bwhere SRI1 is the y-intercept and b is the slope (after Basquin). It is interesting t
30、o note historically that, although invented in 1870 by August Woehler, the S-N curve was not actually displayed graphically until some 30 years later. And it was not until 10 years after that that the curves were characterized in equation form. Our curve actually has two slopes and a transition poin
31、t. If the second slope were zero it would act as a fatigue limit.149Chapter 4: Component S-N AnalysisSet Up the Fatigue AnalysisBatch Entry of Materials DataInput another S-N data set. To illustrate batch mode operation of PFMAT we are going to define the parameters of the second S-N set in a file.
32、Using your a text editor, create a file called bracket.mat in the working directory.Enter the following lines in this file:/OPT=CREATE/INDB=YES/PASS=/MATNO=2/PRI=BRACKET_SN2/UTS=700/E1=205800/SNT=C/SRI=13950/B1=-0.29/NC1=2E7/B2=-0.16/SE=0.14/RRAT=-1/OPT=EXThen from the system prompt or a DOS window
33、issue the following command:pfmat bracket.matASCII Materials File ReaderThe MAT file created above can also be entered in the S-N data set by using the ASCII Materials File Reader. This form can be accessed by going to the Tools pull-down menu and selecting MSC Fatigue (for the MSC Patran version) o
34、r Fatigue Utilities (for the Standalone version). From here, select Material Management and then ASCII Materials File Reader. Table 4-3 Second S-N Data Set for Bracket AssemblyS-N Properties SI ImperialStress Range Intercept, SRI1 (MPa) 13950 2023ksiSlope, b1 -0.29 -0.29Transition life, NC1 (cycles)
35、 2E7 2E7Slope, b2 -0.16 -0.16Standard error, SE 0.14 0.14Stress ratio, RRAT -1 -1MSC Fatigue QuickStart Guide150Set Up the Fatigue AnalysisOn the form that comes up, enter the name bracket.mat into the MAT Filename databox and click the Apply button.Either of the above mentioned two methods will put
36、 the second data set into the database. Graphically compare bracket_sn and bracket_sn2 by running PFMAT interactively and using the Graphical display option. To run interactively you can either just type pfmat at the system prompt or go back to Pre & Post or MSC Patran and spawn it from the MSC Fati
37、gue Material Info. form. Make sure both bracket_sn and bracket_sn2 are loaded as data set 1 and 2 using Load | data set n.Note: The above mentioned MAT file can also be created from scratch by using the “Edit” button on the form shown above.151Chapter 4: Component S-N AnalysisSet Up the Fatigue Anal
38、ysisSpecify the Material for the AnalysisOn the Material Info. form enter the following in the spreadsheet:1. Material: BRACKET_SNSelect the cell under the Material column to activate it and select the S-N curve from the listbox that appears below the spreadsheet. The next cell will become active.2.
39、 Finish: No FinishSelect No Finish from the pull-down menu that appears below the spreadsheet. Finish and treatment are not allowed in a component S-N analysis (they are built into the curve). They will be ignored if you set them. The next cell will become active once you select the finish.3. Treatm
40、ent: No TreatmentSelect No Treatment. The next cell becomes active.4. Region: default_groupHint: If you do not have any S-N data, but only know E and UTS, you can have PFMAT generate generic material properties based on empirical formulas and the type of material. Simply enter E and UTS as if you we
41、re going to enter your own S-N data and the Material Type Number (see the MSC Fatigue Users Guide) and the S-N parameters will be generated automatically for you. (99=steel of unknown heat treatment) Of course you have to turn on the Generate all parameters from UTS toggle.MSC Fatigue QuickStart Gui
42、de152Set Up the Fatigue AnalysisSelect default_group which contains the nodes and elements from the entire model.Close the Material Info. form when you are done by clicking OK.Loading InformationTo create the time history which represents the actual loading conditions of the bracket, use PTIME and t
43、he X-Y points option representing y-values only. The time history will have a maximum of 3000 lbs and a minimum of 7000 lbs. No other information has been given so you can assume that there are no peaks and valleys between these points and that only these two points are required. You will enter the
44、values 0, 3000, 7000, and 0 to create this loading.The 1/2 hour interval can be modeled using the fatigue equivalent units. This is a term relating to the real value of one repeat of the time history. In this case, you can use 30 minutes, 1/2 hours, 1/48 days, etc. The answer will be the same of cou
45、rse, but you can choose the best parameter for reporting the life of your product.Open the Loading Info. form and click the Time History Manager button.153Chapter 4: Component S-N AnalysisSet Up the Fatigue AnalysisDefine the LoadWhen PTIME comes up, select Enter X-Y points as the method of input. A
46、 form will appear that will ask for a name, description and other information. Enter the following leaving defaults for those not mentioned:Note: If you have been working sequentially through this document, then you will already have some entries in the PTIME database. The version of the form that i
47、s displayed will be different than the one shown here. On this form, select Add an entry and then select the option X-y time series, which is the equivalent of selecting Enter X-Y points on the shown form.MSC Fatigue QuickStart Guide154Set Up the Fatigue Analysis1. Filename: BRACKET_LOAD2. Descripti
48、on 1: Bracket Loading3. Load Type: Force4. Units: lbs force5. Number of fatigue equivalent units: 0.56. Fatigue Equivalent Units: HoursWe are defining a single occurrence of this signal as representing 1/2 hour.Click the OK button to go on. Next you will be prompted to enter the Y points. Enter the
49、following numbers with a carriage return after each: 0, 3000, -7000, 0. End by putting in a blank entry and then click the End button.155Chapter 4: Component S-N AnalysisSet Up the Fatigue AnalysisPlot the Time HistoryPTIME returns to its main menu where you can select Plot an entry to make sure it took correctly. Accept the default file, BRACKET_LOAD.Select File | Exit to close the plot and press or double-click the eXit switch in PTIME.Associate the FE Load to its Time VariationNow back on the Loading Info. form you must associate the time variation of the load t
