7 Spot Weld Analysis
7.3 Tutorial: Spot Weld Fatigue Analysis of a Welded Beam
• Load existing file Fatigue_Spotweld_cbar_ini.hm
• Load in User Profiles, OptiStruct
• Change representation to ”1D Detailed Element Representation”
• Edit the existing Material “Steel” (MAT1) with the values as presented below
138
• Define Spotweld properties (σN Curves) by activating the MATFAT box
HINT: Here 1, 2, 3 represent sheet1, sheet2, and nugget
• Create Property PFATSPW; Card Image PFATSPW
139
• set SPTFAIL to SHEET
• Create Load Collectors FATDEF
➢ Card Image FATDEF
➢ set PTYPE to PBARL
140
• Assign Fatigue Property to SpotWeld with FATDEF
• Create Load Collector FATPARM
• Set the card image to FATPARM
• STRESS -> COMBINE - > SGVN
• CERTNTY, Survcert set to 0,5
• SPWLD checked
• Set CORRECT to FKM
• Enable thickness correction by setting THCKCORR to YES
141
• Create Load Step and refer the FATDEF, FATPARM and FATSEQ subcase entries by appropriate load collectors
142
• Start OptiStruct ANALYSIS
• Post-Processing in HyperView, Options, Visualisation, BAR, “Cylinder “
• Make 2 Windows to display
• displacement for static load on left side
• damage for fatigue analysis on rigid side
143
• Edit legend as per the values displayed
144
145
Appendix A
This section discusses new features regarding OptiStruct Fatigue analysis.
Fatigue Analysis based on a Local Submodel
Feature supported version: OptiStruct 14.0.220 or above
In Global-local analysis, a full model is solved using two or more submodels. One submodel represents the full structure, and parts of the structure with small details, which require relatively higher accuracy, can be modeled as local submodels with a fine mesh. Displacements from the coarser global model are interpolated and applied to the finer mesh of the local model at the transfer zone. This allows for the local model to be driven by the results of the global model.
• This feature may help improve results in models with local stress concentrations. It allows faster solution time as only parts of the structure is being resolved with a fine mesh.
• Solid-to-Solid, Shell-to-shell, and shell-to-solid zooming are supported.
• It is an approximate solution, under the assumption that the displacements are correct in the coarser global model.
Until Version 14.0.220 only linear static analysis was supported In V14.0.220 Fatigue analysis is also supported
Setup is similar to setting up any fatigue analysis
146
Can pick either global subcase or a local subcase Can pick either global element set, or local element set Example
• Symmetric Boundary Conditions
• Pressure Load applied on the plate
• Fatigue Analysis of both global and local submodel
147
Results
Amplitude Based MATFAT
Feature supported version: OptiStruct 2017.2.1 or above
The SN curve considered for fatigue calculation could be either Stress-range V/s No.
of Cycles or Stress-Amplitude V/S No. of cycles. Until Version 2017.2, only Stress-range based curves were supported. In Version 2017.2.1, we support both. The default is still stress-range based curve.
148
How to setup in OptiStruct
• The A/R field on the MATFAT card can be used to switch between stress-range and stress-amplitude based curves.
• A/R defines the interpretation of the defined SN curve.
➢ A: The SN curve is defined based on Amplitude.
➢ R: The SN curve is defined based on Range.
Static Loading Applied Sequentially in Fatigue Analysis
Feature supported version: OptiStruct 2017.2.1 or above
Static loads can now be applied sequentially in fatigue analysis from Version 2017.2 Until Version 2017.2, static loads could only be super-imposed in fatigue analysis The SQNTL flag on the FATEVNT entry can be used to switch the applied Static Fatigue loads to sequential loading instead of super-positional loading.
149
TID field on FATLOAD card needs to be blank Format
Random Response Fatigue Analysis with Strain-Life Approach
Feature supported version: OptiStruct 2017.2.2 or above
Static loads can now be applied sequentially in fatigue analysis from Version 2017.2 Strain-Life approach is supported for Random Fatigue Analysis.
Stress life approach is already available.
Only Uniaxial Analysis is supported with combined stress as von-Mises stress.
The TYPE field on the FATPARM Bulk Data Entry can be used to identify stress-life (SN) or strain-life (EN).
150
Multiple Mean Stress SN curve Definition
Feature supported version: OptiStruct 2017.2.2 or above
Multiple SN data can be defined for different mean stress through the FATMCRV entry which can be referenced on the SNCM continuation line of the MATFAT entry. The input on FATMCRV would be the mean stress and TABLEXN IDs. TABLEXN defines stress vs life. Alternatively, TABLEXN can directly be referenced in MATFAT and in that case, there is single SN curve in analysis.
• Stress type - A(Amplitude,) R(Range), or MAX (Maximum stress) is supported.
• Available analysis type is static, transient, and random response analysis. No support for seam weld / spot weld.
• The intermediate curves are interpolated for the damage evaluation.
• Mean stress correction is redundant when Multiple mean stress SN curves are defined.
151
Multiple Mean Stress SN curve Definition: Bulk Cards
Sine Sweep Fatigue Analysis
Feature supported version: OptiStruct 2017.2.2 or above
Sine Sweep Fatigue Analysis: Sine sweep is a well-known method of vibration testing which is conducted to evaluate the vibration behavior of a specimen. It is the study of fatigue life of structures under Sinusoidal Loading.
• The setup is very similar to a Frequency Response Analysis setup, with an additional Fatigue subcase, and additional required Sine Sweep parameters.
The LCID field on the FATLOAD entry references the subcase ID of the Frequency Response Analysis subcase.
• Frequency Response Stress results from the Frequency Response Analysis are used to calculate the frequency-dependent stress range, which are subsequently used to calculate Fatigue damage based on sweep rate.
• Both Stress-Life and Strain-Life approach is available for Sine Sweep Fatigue analysis.
152
Acceleration Profile and Sweep rate
FATPARM Bulk Cards
• The TYPE field on the FATPARM Bulk Data Entry can be used to identify stress-life (SN) or strain-life (EN) sine sweep fatigue analysis.
• The SWEEP continuation line on the FATPARM Bulk Data Entry can be used to define the frequencies (NF or DF fields) at which the sine sweep fatigue calculations are performed. Additionally, the STSUBID field can be used to identify a static subcase for mean stress correction.
FATLOAD Bulk Cards
• The LCID field on FATLOAD Bulk Data Entry references a Frequency Response Analysis subcase.
• The SWEEP continuation line should be specified on FATLOAD Bulk Data Entry, and corresponding sine sweep parameters, via SR (sweep rate) and SRUNIT (sweep rate unit) fields are required.
153
FATSEQ Bulk Cards
• The N# fields on the FATSEQ Bulk Data Entry identifies the number of sweeps of the corresponding FID# reference and the T# fields define the time in sec.