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  1. GMarsh

    Johnson Cook model - flow stress match with UTS at ductility?

    Hi, I am trying to use Johnson Cook model for predicting equivalent flow stress. Just to check its validity at extremities, I input ductility as plastic strain, but I don't see it output stress close to ultimate tensile strength. I used ductility over 1 millisecond to calculate plastic strain...
  2. GMarsh

    Simulating HCF crack propagation using XFEM

    Hi, I am thinking if it is possible to simulate crack growth under High Cycle Fatigue in Abaqus? All the tutorials I have come across on XFEM shows crack growth only with static load. Wondering if there is a way to simulate alternating stresses superimposed on static stresses. Thank you Geoff
  3. GMarsh

    Idealisation of weld joints

    Hi, I am looking for various methods by which one can idealise a fillet weld joint in FE analysis. I searched through previous threads here and some general search on Google. Didn't find relevant info. Currently I use bonded contact between solid bodies which are welded. Then (as I get...
  4. GMarsh

    Thermal analysis using beam elements

    Hi, I am doing a steady state thermal analysis in a model which has solid and beam elements (using Ansys Workbench 14). I get an error "Higher order elements are not valid for current analysis". But this problem is not there if I suppress the beam elements (even though the solid elements have...
  5. GMarsh

    Fatigue criteria for different materials

    Hi, Are certain fatigue criteria more relevant for certain type of materials? I heard that Gerber is good for ductile materials, etc. So can we use criterion based on material? Any particular reference on this? Thank you Geoff
  6. GMarsh

    Bushing joint for springs in 3-directions?

    Hi, I would like to connect surfaces of two bodies with springs in 3-directions (X,Y,Z) in ANSYS software. As the surface normals lie only along Y-direction and that a spring accepts stiffness only along its longitudinal axis, I am wondering if I can use Bushing Joint instead and define...
  7. GMarsh

    Stabilization damping factor in non-linear static analysis

    Hi, I am doing non-linear static analysis of a spring like structure. As the solution convergence is quite difficult, I would like to know what parameters for automatic stabilization - damping factor or dissipation energy fraction - should be given for convergence. And what are the criteria for...
  8. GMarsh

    Will rubber damping increase with compressive force?

    Hi, I have a neoprene rubber sheet mounted on a structure. I am getting FRF using impact hammer testing and measuring damping through half power point method. When I compress this rubber sheet with two different loads I am getting different damping values. Higher compressive load gives more...
  9. GMarsh

    Viscoelastic material for damping high frequencies?

    Hello all, I would like to know if there is any viscoelastic material that is more suited or designed to damp high frequencies - 15000 - 16000 Hz ? I mean having loss factor near to 1 around this frequency range? Most of the commercially available tapes are designed for 1000 Hz range. Thank...
  10. GMarsh

    Is a tuned damper effective in forced vibration?

    Hi, I have a structure on which a tuned damper is mounted, tuned for one of the resonant modes of structure. Now this structure is also being subjected to a forced vibration. Now I am interested to know if the tuned damper will have effect on minimising forced vibration (because of added mass...
  11. GMarsh

    Data error or curve fitting error?

    Hi, I am trying to curve fit FRFs of a cylindrical casing. When I extract the modal parameters and see the mode shape, I am getting half of the mode shape quite amplified, as shown in attached document. Actually the lobes seen in mode shape should have been equally amplified along the...
  12. GMarsh

    Model order in curve fitting

    Hi, Can someone suggest how to decide the model order (or size) while curve fitting in modal analysis? As the curve fitting model order is increased, number of stable poles are increasing. But as some of these are computational modes, I want to know if there are any thumb rules or guidelines in...
  13. GMarsh

    Reasons for phase drift

    Hi, I am noticing phase drift in the FRF acquired on a cylindrical thin shell. When I acquire a vibration signal during machining of that structure, the time domain signal also shows a phase drift. Please find attached snapshot of the FRF and vibration time domain signal. Can someone suggest...
  14. GMarsh

    Frequency resolution and amplitude in FFT

    Hi, I am looking at improving the frequency resolution in FFT through zero padding. Attached is an excel sheet showing an attempt to study the frequency resolution with padding of zeros to data. Any comments ? Another basic (and obviously could be silly and stupid) question: What does the...
  15. GMarsh

    Do resonant frequencies produce harmonics

    Hi, I have a plate vibrating due to some forced excitation. When I take FFT of measured acceleration signal on the plate, I am getting a dominant mode and its harmonics. I see no link to this dominant frequency and exciting frequency (they are very wide and far). But the dominant frequency in...
  16. GMarsh

    Preloading a spring element

    Hi, I am trying to use a spring element to idealise a bolt in a dynamic (eigenvalue & harmonic) analysis. I read all the associated literature on calculating equivalent spring stiffness to represent a bolt. Now I wonder how can I give preloading to a spring element? Commercial FEA software...
  17. GMarsh

    Spring analysis

    Hi, I am trying to do a simple static analysis with 4 springs connected between two blocks. The stiffness of each spring is 1e5 N/m. The bottom block is fixed and top block is pulled with 10 kN. So using simple F=KX,I am suppose to get 25mm of displacement for top block. But I am getting...
  18. GMarsh

    Reading Abaqus odb into Ansys

    Hi, I have a Abaqus (6.9-1) odb file with modal analysis results. Now due to requirement of some downstream applications, I need to get these results from Ansys. I have two questions: 1. Reading Abaqus odb into Ansys - I have Ansys 12 in office. Is there any way where I can import my Abaqus...
  19. GMarsh

    Shell mode or beam mode ?

    Hi, I have a question regarding identification of particular mode as shell or beam mode. I am attaching the picture of mode shape here. Till recently I was thinking it is i=1, j=2 shell mode (i-circumferential, j-axial). But when reading some other literature I am reminded that beam mode...
  20. GMarsh

    Anomaly in drive point FRF

    Hi, I am doing modal testing on a casing (rigid base with thin wall at top) fixed on the base with 8 bolts. Roving hammer test. When I am looking at my drive point FRF (acquired on thin wall at the top) I am finding two anomalies: 1. In the imaginary part of FRF, two modes are appearing on...

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