Answers to Workshop 4

1/1 Introduction to ABAQUS/Standardand ABAQUS/Explicit

WA4.1

ABAQUS

Answers 4Dynamics

Question W41: Are there modes of the physical system that cannot be captured by yourmodel because of limitations in element type or mesh? (Remember that theelements are planar and the mesh is somewhat coarse.)

Answer: Because the model is two-dimensional, it cannot capture the modes thatoccur out of the plane of the model, including torsional modes.The mesh is too coarse to capture modes other than the first five. Use moreelements (at least 10) to look at all 10 requested modes.

Question W42: Do any of the mode shapes for your model look nonphysical?Answer: No.

Question W43: How does this compare with the frequency calculated in the eigenvalueanalysis?

Answer: The frequency calculated from the history plot of the tip displacement isapproximately 5.8, which agrees very closely with the frequency calculatedin the eigenvalue analysis.

Question W43: Why is the NLGEOM parameter set to NO in this analysis?Answer: By default, ABAQUS/Explicit invokes a large-displacement formulation.

Since the ABAQUS/Standard results with which we wish to compare wereobtained using small-displacement theory (which is the default formulationused in ABAQUS/Standard), we must set the NLGEOM parameter to NOin the ABAQUS/Explicit analysis.

Question W44: How do the results compare with one another? What factors contribute tothe discrepancies in the solutions?

Answers to Workshop 4

1/1 Introduction to the ABAQUS Solvers WA4.2

ABAQUS

Answer: The results for the first step are in close agreement: the peak verticaldisplacement in the ABAQUS/Standard analysis is 30.66 units (downward),while the peak vertical displacement in the ABAQUS/Explicit analysis is31.23 units (downward). Closer agreement can be obtained by reducing theloading rate in the first step in the ABAQUS/Explicit analysis. This willfurther reduce the effect of inertia in this step, thereby improving the quasi-static nature of the loading. Reducing the loading rate will lead to increasedcomputation time, however. The user must be careful to balance accuracyand efficiency in this case.The results for the second step show some discrepancies between the twoanalysis methods. In particular, the peak displacements and the frequencyof vibration predicted by the two analysis codes differ slightly. Thedifferences are due to the fact that the fixed time increment that was used tointegrate the equations of motion in the ABAQUS/Standard analysis wastoo large to yield the same level of accuracy that ABAQUS/Explicitprovides. Reducing the time increment used in the ABAQUS/Standardanalysis yields much closer agreement with the ABAQUS/Explicit solution,as illustrated in Figure WA41.

Answers to Workshop 4

1/1 Introduction to ABAQUS/Standardand ABAQUS/Explicit

WA4.3

ABAQUS

Question W45: Was the loading rate small enough to ensure a quasi-static response in thefirst step?

Answer: A comparison of the internal and kinetic energies of the model reveals thatthe kinetic energy is very small relative to the internal energy in the firststep. This indicates that inertia effects are minimal, and the response in thisstep can be considered quasi-static.

STD (DT=0.0001)XPL

Figure WA41. Comparison of the Tip Node DisplacementHistory: Reduced Implicit Time Increment