Page 65 - Using ANSYS for Finite Element Analysis A Tutorial for Engineers

52 • Using ansys for finite element analysis
A poor model may have defects that are not removed by refinement of
the mesh.
A responsible user must understand the physical nature of the
problem and the behavior of finite elements well enough to prepare a
suitable model and evaluate the quality of the results. Competence in
using FE for stress analysis does not imply competence in using FE for
(say) magnetic field problems. The engineer who uses the software, not
the software vendor, even if the results are affected by the errors in the
software, takes responsibility for the results produced.
Figure 1.13 is an example of discrepancies that may appear. A pressure
pulse is applied to a straight beam with hinge supports. The loading causes
the material to yield and the beam to vibrate. Analysis seeks to track the lat-
eral displacement of the midpoint as a function of time. The results plotted
come from 10 reputable analysis codes and were obtained by users regarded
as expert. Yet, if any of the curves is correct, we cannot tell which one it is.
Admittedly, the problem is difficult. The results indicate strong sensitivities
of both physical and computational nature. This example reminds us that
any analysis program is based on theory and approximation, and that a user
may push the program beyond its range of validity.