Interior stress validation · M07 · Learn to Program Ansys APDL Simulations

Choose the correct stress, avoid singularities, and extract values at the top and bottom fibers.

Displacement error

inertia=beam_b*beam_h**3/12
uy_ref=tip_force*beam_l**3/(3*young*inertia)
uy_error=ABS(uy_tip_avg-uy_ref)/ABS(uy_ref)

The comparison retains the sign in the original values and uses magnitudes only in the denominator of the error. Acceptance requires uy_error<0.05.

Which stress should we compare?

Beam theory predicts longitudinal normal stress. Therefore, compare it with S,X, not with von Mises stress:

sigma_x = M(x)·y/I

S,EQV combines components using an energy criterion and is useful for other decisions, but it does not represent the same magnitude as the previous equation.

Do not use the fixed support to characterize the beam

Interior stress section separated from the fixed support singularity — Figure 2. The validation section is located in `x=0.2·L`.

At the fixed support, zero displacement is imposed on an entire face. The transition from that idealization to the deformable field creates mesh-sensitive stress concentrations. Using the maximum stress as the sole validation metric mixes nominal behavior with a local disturbance.

Stress reference in the interior section

x_section=0.2*beam_l
moment_section=ABS(tip_force)*(beam_l-x_section)
sigma_ref=moment_section*(beam_h/2)/inertia

For the base case:

M(0.2L) = 1000·(1−0.2) = 800 N·m
sigma_ref = 9.6 MPa

The section is two element lengths from the fixed support and sufficiently far from the load application.

Extract the upper and lower fibers

NSEL,S,LOC,X,x_section
NSEL,R,LOC,Y,beam_h
CM,stress_top_nodes,NODE

*GET,sx_node,NODE,node_id,S,X

The base mesh contains two nodes across the width at the top fiber and two at the bottom fiber. We average each pair separately. For a force in the −Y direction, we expect:

sx_top_avg > 0     ! tension
sx_bottom_avg < 0  ! compression

Magnitude and antisymmetry

sigma_fea=(ABS(sx_top_avg)+ABS(sx_bottom_avg))/2
stress_error=ABS(sigma_fea-sigma_ref)/sigma_ref
stress_symmetry=ABS(ABS(sx_top_avg)-ABS(sx_bottom_avg))/sigma_ref

The first metric checks the nominal magnitude. The second verifies that the extreme fibers develop opposite stresses of similar magnitude, as predicted by pure bending. Their respective limits are 10 % and 5 %.

PLNSOL vs. PLESOL

PLNSOL,S,X
PLESOL,S,X

PLNSOL represents nodal values averaged across compatible elements.
PLESOL preserves the values of each element and leaves the breaks visible.

A smoother contour is not automatically more precise. Comparing both views helps detect how much the averaging is hiding.

Smoothed nodal S X stress contour from PLNSOL — Figure 4. Smoothed nodal `S,X` stress (`PLNSOL,S,X`) in the interior section.

PLESOL elemental contour of stress S X with jumps at interfaces — Figure 5. Elemental `S,X` stress (`PLESOL,S,X`). The key contrast with PLNSOL is that jumps remain visible at interfaces.