Modeling plastic behavior of Steel, force doesn't increase.

[zhddz]

I would like to analyze stress strain behavior of a steel beam using Static Riks. The Load proportional factor stop increasing shortly after reaching the yield strength. How to I solve the problem? Following is the status file,
STEP INC ATT SEVERE EQUIL TOTAL TOTAL STEP INC OF DOF IF
DISCON ITERS ITERS TIME/ TIME/LPF TIME/LPF MONITOR RIKS
ITERS FREQ
1 1 1 10 1 11 0.0147 0.01467 R
1 2 1 0 3 3 0.0293 0.01466 R
1 3 1 2 1 3 0.0512 0.02192 R
1 4 1 2 1 3 0.0840 0.03277 R
1 5 1 2 2 4 0.133 0.04890 R
1 6 1 2 2 4 0.206 0.07279 R
1 7 1 2 2 4 0.314 0.1079 R
1 8 1 3 2 5 0.473 0.1591 R
1 9 1 3 3 6 0.705 0.2321 R
1 10 1 5 4 9 1.03 0.3248 R
1 11 1U 2 1 3 1.03 -0.002430 R
1 11 2U 2 1 3 1.03 0.02908 R
1 11 3 0 3 3 1.04 0.01480 R
1 12 1 1 4 5 1.06 0.02046 R
1 13 1U 6 1 7 1.06 0.04049 R
1 13 2U 5 1 6 1.06 0.01259 R
1 13 3 0 3 3 1.07 0.0007418 R
1 14 1 0 3 3 1.07 0.001083 R
1 15 1 0 4 4 1.07 0.001557 R
1 16 1 1 3 4 1.07 0.003492 R
1 17 1 0 4 4 1.07 0.002512 R
1 18 1 0 4 4 1.08 0.003599 R
1 19 1 0 4 4 1.08 0.005131 R
1 20 1 1 3 4 1.09 0.007445 R
1 21 1 1 3 4 1.10 0.01062 R
1 22 1 0 4 4 1.11 0.01271 R
1 23 1 0 4 4 1.13 0.01551 R
1 24 1U 3 2 5 1.13 0.04949 R
1 24 2 0 4 4 1.14 0.007427 R
1 25 1 1 3 4 1.15 0.01057 R
1 26 1 0 4 4 1.16 0.01114 R
1 27 1 0 4 4 1.17 0.01149 R
1 28 1U 4 1 5 1.17 0.06784 R
1 28 2 0 4 4 1.17 0.004171 R
1 29 1U 13 0 13 1.17 0.01423 R
1 29 2 1 3 4 1.18 0.001992 R
1 30 1U 1 3 4 1.18 0.0001661 R
1 30 2 0 4 4 1.18 0.0004560 R
1 31 1U 3 3 6 1.18 0.02057 R
1 31 2U 3 3 6 1.18 0.001404 R
1 31 3 0 5 5 1.18 2.428e-005 R
1 32 1 0 4 4 1.18 2.329e-005 R
1 33 1 0 5 5 1.18 2.287e-005 R
1 34 1 0 4 4 1.18 2.231e-005 R
1 35 1U 4 3 7 1.18 0.001117 R
1 35 2 0 3 3 1.18 6.486e-006 R
1 36 1U 2 2 4 1.18 -7.297e-007 R
1 36 2U 2 2 4 1.18 1.956e-006 R
1 36 3 2 3 5 1.18 1.560e-005 R
1 37 1U 0 4 4 1.18 2.242e-005 R
1 37 2 0 4 4 1.18 8.877e-006 R
1 38 1U 0 6 6 1.18 1.238e-005 R
1 38 2 0 3 3 1.18 4.879e-006 R
1 39 1 0 6 6 1.18 6.871e-006 R
1 40 1U 0 6 6 1.18 5.398e-006 R
1 40 2 0 7 7 1.18 1.474e-006 R
1 41 1U 0 6 6 1.18 1.569e-006 R
1 41 2 0 4 4 1.18 4.307e-007 R
1 42 1U 0 6 6 1.18 3.519e-007 R
1 42 2 0 2 2 1.18 1.181e-007 R
1 43 1 0 3 3 1.18 1.516e-007 R
1 44 1U 0 5 5 1.18 1.817e-007 R
1 44 2 0 3 3 1.18 8.040e-008 R
1 45 1U 0 4 4 1.18 1.084e-007 R
1 45 2 0 3 3 1.18 4.501e-008 R
1 46 1 0 4 4 1.18 7.325e-008 R
1 47 1 0 2 2 1.18 1.262e-007 R
1 48 1U 0 8 8 1.18 -1.000e-008 R
1 48 2U 0 6 6 1.18 -6.239e-008 R
1 48 3 0 1 1 1.18 6.670e-009 R
1 49 1 0 1 1 1.18 1.001e-008 R
1 50 1 0 1 1 1.18 1.501e-008 R
1 51 1 0 3 3 1.18 1.000e-008 R
1 52 1U 0 6 6 1.18 -7.588e-008 R
1 52 2 0 1 1 1.18 1.266e-008 R
1 53 1U 0 6 6 1.18 -9.928e-008 R
1 53 2 0 1 1 1.18 7.123e-009 R
1 54 1 0 1 1 1.18 1.068e-008 R
1 55 1 0 1 1 1.18 1.603e-008 R
1 56 1U 0 5 5 1.18 -7.496e-008 R
1 56 2 0 1 1 1.18 9.015e-009 R
1 57 1 0 1 1 1.18 1.352e-008 R
1 58 1U 0 5 5 1.18 -7.421e-008 R
1 58 2 0 1 1 1.18 7.606e-009 R
1 59 1 0 1 1 1.18 1.141e-008 R
1 60 1 0 1 1 1.18 1.711e-008 R

 

[mrgoldthorpe]

At what multiple of yield stress does your stress-strain data end? ABAQUS assumes non-hardening behaviour after that. So you may simply be seeing the achievement of the plastic limit load of your beam - the load factor can be increased no further.