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Initial Stress Field

Initial Stress Field

Initial Stress Field

(OP)
I am applying an initial stress field to a welded skin-stringer panel and I am having a problem with initial stresses which are at the yield stress. If I apply a stress such as this:

*INITIAL CONDITIONS, TYPE=STRESS
STWELDRS,    43290,
STR_WELD,    -15678,
STR_WEB,    -15678,    
PAD_WELD,    ,    43079,
PDWELDRS,    ,    43079,
SKIN_WEL,    ,    -13163,
SKIN,        ,    -13163,

the stresses that appear in the dat file are as follows:

   THE FOLLOWING TABLE IS PRINTED AT THE INTEGRATION POINTS FOR ELEMENT TYPE S8R5 AND ELEMENT SET
   STWELDRS

 ELEMENT  PT SEC FOOT-   S11         S22         S12     
              PT NOTE

     141   1  3      4.0224E+04   581.5     -3.8702E-10
     141   2  3      4.0224E+04   581.5     -1.5481E-09
     141   3  3      4.0224E+04   581.5      3.8702E-10
     141   4  3      4.0224E+04   581.5      3.8702E-10
etc

THE FOLLOWING TABLE IS PRINTED AT THE INTEGRATION POINTS FOR ELEMENT TYPE S8R5 AND ELEMENT SET
   STR_WEB

 ELEMENT  PT SEC FOOT-   S11         S22         S12     
              PT NOTE

     162   1  3     -1.5678E+04 -5.4624E-09 -4.2262E-10
     162   2  3     -1.5678E+04      0.      5.9167E-09
     162   3  3     -1.5678E+04  1.0093E-08 -3.8036E-09
     162   4  3     -1.5678E+04      0.     -2.5357E-09
etc

  THE FOLLOWING TABLE IS PRINTED AT THE INTEGRATION POINTS FOR ELEMENT TYPE S8R5 AND ELEMENT SET
   STR_WELD

 ELEMENT  PT SEC FOOT-   S11         S22         S12     
              PT NOTE

     155   1  3     -1.5678E+04 -8.3310E-10  6.7620E-09
     155   2  3     -1.5678E+04  8.3219E-10 -8.4525E-09
     155   3  3     -1.5678E+04 -8.0739E-08  3.8437E-22
     155   4  3     -1.5678E+04  1.6148E-07  3.2119E-08
etc

   THE FOLLOWING TABLE IS PRINTED AT THE INTEGRATION POINTS FOR ELEMENT TYPE S8R5 AND ELEMENT SET
   PAD_WELD

 ELEMENT  PT SEC FOOT-   S11         S22         S12     
              PT NOTE

      64   1  3       637.9      3.9688E+04 -1.2638E-08
      64   2  3       637.9      3.9688E+04 -6.3181E-09
      64   3  3       637.9      3.9688E+04 -6.3193E-09
      64   4  3       637.9      3.9688E+04 -7.8978E-09
etc

   THE FOLLOWING TABLE IS PRINTED AT THE INTEGRATION POINTS FOR ELEMENT TYPE S8R5 AND ELEMENT SET
   PDWELDRS

 ELEMENT  PT SEC FOOT-   S11         S22         S12     
              PT NOTE

      57   1  3       637.9      3.9688E+04 -1.2637E-08
      57   2  3       637.9      3.9688E+04  6.3187E-09
      57   3  3       637.9      3.9688E+04  1.8956E-08
      57   4  3       637.9      3.9688E+04 -8.6988E-23
etc

   THE FOLLOWING TABLE IS PRINTED AT THE INTEGRATION POINTS FOR ELEMENT TYPE S8R5 AND ELEMENT SET
   SKIN_WEL

 ELEMENT  PT SEC FOOT-   S11         S22         S12     
              PT NOTE

      50   1  3     -1.7162E-09 -1.3163E+04  5.6636E-09
      50   2  3      4.0328E-08 -1.3163E+04  1.7426E-09
      50   3  3      2.2525E-08 -1.3163E+04 -4.3566E-10
      50   4  3      9.0949E-13 -1.3163E+04 -6.9705E-09
etc

   THE FOLLOWING TABLE IS PRINTED AT THE INTEGRATION POINTS FOR ELEMENT TYPE S8R5 AND ELEMENT SET
   SKIN

 ELEMENT  PT SEC FOOT-   S11         S22         S12     
              PT NOTE

       1   1  3      5.2023E-09 -1.3163E+04  1.3070E-09
       1   2  3      6.0600E-09 -1.3163E+04  1.7426E-09
       1   3  3      3.4588E-09 -1.3163E+04  4.3566E-10
       1   4  3      9.0949E-13 -1.3163E+04 -2.1783E-09
etc



For some reason when the initial stress exceeds the first value of the *PLASTIC for the material properties, the initial tensile stress does not reach the value specified in the input file. The material properties are shown below.

STR_WELD, STWELDRS and STR_WEB are material EX-WELD:

*MATERIAL, NAME=EX-WELD
**
*ELASTIC, TYPE=ISO
  1.01257E+7,        0.33
*PLASTIC
34388,    0.000000
36403,    0.000037
38422,    0.000137
39029,    0.000194
39637,    0.000271
40246,    0.000374
40857,    0.000513
41470,    0.000698
42085,    0.000944
42395,    0.001096
42705,    0.001270
43016,    0.001470
43329,    0.001699
43644,    0.001962
44125,    0.002435
44612,    0.003014
45105,    0.003722
45606,    0.004584
46116,    0.005634
46638,    0.006908
47174,    0.008450
47727,    0.010315
48301,    0.012563
48899,    0.015268
49528,    0.018514
50192,    0.022402
50900,    0.027047
51659,    0.032582
52480,    0.039160
53376,    0.046956
54359,    0.056170
55448,    0.067024
56662,    0.079768
58025,    0.094678
59563,    0.112056
**

PAD_WELD, PDWELDRS, SKIN_WEL and SKIN are material SK-WELD:

*MATERIAL, NAME=SK-WELD
**
*ELASTIC, TYPE=ISO
   1.0438E+7,        0.33
*PLASTIC
33354,    0.000000
35368,    0.000036
37385,    0.000122
37992,    0.000170
38599,    0.000232
39207,    0.000315
39816,    0.000425
40426,    0.000568
41039,    0.000755
41654,    0.000998
41963,    0.001146
42273,    0.001313
42584,    0.001504
42897,    0.001720
43210,    0.001964
43698,    0.002408
44190,    0.002944
44688,    0.003591
45192,    0.004371
45705,    0.005308
46227,    0.006431
46760,    0.007776
47308,    0.009382
47872,    0.011297
48455,    0.013574
49063,    0.016276
49698,    0.019477
50366,    0.023259
51073,    0.027719
51827,    0.032965
52635,    0.039122
53508,    0.046329
54457,    0.054742
55495,    0.064538
56639,    0.075909
57905,    0.089066
59316,    0.104242


Has anyone any idea why this is happening. Cheers in advance.

RE: Initial Stress Field

The initial stresses you put into the model would not have been in equilibrium. The first step (presumably the one in which you are printing the stresses to the .dat file) is bringing everything into equilibrium, compatibility and (though I haven't checked) making sure the hardening rule is satisfied.

Regards,
Martin

http://www.analysis.demon.co.uk

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