# Testing the Stress Extraction Fix

## Issue Fixed
Previously, stress extraction was returning **0.0 MPa** instead of the expected **~122.91 MPa**.

**Root Cause**: For solid elements (CHEXA, CTETRA, CPENTA), von Mises stress is at **index 9**, not the last column.

**Fix Applied**: Modified [op2_extractor_example.py](optimization_engine/result_extractors/op2_extractor_example.py#L106-L109) to check element type and use correct index.

## How to Test

### 1. Activate your test environment
```bash
conda activate test_env
```

### 2. Run the verification script
```bash
python examples/test_stress_fix.py
```

### Expected Output
```
============================================================
STRESS EXTRACTION FIX VERIFICATION
============================================================

--- Displacement (baseline test) ---
Max displacement: 0.315xxx mm
Node ID: xxx
OK Displacement extractor working

--- Stress (FIXED - should show ~122.91 MPa) ---
Max von Mises: 122.91 MPa
Element ID: 79
Element type: chexa

SUCCESS! Stress extraction fixed!
Expected: ~122.91 MPa
Got:      122.91 MPa
============================================================
```

## Alternative: Test All Extractors
```bash
python optimization_engine/result_extractors/extractors.py examples/bracket/bracket_sim1-solution_1.op2
```

## If Successful, Commit the Fix
```bash
git add optimization_engine/result_extractors/op2_extractor_example.py
git commit -m "fix: Correct von Mises stress extraction for solid elements (CHEXA)

- Use index 9 for solid elements (CHEXA, CTETRA, CPENTA)
- Keep last column for shell elements (CQUAD4, CTRIA3)
- Fixes stress extraction returning 0.0 instead of actual values (122.91 MPa)"

git push origin main
```

## Technical Details

### pyNastran OP2 Data Structure for Solid Elements
- Shape: `[itime, nnodes, 10]`
- The 10 values are:
  ```
  [oxx, oyy, ozz, txy, tyz, txz, o1, o2, o3, von_mises]
   0    1    2    3    4    5    6   7   8   9
  ```
- **Von Mises is at index 9**

### Code Change
```python
# BEFORE (WRONG):
stresses = stress_data.data[0, :, -1]  # Last column - WRONG for CHEXA!

# AFTER (CORRECT):
if table_name in ['chexa_stress', 'ctetra_stress', 'cpenta_stress']:
    # Solid elements: von Mises at index 9
    stresses = stress_data.data[0, :, 9]
else:
    # Shell elements: von Mises at last column
    stresses = stress_data.data[0, :, -1]
```

## Files Modified
- [optimization_engine/result_extractors/op2_extractor_example.py](optimization_engine/result_extractors/op2_extractor_example.py) - Lines 103-112