Stress-analysis Stress-analysis | HBM

# Stress analysis with PMX

PMX offers a suitable glyph for stress analysis using rosette SGs. The block returns the two principal stresses, the angle of the first principal stress, the maximum shear stress and the von Mises reference stress. In this example three fixed resistors are added to a rosette to form half bridges. The temperature compensation of the rosette SG can be based on either the rosette temperature or a reference SG that is mounted stress-free.

## Introduction

The rosette temperature is provided by an external Pt100 module in this case (for example from LKM electronic) for a 10-V input of the PX401 card. The polynomial for the rosette data sheet returns the temperature-dependent apparent strain with which compensation is performed.

The actual stress calculation takes place in the "SG stress analysis" glyph. It requires the following inputs:

epsA_raw... epsB_raw are the raw values of the rosette strains. The unit must be μm/m. (Note: 0°/60°/120° rosettes are currently not supported.)

The temperature compensation SG returns the reference stress. If this option is not used, as in this case, the input should be continuously connected with 0.

The apparent strain eps_p is returned by the temperature polynomial in μm/m. (If it is not used, this input should also be continuously connected with 0.)

The modulus of elasticity and transverse contraction index should be set as parameters (in this case for aluminum). The unit of measure for the modulus of elasticity is user-defined. The calculated outputs have the unit of the modulus of elasticity.

The block returns the following quantities:

• sigma 1 (principal normal stress 1)
• sigma 2 (principal normal stress 2)
• tau_max (maximum shear stress)-sigma_v (von Mises reference stress)
• phi in angular degrees (angle between measuring grid A (0°) and the principal normal stress 1)

Scaling of the rosette inputs is some what more complicated. The gauge factor is given and strain in μm/m is required.

The first scaling point is simple: 0 mV/V corresponds to 0 μm/m.

The second point is specified at a misalignment of 1 mV/V.

The output values can be reset with the "Reset" button or by a digital signal that is defined under "Reset by".

## Tips

• Although three fixed resistors are added to a rosette to form half bridges at the installation location, the measurement signal is that of a quarter bridge. The relationship between the bridge unbalance, gauge factor and strain is expressed by
• Raw values (epsA_raw...) can be set to zero, but they do not have to be. Resetting the "SG stress analysis" glyph is sufficient.
• The zero point of the "SG stress analysis" glyph is saved with the device parametrization. To do this click the Save icon or save the parameterization on the PC: Menu–Setting–System Device –Device Management –Backup to PC

This makes it possible to restore the zero point.

## Disclaimer

These examples are simply for the purpose of illustration. They cannot be used as the basis for any warranty or liability claims.