Combined Stress test
The aim is to better simulate loadings that can occur in real-life, three dimensional loading. The loadings can be applied as a combination of tensile and compressive forces, also bending moments and torsional loads. By use of special strain gage, rosette, values of three strains will be taken. Rosettes have three strain gages. Main aim is to get information concerning plane stresses using a rosette.
The use of rosette strain gage will be illustrated plus computer aided data acquisition systems so as to experimentally determine plane stresses. Application of superposition in finding the stress will also be illustrated.
Combined Stress test-Introduction/Experimental setup
A closed end steel tube, with fittings was provided for internal pressurization with hydraulic oil. One end of tube is fixed while the other is attached to a lever arm so an axial torque can be applied to the tube. On surface of tube there are rectangular and delta strain gage rosette. First pressure is applied with hydraulic oil pump so as to have 2000psi later the weight and pressure were applied simultaneously. Strains were measured and data recorded on computer.
- Superposition principle applied in this experiment can be used on a member going through different loading at same time. While experiment was ongoing, the tube was undergoing torque and pressure forces simultaneously.
Analytical results were close to the ones calculated with the thick wall equations. σ2 gotten by rectangular rosette was around 1100 psi less than the one that was calculated with the thick wall equation, the others were relatively close. When results from thin walled equations were compared with the experimental ones, the maximum in-plane shear remained close to
- experimental values, but calculated σ1 and σ2 was higher than delta and rectangular rosette.
- A reason for errors is that the formulas are derived experimentally and are an approximation of the stresses found.
Errors could also occur from the readings of the strain values. If the loading or setup of the gages the measurements