Required gage length 3mm, precision o measurement 10^-5, sampling frequency 500 Hz. I tried strain gauges but they have been damaged ofter a few thousands of cycles when the amplitude of strain exceeds 3*10^-3. I also tried mechanical extensometer with base 3.6 mm, but I suppose that the blades are sliding on the steel specimen surface since the calculated Young modulus varied between 195-240 GPa. Any optical system?