Digital photoelastic instrument is a precision optical measurement device based on the principle of polarized light interference, widely used in fields such as material mechanical property testing, stress analysis of transparent components, and industrial non-destructive testing. Its core function is to capture the birefringence phenomenon caused by stress inside the material, generate intuitive color stripe patterns (isochromatic lines), and quantitatively analyze the difference in principal stress. The following are the standardized usage details:
1、 Preparation and inspection before startup
1. Environmental requirements
Ensure that the laboratory is dark and free from strong airflow interference, and avoid temperature fluctuations caused by direct sunlight or air conditioning. The ideal working temperature is 20 ± 5 ℃ and the relative humidity is less than 60%.
Check the stability of the shock-absorbing platform and install shock-absorbing pads if necessary to prevent small vibrations from affecting the alignment of the optical path.
2. Equipment self-test
After turning on the host power, observe whether the laser source starts normally (usually with a red or green beam).
Verify the clarity of the preview image of the CCD camera. If there is noise or dark corners, the lens focal length needs to be readjusted.
Confirm that the initial positions of the polarizer, analyzer, and quarter wave plate have been reset to zero (which can be achieved through software reset function).
2、 Sample preparation and clamping
1. Sample requirements
The sample to be tested must be made of transparent or semi transparent material (such as glass, resin, crystal, etc.), and the surface should be polished to eliminate scattering noise.
Non nominal samples need to be processed into thin sheets (thickness recommended ≤ 5mm), as excessive size may cause stress concentration and distortion at the edges.
2. Precise positioning
Place the sample at the center of the rotating stage and adjust the three-dimensional moving platform to place the sample in the focal area of the optical path.
Align with the side view camera to ensure that the laser beam penetrates the geometric center of the sample vertically, with a deviation of no more than ± 0.1mm.
3、 Parameter Setting and Data Collection
1. Basic configuration
Select a matching wavelength filter (commonly used sodium lamp λ=589nm or semiconductor laser λ=635nm) and match the sensitive wavelength band of the material.
Set exposure time (typical value 50-200ms) and gain coefficient, and prioritize using automatic exposure mode to adapt to samples with different light transmittance.
2. Synchronous control
Activate the continuous rotation polarizer function and cooperate with the high-speed camera to capture real-time multi angle interferometric images.
Select the "Phase unwrapping" algorithm in the software interface to convert the circular stripes into continuous grayscale images for precise calculation.
4、 Data analysis and storage
1. Real time analysis
Using the built-in software to identify the spacing between equal color lines, calculate the principal stress difference according to the formula Δσ=N × f/t (where N is the stripe number and f/t is the material constant).
Fourier transform can be used to separate orthogonal birefringence signals in complex stress fields.
2. Report output
When exporting the original image, keep the EXIF parameters (including exposure time, aperture value, etc.) for easy traceability in the future.
Establish an experimental database, correlate load conditions with stripe evolution processes, and generate dynamic stress cloud maps.
5、 Key points of daily maintenance
Clean optical lenses daily with anti-static brushes and avoid wiping with organic solvents;
Check the power attenuation of the laser tube every month and replace the aging module in a timely manner;
When not in use for a long time, close the shutter to block the laser and cover it with a dust cover to protect the CCD photosensitive chip.
By strictly following the above operating procedures, the high sensitivity advantage of the digital photoelastic instrument can be fully utilized to achieve a stress measurement accuracy of 0.01 MPa resolution. Suggest cross validation based on finite element simulation results to enhance data reliability.