Optical residual stress analysis

ReSA system based on 3D-Micro-DIC stereo sensor for residual stress analysis

 

The hole drilling method in com­bi­na­tion with strain gauges is an estab­lished method for resid­ual stress analy­sis in met­al mate­ri­als. In con­trast a full-field and non-con­tact mea­sure­ment using VIC-3D is espe­cial­ly advan­ta­geous in order to quan­ti­fy the non-homo­ge­neous resid­ual strains and stress­es with­in com­pos­ite struc­tures. The exam­ple above shows the strain mea­sure­ment ε_xx of a car­bon com­pos­ite at dif­fer­ent drilling depths. The right image is super­posed with an FEM mod­el of a quar­ter sec­tion. isi-sys offers a turn-key solu­tion for resid­ual stress analy­sis accord­ing to the hole drilling method in com­bi­na­tion with a spe­cial ver­sion of the 3D Micro DIC stereo sen­sor.

 

 

 

The images below show the defor­ma­tions (U, V, W) of the car­bon fibre com­pos­ite at 1mm drilling depth cor­re­spond­ing to the strain mea­sure­ment above. The exam­ples show the mea­sured defor­ma­tion and strain close to the drilled hole with high spa­tial res­o­lu­tion, even at very small diam­e­ters (2.3mm down to 0.8mm). This is pos­si­ble as the stereo sen­sor set-up is designed for a FOV of about 8.4mm x 7mm @ 5Mpx. This is hard­ly resolv­able using the strain gauge prin­ci­ple. The resid­ual strain analy­sis sys­tem is equipped with a spe­cial drill, oper­at­ing with around 100.000 to 300.000 RPM. The spe­cial ver­sion of the 3D Micro DIC sen­sor allows to drill and mea­sure with­out repo­si­tion­ing of the drill or the sensor.

 

 

ReSA — the new Residual Strain/Stress Analysis solution

 

ReSA system advantages against strain gauge based methods

 

Strain gauge and hole drilling method	ReSA sys­tem based on 3D-Micro-DIC
Mea­sure­ment errors:
- Errors due to mis­align­ment of the drilled hole against the cen­tre of the strain gauge rosette	+ No require­ment on the exact drilling hole posi­tion rel­a­tive to the sen­sor, no cor­re­spond­ing errors
- Fibre matrix direc­tion and strain gauge align­ment required or rel­a­tive direc­tion need be known	+ No adjust­ment between sen­sor and the mate­r­i­al matrix direc­tions is required or needs to be know
- Errors due to indi­vid­ual strain gauge application	+ Repeat­able prepa­ra­tion of the area of interest
Tech­ni­cal consideration:
- Sin­gle point mea­sure­ment of strain, aver­ag­ing across the effec­tive strain gauge area	+ Full field strain and defor­ma­tion mea­sure­ment, high spa­tial res­o­lu­tion close to the hole
- Results depend­ing on the applied eval­u­a­tion model	+ Reli­able post-pro­cess­ing with Vic-3DTM
- Sin­gle point val­ues of resid­ual stress­es only – so not rep­re­sen­ta­tive for inho­mo­ge­neous material	+ Full field resid­ual stress data suit­able for inho­mo­ge­neous materials
Eco­nom­i­cal aspects:
- High cost per mea­sure­ment due to strain gauge	+ Low oper­a­tion cost per measurement
- Prepa­ra­tion time for apply­ing the strain gauge	+ Fast and sim­ple prepa­ra­tion of object surface