DIC Experiment - Application of Digital Image Correlation Technology in Automotive Structural Testing

Background:

Before large-scale production of automotive parts, we need to conduct monitoring, and many components require repeated testing to ensure their integrity after being put into use. Therefore, the application of image correlation technology in the automotive industry has become a trend. Here, we mainly introduce the testing specifically for the open structure's "gust" test (Fig. 1).

Fig. 1

In the Renault Technology Center's workshop, throughDigital Image Correlation (DIC)load tests were conducted on the car door. The purpose of these tests is to perform real-time tracking to quantify the displacement applied to the car door and explore the application of image correlation technology in the automotive industry. Therefore, we seek to quantify the displacement field across the entire structure in a holistic manner to ensure the validity of the corresponding numerical model. To achieve this, the door is stopped, and a progressive force is gradually applied at its end, simulating the user's opening. This rigid body motion extends beyond the door's operational stop block. Additionally, between each loading step, the door returns to zero and back to its initial position.

Displacement sensors (LVDT type) are installed at multiple locations on the car door. The first wire sensor is located at the end, on the horizontal plane of the cylinder that allows displacement. The second wire sensor is located at the bottom of the door, near the hinge axis. The second sensor will be compared with the image correlation results.

Displacement tracking based on image correlation field measurements

As indicated in Fig. 1, a pair of cameras is placed opposite the test door (see Fig. 2). The final dimensions of the structure do not allow for visualization of the entire door, nor do they allow for analysis of all surfaces with sufficient accuracy. Therefore, a reduced study area is determined (as shown in Fig. 3), and this area is analyzed and compared during the study.

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Fig. 3

Despite the significant rigid body motion of the parts, the cameras are able to capture the entire opening motion within the study area, including the position of the wire sensors (green dots in Fig. 3). The results provided in the next section will be displayed proportionally. However, we note that the measurements are directly represented on the finite element mesh of the part, allowing us to immediately compare with the predicted displacement field from the design calculations considering the stresses.

Results and Outlook

The measured displacements are directly projected onto the finite element model provided by Renault, as shown in Fig. 4. As expected, the measured opening motion of the door outward and at the ends is more significant than on the inside. These results can also be discovered by specifically looking at the comparison with the wire sensors. For this purpose, the displacement sensor ofEikoTwincreates a function that places virtual displacement sensors (green in Fig. 4) on the mesh, positioned at the actual locations of the physical sensors. Fig. 5 shows the comparison between the two sensors during the first few loading steps.

Fig. 4

Fig. 5

Here, we see that the displacement values between the actual wire sensors and the DIC measurements are consistent. However, there are differences that become larger as the door opens. This difference is caused by the normals of the wire sensors (sliding normal, varying with the door's opening angle) and the camera normals (fixed). Since there is no need to recalibrate within the model framework, optical measurements on the mesh have an additional advantage for large displacements.

This test is also applicable to focus on the behavior of the door during the relaxation phase. In fact, between each loading phase, operations are performed to return to the initial state. As predicted by the numerical model, the door is expected to return to its initial position between each step, which corresponds to returning to zero displacement in the measurements.

As shown in Fig. 6, in the case of the aforementioned deviations between the actual sensors and the virtual sensors, the door does not return to zero between the two loading phases, which is contrary to the predictions of the numerical model. Additionally, the deviation from the initial position increases with the applied displacement, which is fundamental information for future recalibration of the numerical model.

Fig. 6

In summary, the measurements conducted show the displacement field obtained during the load increase phase, which is consistent with the applied tensile force and the wire sensor instrumentation used for this test. However, deviations occur due to the variable angle between the normal of the wire sensor and the movement of the door (depending on the angle changing over time).

However, the tests also revealed unexpected behavior during the relaxation steps and the presence of residual displacements. Contrary to the predictions of the numerical model, the door does not return to its original position between each step, as shown inimage correlation measurements, but also as indicated by the wire sensors.This field has collected test data for the first time, providing clues for using image correlation technology to address interesting issues in the automotive sector. These tests are encouraging and provide qualitative and quantitative results in previously unmeasured research areas, allowing for updates to the relevant numerical models.

This study is the second collaboration between EikoSim and Renault.