PSI - Issue 42
Dennis Domladovac et al. / Procedia Structural Integrity 42 (2022) 382–389 Domladovac et al. / Structural Integrity Procedia 00 (2019) 000–000
386
5
10 -4
10 -4
-1 0 1 2 3 4 5 6 7 8 w ,, / 1/mm clamping
-1 0 1 2 3 4 5 6 7 8 w ,, / 1/mm clamping
cracktip at 0 mm no gap.
cracktip at 0 mm
width of gap: 40 mm
400
400
200
200
0
0
-200
-200
Q / N
Q / N
-400
-400
-600
-600
-150 -100
-50
0
50
100
150
200
250
300
-150 -100
-50
0
50
100
150
200
250
300
fibreposition x / mm
fibreposition x / mm
0
20
40
60
80
100 120 140 160 180 200 time t / s
0
20
40
60
80
100
120
140
160
180
time t / s
Fig. 4. BFSM results of the specimen without void (left) and specimen with void (right).
3. Results and Discussion
3.1. Gaps localisation
The BFSM delivers the curves shown in Fig. 4: In the top diagrams the second derivative of the deflection curve is plotted, which is proportional to the bending moment and the fibre strain. The values are plotted over the fibre position with the initial crack tip position being set to zero. The plots on the bottom show the transverse force which is proportional to the third derivative of the deflection curve. In the front of the crack tip, i.e., the region without adhesive, the bending moment increases linearly with the distance from load introduction, which is caused by the load introduction, matching with beam theory and, hence, validating the BFSM. The transverse force of the specimen without gap shows the expected result and the region in front of the crack tip exhibits a constant value of the transverse force. The transverse force curves of the specimen with gap has also a constant region in front of the crack, but additionally plateau fitting well with gap position. The measured deflection curve obtained by DIC is shown in Fig. 5. In agreement with the results from BFSM, the deflection curve of the specimen with gap is much higher than for the specimen without gap, caused by the lower joint sti ff ness. The third derivative of these curves is proportional to the transverse force and shown in the diagrams at the bottom of Fig. 5. As well as the BFSM results, only a constant transverse force exists in the region of the gap. Evaluating the measurement data, it became apparent, as also reported by Schrader et al. (2022), the noise increases with the numerical di ff erentiation and filters are needed to compensate these numerical artefacts. Filtering the data is laborious and time consuming, making BFSM more advantageous than DIC. For the BFSM only two derivates needed and, thus, a lower e ff ort of data filtering is required. Fig. 6 shows a comparison of the shearography measurement of a reference sample (top) and a sample with a 40 mm gap (bottom). In the interferogram of the reference sample as well as in the demodulated image, it can be seen that a uniform deformation occurs in the front region. In the interferogram as well as in the demodulated image of the specimen with the 40 mm gap, it can be seen that a larger deformation occurs in the area in front of and behind the gap. Due to the increased deformation in the area of the gap, it can be concluded that the irregularity introduced in the adhesive causes a su ffi ciently high deformation due to the change in sti ff ness to be able to detect it on the substrate surface.
Made with FlippingBook - Online catalogs