PSI - Issue 68

Andreas J. Brunner et al. / Procedia Structural Integrity 68 (2025) 1266–1272 Brunner et al. / Structural Integrity Procedia 00 (2025) 000–000

1269

4

quasi-static loads, Mode II delamination resistance is usually higher than for Mode I. The data in Table 3 seem to suggest that delamination resistance under different modes is not strongly dependent on micro- or mesoscopic defect size. However, a firm conclusion cannot be drawn, pending consideration of, e.g., effective fracture surface area and AE noise signals from sources not related to delamination propagation, discussed in sections 3.2 and 3.3.

Table 2. Simple estimate of average delamination area and diameter per AE signal for GF-EP1 under Mode I tensile opening load Fracture Area ( mm 2 ) Remarks

Total number of AE signals ( - )

Average defect area ( µ m 2 ) per AE signal

Average defect diameter ( µ m ) square-root of area

Specimen 1 Specimen 2 Specimen 3 Specimen 4

1‘103 1'092 1'164 1'103 1'116

64'687 68'711 60'670 78'691 68'190

17'064 15'889 19'187 14'014 16'538

131 126 139 118 128

AE sensor near specimen end AE sensor near specimen end AE sensor near specimen end AE sensor near specimen end

Average

- -

Standard deviation

33

7'732

2'167

8

Table 3. Simple estimate of average delamination area and diameter per AE signal for the GF-EP2 laminate tested under different loading modes according to test standards and test protocols used in ESIS TC4 round robin tests.

Loading mode and test standard

Remarks

Average total number of AE signals ( - ) 51’989±3’978

Average defect area ( µ m 2 ) per AE signal 22’760±1’283

Average Fracture Area ( mm 2 ) 1’180±71

Average defect diameter ( µ m ) square-root of area

Mode I tensile opening, ISO 15024 (2023) Fixed Ratio Mixed Mode I/II ESIS TC4 test protocol (2015) Mixed Mode I/II MMB at 4:3 ratio ASTM D6671 (2022) Mode II in-plane shear ISO 15114 (2014) Average ± Standard deviation

151±4

Sensor near specimen end Sensor near specimen end

721±59

41’855±18’431 22’881±13’909

145±42

425±12

46’505±14’351

9’654±2’500

97±13

Single sensor near specimen end

622±41

45’142±18’613 18’148±10’782

129±38

Sensor near specimen end

-

-

18’361±10'407

131

-

3.2. Possible corrections of simple estimates Several factors may have an effect on the number of recorded AE signals. A first is AE signal amplitude attenuation during propagation (Figure 1a). Signals with low amplitudes at the source location may not exceed the threshold at the sensor, i.e., the effective AE signal number may be higher. Evidence is the higher number of signals from the sensor on the load block, nearer to the delamination, compared to that of the sensor near the specimen end. For GF EP1, this difference amounts to roughly 33%, comparable to the difference for GF-EP2 under Mode I load (29%). Beside larger AE signal attenuation between delamination tip and the sensor near the specimen end due to larger signal propagation distance, noise signals from test machine and load introduction recorded by the sensor on the load block likely contribute to the difference. If a lower limit value (fracture surface area) is divided by an upper limit value (number of all recorded AE signals, independent of source location and mechanism), the "simple" estimate yields a lower bound for average defect size. Limiting AE signal source locations for GF-EP1 specimen 1 in Table 2 to the process zone yielded a significantly lower number of AE signals (25'969 versus 35'094 total located and 64'687 total recorded AE signals) and a higher estimate for the average defect diameter (206 micrometer versus 177 micrometer for all located signals and 131 micrometer for all recorded signals). Of course, some AE signals with source location in the process zone still may come from mechanisms that do not contribute to delamination propagation. Hence, the "real" number of AE signals from delamination propagation may be lower than 25'969. If full AE waveforms are recorded, pattern recognition may provide quantitative numbers for each mechanism (matrix cracking, fiber-matrix