1.Research Background
WSRS-woven spacer structurally reinforced sandwich composites are widely used in shock absorbers in aviation, transportation, and construction due to their energy-absorbing, lightweight, and high delamination resistance characteristics.
The WSRS sandwich structure is complex, and there is a lack of effective methods to characterize its load carrying capacity and failure process in the warp direction (WSRS-WA) and weft direction (WSRS-WE) under mechanical loading.
2.Research Overview
Researchers from the Textile and Clothing Technology Innovation Center of HBKU use acoustic emission (AE) technology to capture the strain energy released by damage under mechanical loading of WSRS with different height specifications (see 3.1 for more details), and then use digital image correlation (DIC) technology to observe in real time the damage pattern of the damaged area as well as the image of synergistic effects of the surface layer, the core pile, and the foam material, and to analyze the mechanical properties, the parameters of AE signals, and the strain graphs, and study to verify the damage mechanism during bending loading.
3. Research Materials and Equipment
3.1 Prepare six WSRS of different sizes and mechanical properties to be used as experimental standard specimens.
3.2 Acoustic Emission (AE) detection equipment, 40 dB amplifier, two RS-2A transducers to capture six AE signals for damage analysis and determination of WSRS failure mechanisms.
3.3 Digital Image Correlation (DIC) strain field measurement equipment, capturing the full-field strain in the WA,WE direction of the specimen, to restore the dynamic damage process of WSRS, and to study the bending response mechanism.
3.4 Universal material strength machine, three-point bending test for WSRS, test standard refer to ISO 1209-1-2007(E).
3.5 Scanning electron microscope, to analyze the final failure of WSRS, to verify the conclusions of AE signal analysis and digital image correlation (DIC) strain analysis.
4.Digital Image Correlation (DIC) Study Validation
The middle region of the six WSRSs (see Fig. 2) was selected for DIC analysis to obtain the local strain changes between the core and the foam, and the WSRSs were monitored in real time using Lagrangian micro-strain to compare the trends of the average micro-strain of the six WSRSs over time (see Fig. 3).
The comparison reveals that WSRS-2 and WSRS-4 have the same strain trajectory, indicating that the compressive strain is greater than the tensile strain, but the micro-strain is more abrupt than that of WSRS-4, suggesting that WSRS-2 produces more shear damage.The damage mechanism of WSRS-6 is not obvious, with a stable loading state and a linear variation of the average micro-strain, and the WSRS-6-WA meridional and WSRS-6-WE latitudinal strains The trend is different, and the average micro-strain of WA in the warp direction exhibits the same compressive strain as that of WSRS-4. The latitudinal direction shows obvious tensile strain.
To further observe the strain trends during the loading of the six WSRSs, strain maps were selected to characterize the failure modes at six different time points, using 20 seconds as the node. The blue region indicates the compressive strain, and the red region indicates the tensile strain caused by the real-time change of micro-strain values. Figures 4 and 5 show that the core layer, as the main failure body, is missing during foam compression.
According to WSRS-WA (Fig. 4), the core layer is mainly affected by tensile strain when compression causes the upper layer to move. As the height increases, the strain graphs show different trends.Within 60 seconds, the core layer of WSRS-2-WA is not strong enough to withstand the stress, causing the compressive strain to shift to the lower layer, which then leads to the compressive strain penetrating through the entire core layer.After 120 seconds, the inner layer of WSRS-2-WA appears to be damaged, and interfacial delamination occurs in the core layer.The compressive strain appears in WSRS-4-WA at 80 seconds, and butterfly-like spreading.
WSRS-6-WA showed compressive strain at 100 seconds, which indicated that the lower layer was weakly affected by the tensile effect due to the height factor, which retarded the compressive strain transfer. Figure 5 shows that WSRS-2-WE and WSRS-4-WE have the same strain trend as WSRS-2-WA and WSRS-4-WA, but all of them are earlier than the latter, with compressive strains appearing in WSRS-2-WE at 40 s and WSRS-4-WE at 80 s, indicating that the overall destruction of WSRS-WE is earlier than that of WSRS-WA. After 80 s, the upper layer is centrally fragmented and the lower layer stretches and appears missing in the strain diagram. At the same time, WSRS-6-WE also showed core pile separation (Fig. 5).
Unlike WSRS-2, WSRS-4 and WSRS-6-WA, WSRS-6-WE has tensile strains on both sides, which is due to the linkage effect of the foam material and the transfer effect of the core piles to increase the flexural strength.
5. Conclusion of the Study
A combination of acoustic emission (AE) technique and digital image correlation (DIC) method revealed the dynamic damage process and bending response of woven spacer structure reinforced sandwich composites (WSRS).
The results show that the overall compressive strain of WSRS-WE is earlier than that of WSRS-WA, and the failure region is more prominent, which indicates that the synergistic effect of the core pile structure in WSRS-WE is better than that of WSRS-WA. When subjected to bending stresses, the WSRS has a higher load carrying capacity in the weft direction than that in the warp direction, and exhibits better integrity and load carrying capacity.
