## 3 Results and Discussion  
### Moisture Absorption Behavior  
The water content M defined as:  
\[\rm M=\frac{\rm W-W_{d}}{W_{d}}\times 100\%, \tag{4}\]  
Figure 8: The strain rate history curves under dynamic loading for specimens after 14, 70, 140, 210 hygothermal aging days under typical strain rate 1700–1800 s\({}^{-1}\). Figure 7: Contrast of the stress-strain curves for the specimens of 14 cyclic hygothermal aging days under the same pressure 0.6 MPa. where Wd is the weight of the dry specimen and W is the weight of the specimen after hygothermal aging. The weights and moisture absorption rates of the specimens can be found in Table 1.  
The 210 days of moisture absorption of 3D four-step braided composite, as a function of square root of time (hour 0.5), is shown in Figure 9, and the moisture absorption of the T700/TDE85 laminated composite after cyclic hygothermal aging by our previous work [35] was also added into Figure 9, in order to analyze contrastively moisture absorption behavior of 3D four-step braided composites more comprehensively. It can be found that the 3D four-step braided composite did not achieve the saturated moisture absorption, and the relationship between moisture absorption and aging time was almost linear; the maximum moisture absorption was 0.6944%. However, the T700/TDE85 laminated composite not only reached the saturated moisture absorption (1.261%, at 180 cyclic hygothermal aging days), but also the maximum moisture absorption (1.261%) [35], which was almost twice that of the 3D four-step braided composite at the same cyclic hygothermal aging days. In this study, the magnitude of the diffusion coefficient of the 3D four-step braided composite could not be computed, for the reason that the saturated moisture absorption was not reached under 210 cyclic hygothermal aging days, according to the Fick law. The reason that the saturation moisture absorption of 3D four-step braided composites was not reached during this study, is that its spatial structure is more sophisticated, which may cause more porosity of the matrices that may act as secondary absorption locations and lead to deviations from the Fick diffusion behavior [38; 39] compared with the laminated composites. It is well recognized that the diffusion mechanism of water molecules with the polymer composites is obviously different from that in a pure polymer, because the interfaces between fibers and resin play a decisive role in the whole moisture absorption. The moisture absorption process of the composites mainly contains three phases: epoxy plasticization, epoxy hydrolysis and interfaces debonding (causing cracks). At the beginning, the water entered into the specimen from the specimen surface, then the water molecules diffused into the resin through microflaws (e.g., void or microcracks). With the absorption process of water molecules continuing, the epoxy had produced plasticization, this is because the weak interactions between water molecules and polymer chains inside epoxy ranged from Van der Waals bonds into single hydrogen bonds. In addition, the water molecules which entered into epoxy acted as a corrosion agent and promoted epoxy hydrolysis. The volume of epoxy increased with the increased polymer chain mobility and disruption of interchain Van der Waals, which produced differential swelling stresses (since the carbon fibers do not absorb water), that might drive the interfaces debonding between fiber and epoxy [18; 40; 41]. Figure 10 presents the damage evolution on the cross section of the 3D four-step composites after different hygothermal aging days. Some broken fibers were observed on the surface of the composite, due to defects caused during the manufacturing process, as shown (Figure 10a). Before hygothermal aging, the surface of epoxy was relatively smooth, except that there were some impurities. After 14 cyclic hygothermal aging days, some potholes appeared on the surface of epoxy, with a length below 100 um, which was the phenomenon after epoxy hydrolysis.