## 3 Results and Discussion
### Hygrothermal Aging Effects for Braided Composites under High Strain Rates  
In order to evaluate the hygrothermal aging effects of 3D four-step braided composites when compressed on the longitudinal direction under high strain rates, in this study, the difference of dynamic mechanical properties of the composites between hygrothermal aging and un-hygrothermal aging (abbreviation, UHA) at three high strain rates that correspond to 1100-1250 s\({}^{-1}\), 1400-1600 s\({}^{-1}\), 1700-1850 s\({}^{-1}\), respectively. Due to the limit on the number of the specimens, the dynamic experimental results of the un-hygrothermal aging specimens are from our previous work [15]. However, the previous work lacked the experimental results under the strain rate 1400-1600 s\({}^{-1}\), therefore, we did the SHPB tests of un-hygrothermal aging specimens under strain rate 1400-1600 s\({}^{-1}\). It is important to emphasize that all specimens, whether they are the specimens in our previous work [15] or the specimens in this study, came from the same batch of 3D four-step braided composites. The validity and objectivity of the experimental tests can be guaranteed this way. Figures 16a, 17a and 18a present the stress vs. strain curves of the composites between hygrothermal aging and un-hygrothermal aging under three high strain rates. The stress vs. strain curves of un-hygrothermal aging specimens also present in a linear manner up to failure and have no clear yield at three high strain rates. The peak stresses are 345.1 MPa, 392.3 MPa, and 404.4 MPa, corresponding to the strain rates 1138 s\({}^{-1}\), 1420 s\({}^{-1}\), and 1847 s\({}^{-1}\), respectively. The peak stress of 3D four-step braided composites without hygrothermal aging also behaves like the strain rate effect. It reveals that the experimental result of the un-hygrothermal aging specimen under the strain rate 1420 s\({}^{-1}\) in this study is in accordance with  
Figure 14: Dynamic compressive stress vs. strain curves for the 3D four-step braided composites after different hygrothermal aging days, (**a**) 14 days, (**b**) 70 days, (**c**) 140 days, (**d**) 210 days. Figure 15: Dynamic mechanical property vs. strain rate for the 3D four-step braided composites under different hygrothermal aging days, (**a**) peak stress vs. strain rate, (**b**) dynamic stiffness modulus vs. strain rate. the conclusion that peak stress has a strain rate effect in our previous work [15]. At the same time, it can obviously be seen that the peak stress and the dynamic stiffness modulus of un-hygrothermal aging specimens are both greater than that hygrothermal aging specimen. It can be concluded that the dynamic compressive strength in a longitudinal direction was weakened when subjected to cyclic hygrothermal aging. Figure 16b,c, Figure 17b,c and Figure 18b,c shows the effects of hygrothermal aging on the peak stress and dynamic stiffness modulus. It can be seen that no matter the peak stress or the dynamic stiffness, the moduli have both decreased significantly after cyclic hygrothermal aging. However, the peak stress and dynamic stiffness modulus have not shown any specific regularity from 14 to 210 cyclic hygrothermal aging days at three high strain rates. Although the peak stress of the specimens under strain rate 1100-1250 s\({}^{-1}\) decreases with respect to the increase of cyclic hygrothermal aging days, the same value shows no obvious up or down trend with the increase of cyclic hygrothermal aging days under strain rate 1400-1600 s\({}^{-1}\) and 1700-1850 s\({}^{-1}\). In particular, the peak stress and the dynamic stiffness modulus reached a minimum value at 210 cyclic hygrothermal aging days, under three high strain rates. The peak stress decreased by 51.06%, 52.43%, 47.90%, compared with that without hygrothermal aging under the strain rate 1100-1250 s\({}^{-1}\), 1400-1600 s\({}^{-1}\), 1700-1850 s\({}^{-1}\), respectively.