Carbon fiber reinforced epoxy composite (CFRP) has many advantages such as low density, high specific strength, high specific stiffness, fatigue resistance, corrosion resistance and good mechanical properties. It is widely used in aerospace and other environmentally harsh structures, damp heat and impact. The influence of environmental factors on materials is increasingly apparent. In recent years, domestic and foreign scholars have carried out a large number of studies on the effects of hot and humid environment on CFRP composites  and the impact of impact on CFRP composites. The study found that the influence of hot and humid environment on CFRP composites includes plasticization of matrix [2, cracking [31 and weakened fiber-matrix interface properties [2'3'5], CFRP composite bending with increasing wet heat treatment time) The mechanical properties of the performance [2, lead and interlaminar shear properties [2, 1 and static tensile properties [3'6'7] showed a downward trend. Woldesenbet et al. [8,9] studied the impact mechanical properties of composites at high strain rates after wet heat treatment, and obtained that the hot and humid environment improved the impact strength of the composites. It is revealed that the moisture absorption of composite materials can improve the impact mechanical properties of materials under certain conditions, which is quite different from the experimental results under quasi-static conditions. The current major research work is the effect of moist heat (including water immersion) on the low velocity impact properties of fiber reinforced resin matrix composites. Pan Wenge et al  studied the compression properties of two-dimensional woven fiberglass/epoxy composite laminates after low-speed impact at room temperature and under hot and humid conditions (65 °C water immersion). 4. The laminate under the hot and humid environment is obtained after low speed shock. The compression performance is significantly reduced. Karasek et al.  studied the effects of humidity and temperature on the impact of graphite/epoxy composites, and obtained them in low temperature and room temperature environments. Humidity has little effect on the initial energy and energy absorption of the damage. Yucheng zhong et al [12,13] performed a low-speed impact test on composite laminates after wet heat treatment. It is concluded that the hot and humid environment significantly reduces the impact damage of the laminate. Improve the impact resistance of laminates. Krystyna et al.  studied the low-speed impact of aramid-glass fiber/epoxy composite after wet heat treatment (70 °C water immersion), and obtained a smaller impact damage area after wet heat treatment. This causes delamination damage inside the sample, which absorbs more energy during impact and inhibits the formation of delamination. It can be seen from the above that the influence of moist heat environment on the impact damage of composite materials has a promoting effect and a weakening effect. Therefore, further research and verification are needed. In terms of impact, Mei Zhiyuan et al  proposed and established a two-stage (shear penetration and continuous penetration) penetration dynamics analysis model of fiber-reinforced composite laminates under high-speed impact. Guiping Zhao et al.  conducted three kinds of different speeds (less than, equal to and greater than the ballistic limit speed) on the impact performance and damage of the specimen after three kinds of laminates, but did not involve the impact of the moist heat environment on the impact damage. . Based on the above literature, the related research on the impact of the wet and hot environment on the fiber reinforced composite laminates has yet to be further explored. In this paper, the impact damage characteristics of wet heat saturated carbon fiber/epoxy composite laminates under 70 °C water bath conditions were studied. The effects of hot and humid environment on the impact failure characteristics of composites were analyzed by comparison with dry room temperature samples. In the experiment, the CFRP laminates were impacted on the CFRP laminates at 45 m/s, 68 m/s and 86 m/s. The velocity before and after the impact was measured. The influence of the hot and humid environment on the energy absorption performance of the laminates was analyzed. Ultrasonic c-scan was used to detect the internal damage of the laminate, and the influence of the impact velocity on the fractured area was analyzed. The scanning electron microscope and the ultra-depth-depth three-dimensional microscopic system were used to observe the mesoscopic characteristics of the sample damage, and the damage of the sample was analyzed by the humid heat environment. The impact of features.
2 Experimental materials and methods
2. 1 Material and preparation
Carbon fiber epoxy resin (T300/EMl 12) composite material, pre-immersion provided by Jiangsu Hengshen Co., Ltd., single-layer pre-immersion thickness of 0. 137 mm with a fiber volume fraction of 66%. The laminate panel is laid on the floor of the layer. , size 115mm x 115mln. The forming process of hot-press tank is used. The curing process diagram prepared by the process is shown in Figure 1. First raise dwelling from room temperature to 80 oC at a heating rate of 1 to 3 oC/min, then keep warm for 30 min, heat to 130 oC at a heating rate of l13 oC/min, keep warm at 120 min, reduce to 60 0C at a constant cooling rate, and then remove the pressure and release, and release.
2. 2 Wet heat treatment
After the preparation of the specimen, the sample was wet-heat-treated in accordance with the specification HB 7401-96.171 “Resin-based composite composite layer wet hot environment moisture absorption experimental method”. First, the specimen is placed in a thermostatic drying chamber at 70 degrees C to dry. Weighing regularly using balances until the quality loss of the specimen is stable at no more than 0. 02%, the recorded value at this time is Engineering Dry Mass G. After drying, the specimen is placed in 70 degrees C of water for wet heat treatment. According to the specification HB 7401. The method specified in 96 “measures the quality of the specimen every day, recorded as Gi, and records the change of moisture absorption Mi. The moisture absorption expression of the CFRP laminate specimen is:
The formula is detailed: Mi is the moisture absorption of the specimen, Gi is the quality after the sample absorbs moisture, g, go is the dry state quality of the specimen engineering.
2. 3 Impact Experiments
The high-speed impact experiment on the CFRP laminate was carried out on a high-speed air cannon with a diameter of 15 mm. High-speed impact test device (see Figure 2) includes high-speed air gun, laser speed measurement device before and after impact, bullet body, specimen installation fixture (top right corner of Figure 2), and bullet body safety recovery device. The bullet body is a cone-headed cylindrical bullet (Figure 2), and the volume of the bullet is 24. 32 g with a diameter of 14. 32 mm; impact speed is 45 m/s (impact energy 46 J), 68 m/s (impact energy 70 J), 86 m/s (impact energy 90 J) impact.
2. 4 Damage detection of specimens
After being affected by the impact, the carbon fiber color epoxy composite composite laminate layer edgout plate is used to detect the internal impact damage of the CFRP laminate plate, and the projection area of the impact damage area is measured by the image analysis software UTwim, and the detailed features of the cross-sectional destruction are observed by scanning electron microscope and ultra-depth of field 3D microscopic system.
3 Results and discussions
3. 1 Specimens moisture absorption characteristics
A total of 37. 7 d, the average of saturated moisture absorption is 1. 780%, with a diffusion rate of 6. 183×10. 7lllnl2/s. The moisture absorption curve of the CFRP laminate specimen is shown in Figure 3. As can be seen from Figure 3, the initial growth rate of the moisture absorption of the specimen is linear, after the linear stage, the growth rate of the moisture absorption begins to decline, reaching a steady state level after about 23 d, and reaching moisture absorption saturation after a period of time. Therefore, the moisture absorption of the specimen conforms to the two-stage moisture absorption mode: the first stage of moisture absorption is due to the joint action of temperature and humidity, moisture through the material itself contains pores, holes, cracks and other defects spread to the inside of the material; The water diffusion is slow and gradually reaches saturation at this stage.
3. The apparent destruction characteristics of the 2-layer laminate board
Impact speed of 86 m/s when the specimen front, the back of the apparent destruction profile map, by the dry room temperature sample, wet hot saturation specimen front destruction shape is more like, the two specimens in the impact, due to the foundation cracks, its destruction along the first layer of fiber have a certain slip. This causes the front to render an elliptical or rectangular shape, and in addition to being able to see the crack in the substrate, the fibers can be seen to break. By the dry room temperature sample, wet hot saturation sample on the back of the destruction of the shape can be seen that the back along the impact direction has a certain bulge, and presents a cross-shaped crack. It is obvious that the fiber fracture, the base cracking and the interlayer fracture (layering) three forms of destruction, the last part of the fiber is lifted but not broken, only layering and fiber/base cracking. The fiber fracture is also different, as can be seen from the comparison of frontal and back damage. The front causes the fracture of fiber and the substrate due to compression and shear. The back is due to stretching caused the fiber to break and layer the substrate. Figure 4 is a shock speed of 45 m/s, 68 m/s, 86 m/s when the specimen internal damage C scan. The area indicated by the approximate round l gray line at the center of the figure is the projected area of the damagehole. The black line above and below each small chart indicates the area for the back peeling area of the specimen. The area marked in the white line in figure (b) (d) (f) is the internal damage to the specimen along the boundary. The graph shows that the impact energy increases as the speed of impact increases. The laminated plate is able to absorb more energy during impact (see Figure 6 for specific values), resulting in an increasing area of laminate damage projection: by comparing the dry room temperature sample with the picture of the wet hot saturation specimen, it can be seen that there is internal damage (white line) of the specimen produced along the boundary in the wet-hot saturation state of the specimen, mainly due to the absorption process. The plasticization of the substrate in the laminate plate and the weakening of the fiber-base interface cause the boundary to have a certain effect on the laminate plate during the impact process. According to the figure, the back peeling area (black line) of the specimen in a dry state is not much different from the wet hot saturation state.
3. The detailed destructive features of the 3-layer panel
The cross-sectional damage feature map of the CFRP layer joint plate, taken by the ultra-depth 3D microsystem and the scanning electron mirror, with the impact speed of 45 m/s, dry and wet and hot, shows that the damage of the specimen in both states includes three forms of destruction: fiber fracture, base cracking and interlayer fracture. But the base of the two specimens is cracked differently. The cracking of the substrate in a dry state is cracked at the connection between the fiber and the substrate. However, the cracking of the substrate after wet heat treatment is accompanied by the fragments of the substrate falling out. Wold-esenbet and other materials in the wet and hot environment of the impact performance of the structure of the structure and fiber substrate interface degradation jointly determined, in the wet hot environment, CFRP layer plate in the resin base experience absorption of a certain amount of water, seeping water will cause the resin substrate to dissolve. Carbon fiber is not absorbent, then there must be wet expansion between the two, this difference weakens the interface between the substrate and the fiber, reduces the strength of the substrate. When subjected to the impact load, the substrate fragments are easily dropped out, resulting in a difference from the dry room temperature sample damage interface. From the detailed structure of the scanned electric mirror, it can be seen that the cracking of the wet and hot post-base body is mainly the loose cracking of the press break, while the cracking before the wet heat is mainly brittle, and the horizontal shear crack between the layers is more obvious. From the optical microscope in the figure, it can be seen that the destruction forms are different in the two cases, and the dry state is per-inter-cutting destruction. To cut the destruction mainly, after wet heat for the form of destruction accompanied by significant layered destruction, the proportion of layered destruction expanded. It can be seen from the angle of destruction mechanism and energy absorption characteristics. Mei Zhiyuan put forward two stages of the projectile invasion: the cutting stage and the continuous invasion stage. The A area in the wet hot sample is the shear intrusion stage destruction, mainly because in the impact process, the layering plate is compressed and sheared the formation of the destruction deformation, b area is the continuous invasion stage destruction. This stage is mainly due to the reduction of the bullet body intrusion speed under the action of the stretch stress component of the fibrous layer, and the energy is mainly converted into the fiber stretch strain energy and the interlayer fracture energy (l 51), so that the fiber break el and the previous fiber break are not in a straight line. In the dry sample, this phenomenon is not obvious, and the damage of the plate is more serious, the layer plate has a cracking state. 3. 4 Absorption energy and damagehole projection area analysis Figure 5 shows the relationship between dry room temperature and wet hot saturation of the launch speed and energy loss of the body, at the incident speed of about 45 m/s, the dry room temperature of the bullet all rebound, so not shown in the figure. As can be seen from Figure 7, when the test is tested under wet thermal saturation, the bullet energy loss is serious, and the suction capacity of the sample after the wet heat treatment increases.
Figure 6 is a graph diagram of the projection area of the bullet body incident speed and the CFRP layer damage hole (the gray line marks part of Figure 4), comprehensive figure (4), (5), (6) can be seen: (1) with the increase of impact speed, the CFRP layer layer damage hole projection area increases ;(2) The projection area of the damagehole in the sample in dry room temperature is larger than that of wet hot saturation; (3) when the impact speed is about 45 m/s, the projection area of the damagehole of the laminated plate after wet heat treatment is much larger than the projection area of the laminated plate damage hole in the dry room temperature state. Wet thermal saturation sample damage l-hole projection area increased by 85. 1% and at a shock speed of about 68 m/s, the laminated plate in a wet and thermal saturation state increased by 18. 10%, the absorption value (Figure 5) increased by 15. 65%; At an impact speed of about 88 m/s, the laminated plate in the wet and thermal saturation state was reduced by 9. 25%, the absorption value still increased by 12. 45%.
Based on the research results of Yucheng Zhong and other products, the moisture absorption of carbon fiber-reinforced composite materials improves the elastic limit and impact resistance of the laminate plate, and combines the projected area of the damagehole hole of the dry room temperature specimen and the wet hot saturation specimen in this paper (Figure 4 in the gray line) The relationship diagram with the bullet-body incident speed and the projection area of the CFRP layer damage hole, and the layered damage of the CFRP layer joining board can be compared when the impact speed is the same and low. The damagehole area of the wet hot saturation specimen is relatively large. This is due to the wet heat treatment makes the CFRP layer substrate plasticization, weakening fiber and substrate interface and interlayer performance, in the impact, the wet heat saturation state of the specimen layered damage expansion, the proportion of damage increased. Based on Wu Yixuan and other experiments know that the impact energy in the vertical paving direction is mainly absorbed by the resin substrate, then the plasticization of the substrate makes the wet and hot saturation specimen absorb more energy during the impact process, improves the impact resistance, and increases the projection area of the damage hole; CFRP laminate damage has not been fully extended, the impact has ended, so when the impact speed is higher, the wet heat treatment on the CFRP laminate damage projection area is no longer serious, but due to the plasticization of the substrate resin, the absorption capacity is still increased.
(1) With the increase of impact velocity, the projected area of the damage hole of carbon fiber reinforced epoxy resin composite (CFRP) laminate increases, and the growth rate of the damage 孑L hole in the sample at dry room temperature is higher than that under wet heat saturation. Large: (2) When the impact velocity is 45 m/s, the damage projection area of the CFRP laminate in the wet heat saturation state is increased by 85. 11%, when the impact velocity is 68 m/s, the damage projection area of the CFRP laminate in the wet heat saturation state is increased by 18% compared with the CFRP laminate in the dry room temperature state. 10%, the impact speed is 86m / s. The wetted-saturated cFRP laminate damage projection area is reduced by 9.9% compared with the dry room temperature cFRP laminate. 25%; (3) After the cFRP laminate is affected by the hot and humid environment, the interlayer performance of the laminate is reduced, resulting in the expansion of the delamination area.
Post time: Jun-24-2019