| Carbon Fiber Reinforced Plastics(CFRP) structures often are inevitably affected by Low Velocity Impact such as falling equipment collision in the process of manufacture, use and maintenance. Be different from the penetration caused by high velocity impact, the low velocity impact is very easy to emerge matrix cracking, delamination and fiber breakage in the composite structures, these damages are often difficult to visible with the naked eye, but can lead to strength, stability and the service life of the structure fell sharply, potentially damaging. In particular, these internal damages will bring deadly hazards during the process of aircraft in service. Therefore, it is particularly important to effectively monitor composite structure damage caused by the low velocity impact.The diameter of Fiber Bragg Grating is small(80~125?m), it has very small effect on the mechanical properties of structure when embedded in composite structure. FBG sensor can accurately reflect the changes of internal strain and temperature in real time, while its high precision, strong anti-interference ability, more applicable to real-time monitoring of composites. For problems of CFRP structures low-velocity impact, in this paper using FBG sensors to monitoring the low-velocity impact damage, combined with FEM software ABAQUS the response process of laminates is simulated under low-velocity impact.In this paper combined with the basic mechanics of composite materials, the problem of low velocity impact of composites is studied, and the response process of CFRP laminated under low velocity impact is observed with ABAQUS. The analysis shows that the stress wave is propagated along fiber direction in laminated plates; the system energy is transmitted with the process of kinetic energy- internal energykinetic energy, and after impact a part of the kinetic energy transmits into internal energy of laminate, this energy may cause the damage of laminate; after impact, the stress distribution forms are completely different in 00 and 900 layer; by comparing the maximum stress of each layer, the stress of laminate surface is maximum, and the more to the middle layer the stress is smaller; the impact contact time is essentially the same under different impact velocity, and will not produce large differences according to the impact velocity; as impact speed increase, the maximum stress of each layer increases substantially linearly.The FBG sensor is essentially a single optical fiber, because of its special mechanical properties, if the FBG sensors were embedded into CFRP laminates without special derivation, protection processing, it is very easy to be broken in the process of embedding and demoulding, making sensor failure, unable to signal transmission. Therefore, firstly the compatibility of FBG sensor and CFRP laminates were studied. The timeliness and stability of FBG sensors are evaluated by the experiment of high temperature curing, tension and bending. Experimental results show that, FBG sensor can respond promptly and effectively to the internal changes of the CFRP laminated in the curing process, and stable output wavelength signal; the output signal of FBG sensor is still continuously and steadily after cooling, the sensor can survive well in high temperature curing; FBG sensors can accurately respond to tension and bending force, indicating that FBG sensors and CFRP laminates are good compatibility.At last, the experiment using FBG sensors to monitor the low velocity impact response of CFRP laminates is implemented. Experimental results show that, the FBG sensors embedded 3 samples were able to accurately capture the transient impact signal; within the range of 0.2 ~ 0.4J, the peak wavelength shift of sensors are gradually increasing, which can roughly determine the size of the impact energy; by analyzing the signal of FBG sensors can determine whether the laminates are damaged and whether the damages are extension in the experiment of multi accumulation impact. |
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