| Because of excellent performance,carbon fiber reinforced plastic(CFRP)is widely used in aerospace fields.The formation of voids in CFRP composites is inevitable due to the manufacturing process.The physical and mechanical properties of the composites will be affected since the porosity(void content in unit volume)exceeds a certain value.Accurate and reliable nondestructive testing technology for porosity is of great significance for controlling the properties and quality in composites.Ultrasonic attenuation method,which is based on establishing the relationship between ultrasonic attenuation coefficient a and porosity P,is the most commonly used nondestructive testing method for porosity of CFRP.The study of ultrasonic scattering attenuation mechanism in composites is the premise and key for the application of ultrasonic attenuation method.The ultrasonic attenuation mechanism researches for composite materials in previous literatures are usually assumed that:? the CFRP matrix is isotropic homogeneous medium;? the void shape is regular,the void size is unique or meet for a distribution function and the void distribution is uniform;? there is no interaction between ultrasonic scattering waves from different voids,a single Rayleigh scattering mechanism was followed in void scattering.Because of the idealized assumptions for matrix and voids in composites and simplified mathematical model for void attenuation in classical researches,a large gap appeares between the theoretical calculation and the experimental results.In order to reveal the mechanism of ultrasonic scattering in CFRP and improve the accuracy of ultrasonic inspection,a real morphology void model(RMVM)was built firstly based on the modeling idea of random void model.In the model,the actual composite properties and the complex void morphology features were considered,such as large range size,irregular shape,random distribution.RMVM can accurately describe the property of matrix and the size,shape,distribution of voids in CFRP with complex and random morphology.Compared with the previous deterministic models,it has a substantial breakthrough,which provides a good model foundation for the study of ultrasonic scattering attenuation mechanism.Subsequently,the influence of porosity and void morphology on the propagation behavior of ultrasonic waves was studied by simulation method,and the non-unique correspondence relationship between ultrasonic attenuation coefficient and porosity due to the complicated void morphology was defined.Considering the various void sizes and the interaction between voids,change rules of a were quantitatively analyzed under different ranges of void size and multi-voids condition.A viewpoint that there are two scattering attenuation mechanisms of Rayleigh scattering and random scattering in CFRP composites was proposed.And the contribution of interaction between scattered waves from voids with different size to ultrasonic scattering attenuation was also explained.Finaly.combining the influence of void morphology on attenuation slope with ultrasoninc attenuation coefficient,an ultrasonic two-parameter method was proposed for characterizing porosity of CFRP.The main research contents and conclusions are as follows:(1)Besed on the randormness and complexity of void morphology in CFRP,a real morphology void model was built using random void model for reference.The void characteristics of CFRP composites were observed by metallographic microscope,and the information of void morphology in micrographs was extracted by digital image processing technology.Four hundred and forty-one models with porosity of 0.58%-3.49%were established after assigning the material properites according to the gray levels.The numerical calculation for ultrasonic testing was carried out using RMVM by finite difference time domain method with 5 MHz probe(ultrasonic wave length ? is 560 ?m).The experimental data were in good agreement with the simulation results for the correspondence between a and P.The validity of modeling idea in RMVM was fully proved.Because of the various size,complex shape and random distribution of void,the model overcomes the problem that the void morphology is too simplified in the previous deterministic models.It provides a more effective model foundation for the study on attenuation mechanism of ultrasonic scattering.(2)The non-unique correspondence between a and P caused by the complex and changeable void morphology was defined by focusing on the propagation behavior of ultrasonic wave.In order to explain the reason of the relationship by ultrasonic scattering mechanism,the influences of different void morphology factors(size,quantity and distribution)on ultrasonic attenuation coefficient were analyzed considering the diversity of void size and the interaction of ultrasonic scattering from voids.The void size D in CFRP composites ranges from a few microns to several hundred microns,so D/?0-?.And,the range of the product of ultrasonic wave number k(k=27?/?)and D is about 0?2?.The research suggests that two kinds of mechanism.Rayleigh scattering and stochastic scattering,are followed when the ultrasonic wave propagates in CFRP.The transition between two kinds of scattering mechanisms is occurred at kD=0.70?1.05.The ultrasonic attenuation is serious with the larger P at the same value of D,and its increase rate in the random scattering mechanism is higher than that of Rayleigh scattering.Moreover,the contributions of voids with different size to ultrasonic scattering are different.The small voids(D<56 ?m)and medium voids(56?m<D<178 ?m)will weaken the interacations while the large voids(D? 178?m)will enhance them.The phenomenon that two kinds of mechanism were followed in CFRP was discovered and the contribution of interaction between voids with different size to ultrasonic scattering attenuation was explained by numerical calculation for RMVM.(3)In order to improve the accuracy and validity of ultrasonic testing for porosity in CFRP composites,the attenuation slope K which is frequency domain parameter was introduced to distinguish the porosity under the same ? for the problem of non-unique correspondence between ? and P.The influences of void size,quantity and distribution on K were discussed under the same a by the simulation method.The ultrasonic attenuation slope K was obtained using spectrum analysis method.Among them,the void size and quantity can be considered as the same influence factors because of the inverse proportion relationship between them.The results show that void size and quantity are important factors affecting the value of K.In low attenuation(??2.8 dB/mm),the quantity of void mainly affects the K value,and the K decreases gradually with the increase of P.The void size turns dominant at the high attenuation(?>2.8 dB/mm).The larger the D is,the smaller the K is.The void distribution has a weak influence on K,and the K will decrease to below 0.5 only in extreme cases,for instance,the voids are arranged in a straight line,the voids are clustered together.(4)The ultrasonic two-parameter method combining a and K is proposed to detect the porosity of CFRP composites according to the rule of attenuation slop changing with porosity.At the same a,the negative correlation between P and K was discovered using the numerical calculation of RMVM.At the same time,an abnormally low value of K shows that the void distribution is serious aggregation(when ??3.6 dB/mm)or the larger voids appear(after?>3.6 dB/mm)in RMVM.Combining ? with K,an ultrasonic two-parameter method was proposed to quantitatively characterize the porosity of CFRP according the negative correlation between P and K.The statistical results show that,the number of models characterized by ultrasonic two-parameter method is 265,accounting for 77%of the total number of models;while two hundred and forty models are effectively characterized by ultrasonic attenuation method,that is,70%of the models is effectively predicted.Therefore,compared with ultrasonic attenuation method,the ultrasonic two-parameter method can effectively improve 7%of the porosity measurement accuracy. |
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