应用声发射技术和图像分形理论对樟子松木材裂纹演化特征的检测

Detection of Crack Evolution Characteristics of Pinus sylvestris var.mongolica Using Acoustic Emission Technology and Image Fractal Theory

以气干状态的樟子松(Pinus sylvestris var.mongolica)木材为试验材料,制备尺寸为(长×宽×厚)300 mm×20 mm×20 mm的试验试件,应用UTM5105型电子万能力学试验机对试件进行三点弯曲加载试验;利用声发射(AE)采集平台和图像采集系统,实时采集樟子松木材在三点弯曲破坏过程中产生的声发射信号及裂纹演变图像;依据原始声发射波形统计声发射计数、声发射能量,并结合图像分析试件在加载过程中的微观损伤及其宏观演化特征;依据图像分形理论计算灰度图像及二值化图像盒计数维数,分析木材表面裂纹起裂和扩展规律;综合运用声发射技术和图像分形理论,构建樟子松木材裂纹演化特征的研究方法。结果表明:声发射计数、声发射能量均能准确地反映宏观破坏阶段;对于微观损伤,声发射计数表征的更为显著。结合声发射计数、声发射能量曲线变化,可以明确材料微观损伤及宏观破坏的起裂时刻。宏观裂纹产生后,灰度图像及二值图像的图像分形维数都大幅上升,且随着裂纹的扩展,裂纹图像的分形维数逐渐上升,尤其是灰度图的分形维数能够明显反映宏观断裂的扩展特性。声发射参数和图像分形维数相结合的方式,可为研究木材损伤断裂过程中裂纹演化行为的损伤机理提供参考。

With the air-dried Pinus sylvestris var.mongolica,the size of the test specimen(length×width×thickness)300 mm×20 mm×20 mm was prepared,and the UTM5105 electronic universal testing machine was used for the test.The acoustic emission signals and crack evolution images in the process of three-point bending failure of samples were collected in real time by using acoustic emission collection platform and image collection system.According to the original acoustic emission waveform,the acoustic emission count and acoustic emission energy were counted,and the microscopic damage and macroscopic evolution characteristics of the specimen during the loading process were analyzed combined with the image;according to the image fractal theory,the fractal dimension of gray image and binarized image was calculated,and the law of initiation and propagation of wood surface cracks was analyzed;and comprehensively acoustic emission technology and image fractal theory were used to construct a research method for crack evolution characteristics of P.sylvestris var.mongolica.The results show that both the acoustic emission count and the acoustic emission energy can accurately reflect the macroscopic damage stage;for the microscopic damage,the acoustic emission count is more significant.Combined with the change of acoustic emission count curve and acoustic emission energy curve,the crack initiation time of the microscopic damage and the macroscopic damage of the material can be determined.After there are macroscopic cracks,the fractal dimensions of both grayscale images and binary images increase significantly,and with the expansion of cracks,the fractal dimensions of cracked images gradually increase,especially the fractal dimension of the grayscale image can clearly reflect the expansion characteristics of macroscopic cracks.The combination of acoustic emission parameters and image fractal dimension can provide a reference for studying the damage mechanism of crack evolution behavior during wood damage fracture process.