中文摘要：

现有的晶粒尺寸超声无损评价方法对被测对象的厚度有严格要求,进而影响方法的实用性及其检测结果的可靠性。基于超声波在材料中的传播规律,结合一次底面波及二次底面波相对于表面波的衰减特性,推导不含厚度信息的超声衰减速率参量,并利用透射系数和反射系数对其参量进行修正,基于超声衰减速率建立平均晶粒尺寸的超声无损评价模型。以核电常用的TP304不锈钢材料验证超声衰减速率法的有效性,将其与传统声速法、衰减法、背散射法进行对比,试验结果表明衰减速率法的灵敏度显著优于声速法;试块厚度小于6 mm时采用衰减速率法的相对误差明显低于背散射法;针对壁厚测量不便的TP304方管,衰减法、衰减速率法及金相法的平均晶粒尺寸评价结果分别为（100.3±2.8）μm、（96.7±3.4）μm和（93.1±1.8）μm,可见衰减速率法能有效抑制壁厚对评价结果的不利影响。

英文摘要：

The current ultrasonic nondestructive methods for evaluating grain size require the thickness of the object to be highly accurate, and thus reduce its practicability and its reliability of the evaluation result. Based on the propagation of acoustic beams in material, the attenuation characteristics of first and second bottom-wall echo as against the front-wall echo are described, and the parameter of ultrasonic attenuation rate exclusive of the thickness is derived, both transmission coefficient and reflection coefficient are used to correct the ultrasonic attenuation rate. Taking the ultrasonic attenuation rate as the acoustic eigenvalue, an ultrasonic nondestructive evaluation model for mean grain size is presented. The proposed method is validated through the use of TP304 stainless steel frequently used in nuclear power plant, and a comparison is conducted between our method and the traditional methods such as ultrasonic velocity, attenuation and backscatter. The results show that the sensitivity of our method is notably better than that of the ultrasonic velocity method and the relative error of the developed method is greatly lower than that of the backscatter method when the thickness of the block is less than 6mm. Furthermore, the presented method effectively restrains the adverse impact on the grain size evaluation induced by the thickness, which is demonstrated by the fact that a TP304 square tube with wall thickness not convenient to measure, the mean grain sizes of measured by the attenuation method, the proposed method and the metallographic method are （100.3±2.8）μm, （96.7±3.4） μm and （93.1±1.8）μm, respectively.