The measurement of acoustic nonlinear response is known as a promising technique to characterize material micro-damages. In this paper, nonlinear ultrasonic approach is used to characterize the evolution of fatigue induced micro-cracks in polymer bonded explosives. The variations of acoustic nonlinearity with respect to fatigue cycles in the specimens are obtained in this investigation. The present results show a significant increase of acoustic nonlinearity with respect to fatigue cycles. The experimental observation of the correlation between the acoustic nonlinearity and fatigue cycles in carbon/epoxy laminates, verifies that an acoustic nonlinear response can be used to evaluate the progressive fatigue damage in the granular polymer bonded explosives. The sensitivity comparison of nonlinear and linear parameters of ultrasonic waves in the specimens shows that nonlinear acoustic parameters are more promising indicators to fatigue induced micro-damage than linear ones. The feasibility study of the micro-damage assessment of polymer bonded explosives by nonlinear ultrasonic technique in this work can be applied to damage identification, material degradation monitoring, and lifetime prediction of the explosive parts.

高聚物粘结炸药（PBX）是一类由炸药造型颗粒高度填充和少量粘结剂构成的复合材料，其内部裂纹的有效检出对结构完整性评价和安全可靠性评估具有重要意义。PBX因其特殊的细观结构特征呈现出声速低、衰减强的特点，利用传统线性超声聚焦成像时，超声波在颗粒边界发生的多重反射会造成重构图像信噪比低，裂纹特征淹没在背景噪声中，难以有效识别。为改善成像信噪比，提出一种基带非线性合成聚焦(BB-NSF)超声相控阵成像算法，充分利用全矩阵数据中各通道接收信号的空间相干性，增强缺陷位置图像强度的同时抑制了背景噪声，再结合PBX曲面构形特征对算法的延时法则进行修正，实现了曲面构形PBX不同取向裂纹缺陷的高信噪比成像，并对重构图像质量和成像效率进行了定量对比评价。结果表明，BB-NSF算法可有效抑制背景噪声，显著改善PBX裂纹检出能力。BB-NSF算法中的信号空间相干度p是影响图像信噪比的关键参数。p值大于2.0时，PBX裂纹缺陷超声重构图像的信噪比相比于传统的全聚焦成像算法可以提升10dB以上。随着p值增加，重构图像的信噪比可以得到进一步提升，而计算效率保持稳定，体现出BB-NSF算法在强衰减复合材料内部缺陷成像方面的优势和应用前景。