The precision error of the Brillouin optical time domain reflectometry (BOTDR)-based distributed fiber optic strain measurement is normally evaluated based on strain change from the initial zero strain state. In many structural health monitoring applications, however, there is initial strain caused by the installation process of a fiber optic sensor cable to a structure. Engineers are interested in the incremental strain profile from the initial strain profile to assess the performance of the structure. The initial strain profile is often not constant throughout the cable length due to the manner that the fiber optic cables are installed (e.g., gluing, clamping, or embedding). This uneven strain distribution causes precision error in the strain incremental values, which in turn leads to difficulty in data interpretation. This paper discusses why large initial strain variation (or initial strain gradient) increases the precision error of the subsequent incremental strain reading and how to evaluate the magnitude of such precision error. A relationship between strain gradient and precision error is demonstrated. A sectional shift method is proposed to minimize the precision error. Results from laboratory tests and a field case study show that the method can reduce the precision error approximately 50% when the strain gradient is large.

中文翻译：

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分布式光纤应变监测重复精度误差分析

基于布里渊光时域反射计 (BOTDR) 的分布式光纤应变测量的精度误差通常基于从初始零应变状态的应变变化来评估。然而，在许多结构健康监测应用中，光纤传感器电缆在结构上的安装过程会导致初始应变。工程师对初始应变剖面的增量应变剖面感兴趣，以评估结构的性能。由于光纤电缆的安装方式（例如，胶合、夹紧或嵌入），初始应变分布在整个电缆长度上通常不是恒定的。这种不均匀的应变分布会导致应变增量值的精度误差，进而导致数据解释困难。本文讨论了为什么较大的初始应变变化（或初始应变梯度）会增加后续增量应变读数的精度误差，以及如何评估这种精度误差的大小。展示了应变梯度和精度误差之间的关系。提出了一种分段移位方法来最小化精度误差。实验室测试和现场案例研究的结果表明，当应变梯度较大时，该方法可以将精度误差降低约 50%。