## 3 Fundamental OBR Principles and the Traditional Method to Analyze the Data  
To measure strain profiles along an optical fiber, Optical Time Domain Reflectometry (OTDR) or Optical Frequency Domain Reflectometry (OFDR) are utilized. Time of flight measurements of a laser pulse are usually used in OTDR to calculate the position of a signal along the fiber. In OFDR, however, the location is determined by the Fourier transformation of a swept frequency pulse that interacts with the fiber. In general, sensors based on Raman, Brillouin or Rayleigh scattering can be used with both methods. In [42; 43], the detailed principles of systems that utilize sensors based on Rayleigh or Brillouin scattering are described. Brillouin scattering-based OFDR measurements have been used to measure the strain development during the hardening of the matrix material of composite structures [44; 45]. The shrinkage of composite materials, however, is considerably less compared to pure matrix material (epoxy), as presented in this work. In addition is the spatial resolution of some centimeters in the cited works, more than one order of magnitude worse than for the results presented here. In this work, an optical time domain reflectometry instrument with a sensor based on Rayleigh backscattering is utilized to measure the local strain of optical fibers along the entire length of the fiber. For simplicity, we use the more general term optical backscatter reflectometry in this article. The employed OBR method uses relative measurements of Rayleigh backscattered light [31; 32; 33; 36]. Hence, it is classified also as distributed fiber-optic sensing. Its use outside of basic research laboratories is a new development after a sufficiently small and easy to use instrument became available some years ago [40]. The method allows measuring one-dimensional strain fields with strain concentrations along the length of the optical fiber much better than other approaches. Spatial resolutions of strain measurements as good as one millimeter can be achieved along the whole length of the fiber and a strain resolution of as good as 0.001% [41]. In OBR, a laser light pulse is sent into an optical fiber. Each fiber contains natural characteristic impurities that lead to Rayleigh backscattering and act as a \"fingerprint\" of each part of the fiber. The returned backscattered light and its arrival time are detected and recorded. Backscattered light from parts of the fiber that are placed further away from the light source arrives later at the detector than from parts close to the source. This time difference allows localizing the reflected signal along the length. The amplitude pattern of the backscattered light along its length is unique for each fiber. Typically, the first record of the reflected light signals is done before a mechanical experiment is carried out on a sample creating a reference record, a kind record of all impurities and their locations. When the sample is mechanically or thermally loaded, the impurities change their location. New light pulses create new scattered light records from the impurities at changed locations, changing the signal. Mechanical strains are subsequently calculated by comparing the actual record with the initial reference record. The result is the strain along the length of the fiber; it is called here a measurement. The measurement gives under ideal conditions a sequence of local strain values along the fiber with a spatial resolution of about 1 mm and a strain resolution of as good as 0.001% [41]. Additional detailed information about the processes involved using this instrument can be found, e.g., in [46; 47]. The following definitions of words are used throughout this document:  
* Record: Storing of the returned light amplitude pattern along the length of the fiber. * Reference record: Record at a reference condition, e.g., at room temperature and when no mechanical loads are applied.