# Fiber Bragg Gratings Based Cyclic Strain Measuring of Weld Toes of Cruciform Joints
## 1 Introduction  
Welding is one of the most common methods in manufacturing for the joining components. It has the main advantages of simple structure, easy process, low costs, superior performance and high reliability. Welded structures are widely used in industries, bridges, buildings, mechanical equipment and so on. Engineering application shows that welded structures are prone to develop fatigue cracks under cyclic loads. Once the cracks appear and the welded structure can't be repaired in time, it will probably cause vital damage. A great deal of research [1; 2; 3; 4; 5] has shown that the fatigue performance of welded structures mainly depends on the type of weld joints and the loads. Fatigue cracks often appear in the local position of the weld, especially at the stress concentration position along the weld toe. Therefore, it is significant to understand the stress-strain state and its response behavior at the weld toe of the welded joint to accurately predict the fatigue life of the welded structure, enhancing the fatigue performance of the welded structure and finally improving the welding procedure. For welded joints, the stress-strain state on the weld section is usually described by the stress concentration factor (SCF). Some researchers [6; 7; 8] have employed theoretical and numerical analysis methods to investigate the geometric parameters for typical welded joints and proposed a variety of formulas to relate the key geometric parameters (such as weld inclination angle, \(\theta\), filled leg length, \(l\), weld toe radius, \(\rho\) and plate thickness, \(t\), etc.) with SCF. Considering the different effects of different welded joints type on SCF, a variety of fatigue life prediction methods have been proposed forcruciform joints [9; 10; 11], T-joints [12; 13], lap joints [14; 15] and butt joints [16; 17], respectively. Other researchers have paid attention to some special issues such as single-sided and double-sided welds [18], load-carrying welds and non-load-carrying welds [19; 20], uniaxial tensile and bending loads [21; 22], weld penetration ratio [23], etc. These investigations not only verified further that the weld toe was the weakest position of the fatigue damage for welded structures but also revealed that the local stress-strain state at the weld toe was affected by various factors including geometry, load, material and its microstructure. However, the above investigations only focused on the SCF effect on the weld section. [PERSON] [24] used 3D laser scanning technology to acquire the real weld toe geometry of some welded specimens, by which Finite Element Analysis (FEA) models were established, and then the SCF along the weld toe was computed. The results showed that the longitudinal discontinuities along the weld geometry also have significant influence on the fatigue performances of the weld joints. In this context, measuring the stresses or strains at the weld toe directly became meaningful and challenging work, which was attempted by some researchers. Generally, stress signals in the mechanical properties testing for materials were obtained by the built-in force sensors of the testing machine, while strain signals were obtained by extensometers. Extensometers essentially get the average strain of the measured area, which cannot reflect the local characteristics. For welded joints, extensometers are difficult to fix on the proper weld region. Therefore, extensometers are not suitable for strain measurement of welded specimens. Recently, a new method called Digital Image Correlation (DIC) has been proposed in the field of strain measuring [25; 26]. To some extent, this method can analyze the distribution gradient of stress and strain by digital image techniques. However, it is also not suitable for weld structures due to the complex surface topography on the weld. So, resistive strain gauge sensors are still widely used in strain testing for welded joints at present.