# Fiber Bragg Gratings Based Cyclic Strain Measuring of Weld Toes of Cruciform Joints
## 1 Introduction
[PERSON] [27] arranged several strain gauges near the weld toe along the longitudinal direction of the weld and monitored the strain time history. The initiation time of the weld toe crack was discussed by analyzing the strain signals, gauges position and the crack morphology. [PERSON] [28] used strain gauges to measure the strain for the low-carbon steel welded specimen under cyclic loads. Amplitude difference technology and phase difference technology were employed to monitor the initiation and growth of the subcritical fatigue cracks. These investigations only captured the initiation time of the fatigue crack based on the changes of the strain signal but did not reveal the stress-strain behavior of the weld toe deeply. [PERSON] [29; 30] arranged two strain gauges vertical to the weld line with two different distances from the weld and calculated the hot-spot stress at the weld toe by linear extrapolation. Essentially, the hot-spot stress is a constructive stress parameter and cannot characterize the actual stress-strain state at the weld toe. Due to inherent deficiencies such as zero drift, resistive strain gauges are generally used for static testing or short-time dynamic testing. There are some inevitable problems for the local strain measuring of the weld toe: (1) The weld toe curve does not have mathematical continuity and the SCF effect along the longitudinal direction of the weld toe is not uniformly distributed. Therefore, several resistive strain gauges are required to capture the characteristic strains along the weld toe. (2) Resistance strain gauges often have bases with non-negligible areas. Even if the gauge has adhered to the weld toe, the actual testing point is still at a certain distance from the actual weld toe point. Therefore, it is difficult to obtain the real strain at the weld toe point. (3) Due to SCF effect on the weld toe, even if the nominal stresses and strains usually obtained by strain gauges are in linear elastic phase, the actual local stresses and strains at the weld toe points may probably excess the plastic phase. So sometimes the stresses and strains obtained by strain gauges are unable to characterize the actual stresses and strains state along the weld toe, which are vital to the life prediction of welded joints. (4) In fatigue testing, specimens are subjected to cyclic loads. The material of the structure has a completely different stress-strain response behavior from that under monotonic loads. In a word, resistive strain gauges are not practical to measure the cyclic strain for welded joints. Recently, optical fiber based sensing technology has gained a growing amount of attention from researchers and engineers, among which FBG sensors are widely used in structural health monitoring(SHM) due to their unique advantages such as a small size, light weight, embedding capability, immunity to corrosion and electromagnetic interference [31; 32; 33; 34; 35]. Aiming at the main problems in this investigation, a measuring method based on FBG was proposed to acquire the local cyclic strains along the weld toe of welded joints. Several FBG sensors were fabricated at multiple characteristic points along the weld toe on the thin plate of the cruciform joints. Fatigue test under cyclic tensile load was conducted, and the strains at each characteristic point were recorded in real-time. Based on the stress-strain constitutive model under the cyclic load of the weld material, the stress-strain response curves were drawn, and the hysteresis loops were analyzed. Finally, the weld toe geometry and fatigue fracture morphology were employed together to reveal the mechanism of weld toe crack initiation.