In steel construction, for both plate and tubular structures, welding is commonly used for joining two or more parts together. In welded joints, defects such as cracks, pores, and slag inclusion can be present from the beginning or generated while in service. Such defects are the weak spots that can lead to structural failures. Therefore, early detection of these defects in welded joints is important. One way of detecting these defects is using ultrasonic non-destructive testing (NDT) techniques. Guided acoustic wave-based techniques have been proven to be effective for damage detection and many studies have been tried out before for damage detection in tubes. Earlier attempts mostly focused on conventional linear ultrasonic techniques. In this research, a newly developed nonlinear ultrasonic technique, called Sideband Peak Count- Index (SPC-I) technique, is carried out. For this investigation, two steel tubes are welded together and four point bending test is conducted under fatigue loading. The welded joint is continuously inspected in real time using strain gauges and Lead Zirconate Titanate (PZT) transducers which is used to generate and receive the guided acoustic waves. The signal is propagated through the specimen in a single sided transmission mode setup. During the test, both the strain gauge values and the nonlinear ultrasonic parameter, the SPC-I values, are continuously monitored simultaneously. The results obtained from the nonlinear ultrasonic NDT measurements are compared with the experimental data obtained from the strain gauge to check if the technique is robust and reliable enough for qualitative inspection of welds.