Digital Image Correlation is a cheap and easy to use measurement method that has been proved to characterise any solid material parameters up to plastic deformation range. The digital image correlation measurement system is based on the tracking of a collective number of pixels from the surface of the reference image to the deformed image. This summation of pixels is called subset. To achieve a reliable measurement in DIC, each of the subsets must contain sufficient speckle pixels. Therefore, there is a firm distinctive intensity pattern contained in a particular subset relative to other subsets. However, with the current DIC method, the user must rely on the intuition and experience for determining these number of speckle pixels. To investigate this problem, a study of measurement and monitoring the behaviour of crossbeam in Orthotropic Bridge due to in-plane quasi-static load is conducted. The reliability of DIC method on measuring strain and deformation in an elastic zone of a steel material is also examined and compared to the other measurement technique such as strain gauge and LVDT. Also, a finite element model is developed to estimate the strain and out-of-plane deformation at the certain location of the crossbeam. The study focuses on the specific location at the crossbeam with ‘haibach’ shape cope hole, where the strain concentration is expected to be significantly high. The applied quasi-static load is set until 250 kN, with strain gauges and LVDT are installed at the same side of the crossbeam (free edge side), and DIC is installed at the exact opposite side (inner side). Parallel with the test on the Orthotropic Deck Bridge specimen, several benchmarking test is also conducted. These tests are used to examine the problems encountered during the initial experiment. The study shows that owing to the quasi-static in-plane load, the crossbeam encounter an out-of-plane behaviour. Therefore, to achieve an accurate measurement, a 3-D DIC measurement system is obliged to use. Strain observed into two different directions: x and y-direction, gives different results in term of accuracy. Due to the low strain limit of DIC system, the result in x-direction can be considered as unreliable. For strain measurement in y-direction, some of the results exceeding the low limit strain of the equipment, give a close match result with the strain gauges, within the acceptable amount of deviation. For out-of-plane deformation, all the measurement show a good agreement between the DIC and LVDT technique. The average speckle pattern size that is used in this research is 0.2 mm, and an average area of 38 pixels2 for a particular speckle pattern. One of the benchmark study shows that under the same condition with same expected level of displacement, an average speckle pattern area between 36 and 45 pixels2 leads to reliable 3D measurement. The finite element model that has been developed in this study also gives close strain estimation for both x and y-direction, compared to the measurement data (DIC and strain gauges).

For cyclically loaded structures, fatigue design becomes one of the important design criteria. The state of art shows that with modification of the conventional structural design methodology, the use of very high strength steels may have a positive effect on fatigue strength of welded connections. However, there is little known about the repair of fatigue cracks in welded connections made of very high strength steels. In this study, the fatigue strength of the repaired base material and repaired fatigue damaged V-shape welded connections made of very high strength steels is investigated. This thesis consists of four parts. In Part I, the emphasis was put on the effects of the material imperfections on the fatigue strength of materials and this part consists of an extensive literature study and a microscopic examination of the fracture surfaces of the base material of very high strength steels. Part II presents a literature survey on the fatigue crack repair methods and the results of the experimental programme of the current study. The experimental programme comprises fatigue tests on the repaired base material and repaired V-shape welded connections made of very high strength steels. The V-shape welded specimens were made of rolled and cast steel plates. In Part III, fatigue strength prediction models were evaluated and a comparison was made between the fatigue strength curves from the prediction models and the fatigue strength curves of the test results. In the last part, conclusions and recommendations from the study are presented. The analysis of test results revealed that the fatigue strength of fatigue damaged V-shape welded connections made of very high strength steels can be recovered by an appropriate repair procedure. @en