Stress relaxation of welded joints can increase the safety of structures. Although it is not absolutely clear why it happens, several researches have proved the phenomenon. This work presents the application of the ultrasonic technique to evaluate the stress relaxation in welds of 5052 aluminum. A special geometry was developed, so each welded plate had uni-axial stresses in the measurement region. Twenty-two plates were tested, eleven in the rolling direction and the remaining in the perpendicular direction. The plates were stress relief using a heat treatment to set an initial stress free reference state. After that, they were welded to create the stress field in the area of investigation. A final step was to cut each of the samples after different periods of time from the welding. The stress was measured in the uni-axial stress region. A calendar of the process was established to record precisely the time for each step. Longitudinal critically refracted waves were used to measure the stresses. The results showed a clear effect of relaxation. The magnitude was not as pronounced as in steel, but kept the same relation with the ultimate strength of material.
Ultrasonic evaluation of stresses has experienced great development in the last decade, mostly because of the new electronic instruments and high-speed data acquisition systems now available. Past applications have been applied mostly using shear waves and the birefringence technique. Longitudinal critically refracted (skimming) waves have proved to be more sensitive to stress variation, and have been applied recently to oil steel pipelines. Oil pipe failures have caused a lot of environmental damage in Brazil. The environmental and economical costs are very high. This work describes part of a research effort to further apply longitudinal critically refracted waves to inspect the welded region in pipes in Brazil through a study of the acoustoelastic constants. Twenty-eight samples were cut from a flat plate of the oil industry class steel (API 5L X70) which would be typically formed into pipe. The samples were prepared for tensile testing. Half of those were cut from what would be the longitudinal direction of the pipe, and the other half from the perpendicular direction. The samples are bars of 760×70×10.8-mm3. Some bars were stress relieved before the test. The results show that there are significant differences between the acoustoelastic constants for the rolling direction and the perpendicular direction. It is clearly important to have the correct value for the acoustoelastic coefficient when applying the technique to stress measurement. There is a minor, but important influence of the stress relief process.
Presto a minha homenagem as seguintes pessoas:Aos meus Pais Joaquim e Meyri e irmãos, Márcia, Mauro e Mauricio, por sempre estarem ao meu lado nos momentos mais difíceis.À Fernanda pelo carinho, companhia e compreensão prestados por todos os anos de dedicação a essa pesquisa.Ao meu Orientador Professor Doutor Auteliano Antunes dos Santos Júnior, pelo incentivo durante a realização deste trabalho, e por sempre acreditar no meu potencial, oferecendo sua ajuda nas horas que mais precisei.
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