Browsing by Author "Tasnim Hassan, Committee Chair"

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Influence of Residual Stress and Heat Affected Zone on Fatigue Failure of Welded Piping Joints
(2009-01-05) Cheng, Pei-Yuan; G. Mahinthakumar, Committee Member; Murthy N. Guddati , Committee Member; Tasnim Hassan, Committee Chair; K. Linga Murty, Committee Member
ABSTRACT CHENG, PEI-YUAN. Influence of Residual Stress and Heat Affected Zone on Fatigue Failure of Welded Piping Joints. (Under the direction of Dr. Tasnim Hassan.) In recent decades, some unexpected fatigue failure occurred in welded joint of metal structures under cyclic loading. In many cases, the cause for the failure could not be detected. A study at NC State University revealed that welding procedure could be one of the factors that was not appropriately considered in current design methodologies. The welding procedure can influence the strain response near weld toe in two ways: one is by generating residual stress, and the other is changing material properties in the heat affected zone (HAZ). It was the primary objective in this investigation to determine the influence of these two factors on strain response by conducting experiments and performing numerical simulations of welded piping joints. On experimental study a series of residual stress data were measured, using x-ray and neutron diffraction techniques, for welded piping joints. The measured results revealed that the initial maximum compressive residual stress of stainless steel piping joints is higher than the yield stress of base metal. Moreover, the axial residual stresses of stainless steel piping joints are mostly relaxed after 5 cycles. The change of mechanical material properties due to high temperature exposure was studied by conducting experiments on tubular specimens. It was obtained that some mechanical material properties changed after subjecting to high temperature cycles, and the changed material was correlated to the peak temperatures. A modified thermo-mechanical material heterogeneity model was then developed to improve the initial residual stress simulation at the weld toe. The heterogeneous material properties coupled with the Chaboche model were used for subsequent fatigue response simulation. Quarter-point elements were applied at the stress concentration locations. The analysis results showed that fatigue response and residual stress relaxation can be simulated well. Final two simulations in this research showed that the presence of initial residual stress influences strain amplitude and strain mean, both of which could influence the fatigue life of welded joints.
Localized Low Cycle Fatigue Failure of Welded Steel Moment Connections
(2007-12-07) Royster, Preston; James Nau, Committee Member; Emmett Sumner, Committee Co-Chair; Tasnim Hassan, Committee Chair
Welded steel moment frames were once thought to be one of the most ductile systems for resisting seismic loading. Welded steel moment connections (WSMCs) consisting of complete joint penetration welds between the beam flanges and the column flange along with a bolted shear tab connecting the beam web to the column flange gave the frame its resistance to lateral loads. However, the unexpected brittle failures initiated by cracks near or at the welds between the bottom beam flange and the column flange during the Northridge (1994) and Kobe (1995) Earthquakes challenged this belief. Over the last decade following the Northridge and Kobe earthquakes researchers have improved the global performance of the connection, however cracking near the weld is still observed in the experiments of these "improved" connections. Through all of these efforts to comprehend the behavior of the welded steel moment connection, no one has successfully understood the local behavior of the joint leading to the global failures that have been observed. This research makes an attempt to observe and understand the local behavior of the welded steel moment connection through analysis and a set of material and structural experimentations. Small coupons of beam flanges, developed according to the ASTM specifications, heat treated to different temperatures were tested to obtain the properties of the steel at different distances from the weld. In addition to the heat treated coupons, experiments were also conducted on the base and weld metal coupons. Structural experiments were conducted on the bottom beam flange tee joint specimens, which were consisted of a short section of the bottom beam flange welded to the column using a complete joint penetration weld. These simpler structural specimens were subjected to unidirectional load reversals along the direction of the beam for inspection of the local failure mechanism in a global specimen. Finite element simulations of the full-scale welded steel moment connections and the simplified bottom beam flange welded joint were conducted using the ANSYS software package with its multilinear material model. These pretest analyses were conducted for guiding the experimental program. Through experimentation of the heat treated coupons this research observed changes in material behavior associated with exposure temperature from welding. Such heterogeneous changes in material behavior in the heat affected zone may influence the formation of cracks near the weld. Brittle failures of the bottom beam flange tee joints were observed under constant amplitude displacement controlled load reversals. Recorded data from the strain gages located near the complete joint penetration welds demonstrated the presence of ratcheting (defined by accumulation of strains with cycles). This strain ratcheting response may also influence the formation of cracks near the weld and thus may also be responsible for the brittle failure of WSMCs. In conclusion, the research presented in this thesis attempts to investigate the local failure of the welded steel moment connection in order to understand the global brittle failure mechanism. The change in material properties and the presence of ratcheting at the WSMCs may influence its local crack initiated brittle failures. However, further research is needed to completely understand the local behavior of the welded steel moment connections.