Eric J. Kaufmann

Fatigue strength of welded AL-6XN superaustenitic stainless steel

Full scale tests of welded AL-6XN superaustenitic stainless steel I-beams were carried out to assess the fatigue behavior of three weld details. A total of 66 full-scale I-beams were tested under constant amplitude loading. The test program was developed to assess the effects of minimum stress, stress range, and residual stress on the fatigue behavior of the longitudinal fillet weld detail, transverse groove weld detail, and a simulated bulkhead attachment detail. Weld defects that initiated fatigue crack growth were characterized. Internal discontinuities included porosity, entrapped oxides, and lack of fusion sites located in the longitudinal fillet welds and the groove welds. Surface cracks initiated at microdiscontinuities at the toe of the attachment fillet welds and at the toe of the groove welds.

Effect of Corrosion on Crack Development and Fatigue Life

Fatigue and fracture as well as loss of section caused by corrosion are time-dependent performance characteristics that have the potential to jeopardize the integrity of bridge structures. During the past 25 years these conditions have developed in a number of bridges, resulting in loss of service, costly repairs, and concern about the safety of these structures. A review of the experience with such time-dependent damage since 1970 is presented. The experience is grouped into three categories: fatigue cracking resulting from changes in structural behavior as a result of corrosion, fatigue cracking resulting from development of corrosion notches in members, and stress corrosion of high-strength steel and weld metal. The examples cited illustrate the role of corrosion phenomena in bridge service and the need to control the corrosion conditions on bridge structures.

The effect of welding discontinuities on the variability of fatigue life

Large-scale I-section beams were welded from A710 (HSLA-80) steel plates. There were plain welded beams, similar beams with two attachment details fillet welded to the tension flange, and similar beams with transverse groove welds. The welding procedures used produced a wide range of welding discontinuities, particularly at groove welds which intersected the longitudinal fillet welds without a cope hole. The 162 beams were fatigue tested in four-point bending in a test matrix which included various stress ranges and minimum stress levels. The discontinuities at the origin of each fatigue crack were identified. These initiating discontinuities included microscopic weld toe discontinuities, porosity, inclusions, lack of penetration and hydrogen cracks. Fracture mechanics fatigue crack growth models were used to calculate the fatigue lives based on the initial discontinuity size. Various methods of idealizing the discontinuities as initial cracks were examined.

Propagation of Very Long Fatigue Cracks in a Cellular Box Beam

Large cellular box beams, 8000 mm long, were fatigue tested in four-point bending. A total of ten tests were performed in a test matrix which included several variations in load ratio. The nominal stress range varied from89 to 219 MPa. The box beams were fabricated from HSLA-80 steel. Crack growth rates in the base metal were obtained from CT specimens of various heats, orientations and thicknesses. In every test, a through-thickness crack developed at an intentionally poor weld detail after less than 30% of the total fatigue life. Nearly fixed crack growth rates were observed for a region of cracking, due to web restraints, redundancy aspects of the box beam design, and welding residual stresses. Cracks 1500 mm or more in total length still grew in a stable manner. The observed crack growth rates were related to macroscopic beach marking of the fatigue crack surface as well as microscopic striations. Correlation of the observed crack growth rates to the square of stress range was relatively good. The cracking history is decomposed into four separate stages. Using the concept of effective stress intensity factor range, crack growth rates predictions for each stage are presented.

Steel moment frame connections that achieve ductile performance

Publisher Summary Several investigations were undertaken after the Northridge earthquake in order to examine various aspects that were believed to be associated with the failures observed in the pre-Northridge connection and to improve connection performance. Two full-scale beam-and-column assemblies with a “pre-Northridge” connection detail were dynamically tested. The column was a W14x311 made of A572 Gr. 50 steel. It was supported by a pin at its bottom and a roller at its top. The beam was an A36 steel W36x150 section with flange yield strength equal to 262 MPa. The first specimen tested was specimen A-1, which was typically pre-Northridge practice. The second specimen, specimen A-2, was the same as specimen A-1, but with the backup bar and weld tabs removed and a 9.5-mm reinforcing fillet weld (E71T-8) added to its weld root. The removal of the backup bars in specimen A-2 slightly improved the connection performance, but brittle fracture of the flange welds again led to failure. Other than the limited yielding at the weld access holes, the connection behaved elastically when the welds of the top and bottom flanges fractured almost simultaneously during reversed loading.