An analytically formulated structural strain method for fatigue evaluation of welded components incorporating nonlinear hardening effects

An analytically formulated structural strain method is presented for performing fatigue evaluation of welded components by incorporating nonlinear material hardening effects by means of a modified Ramberg‐Osgood power law hardening model. The modified Ramberg‐Osgood model enables a consistent partitioning of elastic and plastic strain increments during both loading and unloading. For supporting 2 major forms of welded structures in practice, the new method is applied for computing structural strain defined with respect to a through‐thickness section in plate structures and cross section in piping systems. In both cases, the structural strain is formulated as the linearly deformation gradient on their respective cross sections, consistent with the “plane sections remain plane” assumption in structural mechanics. The structural strain‐based fatigue parameter is proposed and has been shown effective in correlating some well‐known low‐cycle and high‐cycle fatigue test data, ranging from gusset‐to‐plate welded plate connections to pipe girth welds.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146966/1/ffe12900.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146966/2/ffe12900_am.pd

Mixed-mode near-tip fields for elastic perfectly plastic solids.

The small-scale yielding mixed-mode near-tip fields of a stationary crack in an elastic perfectly plastic solid are examined. Full field finite element analyses, employing a small-strain version of the J\sb2 incremental plasticity theory, are carried out to elucidate the details of the limiting stress states near the crack tip. Then, the asymptotic structures of the near-tip fields are generalized, and the corresponding asymptotic solutions are constructed. The current near-tip asymptotic solutions are in excellent agreement with the finite element results in every respect for the whole range of mixed-mode loading. Under both plane-strain and plane-stress conditions, elastic sectors are introduced in constructing the asymptotic near-tip fields. In plane strain there exists one elastic sector bordering the upper crack face for near mode I mixed-mode loading, while in plane stress there exist two elastic sectors bordering both crack faces for pure mode I and near mode I mixed-mode loading, and one elastic sector bordering the lower crack face for near mode II mixed-mode loading. Within the elastic sectors the stresses are nonsingular. The details of the mixed-mode plastic zone sizes and shapes are given. Implications on mixed-mode fractures are discussed. In addition, some issues pertaining to the finite element procedure such as the implementation of the small-scale yielding assumption are addressed in the light of the computational results. A perfect-plasticity solution to the mixed-mode near-tip fields in plane stress is also presented. The mode I crack-tip field, derived from this analysis, differs from Hutchinson's solution by a constant stress sector ahead of the crack tip. The relevance of the near mode II solutions to some important features of the mixed-mode crack-tip fields from early dominant singularity analyses is discussed.Ph.D.Applied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128318/2/8920525.pd

Heat Sink Welding for Preventing Hot Cracking in Alloy 2195 Intersection Welds: A Feasibility Study

Two concepts, stationary cooling and trailing cooling, were proposed to prevent weld intersection cracking. Finite element analysis was used to demonstrate the potential effectiveness of those two concepts. Both stationary and trailing heat sink setups were proposed for preventing intersection cracking. The cooling media could be liquid nitrogen, or pressured air knife. Welding experiments on the small test panel with the localized heat sink confirmed the feasibility of using such a stationary cooling technique. The required cooling was achieved in this test panel. Systematic welding experiments should be conducted in the future to validate and refine the heat sink technique for preventing intersection cracking

Numerical and experimental analysis of hydroelastic responses of a high-speed trimaran in oblique irregular waves

Investigation of hydroelastic responses of high-speed vessels in irregular sea state is of major interest in naval applications. A three dimensional nonlinear time-domain hydroelastic method in oblique irregular waves is developed, in which the nonlinear hydrostatic restoring force caused by instantaneous wetted surface and slamming force are considered. In order to solve the two technical problems caused by irregular sea state, the time-domain retardation function and Proportional, Integral and Derivative (PID) autopilot model are applied respectively. Besides, segmented model tests of a high-speed trimaran in oblique waves are performed. An oblique wave testing system for trimarans is designed and assembled. The measured results of main hull and cross-decks are analyzed, and the differences in distribution of load responses between trimarans and monohull ships are discussed. Finally, from the comparisons, it is confirmed that the present concept for dealing with nonlinear hydroelastic responses of ships in oblique irregular waves is reliable and accurate. Keywords: Hydroelastic responses, Trimaran, Oblique irregular waves, Nonlinear, Segmented model tes

A criterion for fatigue failure mode evaluation of U-rib to deck joints

In this study, a numerical analysis model was established, on basis of the traction structural stress method, for fatigue failure mode prediction of orthotropic bridge deck (OBD) under symmetric three-point bending conditions. The structural stress-based criterion was validated by comparison with a collection of 64 test results. Fatigue failure modes are found to hinge on a dimensionless normalized constraint distance l/lcr that is related to specimen geometric dimensions and supporting arrangements. OBD specimens in symmetric three-point bending fatigue tests fail at toe-U-rib and external toe-deck as the actual constraint distance l is lower and higher than the critical constraint distance lcr where fatigue failure mode changed. The prediction power of the evaluation criterion is not affected by the fatigue load patch width in fatigue tests, U-rib profile shape, or weld penetrations

Analytical Treatment of Distortion Effects on Fatigue Behaviors of Lightweight Shipboard Structures

In this paper, a notional load method is presented for providing analytical treatment of complex distortion effects on fatigue behaviors of lightweight shipboard structures through a distortion decomposition technique. Its applications for analyzing secondary bending stresses caused by nonlinear interactions between four common distortion types induced by welding and remotely applied load are discussed in detail. In addition, two sets of lab-scale specimens and nine full-scale stiffened panel fatigue tests involving complex distortion shapes are also analyzed using the closed form analytical solutions developed. The analytically calculated stress concentration factor results are validated by direct finite element computations in all cases. Furthermore, fatigue test data obtained from both butt-welded thin plate lab specimens and full-scale stiffened panels are shown not only in a good agreement with one another, but also fully contained by the master S-N curve scatter band adopted by ASME Div. 2 since 2007.Peer reviewe