Peter B. Keating

Distortion-induced fatigue cracking of bridge details with web gaps☆

Abstract Distortion-induced fatigue cracking from out-of-plane deformation in web gaps of bridge connection details is examined. This type of fatigue cracking results from interaction of various structural members during live loading of a structure and produces unanticipated distortion and high localized stresses. Although this type of problem is not confined to bridges, this paper focuses on bridge types and members that are susceptible to distortion-induced fatigue cracking at web gaps. This includes both plate-girder and box-girder bridges, as well as tied-arch bridges with box- and plate-girder members. Member details include transverse connection plates for cross-bracing and diaphragms, longitudinal gusset-plate details, and floor beam end connections. The crack development behavior is examined using field observations and measurements, as well as results from large-scale laboratory experiments. Recommendations for retrofitting distortion-induced fatigue damage are given, along with design alternatives to eliminate or minimize the occurrence of distortion-induced fatigue cracking.

Length Effects on Fatigue Behavior of Longitudinal Pipeline Dents

Dent length has been shown to have a significant effect on the fatigue cracking behavior of pipeline dents. Long dents, which experience rerounding and center cracking, have a dramatically shorter fatigue life than otherwise similar short dents, which experience peripheral cracking and little rerounding. Because the fatigue lives of long dents are much shorter than those of short dents, both safety and economy would benefit from improvements in the ability to distinguish long dents from short dents. Based on experimental evidence, a transition between short and long dent behavior is shown to exist. Finite element models are used to further explore the nature of this transition by allowing the examination of cases not available in the experimental record and by permitting stress behavior to be studied. A parametric study is used to quantify the nature of the short dent to long dent transition for a range of cases. Relative dent lengths that bound short and long dent regions of behavior are proposed for these cases.Copyright

High-cycle, long-life fatigue behavior of welded steel details☆

Abstract Results from laboratory fatigue tests of large-scale welded attachments under random variable-amplitude loading have indicated that the existence of a fatigue limit below which no fatigue cracks develop is assured only if none of the stress cycles exceeds the constant-amplitude fatigue limit. In the extreme life region of fatigue behavior, the contribution of the lower cycles of a stress range spectrum to crack propagation is a function of the larger stress cycles. However, the value of the constant-amplitude fatigue limit is not precisely known and in some cases will differ from the assumed value. As a consequence of the variability in the fatigue limit, the lower bound variable-amplitude fatigue resistance on large-scale welded steel details is defined by the straight-line extension of the S-N design curves below the constant-amplitude fatigue limit. A summary of the test data is given and the effect of the variability on the fatigue behavior of welded steel details in the high-cycle, long-life regime is discussed.

Fatigue Life Prediction for Short Dents in Petroleum Pipelines

Pipeline dent fatigue behavior has been shown to be strongly dependent upon dent length and external force dent restraint characteristics. Full-scale laboratory tests have shown that short dents that are unrestrained by an external force typically experience fatigue cracking in the dent periphery outside of the dent contact region. A fatigue life prediction method for short dents is presented here. In order to assess method accuracy, predictions are made for cases in which fatigue life has been measured experimentally. The predictions account for both crack initiation life and crack propagation life. Stress concentration values used in the predictions are determined using finite element modelling on a case-by-case basis for comparison purposes. Appropriate crack initiation life estimates, stress intensity factor predictions, and crack propagation models are taken from existing literature. Predicted and measured fatigue lives are compared for the cases studied.Copyright

Predicting the Fatigue Life of Long Dents in Petroleum Pipelines

A full scale experimental study has demonstrated that long, unrestrained pipeline dents typically experience fatigue cracking in the dent contact region and have significantly shorter fatigue lives compared to other dent types studied. Furthermore, these dents often fully reround under normal pipeline operating pressures, making them difficult to reliably detect and assess using existing depth-based approaches. Several conditions unique to the dent contact region accelerate fatigue damage accumulation and are considered in a case-specific long dent fatigue life prediction method. First, the contact region develops significant bending stresses that contribute to a higher rate of fatigue crack growth. Second, history dependent, thru-thickness residual bending stresses that may have a significant influence on fatigue behavior are present in the contact region as a result of plastic deformation associated with dent formation and subsequent rebounding. A method for predicting the fatigue life of long dents that accounts for these factors is presented here and is used to analyze specific cases for which laboratory data is available. Nonlinear finite element modelling of the dent life cycle, including the indentation and rebounding phases, is used to determine local stress range behaviors and residual stress distributions. The application of appropriate fracture mechanics based models of fatigue is discussed and demonstrated. Fatigue life predictions are made on a case by case basis for situations studied in the laboratory so that the validity and accuracy of the approach presented here may be studied.Copyright