Course on fatigue and fracture mechanics : influence of residual stresses and post-welding improvement methods

Sumário da Lição, apresentada para Provas de Agregação em Engenharia Civil, especialidade de Mecânica das Estruturas e dos Materiais, da Faculdade de Ciências e Tecnologia da Universidade de Coimbr

On the low-cycle fatigue capacity of unanchored steel liquid storage tank shell-to-base connections

Under strong earthquake motions, the liquid stored within large unanchored steel tanks can cause rocking and uplift of the tank base from the supporting foundation. Repeated rocking can lead to low-cycle fatigue fractures in the tank base-plate, and ultimately spillage of tank contents. Due to limited research on tank connection capacity, current European and New Zealand tank standards (EuroCode8 part 4 and NZSEE) impose unjustified rotation limits on tank shell-to-base connections (current limit being 0.2rad). These rotation limits often govern the design of new tanks, and many existing tanks require retrofit to meet compliance. This paper experimentally investigates the fatigue capacity of tank shell-to-base connections, with a focus on the effects from material ductility and applied strain range (rotation range). Twenty-seven tank connections representing two steel material grades are fatigue tested under constant range rotation cycles to generate fatigue-life curves. Tensile loads applied to the base-plate simulate membrane action present in tanks during uplift. Results indicate that the strain capacity of the base-plate base material strongly influences connection fatigue capacity as fatigue fractures originate in the base material away from the weld toe and weld heat affected zone. Increasing base-plate ductility drastically increases connection fatigue life. All connection specimens sustained multiple uplift cycles at rotations greater than the existing EuroCode8 and NZSEE limit

Analytical approach for improving damage equivalence factors

The fatigue design of bridges under variable amplitude traffic loads is not straightforward. To simplify the task, the current Eurocode provisions apply fatigue load model 3 and damage equivalence factors (lambda). However, the fatigue load model and the damage equivalence factors have several shortcomings: (1) the critical span length is defined for limited bridge influence lines, (2) the damage equivalence factors neglect simultaneous presence of several heavy vehicles on bridges, and (3) the safety margin of damage equivalence factors is not uniform for all bridge influence lines. The current study identifies the effective parameters in the damage equivalence factors through a step-by-step analytical approach. Based on the analytical studies, new propositions for two main parameters of the fatigue load model and fatigue equivalent length are made. These modifications not only improve the accuracy of damage equivalence factors (both lambda and lambda(max)) but also extend the application to any bridge influence line. In addition, the proposed modifications are generalized by introducing a partial damage equivalence factor (lambda(5)) which takes into account the effect of repetition in influence lines. The proposed parameters are justified with traffic simulations for various bridge cases. (C) 2013 Elsevier Ltd. All rights reserved

Modelling fatigue crack propagation of a cracked metallic member reinforced by composite patches

The concept of fracture for material elements at front of a crack for fatigue crack propagation was extended to the fatigue crack propagation of a cracked metallic member reinforced with a composite patch in this paper. From static mechanics and linear elastic fracture mechanics, force transfer on a cracked member through a composite patch was analyzed and a formula connecting the stress intensity factor with crack length was obtained. Thereafter, a fracture model for fatigue crack propagation of a repaired cracked metallic member was proposed. A new expression for the fatigue crack propagation rate has thus been derived. The expression was verified objectively by the test data. It is in good agreement with the test results

Residual stress effects in tubular K-joints crack growth

Seeking light and transparent bridge designs, engineers and architects have found an efficient and artistic way to fulfill their requirements: steel tubular bridges. Like any other welded structure, the joints of this kind of bridge suffer from high tensile weld residual stresses. Combined with high stress concentrations, tensile residual stress is a relevant factor in fatigue crack development. Therefore, an experimental study has been carried out on tubular joints in order to characterize the 3D residual stress field, using mainly the Neutron Diffraction method. On the other side, two large-scale welded tubular truss beams were tested under constant amplitude fatigue loading. It is revealed that the peak tensile residual stress, exceeding the yield stress of the material, is localized at ~2 mm under the surface and allows cracks to develop in compressive joints. The influence of residual stresses on stable crack propagation is analyzed based on fracture mechanics theory

Study on crack propagation in tubular joints under compressive fatigue loadings

Large scale tubular truss beams, approximately of 9 m long and 2 m high, were tested under constant amplitude fatigue loading. The beams were made out of circular hollow sections of steel S355, welded to form a uni-planar truss with K-joints, in a shape common to bridge construction. The main goal of these tests was to focus on the fatigue behavior of the joints loaded in compression that is with chord in compression, one diagonal in compression and the remaining diagonal in tension. The tests showed, as in other studies, that fatigue cracks may develop in compressive stress field zones from applied loads due to the presence of large welding tensile residual stresses. An alternate current potential drop system was used to follow during the tests the crack development from the weld toes. This paper contains the results, presented in terms of number of cycles versus crack depth and crack growth rate, from the cracks that developed. In order to evaluate the level of residual stresses near the weld toes, both measurements by hole-drilling and neutron diffraction were carried out. This paper includes the first results of the residual stress field, more measurements being in progress. Among the interesting results, it was observed that crack initiation and growth occurred first from hot spot 1c (weld toe in-between the braces, compression brace side) until the crack reached about 2 mm deep. Then, the crack growth decreased and, simultaneously, a crack started to develop from hot spot 1 (tension brace side). Both crack continued to grow, the failure being triggered by a fatigue crack in a joint on the tension chord. Characteristics of the different cracks (shape, angle of propagation, etc.) are given in the paper. The S-N results are compared with previously obtained results on beams with different tube dimensions

Round robin on local stress evaluation for fatigue by various FEM software

The objective of this Round Robin is to assess the software influence on the structural/local stress values as determined by means of various methods. In particular the influence of (a) FEM software solver, (b) potentially software-depending element characteristics (e.g. exact shape functions) and (c) software-depending post processing procedures (e.g. extrapolation from integration points and stress averaging methods. Two different typical problems are studied, one 2D geometry and one 3D geometry, using FE models with various element types and varying mesh finesse. For each of the models, the participants receive files with node coordinates and element numbering, in order to eliminate variations in mesh geometry. Results calculated by the Round Robin participants using various software are reported for structural/local stress values defined according to the following methods: hot-spot, Xiao and Yamada, Dong, Haibach, critical distance and equivalent notch stress. Eventually, these two problems could be proposed as benchmark examples for use by engineers how are not familiar with these methods in the validation of FE modeling procedures aiming at structural/local stress evaluatio

A probabilistic assessment of the effect of post-weld treatment on the fatigue performance of tubular truss bridges

In the design of tubular truss bridges, engineers have found the fatigue performance of the joints to be a critical aspect. In looking for ways to improve this performance, the use of residual stress-based post-weld treatments has been suggested. Although these treatments have been shown to increase the average fatigue lives of welded details under constant amplitude loading conditions in a number of studies, concerns exist regarding their reliability, in particular under realistic, variable amplitude loading conditions. With this in mind, the effect of post-weld treatment on the fatigue performance of tubular truss bridges is assessed herein using a previously developed probabilistic, fracture mechanics-based model, modified to analyze entire bridge structures under realistic, variable amplitude loading conditions. This model uses a systems reliability approach to consider the influences of the various potential crack sites on the overall reliability of the bridge. By analyzing several variants of a typical tubular truss bridge, it is shown that post-weld treatment can result in a significant fatigue performance improvement for this bridge type. This improvement is quantified herein in terms of either a savings in steel weight or an increase in fatigue life. Several additional studies examine the sensitivity of the results of this assessment to variations in the treatment coverage, intensity, and uniformity. These studies show that similar results can be obtained with a partial treatment strategy to those observed when the entire bridge is treated, and that the treatment benefit depends more on the intensity than the uniformity of the treatment