Abílio M. P. de Jesus

Fatigue Behaviour of Welded Joints Made of 6061-T651 Aluminium Alloy

The purpose of this chapter is to present the main results of an investigation concerning the assessment of the fatigue behaviour of welded joints made of the 6061-T651 aluminium alloy. The 6061 aluminium alloy is one of the most common aluminium alloys for heavyduty structures requiring good corrosion resistance, truck and marine components, railroad cars, furniture, tank fittings, general structures, high pressure applications, wire products and pipelines. Many of these applications involves variable loading, which makes very relevant the study of the fatigue behaviour of this aluminium allow. In particular, the study of the fatigue behaviour of welded joints is of primordial importance since welds are intensively used for structural applications. The proposed investigation focuses in four types of welded joints, made from 12 mm thick aluminium plates, namely one butt welded joint and three types of fillet joints: T-fillet joint without load transfer, a load-carrying fillet cruciform joint and a longitudinal stiffener fillet joint. Traditionally, the fatigue assessment of welded joints, including those made of aluminium alloys, is based on the so-called S-N approach (Maddox, 1991). This approach, which is included in main structural design codes of practice, adopts a classification system for details, and proposes for each fatigue class an experimental-based S-N curve, which relates the applied stress range (e.g. nominal, structural, geometric) with the total fatigue life. Alternatively to this S-N approach, the Fracture Mechanics has been proposed to assess the fatigue life of the welded joints. It is very often claimed that welded joints have inherent crack-like defects introduced by the welding process itself. Therefore, the fatigue life of the welded joints may be regarded as a propagation process of those defects. A relation between the Fracture Mechanics and the S-N approaches is usually assumed. The slope of the S-N curves is generally understood to be equal to the exponent of the power relation governing the fatigue crack propagation rates of fatigue cracks. More recently, the local approaches to fatigue have gaining added interest in the analysis of welded joints (Radaj et al., 2009). In general, such approaches are based on a local damage definition (e.g. notch stresses or strains) which makes these approaches more adequate to model local damage such as the fatigue crack initiation. In this sense, the Fracture Mechanics can be used to complement the local approaches, since the first allows the computation of the number of cycles to propagate an initial crack until final failure of the component. The present research seeks to understand the significance of the fatigue crack initiation, evaluated using a local strain-life approach, on the total fatigue life estimation for four types

Mechanical characterization of ancient Portuguese riveted bridges steels

In repairing and retrofitting processes of ancient riveted steel bridges is crucial to assess the structural state of old metals to guaranty structural safety. Metals under long-time operations (mild, rimmed low carbon steels <0.1% C, puddle irons) and cyclic loading present a tendency for degradation processes. The case-studies of this work are five metallic bridges existing in Portugal (Luiz I, Eiffel, Fao, Pinhao and Trezoi). This work presents the study of some characteristics of materials extracted from the five case-studies, such as: monotonic tensile strength, chemical composition, microstructures, hardness, notch toughness and fatigue crack propagation. In terms of monotonic tensile tests, the materials from Luiz I, Eiffel and Fao bridges are similar to puddle steel and the materials from Pinhao and Trezoi bridges are similar to mild steel. In terms of toughness only the material from the Pinhao bridge exhibits acceptable toughness properties, considering current design requirements. The materials from the other bridges exhibit relatively low toughness properties. The fatigue crack propagation data from the old Portuguese riveted steel bridges were correlated using the Paris’s law and the possibility for a design crack growth rate was discussed.

Fatigue assessment of an existing steel bridge by finite element modelling and field measurements

The evaluation of fatigue life of structural details in metallic bridges is a major challenge for bridge engineers. A reliable and cost-effective approach is essential to ensure appropriate maintenance and management of these structures. Typically, local stresses predicted by a finite element model of the bridge are employed to assess the fatigue life of fatigue-prone details. This paper illustrates an approach for fatigue assessment based on measured data for a connection in an old bascule steel bridge located in Exeter (UK). A finite element model is first developed from the design information. The finite element model of the bridge is calibrated using measured responses from an ambient vibration test. The stress time histories are calculated through dynamic analysis of the updated finite element model. Stress cycles are computed through the rainflow counting algorithm, and the fatigue prone details are evaluated using the standard SN curves approach and the Miners rule. Results show that the proposed approach can estimate the fatigue damage of a fatigue prone detail in a structure using measured strain data.

Numerical analysis of a double shear standard bolted connection considering monotonic loadings

The behaviour of standard bolted steel connections submitted to monotonic loads, through the use of numerical models, is presented in the current paper. The bolted connections allow speed up constructive processes in an increasingly competitive and globalized world in which the costs are a decisive factor in the development of a project. The use of computational tools in the analysis of bolted connections becomes determinant, mainly for new solutions or solutions less explored in terms of design codes. Throughout the years, bolted connections have been suffering transformations resulting from research activities performed by many authors. Rivets have been replaced by bolts, the main achievement being the pre-stressed bolts. Methodologies based on finite element analyses were proposed for double shear bolted connection. The non-linear behaviour of these connections is investigated and their performances are compared. In the numerical modelling of the bolted connection, linear elastic and elastoplastic analyzes reveal that there are two slip levels associated with local non-linearities caused by the contact pairs, which vary with clamping stresses.

Ultra-Low-Cycle Fatigue Behaviour of Full-Scale Elbows

This paper presents an investigation concerning the ultra-low-cycle fatigue (ULCF) characterization of large-scale elbows produced from line pipes subjected to hot bending process. Two distinct pipes were used in this process: a 16” (w.t. 9.5 mm) X60 and an 8 5/8” (w.t. 5.59 mm) X65 pipes that were bent to 45 and 90° elbows (8 tests). Cyclic external loading was applied to the elbows, combined with internal pressure, until failure was observed. The failure was preceded by a local plastic instability (bucking) and resulted due to intense cyclic plastic deformation. In general, the number of cycles to failure was lower than 100 cycles which typifies this failure mechanism as ultra-low-cycle fatigue. Besides the full-size tests, the plain material was investigated under ULCF conditions using both smooth and notched specimens. The thermal process used in the hot bending manufacturing process was also accounted for in the material testing in order to understand the effect of this process on pipe material. Non-linear finite element models of the elbows were constructed to simulate the cyclic behaviour of the elbows using the actual loading histories applied to the elbows. Damage models (e.g. Coffin-Manson, Xue) identified using material test data are applied to simulate the failure cycles of the tested elbows. Besides the use of damage models available in the literature and identified with generated materials experimental data, current ASME VIII Div.2 procedures are also used to compute the failure cycles of the elbows to allow an assessment of these existing procedures.Copyright

Simulation of Monotonic Full-Scale Tests

This chapter presents the results of numerical simulations of monotonic large-scale tests described in Chap. 4, using constitutive models described in Chap. 6 and material properties identified with material (small-scale) test data obtained in Chap. 2.

Small-Scale Monotonic Test Data of Smooth and Notched Specimens

This chapter presents results of small-scale monotonic tests performed with smooth and notched specimens made of various pipeline steels, covering medium to high strength steels and isotropic and anisotropic materials. Tension/compression, torsion and bending loads are explored. The materials investigated cover the API X52, X60, X65, X70 and X80 steel grades.

Small-Scale Cyclic Test Data of Smooth and Notched Geometries

This chapter presents results of small-scale cyclic tests performed with smooth and notched specimens made of various pipeline steels. Both LCF and ULCF behaviours will be investigated. The generated experimental data is used latter in plasticity/damage models calibration/validation which will be later used in finite element simulation of large-scale piping components made of similar materials.

Simulation of Cyclic Full-Scale Tests

This chapter presents results of numerical simulations of cyclic large-scale tests described in Chap. 5, using various constitutive models including some described in Chap. 6 and material properties identified with material (small-scale) test data obtained in Chaps. 2 and 3.

Large-Scale Monotonic Tests of Piping Components

This chapter presents results of a testing programme of large-scale monotonic tests carried out on pipeline components manufactured with X52, X70 and X80 steel grades.