Alfredo S. Ribeiro

Low and High Cycle Fatigue and Cyclic Elasto-Plastic Behavior of the P355NL1 Steel

A normalized fine grain carbon low alloy steel, P355NL1 (TStE355), intended for service in welded pressure vessels, where notch toughness is of high importance, has been investigated. Low and high cycle fatigue tests have been conducted on several series of smooth specimens under both strain and stress control. The monotonic and cyclic elasto-plastic behavior of the material is characterized and described using relations available in the literature. The shape of hysteresis loops are conveniently modeled, taking into account the observed non Masing behavior of the steel. Some important cyclic phenomena, observed for the studied steel, such as the cyclic creep and the cyclic stress relaxation, are illustrated. Strain, stress, and energy based relations for fatigue life prediction until crack initiation, are evaluated based on experimental results. The adequacy of several rules for damage accumulation is also investigated. Finally, along the paper, some comparisons are performed between the cyclic elasto-plastic and fatigue behaviors of the steels P355NL1 and ASTM A516 Gr. 70.

Finite Element Modeling of Fatigue Damage Using a Continuum Damage Mechanics Approach

In this paper, a fatigue model formulated in the framework of the continuum damage mechanics (CDM) is presented. The model is based on an explicit definition of fatigue damage and introduces a kinematic damage differential equation formulated directly as a function of the number of cycles and the stress cycle parameters. The model is initially presented for uniaxial problems, which facilitates the identification of its constants. An extension of the fatigue model to multiaxial problems is also proposed. This model was implemented in a nonlinear finite element code in conjunction with a constitutive model for cyclic plasticity. The cyclic plasticity model considered is based on a J2-plasticity theory with nonlinear isotropic and kinematic hardenings. In order to enhance the description of the cyclic elastoplastic behavior, the superposition of several nonlinear kinematic hardening variables is suggested. Both fatigue and plasticity models are identified for the P355NL1 (TStE355) steel. Finally, the numerical model is used to predict the fatigue crack initiation for a welded nozzle-to-plate connection, made of P355NL1 steel, and results are compared with experimental fatigue data.

Fatigue Damage Behavior of a Structural Component Made of P355NL1 Steel Under Block Loading

The common design practice of pressure vessels subjected to variable amplitude loading is based on the application of a linear damage summation rule, also known as the Palmgren-Miners rule. Even though damage induced by small stress cycles, below the fatigue limit, are often taken into account in design codes of practice by two-slope stress-life curves, the sequential effects of the load history have been neglected. Several studies have shown that linear damage summation rules can predict conservative as well as nonconservative lives depending on the loading sequence. This paper presents experimental results about the fatigue damage accumulation behavior of a structural component made of P355NL1 steel, which is a material usually applied for pressure vessel purposes. The structural component is a rectangular double notched plate, which was subjected to block loading. Each block is characterized by constant remote stress amplitude. Two-block sequences were applied for various combinations of remote stress ranges. Three stress ratios were considered, namely, R =0, R =0.15, and R =0.3. Also, constant amplitude fatigue data are generated for the investigated structural component. In general, the block loading illustrates that the fatigue damage evolves nonlinearly with the number of load cycles and is a function of the load sequence, stress levels, and stress ratios. In particular, a clear load sequence effect is verified for the two-block loading, with null stress ratio. For the other (higher) stress ratios, the load sequence effect is almost negligible; however the damage evolution still is nonlinear. This suggests an important effect of the stress ratio on fatigue damage accumulation.

Cyclic and Fatigue Behavior of the P355NL1 Steel Under Block Loading

Current fatigue analyses of metallic structures undergoing variable amplitude loading, including pressure vessels, are mostly based on linear cumulative damage concepts, as proposed by Palmgren and Miner. This type of analysis neglects any sequential effects of the loading history. Several studies have shown that linear cumulative damage theories can produce inconsistent fatigue life predictions. In this paper, both fatigue damage accumulation and cyclic elastoplastic behaviors of the P355NL1 steel are characterized using block loading fatigue tests. The loading is composed of blocks of constant strain-controlled amplitudes, applied according to two and multiple alternate blocks sequences. Also, loading composed by blocks of variable strain-controlled amplitudes are investigated. The block loading illustrates that fatigue damage evolves nonlinearly with the number of load cycles, as a function of the block strain amplitudes. These observations suggest a nonlinear damage accumulation rule with load sequential effects for the P355NL1 steel. However, the damage accumulation nonlinearity and load sequential effects are more evident for the two block loading rather than for multiple alternate block sequences, which suggests that the linear Palmgren-Miner rule tends to produce better results for more irregular loading histories. Some phenomenological interpretations for the observed trends are discussed under a fracture mechanics framework.

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

Cyclic Elastoplastic Analysis of Structures Concerning a Fatigue Assessment According to the Local Strain Approach: An Overview

This Technical Brief presents an overview about the current methodologies for elastoplastic calculation of structures undergoing cyclic loading, aiming a fatigue assessment according to the local strain approaches. The possibilities and limitations of the plasticity models available in most popular commercial finite element codes are discussed. Also, several methodologies for structural elastoplastic analysis, concerning the derivation of the stabilized material response, are also addressed highlighting their potentialities and drawbacks. The discussion proposed in this Technical Brief is intended as guidance for design engineers needing to ponder all calculation alternatives for assessing fatigue damage parameters before a rational decision is carried out, in the framework of local approaches to fatigue.

Fatigue Modeling of a Notched Flat Plate Under Variable Amplitude Loading Supported by Elastoplastic Finite Element Method Analyses

Although intensive research has been carried out to understand the fatigue behavior of steel notched components, under variable amplitude loading, no definite and general robust models have been derived so far. Therefore, every effort to augment the knowledge in this topic is welcomed. Within this context, existing variable amplitude data, derived by the authors for a notched low carbon pressure vessel steel (P355NL1) flat plate, is used to assess a local approach to fatigue. A linear damage summation framework, supported by elastoplastic finite element analyses, is used. Several variable amplitude loadings were selected and analyzed, using alternative configurations of kinematic hardening plasticity models (e.g., Chaboche’s model with distinct constants superposition). The predictions are assessed using available experimental data and data derived with simplified empirical elastoplastic tools. This paper highlights the difficulties of performing such elastoplastic analysis and compares the obtained results with those obtained using more classical tools for elastoplastic analysis (Glinka and Seeger–Heuler). It was found that fatigue predictions based on an elastoplastic finite element analysis, made using the Chaboche’s model, were significantly more accurate than predictions based on simplified elastoplastic analysis. These results have important practical relevance.

A Critical Analysis of the Plasticity Correction Factors Proposed in the EN13445 Standard for Fatigue Analysis of Unwelded Material

This paper presents a preliminary investigation about the plasticity correction factor, Ke , proposed in the Part 3–Clause 18 of the EN13445 standard, for correction of the elastic stress ranges exceeding twice the yield stress, resulting from mechanical loading. The plasticity correction factors are re-analyzed using a calculation strategy based on a comparison between results from fully-elastic and elastoplastic analyses. Several materials, elastoplastic models and geometries are considered in the study. The analyses revealed, for an important number of the situations, non-conservative results for Ke calculated according to the standard, which suggests the need for improvements in the standard procedures. However, more intensive research will be carried out to support these preliminary results. Nevertheless, authors recommend that a greater emphasis should be dedicated in the standard to the elastoplastic analysis, since the fatigue resistance data presented in the standard is strain-life type data. The elastoplastic analysis should be the preferred approach for evaluation of the strains/stresses for fatigue analysis.Copyright

Strain-based approach for fatigue crack propagation simulation of the 6061-T651 aluminium alloy

Fatigue crack growth models based on elastic-plastic stress-strain histories, at the crack tip vicinity, and strain-life damage models have been proposed. The UniGrow model is a particular case of fatigue crack propagation models. The residual stresses developed at the crack tip play a central role in these models, since they are used to assess the actual fatigue crack driving force, taking into account mean stress and loading sequential effects. The performance of the UniGrow model is assessed based on available experimental constant amplitude crack propagation data, derived for the 6061-T651 aluminium alloy. Key issues in fatigue crack growth prediction, using the UniGrow model, in particular the residual stresses evolution, are discussed. Using available strain-life data, it was possible to model the fatigue crack propagation behaviour for the AA6061-T651, taking into account the stress R-ratio effects. A satisfactory agreement was found, between the predictions and the experimental crack propagation data.

Fatigue Modelling of a Notched Geometry Under Variable Amplitude Loading Supported on Elastoplastic FEM Analyses

Despite intensive research has been carried out to understand the fatigue behavior of steel notched geometries, under variable amplitude loading, no definite and general robust models have been derived so far. Therefore, any effort to increment the knowledge in the topic is welcome. Within this premise, it is proposed an assessment of existing variable amplitude data, which has been derived by authors for a notched geometry, made from a low carbon pressure vessel steel (P355NL1), within the local approaches and linear damage summation framework, and supported by elastoplastic finite element analyses. Several variable amplitude loading are selected and analyzed using alternative configurations of kinematic hardening plasticity models (e.g. Chaboche’s model with distinct constants superposition). The predictions are assessed using available experimental data as well as with predictions made with simplified empiric elastoplastic tools. This paper highlights the difficulties on performing such elastoplastic analysis and compares the obtained results with those obtained using more classical tools for elastoplastic analysis. Fatigue predictions based on elastoplastic analysis made using the Chaboche’s model, with a finite element model, were significantly more accurate than predictions based on simplified elastoplastic analysis. The proposed information has important practical relevance.Copyright