S. Cicero

INCEFA-PLUS (INCREASING SAFETY IN NUCLEAR POWER PLANTS BY COVERING GAPS IN ENVIRONMENTAL FATIGUE ASSESSMENT)

Este proyecto ha recibido financiacion del programa de investigacion y formacion de Euratom 2014-2018 bajo acuerdo de subvencion No 662320. Tambien se reconocen las contribuciones significativas de todos los socios del proyecto INCEFA-PLUS.

On the application of the theory of critical distances to the structural integrity assessment of stress risers

Purpose – The purpose of this paper is to present and validate a methodology for the structural integrity assessment of components containing a variety of stress risers and subjected to static conditions.Design/methodology/approach – The methodology is based on the use of the apparent fracture toughness prediction provided by the theory of critical distances (in this case, the line method), together with a well‐known, widely‐used engineering tool in structural integrity assessments: failure assessment diagrams. In order to validate the proposed methodology, an experimental programme has been conducted, testing 38 specimens made of aluminium alloy Al7075‐T651, each of them containing a certain stress riser. The comparison between the experimental results and the corresponding predictions provided by the proposed assessment methodology has also allowed the situations for which the theory of critical distances provides accurate predictions to be defined.Findings – The results show that the methodology provid...

Analysis of Notch Effect in Fracture Micromechanisms

This paper presents an analysis of the notch effect in fracture micromechanisms. To this end, experimental results obtained in notched specimens are presented, together with the corresponding stress field at fracture and the SEM fractographies. The specimens comprise three materials (structural steel S275JR, high-strength aluminum alloy Al7075-T651 and Polymethyl methacrylate-PMMA) and notch radii varying from 0 mm (cracks) up to 2.5 mm. The results show how the stress relaxation caused by the notch effect is accompanied by a progressive change in the fracture mechanisms, from basically brittle ones in cracked conditions (for the three materials analyzed) to non-linear mechanisms observed for high notch radii, which explain the increase caused by the notch effect in both the load bearing capacity and the apparent fracture toughness. Also the concept of critical radius, that one below which the notch effect is negligible, is justified by SEM observations.Copyright

Effect of Cutting Method on Fatigue Crack Initiation and Fatigue Life of Structural Steel S355M

This paper analyses the effect of the cutting method on both the fatigue crack initiation and the fatigue life of steel S355M. The research covers three cutting methods (plasma, laser and oxy-fuel) and two specimen geometries: plain specimens with rectangular section and cut edges, and specimens with machined edges and a cut hole in the middle section. All the specimens were conducted to failure in a resonance machine by applying fatigue cycles, the stress ratio (R) being 0.1, and the corresponding S-N curves were obtained for each combination of cutting method and specimen geometry. The crack initiation time was estimated by analyzing the evolution of the resonance frequency on each specimen. The results show a significant influence of the cutting method on both the crack initiation and the fatigue life of this particular steel.Copyright

Analysing the Notch Effect Within the Ductile-to-Brittle Transition Zone of S275JR Steel

This paper analyses the notch effect in ferritic-pearlitic steel S275JR in a range of temperatures within the material Ductile-to-Brittle Transition Zone (DBTZ). The notch effect is evaluated in terms of load-bearing capacity, apparent fracture toughness (modeled here using the Theory of Critical Distances) and fracture micromechanisms. The concept of Master Curve in notched conditions is also presented.To this end, experimental results obtained in S275JR notched specimens are presented, together with Scanning Electron Microscopy (SEM) fractographies. The analysis is performed at −50 °C, −30 °C and −10 °C, the material Transition Temperature (T0) being −26.1 °C, with the notch radii ranging from 0 mm (crack-type defects) up to 2.0 mm.The results show how the lower the temperature the larger the notch effect, and also that the evolution of both the load bearing capacity and the apparent fracture toughness is directly related to the evolution of fracture micromechanisms. Moreover, the proposed Master Curve in notched conditions has provided good predictions of the experimental results.Copyright

Estimation of Fracture Toughness by Testing Notched Fracture Specimens and Applying the Theory of Critical Distances

This paper applies a methodology that allows the fracture toughness of a given material to be estimated by testing notched fracture toughness specimens and applying the Theory of Critical Distances, which requires the elastic stress field at the notch tip to be determined by finite elements simulation. This methodology, which is not intended to substitute any standardised fracture characterisation procedure, constitutes an alternative in those situations where pre-cracking processes may be too time-consuming, too expensive or, simply, cannot be performed. It comprises testing two notched specimens with different notch radii, defining the corresponding stress fields at fracture by using finite elements analysis, and applying the Theory of Critical Distances in order to calibrate the material’s critical distance and to apply the corresponding apparent fracture toughness formulation. The methodology has been applied to two different materials, PMMA and Al7075-T651, and the results have proven that, as long as the Theory of Critical Distances has been applied within its validity range, the fracture toughness estimations are highly accurate.

Fracture Loads Prediction on Notched Short Glass Fibre Reinforced Polyamide 6 Using the Strain Energy Density

This paper provides an energetic approach useful for the prediction of critical loads on U-notched components without an ideally linear elastic behaviour. The methodology has been applied to 100 fracture specimens of short glass fibre reinforced polyamide 6 (SGFR-PA6), combining four different fibre contents (5, 10, 30 and 50 wt %) and five different notch radii (0.00, 0.25, 0.50, 1.00 and 2.00 mm). The proposal combines the application of the strain energy density criterion with the use of the whole absorbed energy in the tensile test (elastic-plastic area under the stress-strain curve). With all of this, the fracture loads have been well estimated in this type of material.

Fracture Load Predictions in Short Glass Fiber Reinforced Polyamide 6 U-Notched Specimens Combining the Equivalent Material Concept and the Theory of Critical Distances

This article provides the prediction of fracture loads in single edge notched bending (SENB) specimens made of short glass fiber reinforced polyamide 6 (SGFR-PA6) and containing U-notches. The predictions are obtained through the combination of the equivalent material concept and the theory of critical distances (TCD). The latter is based on the material critical distance (L) and has a linear-elastic nature. This implies that in those materials exhibiting non-fully linear-elastic behavior, the determination of the material critical distance requires a calibration process that may be performed by fracture testing on notched specimens or through a combination of fracture testing and finite elements simulation. This represents a significant barrier for the application of the TCD on an industrial level. The proposed methodology defines an equivalent linear-elastic material on which the TCD may be applied through its basic formulation and without any previous calibration of the corresponding critical distance. It is applied to SGFR-PA6 SENB specimens, providing accurate predictions of the experimental fracture loads.

Proposal of AASHTO Fatigue Detail Categories for Structural Steels Containing Thermally Cut Edges and Cut Holes

AbstractThermal cutting is a common engineering practice that is used to obtain the final shape of structural components. The characteristics of the cut surface and the material transformations caused by the cutting method determine the corresponding fatigue behavior. However, design codes, including the AASHTO LRFD bridge-design specifications, provide fatigue design curves for a limited number of situations. In the case of cut edges, only flame cutting is covered, whereas plasma and laser cutting are not associated to any category. In the case of cut holes, AASHTO specifications only consider the fatigue performance of drilled and punched holes, limiting the use of thermal cutting processes to, for example, produce bolt holes. All this restricts the use of thermal cut technologies in many applications. This paper analyzes the effects of flame, plasma and laser cutting on the fatigue behavior of cut edges and cut holes performed on structural steels. With this aim, an experimental program composed of 300...

A Basic Procedure for the Integrity Assessment of Structural Steels Containing Notches

This paper presents a basic procedure for the integrity assessment of structural steels containing notches. It is based on the work developed by the authors in the last five years analyzing the notch effect in structural steels, with the Theory of Critical Distances as the main theoretical framework. The procedure combines the notch effect corrections provided by this theory with a basic Failure Assessment Diagram, and has been successfully validated through its application to 394 fracture tests performed on 4 different steels working at different temperatures.Copyright