hydrogen embrittlement in advanced high strength steels and ultra high strength steels a new investigation approach

Abstract In order to reduce CO2 emissions and fuel consumption, and to respect current environmental norms, the reduction of vehicles weight is a primary target of the automotive industry. Advanced High Strength Steels (AHSS) and Ultra High Strength Steel (UHSS), which present excellent mechanical properties, are consequently increasingly used in vehicle manufacturing. The increased strength to mass ratio compensates the higher cost per kg, and AHSS and UHSS are proving to be cost-effective solutions for the body-in-white of mass market products. In particular, aluminized boron steel can be formed in complex shapes with press hardening processes, acquiring high strength without distortion, and increasing protection from crashes. On the other hand, its characteristic martensitic microstructure is sensitive to hydrogen delayed fracture phenomena and, at the same time, the dew point in the furnace can produce hydrogen consequently to the high temperature reaction between water and aluminum. The high temperature also promotes hydrogen diffusion through the metal lattice under the aluminum-silicon coating, thus increasing the diffusible hydrogen content. However, after cooling, the coating acts as a strong barrier preventing the hydrogen from going out of the microstructure. This increases the probability of delayed fracture. As this failure brings to the rejection of the component during production, or, even worse, to the failure in its operation, diffusible hydrogen absorbed in the component needs to be monitored during the production process. For fast and simple measurements of the response to diffusible hydrogen of aluminized boron steel, one of the HELIOS innovative instruments was used, HELIOS II. Unlike the Devanathan cell that is based on a double electrochemical cell, HELIOS II is based on a single cell coupled with a solid-state sensor. The instrument is able to give an immediate measure of diffusible hydrogen content in sheet steels, semi-products or products, avoiding time-consuming specimen palladium coating with a guided procedure that requires virtually zero training. Two examples of diffusible hydrogen analyses are given for Usibor®1500-AS, one before hot stamping/ quenching, and one after hot stamping, suggesting that the increase in the number of dislocations during hot stamping could be the main responsible for the lower apparent diffusivity of hydrogen

Design of a non-destructive test for validating models of hydrogen migration.

High-strength steels, despite their excellent mechanical properties in normal conditions, can be susceptible to hydrogen embrittlement. Due to the service loads or residual stresses, hydrogen migrates within the component and accumulates in the regions where the highest tensile hydrostatic stress occurs. As a consequence, component brittle failure can occur even if the initial or mean hydrogen concentration is lower than the critical value. The availability of models predicting the hydrogen diffusion within the component is a crucial task for the design. Several diffusive models have been presented in the literature and some general-purpose finite element codes have implemented some of them. However, the validation of those models is still an open issue due to the difficulty in performing accurate local measurements of the hydrogen concentration. This study deals with the design of a test potentially able to validate hydrogen migration models. In the test, a four-point bending configuration is applied to a properly shaped hourglass specimen, previously charged with hydrogen, extracted from thin high-strength steel sheets. The specimen geometry and the loading configuration were designed to obtain a central region in which the stress and strain field is uniform in plane and exhibits a quasi-uniform gradient in the thickness direction. As a consequence, it is expected a large enough central region of the specimen in which the Hydrogen can migrate only in the thickness direction during the typical duration of the test. The local hydrogen concentration is evaluated by measuring the flux leaving the tensile surface of the specimen by a solid-state hydrogen sensor

First-order correction to counter the effect of eccentricity on the hole-drilling integral method with strain-gage rosettes:

The offset between the hole and the centre of the strain-gage rosette is unavoidable, although usually small, in the hole-drilling technique for residual stress evaluation. In this article, we revi..

EFFECT OF WIND LOADS ON NON REGULARLY SHAPED HIGH-RISE BUILDINGS

Wind loads have historically been recognized as one of the most important issue in high-rise buildings analysis and design. In particular, in regions of low seismic intensity, a high-rise building lateral design is controlled by wind loads. In wind analysis, Computational Fluid Dynamics (CFD) and/or wind tunnel testing are required to calculate the external pressures acting on a building. In this paper, two case studies are presented to show how the wind loads are calculated and applied in design. The first case study is based on the CFD results for the New Marina Casablanca Tower in Casablanca, Morocco. The second case study considers the results from the wind tunnel test studies conducted for the Al- Hamra tower, in Kuwait City, Kuwait. The New Marina Casablanca tower is a 167m tall concrete building, with a unique twisting shape generated from the relative rotation of two adjacent floors. Sloped columns are introduced in the perimeter to follow the tower outer geometry and to support the concrete slabs spanning between the central core and the perimeter frame. The effects of wind loads on the twisted geometry has been studied in details since the pressure coefficients are not easily identified for such a complex form. In addition, the effect of the wind loads on the structure presented unique challenges that required innovative structural solutions. The Al-Hamra tower is a 412m tall concrete building with a sculpted twisting form which optimizes the views to the Arabian Gulf while minimizing the solar heat gain. The complex form is realized using sloped walls and vertical columns on the perimeter and a central concrete core. The unique shape of the tower presented several design challenges related to the wind loads on the structure. This paper will discuss the unique challenges and solutions associated with wind loads effect on buildings of unique form

Funzione peso per una fessura laterale inclinata e deflessa

In questo lavoro viene presentato un metodo per determinare i fattori di intensità degli sforzi (stress intensity factors SIF) per una fessura di bordo inclinata e deflessa in un semipiano. Il problema è stato affrontato mediante un’analisi parametrica ad elementi finiti che ha permesso di studiare l’influenza dei principali parametri che governano il problema: l’angolo d’inclinazione iniziale, l’angolo di deflessione del segmento terminale della fessura ed il rapporto tra le lunghezze dei due segmenti di fessura. Sulla base dei risultati numerici è stata definita una Funzione Peso (Weight Function WF) con struttura matriciale, estendendo una tecnica mista analitico-numerica già sviluppata per lo studio di fessure di bordo inclinate. La correttezza della WF è stata verificata mediante confronto con i risultati numerici ottenuti per condizioni di carico indipendenti. La WF può essere usata per calcolare i SIF per fessure aventi inclinazione iniziale compresa tra 0° e 60 °, angolo di deflessione rispetto al segmento iniziale tra -90° e + 90° e rapporto di lunghezza tra i segmenti iniziale e terminale compreso tra 0.005 e 0.1

Effetti sulla ripartizione dei carichi dovute alla configurazione dei satelliti in rotismi planetari

La conoscenza della ripartizione dei carichi sulle dentature è di cruciale importanza per la progettazione di rotismi planetari, in quanto permette di evitare che la trasmissione operi in condizioni non conformi alle specifiche, in cui possano avvenire rotture catastrofiche. La ripartizione del carico dipende sia dai parametri funzionali del rotismo (quali gioco dei cuscinetti, backlash, rigidezze dei componenti) sia dal numero di satelliti impiegati. Il presente lavoro mira ad investigare come il numero di satelliti possa influenzare la ripartizione dei carichi applicati a una trasmissione tipicamente impiegata in campo eolico e come questi effetti vengano amplificati oppure attenuati dalla variazione di parametri funzionali (quali il gioco dei cuscinetti, le rigidezze dei supporti e dell’ingranamento, ed il backlash) e dagli errori di montaggio. Lo studio è stato condotto impiegando un modello a parametri concentrati interamente parametrico che permette di identificare condizioni di malfunzionamento quali perdita di contatto e/o incuneamento oltre al carico agente su ciascun ingranamento in funzione dell’insieme dei suddetti parametri. Mappe di ripartizione del carico vengono proposte per rotismi aventi da tre a cinque satelliti.The knowledge of the distribution of loads on the teeth is of crucial importance for the design of planetary gearboxes, since it allows to avoid that the transmission operates in conditions that do not comply with the specifications, in which a catastrophic failure can take place. The load distribution depends both on the functional parameters of the gearing (such as bearing play, backlash, stiffness component) and on the number of planets. This paper aims to investigate how the number of planets will affect the distribution of loads applied to a transmission typically used in wind farms and how these effects are amplified or attenuated by the variation of operating parameters (such as bearing clearance, supports and tooth stiffness, and backlash) and the assembly errors. The study was conducted employing a lumped parameter model parametric spaces that allows to identify fault conditions such as loss of contact and/or wedging in addition to the load acting on each meshing on the basis of all of those parameters. Maps load distribution are proposed for gears having from three to five planets

Residual stress measurement and modeling by the initial strain distribution method: Part I - Theory

The paper presents a method for measuring and modeling residual stress fields. The method is based on the evaluation of a hydrostatic initial strain distribution. which is assumed to be the cause of the residual stresses. The initial strain distribution can be used, within a numerical (e.g., finite element) model, to evaluate the complete residual stress distribution of the body. For estimating the initial strain distribution a hybrid (experimental plus numerical) technique is proposed, which is based on measuring the strain relaxed by cutting the body in incremental steps and on sensitivity coefficients obtained by properly elaborating numerical simulations. The numerical and experimental methodologies are presented along with a general discussion of the rational way to be applied in practice