The Weldability of Duplex Stainless-Steel in Structural Components to Withstand Corrosive Marine Environments

There is still a considerable gap in the definition of the weldability of Duplex Stainless Steel (DSS). A lack of clarity that is explained by the standard specification of the maximum content of equivalent carbon that defines a “weldable” steel coupled with the fact that the alloying elements of DSS exceed this defined limit of weldability. In this paper, welding quality in an inert environment and in presence of chlorides is analyzed with the aim of defining optimum welding conditions of 2001, 2304, and 2205 DSS. The same procedure is followed for a hybrid weld between DSS 2205 and a low carbon mild steel, S275JR. As main output, this study defined the optimal welding conditions with tungsten inert gas without filler for each type of DSS weld that showed excellent anti-corrosion performance, with the exception of the DSS 2205-S275JR weld where widespread corrosion was observed. Additionally, this study established a relationship between the thermal input during welding and the content of alloying elements in defect-free joints. Furthermore, it demonstrated that an increase in ferrite content did not lead to a worse corrosion resistance, as expected after passivation.This research was funded by ACERINOX EUROPA through Center for the Development of Industrial Technology (CDTI) within the frame of IISIS project, grant number IPT-20111023

Corrosion Fatigue Numerical Model for Austenitic and Lean-Duplex Stainless-Steel Rebars Exposed to Marine Environments

Steel rebars of structures exposed to cyclic loadings and marine environments su_er an accelerated deterioration process by corrosion fatigue, causing catastrophic failure before service life ends. Hence, stainless steel rebars have been emerging as a way of mitigating pitting corrosion contribution to fatigue, despite the increased cost. The present study proposes a corrosion fatigue semiempirical model. Di_erent samples of rebars made of carbon steel, 304L austenitic (ASS), 316L ASS, 2205 duplex (DSS), 2304 lean duplex stainless steels (LDSS), and 2001 LDSS have been embedded in concrete and exposed to a tidal marine environment for 6 months. Corrosion rates of each steel rebar have been obtained from direct measurement and, considering rebar standard requirements for fatigue and fracture mechanics, an iterative numerical model has been developed to derive the cycles to failure for each stress range level. The model resulted in a corrosion pushing factor for each material, able to be used as an accelerating coe_cient for the Palmgren-Miner linear rule and as a performance indicator. Carbon steel showed the worst performance, while 2001 LDSS performed 1.5 times better with the best cost-performance ratio, and finally 2205 DSS performed 1.5 times better than 2001 LDSS.The authors would like to acknowledge funding support from Centre for Industrial Technological Development CDTI and ACERINOX EUROPA, that funded the materials and experiments through the grant of IISIS project: IPT-20111023 and UPV/EHU PPGA19/61 contract. Besides, they would also like to acknowledge the University of Akron, Fellowship Program FRC-207367, the IT1314-19 (Basque Government) and GIU19/029 (UPV/EHU) research groupsand the Laboratoire des ciencies de l’ingenieur appliquées, Fédération IPRA-EA4581, from the Université de Pau et Pays de l’Adour, for their support setting a cooperation framework for this research

Second-order fatigue of intrinsic mean stress under random loadings

A fatigue process due to random loading that is progressively damaging a certain structural detail will vary in the presence of mean stresses. The variations are already considered in crack propagation laws and by applying equivalent 0-mean stress ranges from the Palmgren–Miner linear rule. Nevertheless, if the mean stress is intrinsic, instead of a direct consequence of the random loading, other second-order effects will have to be taken into account. Those effects are cycle quasi-ordering, histogram variations, and apparent mean tension, which are identified and defined in this study and, finally, developed in a case study for demonstrative purposes

Fatigue of Structural Elements by Random Dynamic Actions in Aggressive Environment

La fatiga es un fenómeno que se da cuando una estructura o elemento estructural se ve sometido a acciones dinámicas que producen una variación de tensiones cíclica en el tiempo, provocando la aparición y propagación de grietas que, con el tiempo, alcanzan el tamaño crítico suficiente para causar el fallo del elemento o estructura a tensiones inferiores al límite elástico. Desde que la fatiga se empezó a estudiar por primera vez por el ingeniero Wöhler en el caso del ferrocarril en 1870, con estudios limitados a ciclos de amplitud constante y tensión media nula, ha habido contribuciones significativas al Estado del Arte, relativas a la consideración de la acumulación de daño en secuencias de carga cada vez más complejas, con ciclos de amplitudes diferentes, de varias tensiones medias, en distintos órdenes, mediante reglas lineales, no lineales, etc. En esta Tesis Doctoral se ha estudiado el Estado del Arte de la normativa de ingeniería estructural, constatando la obligatoriedad y necesidad de considerar el fenómeno de fatiga en las estructuras actuales más avanzadas, determinando los límites de aplicación y sencillez de los modelos aplicados actualmente y justificando así su importancia en la ingeniería de estructuras. Por otro lado, se ha hecho un estudio en profundidad de la literatura científica e ingenieril relativa a la dinámica de estructuras frente a acciones aleatorias, métodos de recuento de ciclos, relativos a la interacción de la corrosión con la fatiga, etc. Como resultado, se ha determinado que los modelos actuales carecen de medios sencillos y aplicables para considerar la tensión media, el desorden de los ciclos y cómo afecta el fenómeno de la corrosión en secuencias de carga dinámicas aleatorias. Por lo tanto, la adaptación sencilla de los métodos de cálculo de fatiga actuales para tener en cuenta estas variables se considera que constituye, sin lugar a dudas, un avance significativo del Estado del Arte. Como consecuencia, para dar ese avance significativo, se estudia y analiza la tensión media, el desorden de los ciclos y el efecto de la corrosión, desarrollando modelos teóricos para considerar esos efectos en el cálculo de la acumulación de daño por la Regla Lineal de Palmgren-Miner, la más utilizada en la actualidad, mediante una serie de coeficientes o factores de empuje y aplicándolos por separado a casos sencillos. Lo cual, además de constituir un avance significativo para la ciencia, será un avance práctico. Finalmente, se consideran y aplican todos los factores en conjunto en un caso particular de estudio para demostrar su aplicabilidad y el efecto que tendrían, constatándose que las predicciones de fatiga, sin tener en cuenta todos estos factores, conducen a previsiones de daño muy inferiores, pudiendo llegar al fallo inesperado, apartando el modelo del lado de la seguridad si se aplicara sin estos factores de empuje correctores.Fatigue is a phenomenon that occurs when a structure or structural element is subjected to dynamic loads causing a cyclic stress variation in time, this triggers the start and propagation of cracks that eventually reach the critical size enough to produce the failure of the element or structure at lesser stresses than the yield stress. Since the fatigue began to be studied for the first time by the engineer Wöhler in the case of the railway in 1870, with studies limited to cycles with constant stress amplitude and zero mean stress, significant contributions to the State-of-the-Art have been done, regarding the consideration of the damage accumulation in more and more complex load sequences, with cycles of different stress amplitudes, distinct mean stresses and various orders, by means of linear rules, non-linear rules, etc. The structural engineering normative has been studied in the State-of-the-Art of this PhD Thesis, noting the mandatory and necessary consideration of the fatigue phenomenon in the more advanced modern structures, determining the applicability limits and the simplicity of the models used today and justifying, therefore, its importance to structural engineering. Besides, a deep study of the scientific and engineering literature relative to the structural dynamics under random loading has been done, cycle counting methods, fatigue and corrosion interaction, etc. As a result, it has been determined that the current models present a lack of easy and applicable methods to consider the mean stress, the cycle disorder and the effect of corrosion phenomenon on dynamic random load sequences. Therefore, the simple adaptation of current fatigue calculation methodologies to take into account all these variables is considered that constitute, indeed, a significant advance of the State-of-the-Art. As a consequence, to do this significant advance, the mean stress is studied and analysed, and so on the cycle disorder and the corrosion effect, developing theoretical models to consider these effects in the fatigue damage accumulation calculation by the Palmgren-Miner Linear Rule, the most widely used today, by means of a number of pushing coefficients or factors, applying them separately to simple cases. This, besides constituting a significant advance for the science, will be a practical advance. Finally, all the factors are considered and applied together in a particular case study to demonstrate their applicability and the effect they would have, confirming that the predictions of fatigue, not considering all these factors, lead to much lower damage estimations that even could reach to unexpected failure, moving away the fatigue model from the safety side if applied without these correction factors push