Ductile failure of X100 pipeline steel- Experiments and fractography.

The growing energy demand in different areas of the world and the distance between gas reservoirs and consumers has increased the need to transport gas from far away regions to the final market. Mainly this transportation is done by pipelines. This demand has moved the pipeline industry to optimizing the methods in transportation. In several occurrences a High Pressure transportation approach joined with the use of High Strength Steel (HSS) pipeline appears as the most convenient way to combine technical feasibility, cost effectiveness and reliability over the whole pipeline lifetime span [1]. Crossing harsh and very often ‘‘unknown or unsurveyed’’ areas involves complex hazards to be faced or anticipated. Depending on the area of application, significant strain demand due to different types of ground movement (seismic wave propagation, lateral spread, landslide and subsidence caused by liquefaction and permanent ground movement), reflects specific requirements for line pipe material in terms of strain resistance capability [1,2]. The other main problem of pipelines is corrosion which causes different types of material loss in the body of pipe and in these cases the remaining load carrying capacity of the pipe is of crucial importance [3–7]. X100 (also known as Grade 690) steel is one of the recently developed materials for production of gas transportation pipelines according to the above mentioned needs. This material is micro alloyed steel with high strength and toughness that have a comparatively high yield stress of 690 MPa and failure strain in simple tension of 142% in longitudinal direction. Moreover it shows a severe orthotropy in plasticity. According to the above mentioned problems, it is really important to characterize the pipeline material thoroughly and have a good estimation of the load carrying capacity and of the amount of deformation the pipe material can undergo before failure in different loading condition. Different failure analysis studies are done on the failure of previous grades of pipelines [8–11], as some recent samples, but few studies are done on the material characterization and failure prediction of X100 grade. Researches by Hashemi et al. [12,13], Tanguy et al. [14] and Liessem et al. [8] are from the main researches done on the characterization and failure of this grade. Two types of approaches have been developed for modelling fracture. The first one, referred to as the ‘‘global approach to fracture’’, is essentially based on linear/nonlinear fracture mechanics (LEFM/NLFM). In this methodology it is assumed that the fracture resistance can be measured in terms of a single parameter, such as KIC, JIC or CTOD. More recently global approaches incorporating a second parameter (T and Q stress) have been introduced [15,16]. This methodology is extremely useful and absolutely necessary, but it has also a number of limitations. The second approach is the so-called ‘‘local approach to fracture’’ (LAF) in which the modelling of fracture is based on local fracture criteria usually established from tests on volume elements, in particular notched specimens [17]. Unlike the ‘‘global approach to fracture’’, which makes the fracture resistance of a component mainly dependent on one or two global parameter, whatever the damage and deformation mechanisms of the specific material under study, the ‘‘local approach to fracture’’ emphasizes detailed experimental analysis of the considered materials and their specific damage mechanisms [18]. In this approach two groups of parameters are introduced; the first one is the constant parameters which sometimes is known as material parameters and the second one is varying parameters that is not constant and can vary from one geometry and loading condition to the other one. The main advantage of local approaches is that, the effect of material is embedded in the constant parameters which depend on the material and not on the geometry. Thus these approaches guarantee a better transferability from specimens to structures. They can still be used when only a small amount of the material is available which is the situation met with the standards applied in LEFM and NLFM [18]. The varying parameters can be calculated in every desired point of a specimen or structure from a finite element analysis. After this analysis it is easy to find the critical points and discuss about failure. As mentioned above, Failure strain of materials is highly variable from one geometry and loading condition to the other one. Mean stress is one of the known factors that affects the failure strain; the effect of this parameter is represented by triaxialty factor parameter which is the ratio of mean stress to equivalent stress [19–22]. Recently it is shown that Lode angle, which embeds the third invariant of deviatoric stress tensor, have an influence on failure strain [23–25]. Triaxiality factor and Lode angle are the variable parameters that change from one geometry and loading condition to the other ones. In this article in order to study the effect of anisotropy, triaxiality factor and Lode angle a series of experiments are conducted. In these tests the above mentioned parameters change and their effects on the failure are studied. Measurements of deformation in three dimensions are done to check the variation of elongation and lateral deformations during loading. Extensive fractography is done using pictures taken by SEM to study the effect of each of micromechanical mechanisms. Moreover, delamination has occurred in some of the specimens. This phenomenon is studied and the causes of delamination are explained in macro scale.