Abstract: A significant factor that influences the creep life of AISI Type 316H austenitic stainless steel components such as headers, and tubes is the initial microstructure. These components typically have a comparable specified composition but different thermo-mechanical fabrication histories. The variations in composition within the nominal range result in initial microstructures which become increasingly divergent during ageing. In this paper we explore effect of these contributions on the long term service aged microstructure and discuss the resulting impact on the overall creep life of these components. The microstructure of specific regions has been characterised with a range of techniques, including high resolution transmission electron microscopy imaging and chemical analyses undertaken using a JEOL ARM instrument operating at 200 KeV fitted with an energy dispersive spectrometer. This provides a unique identification of the service aged precipitates and the distribution of alloying and impurity elements. The results are discussed with respect to the initiation of creep cavities and the associated creep damage accumulation in the context of lifetime assessment of these AISI Type 316H austenitic stainless steel boiler components.
Abstract: A new methodology that enables us to compute the arbitrary shaped 3D crack problems is studied. In the present method, it is possible to analyze the 3D crack problems without preparing mesh data as in ordinary boundary elements but with defining a sequence of nodal points representing the crack front and the internal nodal points that define a crack surface as well as a shape function used for determining unknown variables. The present method has special potential for analyzing a complicated 3D crack geometry which is generally difficult to treat in usual element based methods. In the present research, we apply mesh-free body force method to analyze the growth of 3D planar cracks. In concrete, a crack growth analysis for initially rectangular or elliptical crack existing in an infinite solid under uniform tensile stress perpendicular to the crack surface at infinity is demonstrated
Abstract: The aim of this paper is to understand the effects of the damage criteria modelling on the training phase (performed by means of Finite Element simulations) of an artificial neural network (ANN) enabled to locate impacts onto a CFRP laminate. The developed FE models have been also used to investigate the intra-laminar damage mode, which, among different ones, has the most effects on the residual strength of the panel.
Abstract: Low velocity impacts induce concurring failure phenomena in unidirectional fiber reinforced composites. Hence a refined methodology able to predict the different failure modes and their interaction is mandatory to correctly predict the damage onset and evolution. Indeed, intra-laminar damage and inter-laminar damage often take place concurrently, causing a significant strength reduction up to composite structure collapse. In this paper, a numerical study is proposed which, by means of non-linear explicit FEM analysis, aims to completely characterize the composite reinforced laminates damage under low velocity impacts by introducing a user defined material model in the FEM code ABAQUS. The proposed 3-D numerical investigation allowed to obtain an exhaustive insight on the different phases of the impact event considering the damage formation and evolution.
Abstract: The classical two-dimensional solutions of the theory of elasticity provide a framework of Linear Elastic Fracture Mechanics. However, these solutions, in fact, are approximations despite that the corresponding governing equations of the plane theories of elasticity are solved exactly. This paper aims to elucidate the main differences between the approximate (two-dimensional) and exact (three-dimensional) elastic solutions of crack problems. The latter demonstrates many interesting features, which cannot be analysed within the plane theories of elasticity. These features include the presence of scale effects of deterministic nature, the existence of new singular stress states and fracture modes. Furthermore, the deformation and stress fields near the tip of the crack is essentially three-dimensional and do not follow plane stress or plane strain simplifications. Moreover, in certain situations the two-dimensional solutions can provide misleading results; and several characteristic examples are outlined in this paper.
Abstract: This paper presents a methodology to carry out Reliability-Based Design Optimization (RBDO) in composite stiffened panels. The target is to maximize the reaction force that the panel can withstand before collapse, setting the shortening of failure as the probabilistic constraint. The design variables are the stacking sequence orientations of the composite plies while the random parameters are the elastic properties of the material. In order to predict the collapse load properly, post-buckling and progressive failure analyses are considered within the FE solver employed.
Abstract: Earthquakes are the most unpredictable damaging loads which can affect civil engineering structures. Due to insufficient separation distance between adjacent structures with different dynamic properties, structural collisions may occur during ground motions. Although the research on structural pounding has recently been much advanced, the studies have mainly been conducted for concrete structures. The aim of this paper is to show the results of experimental investigation, focused on dynamic behaviour of closely-separated three models of steel structures which have been subjected to damaging earthquake excitations. The study was performed using three models of steel towers with different dynamic parameters and various distances between the structures. The acceleration time histories of the Kobe and the Northridge earthquakes were applied as the seismic excitation. The unidirectional shaking table, located at the Gdansk University of Technology (Poland), was used in the experimental study. The results have confirmed that collisions may lead to the increase in the structural response, although they may also play a positive role, depending on the size of the separation gap between the structures.
Abstract: Shot peening is a method widely used to improve the fatigue strength of materials, through the creation of a compressive residual stress field (CRSF) in their surface layers. In the present research the gain in fatigue life of AISI 4340 steel, obtained by shot peening treatment, is evaluated under the three different hardnesses used in landing gear. Rotating bending fatigue tests and alternating tension tests were conducted and the CRSF was measured by x-ray tensometry prior and after interrupted fatigue tests. The evaluation of fatigue life after shot peening in relation to the relaxation of CRSF, of the crack initiation sites and surface roughness is done.
Abstract: The present study utilizes a state-of-the-art stochastic modeling with structural health monitoring (SHM) data derived from strain measurements, in order to assess the remaining useful life (RUL) online in composite materials under fatigue loading. Non-Homogenous Hidden Semi Markov model (NHHSMM) is a suitable candidate with a rich mathematical structure capable of describing the composite’s multi-state damage evolution in time. The proposed model uses as input SHM data in the form of strain measurements obtained from the Digital Image Correlation (DIC) technique to a coupon-level constant amplitude fatigue test campaign. The obtained from the stochastic model RUL estimations are compared with the actual RUL and the effectiveness of the prognosis is discussed.