Advanced Materials Research Vols. 891-892

Abstract: The consequences of surface finish and decarburization on the fatigue performance of cast and forged steel components in the railway industry is substantial, and means that fatigue cracking is an ongoing issue across the industry. Examples of loading spectra for coupler forces and track loads are presented, along with data from past investigations showing the severe penalty in terms of fatigue life caused by inadequate surface finish at critical locations of components under fatigue loads. Managers in railway industry need to understand the technical case for increased manufacturing requirements, as costs from operational losses may well have made the economic case for increased requirements more compelling. Various options for improving the surface finish at critical locations that are prone to fatigue are available, and should be explored to reduce the vulnerability of these components to failure via fatigue cracking.

Abstract: Two types of cast steel rail bogies have been affected by cracking and defects. The cast steel design was from a period when bogie loads were lower than the endurance limit. However, the bogies as manufactured included a large number of casting defects and 60 years of operations has resulted in many repairs being applied. Thus like all ageing structure, the original basis for safe operations may be invalid. Structural assurance instructions were developed based on the following work. A condition data survey was undertaken to obtain configurations and defects within the fleet. This was used to assist in identifying critical areas, types and shapes of defects. Two bogies were instrumented to obtain operational loads that were compared to modern standards to provide both a spectrum for future operations and loads for Finite Element Analysis (FEA), Fatigue and Damage Tolerance Assessment (DTA). Finite Element Models (FEM) were developed to provide internal bogie loads and stresses for use in the fatigue and DTA and identify additional critical locations not identified in the condition data survey. DTA were undertaken to determine maximum flaw sizes and to verify the safe period of operation; given the loads, usage and stress data of the previous tasks. Based on the work undertaken, instructions for continued management documenting acceptable flaw criteria, life limits, and ongoing inspections were provided.

Abstract: Repair of damaged rail surface by overlay welding is the common rail maintenance method. But the discontinuity in material between base and weld brings initiation of cracks and they causes a rail fracture. Unfortunately, such cracks are hard to detect on site because the weld boundary prevents the echo signals penetration by reflection. So estimation of the critical crack size (CCS) has been a critical issue in railroad industry to prevent a rail from sudden fracture. In this study, we calculated the critical size of crack which was initiated and propagated underneath of the overlay welded rail by applying linear elastic fracture mechanics. For this purpose, we measured the maximum load carrying capacities of cracked UIC60 by inverted 3 point bend tests and checked the feasibility of the finite element (FE) analysis procedure. We could find the correlation in crack size between the test and 3D FE analysis results and applied the proposed 3D FE analysis model to calculate the CCS of a rail. We calculated the stress intensity factors on cracked rail by increasing the size of crack until the rail broke. The CCS was calculated as around 30.0 mm under the normal railway service operating condition.

Abstract: An evaluation of the potential risk of fatigue damage at the rail underhead radius (UHR) due to the occurrence of a short duration tensile stress peak, as a wheel passes over, has been examined. The tensile stress peak is mainly due to the localised bending of the rail head-on-web and its magnitude is associated with the contact position, lateral and vertical forces and rail head wear (HW). The stresses at the underhead radius have been explored using the finite element method (FEM). The Dang Van (DV) criterion, implemented as a customised computer programme, was used to identify the fatigue damage at the UHR. Fatigue behaviour under heavy haul conditions was compared for heat-treated low alloy, euctectoid and hypereutectoid rail grades in order to predict allowable rail head wear limits.

Abstract: The General Aviation (GA) fleet includes about 150,000 airplanes that were certificated with no fatigue evaluation requirements. The average age of these airplanes is about 40 years, and many are high-time. To mitigate the aging effects on the GA fleet, a probabilistic damage tolerance analysis (PDTA) program has been developed. A PDTA approach also provides a mechanism whereby inspection and maintenance operations can be included into the simulation, thus providing engineers the opportunity to assess the benefits of maintenance actions. This paper describes the probabilistic methodology to be utilized in a computer software program (SMART|DT) that performs risk assessment of small airplanes employing NASGRO® or a user selected code as the crack growth engine. The methodology can assess a range of random variables, calculate the extreme value distribution (EVD) of maximum stress per flight from a general aviation (GA) spectrum, and generate a surrogate model for accurate and fast calculations of crack grow. The main objective is to develop a comprehensive probabilistic methodology such that engineers can conduct a risk assessment of GA structural issues in support of policy decisions.

Abstract: The probabilistic distribution of crack lengths at a critical location is a key input for risk assessment of structural failure of airframes due to fracture. This distribution depends on the initial surface condition of the structural component, the crack growth behaviour of the material, and the applied load history. MIL-STD-1530C suggests the use of a single master crack growth curve to derive the subsequent crack length distributions from the initial crack length distribution, without taking into consideration the well-recognized scatter in crack growth rates due to uncertainties in material properties. In this paper, we report the results of a study that aims to quantify the scatter in crack growth rates of 7075-T7351 aluminium alloy subjected to spectrum loading, and to assess its effect on risk assessment. About 80 middle-tension specimens were planned to be tested under nominally identical conditions, with the crack lengths recorded using crack cameras and direct current differential potential method. Based on the test data and the analyses, crack length distributions are developed for different times. These are then compared to the distributions derived from the master curve approach. This investigation attempts to quantitatively demonstrate the reasonableness, or otherwise, of using a single crack growth curve to derive the distributions of crack lengths for risk analyses, and the conditions under which the observations apply. The results reported here and those from other researchers show that this scatter in crack growth rates cause by variation in material properties cannot be appropriately addressed by the master crack growth curve approach for the purpose of risk analysis.

Abstract: The fully effective utilization of large aluminum forgings in aerospace structures has been hampered in the past by inadequate understanding of, and sometimes inaccurate representation of, bulk residual stresses and their impact on both design mechanical properties and structural performance. In recent years, significant advances in both computational and experimental methods have led to vastly improved characterization of residual stresses. As a result, new design approaches which require the extraction of residual stress effects from material property data and the formal inclusion of residual stresses in the design analysis, have been enabled. In particular, the impact of residual stresses on durability and damage tolerance can now be assessed, and more importantly, accounted for at the beginning of the design cycle.

Abstract: The all-metal web-core sandwich structure consists of two face plates stiffened by one-directional system of web plates. These web core sandwich structures are used in many structural applications such as ship hulls, offshore platforms, bridge decks, and industrial platforms. However, the stress variation caused by the service loadings can be a determinant factor for crack initiation and growth until early failure of the entire structure. This paper presents an experimental study on fatigue crack growth rate in base material from a face plate after rolling and welding. The study is focused on the analysis of the stress ratio and crack closure effect on the fatigue crack growth rate in two directions. There is a significant stress ratio effect on fatigue crack growth rate, much more pronounced in the case of crack propagation in the longitudinal direction than in the transverse propagation. For all tests, the crack closure effect is more pronounced at low stress intensity factor range (in the threshold domain).

Abstract: Cracking in steel road or rail bridges is a major concern for continued safe operation of the bridge. Traditional crack arrest hole (CAH) or drill stops placed at the end of the detectable crack have been shown to be ineffective in stopping cracks. A method adapted from the aerospace industry, which utilizes a high interference fit bushing expanded into the CAH, has been tested in large A36 steel coupons under typical cyclic bridge loading. Expansion of the bushing into the hole yields the surrounding steel and induces a residual compressive stress around the hole to shield it from the cyclic load and prevent further crack growth. Not only did this method completely arrest the crack with a life improvement factor exceeding 60:1 over the conventional CAH but it also increased the load factor capability of the coupon by over 20%. This paper will describe the methodology, present the results of the test program and show how this innovative method dramatically increases the effectiveness of a typical CAH that could forestall major structural failure.

Abstract: In order to assure structural integrity for mechanical structures, it is indispensable to estimate three-dimensional residual stresses quantitatively to asses a crack growth rate of an observed crack. Now, the neutron diffraction method and the DHD (Deep Hole Drilling) method have been proposed to evaluate stress gradient in the thickness direction. However, estimated stresses by these methods can not be input to the FEM (Finite Element Method) model that has been widely used at design time for the assessment of the structural integrity. Then, the eigen-strain method has been proposed. In this method, three-dimensional residual stresses are calculated by an elastic FEM analysis from eigen-strains those can be evaluated quantitatively by an inverse analysis from released strains measured by strain gauges while the geometric boundary condition or material properties of the object has been changed. However, inelastic strains are newly created on the machined surface, the estimation accuracy of this method becomes relatively poor because the eigen-strains before and after measurements have to be the same. In this study, a calculation technique to evaluate not only initial eigen-strains but also processing strains is shown, and effectiveness of this method is demonstrated numerically in the bead flush method based on the eigen-strain method. Although estimation accuracy of processing strains was poorer, three-dimensional residual stresses for whole region could be evaluated accurately from measured strains without measurement errors.