Procedia Structural Integrity最新文献

Among the energy-based approaches to estimate the fatigue life of steel specimens, the experimental method based on the heat dissipation (or intrinsic dissipation) per cycle, Q, proved effective for correlating the effects of geometrical stress concentrations, uniaxial and multiaxial loadings, and mean stress. The mean stress effect requires a properly defined temperature-corrected parameter Q. The parameter Q is readily evaluable using temperature measurements and in this investigation it has been employed for fatigue strength assessment of plain specimens, extracted from a 42CrMo4 Q&T connecting rod big end of a marine engine. Completely reversed, strain-controlled, constant amplitude fatigue tests were carried out and the Q parameter evolution was monitored during each test by suddenly stopping the fatigue test several times and measuring the cooling gradient of material temperature. As result, besides the traditional strain-life (εa-2Nf) curve, the Q-Nf curve was also obtained, which is expected to be applicable for correlating notch and mean stress effects in future investigations.

Bogies of rail vehicles for passenger coaches and traction units commonly contain air spring systems as secondary spring stages. In the development and design of spring stages, it is necessary to ensure precise knowledge about the material properties and fatigue behaviour of the air spring bellows.

The aim of this work is to systematically investigate the damage mechanisms evaluated at air spring bellows on sample level and to analyse the fatigue strength of the base material under different load conditions. The specially developed small-scale sample is biaxially loaded and different layups are examined at varying load levels. In the tests with different parameters, an increase in the mean value of the longitudinal displacement by 20 % has proved to act as a suitable failure criterion. In addition to the purely optical damage analysis, micro computed tomography analysis was carried out.

In this study, four layered samples with a fibre angle of ±15, ±25 and ±35 degrees in respect to the longitudinal direction are examined. A global evaluation of the service life tests reveals that under comparable load conditions, the fibre angle exhibits a clear influence on the fatigue strength. The increase of 10 degree in fibre angle roughly results in a 15 % reduction of the tolerable lateral displacement amplitude at a number of fifty thousand load-cycles which commonly acts as design lifetime. In a second step, a local analysis based on an analytical approach is presented. With the help of the fibre strain amplitude calculated, all fatigue test data points can be unified to a master S/N-curve leading to an elaborated design model of cord rubber composite materials used in air spring bellows of rail vehicles.

With the help of the presented methodology utilizing the developed representative small-scale sample testing procedure and evaluation approach, a time- and cost-efficient fatigue design is facilitated.

Finite Element Analysis (FEA) is widely used to perform fatigue calculations for geometric singularities at welded components. The analysis methodologies are described in design codes and recommended practices such as DNV, IIW and Eurocode. The focus in the present study is the application of hot spot stress methodology on a weld detail located at the cope hole in a pile sleeve connection of a jacket substructure. Finite element analysis is used to calculate the geometric stress where the influence factor (INF) technique has been implemented to calculate the hot spot stress at the weld location. The INF methodology is used as the preferred approach compared to the traditional nominal stress method due to its ability to capture the stress response in complex welded details. Generally, a mid-surface shell model excluding the weld is used to model the welded components in FE analysis and a stress extrapolation method is applied to calculate the hot spot stress at the fatigue critical location. Here a full solid model of the cope hole detail including the weld geometry has been used for fatigue calculation as benchmark to calibrate the weld modeling techniques using shell elements for the analyses. The results confirmed that the weld geometry and stiffness has a significant influence on the hot spot stress calculation at the considered cope hole. Thus, the weld geometry and stiffness must be included into the finite element model for an accurate fatigue damage calculation of such details. The calibrated results showed that the mid surface shell model can still be used if an appropriate weld stiffness is included in the finite element model.

The need for more environmentally sustainable ways of transportation and for a reduction in emissions and fuel consumption make lightweight structures essential. Together with the use of lightweight materials and design optimisation, the use of hybrid joints represents one way to reduce the weight of components and it is becoming increasingly popular in the transportation industry. The name ‘hybrid joint’ refers to a connection where adhesive bonding is used in conjunction with traditional joining techniques, such as spot welds and rivets with the aim of combining and exploiting the advantages of the individual joining techniques. To optimise the design of hybrid joints and minimise the risk of in-service fatigue failures, the transportation industry needs efficient, robust, and easy-to-use approaches for the modelling and fatigue life estimation of hybrid joints.

This work presents two practical methodologies for estimating the fatigue life of hybrid joints that can be easily adopted by companies in the transportation industry. The first methodology neglects the life given by the mechanical joints after failure of the adhesive (the joint is considered failed when the adhesive fails), while the second methodology considers the life of both the adhesive and the mechanical joints. In the first methodology, just one configuration would need to be analysed (‘hybrid’ joint or ‘purely bonded’ joint, if this simplification is considered reasonable). In the second methodology, instead, the analysis of two configurations would be required (the previous configuration followed by a configuration where only the mechanical fasteners are considered). The second methodology would produce more realistic fatigue life estimations compared to the first methodology, but it would be more onerous both in terms of modelling and computationally. For both methodologies, FE modelling guidelines to recover the required stresses are suggested. These guidelines require limited changes to the typical FE modelling strategies currently used, especially in the automotive industry. Furthermore, the proposed modelling guidelines provide FE models that are not computationally too onerous, reasonably mesh insensitive and that do not require congruent meshes. The relevant stresses recovered from the FE model are then used as an input into nCode DesignLife to estimate the fatigue life of the hybrid joints in the analysed structure. The fatigue life estimation is carried out using standard Stress-Life (SN) based nCode DesignLife analysis engines and bespoke SN curves obtained through testing of hybrid joint specimens, representative of the joints in the production parts.

This paper investigates the comparative study of performance of two-way polypropylene fiber reinforced flat slab strengthened with BFRP (Basalt fiber reinforced polymer) fan and laminate against concentric loading. Three slabs of same dimension (1000 x 1000 x 135mm) were casted. Two slabs were reinforced with an optimum percentage (say 0.3%) of polypropylene fiber and one slab is taken as the control specimen. One of the slabs are Strengthened with BFRP string (obtained from basalt fabrics) having a fan shape as a new strengthening technique. For strengthening 16 double strand strengtheners were used and strengthened from a critical distance for punching shear of d/2 from the column face. Another slab is strengthened with basalt strip in a radial pattern.

The result reveals that the suggested strengthening technique increased the load carrying capacity and enhances the ductile behavior of the flat slab in comparison with the unstrengthen slab. These strengthening methods are capable of enhancing both maximum loading capacity and reduce the formation of retraction crack thus avoids brittle failures that may occur under line loading. This method of strengthening improves the bearing capacity of the slab without increasing the size of the structural components.

Plasma Transfer Arc (PTA) process uses the intense heat of electric arc to melt and fuse the 75Ni13.5Cr2.7B-3.5Si hard-facing alloy and the base metal. This process develops substantial residual stresses near the hard-faced surfaces during deposition and subsequent solidification and cool down. Furthermore, when a material interface is present, additional residual stress is formed because of the thermal strain mismatch of the dissimilar materials caused by their different thermal expansion coefficients. These stresses can cause cracks in the overlay during the component’s service life or even earlier during manufacturing which can lead to partial or total loss of the part structural integrity. To start optimizing the process to avoid these defects, it is necessary to know the residual stress distribution in the part and how it is related to the process parameters. Hard-faced components are having distinct microstructures with a step change in material properties, and this makes the residual stress measurement more challenging. This paper presents 2D residual stress maps of the deposit cross sections for PTA hard-faced samples using the contour method. This study is part of an ongoing research on the influence of process parameters on the residual stress and local microstructure of 75Ni13.5Cr2.7B-3.5Si clad 316 stainless steel.

The present work deals with the characterization of ductile tearing resistance of 1 mm thick interstitial free steel sheet using crack tip opening angle (φ), and δ₅ concepts with some experimental modification. CTOA (φ) measurements on the specimen surface at the growing crack tip have been performed using light microscope on both pre-cracked DENT and SENT specimens at three ramp rates following the essence of ASTM E 2472. In order to determine the effect of pre-cracking, CTOA of the notched (ρ:0.1 mm) SENT specimen has also been measured at 10^-4 s^-1. The transferability of φ−Δα and δ₅−Δα between two geometries has been verified. It is concluded that for a specified thickness both φ−Δα and δ₅−Δα plots can be used for crack growth characterisation of sheet metals even without using pre-cracked specimens. However, the angle φ(δ₅) determined from δ₅−Δα curve is not the true measure of optical CTOA,φ as the δ₅ gauge position does not follow the tip position of the growing crack.

In order to mitigate the effect of global warming and climate change by reducing CO₂ emission, clean energy options are being explored. Hydrogen generation using renewable energy like solar and wind is one of the clean energy options being considered. Four pillars of hydrogen economy are hydrogen generation, storage, transportation and consumption. The overall life cycle cost of these technologies will depend on the endurance of the material of construction used. Hydrogen is known to cause embrittlement in steels and in hydride forming metals, which can lead to early failure of the components used in hydrogen economy. The overall life cycle cost of these technologies can be significantly reduced if the operating parameters are so chosen to avoid susceptibility to hydrogen/hydride embrittlement or use materials, which are resistant to hydrogen/hydride embrittlement. Hence, investigation of the hydrogen/hydride embrittlement of the materials used during the hydrogen production, storage and transportation has to be in sync with technologies related to hydrogen energy. Significant work has been reported on hydrogen/hydride embrittlement of structural materials such as high strength steels, Ti-alloys, Zr-alloys, Nb-alloys used in power and process industries. The knowhow of the hydrogen/hydride embrittlement mechanisms of these materials will be of immense help in understanding the hydrogen/hydride embrittlement of newer materials of construction used in hydrogen systems. The mechanisms of hydrogen and hydride embrittlement will be discussed.

Aluminum Alloy is extensively used in aerospace and automotive industry due to its light weight and high strength properties. It is generally joined using Friction Stir Welding, which is a solid-state process. During this process residual stresses are developed in the welded region. It is a critical factor affecting the performance and lifespan of welded parts. Accurate measurement of residual stress is very important for ensuring the structural integrity of welded components. The conventional blind hole drilling method for residual stress estimation using the strain rosette, results error in the strain data capturing and compensating it is a challenging task. The omission of strain rosette is possible using the recently developed Digital Image Correlation in conjunction with Blind Hole Drilling. This paper focuses on the feasibility study of DIC in residual stress measurement. To accomplish this, Aluminum alloy AA6082 friction stir welded butt-joints are prepared. The residual stresses were measured at the top side of the weld joint using the DIC-BHD approach. At the weld top position, the transverse residual stress of -100 MPa approx. and the longitudinal residual stress of 118 MPa approx. were estimated.

Ever since the introduction of damage tolerance requirements in the aviation regulations, efforts continue to be made to prevent catastrophic failures due to damages present in the structure. It has also been realized that damage detection is the weakest link in the whole process of damage tolerance design to maintain continued airworthiness. The major components of the aircraft structure consist of both integral and riveted panels of sheets and stringers which are employed in fuselage skin panels, spar webs and stiffeners. In spite of all precautions, cracks or damages may arise in many of these primary structural members. These cracks cause stiffness degradation and reduce the total load-carrying capacity of the structure. In this paper, the damage tolerance behaviour of fuselage crown panel both integral and riveted stiffened panel configurations of Aluminium alloy 2024-T351 are studied using finite element based tools using crack growth analysis methods. The crack growth behaviour of both integral and riveted stiffened panels of aircraft fuselage having same geometrical configuration and subjected to uniformly distributed tensile loads is investigated. For this, a metallic stiffened panel with eight stringers, representative of crown panel of a transport aircraft fuselage is analysed with a centre skin crack propagating through the stringers. Stress intensity factors and fatigue crack propagation rates at the progressive crack tip of both types of the stiffened panels are computed by using Modified Virtual Crack Closure Integral (MVCCI) method. The stiffened panels fatigue crack growth rate was computed by using Paris law under constant amplitude fatigue loads. The analysis results show that integral stiffened panel causes higher stress intensity factor and less load bearing capability than riveted stiffened panel which has better damage tolerant capability in comparison to the integrally stiffened panel.