Procedia Structural Integrity最新文献

In the light of the energy transition, part of the European natural gas pipeline grid will be converted to hydrogen gas pipelines. One of the challenges of this conversion is the well-acknowledged reduction in mechanical properties of steel in the presence of hydrogen, commonly known as ‘hydrogen embrittlement’. The steels in-use are of variable characteristics (with respect to chemical, microstructural and mechanical properties), resulting in a variety of plasticity and fracture behaviors. The present work investigates the effect of hydrogen on the fracture behavior of a relatively old grade API 5L X56 pipeline steel produced by normalized rolling which resulted in a banded microstructure. Tensile tests were performed on smooth and notched round bar specimens that were hydrogen pre-charged electrochemically (ex-situ), and compared to tests on uncharged specimens as a reference. The fractured specimens were scanned using high resolution X-ray computed tomography (X-ray micro-CT) to visualize and quantify the damage underneath the fracture surface. Statistics regarding the void size distribution and void shapes are provided. The fracture process in the absence of hydrogen is characterised by significant void development. The presence of hydrogen accelerates the fracture mechanisms without fundamentally altering them. This is in contrast with previous results obtained on a different pipeline steel grade, demonstrating the sensitivity of hydrogen embrittlement susceptibility to material characteristics.

The damage detection in structures using modulation transfer phenomena is a topic of increasing interest. However, the lack of comprehensive knowledge and established signal processing methods have hindered its widespread application. This paper explores the potential of the modulation transfer phenomenon for damage localisation by conducting experiments on test stands with two structures: a damaged and an undamaged beam. A well-defined procedure for processing response signals and damage indicators was established. Before the experiments, modal analysis was conducted to select the appropriate excitation frequency. The presented results include spectra and trends of the damage indicators, demonstrating the viability of using the modulation transfer phenomenon for damage localisation. Furthermore, the vibroacoustic modulation phenomenon was observed during the tests. These findings underscore the potential of modulation transfer techniques in structural health monitoring applications.

Standard-based evaluations of tensile tests assume ideal geometry and homogeneous, isotropic material. Based on the Digital Twin concept, a measurement and evaluation system has been built in recent years allowing the monitoring of tensile tests with a fine temporal resolution and full spatial data acquisition technology that provides significantly more detailed data than conventional measurement techniques. This paper investigates whether the theoretical model used in Digital Twin can capture differences between the realistic initial geometry of a specimen and its idealised model. High-precision machining of samples, combined with highly accurate coordinate measurements, results in a fine resolution coordinate map. The geometric imperfections of the finished samples are well within the allowed manufacturing tolerances. Digital Twins of the test specimens were built using two approaches. First, the initial geometry of the specimen's active zone was idealised. For the second, the shape of the test specimen was defined by the best fitting surfaces to the observed results. Simulation results show that computations, based on realistic initial geometry, i.e., considering geometric imperfections inherent in the initial geometry, are much more accurate in tracking time evolution of the specimen geometry –including necking zone location– than computations based on idealised geometry.

Due to the steadily growing use of plastics, also for technically demanding applications, the selection of materials and their design are becoming increasingly important. A major part of the plastic design is the consideration of the temperature, whereas the heat development of the component under mechanical load has hardly been taken into account until today. Standard mechanical methods are often unable to describe molecular processes and the failure dynamics. Fracture mechanics methods in combination with imaging techniques offer the possibility to investigate the local failure much more precisely and represent a useful supplement to the standard testing methods. The unforeseen changes in the individual plastic properties due to the increased internal temperature changes during crack formation and the corresponding local softening can be considered in much greater detail. Thus, it has not yet been possible to clarify whether plastification at the crack tip inhibits or promotes crack growth. In order to be able to investigate this question, a test setup was implemented that determines basic fracture mechanics parameters and, in combination with a high-resolution thermographic camera provide temperature data with spatial and temporal resolution for each point on the so-called crack resistance curves. Three amorphous plastics were investigated in this study. These include a polystyrene and two polycarbonates with different chain lengths. To determine the mechanical properties, a tensile load is applied to pre-notched test specimens. In a first series of tests, the setup was used to determine the temperature change at the crack tip for test speeds between 1 mm/min and 250 mm/min. Due to the different polymer structure and the resulting different forces of attraction between the molecular chains of the polymers, a clear difference in the maximum temperatures at the crack surface between 45°C up to 90°C occurred. In addition, the material behavior had a major influence on the shape of the fracture process zone and showed a difference in the temperature data and strain rate recorded with the digital image system.

Accurately describing the fatigue crack growth rate and fatigue crack growth direction is crucial in determining the residual fatigue life of steel structures in general and for railway rails in particular. The crack growth rate and crack growth direction depend on the crack driving force. The stress intensity factor (SIF) is often considered as crack driving force and it depends on the applied load, the crack length and geometry. This paper concerns a numerical investigation on an inclined edge crack in a rail subjected to a moving patch load to evaluate its growth rate and direction including both normal and tangential stress components. A 2D finite element (FE) model is created including friction between the crack faces. The crack is incrementally extended in the predicted direction after each passage of the moving load. A parametric study is conducted to study the effect of the friction and traction coefficients. The results are compared in terms of predicted crack paths and SIF characteristics. It is shown that both friction and traction have a significant influence on the fatigue crack growth rate and path.

This study aims at investigating the effect of strengthening shear-deficient recycled aggregate concrete (RAC) beams with carbon fiber-reinforced polymer (CFRP) laminates. Five RAC beams were cast, four of which were strengthened with different CFRP shear strengthening configurations: U-wraps bonded at 45°, vertical U-wraps, continuous U-wraps along the shear span, and side-boned laminates. In addition, one RAC specimen was left unstrengthened to act as a benchmark specimen. For comparison purposes, an additional five normal aggregate concrete (NAC) beams were cast, three of which are strengthened with similar CFRP schemes as that of the RAC, and one was left unstrengthened. All beams are loaded under four-point bending tests, and the results in terms of shear force-deflection graphs and failure modes are analyzed and compared. Experimental results indicated that the shear force values obtained in NAC and RAC beams are comparable. In fact, the percentage increase in the shear strength compared to the respective control beam was higher for RAC beams than that of NAC beams. This proves the effectiveness of using different shear strengthening configurations and the viability of using CFRP shear strengthened RAC beams compared to CFRP shear strengthened NAC beams.

Laser Metal Deposition (LMD) ability to precisely fabricate complex geometries layer by layer, along with its capability to repair and enhance existing components, has ushered in new frontiers of design freedom and innovation. As industries continually seek solutions for increased efficiency and performance, LMD offers an avenue to unlock novel possibilities, enabling the production of high-quality, intricately designed parts while simultaneously reducing material waste, with significant build rate when compared to other metal AM processes. The unique properties of nickel-based superalloys, including exceptional high-temperature strength and corrosion resistance, make them indispensable materials for critical applications, particularly in aerospace, power generation, and the energy sector. This research paper presents a comprehensive investigation into the process optimization of laser melting deposition for Inconel 625, a high-performance nickel-chromium-based superalloy. The study employed a center cubic design as a Design of Experiments (DoE) framework, with a primary focus on achieving the maximum tensile strength as the optimization objective. A series of quasi-static tensile tests was conducted to evaluate the mechanical properties of the deposited material, while optical microscopy was utilized to analyze the cross-sectional characteristics, including deposition density and defect sizes.