13 Zentrale Universitätseinrichtungen

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/14

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Institute: search.filters.UBSInstitut.Materialprüfungsanstalt Universität Stuttgart (MPA Stuttgart, Otto-Graf-Institut (FMPA))Start date: 2020

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Now showing 1 - 10 of 28
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    Investigation of oxide layer development of X6CrNiNb18-10 stainless steel exposed to high-temperature water
    (2024) Veile, Georg; Hirpara, Radhika; Lackmann, Simon; Weihe, Stefan
    The oxide layer development of X6CrNiNb18-10 (AISI 347) during exposure to high-temperature water has been investigated. Stainless steels are known to form a dual oxide layer in corrosive environments. The secondary Fe-rich oxide layer has no significant protective effect. In contrast, the primary Cr-rich oxide layer is known to reach a stabilized state, protecting the base metal from further oxidation. This study’s purpose was to determine the development of oxide layer dimensions over exposure time using SEM, TEM and EDX line scans. While a parabolic development of Cr in the protective primary layer and Fe in the secondary layer was observed, the dimensions of the Ni layer remained constant. Ni required the presence of a pronounced Fe-rich secondary layer before being able to reside on the outer secondary layer. With increasing immersion time, the Ni element fraction surpassed the Cr element fraction in the secondary layer. Oxide growth on the secondary layer could be observed. After 480 h, nearly the entire surface was covered by the outer oxide layer. In the metal matrix, no depletion of Cr or Ni could be observed over time; however, an increased presence of Cr and Ni in the primary layer was found at the expense of Fe content. The Nb-stabilized stainless steel was subject to the formation of Niobium pentoxide (Nb2O5), with the quantity and magnitude of element fraction increasing over exposure time.
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    Testing and evaluation of anchor channels under fatigue loading
    (2020) Fröhlich, Thilo; Lotze, Dieter
    Cast-in anchor channels are used to connect steel components to concrete structures e.g., for elevators, cranes or machines, where repeated load cycles require verification against fatigue failure. The fatigue resistance of anchor channels may be determined by tests according to the interactive method, which provides a complete description of the S/N curve from one to infinite load cycles according to the current assessment document. This procedure differs from conventional fatigue concepts, which do not consider loads that are part of low cycle fatigue, but also question the general existence of an endurance limit. An alternative approach presented in this paper is based on the assumption that the S/N curve can be approximated by a bilinear function. The procedure for the evaluation of fatigue tests on anchor channels embedded in concrete is described. A comparison with the current qualification criteria is given by a test example to discuss the applicability of the proposed method.
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    Effect of weld length on strength, fatigue behaviour and microstructure of intersecting stitch-friction stir welded AA 6016-T4 sheets
    (2023) Walz, Dominik; Göbel, Robin; Werz, Martin; Weihe, Stefan
    Friction stir welding is a promising joining process for boosting lightweight construction in the industrial and automotive sector by enabling the weldability of high-strength aluminum alloys. However, the high process forces usually result in large and heavy equipment for this joining method, which conflicts with flexible application. In order to circumvent this issue, a friction stir welding gun has been developed which is capable of producing short stitch welds-either stand-alone as an alternative to spot welds or merging into each other appearing like a conventional friction stir weld. In this study, the influence of the stitch seam length on the strength properties of intersecting friction stir welds is investigated, and the weld is characterized. For this purpose, EN AW-6016 T4 sheets were welded in butt joint configuration with varying stitch lengths between 2 and 15 mm. Both the static and dynamic strength properties were investigated, and hardness and temperature measurements were carried out. The results show a scalability of the tensile strength as well as the fatigue strength over the stitch seam length, while the substitute proof strength is not affected. Hereby, the tensile strength reached up 80% of the base materials tensile strength with the chosen parameter setup. Likewise, the stitch weld length influences the hardness characteristics of the welds in the transition area.
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    Theoretical-numerical investigation of a new approach to reconstruct the temperature field in PBF-LB/M using multispectral process monitoring
    (2024) May, Lisa; Werz, Martin
    The monitoring of additive manufacturing processes such as powder bed fusion enables the detection of several process quantities important to the quality of the built part. In this context, radiation-based monitoring techniques have been used to obtain information about the melt pool and the general temperature distribution on the surface of the powder bed. High temporal and spatial resolution have been achieved at the cost of large storage requirements. This contribution aims to offer an alternative strategy of gaining information about the powder bed’s temperature field with sufficient resolution but with an economical amount of data. The investigated measurement setup uses a spectrometer to detect the spectral radiation intensities emitted by an area enclosing the melt pool and part of its surroundings. An analytical description of this process is presented, which shows that the measured spectral entities can be reconstructed by the Ritz method. It is also shown that the corresponding weighting factors can be physically interpreted as subdomains of constant temperature within the measurement area. Two different test cases are numerically analyzed, showing that the methodology allows for an approximation of the melt pool size while further assumptions remain necessary to reconstruct the actual temperature distribution.
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    Fire rating of post-installed anchors and rebars
    (2020) Mahrenholtz, Philipp; Sharma, Akanshu
    Fire safety is a critical performance aspect of construction products, and post-installed anchors and rebars are no exemption in that regard. During their service life, anchors and rebars are subjected to different kinds of load actions, so they have to be qualified and designed for critical safety performance. While the qualification guidelines for static and seismic loading have matured to conclusive requirements over the past two decades, the requirements for determining the resistance to fire are just about to consolidate. This contribution strives to provide clarity on the fire rating of post-installed anchors and rebars. For this, the current status of the regulations, as well as the underlying background, is reviewed after a brief introduction. Typical examples of fire ratings in the field of post-installed anchors and rebars are given, and recent research undertaken to close the last regulative gaps is briefly presented.
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    A novel runtime algorithm for the real-time analysis and detection of unexpected changes in a real-size SHM network with quasi-distributed FBG sensors
    (2021) Sakiyama, Felipe Isamu H.; Lehmann, Frank; Garrecht, Harald
    The ability to track the structural condition of existing structures is one of the main concerns of bridge owners and operators. In the context of bridge maintenance programs, visual inspection predominates nowadays as the primary source of information. Yet, visual inspections alone are insufficient to satisfy the current needs for safety assessment. From this perspective, extensive research on structural health monitoring has been developed in recent decades. However, the transfer rate from laboratory experiments to real-case applications is still unsatisfactory. This paper addresses the main limitations that slow the deployment and the acceptance of real-size structural health monitoring systems (SHM) and presents a novel real-time analysis algorithm based on random variable correlation for condition monitoring. The proposed algorithm was designed to respond automatically to detect unexpected events, such as local structural failure, within a multitude of random dynamic loads. The results are part of a project on SHM, where a high sensor-count monitoring system based on long-gauge fiber Bragg grating sensors (LGFBG) was installed on a prestressed concrete bridge in Neckarsulm, Germany. The authors also present the data management system developed to handle a large amount of data, and demonstrate the results from one of the implemented post-processing methods, the principal component analysis (PCA). The results showed that the deployed SHM system successfully translates the massive raw data into meaningful information. The proposed real-time analysis algorithm delivers a reliable notification system that allows bridge managers to track unexpected events as a basis for decision-making.
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    Investigation of the influence of moisture content on fatigue behaviour of HPC by using DMA and XRCT
    (2021) Markert, Martin; Katzmann, Josef; Birtel, Veit; Garrecht, Harald; Steeb, Holger
    High-performance concrete (HPC) is a topic of current research and construction projects, due to its outstanding compressive strength and durability. In particular, its behaviour under high-cycle fatigue loading is the focus of current investigations, to further pave the way to highly challenging long-lasting constructions; e.g., bridges or offshore buildings. In order to investigate the behaviour of HPC with different moisture contents in more detail, a mixture of silica sand and basalt aggregate with a maximum grain size of 8 mm was investigated with three different moisture contents. For this purpose, cyclic compressive fatigue tests at a loading frequency of 10 Hz and different maximum stress levels were performed. The main focus was the moisture influence on the number of cycles to failure and the development of concrete temperature and strain. In a further step, only the mortar matrix was investigated. For this purpose, the mixture was produced without basalt, and the moisture influence was investigated on smaller-sized test specimens using dynamic mechanical analysis (DMA) and X-ray computed tomography (XRCT). It was shown that the moisture content of HPC had a significant influence on the fatigue damage behaviour due to the number of cycles to failure decreasing significantly with increased moisture. In addition, there was also an influence on the temperature development, as well as on the strain development. It was shown that increasing moisture content was associated with an increase in strain development. XRCT scans, in the course of the damage phases, showed an increase in internal cracks, and made their size visible. With the help of DMA as a new research method in the field of concrete research, we were also able to measure damage development related to a decrease in sample stiffness. Both methods, XRCT and DMA, can be listed as nondestructive methods, and thus can complement the known destructive test methods, such as light microscopy.
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    Investigation of tool degradation during friction stir welding of hybrid aluminum-steel sheets in a combined butt and overlap joint
    (2024) Göbel, Robin; Schwertel, Stefanie; Weihe, Stefan; Werz, Martin
    Friction stir welding, as a solid-state welding technique, is especially suitable for effectively joining high-strength aluminum alloys, as well as for multi-material welds. This research investigates the friction stir welding of thin aluminum and steel sheets, an essential process in the production of hybrid tailor-welded blanks employed in deep drawing applications. Despite its proven advantages, the welding process exhibits variable outcomes concerning formability and joint strength when utilizing an H13 welding tool. To better understand these inconsistencies, multiple welds were performed in this study, joining 1 mm thick steel to 2 mm thick aluminum sheets, with a cumulative length of 7.65 m. The accumulation of material on the welding tool was documented through 3D scanning and weighing. The integrity of the resulting weld seam was analyzed through metallographic sections and X-ray imaging. It was found that the adhering material built up continuously around the tool pin over several welds totaling between 1.5 m and 2.5 m before ultimately detaching. This accretion of material notably affected the welding process, resulting in increased intermixing of steel particles within the aluminum matrix. This research provides detailed insights into the dynamics of friction stir welding in multi-material welds, particularly in the context of tool material interaction and its impact on weld quality.
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    Experimental investigation on the fatigue design of anchor channels
    (2022) Fröhlich, Thilo; Lotze, Dieter
    Cast‐in anchor channels subjected to repeated cyclic actions need to be verified against fatigue failure. However, the fatigue behavior of these fasteners has rarely been investigated up to now. Consequently, the fatigue verification is not covered by the provisions of EN 1992‐4 and specific design rules apply for this product. This article gives a brief overview of the current design methods for anchor channels under fatigue loading and addresses its limitations. Results of experimental investigations within a recent research project are presented that focused on the fatigue resistance of fastenings under combined static and fatigue loading. Accompanying strain measurements provide insights about the distribution of cyclic tension loads through the channel profile to the anchors. In addition, tests under pulsating and alternating shear perpendicular to the channel axis were performed to outline the suitability under fatigue relevant shear loads. The test results obtained are discussed with respect to the existing state of knowledge. Finally, recommendations for the fatigue design of anchor channels are given.
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    Characterisation of adobe and mud-straw for the restoration and rehabilitation of Persian historical adobe buildings
    (2024) Hejazi, Bina; Luz, Corinna; Grüner, Friedrich; Frick, Jürgen; Garrecht, Harald
    In the restoration or rehabilitation of traditional buildings, compatible materials with known characteristics must be used. However, the existing literature lacks comprehensive studies on the characterisation of Persian mud-straw plaster, focusing primarily on Persian adobe. Moreover, previous research on Persian adobe has primarily employed XRF and XRD tests, neglecting ion chromatography, moisture sorption isotherm determination, and thermogravimetric analysis with differential scanning calorimetry. Consequently, there is a shortage of information regarding the elemental composition, mineralogical characteristics, moisture sorption behaviour, and thermal properties of Persian mud–straw plaster, as well as Persian adobe bricks. This paper aims to address this research gap by examining historical and new adobe bricks and mud–straw plaster used in Iran, utilising a comprehensive array of analytical techniques. The results from XRF analysis reveal relatively similar chemical compositions across all samples, while XRD analysis indicates predominantly similar mineral phases. Ion chromatography results demonstrate higher conductivity and chloride concentrations in the mud–straw samples than the adobe samples, with higher values for new samples than historical ones. Freshly used straw, clay, or soil may have higher chloride concentrations caused by the arid climate and soil salinisation in the area. Additionally, moisture sorption isotherm determination results show that adobe and mud–straw plaster with a higher salt load of chlorides have significantly higher moisture absorption. The increased straw quantity in the samples increases the moisture content. Furthermore, thermogravimetric analysis and differential scanning calorimetry indicate that, at low heating, adobe and mud–straw plaster lose water due to dehydration, and at high heating, they lose carbon dioxide due to decarboxylation. The comprehensive characterisation of Persian adobe and mud–straw plaster in this study fills a significant gap in the literature and offers invaluable insights for informing restoration and rehabilitation processes, ensuring the compatibility of the materials used.