2024
Ascher M., Späth R., Johlitz M. (2024). Elastoplastic Characterization of a Two-Component Epoxy-Based Structural Adhesive. In: Altenbach H, Hitzler L, Johlitz M et al. (eds) LECTURES NOTES ON ADVANCED STRUCTURED MATERIALS 2, Vol 203. SPRINGER INTERNATIONAL PU, [S.l.], S. 291–306.
Abstract
Two-component epoxy-based adhesives are frequently used to bond metals and composite materials in structural lightweight applications. Due to the significant nonlinearity in the joints’ constitutive equations and the complex shapes of the adherents, numerical techniques such as the finite element method (FEM) are the preferred method for strength assessment. This work presents an experimental approach for determining the mechanical response of a two-component epoxy adhesive by means of a multilinear elastoplastic material definition. The tensile behavior was established by conducting static tensile tests using a total of 72 adhesive bulk samples, and the resulting stress–strain curves are approximated using multiple regression lines. The adhesive's high viscosity caused inhomogeneities in the adhesive bulk, which were detected on the fracture surfaces of the samples. By excluding samples with air inclusions larger than 100 µm from the analysis, the scatter in the results is reduced to an acceptable level. Assuming isotropic material behavior, the mechanical response of the adhesive is fully characterized by additionally determining the shear modulus. This is done through torsion testing of additively manufactured butt-bonded cylinders. The design of the test sample failed to sufficiently account for the chemical shrinkage of the adhesive, resulting in frictional effects between the integrated adhesive fill gap spacers during test execution. Additionally, the high stiffness of the adhesive layer is underestimated, which negatively impacted the measurement resolution of the torsional angle. Consequently, the shear modulus values obtained exhibited an unacceptable level of scattering.
URL
https://link.springer.com/chapter/10.1007/978-3-031-49043-9_17
2023
Steinebrunner, Martin; Späth, Ralf. (2023). FE-Analysen zur Lebensdauerermittlung basierend auf 3D-Scans von industriell geschweißten Kreuzstoßproben und der Absicherung mittels Schwingversuchen im Hochfrequenzpulsator. DVS Congress (2023, Essen).
Abstract
In Kooperation mit einem Industriepartner hergestellte Schweißproben wurden 3D-gescannt, mittels der FEM gerechnet und im Schwingversuch getestet. Anschließend wurden die Ergebnisse aus der FEA bezüglich der Korrelation zu den real erzielten Lebensdauern verglichen. Dazu wurden Bereiche mit erhöhter Spannungskonzentration identifiziert und quantifiziert. Einflussparameter waren dabei nicht nur die Prüfbelastung und das Lastverhältnis, sondern auch Belastungen, die bedingt durch geometrischen Verzug, bereits beim Einspannvorgang aufgebracht wurden. Zur Absicherung der FEA wurden statisch an Probekörpern und dynamisch während des Schwingversuches Messungen mittels Dehmesstreifen durchgeführt. Die Abweichung betrug dabei nur 5%. Als Ergebnis konnten Wöhlerkurven im Zeitfestigkeitsbereich erstellt werden. Dies sowohl für die aufgebrachte Nennspannung als auch für die lokalen, rechnerisch aus der FEA ermittelten Spannungen. Die mittels der FEM rechnerisch ermittelte Neigung der Wöhlerkurve korrelierte gut mit den real erzielten Ergebnissen.
Späth, Ralf. (2023). Betriebsfeste Konstruktion und Berechnung von Schweißverbindungen: Leitfaden für die Entwicklung geschweißter Strukturen anhand leistungsfähiger Berechnungsmethoden auch mittels FEM. Wiesbaden: Springer Vieweg, 2023, S. XII, 172.
Abstract
Schweißverbindungen zählen nach wie vor zu den wichtigsten stoffschlüssigen Verbindungen. Für die Betriebsfestigkeitsberechnung dieser Verbindungen gibt es zahlreiche Normen und Vorgaben – das Einordnen der verschiedenen Methoden ist auch für den versierten Ingenieur nicht immer einfach. Darüber hinaus wird die FEM-Methode immer wichtiger – auch hier gibt es eine Vielzahl von Modellierungsansätzen. In diesem Buch werden die Grundlagen und die Zusammenhänge der Betriebsfestigkeit von Schweißverbindungen anschaulich und praxisgerecht erläutert, sowohl auf der Basis von „Handrechnungen“ als auch mit FEM-Methoden. Der Leser ist damit in der Lage auch kritische Schweißverbindungen sicher schwingfest zu gestalten, zu berechnen sowie begleitende Labortests zielgerichtet zu planen.
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Späth, Ralf; Steinebrunner, Martin. (2023). Using 3D-scanners and finite element method to assess the fatigue life of welded joints based on the actual geometry of seam welds: Realistic modelling of notches at the weld toe. Materialwissenschaft und Werkstofftechnik, Vol. 54, No. 4, S. 459-465.
Abstract
To predict the service life of welded joints, the seam welds of samples (cross joint, K-seam, sheet thickness 15 mm and a laser-welded butt joint, sheet thickness 6 mm, both made of steel quality S 355) are recorded using a 3D scanner. The data obtained in this way is processed and transferred to a finite element method program. The meshing of the weld seams is very detailed. A finite-element-method stress calculation is then carried out. The real samples are tested in a fatigue test up to the point of cracking. The results from calculation and test are then compared. A possible crack location can be predicted very well using the finite element method results. The cracks in the real test are always in the area of very high or often the absolute highest calculated stresses. A prediction of the service life in the fatigue test is possible with a certain scatter – close to the usual scatter for welded joints. Based on the results of 19 samples, a fatigue class for local stress (based on the real geometry) of FAT 300 can be preliminary estimated for the cross-joint specimens. Definition according to the specifications of the International Institute of Welding (FAT at 2 mio. cycles, survival probability 97.7 %).
Ascher, Michael; Brenner, Stefan; Pang, Genny A.; Späth, Ralf. (2023). Joining technology of additively manufactured components: Effects on the bonding strength for the adhesive application through inner channels. Progress in Additive Manufacturing, Vol. 8, S. 711-718.
Abstract
The maximum size of additively manufactured (AM) components is restricted due to the confined building space of the manufacturing machines. Component separation and subsequent joining can be an effective way of manufacturing larger components using AM processes. For joining of AM components, adhesive bonding provides great potential for not constraining the adherend’s geometry, as long as the adhesive can still be applied to the adhesive surfaces of the adherends. This work investigates the effectiveness and applicability of additively manufactured inner channels to improve the adhesive application. A circular adhesive single lap joint between a laser-based powder bed fusion (PBF-LB) component made of AlSi10Mg and a cold drawn aluminum round bar was considered. The PBF-LB components were designed with varying geometric complexity to implement different adhesive application concepts. Subsequently, the bonded joints were subjected to static tensile tests. The fracture strength of joints where the adhesive was applied by injection into AM inner channels exceeds the fracture strength of joints where the adhesive was injected into geometries manufacturable by subtractive machining, and also exceeds the fracture strength of joints where the adhesive was pre-applied.
URL
https://link.springer.com/article/10.1007/s40964-023-00430-x
Ascher, Michael; Pang, Genny A.; Späth, Ralf. (2023). Method for the design of additively manufactured inner channels intended for adhesive application. CIRP Design Conference (33., 2023, Sydney). S. 752-757.
Abstract
Joining of metal parts manufactured by laser beam powder bed fusion (PBF-LB/M) eliminates the inherent size restriction of this additive manufacturing process and reduces the manufacturing effort. Adhesive bonding presents great potential for joining of PBF-LB/M parts, as there are no constraints on the shape of the connecting surfaces, as long as the adhesive can be applied. The freedom of design underlying the PBF-LB/M process enables the construction of inner channels in the parts, which can facilitate the adhesive application. The adhesive can be injected into a single inlet on the exterior of a part and directed through inner channels before leaking from multiple outlets into the adhesive fill gap between the pre-aligned adherents. To avoid insufficient adhesive distribution and air inclusions in the adhesive fill gap, both of which reduce the bond strength, a methodical approach for the design of inner channels intended for adhesive application by injection is required. This work provides a corresponding design method based on the theory of fluid mechanics. Moreover, an example case of a PBF-LB/M sleeve to be joined to a circular tube is presented. Results from a CFD analysis show that the inner channels designed using the presented method lead to excellent coverage of the adhesive fill gap with adhesive and minimal air inclusions.
URL
https://www.sciencedirect.com/science/article/pii/S2212827123005541?via%3Dihub
2022
Ascher, Michael. (2022). Joining technology of additively manufactured components: Adhesive application through inner channels. International Conference on Advanced Computational Engineering and Experimenting (15., 2022, Florenz).
Abstract
Depending on the specific additive manufacturing (AM) process the maximum size and complexity of AM components is restricted. Separation and subsequent joining can be an effective way of dealing with these issues. For joining of AM components adhesive bonding provides great potential for not constraining the joining partners geometry. This is only valid if the adhesive can still be applied to the connecting surfaces. This work investigates the effectiveness and applicability of additively manufactured inner channels to improve the adhesive application. The use case considered is a circular adhesive single lap joint (SLJ) between a laser powder bed fusion (PBF-LB) component made of AlSi10Mg and a cold drawn aluminum round bar. The SLJ´s were produced using different adhesive application concepts. Those can be implemented by realizing the joining partners geometry through AM processes, subtractive machining or without any machining. Subsequently the joints were subjected to static tensile tests. The fracture strength of joints where the adhesive was applied by injection into AM inner channels exceeds the fracture strength of joints where the adhesive was injected into geometries manufacturable by subtractive machining and exceeds the fracture strength of joints where the adhesive was pre-applied. Moreover, a stepwise de-powdering procedure for the inner channels was quantified by weighing methods and the resulting cross-sectional area of the inner channels was analyzed on the basis of microscopic images and digital image processing software.
Ascher, Michael; Späth, Ralf. (2022). Joining technology of additively manufactured components: Design measures for optimizing the strength of adhesively bonded joints. Konstruktion für die Additive Fertigung (2021, Hannover). S. 63-81.
Abstract
The size of components manufactured by laser-based powder bed fusion (PBF-LB) is limited by the maximum available powder bed volume. To create larger structures, additively manufactured (AM) components can be combined with inexpensive intermediate products. A comprehensive consideration of the connection task is required. Joints of fibre-reinforced plastic profiles adhesively bonded with LPBF-manufactured aluminium couplers and stiffening elements were investigated as examples. The geometric freedom of design in AM-processes enables optimisation of the joint properties: • Optimisation of the bonding process (application and distribution) • Uniform stress distribution within the adhesive fill space • Strength optimisation of the joined parts To reveal the adhesive distribution during injection, tests with transparent acrylic models where carried out. The AM components’ topology was optimised using FEA with regard to a homogeneous stress distribution within the joint. The fatigue strength was validated using numerous samples. Specific results on adhesive distribution and fatigue behaviour will be presented.
URL
https://link.springer.com/chapter/10.1007/978-3-031-05918-6_5
2021
Ascher, Michael. (2021). Joining technology of additively manufactured components: Design measures for optimizing the strength of adhesively bonded joints. Innovative Product Development by Additive Manufacturing (2021, Hannover).
Abstract
The size of components manufactured by laser-based powder bed fusion (PBF-LB) is limited by the maximum available powder bed volume. To create larger structures, additively manufactured (AM) components can be combined with inexpensive intermediate products. A comprehensive consideration of the connection task is required. Joints of fibre-reinforced plastic profiles adhesively bonded with LPBF-manufactured aluminium couplers and stiffening elements were investigated as examples. The geometric freedom of design in AM-processes enables optimisation of the joint properties: • Optimisation of the bonding process (application and distribution) • Uniform stress distribution within the adhesive fill space • Strength optimisation of the joined parts To reveal the adhesive distribution during injection, tests with transparent acrylic models where carried out. The AM components’ topology was optimised using FEA with regard to a homogeneous stress distribution within the joint. The fatigue strength was validated using numerous samples. Specific results on adhesive distribution and fatigue behaviour will be presented.
2019
Späth, Ralf. (2019). Betriebsfestigkeitsanalyse von Schweißverbindungen anhand von digitalisierten Real-geometrien und FEM-Berechnungen sowie deren Validierung anhand von Schwingversuchen. Große Schweißtechnische Tagung (2019, Rostock). S. 240-245.
Abstract
Für die Lebensdauervorhersage von Schweißverbindungen werden Nähte von Proben mittels eines 3D-Scanners erfasst. Die so gewonnenen Daten werden bearbeitet und in ein FEM-Programm übermittelt. Die Vernetzung der Schweißnähte erfolgt sehr detailliert. Anschließend wird eine FEM-Spannungsberechnung dieser Proben (MAG-geschweißter Kreuzstoß, K-Naht, Blechstärke 15 mm sowie ein lasergeschweißter Stumpfstoß, CO2-Laser, Blech-stärke 6 mm, beide aus Stahlqualität S 355) durchgeführt. Die realen Proben werden im Schwingversuch bis zum Anriss geprüft. Das Spannungsniveau wird so gewählt, dass der Anriss im Zeitfestigkeitsbereich liegt. Anschließend werden die Ergebnisse aus Berechnung und Versuch gegenübergestellt: Ein möglicher Anrissort kann anhand der FEM-Berechnung sehr gut vorhergesagt werden. Die Risse im realen Versuch liegen immer im Bereich sehr hoher bzw. oft der absolut höchsten berechneten Spannungen. Eine Vorhersage der Lebensdauer im Schwingversuch ist nur mit einer Streuung möglich. Hohe Spannungsspitzen in der Berechnung korrelieren meist gut mit einer niedri-gen Lebensdauer und umgekehrt. Für die MAG-geschweißten Proben lässt sich eine FAT-Klasse in Anlehnung an die IIW-Vorgaben von etwa FAT 280 grob abschätzen. Für die lasergeschweißten Proben ist noch keine belastbare Aussage möglich. In beiden Fällen sind weitere Versuche zur besseren statistischen Absicherung nötig. Ausbli-ckend wurde auch eine Modellierung mit nichtlinearem Materialverhalten durchgeführt. Nach ersten Ergebnissen ist damit keine signifikante Verbesserung der Lebensdauervorhersage möglich. Auch hier sind weitere Untersuchungen nötig.
2018
Späth, Ralf. (2018). Entwicklung einer Methode zur schnellen Druckpulsationsprüfung von Hydraulikkomponenten. VDI-Seminar Landtechnik (2018, Herrsching).
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