Selim Mrzljak

Strut diameter is a key geometric parameter in additively manufactured lattice structures and affects their mechanical properties even at constant relative density. To isolate such size effects from the complex geometry of lattice structures, this study systematically investigates PBF-LB/M manufactured 316L stainless steel bulk specimens with diameters of 1, 2, 4, and 6 mm. Microstructural characteristics, microhardness, internal defects, and surface roughness were characterized, and correlated with quasistatic tensile tests and total strain-controlled fatigue tests.The investigated specimens, manufactured using a single set of process parameters, exhibited comparable microstructural characteristics, hardness, defect populations, and absolute surface roughness across the considered diameter range. Nevertheless, a pronounced diameter-dependent mechanical response was observed. Yield strength, ultimate tensile strength, and deformation capacity increased with increasing specimen diameter and approached a saturation-like behavior. Fatigue experiments revealed a similar trend: larger specimen diameters exhibited higher stress amplitudes, withstood higher plastic strain amplitudes, and required more cycles to failure. The results indicate that the increasing geometric significance of process-induced surface features and internal defects with decreasing specimen diameter contributes significantly to the observed size-dependent mechanical behavior. A surface-to-volume-based parameter showed a consistent diameter-dependent trend with the mechanical properties, indicating that process-induced surface features of approx. constant absolute magnitude become geometrically more significant relative to the load-bearing cross-section with decreasing specimen diameter.These results show that geometric size effects must be taken into account when designing and scaling additively manufactured lattice structures and provide a basis for transferring size-dependent mechanical properties from bulk specimens to individual lattice struts.

Abstrakt: In der heutigen Zeit steigt die Nachfrage nach umweltfreundlichen und nachhaltigen Alternativen zu etablierten Werkstoffen in technischen Anwendungsfeldern stetig an. Mit Blick auf das Bauwesen werden nach wie vor hauptsächlich energieintensive Werkstoffe, wie z. B. Beton oder Stahl, verwendet. In den letzten Jahren lässt sich in Deutschland in Ansätzen ein Umdenken hin zu dem nachhaltigeren Bauwerkstoff Holz erkennen. Die Verwendung beschränkt sich hierbei derzeit größtenteils auf Nadelhölzer, die sich in Zukunft allerdings in den heimischen Wäldern aufgrund von wandelnden klimatischen Bedingungen sowie häufiger auftretendem Schädlingsbefall einer deutlichen Verknappung ausgesetzt sehen. Eine vielversprechende Alternative zum Holz stellt Bambus dar. Der Naturwerkstoff besticht durch sein schnelles Wachstum, seine Artenvielfalt und die damit einhergehende globale Verfügbarkeit. Weiterhin überzeugt die Pflanze durch ihren äußerst positiven Einfluss auf die Umwelt und mechanische Festigkeiten, die denen herkömmlicher Hölzer mindestens gleichwertig sind. Das ursprünglich in hohlen Halmen wachsende Material wird häufig zu Bambuswerkstoffen weiterverarbeitet. Ein solcher Bambuswerkstoff ist das untersuchte Bambuslaminat (engl. „laminated bamboo lumber“, LBL), das durch die mechanische Charakterisierung für technische Anwendungen qualifiziert und bekannt gemacht werden soll.

The sustainable and fast-growing natural material bamboo accommodates the potential for partially substituting energy-intensive materials in specific technical applications (e.g., in the building industry), thus playing a significant role in bringing about a turning point in reducing the disconcerting effects of global warming and climate change. Beyond the mechanical properties required for the intended applications and rapid growth, the plant exhibits a notable ability of CO ₂ storage and eco-restoration. Bamboo is available in many different species in large parts of the world, growing under different climate conditions. With each species possessing its own unique characteristics, the Phyllostachys edulis (Moso) from the Asian region and the Guadua angustifolia (Guadua) from South America were contrasted as two of the best-known and most extensively researched species. The following investigations were conducted in accordance with normative standard ISO 22157 that has been previously established. Material responses are typically recorded using displacement sensors or strain gauges, which are spatially limited to the measurement zone. Because bamboo is a natural material with locally varying properties, this spatial limitation can lead to inaccuracies by failing to capture the actual material response during testing. Digital image correlation (DIC) is therefore employed, as it enables the observation of larger specimen areas and offers flexibility in subsequent data analysis. In addition, the area-based and visually interpretable evaluation makes it possible to assess whether loads are transferred uniformly or whether naturally occurring defects cause an uneven stress distribution, as discussed later in this paper. The method also serves to verify the intended load introduction into the bamboo culm and to identify, and thus exclude, local failure caused by improper loading. The mechanical test results confirmed the excellent mechanical properties of bamboo when loaded parallel to the fiber direction, as high strength values were observed particularly in compression and bending tests in this direction. While Guadua exhibited slightly higher compressive strength, Moso showed overall higher strengths in transverse tensile and bending tests. Generally, a marked reduction in strength was observed when comparing tensile tests perpendicular to the grain.

Electron optical imaging shows significant potential as a process monitoring technique in electron beam powder bed fusion. When the primary electron beam interacts with substrate materials, electrons are emitted from the surface. These electrons carry essential surface information that can be used for monitoring. Usually, an additional step is incorporated into the process where the electron beam scans the entire surface, generating optical images layer by layer, known as in situ electron optical imaging. However, the potential of electrons emitted during the melting process, called operando signals, has not been fully utilised. By correlating X-ray computed tomography measurements, in situ, and operando electron optical monitoring, this study reveals a strong correlation between outliers in the operando signal and the start of defect formation. The defect initiation is often associated with strong balling which results in abnormal high operando signal intensity. The defect development leads to topographical features that can be effectively captured by subtracting operando signals from opposite detector pairs. These observations suggest that operando electron optical imaging is a strong tool to identify the initiation of faults in electron beam powder bed fusion.

Fiber metal laminates (FML) combine the high specific properties of fiber-reinforced polymers with the ductility and damage tolerance of metals. However, during the manufacturing process of FMLs, in-plane residual stresses arise in the single layers of the laminate. The stresses mainly originate from the different coefficients of thermal expansion of the two materials and lead to an inhomogeneous stress state in the thickness direction of the laminate. This work investigates the influence of such thermal residual stress (TRS) on the fatigue life in an FML by experimentally varying the TRS in identical layups using modified cure cycles. The experimental investigations show that the amount of TRS directly influences the fatigue performance of such laminates. Reducing the TRS, particularly the tensile residual stresses in the metal layers, slows the fatigue crack growth during cyclic loading. This leads to a significantly delayed crack initiation and slower crack propagation. Knowledge of material behavior is essential for utilizing the full potential of structural health monitoring (SHM) in component lifetime predictions. Predictive models require a set of input parameters to perform this task. Since the TRS in an FML significantly contributes to the component's fatigue life, its consideration in predictive approaches is inevitable. This finding is discussed based on a phenomenological damage accumulation model initially developed for composite materials.