Exploring New Strategies for Ultrahigh Strength Steel via Tailoring the Precipitates

Ultrahigh-strength (UHS) steels have shown great potential in the field of high-end equipment manufacturing in demand of lightweight engineering and performance upgrade. A significant research effort has been directed toward the development of advanced UHS steels with excellent combination of strength and toughness. In the course of development, tailoring precipitates by means of composition design and process optimization is absolutely a critical module. In this mini review, typical UHS steels strengthened by carbides and intermetallics are systematically summarized and discussed. With the increase of strength, the toughness losses of UHS steels strengthened by carbides and intermetallics have been compared in detail. In particular, the in-depth mechanisms leading to various strength/toughness variation trends have been discussed, extracting the bottleneck in developing new-generation UHS steels containing merely one type of precipitate. Meanwhile, prospects on designing advanced UHS steels strengthened by coexisting dispersive precipitates have been proposed to achieve better performance.

An Approximation-Based Design Optimization Approach to Eigenfrequency Assignment for Flexible Multibody Systems

The use of flexible multibody simulation has increased significantly over recent years due to the increasingly lightweight nature of mechanical systems. The prominence of lightweight engineering design in mechanical systems is driven by the desire to require less energy in operation and to reach higher speeds. However, flexible lightweight systems are prone to vibration, which can affect reliability and overall system performance. Whether such issues are critical depends largely on the system eigenfrequencies, which should be correctly assigned by the proper choice of the inertial and elastic properties of the system. In this paper, an eigenfrequency assignment method for flexible multibody systems is proposed. This relies on a parametric modal model which is a Taylor expansion approximation of the eigenfrequencies in the neighborhood of a configuration of choice. Eigenfrequency assignment is recast as a quadratic programming problem which can be solved with low computational effort. The method is validated by assigning the lowest eigenfrequency of a two-bar linkage by properly adding point masses. The obtained results indicate that the proposed method can effectively assign the desired eigenfrequency.

Electrostatic charge distribution in armchair and zigzag carbon nanotubes: a numerical comparison of CNT charge models

In engineering—especially in mechanics, lightweight engineering, aerospace engineering, electrical engineering as well as bioengineering—there is a desire of developing materials enabling an excellent performance with respect to mechanical, thermal, and electrical properties. One of the most promising materials are carbon nanotubes (CNTs), as they show excellent mechanical and electrical properties. To improve the understanding of the electrical behavior, i.e., the charge distribution in single-walled carbon nanotubes, both open ended armchair and zigzag types are investigated. In the present research, three different modeling approaches, a classic electrostatic model, the model by Li and Chou and the model by Mayer, are analyzed and compared with respect to their further applicability. In the numerical investigations, different test cases are performed: (i) the carbon nanotubes are charged with an overall charge, (ii) the CNTs are exposed to an external electric field, and (iii) the test cases (i) and (ii) are combined. Furthermore, the influence of different geometric parameters is investigated. It is shown that the charge applied to the CNTs distributes over the whole CNT having maxima at the ends of the tubes. These maxima can be influenced by both, the geometric parameters and the electric field strength.

Development from Alloys to Nanocomposite for an Enhanced Mechanical and Ignition Response in Magnesium

The current study reports on the evolution of microstructure, variations in compressive properties and the ignition resistance of Mg through compositional variation, using alloying elements and nanoreinforcement. The alloys were designed with the use of a singular alloying element, Ca, and a binary alloying element, Ca+Sc, to develop Mg1Ca (wt.%) and Mg1Ca1Sc (wt.%) alloys. B4C nanoparticles were addedas the reinforcement phase in the Mg1Ca1Sc alloy to create the Mg1Ca1Sc/1.5B4C (wt.%) nanocomposite. The most effective compressive properties and level of ignition resistance was displayed by the developed composite. The grain sizes were significantly reduced in the Mg alloys (81%) and the composite (92%), compared with that of the Mg. Overall, the microstructural features (i.e., grain refinement, the formation of favorable intermetallic compounds, and hard reinforcement particles with an adequate distribution pattern) enhanced both the compressive strength and strain of the alloys and the composite. The ignition resistance was progressively increased from the alloys to the nanocomposite, and a peak ignition temperature of 752 °C was achieved in the composite. When compared with the ignition resistant of Elektron 21 (E21) alloy, which met the Federal Aviation Administration (FAA) requirements, the Mg1Ca1Sc/1.5B4C nanocomposite showed a higher specific yield strength and better ignition resistance, asserting it as a potential candidate material for lightweight engineering applications, including aerospace and defense sectors.

Numerical and Experimental Investigation of Cumulative Fatigue Damage under Random Dynamic Cyclic Loads of Lattice Structures Manufactured by Laser Powder Bed Fusion

Lattice structures are lightweight engineering components suitable for a great variety of applications, including those in which the structural integrity under vibration fatigue is of paramount importance. In this work, we experimentally and numerically investigate the dynamic response of two distinct lattice configurations, in terms of fatigue damage and life. Specifically, Face-Centered-Cubic (FCC) and Diamond lattice-based structures are numerically studied and experimentally tested under resonant conditions and random vibrations, until their failure. To this end, Finite Element (FE) models are employed to match the dynamic behavior of the system in the neighborhood of the first natural frequency. The FE models are employed to estimate the structural integrity by way of frequency and tip acceleration drops, which allow for the identification of the failure time and a corresponding number of cycles to failure. Fatigue life under resonant conditions is well predicted by the application of conventional multiaxial high cycle fatigue criteria to the local state of stress. The same approach, combined with the Rainflow algorithm and Miner’s rule, provides good results in predicting fatigue damage under random vibrations.

On the use of a trilinear traction-separation law to represent stitch failure in stitched sandwich composites

Modern aircraft employ the use of lightweight engineering materials such as sandwich composites to increase the flexural rigidity of their structural components. These sandwich composites are limited by their low interfacial strength between the outer facesheets and internal core, which can result in facesheet-core debonding at relatively low out-of-plane loads. In this study, sandwich composites that are reinforced with through-the-thickness stitching are considered. Stitched sandwich composite specimens, fabricated from 110 kg/m3 perforated foam core with cross-ply carbon/epoxy facesheets, were manufactured with different combinations of stitch densities (0.0016–0.01 stitches/mm2) and linear thread densities (400–1200 Denier) of through-the-thickness reinforcement. Single cantilevered beam (SCB) tests were performed to characterize the facesheet-core debonding within the stitched sandwich composites. Unique fracture morphologies were observed that exhibit dependency on stitch processing parameters. A discrete cohesive zone modeling approach is used to simulate the separation of the facesheet from the core. Three-dimensional finite element analysis reveals crack curvature near the stitching. Good agreement between predicted and experimental measurements were obtained.

Modeling, Design and Optimization of Flexible Mechanical Systems

Performance, efficiency and economy drive the design of mechanical systems and structures and has led lightweight engineering design to prominence [...]

Fatigue behaviour of material-adapted fibre-reinforced polymer/metal joints

By regarding the needs and requirements in modern multi-material joining, the Flow Drill Joining Concept (FDJ) was developed at the Chemnitz University of Technology. The technology allows an efficient and material-adapted joining of thin metal sheets with continuous fibre-reinforced thermoplastics, as required in modern lightweight engineering. For a better understanding of their fatigue behaviour, single-lap FDJ joints were examined in quasi-static and dynamic tests regarding shear loads, cross tension and superimposed shear/cross tension loads. By way of example, joints between micro-alloyed steel with high yield strength for cold forming and a continuous glass/carbon fibre-reinforced polyamide 6 were investigated. The fatigue curves show inclinations between k = 8.01 (shear loads) and k = 5.17 (cross tension loads), depending on the applied load angle. The results of the fatigue testings represent a basis for the enhancement of a failure criterion for FRP/metal joints in highly stressed multi-material designs.