Lightweight Construction Potential Research Articles

High potential: lightweight optimised structural design of car bodies for railway vehicles with alternative drive systems

Climate change and related goals and measures require a shift away from fossil fuels as an energy source. One solution for non-electrified lines for rail transport is the use of alternative drive technologies such as battery or hydrogen drive instead of diesel units. In order to use these efficiently in rail vehicles, it is imperative to make adjustments to the body structure. Existing car body structures must be adapted to integrate the space-intensive and heavy additional equipment components. Within the framework of the project AnoWaAS those with the highest lightweight construction potential are methodically identified from a large number of different derivatives of car body designs. The high influence of a force flow-compliant design on the lightweight potential is analysed in depth. Possible adapted construction methods for further exploitation of the lightweight potential are shown and analysed with regard to their potential. With the help of trapezoidal windows and the structural integration of equipment enclosures, it is possible to reduce the structural mass of a car body by more than 20%.

Investigations to improve the tool life during thermomechanical and incremental forming of steel auxiliary joining elements

Modern vehicles as well as structures consist more and more often of a multi material mix. In addition to the use of light materials (e. g. aluminium) to optimise lightweight construction potential, higher strength alloys are being used more and more frequently. With the different material combinations, the requirements for the joining technology also increase. Thus (due to restrictions in the area of thermal joining processes) the joining of different material combinations is often realised by mechanical joining technology. Even though different joining tasks (dissimilar material or sheet thicknesses) can be solved by mechanical joining technology in general, it is often required to switch between appropriate auxiliary joining elements and even joining technique. To adapt the auxiliary elements to the individual requirements of each joint an innovative manufacturing process of customisable auxiliary joining elements has been developed. The use of the incremental, thermomechanical manufacturing process called friction-spinning for the manufacturing of adaptive auxiliary joining elements enables to individualise each element for the subsequent joining process. However, this advantage comes at the cost of a high mechanical and thermal loads on the forming tools. These loads also depend on the process design (trajectory, feed, rotational speed) and have to be controlled in order to improve the tool life. For a more profound process knowledge and to better understand the corellation of the individual process variables, a series of tests with varying process parameters were carried out in a partial factorial design of experiments. The results show not only an possible improvement in the tool life, but also the required process design for a higher reproducibility of the auxiliary joining element geometry and the possibility of positively influencing the geometry to achieve a force-closure of the joint.

Hybrid joining of cast aluminum and sheet steel through compound sand casting and induction heating to enable thin-walled lightweight structures

Combining different joining processes to form a hybrid process offers new manufacturing possibilities. Adding induction heating to compound sand casting with additively manufactured lost sand moulds to preheat a metallic solid insert increases the degree of the metallic bond between sheet metal and casting metal. In this study, the manufacturability of thin-walled sheet steel/cast aluminum structures with reduced cast wall thickness in sand casting is characterized for the first time. Enabling lower wall thicknesses of sheet metal/cast metal structures in sand casting shifts the current limits and offers more significant lightweight construction potential. Shear tensile, compression shear, and pullout tests characterize the mechanical properties of the joints. Light microscopic imaging of metallographic samples quantifies the compound zone intermetallic (IMC) thickness. The shear tensile test specimens fail at wall thicknesses below 10 mm in the cast material, so metallurgical bond strength characterization does not occur. Therefore, the compression shear test is used to evaluate the metallurgical bond. Sound metallic bonding with smaller cast wall thicknesses of 8, 6 and 4 mm is achieved. Pullout specimens with 3 mm cast wall thickness further investigate the force-transmitting mechanisms of metallic bond, force-fit and form-locking. It is shown that metallic bonding is the predominant mechanism for force transmission when the compound sand casting process is enhanced by induction heating.

Characterization and Separation of Damage Mechanisms of Extruded Case-Hardening Steel AISI 5115 under Cyclic Axial-Torsional Loading

Due to the emerging relevance of topics such as climate change or scarcity of resources, the requirements for energy efficiency, emissions and resource conservation are increasing. In this context, components manufactured by metal forming offer a high potential for lightweight construction, cost effectiveness and resource efficiency. The defects resulting from forming processes e.g. in form of micropores and their growth are currently not taken into account. A commercial design is usually based on mechanical material properties and additional safety factors. The knowledge of the ductile forming-induced damage in the component design enables an improved design. In this study, the influence of different forming process parameters during full forward rod extrusion on the structural damage and the fatigue properties were investigated for the case-hardened steel AISI 5115 (16MnCrS5, 1.7131). The intention was to compare the fatigue properties of different damage states under cyclic axial and axial-torsional loading including the identification and separation of underlying damage mechanisms. A significant effect of superimposed cyclic torsional loading on cyclic axial properties and mechanisms was found, which was associated with a decrease of 38 % in the lifetime. Axial-torsional fatigue tests were conducted at various test temperatures to determine the effect of forming-induced damage and test temperature on the fatigue strength. In addition, differences in microstructure as a result of forming-induced and fatigue-induced damages were validated by using scanning electron microscopy.

Biomimetics Design of Sandwich-Structured Composites

In the context of energy efficiency and resource scarcity, lightweight construction has gained significant importance. Composite materials, particularly sandwich structures, have emerged as a key area within this field, finding numerous applications in various industries. The exceptional strength-to-weight ratio and the stiffness-to-weight ratio of sandwich structures allow the reduction in mass in components and structures without compromising strength. Among the widely used core designs, the honeycomb pattern, inspired by bee nests, has been extensively employed in the aviation and aerospace industry due to its lightweight and high resistance. The hexagonal cells of the honeycomb structure provide a dense arrangement, enhancing stiffness while reducing weight. However, nature offers a multitude of other structures that have evolved over time and hold great potential for lightweight construction. This paper focuses on the development, modeling, simulation, and testing of lightweight sandwich composites inspired by biological models, following the principles of biomimetics. Initially, natural and resilient design templates are researched and abstracted to create finished core structures. Numerical analysis is then employed to evaluate the structural and mechanical performance of these structures. The most promising designs are subsequently fabricated using 3D printing technology and subjected to three-point bending tests. Carbon-fiber-reinforced nylon filament was used for printing the face sheets, while polylactic acid (PLA+) was used as the core material. A honeycomb-core composite is also simulated and tested for comparative purposes, as it represents an established design in the market. Key properties such as stiffness, load-bearing capacity, and flexibility are assessed to determine the potential of the new core geometries. Several designs demonstrated improved characteristics compared to the honeycomb design, with the developed structures exhibiting a 38% increase in stiffness and an 18% enhancement in maximum load-bearing capacity.

Fine Blanking Limits of Manganese-Boron-Steel in Fine Blanking Compared to Tempered Steel with Variation of Sheet Metal Temperature

Fine blanking is a key technology for the near-net-shape production of sheet metal components. It is often used in combination with processes such as deep drawing. With increasing environmental requirements, the processing of materials with lightweight construction potential is becoming more and more important. A group of materials with high potential for lightweight construction, not only due to their suitability for hot stamping, is the group of manganese-boron steels. The fine blanking properties of these steels have not yet been exhaustively investigated. In this work, the fine blanking properties of the manganese-boron steel 40MnB4 were investigated in comparison to the quenched and tempered steel 42CrMo4. This was done using a star-piece fine blanking layout with investigation of die roll as well as tooth tip cracking. Furthermore, an investigation of the high-temperature fine blanking properties were investigated by means of inductive heating prior to fine blanking. The process forces were evaluated depending on the sheet metal temperature. A good fine blanking capability of 40MnB4 could be confirmed. Process forces and product quality were comparable to 42CrMo4 steel. Accordingly, an industrial application of fine blanking to manganese-boron steels seems highly promising.

Investigation of the Mechanical Properties of Vinylester-Based Sheet Moulding Compound (SMC) Subject to Particle Recycling

In light of climate change, fiber-reinforced plastics (FRP) are becoming increasingly important due to their lightweight construction potential. This benefit is additionally expanded on with the low cycle times, scrap rates and material costs of Sheet Moulding Compounds (SMC), making it the most often used material on the FRP market. While extensive studies regarding the recycling of SMC with unsaturated polyester-based matrices (UP-SMC) have been conducted in the past, this is not the case for vinylester-based SMC (VE-SMC). In this research, VE-SMC components were subject to a particle-recycling approach. Recyclate in the form of VE-SMC regrind was used to substitute SMC matrices during the compounding process. The dependence of the mechanical properties and failure behavior of VE-SMC containing regrind was investigated by means of microscopic and mechanical test methods for varying mass proportions of SMC regrind. Due to the addition of SMC regrind, a decreased fiber/matrix adhesion was observed. Furthermore, an increase in pore formation was observed with an increasing proportion of SMC regrind. The flexural modulus increased by 20% with a low percentage of regrind (ωrSMC = 10 wt.%) in comparison to virgin SMC. In contrast, the tensile properties decrease (up to 30%) with the addition of SMC regrind independent of the investigated proportions of SMC regrind.

In-situ characterization of impact damage in carbon fibre reinforced polymers using infrared thermography

Passive Infrared Thermography (pIRT) was evaluated for its capability for in situ monitoring of impact events in thin-walled fiber reinforced polymer composites. Low velocity impact events with low impact energy lead to barely visible, or even invisible from the outside damages in fiber reinforced polymer composites, severly limiting its light weight construction potential. A new method for evaluation of thermograms of pIRT in the frequency domain is proposed to gain insight in the dynamic of crack formation as well as precise quantification of damage extent in situ during impact testing. Results on aircraft grade carbon fiber reinforced composites illustrates the potential of this method.

Determination of Strain Limits for Dimensioning Polyurethane Components.

Within the scope of this contribution, a method for the determination of a strain limit for designing components made of elastomeric polyurethane systems is presented. The knowledge of a material-specific strain limit is essential for the structural-mechanical calculation of plastic components in the context of component design. Compared to a commonly used component design, based on a simplified dimensioning approach taking only linear viscoelastic deformations into account, the strain limit determined in this study allows an improved utilisation of lightweight construction potential in the dimensioning of technical components made of polyurethanes through the consideration of permissible nonlinear viscoelastic deformations. The test method comprises a sequence of quasi-static loading and unloading cycles, with a subsequent load-free recovery phase, allowing the relaxation of the viscoelastic forces. Standardised tensile and simple shear test specimens and a dynamic mechanical thermal analyser (DMTA) are used within the tests. The strain limit is determined by means of the so-called residual energy ratio, which is a characteristic quantity for the evaluation of hystereses of load–unload cycles. These hystereses are increasingly formed by deformations outside the range of linear viscoelastic deformations. The residual energy ratio relates the proportion of deformation energy recovered during unloading to the deformation work that is applied. In this contribution, the residual energy ratio is successfully used to detect a significant evolution of loss energy under increasing load and to correlate this transition to a characteristic strain. The latter is used as a dimensioning parameter for the design of components made of elastomeric polyurethane materials for quasi-static load cases. The determination of this strain limit is performed under consideration of the criterion of reversibility of deformation.

Temperature-controlled tools for multi-stage sheet metal forming of high-strength aluminium alloys

The high-strength aluminium alloys EN AW-6082 and -7075 possess great light-weight construction potential due to their high specific strength. However, their range of applications is limited by the low cold formability and high springback in the ultra-high-strength T6 state. To extend formability, either preconditioned semi-finished products, requiring a subsequent heat treatment or temperature-supported process routes such as warm forming or hot forming die quenching (HFQ) are used. If the forming degrees achievable with these methods are not sufficient, multi-stage forming is required. The forming degrees are limited by strain hardening in case of a cold forming operation of preconditioned semi-finished products and by heat transfer between the component and the tool parts in case of temperature-supported process routes. In order to reduce or even eliminate the cooling of the component in the subsequent forming stages, a four-stage forming tool with temperature-controlled active parts was developed and its influences investigated in more detail. The paper will demonstrate how precise temperature control of the tool components contributes to an extension of the formability. In addition, reproducible and robust process conditions can be guaranteed throughout the entire process chain.

Using Quantitative Passive Thermography and Modified Paris-Law for Probabilistic Calculation of the Fatigue Damage Development in a CFRP-Aluminum Hybrid Joint.

Although metal to Carbon-fiber-reinforced-polymer (CFRP) hybrid-joints possess a high lightweight construction potential, their extensive application has to deal with interfacial stress concentrations promoting fatigue damage. Furthermore, the underlying damage processes and their influencing factors are still not completely understood. Besides interfacial property-gradients, generic shapes counteract a precise determination of local stresses or strains, respectively. Hence, new methods are required that combine non-destructive testing and fracture mechanics to account for the fatigue damage. In this work, data of mechanical fatigue testing of an aluminum-CFRP hybrid-structure is presented by means of the dynamic stiffness and the mechanical hysteresis. Additionally, in situ passive thermography allows for capturing the heat development due to delamination growth. Correlating the obtained data implies that faster delamination growth coincides with higher amplitude values of lock-in thermography and higher mechanical hysteresis. Supported by this observation, a model is formulated to calculate the local dissipation per loading cycle. Further integration into a Paris-law like formulation results in a calculation model to account for the mode-I fatigue delamination growth. Additional validation of the model parameters shows good agreement with the experimental data.

Novel Low-Twist Bast Fibre Yarns from Flax Tow for High-Performance Composite Applications.

The use of natural fibres for components subjected to higher mechanical requirements tends to be limited by the high price of high-quality semi-finished products. Therefore, the present study deals with the development of more cost-effective staple fibre yarns made from flax tow. In the subsequent processing stage, the yarns were processed into quasi-unidirectional (UD) fabrics. The results of the fibre characterisation along the process chain have shown that no significant mechanical fibre damage occurs after slivers’ production. Fibres prepared from yarns and fabrics show comparable characteristics. The yarns were processed to composites by pultrusion to verify the reinforcement effect. The mechanical properties were comparable to those of composites made from a high-quality UD flax roving. The fabrics were industrially processed into composite laminates using a vacuum infusion and an autoclave injection process (vacuum injection method in an autoclave). While impact strength compared to a reference laminate based on the UD flax roving was achieved, tensile and flexural properties were not reached. An analysis showed that the staple fibre yarns in the fabric show an undulation, leading to a reorientation of the fibres and lower characteristic values, which show 86–92% of the laminate made from the flax roving. Hybrid laminates with outer glass and inner flax layers were manufactured for the intended development of a leaf spring for the bogie of a narrow-gauge railroad as a demonstrator. The hybrid composites display excellent mechanical properties and showed clear advantages over a pure glass fibre-reinforced composite in lightweight construction potential, particularly flexural stiffness.

Structural optimization of a wheel force transducer component for more realistic acquisition of vehicle load data and fracture mechanical evaluation

Special measuring wheels are used to record vehicle load data. It is important that the data is recorded unaltered and as true to the original as possible. Due to the sensor technology, the measuring wheels are usually heavier than standard rims, which results in an influence on the recorded measurement data.In this paper an approach is shown to develop a structure with the same performance but reduced mass by using CAD and numerical analysis. Due to the geometrically complex structure, the selective laser melting process is chosen as the manufacturing method for the optimized part. The lightweight construction potential can be fully exploited by homogeneously stressing the optimized part and by using the Ti6–4 alloy, which has a higher Young's modulus than the aluminum alloy used so far. The interaction of the methods of technical mechanics, CAD, FEM, and additive manufacturing results in a safety-relevant component that has a mass reduced by almost 40% compared to the previously used component while maintaining the same stiffness. A final crack growth simulation confirms the operationally safe design. Finally, the component was first tested experimentally on a test rig and then installed in a vehicle. All tests were passed successfully.

Technology fusion of metal injection moulding and selective laser melting for the manufacture of complex and multifunctional (hydraulic) components

Selective laser melting (SLM) and metallic injection moulding (MIM) are established processes for the production of high-performance metallic components for small and large series. In the aerospace industry, where very high demands are placed on materials and components, both processes are still considered to be relatively new. In both processes, the conventional titanium alloy Ti-6Al-4V can be used in the form of powder. Currently, both technologies are only considered separately. By fusing components of the same type, multifunctional components with a high lightweight construction potential can be produced.
 In order to generate direct material fusion, the MIM component must be mechanically processed accordingly. In addition, suitable SLM process parameters must be developed in order to ensure both generative construction and high joint strength. To this end, a characterisation of the joining zone and the static joint strength was carried out. Furthermore, pressure test samples were designed and examined both statically and for fatigue strength. Thus, a high static joint strength could be proven. The compression test samples also withstood a fatigue strength of over 1 million cycles.

Improvement of the adhesive bonding properties of an polyamide 6 injection molded fiber reinforced plastic component by laser beam pre-treatment

Fiber-reinforced plastics (FRP) are widely used in the automotive industry due to their lightweight construction potential. Especially, injection-molded components are very popular because of their short cycle times. Unfortunately, this manufacturing process results in some disadvantages for delayed adhesive bonding. On the one hand, such thermoplastics have a very low surface energy (Polyamide 6 (PA6) 36 mJ/m), on the other hand residues of release agents on the surface hinder adhesion between product and adhesive. These disadvantages have to be overcome by the right surface pre-treatment process. Currently, a solvent-containing process is usually used for this purpose. If necessary, the use of primer is common. In the presented paper, the pre-treatment process of an injection-molded component made of polyamide 6 reinforced with glass short-fibers is presented. First, material-specific properties are measured. The influence of laser pre-treatment at 355 nm and 1064 nm wavelength is investigated. Measurements of surface free energy and surface roughness are conducted. The bonding strength is validated by peel test and tensile shear tests. The shear strength was doubled by laser pre-treatment at a medium energy level, partially cohesive failure of the adhesive was achieved. Both investigated wavelengths led to a great improvement of bonding strength at suitable parameters.

Mechanical Properties and Impact Toughness of Molybdenum Alloyed Ductile Iron

Grades of high silicon ductile iron offer excellent combinations of static strength and ductility as well as good machinability due to their fully ferritic, solution strengthened matrix. As a result of elevated silicon contents, however, the ductile-to-brittle transition temperature in the Charpy impact test is significantly increased. Thus, minimum required Charpy impact energies cannot be met for many applications by using high silicon ductile iron. Therefore, alloys with lower strength and higher toughness properties are commonly used for many technical applications. The enormous lightweight construction potential of high silicon ductile iron can therefore not be fully exploited. The present investigation pursues the metallurgical approach of partially substituting silicon with molybdenum as an alternative strengthening element in order to improve the toughness properties while maintaining similar static mechanical properties. Molybdenum serves as a carbide-stabilising element in ductile iron, while simultaneously promoting ferrite formation and is therefore regarded to be suitable alloying element. In Charpy impact tests, the ductile-to-brittle transition temperature could be reduced by about 55 °C by reducing the silicon content to 2.95 wt% and adding 0.21 wt% molybdenum compared to a high silicon alloy. Additionally, it was possible to mathematically describe the transition behaviour of the studied alloys using nonlinear regression functions and to achieve a sufficient correlation of empirically determined and calculated data. This present metallurgical concept offers a promising metallurgical tool for further improving the toughness properties of alloyed ductile iron.

Statistical analysis of S/N-curves by means of a fatigue database for polypropylene

Material databases of plastics are becoming more and more the focus of applied science and commercial use in industry. Material properties of material manufacturers are often provided in publicly accessible material databases, which usually contain processing and mechanical properties under static loadings. Fatigue strength values are usually not accessible. The fatigue data for thermoplastics is of particular interest, as these materials have a particularly high lightweight construction potential and can be processed with a high degree of automation and reproducibility.Individual fatigue strength parameters for a specific material, environmental condition, geometry and loadings have been investigated in numerous publications. However, no work has been found in which fundamental interactions of different materials, environmental conditions, geometries and loadings on the course of the S/N-Curve have been investigated.In this paper, different effect relationships between temperature, filler type and filler content, fiber orientation and load ratio will be presented for the material Polypropylene (PP). A fatigue strength database of 11 different material manufacturers, from 71 different S/N-Curves with 606 tested samples, serves as a basis. The fatigue database enables a digital twin, which is used for the design of structural components to add a third dimension with artificial intelligence, and which is trained by an engineer. From the determined effect relationships, fatigue factors are to be derived and can be used to evaluate the fatigue strength of a component in the design process and to train the digital twin. The fatigue-strength values from the database also allow a statistical consideration of the slope k and the scattering of the S/N-Curve. The different S/N-Curves are transferred into a Haigh diagram, from which the functional course of the mean stress is determined.

Air inclusions in the polymer melt functioning as intrinsic physical blowing agents for the generation of foams in rotational molding

In recent years, foams have experienced a major economic uprise, not least due to their lightweight construction potential. In this article, a new process variation is presented, which enables the generation of foamed structures in rotational molding by the utilization of vacuum. The novel method is based on entrapped air in the melt as an intrinsic physical blowing agent. By applying negative pressure in the cooling or solidification phase, the air bubbles expand. The crystallization freezes the existing conditions and thus forms the foamed structure. The investigations presented consider influences by different pressures as well as the temperature at which the vacuum is applied. The results with polyethylene show that by varying the pressure as well as the application temperature of the vacuum, components with different densities and cell characteristics result. The resulting foamed components excel by an improved stiffness per unit weight ratio.

Ductile viscose fibres and stiff basalt fibres for composite applications – An overview and the potential of hybridisation

In the present study, the hybridisation of ductile viscose (Danufil) and stiff basalt fibres was investigated. The aim was to combine the positive properties of both fibre types in an epoxy matrix. Composites (round rods) were produced with a fibre volume fraction of 40% with a pultrusion technique from Danufil, basalt and a mixture of Danufil and basalt fibres. The hybrid composites were produced on the one hand with a unidirectional (UD) Danufil core and a UD basalt outer layer and vice versa. On the other hand, a UD core layer of Danufil is covered with a twisted basalt outer layer and vice versa. The results have shown that the most successful hybridisation was realised with the UD composite having a Danufil core and a UD basalt outer layer. An impact strength on the level of the basalt fibre-reinforced composite could be achieved whereby the lightweight construction potential could be improved. The spontaneous failure of the basalt fibre-reinforced composite under bending load could be considerably delayed by the addition of Danufil, whereby the strength, stiffness and temperature resistance were significantly improved by the presence of basalt. For the application of round rods, e.g. in poles for mobile architecture, the load-bearing capacity and the safety aspects could be significantly improved by hybridisation.

Analysis of incremental die bending of wires and tubes

The use of bent wires, tubes, and profiles has a broad application in the industry as it is based on a great number of possible applications. Both structures with a high light-weight construction potential as well as functional and aesthetic geometry can be realised expediently. Known processes are often limited in case of tight radii and complex bending contours that cut each other or that are similar to a loop. The so called incremental die bending provides the remedy. It is an innovative process for the production of complexly shaped workpieces in the form of bent wires, tubes, or profiles. The bending process in case of incremental die bending is different to conventional processes, since a die is reshaped. With this process the semi-product is put into a hard bending shape by a moving feeding unit. The process is very accurate and at the same time cost-effective, since almost any bending contours can be produced form-based in one working stage. Furthermore, the die geometry enables the manufacture of contours that were previously very work-intensive or not producible at all.