High Specific Bending Stiffness Research Articles

Reliability analysis and design of randomly imperfect thin cylindrical shells against post-critical drops

Thin-walled cylindrical shells are employed for high specific bending stiffness and light design. Elastic buckling of shell is a critical design consideration. The Knock Down Factors (KDFs) are stipulated for accounting the markedly reduced experimental critical load by multiplying the linearized critical load by the KDF. Key to such reductions is the nonlinearity induced modal interactions among the closely spaced buckling modes, over which, the geometric imperfections act in synergy. Being the lower bound of an extensive experimental dataset; the stipulated design KDFs are largely over-conservative; which can be relaxed by accounting for imperfections. With this as the eventual goal, this study first demonstrate the feasibility of modeling the KDFs as uniformly distributed Uncertain-but-Bounded (UBB) type variable. The UBB description is employed to obtain the reliability bounds and the design variables (radius-to-thickness R/t and L/R ratio) for achieving a target reliability index through Reliability Based Design (RBD). Repeated computations of KDFs for reliability assessment are bypassed by fitting the KDFs vs. R/t relations for given L/R. The approach is demonstrated on a thin cylindrical shell with a zero-mean, stationary, gaussian, stochastic imperfection fields. The Riks method is employed for the nonlinear finite element analysis of the shell using the code ABAQUS. The KDFs and reliability bounds are provided for varying imperfections and are compared with an existing study on refining the KDFs. Furthermore, the RBD presents the desired R/t for a given L/R for achieving a target reliability index and the respective KDF for a given imperfection level. The L/R dependency is shown to reduce the over-conservatism in the design stipulation.

Heat Treatment of Titanium-Aluminum-Compounds Made by Co-Extrusion of Asymmetric Compound Profiles

Combining different materials allows designing lightweight structures with tailor-made properties at the macroscopic as well as at the microscopic scale. In this context co-extrusion by lateral angular co-extrusion process (LACE) offers a great potential for advanced products by joining different light alloys, such as titanium and aluminum alloys. While titanium alloys show particular high mechanical strength values and a good corrosion resistance, aluminum alloys feature a considerable high specific bending stiffness at low materials costs. Within the presented research study metallic compounds were manufactured by co-extrusion using a LACE process. Since the mechanical properties of metallic composites highly depend on the existence and formation of an intermetallic layer, the bonding zone of the compounds were investigated. Experiments were carried out at the example of the material combinations Al99.5 & Ti99.2 and AlSi1MgMn & Ti99.2, respectively. The results of co-extruded samples concerning the intermetallic layer that was formed during heat treatment were compared with those directly after the co-extrusion. The investigations were focused on the formation of the bonding zone after both co-extrusion and the subsequent heat treatment. The bonding zone was characterized by scanning electron microscopy as well as electron probe micro analysis. Finally, the bond strength of each compound variant was determined and evaluated.

Characterization of the interface of co‐extruded asymmetric aluminum‐titanium composite profiles

The combination of different light alloys enables lightweight solutions with tailor‐made properties at the macroscopic global as well as at the microscopic scale. In this context co‐extrusion by a lateral angular co‐extrusion (LACE) process offers a great potential for advanced profiled structures. While titanium alloys show particular high mechanical strength and good corrosion resistance, aluminium alloys provide a considerable high specific bending stiffness along with low materials costs.The mechanical properties of metallic compounds strongly depend on the bonding mechanisms, which are initiated during processing. In order to enable a rigid adhesive bond between Al and Ti in this study the extrusions were processed by means of a newly developed lateral angular co‐extrusion that is a modified equal channel angular pressing process (ECAP). Latter is known for promoting diffusion bonding by reducing the activation energy. Hence, the study is focused on the general manufacturing of Al–Ti‐compounds by lateral angular co‐extrusion. The second aim of the study was to investigate the development of the bonding zone during processing. Experiments were made with the material combination Al99.5 and titanium grade 2. The bonding zones of the co‐extruded samples were analyzed by scanning electron microscopy and energy dispersive X‐ray analysis. The bond strength was determined by quasi‐static tensile tests. Compared to the as‐extruded condition an intermetallic layer was formed during heat treatment. The layer was characterized by scanning electron microscopy and electron probe micro analysis.

Heat Treatment of Aluminum-Titanium-Compounds Made by Co-Extrusion and Friction Welding

The combination of aluminum and titanium alloys allows for designing lightweight structures with tailor-made properties at the macroscopic global as well as at the microscopic scale. In this context both co-extrusion and friction welding offer a great potential for advanced solutions for products with material combinations of aluminum and titanium. While titanium alloys show particular high mechanical strength and good corrosion resistance, aluminum alloys provide a considerable high specific bending stiffness along with low materials costs. Since the mechanical properties of metallic composites highly depend on the existence and formation of the intermetallic layer in the bonding zone compounds were processed by co-extrusion and friction welding and subsequent heat treatment to investigate the strength and the composition of the bonding zone. The results of friction welded samples concerning the intermetallic layer that was formed during heat treatment were compared with those directly after the co-extrusion. In this layer an enrichment of elements which origin from the aluminum alloy, particularly silicon, was observed. The layer was characterized by optical microscopy, scanning electron microscopy as well as electron probe micro analysis. The mechanical properties were determined by tensile tests.

All-Thermoplastic Composite Sandwich Panels – Part I: Manufacturing and Improvement of Surface Quality

All-thermoplastic sandwiches with composite faces and thermoplastic foam as core material present great potential for future automotive mass applications. They offer many advantages such as high specific bending stiffness, good insulation properties, high impact resistance,... Furthermore, the use of one single polymer throughout the sandwich brings on optimal face–core compatibility, enhanced recyclability and post-formability. The aim of this first part is to investigate the processing of thermoplastic sandwich structures with improved surface quality manufactured by isothermal compression moulding.

Manufacturing of all‐thermoplastic sandwich systems by a one‐step forming technique

AbstractThermoplastic sandwich composite systems offer great potential for future automotive applications. High specific bending stiffness, good insulation properties, high specific energy absorption and impact resistance can be achieved by the use of fabric reinforced sheets as sandwich faces and thermoplastic foam as core material. Recycling is possible by chopping and injection molding. Fabric‐reinforced thermoplastic sheets consisting of 10 layers glass fabric were impregnated continuously with a polyamide‐12 matrix using isobaric double belt press technique to 2‐mm‐thick faces with a fiber content of 50 vol%. Sandwich angles and hemispheres were manufactured by heating the faces and the 13‐mm‐thick foam core above the melting temperature and high‐speed stamp forming with matched metal tools. Cycle times for forming and cooling were below 40 s/part. Different heating methods were matter of investigation. Best results could be achieved by conductive heating. This new material system and its processing technology are promising methods to reduce structural car weight and enhance passenger safety.