Hierarchical thermoplastic sandwich materials for even more efficient lightweight construction

Lightweight materials are used whenever high-quality products are required that offer an optimum balance between strength, weight, and reliability. Among other areas, this applies to aircraft, vehicle, ship, and container construction or the manufacture of wind power rotor blades. The Fraunhofer Institute for Microstructures of Materials and Systems IMWS and ThermHex Waben GmbH are now working together on a novel basic technology. They are focusing on hierarchical sandwich structures in which the cover layers themselves are sandwich structures in turn with the aim of making lightweight construction even more efficient.

Operating principle and structure of a sandwich structure with a honeycomb core

Currently, sandwich structures are particularly effective for use in lightweight construction and are especially suitable for large, flat structures. They consist of a light core and two solid thin cover layers. Plastic sandwich structures usually consist of a honeycomb core or polymer foam and cover layers made of fiber-reinforced plastics. This combination provides high bending and dent resistance with a very low weight.

The Fraunhofer IMWS and ThermHex Waben GmbH, which manufactures cost-efficient polypropylene (PP) honeycomb cores using a patented production process, have been cooperating in the field of lightweight construction with sandwich materials since the end of 2015. In the current joint project »HPHex - Basic Investigation of Thermoplastic Lightweight Sandwich Materials from Hierarchical Honeycomb and Cover Layer Structures for High-Strength and Cost-Effective Lightweight Structures,« sandwich materials with novel hierarchical structures are being studied with the aim of achieving a significant increase in mechanical performance.

High-performance and cost-effective core materials are already available in the form of the thermoplastic honeycomb cores currently used in lightweight construction. However, their mechanical performance is limited, especially compared to more cost-intensive honeycomb cores, making them more suitable for use in less heavily stressed trim and interior components. This is where the innovative approach of cooperation partners ThermHex Waben GmbH and Fraunhofer IMWS comes in. They intend to overcome the hitherto low buckling strength of thin thermoplastic honeycomb cell walls by using a hierarchical material design. The honeycomb cell walls themselves are formed by a honeycomb sandwich, thus significantly increasing the stability of the honeycomb core while maintaining or even reducing its weight. The project will also explore the approach of hierarchically structured cover layers, which offer advantages in terms of load distribution and surface quality. The investigation of such sandwich structures will then allow further physical properties to be described and evaluated. The project partners already have a EUREKA label entitled »Hierarchical Hexagonal Sandwich Honeycomb Core (HiHex).«

Project leader Dr.-Ing. Ralf Schlimper, who heads the group »Evaluation of Fiber Composite Systems« at the Fraunhofer IMWS, comments on the project start: »We are pleased to have another productive cooperation with ThermHex GmbH and to have been awarded the EUREKA label. Together, we can advance and coordinate the development of cost-efficient and high-performance hierarchical sandwich structures in the long term. The novel thermoplastic honeycomb structures will ensure a high degree of automation in production and low material costs as an alternative to previous cost-intensive construction methods.«

»The improvement of material properties, especially the increase of weight-specific properties by means of hierarchical structuring, is an approach that is becoming increasingly important worldwide,« says Dr.-Ing. Jochen Pflug, Managing Director of ThermHex Waben GmbH.