Selective Laser Melting of Honeycombs with Negative Poisson’s Ratio

Laser Freeform Fabrication allows additive manufacturing of functional parts from metallic powder materials. This yields potential for product innovations due to increased geometric degrees of freedom. Additional advantages are reduction of throughput time and economical manufacturing of small lot sizes. Hence, these processes are capable of producing structures of complex freeform geometry that cannot be manufactured at all with conventional processes This provides the possibility to manufacture designs that show a negative Poisson's ratio . It gives the relation of the negative transverse strain to the longitudinal strain of a material exposed to uniaxial tension or compression. The longitudinal strain occurs in the load direction, whereas the transverse strain is found normal to this axis. Typically, the Poisson's ratio is greater than zero, i.e. in tension a material's volume can increase or remain constant while the behaviour in compression is vice versa. However, some materials can exhibit negative values between -1 and 0 meaning that they show unexpected behaviour by expanding transversely under tensile forces and vice versa under compression. This circumstance can be exploited in applications where parts are subject to compressive deformation but which are not allowed to expand laterally in order to not cause damage in adjacent components. Examples are crash impact absorbers and artificial intervertebral discs, etc. Experimental results were obtained from new honeycomb structures that were manufactured with Selective Laser Melting from stainless steel powder and tested in a universal testing machine under elastic, compressive deformation. Thus rules for successful design of honeycomb structures with negative Poisson's ratio were derived.