S. McKown

The Influence of Processing Parameters on the Mechanical Properties of Selectively Laser Melted Stainless Steel Microlattice Structures

The rapid manufacturing process of selective laser melting has been used to produce a series of stainless steel 316L microlattice structures. Laser power and laser exposure time are the two processing parameters used for manufacturing the lattice structures and, therefore, control the quality and mechanical properties of microlattice parts. An evaluation of the lattice material was undertaken by manufacturing a range of struts, representative of the individual trusses of the microlattices, as well as, microlattice block structures. Low laser powers were shown to result in significantly lower strand strengths due to the presence of inclusions of unmelted powder in the strut cross-sections. Higher laser powers resulted in struts that were near to full density as the measured strengths were comparable to the bulk 316L values. Uniaxial compression tests on microlattice blocks highlighted the effect of manufacturing parameters on the mechanical properties of these structures and a linear relationship was found between the plateau stress and elastic modulus relative to the measured relative density.

The Mechanical Properties of Sandwich Structures Based on Metal Lattice Architectures

A range of metallic lattice structures were manufactured using the selective laser melting (SLM) rapid prototyping technique. The lattices were based assemblies of repeating unit-cells with their strands oriented at 0°, ±45°, and 90° to the vertical when viewed from the front. Mechanical tests on the strands and the lattice blocks showed that these systems exhibit a high level of reproducibility in terms of their basic mechanical properties. An examination of the compression failure mechanisms showed that the [±45°] and [±45°, 90°] lattices failed in bending and stretching modes of failure, whereas the [0°, ±45°] lattices failed as a result of buckling of the vertical pillars. Sandwich structures were manufactured by binding woven carbon-fiber reinforced plastic to the lattice structures. Subsequent three-point bend tests on these structures identified the principal failure mechanisms under flexural loading conditions. Here, cell crushing, hinge rotation, and gross plastic deformation in the strands were observed directly under the point of loading. Low-velocity impact tests were conducted on the sandwich beams and a simple energy-balance model was used to understand how energy is absorbed by the sandwich structures. The model suggests that the majority of the incident energy of the projectile was absorbed in indentation effects, predominantly in the core material, directly under the steel indenter.

Investigation of Strain-rate Effects in Self-reinforced Polypropylene Composites

The strain-rate sensitivity of a hot-compacted formed self-reinforced thermoplastic composite, based on polypropylene fibers in a polypropylene matrix, was investigated through a series of quasi-static and dynamic tensile tests. Characterization of the mechanical property dependence on strain-rate for a self-reinforced thermoplastic is an important issue when the material forms one part of a hybrid system, as in the case of a thermoplastic fiber—metal laminate that is prone to localized impact loading events. Strain-rates in the range from quasi-static (10—4 s—1) up to 10 s—1 were achieved in the gauge region of rectangular specimens loaded in a servo-hydraulic test machine. A measurable rate effect was observed in key mechanical properties of the self-reinforced composite, and constitutive equations were successfully applied to characterize the strain-rate behavior of the yield stress. Failure of the longitudinal ply fibers was the dominant failure mechanism, whilst the degree of inter-ply delamination varied over the dynamic loading range.

Investigation of Scaling Effects in Fiber—Metal Laminates

The results of a series of quasi-static and impact tests on four scale-model sizes of fiber—metal laminate (FML) are compared to a scaling law that predicts response parameters based on a simple geometrical relationship of the input parameters. The FMLs consist of an aluminum alloy and a self-reinforced thermoplastic composite based on polypropylene fibers in a polypropylene matrix. The scaled FML laminates were arranged in a 2/1 configuration, and ply-level scaling of the FML constituent materials was employed to yield specimens with a nominally constant composite volume fraction, as well as correctly scaled in-plane and bending stiffness properties. In the initial part of this experimental program, the tensile and flexural properties of these hybrid materials were investigated at quasi-static rates of loading. Here, no significant scaling effects were observed in the mechanical response of the laminates. Following this, simply-supported scaled beams and plates were subjected to low-velocity impact loading in order to investigate scaling effects in the processes of damage development and target perforation. Here, response parameters such as the target deflection, the impact force, and the damage threshold energy were found to obey the scaling law. It is believed that experimental data of this nature will give greater confidence to engineers involved in the design of components based on hybrid materials such as fiber—metal laminates.

Local Effects during Indentation of Fully Supported Sandwich Panels with Micro Lattice Cores

This paper discusses the penetration behavior of fully supported sandwich panels with micro-lattice and foam cores, and composite skins. This behaviour is of importance during foreign object impact and perforation of sandwich structures. Experimental results are given for quasi-static penetration of micro-lattice and foam blocks, and it is shown that these two cellular materials are comparable. Experimental results are also given for drop weight penetration of fully supported skinned panels, and it is shown that skin failure and core penetration are also similar for the two core materials. It is concluded that there is scope for improving the performance of micro-lattice structure and so making such material superior to that of aluminium foam.

Measurement of Material Properties for Metal Foam Cored Polymer Composite Sandwich Construction

Material properties are required for the numerical simulation of the impact progressive collapse of metal foam cored polymer composite sandwich beams, using LS-DYNA. As far as the metal foam, Alporas, is concerned, multi-axial tension and compression data is required. This includes large scale crush and tensile rupture. An Arcan test fixture was developed, in which a sample of foam can be subject to tensile and shear deformation simultaneously. The data was also used to calibrate the crushable foam material model in LS-DYNA. For the skin, tensile and compression data was generated for a cross ply glass fibre thermoplastic laminate. The data was then used to calibrate the composite damage material model in LS-DYNA. Inclusion of calibrated material models into the simulation of the progressive collapse of metal foam cored composite sandwich beams will be briefly discussed.

On the Progressive Collapse of Micro Lattice Structures

Foam and honeycomb materials have been used for many years as cores in sandwich construction. Foam materials range from polymeric materials (Divinycell, Rohacell), through metals (Alporas aluminium foam), to graphite. Similarly, materials for honeycomb can be aluminium (Hexcel) or aramid (Hexcel). The main design variable for these cellular materials is density, but in general the microstructures of these materials are restricted to one, or a few, geometries. More recently, rapid prototyping manufacturing processes, such as selective laser melting, have been developed that allows the realisation of metallic open cellular lattice structures with resolution of 50 micro meters

Verification of a Finite Element Simulation of the Progressive Collapse of Micro Lattice Structures

Stainless steel micro lattice structures, manufactured by selective laser melting, have the potential to be used as core materials in twin skinned structures. The configuration considered here is body centred cubic. One of the major structural performance requirements for such twin skinned structures is foreign object impact. The paper describes the series of steps taken to simulate (using DYNA) and validate the low velocity foreign object impact behaviour of twin skinned panels with micro lattice cores. This includes the validation of a three beam model for each micro strut, the modelling of node behaviour, and the modelling and validation of compressed micro lattice blocks, and of full foreign object impact panel behaviour.

Measurement of the Large Strain Behaviour of Aircraft Tyre Rubber

The paper describes mechanical property tests on a Concorde aircraft tyre rubber. The tyre rubber is taken from the tyre tread, and consists of nylon reinforcement, laid up in an angle ply form. The constitutive behaviour of the rubber is characterised using the Mooney Rivlin approach, in which deformation is expressed in terms of strain energy. Static and dynamic tensile tests are conducted along the major reinforcement and minor reinforcement axes in the plane of the tread, and compression tests are conducted through the tread thickness. This data is then input into a finite element model of the tyre, using DYNA.