Robert A.W. Mines

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.

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.

Strain Rate Effects in Crushable Structural Foams

Structural foams are used as cores in sandwich construction. In the application of foreign object impact loading of sandwich structures, the core will suffer dynamic multi axial deformation and crush. This means that experimental study is required for the crush behaviour of structural foams at various strain rates, and numerical simulation foam models need to be calibrated with dynamic data. A number of foams are considered, namely Divinycell PVC foam, Rohacell PMI foam and Alporas aluminium foam. Also, new generation metallic micro lattice structures are discussed

Low-velocity impact performance of lattice structure core based sandwich panels:

This paper outlines the findings of a study on a range of stainless steel and titanium alloy lattice structures manufactured using the selective laser melting technique. The effect of varying key manufacturing parameters on the properties of lattice strands was studied through a series of single-filament tensile tests. The resulting failure mechanisms were investigated using a scanning electron microscope. The resulting observations have shown that the properties of these lattice strands are determined by the laser energy during the manufacturing process, which in turn is controlled by the laser power and laser exposure time. The quasi-static and low-velocity penetration behaviour of lattice core-based sandwich panels has been examined, and an aluminium foam and an aluminium honeycomb were chosen to benchmark their performance. The impact resistance of the lattice core-based sandwich structures were shown to be dependent on both the manufacturing parameters and lattice unit-cell geometry of the lattice structure. The impact resistances were improved by increasing manufacturing laser energy and lattice core density. A series of drop-weight tests at velocities up to 6 m/s have shown that the penetration behaviour of the titanium alloy lattice cores and the aluminium honeycomb cores is similar.

Comparison of the Drop Weight Impact Performance of Sandwich Panels with Aluminium Honeycomb and Titanium Alloy Micro Lattice Cores

This paper is a study of the drop weight impact behaviour of small sandwich panels of carbon epoxy skins with aluminium honeycomb and titanium alloy micro-lattice cores. A series of experimental tests have shown that the specific impactor penetration behaviours are similar for both cores. The reasons for this are a result of the detailed deformation and rupture behaviour of the two types of core. The deformation and rupture mechanisms of honeycomb and micro-lattice structures will be discussed in general terms, and these observations will be used to inform discussion of actual deformation and rupture in the panel tests. In this way, micro energy absorbing mechanisms will be related to panel performance, and conclusions on the way forward for improved penetration performance using other core materials and geometries will be identified.

The Properties of Lattice Structures Manufactured Using Selective Laser Melting

This paper outlines the findings of an on-going research study investigating the properties of a range of steel and titanium-based micro-lattice structures manufactured using the selective laser melting (SLM) technique. Initially, tension tests have been conducted on strands manufactured at different build angles. Micro-lattice block structures, with struts oriented at +/-45o were then tested in compression at quasi-static rates of loading. The failure mechanisms have been investigated using both optical and scanning electron microscopy. These tests have highlighted the attractive properties offered by these complex architectures.

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

Comparison on Compressive Behaviour of Aluminium Honeycomb and Titanium Alloy Micro Lattice Blocks

The paper discusses the compressive behaviour of two materials, the conventional aluminium honeycomb and the new titanium alloy micro lattice blocks. The new titanium alloy micro lattice structure is being developed as core material candidate in sandwich construction for aerospace application. Experimental tests have been done on the blocks in order to compare its property with the aluminium honeycomb. Compression strength as well as compressive behaviour of both materials are compared and observed. The mechanisms that contributed to the differences in their performance are discussed and this will be used to improve the geometrical and structural design of micro lattice structure in order to achieve properties that are superior or at least comparable with that of aluminium honeycomb.