Mechanical behaviour of additive manufactured lattice structures

Tshikwand, Georgino Kaleng (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Lattice structures are open-cell, strut-based structures made up of unit cells that are tessellated in 3 orthogonal directions. Depending on the physical design of the unit cell, the lattice structure can be designed for customized stiffness, strength, and specific strain energy absorption. This allows for the design of lightweight, load-bearing structures, suitable for functional engineering applications. 5 X 5 X 5 octet-truss and diamond lattice structures of a specified relative density were designed using equations that relate the relative density to the strut dimensions. Computer-aided design (CAD) models of the structures were created and used to produce these lattice structures of Ti6AL4V using the additive manufacturing (AM) technique: laser powder bed fusion (L-PBF). Finite element analysis (FEA) was used to simulate the uniaxial compression of the lattice structures, yielding predictions for the stress distribution in the lattice struts, and allowing for the prediction of the deformation and failure modes. 3D solid and 1D beam elements were used for this purpose. A prediction of the global mechanical properties and deformation mechanism of the lattice structures was established for both types and compared. A comprehensive flowchart describing the FEA approach taken in order to predict the mechanical behavior of L-PBF lattice structures is provided. Mechanical uniaxial compression of the as-built lattice structures was conducted. Global mechanical properties were determined from the load-deformation data. The progressive collapse of struts under the load was analyzed in order to categorize the failure as stretch- or bending-based. Micro-computed tomographic (μCT) analysis of the as-built structures showed that the struts were thicker as compared to the CAD structures. Thus new CAD models were created with the strut thickness correlating to the actual average dimension. This resulted in improved prediction of the mechanical response of the as-built structures. A comparison of the FEA predictions and the experimentally measured mechanical properties of the lattice structures was carried out. It was determined that both the 3D solid and 1D beam structures predicted the actual mechanical properties of the octet-truss lattice structure within an error margin of less than 25 %. However, for the diamond lattice structure, only the 3D solid structure predicted the actual mechanical properties with an error margin of less than 20 %. A study of the stress distribution across individual struts in the structure was used to explain the deformation mechanisms observed in the respective lattice structures. The octet-truss structure was found to deform by a combination of 45° and 135° shear bands caused by the stretching of the horizontal struts in those planes, whereas the diamond lattice structure was found to deform by 45° shear bands caused by strut bending.

AFRIKAANSE OPSOMMING: Roosterstrukture is oopsel strukture waarvan stutte die basis struktuur maak, wat van eenheidselle wat in 3 ortogonale rigtings getëel word. Afhangend van die fisieseonterwerp van die eenheidsel, kan die roosterstruktuur pasgemaak word vir styfheid, sterkte en spesifieke opname van spanningenergie. Dit maak voorsiening vir die ontwerp van liggewig, las-draende strukture, geskik vir funksionele ingenieurswese-toepassings. 5 x 5 x 5 Octetstut- en diamant-roosterstrukture van ʼn spesifiseerde relatiewe digtheid is ontwerp met behulp van vergelykings wat die relatiewe digtheid met die stutafmetings verwant. Rekenaargesteunde ontwerp (CAD) modelle van die strukture is gebou en gebruik om hierdie roosterstrukture te vervaardig uit Ti6Al4V, met die gebruik van die bytoevoeging vervaardiging (AM) tegniek: laserpoeierbedversmelting (L-PBF). Eindige elementanalise (EEA) is gebruik om eenassige samedrukking van die roosterstrukture te simuleer, wat voorspellings vir die spanningsverspreiding in die roosterstutte en toelaat dat die deformasie en failingsmodusse voorspel kan word. 3D-soliede en 1D-balk-elemente is vir hierdie doel gebruik. Vir beide tipes is 'n voorspelling van die globale meganiese eienskappe en vervormingsmeganisme van die roosterstrukture vasgestel en vergelyk. 'n Omvattende vloeidiagram word gegee wat die EEA-metodiek wat geneem word om die meganiese gedrag van L-PBF-roosterstrukture te voorspel. Meganiese eenassige samedrukking van die vervaardigde roosterstrukture is uitgevoer. Globale meganiese eienskappe is bepaal. Die opeenvolgende inval van die stutte onder die las is geanaliseer om die faling as op strek- of buig-belasting te kategoriseer. Mikro-berekende tomografiese analise van die vervaardigde strukture het getoon dat die stutte groter was in vergelyking met die CAD strukture. Dus is nuwe CAD modelle gebou waar die stut dikte soortgelyk as die werklike gemiddelde afmeting gestel word. Dit het die voorspelling van die meganiesegedrag van die vervaardigde strukture verbeter. 'n Vergelyking van die EEA-voorspellings en die eksperimenteel gemete meganiese eienskappe van die roosterstrukture is uitgevoer. Daar is bepaal dat beide die 3D-soliede en 1D-balkstrukture die werklike meganiese eienskappe van die octet-stut-roosterstruktuur voorspel het binne 'n waarnemingsfout van minder as 25%. Vir die diamantroosterstruktuur het slegs die 3D-soliede struktuur egter die werklike meganiese eienskappe voorspel met 'n waarnemingsfout van minder as 20%. 'n Studie van die spanningsverspreiding oor individuele stutte in die struktuur is gebruik om die vervormingsmeganisme wat in elkeen van die strukture waarneem is te verduidelik. Daar is gevind dat die octet-stut struktuur vervorm is deur 'n kombinasie van skuifbande van 45 ° en 135 °, wat veroorsaak word deur die strek van die horisontale stutte in daardie vlakke, terwyl dit gevind is dat die diamantroosterstruktuur vervorm is deur skuifbande van 45 ° wat veroorsaak is deur stutbuiging .

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108149
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