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Telefono: 02.2399.8629
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E-mail: laura.boniotti(at)polimi.it

Machine and Vehicle Design

Relatore: Prof. Stefano Foletti

Tutor: Prof. Giorgio Colombo

Università di Provenienza: Università degli Studi di Brescia - Ingegneria Edile-Architettura

Titolo della Tesi: Investigation on the fatigue strength of SLM-printed micro-lattice materials by experimental tests and µCT-based finite element analyses

Investigation on the fatigue strength of SLM-printed micro-lattice materials by experimental tests and µCT-based finite element analyses

Introduction

Additive Manufacturing (AM) technologies offer nowadays new and unusual possibilities to produce lightweight parts and promising micro-lattice materials, characterised by very complex and thin shapes and by a new range of properties that can not be reached in traditional manufactured parts.
The use of SLM-printed micro-lattice materials is still restricted, due to the lack of knowledge about their fatigue strength and fatigue behaviour: standards and accurate guidelines are not available for the mechanical testing of these materials. As a consequence, a specimen design has to be developed to perform experimental tests: the size effect has to be avoided and the specimen has to be representative of the properties of the material.
The as-manufactured geometry of a micro-lattice material is very different from the as-designed one: porosity, irregular dimensions and local irregularities derive from the printing process and lead to cracks and failures in micro-lattice structures affecting the fatigue properties.
The fatigue strength prediction of SLM micro-lattice materials has to take into account these surface features and can be very challenging: numerical models are not available in literature to predict their fatigue properties, neither under uniaxial nor multiaxial cyclic loads.

Objectives

  • Study of an appropriated specimen design for the mechanical characterisation.
  • Experimental investigation of the fatigue strength in tension and in compression of micro-lattice structures at different stress ratios.
  • Failure analysis of the specimens to understand the role of geometrical irregularities.
  • Analysis of strain and stress concentrations.
  • Development of a numerical model for the fatigue strength prediction starting from microCT-based finite element analyses.
  • Numerical investigation on the fatigue strength under uniaxial and multiaxial in-phase and out-of-phase loads.

Results

  • Specimen design for experimental tests (SC8-FCC and SC-BCC) (Fig.1).
  • Fatigue cracks and failures start from surface irregularities, regardless the internal porosity and the microstructure (Fig.2, Fig.3).
  • Experimental results of HCF tests on AlSi10Mg micro-lattice specimens: load range vs number of cycles at failure (Nf) and damage maps - life spent in damage initiation (Ni/Nf) vs total fatigue life (Nf) - of SC-BCC and SC8-FCC micro-lattice materials (Fig.4, Fig.5, Fig.6, Fig.7).
  • DIC results: measure of strain concentrations in a SC-BCC micro-lattice material under a static load (Fig.8).
  • DVC results: measure of strain concentrations in a SC-BCC micro-lattice material during a HCF test (Fig.9).
  • Numerical fatigue strength prediction: normal probability plot and local values in a SC-BCC cell under uniaxial loads applied in different directions (Fig.10, Fig.11).
  • Numerical fatigue strength prediction: percentage of points exceeding the fatigue strength under uniaxial and multiaxial loads (Fig.12, Fig.13).

Conclusions

  • A specific shape and dimension of a feasible specimen for mechanical testing of micro-lattice materials has been achieved.
  • Damage in micro-lattice materials under cyclic loads has been analysed: damage is much faster in tension than in compression; damage is permitted in run-outcompression-compression tests, while no damage is permitted in the run-out condition in tension.
  • Fatigue cracks and failures occur due to the surface irregularities, regardless the internal porosity and the microstructure.
  • Strain and stress localisations characterize micro-lattice materials due to the heterogeneity of the struts.
  • The numerically computed fatigue strength under uniaxial and multiaxial loads depends on several parameters: the loading direction (in respect to the printing angle of the struts), the stress ratio and the loading phase (in-phase and out-of-phase multiaxial loads).

References

L. Boniotti, S. Foletti, S. Beretta, L. Patriarca, Strain concentrations in BCC micro lattices obtained by AM, Third international Symposium on fatigue design and material defects (FDMD3), (2017).
L. Boniotti, S. Beretta, L. Patriarca, L. Rigoni, S. Foletti, Experimental and numerical investigation on compressive fatigue strength of lattice structures of AlSi7Mg manufactured by SLM, Int. J. Fatigue, (2019).
L. Boniotti, S. Foletti, S. Beretta, L. Patriarca, Analysis of strain and stress concentrations in micro-lattice structures manufactured by SLM, Rapid Prototyping J., (2019).
M. Gavazzoni, L. Boniotti, S. Foletti, Influence of specimen size on the mechanical properties of microlattices obtained by selective laser melting, Proceedings of the Institution of Mechanical Engineers, (2019).