Low-velocity impact damage in fibre metal laminates - a study of the impact resistance and metal-composite debonding

Tuomas Pärnänen

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Fibre metal laminates (FMLs) refer to a hybrid material concept that combines layers of thin metal sheets and fibre-reinforced plastics. This thesis concentrates on the damage characteristics of FMLs subjected to collision-type low-velocity impacts. The damage characteristics are studied by drop-weight impact and quasi-static indentation experiments and using finite element simulations. The work is divided into two parts. In Part I, specific interest is laid on the impact resistance of AZ31B-H24 magnesium alloy-based FMLs. The results indicate a disadvantage for the magnesium alloy, as the impact energy limits for metal cracking are significantly lower when compared to the state-of-the-art FML structure Glare 5 with 2024-T3 aluminium alloy layers. The main reason for the difference in cracking energies is concluded to result from the metal tensile response over the plastic region. The lower rate of strain hardening for the magnesium alloy enhances the local strain accumulation over the impact point, which leads to premature cracking. In addition, the impact damage process is experimentally studied in Part I by using a wide range of impact energies. The purpose of the damage process study is to gain an overall view of the impact damage evolution in FMLs. Finally, impacts on FMLs are studied via finite element simulations.Part II focuses on the debonding occurring in FMLs during impact loading, i.e. on the damage process along the metal-composite interface. The study reveals that the debond at the lowermost metal-composite interface forms in two stages. During the impactor loading phase, a debond tends to form as a fracture mode II cracking caused by out-of-plane shear forces. As the impactor rebound phase follows, the linear-elastic composite layers force the FML to straighten. On the contrary, the lowermost metal sheet has deformed plastically and tends to maintain its deformed shape. The discrepancy in the material response leads to peeling forces and consequently to fracture mode I dominated debonding. In the current literature, the debonding along the metal-composite interface is not commonly taken into account in the impact damage modelling of FMLs, and at times the debonding by shear forces is only considered. The energy absorption by the debonding may not be significant in the impact process, but the results of this thesis indicate that the debonding substantially affects deformations and the post-impact strain state. A realistic prediction of the post-impact damage state is especially important for successful investigations of damage tolerance. Therefore, a modelling methodology that takes into account the debonding with the described two-stage formation process defined in this thesis is suggested for future models.
Translated title of the contributionLujitemuovi-metalli-yhdistelmälaminaattien iskuvaurioituminen – tutkimus iskunkestävyydestä ja kerrostenvälisestä murtumisesta
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Saarela, Olli, Supervising Professor
  • Kanerva, Mikko, Thesis Advisor
Publisher
Print ISBNs978-952-60-7189-3
Electronic ISBNs978-952-60-7188-6
Publication statusPublished - 2016
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • fibre metal laminates
  • hybrids
  • impact
  • impact resistance
  • debonding

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