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Design and evaluation of a novel instrumented drop-weight rig for controlled impact testing of polymer composites
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. Odqvist Laboratory, KTH.ORCID iD: 0000-0002-8336-8294
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. Odqvist Laboratory, KTH.ORCID iD: 0000-0002-1187-4796
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Odqvist Laboratory, KTH.ORCID iD: 0000-0001-8667-0520
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. Odqvist Laboratory, KTH.ORCID iD: 0000-0002-9207-3404
2018 (English)In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 68, p. 446-455Article in journal (Refereed) Published
Abstract [en]

A drop-weight rig (DWR) intended to test the true impact response of laminated compositematerials is presented. The test setup is designed to prevent the transfer of unwanted mechanical noise, e.g. vibrations, into the load cell that is used to measure the load during the impact event. A novel catch mechanism preventing secondary impact is also implemented in the DWR design. A detailed evaluation is performed both in terms of the experimental modal and uncertainty analysis of the measured results from the DWR. The results demonstrate that the rig is capable of capturing the true impact response, providing highly resolved and noise-free force-time measurements where even subtle details of the impact event are visible. The rig also enables impact testing with good repeatability.

Place, publisher, year, edition, pages
Elsevier, 2018. Vol. 68, p. 446-455
Keywords [en]
Low velocity impact, Vibration, Uncertainty analysis, Modal analysis, Free-fall
National Category
Composite Science and Engineering
Research subject
Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-228484DOI: 10.1016/j.polymertesting.2018.04.022ISI: 000437076000053Scopus ID: 2-s2.0-85047266915OAI: oai:DiVA.org:kth-228484DiVA, id: diva2:1209986
Projects
DAMTISS
Funder
VINNOVA, 2013-01132
Note

QC 20180525

Available from: 2018-05-25 Created: 2018-05-25 Last updated: 2018-11-09Bibliographically approved
In thesis
1. Damage Tolerance of Impacted Composite Sandwich Structures
Open this publication in new window or tab >>Damage Tolerance of Impacted Composite Sandwich Structures
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with damage tolerance of impacted sandwich structures for load bearing applications. Composite sandwich structures find wide application as lightweight solutions in aerospace components, since weight reduction and less fuel emissions are primary concerns for aircraft manufactures. Sandwich structures are built of stiff face-sheet materials bonded to a low-density core material. In this thesis, the face-sheets are composite materials reinforced with carbon fibre non-crimp fabrics whereas the core consists of a closed cell foam material. Sandwich structures are susceptible to impact damage and even a small amount of damage can reduce the residual strength of components significantly. Therefore, damage tolerance assessment of such structures is essential and needs to be taken into account in the design process.

Main objective of this thesis is assessment of test methodologies for estimation of compressive properties of foam core materials. An extensive experimental study of different densities of closed cell foam materials is presented and existing test standards are evaluated in this regard. Two different test methods were investigated for strain measurements of the foam material during compression testing assisted by a digital image correlation technique. A parametric study was also performed to investigate the effect of in-plane specimen size on the compressive modulus measurements. Both homogenized and stochastic finite element models are used to back the experimental observations. Different types of boundary conditions were used to simulate the effects of in-plane specimen size and prediction of compressive modulus. The findings were also used as basis for recommendations for updating current test standards.

A part of the thesis work concerns the design and construction of a new drop-weight impact rig for low-velocity impact testing of sandwich structures. A test setup was designed to capture the true impact response without adulteration by oscillations. A novel catch mechanism was designed and implemented for preventing secondary impact. A detailed experimental evaluation and uncertainty analysis was also performed to evaluate the drop-weight rig in terms of repeatability and precision.

The developed drop-weight rig was used to perform low-velocity impact characterization of sandwich structure with different face-sheet thicknesses. A range of impact energies were investigated for the identification of low level damage (LLD), barely visible impact damage (BVID) and visible impact damage (VID). A thorough fractography study was performed to understand the damage mechanisms at different energy levels and for different face thicknesses. A finite element model was developed to simulate the impact response and delamination extent, including both inter-laminar and intra-laminar damage modes.

Finally, the impact damaged specimens were tested for damage tolerance assessment. Both symmetric and asymmetric specimen configurations with different face-sheet thicknesses were investigated. The effect of face-sheet thickness on the residual strength of sandwich structures was studied. Three different test methodologies for damage tolerance testing were investigated and the results were compared. A finite element model was developed for simulation of the edgewise compression test methods and the residual strength predictions were compared with the experimental results.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 41
Series
TRITA-SCI-FOU ; 2018:46
Keywords
composite sandwich, cellular, polymer, material characterization, modulus, non-crimp fabric, low velocity impact, vibration, uncertainty analysis, drop-weight impact, fractography, barely visible impact damage, X-ray microtomography, compression-after-impact, bending-after-impact, high speed image processing, damage resistance, damage tolerance
National Category
Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-238738 (URN)978-91-7873-020-9 (ISBN)
Public defence
2018-12-10, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
DAMTISS
Note

QC 20181109

Available from: 2018-11-09 Created: 2018-11-09 Last updated: 2018-11-09Bibliographically approved

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Burman, MagnusSegalini, AntonioHallström, Stefan

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