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Damage Tolerance of Impacted Composite Sandwich Structures
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.ORCID iD: 0000-0002-8336-8294
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 [en]
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: urn:nbn:se:kth:diva-238738ISBN: 978-91-7873-020-9 (electronic)OAI: oai:DiVA.org:kth-238738DiVA, id: diva2:1262068
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
List of papers
1. Compression of structural foam materials: Experimental and numerical assessment of test procedure and specimen size effects
Open this publication in new window or tab >>Compression of structural foam materials: Experimental and numerical assessment of test procedure and specimen size effects
2017 (English)In: Journal of Sandwich Structures and Materials, ISSN 1099-6362, E-ISSN 1530-7972Article in journal (Refereed) Published
Abstract [en]

This study was initiated based on the observation that standardized test for flatwise compression of foam materials, give significantly different test results for the measured moduli, and that these standards to date lack adequate instructions on how the strain should be measured and what specimen size should be used. A brief review of previous work shows that existing test methodologies provide significantly different results for the compressive moduli of foams depending on how the strains are measured. A thorough experimental study of the out-of-plane compressive properties is conducted on three different closed-cell foam materials, where strains measured with two different extensometer placements, and with digital image correlation, come out significantly differently. A parametric study is also performed showing that the results vary considerably with in-plane specimen dimensions, indicating effects of finite size and localized strain at edges. Both stochastic amorphous and homogenized finite element models of foam back the experimental observations by illustrating the effects of finite size and various boundary conditions on the measured properties.

Place, publisher, year, edition, pages
Sage Publications, 2017
Keywords
Cellular, polymer, material characterization, test method, compressive modulus, Poisson’s ratio
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-203868 (URN)10.1177/1099636217690500 (DOI)
Note

QC 20170320

Available from: 2017-03-18 Created: 2017-03-18 Last updated: 2018-11-09Bibliographically approved
2. Design and evaluation of a novel instrumented drop-weight rig for controlled impact testing of polymer composites
Open this publication in new window or tab >>Design and evaluation of a novel instrumented drop-weight rig for controlled impact testing of polymer composites
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
Keywords
Low velocity impact, Vibration, Uncertainty analysis, Modal analysis, Free-fall
National Category
Composite Science and Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-228484 (URN)10.1016/j.polymertesting.2018.04.022 (DOI)000437076000053 ()2-s2.0-85047266915 (Scopus ID)
Projects
DAMTISS
Funder
VINNOVA, 2013-01132
Note

QC 20180525

Available from: 2018-05-25 Created: 2018-05-25 Last updated: 2018-11-09Bibliographically approved
3. Experimental and numerical study of the response to various impact energy levels for composite sandwich plates with different face thicknesses
Open this publication in new window or tab >>Experimental and numerical study of the response to various impact energy levels for composite sandwich plates with different face thicknesses
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Composite sandwich structures find wide application in the aerospace sector thanks to their lightweight characteristics. However, composite structures are highly susceptible to low velocity impact damage and therefore thorough characterization of the impact response and damage process for the used material configurations is necessary. The present study investigates the effect of face-sheet thickness on the impact response and damage mechanisms, experimentally and numerically. A uni-directional, non-crimp fabric (NCF) is used to build the face-sheets, and a closed cell Rohacell 200 Hero material is used as foam core material. Low-velocity impact tests are performed in a novel instrumented drop-weight rig that is able to capture the true impact response. A range of impact energies are initially utilized in order to identify when low level damage (LLD), barely visible impact damage (BVID) and visible impact damage (VID) occur. A thorough fractography investigation is performed to characterize the impact damage using both destructive and non-destructive testing. The damage from the impacts in terms of dent depth, peak contact force, deflection and absorbed energy is measured. The results show bilinear responses in dent depth vs. impact energy and absorbed energy vs. impact energy. It is found than the BVID energy works well as an indication for the onset of excessive damage. Fractography reveals that there is a failure mode shift between the LLD and the VID energy levels, and that delaminations predominantly grow along the fiber direction and rotate in a spiral pattern through the thickness, following the laminate ply orientations. Finally, a progressive damage finite element model is developed to simulate both the impact response and the delamination extent, incorporating both intra-laminar and inter-laminar damage modes. The simulation shows good agreement with the experiments.

Keywords
Non-crimp fabric, drop-weight rig, low-velocity impact, fractography, barely visible impact damage, X-ray micro-tomography, foam core
National Category
Engineering and Technology
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-238476 (URN)
Note

QC 20181109

Available from: 2018-11-01 Created: 2018-11-01 Last updated: 2018-11-09Bibliographically approved
4. Evaluation of test methods and face-sheet thickness effects in damage tolerance assessment of composite sandwich plates
Open this publication in new window or tab >>Evaluation of test methods and face-sheet thickness effects in damage tolerance assessment of composite sandwich plates
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Composite sandwich materials are used in load-carrying components in commercial aircraft due to their lightweight characteristics and high strength-to-weight ratio. However, composite structures are vulnerable to impact damage which can drastically reduce their load-bearing capability. Consequently, damage tolerance assessment of composite sandwich structures is of utmost importance. At present, there is a lack of standardised test methods for such assessments of sandwich structures. This study compared test methods for damage tolerance by means of compression after impact (CAI) and how the residual strength varied with face-sheet thickness. Three different test methods, including two types of CAI, were used i.e. CAI-single skin (CAI-SS) and CAI-sandwich (CAI-SW), and one four-point bending-after-impact (BAI) test method was used for residual strength measurement. A thorough experimental campaign was performed on different face-sheet configurations utilising both symmetric and asymmetric sandwich panels. Test specimens were subjected to impact energy levels causing barely visible damage. Then they were tested using the different methods and the differences in the results were highlighted. In all three test methods, the specimens failed in compression showing prominent fibre fracture and delamination. This compressive failure was initiated at the impact zone and grew across the width of the specimen, perpendicular to the loading direction. The CAI-SW test method showed a by-pass load effect due to the parallel compression of the intact face-sheet. The CAI-SS test method was used to study the effect of face-sheet thickness on the damage tolerance assessment. The BAI test method showed similar failure stresses to the CAI-SS tests. A progressive damage finite element model was used to estimate by-pass load and post impact strengths. The simulations showed good agreement with the experiments.

Keywords
Low-velocity impact, sandwich material, barely visible impact damage, compression-after-impact, bending-after-impact
National Category
Engineering and Technology
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-238477 (URN)
Note

QC 20181109

Available from: 2018-11-01 Created: 2018-11-01 Last updated: 2018-11-09Bibliographically approved

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