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Compressive failure of impacted NCF composite sandwich panels - Characterisation of the failure process
KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
2004 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 38, no 6, 495-514 p.Article in journal (Refereed) Published
Abstract [en]

In the present study, non-crimp fabric (NCF) composite face sheet sandwich panels have been tested in compression after impact (CAI). Damage in the face sheets was characterised by fractography. Compression after impact loaded panels were found to fail by plastic fibre microbuckling (kinking) in the damaged face sheet. Studies of panels for which loading was interrupted prior to failure revealed extensive stable kink band formation at several positions and in numerous plies. Kink bands initiated and propagated within a wide region close to the point of impact. In addition, kink bands initiated in zones with high shear stresses, away from the impact centre line. Consequently, the fractographic results from this investigation do not support the assumption of modelling the impact damage as an equivalent hole. To achieve accurate predictions of kink band initiation, the stress field must be known. The results from this study imply that bending effects caused by remaining dent or material eccentricities in the damaged region must be considered.

Place, publisher, year, edition, pages
2004. Vol. 38, no 6, 495-514 p.
Keyword [en]
CAI, CFRP, Damage tolerance, Fractography, Sandwich
National Category
Reliability and Maintenance
URN: urn:nbn:se:kth:diva-5616DOI: 10.1177/0021998304040559ISI: 000220671000003ScopusID: 2-s2.0-1842526619OAI: diva2:10043

QC 20110119

Available from: 2006-04-27 Created: 2006-04-27 Last updated: 2012-09-11Bibliographically approved
In thesis
1. Physically Based Engineering Models for NCF Composites
Open this publication in new window or tab >>Physically Based Engineering Models for NCF Composites
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Non-Crimp Fabrics - NCF – are increasingly being used as reinforcements in high performance composite materials. NCF offer the manufacturing advantages from textile preforms in combination with excellent mechanical performance. This study concerns the mechanical performance of NCF composites. Through a combination of experimental work and theoretical studies the mechanisms controlling the mechanical behaviour are explained. Fractography is used as a tool to identify governing mechanisms and link these to the material internal structure. Based on the experimental findings, engineering models are suggested predicting the mechanical behaviour of NCF composite laminates.

A simplified constitutive model is presented that accounts for the fibre tow out-of-plane waviness. The model is based on Timoshenko beam theory applied on curved beams representing wavy tows in a NCF composite lamina. The model calculates stiffness knock-down factors to be applied on lamina homogenised properties.

Experiments show compressive failure of NCF composites to be governed by formation and growth of kink bands. For this reason, a failure criterion predicting kinking failure under multiaxial loading is proposed and validated for a NCF composite system. The criterion is to be used on lamina level in a multiaxial NCF laminate. A test method is proposed for extraction of strength parameters valid for the lamina material in a multiaxial laminate.

Compression-after-impact (CAI) behaviour of NCF composite laminates, as monolithic skins and sandwich panel face sheets, is investigated. Fractographic studies show CAI failure to be controlled by formation of kink bands. The experimental studies reveal that kink bands form at relatively low loads and grow gradually during compressive loading. It is suggested that the notch effect from the gradually developing kink bands cause final catastrophic failure in sandwich panel skins. Finite element analyses, simplistically representing the damage with an idealised notch, are shown to predict panel residual strength with reasonable accuracy.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. ix, 27 p.
Trita-AVE, ISSN 1651-7660 ; 2006:17
National Category
Reliability and Maintenance
urn:nbn:se:kth:diva-3927 (URN)91-7178-316-4 (ISBN)
Public defence
2006-05-05, Sal F3, Lindstedtsvägen 26, Stockholm, 13:15
QC 20110120Available from: 2006-04-27 Created: 2006-04-27 Last updated: 2011-01-21Bibliographically approved

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Edgren, FredrikBull, Peter Hoaas
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