On the use of a woven mat to control the crack path in composite sandwich structures

Christian Lundsgaard-Larsen (Invited author), Christian Berggreen (Invited author), Leif A. Carlsson (Invited author)

Research output: Chapter in Book/Report/Conference proceedingConference abstract in proceedingsResearchpeer-review


In the last couple of decades the use of sandwich structures has increased tremendously in applications where low weight is of importance e.g. ship structures, where sandwich panels are often built from fiber reinforced faces and foam cores. An important damage type in sandwich structures is separation of face and core (debonding). Debonds can arise as a result of defects from production when an area between face and core has not been primed sufficiently resulting in a lack of adhesion. In use, impact loading, e.g. due to collision with objects, can result in formation of a debond crack, followed by growth due to continued loading. With debonds present the structure might fail under loads significantly lower than those for an intact sandwich structure [1, 2]. A debond crack in a foam cored sandwich can propagate self similarly or kink away from the interface into either the face or core. Whether or not kinking occurs is governed by the stress state at the crack tip, e.g. described by the mode-mixity of the complex stress intensity factor and the properties of the face, core and adhesive [3]. The criticality of an existing crack can be highly dependent on the crack propagation path, since the fracture toughness of the face, core and interface are often very different. As the crack propagates in the interface or laminate the fibers in the face laminate can form a bridging zone behind the crack tip. This can increase the fracture toughness significantly since the bridging fibers provide closing tractionsbetween the separated crack surfaces [4, 5]. The outline of a crack propagating under large scale bridging in a sandwich structure can be seen in Figure 1. The fiber bridging mechanism possesses an increased potential damage tolerance capacity for the sandwich structure if it can be predicatively initiated and exploited in a controlled manor. The crack path is of high relevance when utilizing the increased fracture toughness from the bridging fibers, since the crack should propagate in a self similar manner and not kink into the core or laminate. Earlier work by the authors shows that for some mixed mode load cases, a crack originally located at the interface will kink into the face and penetrate the laminate by kinking all the way through, which substantially reduces the strength of the sandwich panel. This study proposes a method for controlling the crack propagation path in the sandwich laminate. It is investigated whether crack kinking can be prevented by a thin woven mat, placed in the laminate during manufacturing. The effectiveness of this "crack stopper" is tested for sandwich specimens loaded in a variety of mixed mode conditions. The test is conducted using a modified double cantilever beam specimen loaded by unequal bending moments (DCB-UBM specimen) [6, 7], see Figure 2. The specimen is loaded by a roller-wire system mounted in a tensile test machine. The ratio between the two applied moments (M1/M2) is dictated by the ratio between roller distances (I1/I2), see Figure 2. The sign of the moment ratio can be reversed by changing the mounting direction of the wire. If moments with opposite signs are applied e.g. M1/M2 = -1, crack opening in the normal direction is dominating (mode I). If moments with the same sign are applied the crack opening in the tangential direction is more dominating (mode II). It is possible to vary the loading and hereby the normal-tangential crack opening ratio (see Figure 1) to almost any desired mixed mode value. The test is conducted at different mode mixitiesand the kinking of the crack is explored for specimens with and without the woven mat in the face laminate.
Original languageEnglish
Title of host publicationASME International Mechanical Engineering Congress and Exposition, Proceedings
VolumeVol.10 / PART B
PublisherAmerican Society of Mechanical Engineers
Publication date2008
ISBN (Print)9780791843048
Publication statusPublished - 2008
EventASME International Mechanical Engineering Congress and Exposition - Seattle, Washington, USA
Duration: 1 Jan 2008 → …


ConferenceASME International Mechanical Engineering Congress and Exposition
CitySeattle, Washington, USA
Period01/01/2008 → …

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