Christian Lundsgaard-Larsen

Non-uniform Compressive Strength of Debonded Sandwich Panels – I. Experimental Investigation:

Face/core debond-damaged sandwich panels exposed to non-uniform compression loads are studied. The panel geometry is rectangular with a centrally located circular debond. The study primarily includes experimental methods, but simple finite element calculations are also applied. The complexity of applying a controlled non-uniform compressive load to the test panels requires a strong focus on the development of a suitable testrig. This is done by the extensive use of product development methods. The experimental results based on full-scale testing of 10 GFRP/foam core panels with prefabricated debonds show a considerable strength reduction with increasing debond diameter, with failure mechanisms varying between fast debond propagation and wrinkling-introduced face compression failure for large and small debonds, respectively. Residual strength predictions are based on intact panel testing, and a comparison between a simple numerical model and the experimental results shows fair agreement.

Residual Strength of In-Plane Loaded Debonded Sandwich Panels: Experimental Investigation

Face/core debond damaged sandwich panels exposed to uniform and non-uniform compression loads are studied experimentally. The panel geometry is full-scale rectangular with a centrally located circular prefabricated debond. The results show a considerable strength reduction with increasing debond diameter, with the failure mechanisms varying between buckling driven debond propagation and face compression failure for large and small debonds respectively.

On the Use of a Woven Mat to Control the Crack Path in Composite Sandwich Specimens With Foam Core

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 tractions between 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.Copyright

Measuring Mixed Mode Cohesive Laws for Interfaces in Sandwich Structures

The purpose of this work is to establish a test procedure that allows fracture mechanics characterization of interface cracking in sandwich structures. This often entails large scale fiber bridging, which increases the size of the process zone beyond a point where linear elastic fracture mechanics (LEFM) is applicable. Instead of assigning the fracture processes to a single point at the crack tip (as in LEFM) the fracture process zone is represented by a cohesive zone in which the traction separation law (cohesive law) needs to be determined. The cohesive law can be measured, either indirect by modeling experiments and fitting parameters until numerical and experimental results coincide, Li et al. [1], or by direct measurement by e.g. the J integral approach, Sorensen and Kirkegaard [2]. In this study mixed mode cohesive laws are measured directly by relating the J integral to the normal and tangential opening of the initial crack tip. The test method is based on a double cantilever beam (DCB) specimen loaded by uneven bending moments, see Fig. 1. The test is conducted under displacement control in a tensile test machine, and the moment ratio is kept constant throughout one test. By varying the moment ratio the crack opening can be varied from pure normal opening to pure tangential crack surface displacements. A detailed description of the test method can be found in [2].