Stefan Hallström

Failure Mechanisms and Modelling of Impact Damage in Sandwich Beams - A 2D Approach: Part I - Experimental Investigation

This paper addresses the effect of low velocity impact damage on post-impact failure mechanisms and structural integrity of foam core sandwich beams subjected to edgewise compression, shear and bending load cases. The study deals with a 2D configuration, where a sandwich beam is impacted by a steel cylinder across the whole width of the specimen. The impact damage is characterised as indentation of the core with sub-interface damage seen as a cavity while the GFRP faces remain virtually unaffected by the impact. Digital speckle photography (DSP) analysis is employed for in situ monitoring of crushing behaviour in the foam core during static indentation of sandwich specimens. The static shear strength of impact-damaged sandwich beams is compared with specimens with fabricated sub-interface cracks of the same length. DSP analysis reveals that the face-core interface in the peripheral regions of 2D impact damage is not entirely separated. The crack analogy is thus not fully representable since the surfaces remain bridged resulting in higher strength, when compared with fabricated cracks. The post-impact resistance to compressive loads is lower than for the specimens with fabricated cracks due to the presence of the cavity and the crushed core with reduced foundation stiffness support. The properties of crushed foam core are experimentally determined as they appear to be important for accurate modelling and analysis of the residual strength of sandwich beams. Modelling and post-impact analysis of the specimens with impact damage is elaborated in detail in part II of this study.

Mixed Mode Fracture of Cracks and Wedge Shaped Notches in Expanded PVC Foam

Fracture initiated from a sharp crack or wedge shaped notch in a homogeneous material, subjected to different loading is considered. Singularities in the stress fields at edges and vertices are discussed. A point-stress criterion is used to predict fracture for sharp cracks as well as 90° wedge notches in expanded PVC foam. The point-stress criterion is formulated in a manner allowing failure predictions in general 3D stress situations. The influence of nonsingular T-stress at cracks is discussed and substantiated by experimental results.

Strength prediction of beams with bi-material butt-joints

Failure of bi-material interfaces is studied with the aim to quantify the influence of the induced stress concentrations on the strength of the interfaces. The suggested approach is applied to a sp ...

On cracks emanating from wedges in expanded PVC foam

Abstract An experimental and analytical study was made on the effect of stress singularities on the strength of expanded PVC foam materials of different densities. Experiments were performed on specimens with different wedge geometries ranging from sharp cracks, with the ordinary inverted square root stress singularity, to shallow reentrant corners with weak singularities. A brittle fracture criterion based on a generalized stress intensity factor, called Q, at the wedge tip was fitted to experimental data. The critical stress intensity factor, Qcr for crack initiation depends on the wedge geometry. This dependence was estimated from simple point-stress criteria and a criterion due to Seweryn [Brittle fracture criterion for structures with sharp notches. Engng Fracture Mech. 47, 673–681 (1994)] and good agreement with experimental data was obtained. When the point-stress criterion was applied to Mode II sharp cracks, poor agreement with published data was found. A critical study of the Mode II crack specimen was therefore initiated, leading to the conclusion that the commonly used specimen gives erroneous values of KIIc and the reason seems to be due to crack surface friction. A new Mode II crack specimen which eliminates crack surface friction was proposed and tested, and good agreement with the point-stress criterion was obtained. A criterion for homogeneous materials proved to be adequate also for the porous PVC foams.

Failure Mechanisms and Modelling of Impact Damage in Sandwich Beams - A 2D Approach: Part II - Analysis and Modelling

This study addresses the effect of low velocity impact damage on the post-impact residual strength and failure mechanisms of sandwich beams with Rohacell WF51 foam core. The considered impact damage has a form of a subinterface cavity surrounded by crushed core while the face sheet remains virtually undamaged. Part I of this study deals with experimental investigation of impact-damaged beams tested in transverse shear, bending and edgewise compression. It is shown that the crushed core and the bridging condition in the peripheral regions of the impact damage exert a significant effect on the post-impact critical loads and failure mechanisms. In this paper, parameterised finite element (FE) models of impact damage with implemented crushed core properties are developed for numerical analyses of post-impact failure. In the analysis of the shear case, a model for II bridging condition in the peripheral regions of impact damage is introduced. A point-stress criterion is applied for predictions of failure loads and crack kink angle. Geometrically nonlinear FE analysis is employed for evaluation of critical loads for local buckling in the beams with impact damage. The FE analyses demonstrate good agreement with experimental results.

Failure analysis of laminated timber beams reinforced with glass fibre composites

SummaryA qualitative analysis is presented of failure, perpendicular to the grain, in laminated timber reinforced with a glass fibre composite. The study is focused on beams with holes of different shape. The stress by corners, infinitesimal cracks and finite cracks are investigated. An initial crack model is suggested that brings about some of the phenomena observed in earlier performed experiments. A crack appears to propagate in the wood but is retarded in the reinforced beams. Eventually, the composite will fracture and failure of the beam follows. Finite element computations suggest that the reinforcement decreases the stress intensity at cracks in the wood and acts as a crack stopper. The reinforcing effect increases with the crack length. A point stress criterion is used to predict failure in the fibre composite.

Shear Characterization of Sandwich Core Materials Using Four-point Bending

A new shear test method for sandwich core materials is proposed and evaluated. Sandwich beams are loaded in four-point bending, and the shear deformation is measured with two rotary sensors. Conditions of idealized sandwich theory are assumed to prevail, and the accuracy of the proposed methodology is thus dependent on a few mechanical and geometric relations between the sandwich constituents. The stress-strain responses for two polymer foam core materials, one relatively brittle and one relatively ductile, are extracted and compared with results from single-block shear tests of the same material batch. The new method provides several benefits with respect to the block shear test. It does not suffer from extreme stress concentrations and the specimens are tested under in-service conditions. Problems arise, however, for the ductile material, predominantly related to large deformations during the test eventually resulting in bending failure of the face sheet instead of shear failure of the core.

Glass fibre reinforced holes in laminated timber beams

SummaryAn experimental investigation of glass fibre reinforcement of glued laminated timber beams is presented. A polyester resin is used both as matrix and adhesive between the reinforcement and the wood. The main part of the work considers beams with large holes tested in three point bending. Circular and rectangular holes, centred at quarter length of the beams make the strength of wood perpendicular to the grain become critical. Great improvements of strength are obtained with the glass fibres. A comparison between various kinds and combinations of glass fibre reinforcement is made. Further, the reinforcement applied as repair of earlier cracked beams is investigated with positive results. One series of beams without holes is reinforced and tested in four-point bending.

Weight-balanced drop test method for characterization of dynamic properties of cellular materials

A novel weight-balanced drop rig used to evaluate the response of cellular materials subject to dynamic compression is presented. The testing method utilizes approximately constant velocity throughout the major part of the compression phase and the results compare well with results from other methods, reported in the literature. The repetitiveness is excellent, the rig is simple and the results are easily extracted. The applicability of the method for determination of elastic modulus is however limited to materials with relatively low stiffness. Accurate modulus measurements for stiff materials at high strain-rates require a very rigid and lightweight test set-up.

Generation of periodic stochastic foam models for numerical analysis

Stochastic cellular models of rigid foam based on Voronoi spatial partitioning are generated and investigated for potential use in numerical analysis using finite element methods. Such partitions are deterministic once a distribution of cell nuclei has been defined. A drawback is that the models tend to exhibit a significant share of short edges and small faces. Such small geometrical features are not likely to occur in real foams since they are unfavorable from a surface energy point of view and they also generate problems in numerical analysis due to associated meshing challenges. Through minimization of the surface area, using the computer software Surface Evolver, the Voronoi models are brought to better resemblance with ideal dry foam and the occurrence of small geometrical features is strongly reduced. It is generally seen that different seed point distribution algorithms result in different model topologies. The presented methodology is systematic, parameterized and the results are very promising. Good grounds are provided for modeling of real rigid foam materials, that do not necessarily fully resemble ideal dry foam.