Stephen R Hallett

Experimental Investigation of Progressive Damage and the Effect of Layup in Notched Tensile Tests

The presence of subcritical damage in notched composites significantly affects the ultimate failure mode and strength. This article presents a detailed study of four different layups of E-glass/913 tested using a double-edge-notched specimen loaded in tension. For each layup three different in-plane dimensions are tested. Results are presented in terms of failure mode, strength, and subcritical damage development. Subcritical damage development is consistent between the layups and between scaled specimens of a single layup. Ultimate failure, however, shows some variations both with layup and size and this is examined in some detail. The trend of decreasing strength with increasing specimen size is observed for all cases except those where there is a change in failure mode between different size tests. The strengths are compared with predictions from two analytical techniques, which show some ability to achieve correlation across a subset of the test data. Correlation is not possible where variations in failure mode occur for a single layup.

Scaling Effects in Notched Composites

A program of scaled tests on unnotched and open-hole tension and compression specimens is summarized. Quasi-isotropic IM7/8552 carbon-fiber epoxy specimens have been tested using two different scaling techniques: sub-laminate-level ([45/90/ - 45/0]ns) and ply-level scaling ([45n/90 n/ - 45n/0n]s), independently varying the thickness and in-plane dimensions. Significant scaling effects are shown, with both strength and failure mechanisms changing with specimen size and the thickness scaling method having a particularly important effect. Failure mechanisms and scaling behavior are compared between tension and compression and models presented that predict the observed size effects from fundamental material parameters without any fitting factors.

Computational modeling of complex failure mechanisms in laminates

A computational framework for the simulation of progressive failure in composite laminates is presented. The phantom-node method (a variation to the XFEM) is used for a mesh-independent representation of matrix cracks as straight discontinuities in the displacement field. Furthermore, interface elements are used for delamination and a continuum damage model for fiber failure. The framework is validated against experimental observations for open-hole tests and compact tension tests. It is shown that different failure mechanisms are captured well, which allows for the prediction of size effects.

Strength Prediction in Open Hole Composite Laminates by Using Discrete Damage Modeling

The present paper addresses the issue of direct simulation of complex local failure patterns in laminated composites. A model capable of discrete modeling of matrix cracking, delamination, and the interaction of these two damage modes is proposed. The analytical technique uses the eXtended Finite Element Method (X-FEM) for the simulation of matrix crack initiation and propagation at initially unknown locations, as well as a cohesive interface model for delamination. The model is capable of representing the complex kinematics of crack networks in composite laminates without previous knowledge of the crack locations or user intervention. An important feature of the technique is that it uses independently measured standard ply-level mechanical properties of the unidirectional composite (stiffness, strength, fracture toughness). Failure simulations of composites containing open holes are presented. Although the process of crack initiation is impossible to capture precisely due to local material variations, the proposed method exhibits excellent agreement with experimental data for matrix crack growth in unidirectional graphite-epoxy composites.

Development of Domain Superposition Technique for the Modelling of Woven Fabric Composites

A Domain Superposition Technique (DST) is proposed for the simulation of woven fabric composites. Instead of modelling the tows and the likely degenerated resin pocket regions among tows explicitly, DST separately models the tow domain and the global domain which are both non-degenerated, and can thus be easily discretised using the traditional solid elements. During the solution process, the two domains are superimposed by coupling them together to produce the exact results. Numerical simulation shows that the results of DST correlate very well with the results of conventional finite element analysis.

Modelling the effect of gaps and overlaps in automated fibre placement (AFP)-manufactured laminates

Abstract In automated fibre placement (AFP) process, gaps and overlaps parallel to the fibre direction can be introduced between the adjoining tapes. These gaps and overlaps can cause a reduction in strength compared with pristine conditions. Finite element modelling is an effective way to understand how the size and distribution of such gaps and overlaps influences the strength and failure development. Many modelling work showed that out-of-plane waviness and ply thickness variations caused by gaps and overlaps play an important role in inducing the strength knock-down; however, there has been a lack of effective way to explicitly model the ply waviness, which constrained the relevant research. In this work, 3D meshing tools were developed to automatically generate ply-by-ply models with gaps and overlaps. Intra-ply and inter-ply cohesive elements are also automatically inserted in the model to capture the influence of splitting and delamination. Out-of-plane waviness and ply thickness variations caused by gaps and overlaps are automatically modelled. Models with various sizes and distribution of gaps and overlaps were built to predict the reduction of strength as a function of the magnitude and type of the defects. Results of gap and overlap models will be used to guide future experimental characterization of simulated AFP process defects, manufactured by hand layup from pre-preg tape.

Effect of Stacking Sequence on Open-Hole Tensile Strength of Composite Laminates

Theeffectofaplystackingsequenceontheopen-holetensilestrengthofcompositelaminateshasbeeninvestigated both numerically and experimentally. A finite element technique has been developed that includes the subcritical damage within and between the plies. It is thus able to capture, in some detail, the effects of variation in stacking sequence. All possible permutations of 0, 90, and 45deg plies in a quasi-isotropic layup have been analyzed. Variations in strength have been observed andexplained in terms of subcritical damage development. Two stacking sequences showing different behavior were chosen for scaling in the thickness direction by increasing the ply thickness. Analysis of the first predicted a significant decrease in strength and a change in failure mode from fiberdominated to delamination-dominated failure with increasing thickness. The second predicted no such decrease in strength or change of failure mode. Both of these stacking sequences were then tested experimentally at both thicknesses, and excellent agreement with the numerical models was obtained, both in terms of damage mode and failure stress.

Predicting progressive delamination via interface elements

Publisher Summary This chapter reviews some of the background behind the development of interface elements with application to composites delamination. Interface elements are elements which model a thin or zero thickness layer between continua in a finite element analysis. The location of the interface elements are not themselves a prediction of the crack path but a plane of potential delamination which may or may not fail depending on the loading and the relevant failure criteria. For laminated composites this approach works well as the planes of possible delamination are generally well defined between the plies or at adhesive bond lines. The relative nodal displacements under an applied load can be separated out into normal and shearing components. The normal component represents a mode I type crack opening and the shearing a combination of mode II and III since these cannot be distinguished without knowledge of the direction of the crack front. The constitutive relation of the interface element is based on a traction-displacement law which is generally elastic up to a stress-based failure criterion (initiation) and then undergoes a softening behavior which describes an area under the curve equal to the critical fracture energy (GC) at complete failure (propagation).

The Effect of Stacking Sequence on Thickness Scaling of Tests on Open Hole Tensile Composite Specimens

The effect of ply stacking sequence on the open hole tensile strength of composite laminates has been investigated both numerically and experimentally. A finite element technique has been developed which includes the sub-critical damage within and between the plies. It is thus able to capture in some detail the effects of variation in stacking sequence. All possible permutations of 0, 90 and ± 45 ° plies in a quasi-isotropic layup have been analysed. Variations in strength have been observed and explained in terms of sub-critical damage development. Two stacking sequences of interest were chosen for scaling in the thickness direction by increasing the ply thickness. The first showed a significant decrease in strength and a change in failure mode from fibre dominated to delamination dominated failure with increasing thickness. The second showed no such decrease in strength or change of failure mode. Both of these stacking sequences were tested experimentally at both thicknesses and excellent agreement with the numerical models was obtained, both in terms of damage mode and failure stress.

Dynamic Mode II Delamination in Through Thickness Reinforced Composites

Through thickness reinforcement (TTR) technologies have been shown to provide effective delamination resistance for laminated composite materials. The addition of this reinforcement allows for the design of highly damage tolerant composite structures, specifically when subjected to impact events. The aim of this investigation was to understand the delamination resistance of Z-pinned composites when subjected to increasing strain rates.