Scott E. Stapleton

Crush Initiators for Increased Energy Absorption in Composite Sandwich Structures

Sandwich composites loaded in edgewise compression often display low energy absorbing failures due to facesheet debonding and/or buckling-type failure modes. Although crush initiators have been used to trigger progressive crushing in monolithic composites, little research has been performed to investigate the use of crush initiators with sandwich composites. In this investigation, fixture-based crush initiators were evaluated for producing progressive crushing failures and high levels of energy absorption under dynamic edgewise compression loading. Four different sandwich configurations were investigated, fabricated using woven or random mat carbon/epoxy facesheets with foam or balsa wood cores. Results show that the effectiveness of crush initiators is highly dependant upon both the facesheet and core materials used in the sandwich construction. Additionally, the initiation of progressive crushing of a sandwich composite appears to be dependent on the stiffness and strength properties of both the facesheets and core as well as the strength of the facesheet/core interfaces. These findings suggest that a sandwich composite may be designed for enhanced energy absorption through the proper selection of facesheet and core materials and geometries such that high-energy absorbing failure progressions are produced, especially with the application of a crush initiator.

Structural enhancements for increased energy absorption in composite sandwich structures

Facesheet debonding and buckling failure modes are often the cause of low energy absorption in composite sandwich panels under edgewise compression loading. To control the failure mode and initial failure location, the use of structural enhancements coupled with crush initiators were investigated. Three types of structural enhancements (end bevels, stitching, and core webbing) were incorporated into four composite sandwich configurations consisting of woven or random mat carbon/epoxy facesheets with foam or balsa wood cores. The effectiveness of these structural enhancements, combined with the use of crush initiators, was evaluated using dynamic edgewise compression testing. Results show that structural enhancements coupled with crush initiators may be used to increase the energy absorption capabilities of composite sandwich structures. However, the effectiveness of the structural enhancements investigated is highly dependant upon the stiffness and strength properties of the facesheets and core as well as the strength of the facesheet/core interfaces. These findings suggest that composite sandwich structures may be designed for enhanced energy absorption through the proper design of the sandwich configuration and the incorporation of a suitable structural enhancement.

Core Design for Energy Absorption in Sandwich Composites

To achieve high energy absorption under edgewise compression loading, sandwich composites must experience a progressive failure that includes compression failure of the facesheets. This investigation focused on sandwich core design for energy absorption through the use of failure mode maps. Predictive models for global buckling, facesheet wrinkling, and facesheet compression failure were used to construct failure mode maps as a function of sandwich core thickness and density. Predicted failure loads and initial failure modes were found to be in good agreement with results from edgewise compression testing. Sandwich configurations experiencing facesheet compression failure as the initial failure mode were found to produce the highest energy absorption. These results suggest that sandwich composites may be designed for enhanced energy absorption through the proper selection of core materials and geometries.

Macroscopic Finite Element for a Single Lap Joint

*† Macroscopic finite elements are elements with an embedded analytical solution used to carry out efficient, mesh independent finite element analysis. In the present study, this method of macro elements was applied to a single lap joint. The adherends were modeled as Euler-Bernoulli beams, and the adhesive layer assumed to be in a state of plane stress. The field equations were derived using the principle of minimum potential energy, and the resulting solutions for the displacement fields were used to generate shape functions and a stiffness matrix for a single finite element. Simplifying assumptions incorporated in the model development were evaluated by comparing with several corresponding 2-D finite element models with different joint parameters. The results showed that the derived macroelement results in considerable cost savings in computational modeling of structural systems that contain multiple lap joints.

A novel tensile test device for effective testing of high-modulus multi-filament yarns:

The paper introduces a novel clamp adapter with the goal to improve the quality of the tensile test setup for high-modulus multi-filament yarns. Common tensile test machines damage the yarns initially or prematurely due to non-uniform load introduction which causes stress concentrations. As a result, the theoretical yarn strength (perfectly clamped filaments at a unique length and no initial damage) is underestimated. With the new clamp adapter, higher strengths close to the theoretical values can be measured since the adapter largely eliminates the problems with non-uniform load introduction. A test series comparing yarns strengths tested with the clamp adapter and with commonly used test methods has been performed and the results are discussed in this paper. Furthermore, they are compared with theoretical values using the Daniels’ fibre-bundle model.

Effect of reinforcement volume fraction and orientation on a hybrid tissue engineered aortic heart valve with a tubular leaflet design

Transcatheter aortic valve implantation of fibrin-based tissue engineered heart valves with a tubular leaflet construct have been developed as an alternative to invasive traditional surgical heart valve implantation. In general, they are well suited for the pulmonary position, but display insufficient mechanical properties for the aortic position. To enable the application of tissue-engineered valves in the systemic circulation, the tissue is reinforced with a textile scaffold. The current study seeks to compare the effect of varying the fiber volume fraction and orientation of bidirectional textile reinforcement on the closed-valve configuration. An anisotropic large deformation material model based on structural tensors was chosen and the materials were characterized. A finite element model was constructed of the heart valve, and the pinching and suturing of the corners along with application of pressure was simulated. Virtual experiments were conducted with fiber volume fractions of 0.1, 0.01, 0.001, and 0.0001 for ±45° fiber orientations. Furthermore, volume fraction was held at 0.01 and fiber orientations of 0°, ±15°, ±30°, ±45°, ±60°, ±75° and 90° from the tube’s axial direction were simulated and compared. It was shown that increasing the fiber volume fraction decreased the maximum principle strain in the valve, but lead to less closure. Additionally, the effect of fiber orientation affected the strains differently at different locations, depending on the local deformed geometry. This indicates that a non-uniform fiber distribution using tailored fiber placement could be used to optimize reinforcement design.

Design of Tailored Non-Crimp Fabrics Based on Stitching Geometry

Automation of the preforming process brings up two opposing requirements for the used engineering fabric. On the one hand, the fabric requires a sufficient drapeability, or low shear stiffness, for forming into double-curved geometries; but on the other hand, the fabric requires a high form stability, or high shear stiffness, for automated handling. To meet both requirements tailored non-crimp fabrics (TNCFs) are proposed. While the stitching has little structural influence on the final part, it virtually dictates the TNCFs local capability to shear and drape over a mold during preforming. The shear stiffness of TNCFs is designed by defining the local stitching geometry. NCFs with chain stitch have a comparatively high shear stiffness and NCFs with a stitch angle close to the symmetry stitch angle have a very low shear stiffness. A method to design the component specific local stitching parameters of TNCFs is discussed. For validation of the method, NCFs with designed tailored stitching parameters were manufactured and compared to benchmark NCFs with uniform stitching parameters. The designed TNCFs showed both, generally a high form stability and in locally required zones a good drapeability, in drape experiments over an elongated hemisphere.

Advanced modeling of the behavior of bonded composite joints in aerospace applications

Abstract: Adhesively bonded structural components are increasingly being considered for lightweight aerospace structures. This chapter provides a state-of-the-art description of how contemporary advances in computational mechanics, based on the finite element method, can be used to obtain a basic understanding of the deformation response of adhesive joints in aerospace composite structures. Engineering approaches that can be used to design bonded structural joints are also described, with particular attention being given to bonded joint analysis using cohesive zone modeling. As an example for a real-life design implementation of cohesive zone based bonded joint analysis, an integrated computational analysis approach is presented.

Bonded Joint Elements for Structural Modeling and Failure Prediction

Elements based on the exact stiffness matrix method contain an embedded analytical solution that can capture detailed local fields, enabling more efficient mesh independent finite element analysis. In the present study, this method was applied to adhesively bonded joints. The adherends were modeled as Euler-Bernoulli beams, and the adhesive layer was modeled as a bed of linear shear and normal springs. The field equations were derived using the principle of minimum potential energy, and the resulting solutions for the displacement fields were used to generate shape functions and a stiffness matrix for a single joint finite element. Additionally, the capability to model non-linear adhesive and adherend constitutive behavior was developed, and progressive failure of the adhesive was modeled by using a strain-based failure criteria and re-meshing the joint as the adhesive fails. Example joint configurations were analyzed to demonstrate element convergence and the modeling of functionally graded adhesives.

Fluid-structure interaction simulation of artificial textile reinforced aortic heart valve: Validation with an in-vitro test

Prosthetic heart valves deployed in the left heart (aortic and mitral) are subjected to harsh hemodynamical conditions. Most of the tissue engineered heart valves have been developed for the low pressure pulmonary position because of the difficulties in fabricating a mechanically strong valve, able to withstand the systemic circulation. This necessitates the use of reinforcing scaffolds, resulting in a tissue-engineered textile reinforced tubular aortic heart valve. Therefore, to better design these implants, material behaviour of the composite, valve kinematics and its hemodynamical response need to be evaluated. Experimental assessment can be immensely time consuming and expensive, paving way for numerical studies. In this work, the material properties obtained using the previously proposed multi-scale numerical method for textile composites was evaluated for its accuracy. An in silico immersed boundary (IB) fluid structure interaction (FSI) simulation emulating the in vitro experiment was set-up to evaluate and compare the geometric orifice area and flow rate for one beat cycle. Results from the in silico FSI simulation were found to be in good coherence with the in vitro test during the systolic phase, while mean deviation of approximately 9% was observed during the diastolic phase of a beat cycle. Merits and demerits of the in silico IB-FSI method for the presented case study has been discussed with the advantages outweighing the drawbacks, indicating the potential towards an effective use of this framework in the development and analysis of heart valves.