1,721,026 research outputs found
An improved analysis of a STB specimen for fracture characterization of laminates and foam-cored sandwich composites under mode III loads
No full textThis work presents the development of an improved data reduction method for the shear-torsion-bending (STB) test designed for monolithic laminates. The analytical derivation extends the applicability of the STB rig to composite sandwich specimens subjected to an out-of-plane shear loading. The data reduction method consists of an analytical model that expresses the global energy release rate in terms of applied loads, specimen geometry and material properties. The mathematical derivation of the energy release rate relies on first order shear deformation theory, Vlasov theory for non-uniform torsion of beams and near tip effects are also taken into consideration by the analytical model. Face sheets and core are modelled as homogeneous, linear elastic and orthotropic materials. The analytical expression is verified using the energy release rate extracted from a high-fidelity 3D FE based fracture mechanics model of the specimen. A compliance based method is used to generate global predictions, while local predictions are extracted using a displacement-based mode separation method. Local predictions are used to discuss accuracy and limitations of the approximate analytical model.</p
Assessment of foam fracture in sandwich beams using thermoelastic stress analysis
No full textThermoelastic Stress Analysis (TSA) has been well established for determining crack-tip parameters in metallic materials. This paper examines its ability to determine accurately the crack-tip parameters for PVC foam used in sandwich structures
The effects of shear on the fracture parameters of face/core debonds in sandwich beams
No full textThe effects of shear on energy release rate and mode mixity in a debonded sandwich beam are investigated through a semi-analytic approach which uses exact two-dimensional elasticity and structural theories. The expressions for the shear components of the energy release rate and mode mixity will be combined with earlier results for three-layers subjected to bending-moment and axial forces to obtain solutions for general loading conditions
Fracture mechanics solutions and operative formulae for isotropic bi-material layers with large elastic mismatch
No full textFollowing previous works in the literature on the fracture of layered composites, semi-analytical expressions for energy release rate and mode-mixity phase angle in isotropic bi-material layers subjected to arbitrary end loadings are extended to materials characterized by a large elastic mismatch described by Dundurs’ parameters |α| ≤ 0.99 and |β| ≤ 0.4. The solutions depend on elementary bending, axial and shear loads acting at the crack tip cross section, elastic constants and relative thickness of the layers, and dimensionless coefficients describing the fracture parameters of single elementary loads. The coefficients associated to the shear forces are presented in the literature only for combinations with β = 0; here they are determined numerically for different values β not =0 to describe materials used in current applications. Simplified fitting formulae having general validity, as polynomials, which highlight the dependence on Dundurs’ parameters and are practical to support laboratory tests, are derived for layers with mid-thickness delaminations and thin layers on semi-infinite substrates. Applications to some mixed mode fracture specimens highlight the effects of β not = 0 on the fracture parameters and the limitations of estimates based on the commonly adopted assumption β = 0. As expected, the results for mid-thickness delaminations show that such effects are stronger on mode-mixity phase angle than on energy release rate
Experimental investigation of interfacial crack arrest in sandwich beams subjected to fatigue loading using a novel crack arresting device
No full textA recently proposed face-sheet/core interface crack arresting device is implemented in sandwich beams and tested using the Sandwich Tear Test (STT) configuration. Fatigue loading conditions are applied to propagate the crack and determine the effect of the crack stopper on the fatigue growth rate and arrest of the crack. Digital image correlation is used through the duration of the fatigue experiment to track the strain evolution as the crack tip advances. The measured strains are related to crack tip propagation, arrest, and re-initiation of the crack. A finite element model is used to calculate the energy release rate, mode mixity and to simulate crack propagation and arrest of the crack. Finally the effectiveness of the crack arresting device is demonstrated on composite sandwich beams subjected to fatigue loading conditions
Interfacial crack arrest in sandwich panels with embedded crack stoppers subjected to fatigue loading
No full textA novel crack arresting device has been implemented in sandwich panels and tested using a special rig to apply out-of-plane loading on the sandwich panel face-sheets. Fatigue crack propagation was induced in the face-core interface of the sandwich panels which met the crack arrester. The effect of the embedded crack arresters was evaluated in terms of the achieved enhancement of the damage tolerance of the testedsandwich panels. A finite element (FE) model of the experimental setup was used for predicting propagation rates and direction of the crack growth. The FE simulation was based on the adoption of linear fracture mechanics and a fatigue propagation law (i.e. Paris law) to predict the residual fatigue life-time and behaviour of the test specimens. Finally, a comparison between the experimental results and the numerical simulations was made to validate the numerical predictions as well as the overall performance of the crack arresters
Estimation of fracture parameters in foam core materials using thermal techniques
No full textThe paper presents some initial work on establishing the stress state at a crack tip in PVC foam material using a non-contact infra-red technique known as thermoelastic stress analysis (TSA). A parametric study of the factors that may affect the thermoelastic response of the foam material is described. A mode I simulated crack in the form of a machined notch is used to establish the feasibility of the TSA approach to derive stress intensity factors for the foam material. The overall goal is to demonstrate that thermal techniques have the ability to provide deeper insight into the behaviour of the cracks in foam and the potential to determine stress intensity factor
Towards high velocity deformation characterisation of metals and composites using Digital Image Correlation
Full text linkCharacterisation of materials subject to high velocity deformation is necessary as many materials behave differently under such conditions. It is particularly important for accurate numerical simulation of high strain rate events. High velocity servo-hydraulic test machines have enabled material testing in the strain rate regime from 1 – 500 ?/s. The range is much lower than that experienced under ballistic, shock or impact loads, nevertheless it is a useful starting point for the application of optical techniques. The present study examines the possibility of using high speed cameras to capture images and then extracting deformation data using Digital Image Correlation (DIC) from tensile testing in the intermediate strain rate regime available with the test machines. Three different materials, aluminium alloy 1050, S235 steel and glass fibre reinforced plastic (GFRP) were tested at different nominal strain rates ranging from quasi static to 200 ?/s. In all cases DIC was able to analyse data collected up to fracture and in some cases post fracture. The use of high-speed DIC made it possible to capture phenomena such as multiple necking in the aluminium specimens and post compression failure in GFRP specimen
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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