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Prediction of Delamination Onset and Critical Force in Carbon/Epoxy Panels Impacted by Ice Spheres
Get PDFPolymer matrix composite structures are exposed to a variety of impact threats including hail ice. Internal delamination damage created by these impacts can exist in a form that is visually undetectable. This paper establishes an analysis methodology for predicting the onset of delamination damage in toughened carbon/epoxy composite laminates when impacted by high velocity ice spheres (hailstones). Experiments and analytical work focused on ice sphere impact onto composite panels have determined the failure threshold energy as a function of varying ice diameter and panel thickness, and have established the ability to predict the onset of delamination using cohesive elements in explicit dynamic finite element analysis. A critical force associated with damage onset was found to be independent of the ice diameter and thus can be expressed as a function of basic panel-describing parameters, namely bending rigidity and interlaminar fracture energy. Critical force can be used as a failure criterion in simpler models (e.g., shell elements) when predicting the onset of delamination by high speed spherical ice impact
From Ordered to Disordered: The Effect of Microstructure on Composite Mechanical Performance
Get PDFThe microstructural variation in fiber-reinforced composites has a direct relationship with its local and global mechanical performance. When micromechanical modeling techniques for unidirectional composites assume a uniform and periodic arrangement of fibers, the bounds and validity of this assumption must be quantified. The goal of this research is to quantify the influence of microstructural randomness on effective homogeneous response and local inelastic behavior. The results indicate that microstructural progression from ordered to disordered decreases the tensile modulus by 5%, increases the shear modulus by 10%, and substantially increases the magnitude of local inelastic fields. The experimental and numerical analyses presented in this paper show the importance of microstructural variability when small length scale phenomena drive global response
On Uniform Approximate Solutions in Bending of Symmetric Laminated Plates
Get PDFA layer-wise theory with the analysis of face ply independent of lamination is used in the bending of symmetric laminates with anisotropic plies. More realistic and practical edge conditions as in Kirchhoff's theory are considered. An iterative procedure based on point-wise equilibrium equations is adapted. The necessity of a solution of an auxiliary problem in the interior plies is explained and used in the generation of proper sequence of two dimensional problems. Displacements are expanded in terms of polynomials in thickness coordinate such that continuity of transverse stresses across interfaces is assured. Solution of a fourth order system of a supplementary problem in the face ply is necessary to ensure the continuity of in-plane displacements across interfaces and to rectify inadequacies of these polynomial expansions in the interior distribution of approximate solutions. Vertical deflection does not play any role in obtaining all six stress components and two in-plane displacements. In overcoming lacuna in Kirchhoff's theory, widely used first order shear deformation theory and other sixth and higher order theories based on energy principles at laminate level in smeared laminate theories and at ply level in layer-wise theories are not useful in the generation of a proper sequence of 2-D problems converging to 3-D problems. Relevance of present analysis is demonstrated through solutions in a simple text book problem of simply supported square plate under doubly sinusoidal load
Shape-Based Approach for Full Field Displacement Calculation of Cellular Materials
Get PDFIn this paper, we propose a new approach of optical full-field measurement for displacement calculation on the surface of a cellular solid. Cell boundary points are sampled as nodes in the analysis. To find the nodal values of displacements the nodes are to be mapped onto their corresponding points in the deformed cell boundary by shape based point matching. A thin plate spline based robust point matching (TPS-RPM) approach is used instead of correlation of intensity pattern between two regions in traditional displacement measurement methods. The proposed approach involves multiple-step image processing including cell region segmentation, cell region matching and node matching. Consequently displacements at a given node can be found easily. Two numerical examples of cellular solids under compressive loading are considered for assessing the effectiveness and accuracy of the proposed algorithm. The results show that local displacements around cell boundaries on the surfaces of the specimen can be effectively determined with the shape based method, thus it appears that the proposed methods is promising for predicting displacements of complex cellular materials
Failure Analysis of Bolted Joints in Cross-ply Composite Laminates Using Cohesive Zone Elements
Get PDFA strength prediction method is presented for double-lap single fastener bolted joints of cross-ply carbon fibre reinforced plastic (CFRP) composite laminates using cohesive zone elements (CZEs). Three-dimensional finite element models were developed and CZEs were inserted into subcritical damage planes identified from X-ray radiographs. The method makes a compromise between the experimental correlation factors (dependant on lay-up, stacking sequence and joint geometry) and three material properties (fracture energy, interlaminar strength and nonlinear shear stress-strain response). Strength of the joints was determined from the predicted load-displacement curves considering sub-laminate and plylevel scaling effects. The predictions are in a reasonable agreement with the experimental data
A Higher Order Synergistic Damage Model for Prediction of Stiffness Changes due to Ply Cracking in Composite Laminates
Get PDFA non-linear damage model is developed for the prediction of stiffness degradation in composite laminates due to transverse matrix cracking. The model follows the framework of a recently developed synergistic damage mechanics (SDM) approach which combines the strengths of micro-damage mechanics and continuum damage mechanics (CDM) through the so-called constraint parameters. A common limitation of the current CDM and SDM models has been the tendency to over-predict stiffness changes at high crack densities due to linearity inherent in their stiffness-damage relationships. The present paper extends this SDM approach by including higher order damage terms in the characterization of ply cracking damage inside the material. Following the SDM procedure, predictions are aided by suitable micromechanical computations of crack opening displacements. A nonlinear SDM model is developed and applied for multiple classes of composite laminate layups. Stiffness predictions for damaged laminates using the developed model are compared with the experimental data for cross-ply ([0m/90n]s), angle-ply ([±θm/90n]s), off-axis ([0/±θ4/01/2]s) and quasi-isotropic ([0/90/±45]s) laminates. A comparison with current linear damage models showcases the usefulness of the proposed nonlinear SDM approach
Estimation of the Mechanical Property of CNT Ropes Using Atomistic-Continuum Mechanics and the Equivalent Methods
Get PDFThe development in the field of nanotechnology has prompted numerous researchers to develop various simulation methods for determining the material properties of nanoscale structures. However, these methods are restricted by the speed limitation of the central processing unit (CPU), which cannot estimate larger-scale nanoscale models within an acceptable time. Thus, decreasing the CPU processing time and retaining the estimation accuracy of physical properties of nanoscale structures have become critical issues. Accordingly, this study aims to decrease the CPU processing time and complexity of large nanoscale models by utilizing, atomistic-continuum mechanics (ACM) to build an equivalent model of carbon nanotubes (CNTs). The results of tensile and modal analyses agree with previous experimental results indicating that the ACM model can accurately describe mechanical properties. This study also adopted three definitions of cross-sectional area to explore whether the structure properties of CNT ropes depends on the definitions of cross-sectional area. Results indicate that the Young’s modulus distribution based on the circumcircle assumptions agrees well with most of the experimental results. Hence, most experimental methods adopted the circumcircle to obtain the Young’s modulus of the CNT ropes. The circumcircle assumption involves the distribution of the tubes and the gap between each tube. The ratio between the gap and tube areas becomes a stable value when the diameter of the CNT ropes is increased. Therefore, a larger diameter of CNT ropes that represents the Young’s modulus becomes a stable value, as mentioned in literature. This study also investigated the equivalent solid, shell, and beam models to generate similar mechanical behaviors with the ACM model. The similar mechanical behavior of the equivalent model includes the model under tensile, torsion, or shear external loading. These equivalent models can significantly reduce the required total element number and CPU processing time to investigate a larger nanoscale structure
A Novel Metamaterial FSS-based Structure for Wideband Radome Applications
Get PDFA novel metamaterial based FSS (frequency selective surfaces) structure is presented in this paper for wideband airborne radome applications. The proposed metamaterial-FSS structure consists of three layers, where a DPS (double positive sign) layer is sandwiched between a MNG (μ-negative) and ENG (ε- negative) layer, exhibits very good bandpass characteristics inside the operational band along with excellent roll-off characteristics outside the band. The EM performance analysis of the proposed structure has been carried out using transmission line transfer matrix (TLTM) method, which shows excellent bandpass characteristics over a wide frequency range. The transmission efficiency is over 95% both at normal incidence and at high incidence angles of 30°, and 60°. The frequency range extends from S- to X-band (2.5-9.9 GHz). In view of streamlined airborne radome applications, the reflection properties and insertion phase delay (IPD) are also determined at high incident angles
Effects of High Magnetic Field on the Structure and Magnetic Properties of Molecular Beam Vapor Deposited Fe<sub>60</sub>Ni<sub>40</sub> Thin Films
No full textThe Fe60Ni40 (in atomic %) polycrystalline thin films with 90 nm thickness were prepared on 200 °C quartz substrate by using molecular beam vapor deposition method. The influence of 0 T and 6 T magnetic fields on the structural evolution and magnetic properties of thin films was studied by using EDXS, XRD, AFM and VSM. In this study, only α phase was formed in both thin films. It was found that the application of a 6 T magnetic field obviously decreases the RMS of surface roughness and the grain size. For the magnetic properties of the thin films, the 6 T magnetic field increases the saturation magnetism Ms in-plane and the squareness (Mr/Ms) of the hysteresis loop and decreases the coercive force Hc. This indicates the soft magnetic properties of the thin films have been notably enhanced in-plane by a high magnetic field. The relationship between the structural evolution and magnetic properties was discussed in details
An Experimental Study on Drying of Pistacia Terebinthus in a Fixed Bed Dryer
Get PDFIn this study, drying behaviours of the outer shell peeled Bittim (Pistacia terebinthus) with initial moisture content of 42.2% (dry basis (d.b)) was investigated in a novel fixed bed drying system. The drying experiments were performed at different temperatures (40 °C, 60 °C and 80 °C), air velocities (0.5 m/s and 1 m/s) and weights (30 g and 40 g). A constant rate period was not observed in the drying of bittims; all the drying process occurred in falling rate period. Three models in literature were selected to fit the experimental data. The fit quality of models was evaluated using the coefficient of determination (R2), sum square error (SSE) and root mean square error (RMSE). Balbay and Sahin model has a good agreement with the experimental data and gave the best results for bittims in a fixed bed system