1,721,188 research outputs found
Damage detection in composite materials using lamb wave methods
No full textCost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate methods for in situ damage detection of composite materials. Experimental results are presented for the application of Lamb wave techniques to quasi-isotropic graphite/epoxy test specimens containing representative damage modes, including delamination, transverse ply cracks and through-holes. Linear wave scans were performed on narrow laminated specimens and sandwich beams with various cores by monitoring the transmitted waves with piezoceramic sensors. Optimal actuator and sensor configurations were devised through experimentation, and various types of driving signal were explored. These experiments provided a procedure capable of easily and accurately determining the time of flight of a Lamb wave pulse between an actuator and sensor. Lamb wave techniques provide more information about damage presence and severity than previously tested methods (frequency response techniques), and provide the possibility of determining damage location due to their local response nature. These methods may prove suitable for structural health monitoring applications since they travel long distances and can be applied with conformable piezoelectric actuators and sensors that require little power
Damage detection in composite materials using frequency response methods
No full textCost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate methods for the in situ detection of damage in composite materials. The experimental results are presented for the application of modal analysis techniques applied to graphite/epoxy specimens containing representative damage modes. Changes in natural frequencies and modes were found using a laser vibrometer, and 2-D finite element models were created for comparison with the experimental results. The models accurately predicted the response of the specimens at low frequencies, but coalescence of higher frequency modes makes mode-dependant damage detection difficult for structural applications. The frequency response method was found to be reliable for detecting even small amounts of damage in a simple composite structure, however the potentially important information about damage type, size, location and orientation were lost using this method since several combinations of these variables can yield identical response signatures
Postbuckling analysis and optimization of laminated composite plates with applications in aerospace
No full textIn aerospace applications, thin plate-like structures are widely used and often undergo large transverse deflections and have to carry considerable loads. In conventional design practices, buckling is often considered as a final failure of structural members. However, there may be considerable load capacity, particularly for plate-like structures, beyond buckling limit before final failure occurs. Therefore, it has been a common practice in aerospace to further explore the load-carrying capacity, or, in other words, the weight-savings by allowing the structures to operate in their postbuckling regime. The further load-carrying capacity of thin-walled plates/structures in the postbuckling regime makes them very attractive in the design of lightweight aero-structures, which have continuously been in demand in the aerospace industry.For example, lighter, thinner, still stronger skins can be used in wings and fuselages. On the other hand, solid knowledge and deep understanding of the postbuckling behavior of the thin-walled plates/structures must be acquired before performing the optimal design of such nonlinear structural performance. Laminated composite materials are increasingly used in the aviation and aerospace industry as the primary load-carrying components, due to their high strength-to-weight ratios and large stiffness tailoring flexibility. Driven by advanced manufacturing technologies, many novel composite materials with increased design flexibility and functionality, such as functionally graded materials, variable angle tow composites, and 3D-printed composites, have been developed. These novel composite materials provide extensive opportunities for designers to obtain desirable postbuckling performance for future aerospace structures
Predicting fracture of laminated composites
No full textThe compressive strength of currently used carbon fiber-reinforced plastics is generally 30–40% lower than the tensile strength due to fiber microbuckling, thus it is recognized that the compressive strength is often a design-limiting consideration. The zones of compressive stresses can appear in composite structures even under tensile loads. They could be due to the presence of holes, cut-outs and cracks, or generated by impact. It has been revealed that a possible mechanism of failure initiation is fiber or layer microinstability (microbuckling) that might usually occur in regions where high stress gradients exist, for instance, on the edge of a hole or near free edges. A better understanding of the compression failure mechanisms, specific only to heterogeneous materials, is crucial to the development of improved composite materials. The task of deriving Three-Dimensional [“3-D”] analytical solutions to describe the compressive response has been considered as one of great importance. Such solutions, if obtained, enable to analyze the behavior of a structure on the wide range of material properties, and kinematic and loading boundary conditions, without the restrictions imposed by simplified approximate methods
Analysis of delamination in laminates with angle-ply matrix cracks
No full textThe failure process of composite laminate under quasi-static or fatigue loading involves sequential accumulation of intra- and interlaminar cracking. Matrix cracking parallel to the fibres in the off-axis plies is the first damage mode observed. It triggers development of other harmful resin-dominated modes such as delaminations. In this chapter, analytical modelling of crack-induced delaminations in composite laminates subjected to general in-plane loading is presented and discussed. A two-dimensional shear lag analysis is used to determine ply stresses in a representative segment and the equivalent laminate concept is applied to derive expressions for mode I and mode II and the total strain energy release rate associated with uniform local delaminations. These expressions could be used with appropriate fracture criteria to estimate the onset of local delamination in an already cracked off-axis laminate. Dependence of strain energy release rate on crack density, delamination area and ply orientation angle in unbalanced symmetric laminates is examined and discussed, and the effect of crack-induced delamination on the laminate stiffness is predicted
Modelling damage in laminate composites
No full textThe failure of Glass- and Carbon-Fiber Reinforced Plastic [“GFRP” and “CFRP”] laminates subjected to static or cyclic tensile loading acting in the plane of reinforcement, and also under thermal fatigue, is a complex process. It involves sequential accumulation of various types of intra- and interlaminar damage, which gradually lead to the loss of the laminate's load-carrying capacity. The main damage mechanisms, exhibited in composite laminates, are matrix cracking, delamination, fiber debonding, and fiber breakage. Damage mechanisms in composite laminates can be studied theoretically following two approaches. Using the continuum damage mechanics approach, various types of damage are accounted for via the damage tensor. A composite is described as a continuum with mechanical properties depending on the damage tensor. Using the damage micromechanics approach, stress analysis of the damaged composite is carried out in the explicit presence of damage. Various types of damage are analyzed directly with the aim to predict their onset and growth, and also their effect on the properties of the laminate. While for homogeneous isotropic materials it is possible to obtain exact solutions within the linear elasticity theory, stress analysis of damaged composite laminates is approximate in the majority of cases. If interaction between various types of damage is especially complex, stress field can only be determined by numerical methods such as the finite-element method
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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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