78 research outputs found

    Fracture of sandwiched composites

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    Fracture of a pair of collinear cracks in various materials, such as an isotropic strip, an orthotropic strip, a bonded isotropic adhesive layer, and sandwiched orthotropic layers, is investigated. The crack surfaces are subjected to an arbitrary opening pressure p(x). The problems are formulated in terms of Fredholm integral equation of the second kind by making use the techniques of Fourier transform and finite Hilbert transform. In case of uniform opening pressure p(x) = [sigma], exact expressions for the stress intensity factors and the shape of deformed crack are obtained. Numerical calculations are carried out to study the effects of various boundary geometries and material properties on the fracture of the chosen materials

    Hypersingular integral equation for triple circular arc cracks in an elastic half-plane

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    Triple circular arc cracks problems subjected to shear stress in half-plane elasticity is investigated. Modified complex potentials (MCP) with the free traction boundary condition are applied to formulate the hypersingular integral equation (HSIE) for the problems. The unknown crack opening displacements (COD) of the HSIE are solved numerically by using the appropriate quadrature formulas. Mode I and Mode II of nondimensional stress intensity factor (SIF) at all cracks tips are presented for the problems of three adjacent circular arc cracks, three circular arc cracks with dissimilar radius and three circular arc cracks in series in a half-plane. The results exhibit that as the crack opening angle increases and the distance of cracks closer to the boundary of half-plane, the nondimensional SIF increases. This indicates that the strength of material becomes weaker and the tendency of material to fail is higher

    EXPERIMENTAL AND NUMERICAL SIMULATION OF SPLIT HOPKINSON PRESSURE BAR TEST ON BOROSILICATE GLASS

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    This study is an extension to the design of ceramic materials component exposed to bullet impact. Owing to the brittle nature of ceramics upon bullet impact, shattered pieces behave as pellets flying with different velocities and directions, damaging surrounding components. Testing to study the behavior of ceramics under ballistic impact can be cumbersome and expensive. Modeling the set-up through Finite Element Analysis (FEA) makes it economical and easy to optimize. However, appropriately incorporating the material in modeling makes laboratory testing essential. Previous efforts have concentrated on simulating crack pattern developed during 0.22 caliber pellet impact on Borosilicate glass. A major concentration of work is on study of mesh pattern and size. The maximum principal strain has been considered to define the failure criteria which doesn’t correspond to theoretical properties. To appropriately incorporate material properties, the behavior of ceramics under ballistic impact could be tested through controlled impact Split Hopkinson Pressure bar (SHPB) testing setup. This paper discusses the results of SHPB bar testing on 1018 cold rolled steel to validate the experimental procedures and result analysis. The work has been extended to conduct testing on borosilicate samples under different input conditions. Strategies for improving the test result are proposed in the paper. The paper extensively covers the dynamics of glass material under ballistic impacts, various test procedures to obtain material model constants. Incorporating the material model in the previous FEA simulation makes it susceptible to numerous factors affecting the result. FEA characterization of SHPB test makes it suitable for modeling and correlating with the testing result of borosilicate glass. The FEA set-up is simplified to incorporate all the parameters affecting the test. Comprehensive analysis of the loading pulse is conducted to validate the model. This paper discusses specimen analysis through the standard material model in LS-Dyna MAT_110 for five different classes of ceramics. Inconsistencies between testing result and simulation have been identified and presented in this paper. The gaps in the study have been highlighted and means to obtain a good correlation is proposed in this paper to guide future work

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    The significance of embrasure design on the fracture load of fixed denture prosthesis: an in vitro study

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    OBJECTIVE: This study evaluated two embrasure designs by measuring their differential effect on load at failure of provisional fixed partial dentures (FPDs) fabricated of five commercially available polymer-based restorative materials. METHODS: Five provisional C&B materials were selected to fabricate FPDs with two different embrasure designs: sharp vs. rounded embrasures (n=12 for each material). The test materials included: Telio CAD (Ivoclar-Vivadent), Coldpac (Motloid), Protemp Plus (3M), VersaTemp (Sultan), and Turbo Temp (Danville). The embrasures were formed using prefabricated cutters with measured Radii (0.002r and 0.03r) and a fixture to hold each provisional FPD in place for the uniform standardized cuts. Molds for the CAD/CAM provisional FPDs were used to fabricate the syringeable temporary materials and form bridges with the same geometric design. All provisional bridges were cemented using Temp-Bond (Kerr) to the corresponding standardized abutments and tested to failure in a universal Instron testing machine by loading each specimen compressively in the mid pontic region. The load at break was recorded in Newton. A one-way analysis of variance (ANOVA) was used to compare the difference in each group’s mean. RESULTS: A significant difference in fracture load was found between the two groups of designs, in which the round embrasure was significantly stronger than was the sharp. A significant difference also was found between the type of temporary material used to fabricate the bridge in the two groups, and except for Coldpac, no significant difference between the embrasure anatomies was found. Fatigue loading did not appear to influence the two bridges’ fracture load, but it did show a significant difference with respect to the modulus of elasticity, in that the bridges that underwent fatigue loading showed a higher elastic modulus by comparison to the control group. Another variable that influenced the modulus of elasticity was the type of temporary material used to fabricate the bridge, in which TelioCAD was found to be the stiffest. However, the embrasure design did not seem to affect the bridges’ rigidity. CONCLUSION: A significant difference was found in fracture strength between the rounded and sharp embrasure design. Except for Coldpac, the rounded embrasure showed higher fracture toughness than did the sharp. No significant correlation was found between the two embrasure designs and the modulus of elasticity. Interestingly, the fatigued bridges that underwent cyclic loading showed a higher modulus of elasticity. The sharp embrasure design showed no fracture in the pontic region, while the rounded design did in 5.47% of the sample. This may be explained by the photoelastic bridges, in which the stress diffuses in the rounded design to include the pontic region, while in the sharp design, the stress is concentrated on the connector area. Stress analysis, both by means of photoelastic and finite element analysis, demonstrated that the bridge with the sharp embrasure design’s stress was high in the connector area compared to the round embrasure design

    Mixed-mode partition theories for one-dimensional fracture

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    Many practical cases of fracture can be considered as one-dimensional, that is, propagating in one dimension and characterised by opening (mode I) and shearing (mode II) action only with no tearing (mode III) action. A double cantilever beam (DCB) represents the most fundamental one-dimensional fracture problem. There has however been considerable confusion in calculating its mixed-mode energy release rate (ERR) partition. In this work, new and completely analytical mixed-mode partition theories are developed for one-dimensional fractures in isotropic homogeneous and laminated composite DCBs, based on linear elastic fracture mechanics (LEFM) and using the Euler and Timoshenko beam theories. They are extended to isotropic homogeneous and laminated composite straight beam structures and isotropic homogeneous plates based on the Kirchhoff-Love and Mindlin-Reissner plate theories. They are also extended to non-rigid elastic interfaces for isotropic homogeneous DCBs. A new approach is used, based on orthogonal pure fracture modes. Two sets of orthogonal pairs of pure modes are found. They are distinct from each other in the present Euler beam and Kirchhoff-Love plate partition theories and coincide on the first set in the present Timoshenko beam and Mindlin-Reissner plate partition theories. After the two sets of pure modes are shown to be unique and orthogonal, they are used to partition mixed modes. Interaction is found between the mode I and mode II modes of the first set in the present Euler beam and Kirchhoff-Love plate partition theories. This alters the ERR partition but does not affect the total ERR. There is no interaction in the present Timoshenko beam or Mindlin-Reissner plate partition theories. The theories distinguish between local and global ERR partitions. Local pureness is defined with respect to the crack tip. Global pureness is defined with respect to the entire region mechanically affected by the crack. It is shown that the global ERR partition using any of the present partition theories or two-dimensional elasticity is given by the present Euler beam or Kirchhoff-Love plate partition theories. The present partition theories are extensively validated using the finite element method (FEM). The present beam and plate partition theories are in excellent agreement with results from the corresponding FEM simulations. Approximate 'averaged partition rules' are also established, based on the average of the two present beam or plate partition theories. They give close approximations to the partitions from two-dimensional elasticity. The propagation of mixed-mode interlaminar fractures in laminated composite beams is investigated using experimental results from the literature and various partition theories. The present Euler beam partition theory offers the best and most simple explanation for all the experimental observations. It is in excellent agreement with the linear failure locus and is significantly closer than other partition theories. It is concluded that its excellent performance is either due to the failure of materials generally being based on global partitions or due to the through-thickness shear effect being negligibly small for the specimens tested. The present partition theories provide an excellent tool for studying interfacial fracture and delamination. They are readily applicable to a wide-range of engineering structures and will be a valuable analytical tool for many practical applications

    Analysis and optimization of cracked composite laminates

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    This thesis deals with three inter-related topics. The first topic is concerned with the solution of a cracked transversely isotropic—orthotrOpic composite laminate of finite thickness. The transversely isotropic sublaminate which is sandwiched between orthotropic outer sublarninates contains a central crack or an array of periodically distributed cracks which are perpendicular to the interfaces between the two media. When the crack is wholly within the sublaminate, solutions are obtained for the stress intensity factors and the crack-induced interfacial stresses in three loading modes. When the crack tips touch the interfaces, the stress singularities are no longer the usual square-root type but are determined by the mechanical properties of the media. In this case, solutions are obtained for stress singularities and the corresponding stress intensity factors. The degenerate case when the outer sublarninates are also isotropic but dissimilar from the central sublaminate is also solved. The second topic concerns the application of the above fracture mechanics solutions to crack problems of laminates composed of unidirectional fibre-reinforced composites. In view of the fact that unidirectional fibre-reinforced composites are prone to transverse cracking and that laminates made from unidirectional plies are prone to delamination, a cracked [(il9),,2 /(900)n1 ]s symmetric laminate is studied with a view to examining the mutual constraining effect of plies on transverse cracking and the role of transverse cracks in causing delamination. The fracture mechanics framework is used to reveal the mechanisms behind the enhancement of the socalled in situ strengths of unidirectional laminae in multidirectional laminates. When the tips of a transverse crack touch the interfaces, the effect of the properties of the constraining sublarninates on the stress singularities and stress intensities at the tips of the crack is investigated. The third topic is concerned with two types of optimum strength design of composites laminates. First, for a fibre-reinforced antisymmetric [(:l:0°)n2 / (90°)n1 / (2120),,2] angle-ply laminate, the design variables of the laminate, viz. the ply angle 0 and relative ply thickness, are chosen in such a way as to minimize the stress intensity factor at the crack tip in the (900),“ lamina without exceeding the interfacial maximum principal tensile stress. Secondly, based upon the extensive fracture mechanics analysis (from topics one and two), a set of in situ strength parameters for unidirectional laminae in a multidirectional laminate is proposed. The in situ strength parameters take into account the influence of adjacent laminae and thickness of a particular lamina upon its transverse tensile and in-plane shear strengths when it is used in a multidirectional laminate. These strength parameters are then employed to calculate a stress norm which determines how close the stress state in the lamina is to its failure state. The stress norm is incorporated into the formalism of an optimization problem in order to enhance the load bearing capacity of multidirectional laminates

    Fiber-fiber bond strength : a study of a linear elastic model structure

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    Adhesive Joints in Wind Turbine Blades

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