Autar Kaw

Optimization of Laminate Stacking Sequence for Failure Load Maximization Using Tabu Search

In this paper, design of a laminate is treated as a combinatorial optimization problem. Optimization of the stacking sequence of a laminate for buckling response, matrix cracking, and strength requirements is conducted. Optimum designs are generated using a heuristic search technique known as Tabu search. Preliminary studies used to examine the effect of different aspects of Tabu search are presented. Three load cases, which have been used in previous studies with Genetic Algorithm, were utilized in this work. The relative performance of Tabu search compared to Genetic Algorithm was found to depend on the load case. For the first two cases, and for average of all the three load cases, Tabu search was found to reduce the computing cost of generating equivalent results by 26, 55 and 1%, respectively. For the third load case, the computing cost with Tabu search was higher by 302%.

Axisymmetric thermoelastic response of a composite cylinder containing an annular matrix crack

An elasticity solution to the problem of an annular crack in a concentric cylinder under a remote uniform axial strain and constant temperature change is found. Stress intensity factors at the crack tips and the stresses at the interface are studied to de termine the nature of the crack growth in the composite. These parameters are studied as a function of the elastic moduli and the thermal expansion coefficients of the constituents, the fiber volume ratio, crack geometry, and interface properties.

Mechanics of multiple periodic brittle matrix cracks in unidirectional fiber-reinforced composites

Abstract A fracture model based on two-dimensional plane stress-strain elasticity theory is developed for the problem of periodic, interacting and regularly spaced matrix cracks in a unidirectional fiber-reinforced brittle matrix composite. The solution is obtained in terms of a hypersingular integral equation. The effects of the fiber reinforcement, and the spacing, the location and the length of cracks on the stress intensity factors at the crack tips and the maximum crack opening displacement in the composite are studied.

Holistic but customized resources for a course in numerical methods

Prototype web based resources have been developed for an undergraduate course in Numerical Methods. The web modules are holistic, that is they include pre‐requisite information, real‐life applications, presentations and notes, simulations, and self‐assessment. The student interest and learning are maximized by providing customization of content based on a students engineering major and computational system of choice.

Fracture mechanics of composites with nonhomogeneous interphases and nondilute fiber volume fractions

Abstract A linear elastic fracture mechanics model is developed for a periodic composite geometry with a single cracked layer under uniform longitudinal strain. The interface between the fiber and the matrix can be nonhomogeneous, homogeneous or perfect. Four fracture mechanics models are considered: (1) dilute fiber volume fraction composite with nonhomogeneous interphases. (2) periodic composite with nondilute fiber volume fraction. (3) periodic composite with irregular fiber spacing near a matrix layer. (4) hybrid composites with more than one type of fiber. The stress intensity factors at the crack tips and the interface stress fields are studied to understand the fracture mechanics of composites as a function of the relative elastic moduli of the fiber, the matrix and the interphase, and the global and local fiber volume fractions.

Assembly Procedures of Trunnion-Hub-Girder for Bascule Bridges

Abstract This work is concerned with avoiding failures during the assembly of a trunnion–hub–girder (THG) for bascule bridges. In the current assembly procedure, AP#1, the trunnion is shrunk fit into the hub, followed by the shrink fitting of the trunnion–hub assembly into the girder. Two separate incidents during assembly prompted this study. The first incident involved the development of cracks in the hub during the assembly process using AP#1. The second incident involved the trunnion getting stuck in the hub before the trunnion could be fully inserted. A complete analytical, numerical, and experimental study was conducted to understand these failures, and the results were used to develop specifications and recommendations for assembly. The causes of failures include the development of high stresses at low temperatures during assembly, while noting that fracture toughness of THG materials decreases with temperature. Recommended specifications included following an alternative assembly procedure that nearly doubles allowable crack length, and that lowers cooling temperatures to avoid trunnion sticking in the hub.

Measuring student learning using initial and final concept test in an STEM course

Effective assessment is a cornerstone in measuring student learning in higher education. For a course in Numerical Methods, a concept test was used as an assessment tool to measure student learning and its improvement during the course. The concept test comprised 16 multiple choice questions and was given in the beginning and end of the class for three semesters. Hakes gain index, a measure of learning gains from pre- to post-tests, of 0.36 to 0.41 were recorded. The validity and reliability of the concept test was checked via standard measures such as Cronbachs alpha, content and criterion-related validity, item characteristic curves and difficulty and discrimination indices. The performance of various subgroups such as pre-requisite grades, transfer students, gender and age were also studied.

Effect of extrinsic and intrinsic factors on an indentation test

The effect of intrinsic and extrinsic factors on the measured results, such as load-displacement curves and interfacial stresses, from indentation tests of composite materials is studied using both analytical and finite element models. The intrinsic factors include properties of the fiber-matrix interface and the material symmetry of the fiber (transversely isotropic or isotropic). The extrinsic factors include the radius of the hole through which the fiber is pushed in, and the size and shape of the indentor. Out of the above factors, only the radius of the hole is found to have a negligible effect on the results of the indentation test.

Asymptotic stresses around a crack tip at the interface between planar or cylindrical bodies

Closed-form equations are derived for the asymptotic stresses in the neighborhood of a crack tip impinging on an interface between two isotropic materials. The symmetric problem is considered and follows from an exact elasticity solution formulated by Gupta [1]. The equations are valid for the planar problem, where the interface is straight and also for an axisymmetric problem in the presence of an annular or penny-shaped crack. The equations may serve to establish a tentative criterion that defines the subsequent direction of a crack impinging on a bimaterial interface. The ambiguity of the asymptotic stress state is highlighted and plausible application of the results is discussed.

The stress field in a cracked brittle matrix composite cylinder with a frictional interface

Abstract The effect of a frictional interface on the response of a unidirectional ceramic matrix composite under a remote axial tensile strain and a temperature change is studied. The geometry of the composite is approximated by a concentric cylinder model with an annular crack in the axial plane of the matrix. The fiber-matrix interface follows the Coulomb friction law. On applying the boundary and the interface continuity conditions, the solution is obtained in terms of coupled integral equations and inequality conditions. The extent of the interfacial damage and the stress fields in the fiber and the matrix along the interface are studied for a SiC/CAS composite system as a function of the coefficient of friction, temperature change, and remote uniform axial strain. These results are also compared with a shear lag analysis model for an identical geometry and loading.