Knowledge-based modeling of fracture of materials and structures

Knowledge-based systems may be very useful, as shown in this publication, for determining the fracture characteristics of materials and structures. Such values as stress intensity, crack length, critical crack length, critical load-carrying capacity, etc. can be found easily and automatically by specifying relatively few items, such as material, geometry and loads. In addition, valuable information can be provided to NDE investigators. Although this research is in its infancy, valuable results have already been found which help in forecasting promising avenues of research in the development of knowledge-based systems. Also, if developed satisfactorily, future handbooks may take a major turn in drawing upon black box capabilities of knowledge-based systems.     

Theoretical and computational hierarchical nanomechanics of protein materials: Deformation and fracture

Proteins constitute the building blocks of biological materials such as tendon, bone, skin, spider silk or cells. An important trait of these materials is that they display highly characteristic hierarchical structures, across multiple scales, from nano to macro. Protein materials are intriguing examples of materials that balance multiple tasks, representing some of the most sustainable material solutions that integrate structure and function. Here we review progress in understanding the deformation and fracture mechanisms of hierarchical protein materials by using a materials science approach to develop structure-process-property relations, an effort defined as materiomics. Deformation processes begin with an erratic motion of individual atoms around flaws or defects that quickly evolve into formation of macroscopic fractures as chemical bonds rupture rapidly, eventually compromising the integrity of the structure or the biological system leading to failure. The combination of large-scale atomistic simulation, multi-scale modeling methods, theoretical analyses combined with experimental validation provides a powerful approach in studying deformation and failure phenomena in protein materials. Here we review studies focused on the molecular origin of deformation and fracture processes of three types of protein materials. The review includes studies of collagen - Nature’s super-glue; beta-sheet rich protein structures as found in spider silk - a natural fiber that can reach the strength of a steel cable; as well as intermediate filaments - a class of alpha-helix based structural proteins responsible for the mechanical integrity of eukaryotic cells. The article concludes with a discussion of the significance of universally found structural patterns such as the staggered collagen fibril architecture or the alpha-helical protein motif.     

Analogies between progressive collapse of structures and fracture of materials

The analogy between structural progressive collapse and Fracture Mechanics is consistent either for phenomenological, technological and theoretical aspects. In this paper a general energy criterion suitable for fracture in heterogeneous materials is applied to study the progressive collapse of simple structures with cohesive post peak behavior: elementary frames and fiber bundles. The analyses put into evidence some interesting scale effects induced by ductility and dynamics. In particular, a power law describing the decrease of the reduced dynamic critical load with the structural scale and a second order ductile-brittle transition, have been found. These results can be usefully applied in robustness oriented structural design. Moreover, the study of the influence of the extent of the starting damage in structures with different sizes suggests that, the elementary cells of complex framed structures can play a role similar to the microstructure of materials. In conclusion, a new approach to the problem of collapse into complex structures by means of the tools of Fracture Mechanics is proposed.     

Smart structures — vibration of composites with piezoelectric materials

A manufacturing technique is developed for embedding piezoelectric material in composite laminates while maintaining the structure strength and piezoelectric effectiveness. An ultrasonic C-scan test is applied to screen out the specimen with possible delamination along the interface of the piezoelectric material and glass fiber layer. It is shown that the problem of electrical insulation and piezoelectric material cracking can be prevented. In addition, tensile and static tests are conducted to validate the manufacturing technique. An analytical model is also presented to predict the natural frequencies and mode shapes of a composite structure with embedded piezoelectric materials, and the predictions are verified by modal testing.     

Simple lattice model for numerical simulation of fracture of concrete materials and structures

In this paper a numerical model is presented for simulating fracture in heterogeneous materials such as concrete and rock. The typical failure mechanism, crack face bridging, found in concrete and other materials is simulated by use of a lattice model. The model can be used at a small scale, where the particles in the grain structure are generated and aggregate, matrix and bond properties are assigned to the lattice elements. Simulations at this scale are useful for studying the influence of material composition. In addition the model seems a promising tool for simulating fracture in structures. In this case the microstructure of the material is not mimicked in detail but rather the lattice elements are given tensile strengths which are randomly chosen out of a certain distribution. Realistic crack patterns are found compared with experiments on laboratory-scale specimens. The present results indicate that fracture mechanisms are simulated realistically. This is very important because it simplifies the tuning of the model.     

Critical review on the assessment of fatigue and fracture in composite materials and structures

This paper provides a critical review of common approaches to assess fatigue and fracture in composite materials and structures. It explains how fatigue in composite materials and structures should be understood based on the observed fatigue phenomena and mechanisms. In relation to these phenomena, the selection of proper similitude conditions for predictions is discussed and it is explained how mechanistic models can be developed that describe fatigue damage growth in its sequence from first initiation to final failure. An explanation is given for the fact that these mechanistic damage growth approaches have not yet led to suitable prediction models. In addition, this paper illustrates that in-depth understanding of individual damage mechanics may pave the road towards further material optimization with respect to fatigue and durability.     

Simulating the pervasive fracture of materials and structures using randomly close packed Voronoi tessellations

Under extreme loading conditions most often the extent of material and structural fracture is pervasive in the sense that a multitude of cracks are nucleating, propagating in arbitrary directions, coalescing, and branching. Pervasive fracture is a highly nonlinear process involving complex material constitutive behavior, material softening, localization, surface generation, and ubiquitous contact. A pure Lagrangian computational method based on randomly close packed Voronoi tessellations is proposed as a rational and robust approach for simulating the pervasive fracture of materials and structures. Each Voronoi cell is formulated as a finite element using the Reproducing Kernel Method. Fracture surfaces are allowed to nucleate only at the intercell faces, and cohesive tractions are dynamically inserted. The randomly seeded Voronoi cells provide a regularized random network for representing fracture surfaces. Example problems are used to demonstrate the proposed numerical method. The primary numerical challenge for this class of problems is the demonstration of model objectivity and, in particular, the identification and demonstration of a measure of convergence for engineering quantities of interest. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.     

The features of fracture of heterogeneous materials and frame structures. Potentialities of MCA design

A new promising numerical method named movable cellular automata (MCA) is described. Because this approach is based on the discrete concept, in contradistinction to FEM-based software, the software based on the MCA concept has a few clear advantages. The main one is connected with modeling of real fracture process. The MCA method has been successfully used for modeling dynamic loading of heterogeneous materials and structures. The results of simulations agree closely with the experimental data. The results show that the MCA approach could be really useful to solve a lot of civil engineering problems from materials to constructions. Special software has been developed on the basis of this method. Due to its potentially unique abilities, the MCA method could be considered as a breakthrough in numerical techniques and a new tool of engineering mechanics.     

Research and application problems in fracture of materials and structures in the united states air force

This paper reviews current and projected Air Force R&D activities in the area of fracture mechanics for materials and structures and cites systems problems which have been essentially responsible for the recent flurry of activity in this area. The specific technology areas reviewed in this paper range from basic material property data generation to the improvement of fracture analysis procedures which account for the complex structural geometries, chemical and stress environment. The emphasis, direction and guidance for selecting specific topics of research has been assisted by recent past failure incidents, and knowledge of the technological deficiencies gained from current attempts to formulate and apply requirements to design, analyze and test for safe crack growth and residual strength.While the current R&D efforts are directed toward improving the material data base and analytical capability needed to effectively implement fracture control procedures, unknowns and areas of concern still exist. Examples of these include the accurate assessment of cost weight and performance impacts caused by the imposition of fracture requirements, the need for, and magnitude of safety or confidence factors for residual strength or safe crack growth analyses and the scope and magnitude of test and/or analysis required to prove conformance with specifications.     

Smart magnetic structures for MEMS

By exploiting the special properties of magnetic materials, magnetic microelectromechanical system (MEMS) technology offers many challenging opportunities for useful device development in the future. This article discusses some of the magnetic materials used in MEMS devices and methods of fabricating them. Some key design issues are addressed, and applications of these technologies to electromagnetic devices developed at RMIT and to thermally controlled magnetic devices are examined.     

A computational method for quasi-static fracture

A direct method for solving quasi-static, mixed-mode fracture problems is presented. The element-free Galerkin method is used in order to allow for crack growth without remeshing. An expression for the normalized, critical traction is derived in terms of the fracture resistance (R-curve) and a crack-dependent function. Sample problems demonstrate the capability of this method to accurately compute the post-peak equilibrium paths for structures with growing cracks.     

Integrated structures and materials design

This paper introduces the concept of␣Integrated Structures and Materials Design (ISMD). ISMD combines materials engineering and structural engineering for the purpose of more effectively achieving targeted structural performance, by adopting material composite properties as the shared link. An application example, design of a bridge deck link-slab, is used to illustrate the essential elements of ISMD. It is shown that the composite hardened properties—tensile strain capacity, microcrack width, and Young’s Modulus, as well as composite self-consolidating fresh properties, are amongst the most important composite parameters that govern the targeted structural performance of safety, durability and ease of design and implementation. These are also properties that can be controlled in an Engineered Cementitious Composite—an ultra ductile concrete, by tailoring the ingredients for desired fiber, matrix and interface micromechanical parameters. Broad implications of ISMD on educational approach, research collaboration, and next generation infrastructure development, are briefly discussed.     

Design of fracture resistant structures

Damage accumulation and fracture of structures represented an actual mechanical problem that is needed in development of theoretical and computational methods. One of the most important problems in this direction is the problem of optimal structural design, when in optimization process it is necessary to take into account initial structural defects, arising cracks and damage accumulation. This problem is characterized by incomplete information concerning initial cracks size, cracks position and its orientation. In this context it is necessary to develop the statements of the optimization problems based on guaranteed (mini–max), probabilistic and mixed probabilistic-guaranteed approaches for considered problems with incomplete information. For many realistic it is reasonable to use variants of the mini–max optimization, named as optimization for “the worst case scenario” (see Banichuk et al. Mech Struct Mach 26(1):149–188, 1997; Mech Based Des Struct Mach 31(4):459–474, 2003; Meccanica 40:135–145, 2005a; Mech Based Des Struct Mach 33(2): 253–269, 2005b). Considered problem consist in finding the shape and thickness distribution of axisymmetric quasi-brittle shells with arising cracks in such a way, that the cost functional (volume or weight of the shell material) reaches the minimum, while satisfying some constraints on the stress intensity factor and geometrical constraints. In the case of cycling loadings we consider the number of loading cycles before fracture as the main constraint. Some examples of problems formulations, analytical and numerical solutions based on genetic algorithm are presented.     

Nonlinear fracture of cohesive materials

The cohesive crack is a useful model for describing a wide range of physical situations from polymers and ceramics to fiber and particle composite materials. When the cohesive zone length is of the order of the specimen size, the influence method—based on finite elements—may be used to solve the fracture problem. Here a brief outline of an enhanced algorithm for this method is given. For very large specimen sizes, an asymptotic analysis developed by the authors allows an accurate treatment of the cohesive zone and provides a powerful framework for theoretical developments. Some recent results for the zeroth order and first order asymptotic approaches are discussed, particularly the effective crack concept and the maximum load size effect. These methods are used to analyze the effect of the size and of the shape of the softening curve on the value at the peak load of several variables for three point bent notched beams. The results show, among other things, that for intermediate and very large sizes the size effect curves depend strongly on the shape of the softening curve, and that only the simultaneous use of asymptotic and influence methods may give an adequate estimate of the size effect in the intermediate range.