Gustavo V. Guinea

The cohesive zone model: advantages, limitations and challenges

Abstract This paper reviews the cohesive process zone model, a general model which can deal with the nonlinear zone ahead of the crack tip––due to plasticity or microcracking––present in many materials. Furthermore, the cohesive zone model is able to adequately predict the behaviour of uncracked structures, including those with blunt notches, and not only the response of bodies with cracks––a usual drawback of most fracture models. The cohesive zone model, originally applied to concrete and cementitious composites, can be used with success for other materials. More powerful computer programs and better knowledge of material properties may widen its potential field of application. In this paper, the cohesive zone model is shown to provide good predictions for concrete and for different notched samples of a glassy polymer (PMMA) and some steels. The paper is structured in two main sections: First, the cohesive model is reviewed and emphasis is on determination of the softening function, an essential ingredient of the cohesive model, by inverse analysis procedures. The second section is devoted to some examples of the predictive capability of the cohesive zone model when applied to different materials; concrete, PMMA and steel.

Stress Intensity factor, compliance and CMOD for a General Three-Point-Bend Beam

New simple and general expressions for the stress intensity factor, compliance and crack mouth opening displacement for three-point bend specimens are computed. Inverse functions giving the crack length as a function of load-point displacement or crack mouth opening displacement are also included. The expressions are valid for any crack length and for any span-to-depth ratio larger than 2.5. The expressions are checked by comparing them to direct finite element computations and to available expressions by other authors. The accuracy of the new expression is equal to or better than available formulas when compared with finite element computations, and its range of applicability is much larger. Moreover, all the new expressions exhibit the correct asymptotic behaviour for very shallow and very deep cracks.

Mixed Mode Fracture of Concrete under Proportional and Nonproportional Loading

A novel testing procedure for mixed mode crack propagation in concrete is presented: four point bend of notched beams under the action of two independent force actuators. In contrast to classical procedures, this method allows nonproportional loading and crack trajectory modifications by changing the action of one actuator. Different experimental crack trajectories, under mixed mode and nonproportional loading, are presented together with the corresponding curves of load-CMOD and load-displacement. The tests were performed for three homotetic specimen sizes and two mixed mode loading conditions. The results are useful for checking the accuracy of mixed mode fracture analytical and numerical models. The models should predict the crack trajectory and a complete group of experimental records of load and displacements on several control points in the specimen.

Generalizations and specializations of cohesive crack models

Abstract This paper presents an overview of the cohesive crack model, one of the basic models used so far to describe the fracture of concrete and other quasibrittle materials. Recent developments and needs for further research are discussed. The displayed evidence and the discussion are based on considering the cohesive crack model as a constitutive assumption rather than an ad hoc model for the behaviour ahead of a preexisting crack. Topics addressed are fracture of unnotched specimens, mixed mode fracture, diffuse cracking, anomalous stress–strain curves, size effect and asymptotic analysis, and strength of structural elements with notches.

Controlled supercontraction tailors the tensile behaviour of spider silk

The interest in the production of fibres that mimic the behaviour of natural silks has been boosted by the first successful attempts of spinning fibres based on spider drag line silk proteins. However, both the processing of biomimetic silk fibres and the basic studies on silk are hampered by the large variability of the fibre properties. Here we show that the tensile behaviour of spider silk can be predictably and reproducibly tailored by controlling the supercontraction effect, a large shrinkage of the longitudinal dimension of the fibre if unrestrained by its ends and immersed in water. This procedure allows to reproduce the tensile behaviour of natural drag line fibres and offers the possibility of obtaining silk fibres with predictable tailored properties in large quantities for experimental use. These results can be interpreted in the frame of the molecular model of drag line silk, as the result of re-orientation of the protein chains, leading to an explanation for the observed variability of natural drag line fibres.

KI evaluation by the displacement extrapolation technique

Abstract This paper shows the influence of element size, element shape, and mesh arrangement on numerical values of K I obtained by the displacement method, and gives some guidelines to obtain K I values as good as the most accurate energy based estimations, typically within a few percent difference of the exact value. Three different displacement-based extrapolation techniques are analyzed. The influence of stress state is also shown.

Stretching of supercontracted fibers: a link between spinning and the variability of spider silk.

SUMMARY The spinning of spider silk requires a combination of aqueous environment and stretching, and the aim of this work was to explore the role of stretching silk fibers in an aqueous environment and its effect on the tensile properties of spider silk. In particular, the sensitivity of the spider silk tensile behaviour to wet-stretching could be relevant in the search for a relationship between processing and the variability of the tensile properties. Based on this idea and working with MAS silk from Argiope trifasciata orb-web building spiders, we developed a novel procedure that permits modification of the tensile properties of spider silk: silk fibers were allowed to supercontract and subsequently stretched in water. The ratio between the length after stretching and the initial supercontracted length was used to control the process. Tensile tests performed in air, after drying, demonstrated that this simple procedure allows to predictable reproduction of the stress-strain curves of either naturally spun or forcibly silked fibers. These results suggest that the supercontracted state has a critical biological function during the spinning process of spider silk.

Review of the splitting-test standards from a fracture mechanics point of view

Abstract This paper analyzes by means of fracture mechanics the current splitting-test standards for concrete. The cohesive crack model, which has shown its utility in modeling the fracture of concrete and other cementitious materials, has been used to assess the effect of the specimen size, the specimen shape and the width of the load-bearing strips on the conventional splitting tensile strength, f st . The results show that, within the ranges recommended in the standards, the values of the splitting tensile strength can differ by up to 40%, and, consequently, f st can hardly be assumed to be a material property. Empirical formulae for concretes with different compressive strength (10–80 MPa) and maximum aggregate size (8–32 mm) have been used to show that f st is nearly specimen independent only for certain compositions, such as high-strength concretes, or when the aggregate size is under 16 mm. New closed-form expressions for f st are given in this paper to incorporate the effect of material properties, and some recommendations are drawn to minimize the influence of the width of the load-bearing strips.

Decellularization of pericardial tissue and its impact on tensile viscoelasticity and glycosaminoglycan content

Bovine pericardium is a collagenous tissue commonly used as a natural biomaterial in the fabrication of cardiovascular devices. For tissue engineering purposes, this xenogeneic biomaterial must be decellularized to remove cellular antigens. With this in mind, three decellularization protocols were compared in terms of their effectiveness to extract cellular materials, their effect on glycosaminoglycan (GAG) content and, finally, their effect on tensile biomechanical behavior. The tissue decellularization was achieved by treatment with t-octyl phenoxy polyethoxy ethanol (Triton X-100), tridecyl polyethoxy ethanol (ATE) and alkaline treatment and subsequent treatment with nucleases (DNase/RNase). The quantified residual DNA content (3.0±0.4%, 4.4±0.6% and 5.6±0.7% for Triton X-100, ATE and alkaline treatment, respectively) and the absence of nuclear structures (hematoxylin and eosin staining) were indicators of effective cell removal. In the same way, it was found that the native tissue GAG content decreased to 61.6±0.6%, 62.7±1.1% and 88.6±0.2% for Triton X-100, ATE and alkaline treatment, respectively. In addition, an alteration in the tissue stress relaxation characteristics was observed after alkaline treatment. We can conclude that the three decellularization agents preserved the collagen structural network, anisotropy and the tensile modulus, tensile strength and maximum strain at failure of native tissue.

Self-tightening of spider silk fibers induced by moisture

Abstract Spider dragline silk has a unique combination of desirable mechanical properties—low density, high tensile strength and large elongation until breaking—that makes it attractive from an engineering perspective [Nature 410 (2001) 541]. Nevertheless, this outstanding performance is threatened by the way mechanical properties are affected by a wet environment, particularly if the stress of these fibers can relax when exposed to moisture. Tests on spider dragline silk ( Argiope trifasciata ) performed by the authors have shown that when the fiber is clamped and exposed to a wet enough environment non-vanishing supercontraction forces develop. When the moisture is removed the residual stresses increase, and this effect has proven long lasting, as the fiber remains stressed for hours. In addition, the tensile properties of the fiber remain unaffected by the residual stresses build up after removing the moisture or after a wetting and drying cycle. These tests give support to the thesis that supercontraction helps to keep the spider webs tight and opens new applications for synthetic analogs.