Manuel Elices

Mechanical properties of single-brin silkworm silk

Mechanical tests were performed on single brins of Bombyx mori silkworm silk, to obtain values of elastic modulus (E), yield strength, tensile breaking strength, and shear modulus (G). Specimen cross-sectional areas, needed to convert tensile loads into stresses, were derived from diameter measurements performed by scanning electron microscopy. Results are compared with existing literature values for partially degummed silkworm baves. The tensile modulus (16 ± 1 GPa) and yield strength (230 ± 10 MPa) of B. mori brin are significantly higher than the literature values reported for bave. The difference is attributed principally to the presence of sericin in bave, contributing to sample cross-section but adding little to the fibers ability to resist tensile deformation. The two brins in bave are found to contribute equally and independently to the tensile load-bearing ability of the material. Measurements performed with a torsional pendulum can be combined with tensile load-extension data to obtain a value of E/ that is not sensitive to sample cross-sectional dimensions or, therefore, to the presence of sericin. The value of E measured for brin can be used together with this result to obtain G = 3.0 ± 0.8 GPa and E/G = 5.3 ± 0.3 for brin. The latter value indicates a mechanical, and therefore microstructural, anisotropy comparable to that of nylon.

Particulate fracture during deformation

The mechanisms of deformation and failure in a 2618 Al alloy reinforced with 15 vol pct SiC particilates were studied and compared with those of the unreinforced alloy, processed by spray forming as well. Tensile and fracture toughness tests were carried out on naturally aged and peak-aged specimens. The broken specimens were sliced through the middle, and the geometric features of fractured and intact particulates were measured. The experimental observations led to the conclusion that failure took place by the progressive fracture of the particulates until a critical volume fraction was reached. An influence of the particulate size and aspect ratio on the probability of fracture was found, the large and elongated particulates being more prone to fail, and the fracture stress in the particulates seemed to obey the Weibull statistics. The dif- ferences in ductility found between the naturally aged and peak-aged composites were explained in terms of the number of broken particulates as a function of the applied strain. Numerical simulations of the deformation process indicated that the stresses acting on the particulates are higher in the peak-aged material, precipitating the specimen failure. Moreover, the compressive residual stresses induced on the SiC during water quenching delayed the onset of particulate breakage in the naturally aged material.

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.

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.

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.

Fracture of components with V-shaped notches

An experimental and numerical study was performed to ascertain the influence of V-notches on the strength of different brittle materials. Critical loads were computed using the cohesive process zone model that allows extension of classical methods, based on linear elastic fracture mechanics, to rounded V-notches and contained plasticity. A large experimental programme, with more than one hundred tests, was performed with PMMA samples to support the numerical predictions. All the experimental results are reported. In addition, experimental results of notched steel samples, performed by Strandberg, [Engng. Fract. Mech. 69 (2002) 403], and notched samples made with expanded PVC foams, performed by Grenestedt, Hallstrom and Kuttenkeuler [Engng. Fract. Mech. 54 (1996) 445], are in very good agreement with the numerical computations, adding further support to the cohesive process zone model.

Mechanical properties of silkworm silk in liquid media

Abstract Tensile tests have been performed on silkworm silk fibres submerged in liquid environments (water, acetone, ethanol and isopropanol). Liquid media were initially chosen in order to weaken non-covalent interactions specifically. However, only immersion in water leads to a decrease in the mechanical properties of silk, indicating the weakening of hydrogen bonds. Immersion in acetone, ethanol and isopropanol leads to an increase in the stiffness of the fibre. In addition, all three organic solvents produce similar force–displacement curves, which can be explained by the desiccating effect that these solvents exert on silk. These results indicate that water disrupts hydrogen bonds initially present in the amorphous phase, while the other solvents eliminate water and contribute to the formation of new hydrogen bonds in the amorphous phase of silk. This interpretation was developed through the shear lag model of the elastic modulus ( E ) of silk, and a good agreement has been found between the model and the experimental values of E .

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.

Fractographic analysis of silkworm and spider silk

An investigation is presented of the fracture surfaces of three different silks produced by two silkworms (Attacus atlas (A. atlas) and Bombyx mori (B. mori)) and one spider (Argiope trifasciata (A. trifasciata)). Tensile tests up to fiber failure were performed at a strain rate of 0.0002 s � 1 , and the fracture surfaces of the broken fibers were analyzed through a scanning electron microscope. The nominal relative humidity during the tests was 60% and the average temperature was 20 C. A. atlas silk was formed of bunches of microfibrils of � 1 lm in diameter embedded in a soft matrix, which were pulled out from the matrix during fracture. B. mori fibers were made up of two brins of irregular shape embedded in a proteinaceous coating. Failure occurred by fracture of the brins, whose fracture surface presented a fine globular structure corresponding to the ends of the nanofibrils of 1–2 lm in length and � 100 nm in diameter, which form the B. mori silk brins according to the analysis of the brins by atomic force microscopy. A. trifasciata fibers were circular and exhibited a defined core-skin structure. The skin fracture surface was featureless while the core showed a globular structure similar to that of B. mori although slightly shallower. The fractographic observations were discussed in the light of current knowledge of the microstructure of each fiber and the corresponding mechanical properties. 2002 Elsevier Science Ltd. All rights reserved.