Stéphane Morel

Influence of the specimen geometry on R-curve behavior and roughening of fracture surfaces

Recently the R-curve behavior observed on quasi-brittle materials was proposed to be related to the roughness development of fracture surfaces. However, many experiments have shown that the R-curve behavior is not a material property but depends on the specimen shape. It is also expected that the roughening of fracture surfaces is influenced by the specimen geometry and so R-curve behaviors related to this roughening. From mode I fracture tests on wood specimens of three different shapes, R-curves are estimated and the morphology of the crack surfaces are analysed. We show that the scaling exponents of the anomalous scaling law used to describe accurately the roughness development of crack surfaces, are not influenced by the specimen geometry and appear as material dependent parameters. Nevertheless, the fracture surfaces exhibit a roughness growth region that reduces with the average stiffness of the specimens. Accordingly, the maximum roughness magnitude of fracture surfaces is a function of the initial stiffness: the higher the stiffness, the smaller the maximum magnitude of the roughness. We show that the analytical R-curves deduced from the roughnening of fracture surfaces provide good fits of the experimental macroscopic R-curves. However, if the scaling exponents obtained from the R-curve fits are close to those measured from the microscopic roughness analysis, the description of the experimental R-curves requires a magnification of the real area of the main crack. This magnification can be explained by the weakness of the assumption of an energy only dissipated by a single crack and not by a process zone. Finally, we argue that an approach where the energy is dissipated by a set of microcracks that follow the same anomalous scaling, could fully explain the experimental R-curves.

R-curve behavior and roughness development of fracture surfaces

We investigate the idea that the fractal geometry of fracture surfaces in quasibrittle materials such as concrete, rock, wood and various composites can be linked to the toughening mechanisms. Recently, the complete scaling analysis of fracture surfaces in quasibrittle materials has shown the anisotropy of the crack developments in longitudinal and transverse directions. The anomalous scaling law needed to describe accurately these particular crack developments emphasizes the insufficiency of the fractal dimension, usually used to characterize the morphology of fracture surfaces. It is shown that a fracture surface initiating from a straight notch, exhibits a first region where the amplitude of roughness increases as a function of the distance to the notch, and a second one where the roughness saturates at a value depending on the specimen size. Such a morphology is shown to be related to an R-curve behavior in the zone where the roughness develops. The post R-curve regime, associated with the saturation of the roughness, is characterized by a propagation at constant fracture resistance. Moreover, we show that the main consequence of this connection between anomalous roughening at the microscale and fracture characteristics at the macroscale is a material-dependent scaling law relative to the critical energy release rate. These results are confirmed by fracture experiments in Wood (Spruce and Pine).

Anomalous roughening of wood fractured surfaces

Scaling properties of wood fractured surfaces are obtained from samples of three different sizes. Two different woods are studied: Norway spruce and Maritime pine. Fracture surfaces are shown to display an anomalous dynamic scaling of the crack roughness. This anomalous scaling behavior involves the existence of two different and independent roughness exponents. We determine the local roughness exponents z loc to be 0.87 for spruce and 0.88 for pine. These results are consistent with the conjecture of a universal local roughness exponent. The global roughness exponent is different for both woods, z51.60 for spruce and z51.35 for pine. We argue that the global roughness exponent z is a good index for material characterization. @S1063-651X~98!00512-1#

Scaling of crack surfaces and implications for fracture mechanics

The scaling laws describing the roughness development of crack surfaces are incorporated into the Griffith criterion. We show that, in the case of a Family-Vicsek scaling, the energy balance leads to a purely elastic brittle behavior. On the contrary, it appears that an anomalous scaling reflects an R-curve behavior associated with a size effect of the critical resistance to crack growth in agreement with the fracture process of heterogeneous brittle materials exhibiting a microcracking damage.

Size effect in fracture: Roughening of crack surfaces and asymptotic analysis

Recently the scaling laws describing the roughness development of fracture surfaces were proposed to be related to the macroscopic elastic energy released during crack propagation. On this basis, an energy-based asymptotic analysis allows us to extend the link to the nominal strength of structures. We show that a Family-Vicsek scaling leads to the classical size effect of linear elastic fracture mechanics. On the contrary, in the case of an anomalous scaling, there is a smooth transition from the case of no size effect, for small structure sizes. to a power-law size effect which appears weaker than the linear elastic fracture mechanics one, in the case of large sizes. This prediction is confirmed by fracture experiments on wood.

Mixed mode fracture of glued-in rods in timber structures

Glued-in-rods in timber structures lead to overcome the use of traditional bolted connections, preserve a large part of the original timber and offer aesthetic benefits. Several research programs were achieved to improve the mechanical knowledge of this technique, exhibiting experimentally the influence of materials and the effect of the geometric configuration. From these experimental results, some design rules predicting the axial strength are available, but a common criterion is still lacking. This paper relates to experimental investigations and finite element computations on glued-in rods, with the aim of providing a better knowledge about their mechanical behavior until failure. An experimental campaign is carried out on single glued-in rod connections. The finite element modeling reproduces the experimental configuration: it exhibits significant normal stress (to the interface) at the onset of the bonding, in comparison with shear stress. Within the framework of equivalent linear elastic fracture mechanics, resistance curves in mode I and mode II are established for each specimen. Finally, a mixed mode fracture criterion (I/II) is used to describe the fracture process zone development at the wood-adhesive interface (failure zone). An analytical formulation is then proposed allowing the evaluation of peak load of each specimen, which highlights a new approach for the design of such connections.

Laboratory evaluation of pavement macrotexture durability

ABSTRACT The aim of this paper is to present a study concerning the evaluation of the durability of the asphalt concrete macrotexture in laboratory. An experimental procedure was developed. This experimental procedure consists in a mechanical test which aims to alter the upper face macrotexture of a cylindrical specimen through the application of a sinusoidal repeated load on its surface. In order to quantify the macrotexture evolution produced we use a laser texture evaluation system. This system allows the scanning of surface specimen before and after mechanical solicitation. In this paper, we first present the materials and the experimental procedure. Then, we expose the various surface indicators analysis and finally we discuss the experimental results.

Wood fracture characterization under mode I loading using the three-point-bending test. Experimental investigation of Picea abies L.

Mode I fracture characterization was performed in wood using the single-edge-notched beam loaded in three-point-bending (SEN-TPB). A data reduction scheme based on equivalent linear elastic fracture mechanics concept was used to evaluate the Resistance-curve instead of classical methods. The method is founded on assessment of an equivalent crack from specimen compliance using beam theory and the existence of a stress relief region in the crack vicinity. Crack length monitoring is unnecessary during the loading process, providing a complete Resistance-curve which is essential for a clear identification of the fracture energy. Experimental results were obtained from fracture tests involving geometrically similar SEN-TPB specimens of different sizes. It was observed that the smallest tested specimen is inadequate to estimate the fracture energy due to fracture process zone confinement. Contrarily, the other two are suitable for such purpose. An inverse method was used to determine a bilinear cohesive law representative of wood material fracture. It was concluded that a unique cohesive law is able to mimic the fracture behaviour of considered specimen sizes.

An Original Evaluation of the Wearing Course Macrotexture Evolution using the Abbot Curve

ABSTRACT The wearing course is the top surface layer of the road. The road surface has multiple functions the main one being traffic safety, which is conditioned by skid resistance. This skid resistance is conditioned by pavement texture. This paper presents an experimental procedure allowing to evaluate surface macrotexture evolution of an asphalt concrete specimen by laboratory testing. Specimen surface changes through the application of a sinusoidal repeated load at high temperature. The studied surface is scanned before and after mechanical testing using a laser profiler. Texture maps are analysed using Abbott curve which is also called bearing ratio curve. The 3D analysis enables to quantify macrotexture evolution under repeated traffic conditions and to analyze the influence of both temperature and binder.

Wood heterogeneities and failure load of timber structural elements: a statistical approach

Abstract In this work, the effect of spatial location and geometrical characteristics of wood defects (only knots) on the failure load of timber structural elements is addressed. For a large set of timber structural elements (188 Maritime pine full size beams), spatial location and geometrical characteristics of knots were estimated from the analysis of external images of each element, and then, modulus of elasticity (MOE) and failure load obtained from a four-point bending test were experimentally estimated. From this large database, correlations between the failure load, MOE and geometrical characteristics of knots were studied on the basis of a polynomial regression and a nonlinear regression based on neural network. It was shown that accounting for simple geometrical characteristics of knots, such as the area, the circularity and the location, in addition to the MOE, significantly improves the prediction of the failure load of structural elements.