Octavian Pop

Characterization of mixed-mode fracture based on a complementary analysis by means of full-field optical and finite element approaches

This paper focuses on the characterization of mixed-mode fracture parameters through use of two formalisms based on Crack Relative Displacement Factors and Stress Intensity Factors, respectively. The evaluation of Crack Relative Displacement Factors is based on a kinematic approach that integrates the experimental displacement field measured by a digital image correlation method. In parallel with this step, the stress intensity factor is calculated from a finite element analysis. The coupling between these two approaches allows for the identification of fracture parameters in terms of an energy release rate without any prior knowledge of material elastic properties. Depending on the mixed-mode configuration, the proportion of the energy release rate corresponding to opening and shear modes can be calculated. Moreover, the proposed formalism allows determining, in addition to fracture parameters, the local elastic properties in terms of reduced elastic compliance directly from the test sample. Experimental protocols are carried out using a Single-Edge notched specimen made from a rigid Polyvinyl Chloride polymer loaded at various mixed-mode ratio values.

J-integral evaluation in cracked wood specimen using the mark tracking method

In this paper, an experimental analysis of the fracture parameters via the invariant J-integral for a cracked specimen made of wood is presented. The experimental test is realized using a sample made of Douglas fir loaded in opening mode. The sample geometry is a mix between the single edge notch and wedge splitting specimens, and the crack advances in the radial-longitudinal system, parallel to the wood rings. By using the optical mark tracking method, the displacement field evolution close to the crack tip is recorded during the test. The stress and strain fields are calculated using a finite element model generated from the experimental displacement fields. Further, the energy release rate is evaluated for different circular paths or crowns defined around the crack tip and for different loading values.

Acoustic emission technique for fracture analysis in wood materials

Understanding the failure mechanisms of construction materials, as well as their damage evolution, constitute two key factors to improving structural design tools. Depending on the failure modes to be highlighted and studied, several test methods and analysis tools have been developed. One such development, the acoustic emission technique (AET), is an experimental tool appropriate for characterizing material behavior by means of monitoring the fracture process. Despite the widespread uses of acoustic emission techniques to characterize and monitor the damage evolution of composite materials, only a few research studies have focused on using AET to characterize the mechanical behavior of wood materials. In the present work, the failure process in Douglas fir under monotonic loading is studied by comparing three experimental methods: force-displacement curve analysis, acoustic emission measurements, and digital image acquisition. First of all, results show good correlation and complementarities among the methods employed. Second, analyzing acoustic emission signals by considering the event number and the cumulative events yields interesting information on crack initiation and growth without the material. Moreover, an additional analysis of acoustic emission data (involving the determination of source locations and the study of amplitude distributions) opens the possibility to characterize the fracture process zone which is a key damage mechanism in wood materials.

Refinement of digital image correlation technique to investigate the fracture behaviour of refractory materials

Refractory materials exhibit a heterogeneous microstructure consisting in coarse aggregates surrounded by fine grains that form an aggregate/matrix composite. This heterogeneous microstructure often leads to a complex mechanical behaviour during loading. This paper is devoted to the study, thanks to an optical method, Digital Image Correlation (DIC), of the fracture behaviour of two industrial refractory materials in relation with their microstructure resulting from both the chosen constituents and the sintering process. The aim is here, specifically, to highlight and to characterize the evolution of kinematic fields (displacement and strain) observed at the surface of sample during a wedge splitting test typically used to quantify the work of fracture. DIC is indeed a helpful and effective tool, in the topic of experimental mechanics, for the measurement of deformation in a planar sample surface. This non-contact optical method directly provides full-field displacements by comparing the digital images of the sample surface obtained before and during loading. In the present study, DIC has been improved to take into account the occurrence of cracks and performed so as to better identify the early stage of the cracking behaviour. The material transformation, usually assumed homogeneous inside each DIC subset, is thus more complex and a discontinuity of displacement should be taken into account. Then each subset which crosses a crack can be cut in two parts with different kinematics. By this way, it is possible to automatically find the fracture paths and follow the crack geometries (length, opening).

Spreading of transverse compressive stresses in glued laminated timber

This paper presents a new approach for understanding localized compressive behavior perpendicular to the grain based on beams subjected to three-point bending. It was intended to take into account the coupling between compressive stresses perpendicular to the grain due to support reactions and shear stresses. To validate this, model, results of compression tests have been examined. The digital image correlation (DIC) method has been applied in order to analyze the deformation fields in support areas. The tests allow confirming the presence of spreading effect in the vicinity of support areas according to the DIC method. An analytical formulation has therefore been proposed to quantify distribution versus increasing compressive strength perpendicular to the grain, including shear effects. A scale effect and support conditions have also been highlighted using three different geometries and two types of tests.

Detection of cracks in refractory materials by an enhanced digital image correlation technique

This paper is devoted to the study of the fracture behaviour of two industrial refractory materials thanks to the development of a new technique of digital image correlation (DIC). DIC, already known as a helpful and effective tool for the measurement of displacement and deformation fields in materials, has been adapted to take into account displacement discontinuities as cracks. The material transformation, usually assumed homogeneous inside each DIC subset, is thus more complex, while each subset can be cut in two parts with different kinematics. By this way, it is possible to automatically find the fracture paths and follow the crack geometries (length, opening) during the loading with a higher spatial resolution than the one obtained by standard DIC. After having presented the principle of the new technique, its metrological performances are assessed from synthetic images and the choice of crack detection criterion is discussed. The capacity of this new technique is shown through a comparative study with standard DIC. Its application is led on magnesia-spinel refractory materials, specifically to highlight and to characterize the evolution of kinematic fields (displacement and strain) observed at the surface of sample during a wedge splitting test typically used to quantify the work of fracture. We show that refractories with aggregates of iron aluminate spinel present a fracture mechanism with crack branching and can dissipate more energy thanks to a longer crack network.

Determination of Asphalt Pavements Fracture Parameters Using Optical Approaches

This paper deals with the characterization of mode I fracture parameters using a kinematic approach integrating the experimental displacement measured by optical techniques. Tests are carried out using a wedge splitting sample made in asphalt concrete. The WST-cube specimen is a bi-layered asphalt concrete AC12 with Carbon Fiber grid at the interface. The tested material was provided in the RILEM-SIB. During the test the displacement field evolution close to the crack tip was recorded by using the optical techniques. An adjustment procedure was also used to improve the displacement fields and avoid experimental noise. Based on the experimental optical measurements, the energy release rate was performed by means the Crack Relative Displacement Factor and Stress In-tensity Factor. The Mark Tracking method is employed in order to measure the Crack Opening Displacement. This approach allows to consider the assessment of fracture parameters for the real structures.

Characterization of Wood Fracture Using Optical Full Field Methods

In this paper a new formalism based on the complementarity between the optical full field techniques and integral invariant Mtheta is proposed in order to evaluate the fracture parameters in cracked specimen made of wood, under mixed mode loadings. The coupling between the experimental and numerical approaches allows identifying the fracture parameters in terms of energy release rate without any the material elastic properties such as the elastic modulus and the Poissons ratio. The proposed formalism allows also determining, in addition with the fracture parameters, the local elastic properties in terms of reduced elastic compliance. The fracture mixed mode tests are realized using a Single Edge Notch sample made in Douglas with the Arcan fixtures and dried to 11% moisture content and the crack is cutting in Radial-Longitudinal system.