@article {150, title = {Nano-chemo-mechanical signature of conventional oil-well cement systems: Effects of elevated temperature and curing time}, journal = {Cement and Concrete Research}, volume = {67}, year = {2015}, month = {Jan-2015}, pages = {103 - 121}, abstract = {

With ever more challenging (T,p) environments for cementing applications in oil and gas wells, there is a need to identify the fundamental mechanisms of fracture resistant oil well cements. We report results from a multi-technique investigation of behavior and properties of API class G cement and silica-enriched cement systems subjected to hydrothermal curing from 30\ \°C to 200\ \°C; including electron probe microanalysis, X-ray diffraction, thermogravimetry analysis, electron microscopy, neutron scattering (SANS), and fracture scratch testing. The results provide a new insight into the link between system chemistry, micro-texture and micro-fracture toughness. We suggest that the strong correlation found between chemically modulated specific surface and fracture resistance can explain the drop in fracture properties of neat oil-well cements at elevated temperatures; the fracture property enhancement in silica-rich cement systems, between 110\° and 175\ \°C; and the drop in fracture properties of such systems through prolonged curing over 1\ year at 200\ \°C.

Fig.1. Particle size distribution of the silica flour (ground quartz) incorporated{\textellipsis}

Fig.2. Location of the {\textquotedblleft}chemical poles{\textquotedblright} of the cement clinker and cement hydration{\textellipsis}

Fig.3. Simplified schematic of: (a) the cement paste microstructure cured at room{\textellipsis}

Fig.4. A schematic of the effect of adding an extra dimension, Al, to the statistical{\textellipsis}

Fig.5. X-ray powder diffraction spectra with qualitative analysis of the major phases{\textellipsis}

Fig.6. Thermal gravimetric analysis (TGA) carried out on samples cured in the{\textellipsis}

}, issn = {00088846}, doi = {10.1016/j.cemconres.2014.08.008}, author = {Konrad J. Krakowiak and Jeffrey J. Thomas and Musso, Simone and James, Simon and Ange-Therese Akono and Franz-Josef Ulm} } @article {306, title = {Discussion: Strength-to-fracture scaling in scratching}, journal = {Engineering Fracture Mechanics}, volume = {119}, year = {2014}, month = {Mar-2014}, pages = {21 - 28}, abstract = {

In their paper, Lin and Zhou (2013) add a new dimension to the scratch test analysis, challenging the applicability of Linear Elastic Fracture Mechanics to scratch tests. The question raised is how to integrate the three-dimensionality of scratch tests into the energetic Size Effect Law (SEL) formulated by Ba{\v z}ant for quasi-brittle materials. We show that Lin and Zhou\’s analysis, although relevant is incomplete, as it neglects the blade width, which is critical for the fracture property assessment. In return, if the blade width is properly taken into account, the SEL here proposed is a formidable means to ascertain the fracture toughness from scratching.

Fig. 1. Description of macroscopic scratch tests

Fig. 2. Influence of cutter width w on the intrinsic specific energy ε and the{\textellipsis}

Fig. 3. Strength scaling of scratch tests for different geometries (straight vs

Fig. 4. (a) Presence of a horizontal propagating crack during scratch testing{\textellipsis}

Fig. 5. Application of Ba{\v z}ant{\textquoteright}s Size Effect Law to scratch tests for different{\textellipsis}

}, issn = {00137944}, doi = {10.1016/j.engfracmech.2014.02.025}, author = {Ange-Therese Akono and Franz-Josef Ulm and Zden{\v e}k P Ba{\v z}ant} } @article {311, title = {An improved technique for characterizing the fracture toughness via scratch test experiments}, journal = {Wear}, volume = {313}, year = {2014}, month = {May-15-2014}, pages = {117 - 124}, abstract = {

The scratch test consists in pushing a tool across the surface of a weaker material at a given penetration depth; and it has several applications in Science and Engineering including strength testing of rocks and ceramics, damage of polymers and metals and quality control of thin films and coatings. Despite numerous attempts in the scientific literature, the application of scratch tests to the characterization of fracture properties remains a challenge and a heavily controversial topic. Therefore, this investigation aims at articulating a rigorous theoretical and experimental framework in order to assess the fracture toughness at both the macroscopic and the microscopic length scales, using scratch tests. First, we apply optical microscopy and scanning electron microscopy to investigate the physical evidence of crack initiation, crack propagation and material removal mechanisms during scratch tests. Then, we employ Finite Element simulations of crack growth during macroscopic scratch tests to assess the influence of the blade back-rake angle, the friction coefficient between the blade and the material and the wear flat of the blade on the scratching forces, thus testing the robustness of our Linear Elastic Fracture Mechanics scratch model. Finally, at the microscopic scale, a meticulous scratch probe calibration procedure is described to improve the accuracy of the fracture properties determination by addressing important issues such as moisture content, specimen surface cleanliness and choice of reference material. In summary, we bring forward a robust, convenient and accurate method that is applied to polymers, ceramics and metals and can be further applied to the multi-scale study of fracture processes in complex and challenging materials such as gas shale, cement paste and cortical bone.

Fig.1. (color online) (a) Macroscopic scratch tests experiments on paraffin wax with a{\textellipsis}

Fig.2. (color online) Fracture processes in scratch tests at both the macroscopic and{\textellipsis}

Fig.3. (color online) (a) Numerical Scratch Fracture model: 0<=θ<=20{\textdegree} is the blade{\textellipsis}

Fig.4. (color online) (a) Ratio of the stress intensity factor in mode II (in-plane{\textellipsis}

Fig.5. (color online) Wear flat configuration: the interface between the blade and the{\textellipsis}

Fig.6. (color online) Example of damage to the scratch probe as a result of misuse or{\textellipsis}

}, issn = {00431648}, doi = {10.1016/j.wear.2014.02.015}, author = {Ange-Therese Akono and Franz-Josef Ulm} } @article {259, title = {Experimental determination of the fracture toughness via microscratch tests: Application to polymers, ceramics, and metals}, journal = {Journal of Materials Research}, volume = {27}, year = {2012}, month = {Jan-2012}, pages = {485 - 493}, abstract = {

This article presents a novel microscratch technique for the determination of the fracture toughness of materials from scratch data. While acoustic emission and optical imaging devices provide quantitative evidence of fracture processes during scratch tests, the technique proposed here provides a quantitative means to assess the fracture toughness from the recorded forces and depth of penetration. We apply the proposed method to a large range of materials, from soft (polymers) to hard (metal), spanning fracture toughness values over more than two orders of magnitude. The fracture toughness values so obtained are in excellent agreement with toughness values obtained for the same materials by conventional fracture tests. The fact that the proposed microscratch technique is highly reproducible, almost nondestructive, and requires only small material volumes makes this technique a powerful tool for the assessment of fracture properties for microscale materials science and engineering applications.

}, issn = {0884-2914}, doi = {10.1557/jmr.2011.402}, author = {Ange-Therese Akono and Randall, Nicholas X. and Franz-Josef Ulm} } @article {301, title = {Fracture scaling relations for scratch tests of axisymmetric shape}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {60}, year = {2012}, month = {Mar-2012}, pages = {379 - 390}, abstract = {

Scratch testing and scratch test analysis continues to gain momentum in Applied Mechanics, due to the possibility offered by this method to assess fracture properties at very fine scales. In this paper, we derive general scratch force scaling relations for axisymmetric scratch probes defined by single variable monomial functions. These relations are used to define fracture criteria with and without consideration of the development of shear stresses at the probe\–material interface. The approach is illustrated for common scratch probe geometries: conical probe, flat punch, and hemi-spherical probe. Application of the proposed method to micro-scratch tests on two materials (an aluminum alloy and a thermoplastic polymer) using a Rockwell probe (a conical probe ending in a hemi-spherical shape) illustrates the versatility of the approach: First, the scratch force-depth scaling relations provide a means to determine the degree of the homogeneous function characterizing the scratch probe. Second, the fracture criteria enable an experimental assessment of the fracture toughness. The good agreement between the fracture toughness determined by scratching and values reported in the open literature show the potential of the proposed method for determining fracture properties of materials at even smaller scales.

Fig.1. Axisymmetric scratch probe geometry

Fig.2. Scaling of the normalized fracture force and the perimeter function, β, of the{\textellipsis}

Fig.3. Forces in three scratch tests on aluminum alloy 2024 (AA 2024) obtained with a{\textellipsis}

Fig.4. Fracture toughness determination from the scratch response of AA 2024 with a{\textellipsis}

Fig.5. Response and analysis of three scratch tests on a thermoplastic polymer (Delrin{\textellipsis}

}, issn = {00225096}, doi = {10.1016/j.jmps.2011.12.009}, author = {Ange-Therese Akono and Franz-Josef Ulm} } @article {254, title = {Scratch test model for the determination of fracture toughness}, journal = {Engineering Fracture Mechanics}, volume = {78}, year = {2011}, month = {Jan-2011}, pages = {334 - 342}, abstract = {

We revisit the scratch test within the framework of linear elastic fracture mechanics. In the analysis, we employ an Airy stress function approach to determine stresses and displacement in the vicinity of the scratch-blade\–material interface, which serve as input for the evaluation of the energy release rate by means of the J-Integral. In contrast to previous models, the energy release rate thus found scales with the sum of the applied forces squared. This entails a linear relation between the applied forces and , where w is the scratch width and d the scratch depth. This analytical scaling is validated using experimental scratch data on cement paste and sandstone, which shows that the proposed approach provides a convenient way to determine the fracture toughness from scratch tests carried out with different scratch widths and depths

}, issn = {00137944}, doi = {10.1016/j.engfracmech.2010.09.017}, author = {Ange-Therese Akono and Franz-Josef Ulm} } @article {255, title = {Scratching as a Fracture Process: From Butter to Steel}, journal = {Physical Review Letters}, volume = {106}, year = {2011}, month = {May-20-2011}, pages = {Article Number: 204302}, abstract = {

We present results of a hybrid experimental and theoretical investigation of the fracture scaling in scratch tests and show that scratching is a fracture dominated process. Validated for paraffin wax, cement paste, Jurassic limestone and steel, we derive a model that provides a quantitative means to relate quantities measured in scratch tests to fracture properties of materials at multiple scales. The scalability of scratching for different probes and depths opens new venues towards miniaturization of our technique, to extract fracture properties of materials at even smaller length scales.

Figure

Figure

Figure

Figure

}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.106.204302}, author = {Ange-Therese Akono and Reis, P. M. and Franz-Josef Ulm} }