Fluid jet erosion as a non-linear fracture process: a discussion
Introduction
The erosion capability of high-speed fluid jets is widely used for many applications in modern industry. Due to several operational advantages, such as low cutting forces, selective removal capability, high efficiency, dust-free, heat-free and vibration-free performance, this technology increasingly replaces conventional mechanical techniques, such as sawing, grit-blasting, jackhammering and milling [1], [2], [3]. Table 1 lists typical applications of the fluid jet technology related to tension-softening materials.
The removal of a brittle, tension-softening material by a high-speed fluid jet is predominantly a fracture mechanical process. This is verified for cement-based composites and rocks [4], [5], [6], [7]. The jet, due to the establishment of a stagnation pressure, enters pre-existing flaws in the material, especially microcracks in the interfacial zones between matrix and inclusions. As the fracture process is introduced, the cracks grow until they intersect and form wear debris. It has been noticed for waterjet and cavitation erosion that the material resistance combines physical and structural features [8], [9], [10]. A physical parameter alone, such as compressive strength or Young’s modulus, cannot give information about the way the material fails and how the efficiency may be. The same is true if only a structural feature, such as inclusion size or distribution, is given. This problem is illustrated in Fig. 1. In this figure, the removal rate of concrete samples eroded by waterjets are plotted against the compressive strength; these results are taken from [9]. Note that the compressive strength does not allow an evaluation of the material removal rate. This situation is a serious drawback of the technology as far as the mass removal is concerned: practitioners often cannot design an appropriate performance as, in many projects, the compressive strength is the only available parameter.
Section snippets
Tension-softening materials
The materials discussed in this paper are characterized by a so-called ‘fracture process zone’ (referred to as FPZ in the paper). The FPZ is a zone characterized by progressive softening, for which the stress decreases at increasing deformation. It is surrounded by a non-softening non-linear zone characterized by hardening. Together, these two zones form a non-linear zone as illustrated in Fig. 2. Depending on the relative size of these zones and of the structure, three basic types of fracture
Energy-dissipative processes during fluid jet erosion
An SEM-study has been performed to verify energy-dissipative fracture processes during the fluid jet erosion of samples of hardened cement paste, mortar and concrete. The specimens have been subjected to high-speed waterjets at a velocity of 470 m/s and a diameter of 0.215 mm. The jets were generated in a commercial high-pressure water intensifier as described in [3]. The experimental conditions for these experiments are listed in Table 3, Table 4. The local exposure time was given through the
Characteristic length
For concrete materials the following empirical relationship between the characteristic length, compressive strength, and aggregate size is derived in [22]:Here, α is an empirical coefficient that takes into account the maximum aggregate size, and σC is the mean compressive strength in MPa. Eq. (4) is valid for the compressive strength range between σC=5 and 100 MPa. However, the relationship is best documented for a maximum aggregate size mm. The relationship between α and
Cratering of rocks by high-speed waterjets
A method for cratering rocks by waterjets is developed in [23]. The authors proposed a so-called ‘cratering toughness’ corresponding to the minimum value of jet variables to produce a crater. This toughness is given byIn that equation, ρ is the fluid density, vJ the jet velocity, dJ the jet diameter, and L the initial crack length. The higher the toughness the higher the material resistance against waterjet erosion. Hence, the cratering toughness should drop with an increase in the
Conclusion
The characteristic length of a tension-softening material is a good indication of its resistance against fluid jet impact. For concrete, this resistance parameter can easily and comfortably be applied under practice conditions to evaluate hydrodemolition processes. A preliminary comparison of other fluid jet applications taken from the reference literature suggests that the characteristic length can also be used in the field of rock cratering and drilling, and for the machining of refractory
Acknowledgements
The author is thankful to the German Research Association, Bonn, and to the Aachen University, Aachen, Germany, for financial and administrative support through a Habilitation-Fellowship. Thanks is also addressed to the Industrial Research Institute Swinburne, Melbourne, Australia, for the permission to use experimental facilities.
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