On borehole indentor (BHI) measurements and analysis

Cold Regions Science and Technology 76–77 (2012) 109–120

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On borehole indentor (BHI) measurements and analysis Nirmal K. Sinha a,⁎, Karl Shkhinek b, Victor Smirnov c a b c

National Research Council Canada, 1200 Montreal Road, Bldg. M17, Ottawa, Ontario, Canada K1A 0R6 St. Petersburg State Polytechnic University, St. Petersburg, Polytechnitcheskaya 29, Russia Arctic and Antarctic Research Institute, St. Petersburg, Bering str 38, Russia

a r t i c l e

i n f o

Article history: Received 26 May 2011 Received in revised form 17 January 2012 Accepted 19 January 2012 Keywords: Ice strength In-situ Borehole indentor Failure classification Acoustic-emissions Microstructural analysis

a b s t r a c t In situ measurements on stress–indentation curves conducted with the National Research Council (NRC), Canada and the Arctic and Antarctic Research Institute (ARRI), Russia borehole indentors (BHI) are analyzed and classified. This establishes harmony with laboratory observations on stress–strain diagrams and some compatibility with the recommendations (ISO/DIS 19906) on estimating uniaxial ice strengths from BHI strengths. The analysis is devoted mainly to consider the influence of local ice conditions and the indentation rates on the pressure–indentation curves. Simultaneous records of the acoustic emission (AE) detected by accelerometers installed on the ice surface, within 1.5 m of the indentation plate, indicated that major cracks are nucleated at the ice/plate interface. Microstructural analysis of the indented ice confirmed this important conclusion in addition to revealing recrystallization as well as healing activities in the indented ice. An attempt, with extremely limited success, has been made in applying conventional ice failure criteria for predicting the observed stress–indentation curves. Phenomenologically, however, a power-law between the indentation-rate and upper-yield strength exhibits the same rate sensitivity (about 3) usually obtained for strain-rate dependence of uniaxial strengths. Numerical solutions of the rate-sensitive indentation processes must be developed (as has successfully been achieved for uniaxial tests) on microstructure-property based mathematical (rheological) model that includes the effects of the rate-dependent kinetics of deformation, microcracking and crack-enhanced creep. Premature brittle fractures are contact problems and modeling must consider the nucleation of cracks in ice at the ice/plate contact surface. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction In situ borehole indentor (BHI) tests involve loading a large volume of ice. For a given thermal and mechanical state of an ice cover, the micromechanics of failure processes and consequently the stress–indentation response of ice depend very strongly on the rate of indentation (Sinha, 2011). Since the real structures in ice-infested waters are subjected to a wide range of environmental conditions, such as temperature, types of ice, the mobility of the ice bodies, etc., it is desirable to obtain detailed characterization of ice and understanding the state of the material. No doubt, the pre- and post-test microstructural analyses of the indented ice (Sinha, 2011) and monitoring of acoustic emissions in ice during BHI tests (Shkhinek et al., 2010; Smirnov et al., 2009) provide chances to witness ice–structure interactions and more or less realistic description of nature. The BHI strength over a wide range of indentation rates provides an “index and its rate sensitivity” that may be considered as a measure of the bulk properties relevant to many ice–structure interactions. In fact,

⁎ Corresponding author. E-mail addresses: [email protected] (N.K. Sinha), [email protected] (K. Shkhinek), [email protected] (V. Smirnov).

this was proven to be the case in 1989 during low-indentation rate, macro-scale and medium-scale tests in consolidated multi-year (MY) sea ice at Hobson's choice Ice-Island (a tabular iceberg calved from the Ward Hunt ice shelf in Canada). The characteristics of large-scale, up to 20 m in length, fracturing were predicted by conducting scaled-down experiments in a 0.3 m deep trench with the 0.09 m diameter National Research Council (NRC)-BHI (Sinha, 1999) before performing the indentation test with a medium scale (1.0 m diameter) indentor in a 3.0 m deep trench (Frederking et al., 1990). The NRC-BHI tests and post-test microstructural analysis were conducted before performing the big tests to figure out the extent of damage in the 3.0 m deep, 3.0 m wide and 90 m long trench in the ice. The big ditch was very expensive in terms of time, manpower, and money. In general, the BHI tests can provide opportunities to examine the development of loads on a structure under different loading rates. This, in turn, may thereby assist in developing a theoretical model and numerical simulation that would adequately describe the real ice–structure interaction processes. Due to augmented interests in shipping through sea-ice infested waters, due primarily to the decrease in the extent of sea ice and the duration of the cold seasons caused by climate change and global warming, NRC-BHI is increasingly being used in the Arctic for over twenty years to measure the in-situ bulk strength of the ice and its

0165-232X/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.coldregions.2012.01.009

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seasonal variation (Johnston, 2006; Johnston, et al., 2001, 2003; Sinha, 1991, 1997; Sinha, et al., 1993). The BHI or borehole jack (BJ) strength has recently been recommended in ISO/FDIS, 19906:2010(E) for determining in situ confined compressive strength of ice for estimating the equivalent unconfined ice strength as a reference parameter. The methodology considers the unconfined ice strength as a reference parameter that can characterize ice conditions. Suggestion was made to divide the average of the BHI strength obtained at a given site of interest by 3 to obtain the uniaxial compressive strength. The type of failure (ductile or brittle fracture) and the rate of loading, however, are not mentioned in the proposed ISO/FDIS, 19906:2010(E) codes. The ratio of the BHI and the uniaxial strength depends on a number of factors and could be as high as 5 (Sinha, 1987; Sinha, et al., 1986; Smirnov et al., 2009). Moreover, for a given ice cover at its ambient state of health and temperature, the BHI strength is rate sensitive and provides only an “index” to measure three-dimensional confined compressive strength of quasi “semi-infinite” natural ice cover under complex stress state. Nonetheless, workable methods have been developed and a measure of changes in strength during the spring and summer as the ice becomes warmer and decay occurs in its structure (Johnston, 2006; Johnston et al., 2001, 2003; Sinha, 1990). The NRC-BHI was built in 1985 and was designed by the first author, as described in Sinha (2011), after examining all the existing in situ test systems, especially the Fenco borehole jack, FBJ (the brainchild of Dr. H.R. Kivisild (1975)), designed on the basis of Goodman borehole jack developed for rocks. This was followed by the fact that the first systematic field evaluation of the FBJ in offshore regions was made by this author and his team at the special request of Dr. Kivisild (Sinha, 1987; Sinha et al., 1986). Although FBJ has been used extensively for many years and by a number of investigators for almost ten years before 1984, its potentials for determining the rate sensitivity of in-situ confined compressive strength of various types of ice (see Sinha, 2011 for details) were never addressed. Moreover, the movable indentation plate in FBJ, with eight big bolt-holes as the possible sources of stress concentrations and nucleation of cracks at the plate–ice contact area, makes the analysis of test data very difficult unless the holes are eliminated, say with ice plugs for tests at low temperatures, as discussed in Sinha (2011). Recently a new BHI has been designed and built at the Arctic and Arctic Research Institute (ARRI) and is being used on sea ice (Smirnov, et al., 2009) and an attempt was made to develop a numerical solution for assessment of BHI measurements (Shkhinek et al., 2010). One of the objectives of this paper is to briefly present the NRCBHI and ARRI-BHI test systems and procedures for improving the test systems. The other objectives are to discuss the BHI results and analytical methods to be developed to present them in a rational manner, keeping in mind the micromechanics of ice deformation and failure processes involved during indentations. However, the primary objective is to combine the knowledge base developed on physics and mechanics of ice, and the modeling capabilities of the three authors.

fresh-water granular or snow-ice (Hawkes and Mellor, 1972) and on columnar-grained S2 ice (Sinha, 1981) indicated, without any doubts that such fractures are nucleated in the ice at the platen/ice interfaces and are not a fundamental material property of ice. The NRC-BHI has two independent pressure chambers and uses two movable plates with polished surfaces (Fig. 1a). When lowered in the borehole, the indentor freely hangs in the hole and this feature provides an opportunity to align the piston heads with the wall of the borehole when the pressure is applied. The polished surfaces are a requisite for good compression tests. The complete system consists of four major components: 1) a fibre-glass motor-driven ice corer, 2) a completely stainless steel indentor with smooth and polished indenting surfaces (surface area of 6.5 × 10 3 mm 2 for which the maximum working pressure is 40 MPa) and two chambers with an extensometer in each of them, 3) a 3800 W electric hydraulic pump with a specially designed flow control valve to move each of the pistons to an extension of 25 mm at a speed of 0.005 to 0.5 mm s − 1 or equivalent to floating ice moving with velocity of 1.8 × 10 − 4 to 1.8 × 10 − 2 km h − 1 and 4) a portable 110 V power supply for the hydraulic system rated for −65 °C and a digital recorder capable of displaying pressure and displacement on a screen for monitoring the tests and 5) a transducer in the hydraulic system, calibrated to give the applied average pressure on the plates and a mechanical pressure gauge connected to the hose for monitoring the tests in case of malfunction of the electronic data logger. The light fibre-glass core auger, for the NRC system, makes 150 mm diameter, smooth-walled, vertical boreholes in the ice, and provides 100 mm diameter ice cores. Extracting ice cores is very

2. NRC-BHI and ARRI-BHI 2.1. NRC-BHI The NRC borehole system was designed and fabricated in 1985. It was developed to evaluate the ‘rate sensitivity’ of in situ ductile strength of ice in its natural state and compare the results obtained from laboratory tests. This would then help in establishing the micromechanics of deformation properties of natural ice with those at the level of the mobility of ice-lattice dislocations and the kinetics of intercrystalline microcracking leading to ductile high-temperature failures. As for brittle compressive fractures, laboratory tests on

Fig. 1. Schematic diagrams of NRC-BHI (a) and ARRI-BHI (b).

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important, because it permits the tested ice to be thoroughly characterized in terms of temperature, salinity and microstructure. To avoid any change in the thermal regime of the ice, each hole is drilled immediately, within a few minutes of the strength test. Where possible, drilling should be terminated before reaching the bottom ice surface, to prevent water from intruding into the borehole and altering the thermal regime of the ice. Moreover, this procedure reduces the time to clean the frozen slush after each test if the ambient air temperature is very low and the wind is blowing snow. From the beginning of drilling, to the start of a test, the total time could be reduced to a few minutes for meter-deep holes. Longer times are required, however, to perform tests in holes deeper than one meter. One of the most important factors in NRC-BHI tests is that it takes only a two-person team to conduct the tests. 2.2. AARI-BHI In case of ARRI-BHI, the main goal of development was to determine the in-situ compressive strength at a given rate of loading (indentor speed of 4.2 mm/s) — not necessarily to focus on the influence of strain rate and micromechanics of failure, as was the prime objectives of the NRC-BHI. The main questions raised at ARRI were: (1) What do BHI measure and how can it be used for structures design? 2) How do the borehole indentor parameters influence the records? 3) How do the local conditions (ice heterogeneity) and strain rate influence records? 4) Can the phenomenon be described numerically with the conventional ice models? The AARI-BHI is a single piston device with two plates (Fig. 2b), one movable (indenting) with a diameter significantly smaller than the fixed supporting plate (Shkhinek et al., 2010). The surfaces of the stainless steel plates are smooth and polished. The indenting plate is removable and can have a diameter of 65 mm, 90 mm or 120 mm. The curvature of the plates in the vertical plane matches the 250 mm diameter hole in the ice; the boreholes are made with an ice auger, not a core auger. The surface area of the fixed plate (9.0 × 10 4 mm 2) is more than 12 times the area of the indenting plat; the depth of indentation at the rear surface is noted to be less than 1 mm. A potentiometer is used to monitor the displacement and recorded in a PC. The potentiometer is situated not inside of the device, as in case of the NRC-BHI, but on the external part of the indenter. Therefore the effect of cold temperature on oil and other

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parts of the system does not affect its mobility. The main feature of this test system is a constant speed of penetration of the indenter into ice. This is achieved by the application of a constant force using a special pump. A portable power supply (380 V) runs the hydraulic pump and the PC. 2.3. Comparison of NRC-BHI and AARI-BHI The ARRI-BHI is capable of delivering a constant indentation rate to an ultimate penetration depth of 50 mm and a pressure of 50 MPa. This is comparable to the maximum working pressure of 40 MPa and the total extension of 50 mm (if used as single acting system) of the NRC-BHI. However, the ARRI-BHI is capable of delivering a significantly higher speed of indentation than the present configurations of the NRC-BHI system and this is certainly a desired feature. However, so far all the tests were performed by ARRI at a fixed indentor velocity of 4.2 mm s − 1, which is about ten times higher than the highest speed (in one direction) used, until recently, with the NRCBHI system. Most of the recent ARRI-BHI tests were carried out using the 90 mm diameter plate with a surface area of 6.4 × 10 3 mm 2, which is comparable to the surface area of 6.5 × 10 3 mm 2 used in all the NRC-BHI tests. However, in conjunctions with a 90 mm diameter plate, with appropriate curvature, at one end (to act as the indentor) and a large plate with the same curvatures at the other end, the configuration of the NRC-BHI can readily be modified to be equivalent exactly to the single-piston AARI borehole indentor. The maximum depth of tests using the ARRI system has been to a depth of 5 m compared to the maximum depth of 8 m reached in decaying sea ice by the NRC system. NRC-BHI tests are conducted in 150 mm diameter holes for which 100 mm diameter ice cores are also obtained for examining the ice at the indentation depth. This is not the case for the ARRI-BHI tests because the 250 mm diameter bore holes, required for performing the tests, are prepared by an ice auger — not a specially designed ice–core auger as in the case of the NRC-BHI system. The strongest point in ARRI-BHI tests is the use of three accelerometers installed in the ice sheet surface, in the vicinity of the indentor, to register acoustic emissions (AE) indicating crack formation during the tests. The seismometers (AПT1-M) register signals in the range of frequencies up to 100 Hz. 3. Ice response to loading during BHI A number of failure processes occur in ice, regardless of test configuration (laboratory unconfined or biaxial and in situ BHI), temperature, ice types and the structural state of ice. The stress–strain or stress–indentation (plate displacement) curves are used for classifying the failures. These processes may be divided into the four commonly observed (idealized) modes, as described below and illustrated in Fig. 3.

Fig. 2. NRC-BHI inside a 150 mm diameter (D) hole (a) and ARRI-BHI inside a 250 mm diameter hole (b).

Flow Stress (Type 1) — This is characterized by the pressure (stress) continually increasing with increasing strain in case of laboratory tests or indentation in case of BHI tests. Flow Stress is commonly observed during uniaxial or biaxial low strain-rate tests in laboratories or BHI tests in warm (close to the freezing point), young (sea-ice cover less than 300 mm in thickness), first-year (FY) defined as sea-ice cover 300 mm or more in thickness or deteriorating/decaying (with signs of melting) FY and old (second-year or older) sea ice. Therefore, it is appropriate to describe the strength in terms of pressure or Flow Stress, (σD) corresponding to a chosen indentation or displacement, D, equal to 3 mm or 5 mm. This idea corroborates with the concept of determining “Yield Strength”, corresponding to the 0.2% offset strain

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4. Microstructural (forensic) investigations in failed ice at NRC

UPPER YIELD

σ

ASYMPTOTIC FLOW STRESS PREMATURE FAILURE DISPLACEMENT

Fig. 3. Four common types of stress–displacement diagrams seen during uniaxial, confined biaxial or 3D confined borehole indentation tests on ice.

yield stress, σ0.2% commonly used in metals and alloys (ASTM, 1998) for tensile tests. Asymptotic (Type 2) — The pressure increases with increasing displacement or indentation to a maximum value that does not change significantly with further increases in displacement when considerations are given to the drag forces generated at the sidewall in case of BHI tests. This is called the “Asymptotic strength, σAS”. This type of response is commonly observed in warm and/ or decaying ice even at the highest rate of displacement that can be delivered by the test system. In this case, the limitation is imposed by the test system. Upper Yield (Type 3) — The stress increases with an increase in strain or indentation (displacement) to a maximum value, and then decreases. With further increases in displacement, the stress may rise again. The first peak stress is called the “Upper Yield” (UY) strength, σUY or simply ductile failure stress σf in analogy with the “Ultimate Tensile Strength or UTS” used in metals and alloys (ASTM, 1998). The ‘ductile’ failures involve microcracking and crack-enhanced viscous deformation (Sinha, 1988). Premature Failure (Type 4) — The stress increases with an increase in strain or indentation (displacement) and then abruptly, in a brittle manner, decreases to a low value. Further increases in displacement may cause the pressure to increase again, leading to two or more peaks. This will be referred to as “Premature Fracture or Failure”. The first peak stress is defined as the “Fracture Strength, σf”. While discussing stress–indentation response during BHI tests, it is crucial to mention that, a BHI cannot be used to evaluate the “elastic properties” of ice floating on its own melt or not far from its melting point. BHI tests in ice are relatively slow. Elastic theories of indentation are applicable only for loading times of “fractions” of 1 s due to high homologous temperatures of natural ice. The slope of stress–displacement curves near melting point provides only a measure of ‘deformation or effective modulus’. This depends on temperature, rate of loading and stress level selected and could be significantly less, even 10% or lower, than the true Young's modulus, E. The engineering physics of the elastic properties of various types of natural freshwater and sea ice (granular, S1, S2 and S3 with different degrees of mean c-axis orientation), and their temperature dependence, are well understood (Sinha, 1989) — and can only be determined from dynamic methods (correctly emphasized by Timco and Weeks, 2009). Because ice in nature contains air bubbles and cracks, the need for dynamic methods is even more crucial for field evaluations of bulk elastic properties required for applications to sonar devices (for example, the AE sensors used during ARRI-BHI tests) and rapid ice–structure interactions as discussed in Sinha (1989).

Ice mechanics is a part of a huge field of high-temperature materials science, metallurgy, geology and geophysics. In mechanical/metallurgical engineering, the intergranular or grain-boundary cracking readily observed in ice and illustrated in Fig. 4, is known as ‘high-temperature grain-boundary embrittlement’. Unlike metallurgical engineering, however, ice engineering tends to divide the failure processes in ice into two basic types — ductile and brittle. Perhaps the main reason for taking such an approach was because iceengineering problems were faced primarily by practicing engineers with knowledge and experience in the general field of civil or mechanical engineering. Moreover, without the benefit from any microstructural examinations, the casual observations tend to bolster this idea. Indeed, natural ice in freshwater lakes and rivers, or in icebergs, may look and break like glass whereas in the case of glaciers it appears to flow like butter. The so-called ductile failures, for strain rates greater than about 1 × 10 − 7 s − 1 (during uniaxial tests at −10 °C) were shown to be induced by intergranular cracking activities (Sinha, 1981). Simple brittle–ductile approach, therefore, is in violation to the known physics of deformation and fracture processes commonly observed in ice. Thus a “black-and-white”, brittle–ductile approach does not apply to ‘Hot Materials’ like natural ice at temperatures close to its melting point (to be clarified next). In the field of materials science of crystalline materials, “high-temperature” is defined as ‘homologous’ temperatures Th (=Tw / Tm) greater than about 0.4Tm, where Tw and Tm are restrictively the working temperature and the melting point in Kelvin. For example, pure ice with Tm = 273 K, working ice temperature of Tw = 263 K is equivalent to Th (=263/273) of 0.96Tm. Thus, 263 K or − 10 °C is certainly cold for human bodies, but for ice it is extremely hot — only 4% below its melting point. The physics and mechanics of ice, like any hightemperature crystalline materials, such as titanium- or nickel-base superalloys used in jet and rocket engines, are extremely complex. However, contrary to the popular belief in the field of ice engineering, the properties of ice as a high-temperature material are better understood than most other materials, such as metals, metallic alloys and ceramics at comparable (in homologous scale) elevated temperatures. This is primarily due to the fact, “unique to ice engineering”, that ice crystal are transparent and birefringent, and consequently forensic type of microstructural investigations of failure can be carried out in ice (Sinha, 1988, 1990, 1999) at the working (in-situ) high

Fig. 4. Grain-boundary cracks, immediately after UY failure in S2 ice under uniaxial compression (Sinha, 1988 for test details), revealed by combined scattered and transmitted polarized light in conjunction with solid-state double microtoming technique for thin sectioning. Cracks extending to the transgranular space or along three neighbouring grain boundaries may erroneously be called as ‘wing cracks’ without performing forensic microstructural analysis.

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homologous temperatures — higher than 85% of its melting point (or higher than 0.85Tm). Forensic analysis in engineering is simply microstructural investigations into mechanical, structural, design or manufacturing failure (Mastromatteo, 2011). The classification of ice types, such as S1, S2, S3, etc. (Michel and Ramseier, 1971), on the basis of growth history and crystal structure, and test techniques like ‘short-term creep and recovery’, now known as Strain Relaxation and Recovery Test (SRRT), are also examples of the progress made in the field of ice mechanics. Fig. 4 illustrates an example of high-temperature embrittlement (intergranular microcracking) observed at 263 K (or 0.96Tm) under uniaxial compressive loading in transversely isotropic pure S2 ice immediately after reaching a ‘ductile’, UY failure. Note the absence of any large cracks at this stage of loading. The cracks are predominantly oriented parallel to the vertical plane, the compression axis, and along the grain-boundaries in this micrograph. This type of intercrystalline cracks develops in crystalline materials at homologous temperatures, higher than about 0.4Tm. Compare this micrograph with the observations in Fig. 5 illustrating the deformation and fracture characteristics developed during a slow test in Ward Hunt shelf ice using the NRCBHI. Also examine the microstructural features in Fig. 6 exhibiting recrystallization and microfracturing and deformation within subgrains (straight tilt-boundary lines in Fig. 6c) at the level of dislocation interactions at brines pockets in first-year (FY) sea ice. Now compare the features in Figs. 4, 5 and 6 with the characteristics observed during a premature, brittle-like fracture in S1 ice during a rapid NRC-BHI test shown in Fig. 7. Brittle, or rather premature fractures (introduced in Sinha, 1981) during ice–structure interactions are commonly addressed in ice engineering. A detailed discussion on premature failures during BHI tests is presented in Sinha (2011). A brief presentation, on the basis of Figs. 7 and 8, may be beneficial before we discuss the measurements carried out on acoustic emissions (AE) during the ARRI-BHI tests (Shkhinek et al., 2010). Stress–displacement diagrams, like Fig. 8a, the basis of most engineering analysis and the development models, do not provide information on ‘time’, extremely important for operation of high temperatures components in real structures, such as gas-turbine engines. Ice engineering is not an exception and lessons learnt in aerospace, nuclear and power-generation industries are equally applicable to ice engineering. The first peak stress in Fig. 8a, defined as the premature fracture strength (24 MPa) occurred at tf of 12.4 s and indentation, Df of about 2 mm as can be seen in Fig. 8b. The second and the third peak stresses of 19.7 MPa and 22.7 MPa occurred at 16.8 s and 22.2 s respectively. The complex histories of stress and indentation (Fig. 8b)

a

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and microstructural observations (Fig. 7) of ice that fractured prematurely, exhibiting saw-tooth type of stress–displacement response (Fig. 8a), assist in-depth, forensic type (Mastromatteo, 2011) of analysis of fracture processes in ice. The relatively shallower zone in the central section and triangular zones of recrystallized ice are prominent in Fig. 7a and b. Fig. 7a displays five major cracks originating from the arc near the left end representing the spherical platen-ice contact area. Two prominent cracks, numbered as ‘1’ and ‘2’, (to be called radial cracks) are at angles of about ±20° from the major axis of indentation. There are two closely spaced large cracks, ‘4’ and ‘5’ (to be called central cracks) emanating from the central area of contact and oriented nearly parallel to the axis of symmetry. The crack ‘3’ is certainly a bifurcation of the radial crack ‘1’. Question is when did these major cracks develop? These are the forensic type of enquiries that can be addressed by microstructural analysis (Mastromatteo, 2011). The microstructural observations (Fig. 7) in conjunction with the loading histories in Fig. 8b provide clues as to the formation of the major cracks. The relatively clear zone beyond the curved contact surface at the left end in Fig. 7a indicates a zone of HIPped (Hot Isostatically Pressed) ice that must have developed during three-dimensionally confined, continued deformation after the formation of the radial cracks (1, 2 and 3). HIPed is a new term for ice mechanics people and was introduced in ice engineering by Sinha (2011) to emphasize that a great deal about crystalline materials at high homologous temperatures can be learnt from ice mechanics. Historically speaking, the term and the process of HIPing were originally developed by metallurgists working in the field of nickel-base superalloys. It was developed for the removal (hence densification) of gas bubbles entrapped during ‘directional solidifications’ (DS) of the melt to produce columnar-grained materials. The DS process in superalloy metallurgy was developed during the early periods of 1960s. It is highly possible that the metallurgists were unaware of the DS processes in the vast oceans of ice-covered waters. It's, therefore, appropriate to point out that Mother Nature – who knows for how many billions of years – has been producing columnar-grained S1, S2 and S3 types of ice by the DS of water from the top surface! Anyway, the superalloys are used in the manufacture of columnar-grained DS gas-turbine engine blades. During operations at extremely high temperatures, these blades are damaged by the formation of intergranular voids and microcracks, and are routinely removed from service. Eventually the HIPing process found its usefulness in repairing and rejuvenating the used turbine blades by sealing the defects generated during operations. It is appropriate to mention here that the developments made in the field of physics and mechanics of ice are now applied directly for developing new experimental and analytical techniques for

b

TRANSITION Fig. 5. Recrystallization and transgranular radial cracking during a slow NRC-BHI test in granular large-grained, Ward–Hunt Shelf ice, (a) vertical section up to 50 mm in depth and (b) 50 mm wide (scale on right) horizontal section, along the cut-mark (arrow in a), exhibiting transition between recrystallized, highly micro-fractured and HIPed (Hot Isostatically Pressed) ice, and crystals with radial transgranular cracks.

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a

b

c

50mm

10mm

1mm

Fig. 6. Post-BHI test structure of first-year, S3 type sea ice showing (a) recrystallization on the left, near the contact surface, and highly deformed grains on the right, (b) radial microcracks within sub-grains on the right of ‘a’, revealed by scattered light, and (c) early stages of recrystallization on the right, revealed by thermal etching, exhibiting tiltboundaries or straight lines, parallel to the c-axis (double white arrow) of the sub-grains, generated primarily from strain (stress) concentrations at the brine pockets (dark inclusions) and the sub-grain boundaries; the absence of air pockets are noticeable.

measuring and predicting strength and deformation of gas-turbine engine materials (Sinha and Sinha, 2011). Going back to Fig. 7a of S1 ice, it is speculated that the cracks ‘1’ and ‘2’ were probably nucleated at about the time of the first peak stress (tf = 12.4 s). The crack ‘3’ must have nucleated shortly after tf. The HIPping process healed these three cracks in the HIPed (relatively clear) zone. The relatively less healing of crack ‘4’ and practically no healing of ‘5’ (see details in Fig. 7a) suggest that the cracks ‘4 and 5’ were nucleated at 16.8 s and 22.7 s respectively corresponding to the second and the third peak stresses. Microstructural analysis provides windows of opportunity for observations after the completion of the BHI tests. However, records of acoustic emissions during the BHI tests provide opportunities for an understanding of the kinetics of micro- and macro-cracking activities leading to failures. This is presented in the next section. 5. ARRI field experience and records analysis Results obtained from the ARRI-BHI experiments are in general agreement with those of the NRC-BHI tests. Both ARRI and NRC

a

laboratories, therefore, found a common ground for agreement in the classification of stress–displacement records into four failure types, as suggested in Fig. 3. The strongest point in the ARRI-BHI tests is the installation of three-component seismometer/accelerometer on the ice surface at about 1.5–2 m away from the borehole (Fig. 9). Moreover, the measurements indicated that the rate of indentation during the tests was indeed constant (Fig. 10) for the displacement rate of 4.2 mm s − 1 used so far. Recordings of seismo-acoustic signals (pulses) during the borehole indentation tests, and particularly their locations, confirmed the formation of internal cracks in ice at the ice–plate interface — and provide credibility to conclusions drawn from forensic microstructural investigations presented in Fig. 7. Note the differences in the amplitudes of the acoustic pulses coming from different planes. Measurably lower amplitudes in the vertical plane (Z-axis) in comparison to those corresponding to the horizontal plane (X–Y), for a given large AE ‘seem’ to indicate that the major cracks (presumably producing large-amplitude AEs) are oriented parallel to the major compression axis. In fact, this ‘may’ also be applicable to the minor

b HIPed

1 3 5

4 UNDAMAGED

2 Fig. 7. Scattered light (a) and cross-polarized light (b) micrographs of a vertical section of NRC-BHI tested S1 ice showing fine-cracks radiating from contact surface, partial closer of major cracks (removal of large air bubbles commonly seen in S1 ice) in the HIPed (Hot Isostatically Pressed) and recrystallized zone, and undamaged ice in the wedge.

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a 30

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NRC-BHI TEST F302 DOWS LAKE, S1 ICE

tf = 12.4s

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20 15 10 5 0

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STRESS, σf

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INDENTATION, D, mm

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0 25

TIME, s Fig. 8. a. Stress–indentation curve for NRC-BHI test on S1ice corresponding to Fig. 7.b. Histories of stress and indentation for NRC-BHI stress–indentation result of 'a'.

cracks related to the low-amplitude or grass-root (that ‘appear’ to be noise) acoustic emissions (relatively higher amplitudes in the X–Y plane than the Z axis). Refined measurements and analyses (amplitude, rise time, frequency-domain, etc.) of the AE signals are required before making any conclusions. In winter periods, the ice was strong and brittle. The unconfined strength (measured on 17 March 2009) of granular sea ice samples from the depths of 0.6 m, with salinity of 5 parts per thousand (ppt) at −2 °C, could reach up to 5 MPa for a stress rate of 1 MPa s − 1.

Fig. 10. Histories of stress and indentation for an ARRI-BHI test at a depth of 0.8 m and indentation rate of 4.2 mm s− 1 exhibiting UY failure and constancy in the indentation rate; note the kinetics of acoustic emissions before and after failure.

Under these conditions of depths, for example at a depth of 0.8 m, the first relatively large-amplitude AEs due to cracking developed during BHI test at 4.2 mm s − 1 corresponding to the indenter penetration of 4–8 mm (that is, in 1–2 s) after the beginning of the tests. These AEs are always recorded before the pressure reached the maximum values. One of the most important observations, for theoretical development, is the fact that the UY stresses, as a rule, have been accompanied by the seismo-acoustic signals as can be seen in Figs. 9 and 10. Only the preliminary measurements and analysis of the acoustic emissions have been made and a detailed analysis will be presented in future. For weak (decayed) ice in hummocks and at depths more than 1.5–1.8 m, stress–displacement, Type 3 response with practically no high-amplitude acoustic pulses (Fig. 11) was registered. Similar or Type 2 response with only single or very few AEs was recorded during the tests in spring–summer periods (warm ice). These acoustic impulses formed in 2–3 s after the penetration starts at ice depths of 0.3–0.8 m.

Fig. 9. Location of the three-component seismometer/accelerometer located on ice surface and recorded history of stress and acoustic emissions (indicating formation of minor and major cracks) for an ARRI-BHI test at a depth of 0.6 m in ice at − 2 °C at indentation rate of 4.2 mm s− 1.

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6. Numerical simulations of interaction process

Fig. 11. Records obtained during an ARRI-BHI test at indentation rate of 4.2 mm s− 1 in weak (decaying) hammock keel ice at a depth of 1.5–1.8 m during winter, exhibiting UY or Type 3 stress–displacement response with practically no high-amplitude acoustic emissions.

Fig. 12. The schematic diagram for computation; the investigated area is considered as a cylinder and is located normal to wall of the borehole, and the indentor with a diameter, d = 90 mm, is considered as a flat circular plate located in the centre of the cylindrical design area.

It follows from the experimental results presented in Sections 4 and 5 that different approaches have to be used for numerical simulations. No doubt, the premature failures of the type 4 in Fig. 3 should be analyzed on the basis of fracture processes starting from the nucleation of cracks at the ice–plate interface. Preliminary experiments and mathematical analyses of penetration in the brittle ice piece were attempted by Goldstein and Osipenko (2009). Penetration in the ‘elastic–ductile’ ice was considered by Shkhinek et al. (2010) and axially symmetric Finite Difference solutions were developed. It should be mentioned here that the word, ‘ductile’ was used to be compatible with the description popularly used in ice engineering — assuming (erroneously though) no damages due to cracking prior to failure. Apparently ductile failures, such as UY, actually involve significant cracking activities as can be seen clearly in Figs. 9 and 10, and the degree of crack-damage at UY point depends on strain rate (Sinha, 1988). The computational scheme for the problem is given in Fig. 12. In the proposed solution, the ice is modeled as a cylinder with dimensions exceeding the indenter radius by at least a factor of ten. Displacements over the side and the back surfaces are restricted in order to model the confinement conditions during the indenter penetration and to prevent rigid body displacement of the ice feature. The indenter is modeled as a flat circular plate located in the center of the cylinder front plane. It is proposed to move with a constant velocity V inside the ice. The Finite Difference method is used for the problem solution. As the problem is axis-symmetrical, the phenomena in the longitudinal cross section are considered. The triangle cells are used and special methods described in Marti and Cundall (1982) are applied to avoid an unrealistically high stiffness of the triangle elements. Very large deformations of the grid elements in the Lagrangian coordinates occur in the process of calculation. Therefore the Lagrangian–Eulerian system of coordinates (Nazem et al., 2008) is used to avoid instability of the computation scheme due to great deformation of the cells: the finite-difference grid is repeatedly updated and the recomputed values of variables are adjusted in a new grid. Applicability of the conventional failure criteria Tresca (T), Mohr– Coulomb (M–C), Mohr–Coulomb–Tresca (M–C–T), and Nadreu– Michel as adopted by Fish–Zaretsky (F–Z, 1997), as illustrated in Fig. 13, were examined. All of these models determine the failure surface in coordinates p–τ, where p = − (σ1 + σ3)/2 is the mean pressure, τ = (σ1 − σ3)/2 is the shear stress where σ1, σ3 − are the principal stresses (positive under tension and σ1 ≥ σ3. Models (T) and M–C are well known. The model (T) was used by Masterson (1996) for the analysis of the BHI records. Some additional information should be used for two other models.

τ

T M-C M-C-T F-Z

C

p p*

pm Fig. 13. Failure criteria considered in conventional approaches in ice engineering.

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25 20

p, MPa

h=0.25d h=0.5d

15

h=1d

10

h=1.5d h=3d

5 0 0.00

0.04

0.08

0.12

0.16

Path, m

Combination of the Mohr–Coulomb and Treska (M–C–T) models. When the confinement increases and reaches a certain level, then the Mohr–Coulomb law transforms into the Treska law. This assumption is confirmed by experimental data (Jones, 1982; see also Sanderson, 1988 p. 96; Schulson et al., 1991; Smith and Schulson, 1993). According to different investigations the transition from M–C to M–C . T occurs when main stresses are compressive (negative) and σ3 / σ1 > 0.2 or 0.3. Nadreu–Michel's (F–Z) model is more complicated. The model takes into account the possibility of ice melting at very high pressures. The following two problems were analyzed by Shkhinek et al. (2010). 1. Which traditional mathematical model better describes the experimental data? 2. How do weak zones in ice influence the borehole indenter records? The assumed criterion for choice of the suitable model should be based on how well the calculated results compare with the experimental observations. Determination of model parameters on the basis of the above-mentioned criteria is actually inverse problems. These problems are often confusing because several combinations of input data may approximately correspond to the same final result. The following results obtained from the ARRI-BHI measurements were used in the analysis carried out: a. The AARI experiments show that the maximum pressures measured during BHI tests (at a fixed indentation rate of 4.2 mm s − 1) are in the range of 18–20 MPa. b. The ratio of the maximum BHI pressure, ‘p’, and the unconfined strength, ‘Rc’, for these conditions, according to AARI measurements, is in the range of 4–5. The AARI measurements show that the unconfined ice strength (Rc) may vary in the range, 3–5 MPa. However, analytically these strength values can be reached by different combination of cohesion and angle of internal friction. Therefore different sets of these parameters were considered to reach unconfined strength in the mentioned above limits. Two principles of combinations formation were used: constant cohesion and different angles of internal friction (different strengths) or constant strength and different combinations of cohesion and angle of internal friction. For each combination, the borehole calculation was conducted numerically. Results show that it was difficult to satisfy simultaneously to both requirements (the level of the p/Rc ratio and the maximum pressure in experiment) for all models besides the F–Z. If it was possible to reach p/Rc about 3–5 then the total pressure was too high. Perhaps ice melting under localized high pressure, which is considered by the F–Z model, could play an important role. Anyway, wide parametric analysis was conducted. Both cohesion and angle of internal friction were varied to reach unconfined strength in the limits measured by AARI (3–5 MPa). All models (besides the F–Z ) could not satisfy simultaneously

Fig. 15. Influence of the weaker layer in hummock's keel on BHI pressure–indentation curve.

requirements p/Rc ≈ 4–5 and p ≈ 20 MPa . Results for the F–Z model are shown in Fig. 14. The test results in Fig. 11 provide a unique opportunity to look into the behavior of ice in hummock keel. It was noticed during the borehole drilling that the bulk of the ice below the surface layer was weak due to the presence of slush and voids. Because of the existence of weak ice in the bulk of the hummock below the relatively stronger surface layer, the recorded pressure increases initially and then sharply decreases. The question is whether this dependence help in the estimation of ice block dimension? To clarify the situation, calculations for the two-layered medium were conducted. The strong upper ice layer with thickness, ‘h’, is located on the weaker material; the indenter diameter ‘d’ is assumed to be 90 mm. The upper layer's parameters are assumed to be: bulk modulus of 3.33 GPa and shear modulus of 1 GPa. The ratio of the properties of the relatively stronger upper layer to the weaker lower layer is assumed to be: bulk modules 1.5/1, shear modules 10/1 and the Poisson ratio 0.25/0.48. Results of calculation (elastic–plastic analysis) are presented in Fig. 15. Curves of different colors correspond to different thickness of the upper layer. The vertical lines in the end of curve show location of the boundary between layers. Fig. 15 shows that the existence of the weaker material significantly influences the amplitude and duration of the record. The maximum pressure in each curve depends on the thickness of the strong layer, the indenter dimension and properties of both layers. For the applied input data, this dependence is plotted in Fig. 16. Here pm is the maximum pressure in the considered record and poo is the maximum pressure corresponding to indenter penetration of 0.05 m in homogeneous space (which has the properties of the upper layer). The results show that the weak zone influences the records if they located on the distance less than 3 indenter diameters. These results show that in future after conduction of more detailed investigations BHI can be used not only for pressure registration but also for determination of the local peculiarities in ice mass. 1 0,8

pm/poo

Fig. 14. Comparison of calculated results (F–Z model, ϕ=20°) and the AARI records.

0,6 0,4 0,2 0

0

1

2

3

h/d Fig. 16. Calculated maximum pressure versus h / d for first-year hummock's keel ice, where h is the thickness of the strong surface layer and d is the diameter of the indenting plate.

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7. Discussions It is shown in this paper that the stress–indentation records obtained with BHIs developed at NRC and AARI are complimentary. This provided the opportunity for a mutually-agreed classification of failure processes depending on the physics of registered stress–indentation curves (Fig. 3). It is important to mention that the results (Figs. 9–11) obtained from the accelerometers installed in the vicinity of boreholes helped in confirming the suggested classification. The quantitative correlations require detailed investigations because of some differences in the construction of the two devices. All the reported ARRI-BHI tests were carried out using the 90 mm diameter plate with a surface area of 6.4 × 10 3 mm 2. This is almost exactly the same as that (surface area of 6.5 × 10 3 mm 2) used in all the NRC-BHI tests. Thus the stress–indentation curves obtained with these two systems, for the same conditions of ice (type, density, temperature, etc.) and indentation rates, should be comparable. Since the radius of curvature in the NRC-BHI is 75 mm compared to 125 mm in the ARRI-BHI, a small effect is expected due to the differences in the curvature of the indenting plates. However, one-to-one correspondences cannot be made at this time because all the ARRI tests were performed at a fixed indentation rate of 4.2 mm s − 1 and this is about ten times higher than the highest speed (in one direction) used, so far, with the NRC-BHI system. Replacing one of the plates in the NRC-BHI with a surface area of 9.0 × 10 4 mm 2 (the same as that of the fixed plate in ARRI-BHI), changing the curvature of the plates and adding the output of the two displacement transducers as a measure of the penetration, the NRC-BHI system will be the same as the ARRI-BHI. Since the plates are screwed on to the actuator, the NRC system is very flexible. However, NRC-BHI tests are conducted in 150 mm diameter holes for which 100 mm diameter ice cores are also obtained for examining the ice at the indentation depth. This is not the case for the ARRIBHI tests because the 250 mm diameter bore holes, required for performing the tests, are prepared by an ice auger — not a specially designed ice–core auger as in the case of the NRC-BHI system. Presently, ARRI recovers 600 × 600 mm ice blocks using chain saws for making 184 mm diameter cylindrical specimens for unconfined tests. Consequently there is a room for further developments in future for the ARRI-BHI test system. An ice–core auger that can make 250 mm diameter boreholes and also provides 200 mm diameter (say) ice cores will be a good solution for the ARRI-BHI system. These large diameter cores can be used not only for examining the ice (type, temperature, salinity, etc.) at the test depth, but also for making horizontally oriented prismatic specimens for determining uniaxial, unconfined (or biaxially confined) strength and deformation properties of ice appropriate for the major axis for the BHI tests. In fact, large diameter (300 mm) cores were indeed used in conjunction with the BHI tests (Sinha et al., 1986), but that 350 mm diameter core auger and its driving system was very heavy (not really portable) and required a bigger team than a two-person team presently used for the NRC-BHI tests. Up to now BHIs were used mainly for consideration of physics of processes which follow the intrusion in ice. The practical needs demand clarification of level of parameters which are necessary for sea structures design. The attempts to link BHI pressure with the ice unconfined strength cannot be considered as successful. The great difference in the empirical transitional coefficient (3–5 and even 6) obtained in different regions may lead to significant over or under estimation of the ice loads exerted on structures. Moreover, physicsbased justifications for the values of the coefficient have to be developed. As the activity in the Arctic regions increases, transition from pure experimental methods of estimating loads on structures and integration with approaches of their evaluation on fundamental basis (experiment + theory) is inevitable. The theoretical development must

be based on three-dimensional constitutive equations incorporating the effects of the kinetics of microcracking and crack-enhanced creep (dislocation creep) similar to the algorithms developed for uniaxial tests (Sinha, 1988; Zhan et al., 1994a,b,1996) capable of predicting rate sensitivity of upper yield strength and hence phenomenological relationship like Eq. (1) described below. Detailed analysis of the acoustic impulses during BHI testing will be an asset. Anyway, it seems that the first approach in this direction has been attempted in this paper. However, it is only a first step. Whereas premature fractures, due to the complex and variable contact problems are unpredictable, the upper yield (UY) type of failures depends on indentation rate and can be quantified (Sinha, 2011). The imposed condition (the independent variable) during a BHI test is the rate of indentation. Ideally the rate of indentation should be constant. For phenomenological relationship, the average indentation rate to failure, D_ af (=Df / tf), is appropriate for the analysis of BHI results. The dependence of σf on D_ af , for UY type of failures, can be described (Sinha, 2011) by a power-law, " #b D_ af σf ¼B σ1 D_ 1

ð1Þ

In Eq. (1), σ1 is the unit stress (=1 MPa) and D_ 1 is the unit displacement rate (=1 mm s − 1). In SI system, 1 m s -1 should be used, but the chosen unit is practical from the experimental point of view. This makes the coefficient ‘B’ equivalent to the UY stress for 1 mm s − 1. Eq. (1) can also be written as:  1=b −1=b σ f D_ af ¼ D_ 1 B σ1

ð2Þ

Eq. (2) is identical to the dependence of viscous flow rate on stress in uniaxial creep or strain-rate dependence of UY strength for compression tests. The stress exponent, 1/b of about 3 is well known for constantstress dislocation creep in pure polycrystalline ice under uniaxial (1D) loading conditions. A value of 3 was also obtained for 1D creep in FY sea ice (Sinha, et al., 1995). Experimental NRC-BHI values of ‘b’ of 0.32 (1/b = 3.1) for first-year S3 ice at −10 °C (Sinha et al., 1993) and 0.31 (1/b = 3.2) for multi-year ridge-ice at −19 °C (Sinha, 1991) confirmed that the fundamental mechanisms of deformation under 3D conditions are the same as in 1D conditions. Modeling of BHI tests should be based accordingly and this will be briefly presented later. Eq. (2), however, does not allow a rational approach for comparison of BHI strengths with the laboratory compression (for example, uniform stress in between two plates) test results without giving considerations to the actual cross-head or actuator displacement rates and differences in stress or strain distributions in ice. Moreover, the premature failures in BHI tests may not be discernable from the UY failures at about the maximum rate of indentation (0.5 mm s − 1) used for NRC-BHI tests, depending on ice conditions (Sinha, 2011). Additionally, the measurement (or recording) of indentation is always subjected to errors caused by the initial sitting (contacting) conditions as the indenting plates (in case of NRC-BHI) try to adjust their positions as the pressure increases. Although the NRC core auger produces smooth-walled holes (as can be judged from the surface finish of the recovered ice cores), the borehole wall may get covered with snow or ice-chips when the indentor is lowered in the hole. ARRI-BHI and NRC-BHI should be tested side by side in the same ice. In conjunctions with a 90 mm diameter plate, with appropriate curvature, at one end and a large plate at the other, with the same curvatures, the configuration of the NRC-BHI can readily be modified to be equivalent exactly to the single-piston AARI borehole indentor

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(Shkhinek et al., 2010; Smirnov, et al., 2009). In such a case, the sum of the displacement recorded by the two LVDTs in NRC system will provide the depth of penetration. Actually almost an identical test situation was created when performing the macro-scale tests in multiyear sea ice to compare the results from low-speed, medium-scale tests with 1.0 m diameter indentor (Sinha, 1999).

plate/ice interface confirms NRC's observations. The ARRI system also provides the kinetics of microcracking activities during tests and this will be extremely important, in future, for developing numerical methods, based on NRC-developed elasto-delayed-elasticviscous (EDEV) model and crack-enhanced creep, for simulations of BHI results.

8. Conclusions

References

Use of the borehole indentors (BHI) for the investigations of insitu ice properties has many advantages in comparison to other widely used methods. This instrument avoids many of the problems of ice sampling and specimen preparation, and thereby increases the reliability of the experimental observations. The results provide information on the effect of in-situ factors like temperature, salinity, ice microstructure including cracks, voids, etc., and the effect of the indentor displacement rate on three-dimensional confinement and modes of failure. Four failure types were identified on the basis of pressure–displacement (indentation) curves. BHI permits the determination of ice properties as functions of depth usually encountered in floating ice including hammocks keel. They provide information not only about ice properties but also may reflect certain gross features related to the structure of any ice body (location of large cracks, border between the consolidated layer and the hummock's keel, anisotropy, heterogeneity of the ice mass, etc.). Construction of two in-situ borehole indentor (BHI) systems for ice has been described. One was developed at the National Research Council (NRC) of Canada in 1985 and the other at the Arctic and Antarctic Research Institute (ARRI) of Russia in 2006. It is shown that both of these indentors have essentially the same features and their records are qualitatively similar. In fact, the classification of BHI failure modes into four general types was mutually agreed between the Russian and Canadian investigators. Nonetheless, tests should be performed in ice simultaneously with both of these test systems for quantitative correlations. The BHI may be considered as a small structure interacting with the natural ice. The analysis of BHI results gives an opportunity for the selection of a more reliable ice model. The first step in this direction, albeit based on the traditional concepts of yield-criteria, has been taken in this work. A number of ice failure criteria have been suggested in models used in ice engineering. These models were examined, but their justification faced difficulties. More complicated investigations should be conducted in future. Consequently, ice loads for design of offshore structures in the Arctic regions should be determined both experimentally and theoretically. Theoretical treatments of ice loads require appropriate constitutive equations capable of describing ice behaviour and failures under environmentally induced different conditions. NRC-BHI was used to develop test techniques for examining the failure processes in ice, micromechanics involved in the failure processes, the rate sensitivity of upper-yield (UY) strength of ice and quantifying the degree of maturity for premature brittle-like fractures. Four general modes of BHI failures also occur during unconfined tests, whereas the ratio between the BHI and unconfined strength may vary widely. However, determination of similar rate sensitivity for UY failures unifies the in-situ BHI tests to laboratory uniaxial and confined tests — an issue that could be vital to the recent ISO codes. NRC tests concluded that the ductile UY failures during BHI tests involve microfracturing, recrystallization and HIPing (Hot Isostatically Pressing), but the premature ‘brittle’ fractures are caused by nucleation of crack at the indentor plate/ice interface and propagation of these cracks in the bulk of the ice body. The ARRI tests involving additional detection of acoustic emissions (AE), especially the identifications of major cracks and locating their source of nucleation at the

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