J OllTllalofGlaci%gy, Vol. +5, Ao. 151, 1999
Microstructural change in ice: Ill. Observations from. an iceberg itnpact zone
K.]. M UGGERIDGE,· 1.]. JORDAAN Ocean E ngineering Research Cell /re, FaCllL£J' d Engineering and Ajlplied Science, l\JemoriaL L'l1il'frsily d. '\fewjolllldLand, St J ohn 's, Nwfoul1 dla nd AlE 3X5, Ca nada
ABSTRACT. During a full-scale ice berg-impact slUd y conducted inJuly 1995 o n the L abrador coast, Canada, a sample of ice was retrieved from the impacted surface o f a n iceberg. The sample was thin-sectioned and the obsen 'ations of the contact-zone micro structure arc presented in this paper. Thin sections were prepa red from two slabs c ut parall el to the impacted surface. In each of the thin ecti ons taken from the impacted-surface slab, fin e-g ra ined materia l was found to surro und parent-size g rains (as obsef\Td in the second slab). A bounda ry between the pa rent grains a nd the g rains of modified micros tructure was found r unning approxim ately pa rallclto the impacted surface in each of"th e thin secti ons taken fr om the impac ted-surface slab. This bo unda ry was pronounced towa rds the edges of the contact zone. Latera l mO\'e ment of grains o ut ward along this boun dary was obsen 'ed in thin scc ti ons near the edges but not near the ce ntre of the contact zo ne. The thin sections we re compa red to the res ults of medium-scale indentation tes ts in 1989 and 1990 from the Arcti c O cean. The sa me type of fi ne-gra ined m ateri al and layer formati on of m oclifi ed micros tructure was found in the contact zon es.
INTRODUCTION T he microstructura l changes found in thi s cO lllact zone a rc of interes t for compa rison with laboratory-tested speci mens. This paper is p resented as one of three compa nion During summer 1995, full- scale iceberg-impact tes ts were papers with Mela nson a nd oth ers (1999) and r..1cglis a nd conducted on Grappling Isla nd (near Cartwri ght), Labra others (1999). These papers present some of the recent la dor, Canada, by the Centre for Cold Ocean R esources bo ratory ice-tes t progra ms co nducted a t r..lemori al Univer Enginee ring (C -C ORE), Mem orial Uni ve rsity of New sity o f":\Iewfoundla ncl . foundl and, a nd K . R. Croasd ale and Associates Ltd (Cracker and others, 1997). The aim of thi s fi eld test pro gram was to measure the loads generated during the impact BACKGROUND of LOwed icebergs again st a fi xed instrumented pa nel. The need for full-scale testing of iceberg impacts was recogni zed De\'el opment of the o il a nd gas industry operating off the in the earl y 1980s, because uncertainties associated with ex cast coast or Canada has been spurred by the rece nt Hiber trapolating ice-crushing streng th measurements from ni a projec t, starting from the ea rl y exploration drilling con small-scale laboratory tes ts LO full-scale ofE h ore structures ducted in th e 1970s a nd ea rl y 19808. T he first oil has been led to conservative a nd thus costl y des igns (Cracker, 1995). recm'ered at the Hibe rnia site ahead o f schedule, in Novem The goal of this paper is to present the microstructural ber 1997, and th e Terra Noya project is well undcrway. Other changes found in a sample taken from the contact zone of an fi eld s such as \\'hiterose, Bonne Bay, H ebron and Ben Nevis impac ted iceberg. A sample of ice containing a contact zone arc expected to provide significant future di scoveri es. Oil was co ll ected following an impact tes t at the tes t site on a nd gas operati ons in the G rand Ba nks a rea require pa rti Grappling Island. The sa mple was stored in a freezer, a nd cul a r attenti on to environmental conditions in the a rea. later thin-secti o ned in a co ntrolled laboratory setting in St These incl ude icebergs and seyere sto rm-generated wind J ohn's, Newfo undla nd. The microstructure of the contact a nd waye co nditions. The consequences of iceberg impacts zone is presented through a seri es of thin secti ons ta ken can be se\'ere, and considerati on must be give n to the prob along the impacted surface of the contact zone. The thin ability of these impacts a nd th e associated loads based on secti ons show the vari ati on in microstructure along the im the type of structure, locati on of producti on site and time pacted surface o f the contac t zone. A second se t of thin sec of o pera ti ons. tions was ta ken from ice directly below the first slab of ice to Onc of the main achievements of the ice group at iVl em obse rve the microstructure behind the centre a nd edge of ori a l U ni\'e r: it y is the compl etion of a com prehensi\T m odel the contact zone. for estimating ice berg design loads fo r ya ri ous types of pro ducti on systems (?-.1em orial Unive rsity of New foundla nd, 1996). This includes esti m ati on of globa l a nd local ice load s Present address: C -CORE, M emori al Uni\Trsity o f New ror different producti o n systems at a number ofprabability found land, StJohn's, ;\Tewfoundla nd AIB 3X 5, Canada. of exceedance \·alues. It was show n tha t design global a nd 449 Downloaded from https://www.cambridge.org/core. 27 Sep 2021 at 11:25:32, subject to the Cambridge Core terms of use. J ournalojGlaciology
local loads could be reduced substanti all y compared to mou nted on a near-vertical elifT face with most of the p anel those previ ously used by industry, leading to more econom below the water surface. The panel consisted of 36 triangu ical designs. The reduction is associated mainly with im lar subpanels, each of which had a single load ce ll located at proved analys is of the ice-failure process. Past assessments every corner (a total of 108 load cells for the panel ) capable have generally been based on a conservati ve design premise, of measuring a uniaxial load. A sc hem atic of the test config for example using conservative estimates of ice pressure. As uration is shown in Figure 2. A rope was attached to the ice knowl edge and understanding of the ice-failure processes berg, passed through a snatch block located just above the improve, there is potenti al for m a king more realistic assess centre of the panel, a nd then connected to the tow ing vessel. ments, and for reducing unce rtainty in the design-load esti A p endulum line was used to align the iceberg with the mates. Fu ll-scale tes t data and information from impacted centre of the panel, as shown in Fi gure 2. Following impact, icebergs are key to obtaining a better understa nding of th e the p endulum line was released and the iceberg was towed behavior of ice during impact. into position for another impact or moved ofT site so that The fai lure process of ice is complex, with a number of testing could proceed with a new iceberg. Because towing a processes occurring during the interaction. Fractures in the single iceberg to the site often took several hours, each ice form of spalls reduce the nomina l a rea of contact, a nd high berg was typically impacted a number of times, providing it press ure zones a re produced during the impact. High-pres was sti ll of significant size. sure zones a re areas of high pressure surrounded by areas of relati vely low pressure (jordaan and others, 1999). High press ure zones accommodate large shear stresses as well as high confin ing press ures towards the centre of these zones. As the compressive failure occurs within the high-pressure zone, evidence of microcracking and press ure melting at the grain boundaries has been observed in the field during medium-scale indentation tests a nd small-scale laboratory tests. As the high-press ure zone (s) fai l, recrystallization of grains and refreezing can occur. Small ice pieces are ex Grappling Island truded at the free edges of the contact zone with the release of the confining press ure. Extrusion of fin e ice partieles has Fig. 2. Schematic qf iceberg-impact test configuration (plan been observed in the fi eld at the Molikpaq structure in the view). Beaufort Sea a nd during medium-scale indenta tio n te ts. A schematic of a high-press ure zone and the extrusion of A total of 30 iceberg impacts was recorded on the panel. crushed ice is shown in Figure 1. See Melanson and others The icebergs were in the range 500- 1500 t, with impact (1999) for further disc uss ion on high-press ure zones. speeds reaching 1.5 m S- 1 (Crocker, 1995). Photographs were taken of the icebergs as they impacted the panel. Plate 11 - shows the second impact of "Steve B", an iceberg with a mass of approximately 700 t at the time of testing. Pieces of ice can be seen fl ying in the air quite some distance from the point of contact. High Pressure Zones Under High Sam.ple collec tion and preparation Confinement Icebergs genera ll y impacted the panel well below the water Ice Feature line, and their contact zones normally remained under Structure water. Samples could be retrieved only if the iceberg rolled and exposed the contact zone, which did not occur often. After the fifth impact of iceberg "M a rlene", a contact zone was spotted above the surface of the water. Iceberg "Mar lene" had a mass of approximately 500 t at the time of tes t ing. It is believed that the impact occurred with the eliff face rather than the pa nel, since the impacted surface was brown in colour with traces of marine plant life. For this reason, there are no load-time data available for this impact. The sp eed of the iceberg was approximately 1 m S- I The a mbient Fig. 1. Sclzematic qfhi gh-pressure zone. temperature was approximately lO oC and th e water tem perature approximately 5°e. The temperature of the ice in PROCEDURE the area of the contact zone is estimated to be between - 5 ° and - lO °e. A bl ock of ice containing the contact zone was extracted Im.pac t experim.ent from ice berg "M a rlene" using a chain-saw. The contact zone on" Marlene" is seen as a brown spot in Plate 12. The block of The test site was located on Grappling Island approxi mately 14 km by boat from Cartwright. A steel panel, meas uring 6 m by 6 m and weighing approximately 30 t, was instrumented to measure impact loads. This p anel was • For colour plates see section before previous paper. 450 Downloaded from https://www.cambridge.org/core. 27 Sep 2021 at 11:25:32, subject to the Cambridge Core terms of use. i\I!ugge ,-idge andJordaan: Microstructural change in ice: III
ice was immediately transferred to a cooler, and was stored continuous bead of wa ter and th en mierotomed to a thick in a fr eezer for 3 days before being transported to Memorial ness of < I mm. Thin sections 5 and 7 o f the impacted-sur University for microstructural ana lysis. face slab and thin section 2 of slab 2 were lost during the mierotoming process. The thin sections from this sample Thin-section analysis of contact-zone material were found to be extremely fragil e, p a rticul arl y the ones closest to the centre of the contact zon e. Each of the thin sections menti oned was photographed The sample was stored at - 30°C at M emorial University to between cross-polarized filters at a number of standa rd prevent significant modification of the micros tructure prior m agnifications. The o rientati on of the thin sec ti ons was to preparation of thin sections approximatel y 3 weeks later. such that the top of the photograph represented the top of Thin sections were prepared from two slabs taken from the the impacted surface for thin sec tions ta ken fr om the im sample containing the contact zone. A schematic of the ice pacted-surface slab. The ori entati on of thin section 3 ta ken sample showing the contact zone a nd the numbered thin from slab 2 was inven ed in the photog raph. Global photo sections is give n in Figure 3. The contact zone was approxi graphs of all thin secti o ns were taken. A number of addition mately oval in shape, with the m aj or axis measuring 160 mm al photographs were ta ken of each thin sec ti on at two and the minor axis 110 mm. The ice sa mple was first cut par m agnificati on se ttings. A scale in mm is shown in all of the a llclto the pla ne of the impacted surface to form a slab of ice photographs. approximately 80 mm thick. This slab was called the "im A few additional photographs of selected thin sections pacted-surface" slab and denoted IS. The approximate were taken with pl ain li ght shone from the side to look at the centre line of the contact zone was marked to indicate the predominant crac k pattern. These photographs we re taken relative positions of the thin secti ons along the impacted approximately I year later. The thin secti o ns we re LO O fragil e surface. Thin secti ons numbered 10 through I, and la to microtome on both sides, so onl y the top side was miero through Id, were taken from the impacted-surface slab as tomed. E\'en though the storage temperature was - 30°C, shown in Figure 3. A second slab, denoted slab 2 (S2), some sublimation occurred betwee n the thin sec ti on and the 80 mm thick was cut para ll el to the first slab. The centre line glass plate. Sampl e photographs have been included since the of the impacted surface was m aked slab 2, and the general pattern of cracking can be eas il y seen. The other detail s in a li gnment of thin sec ti ons ta ken from this slab in rela tion to these photographs will be ignored, since they arc readily vis those taken fr om the impacted-surface slab is shown in Fig ible in the photographs taken of the same thin secti ons di r ure 3. The labelling of thin secti ons sta rts with eZI (contact ectly a ft er the ori gina l thin-sec ti oning process. zone I), foll owed by the slab, IS or S2, and the thin-secti on number. For example, thin secti on 8 from the impacted-sur face slab is denoted e ZI /IS/TS8. OBSERVATIONS AND INTERPRETATION
For brevity, onl y selected photographs a rc presented. Photo graphs of the globa l \'iew of particul a r thin sec ti ons a rc o 11~ shown in Plate 13. M agnified views of selected thin sections 16an a rc shown in Pl ate 14. Selec ted photographs under pla in Plan view of Impacted surface a nd side-lighting conditions arc shown in Plate 15. In all of (contact zone of Iceberg) the g loba l views, the microstructure a t the immedi ate ed ge of the thin sec ti ons corresponds to the frozen bead of water
Impacted Surface used to weld the thin section to the glass pl ate. (IS) Slab Impacted Surface In the global \'iews of the thin sections taken along the impacted surface of the contact zo ne (Plate 13a- d), fin e Slab 2 (S2) 8an grained materi al was found to be present in all of the thin secti ons. Thin secti ons taken from . Iab 2 (Plate 13e and f) in dicate the size of the parent grains of the iceberg, whi ch is Centreline of simila r to that of some of the unaffected g rains in the thin impacted surface sec ti ons taken from the impac ted surface. The ave rage size of the pa rent grains is 10- 15 mm . The average size of grains Elevation view of iceberg sample within the fin e-grain m ateri al is considerabl y less than I mm in diameter. During the microtoming process, it was noti ced Fig. 3. Schematic qf icebelg sample containing contact zone th at the thin sec ti ons cl oser to the centre of the co ntac t zone and position qf th in sections. were more fr ag ile than those nea r the edges. All the thin secti ons were microtomed to approximatel y A boundary between the fin e-gra ined m ateri al and the I mm thickn ess or less and then photographed between pa rent g rains was fo und in each of the thin sec ti ons take n cross-polari zed filters the same day they were prepa red. fr om the impacted-surface slab. This bo unda ry ran approx i Each section was origin all y cut with a band-saw to approxi mately parallel to the impacted surface at about 80 mm m ately 10 mm thickness. The microtoming process began by depth at the ce ntre of the contact zone a nd 50 mm depth attaching the secti on to a glass plate with three or fo ur drops near the edge of the contact zo ne. Amongst the fin e matrix of water appl ied a long the perimeter of the thin section. The of mall g rains, large pa rent-sized grains were found. Pla te thin sec tions were mi crotomed until a smooth, level surface 13a shows an example of a large amount o f fin e-grained m a was obtained. They were then turned over on the glass pl ate teri a l found to exist between large una ffected grains. so that the Oat side of the thin secti on was against the glass A sig nificant difference is see n in this main bounda ry plate. The thin section was attached to the glass pl ate with a pa ra llel to the impacted surface when onc compares thin sec- 451 Downloaded from https://www.cambridge.org/core. 27 Sep 2021 at 11:25:32, subject to the Cambridge Core terms of use. J ournal q/Glaciology
ti ons taken from nea r the centre with those ta ken fr om the tion to the parent g rains at the top of the thin sec tion close to outside edges of the co ntact zone. In thin sections ta ken near the interacti on surface. the centre, the boundary was not as distinctive, a nd lateral \Vithin the layer of most microstructural change found moti on of grains a long the bounda ry was not visible. In thin in the thin sections from the impacted-surface slab, there sec tions ta ken near th e edge of the cont ac t zone, the bound were parent-size grains amongst fin e-grained material. ary was pronounced. The grains abO\-c the bounda r y appea r Based on the ori entation of groups o f bubbles in the pa rent to have shifted outwa rds in a la teral directi on. Pl ate 14a- d size grain s, so me of these grains were found to have rotated illustrates the moyement of g ra ins along the boundary in during the impact process. The bubble orientation in the thin sec ti ons I and la taken from the impacted-surface slab. pa rent grains shown in thin sec tions taken from slab 2 is ap This is consistent with the extrusion process at the fr ee edge proximately 45° from norma l. The bubbles are seen as of the high-press ure zone (see Fig. I). In the m agnified views elongated elliptical shapes in Plate 14c and d, which gives of thin secti on la, shown in Plate 14c and d, the la rge, gree n the magnifi ed view of thin section la from the impacted sur coloured grain in the bottom right quarter of the photograph face, and in Plate 13f, which gives the global \·ie w of thin sec can be seen to be offse t along the boundary by approxi tion 3 from slab 2. The unaffec ted g rains within the layer of mately to mm. This boundary may be considered to be most microstructural change with the sa me ori entation as similar to an extrusion plane with a "crack-like" boundary the parent grains (based on the bubble ori entation) may and the grains within bein g forced outwards towa rds a fr ee have their c axis in an ori entation which makes them less surface. likely to rotate or break down into smaller grains. The microstructural cha nges obse rved in the thin sec There is fin e-grained material a long the main boundary ti ons taken from the co ntact zone of the impacted iceberg which di vides the pa re11l grains from the grains which have suggest that thc process of generating fin e-gr ained materi al undergone the m ost microstructura l change. This can be seen plays an important role in the deform ation m echanisms in Pl ate 14a and b which gives the magnilied views of thin during iceberg impac ts. The increased depth of the zone of section I (CZI/IS/TS1). Fine-grained materi al along the main fin e-grained materi al in the centre of the contact zone, as boundary line m ay be expl ained by the "c hipping off" of compared to the edges of the contact zone, is consistent with g rains as two or m ore grains are forced to slide along each other. The res ulting fragments are expected to have been the configuration of a single hig h-press ure zone. The centre roll ndrd through the process of sIippin g of the grains a long of the contact zone lVo uld correspond to the centre of th e the shear plane bo undary. The fin e-grained structure a long high-pressure zone wh ere the confining pressure is higher. the boundary m ay also help to facilita te further slip. This ma At the edges of the zo ne the confining press ure is signifi teri a lmay also be a res ult of nucleation of new grains. cantl y reduced, corres ponding to a lower proportion of C racking is apparent throughout the thin sections a long fin e-grained m ateri al. Lateral m oti on of line-grained mate the impacted surface and in thin seetions taken from slab 2 ri al occurs as the confining pressure is suddenly released. (see Plate l3e and f). There are randomly ori entated cracks The p rocess which generated the fin e-gra ined materi al a pparent throughout all the thin sections. These a rc be in the thin secti ons taken fr om the impacted surface is co m lieved to be pre-existing cracks because of their ra ndom pl ex. This material is opaque in nature and appears to exist orientati on a nd thei r presence in the parent ice. Cracks in bands which nolV past una ffected grains. For example, which are out-of-plane may not b e \·isible in the thin sec thin sec ti on 8 from the impacted surface (see Pl ate 13a) tions or may appear as thick grey lines as in thin section shows mostl y opaque fin e-grained materi al which looks grey CZI/S2/TS3 shown in Plate 13f. R andomly orientated in colour whcn viewed between cross-polari zed filt ers. The cracks ha\·e been observe d in natura lly occurring iceberg small grain structure may be caused by fin e ba nds of mi cro ice. The crac ks pa rallel to the impacted surface a re assumed fractures which occurred within the grain s. Under high con to be a direct result of the ice berg impact. lining pressure in the centre of the co ntact zone, it is It is in teres ting to examine the cracking paUerns readily expected that some press ure melting wo uld occur during visibl e in the selected photographs in Plate 15a and b taken the impact. There is likely also to be dynami c recrystalliza under plain- a nd side-lighting conditions, res pectively. As ti on along gra in boundaries a nd refr erz ing of the g rains. See m entioned earlier, since these photographs were ta ken some the disc ussion in Mcgli s and others (1999) on the occurrence time aft er the sections we re made, only the pattern of crack of similar shear so ftening in other materi als (e.g. myloni tes ing should be noted. Other deta ils in these photographs will in rocks). be neglected, si nce they arc readi Iy visible in the photo In all thin secti ons ta ken from th e impacted surface g raphs of the same thin sec ti ons taken aft er the original there appears to be a layer in which most of the microstruc thin-secti oning process. In Pl ate 15, very fin e dots in the tural cha nges in the ice occurred. These thin sections repre o rder of 0.1 mm in di ameter should be ignored. These are sent a "s napshot" of the ice a fter impac t, a nd if the iceberg the res ult of sublimation and cr ys ta llization of the ice over were to continue its intcracti on with the structure, the layer time between the thin sec ti on and the glass pl ate. Cracks would prog ress further into the iceberg. The general layo ut throughout thin secti on 8 and thin secti on Id from the im of these layers may remain the same, changing o nly in space pacted-surface slab are seen in Plate 15a and b. and time. "Vhat is obse rved in the thin sectio ns taken from In Pl ate 15a, two large "rectangul ar-shaped" grains meas the impacted surface is a layer of highl y rearra nged and uring approx imately 35 mm ac ross can be seen in the centre broken-down g rains followed by parent g ra in materi al. of thin sec ti on 8. These actually consist of a number of grain s, Near the outer edges of the contact zo ne, the layer is nar as can be seen under cross-pol a rized filters shown in Plate rower, but there is still a sig nificant amount of fine-grained 13a. The grains which make up each of the larger g rains ap materi al conta ined in the layer (see, e.g., Plate 13d ). It is also pear to ha\·e moved as a group. This movement has been de noted tha t in so me cases thin sections taken from the edges termined throug h close examination of the crack lin es of the contact zone have gra ins similar in size a nd ori enta- through the two la rge grains. It appears that the two large
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grains were I)]"e\'iou sly side by side and that the grain on the abl y with Plate 4b. The laboratory cree p tes t was at a low left has mO\'ed down rcla ti\'e to the other large grain. T his confining pressure (Pc = 5 MPa ) with a deviatoric stress of move ment of groups of gra ins sugges ts that there is some re 15 MPa, a strain rate of 10 I to 10 2 S I and a stra in of about arrangement and breakdown of grains within the ee J1lra l 10 %. This va lue of devi atoric stress is representative of the region of the contac t zone which may e\-c ntuall y ex trude out value expected towards the edge of the co ntact zone. Plate th e edges of the contact zone as the ice ack a nces. 4b shows extensive microcracking similar to that seen in In Plate 15b, many microcracks can be seen in thin sec Pl ate 15 b (th i n sec ti on Id taken from the edge of the contact ti on Id ta ken ri'om the impacted-surface slab. The layer of zo ne with side lighting). At hi gher confining press ure corres hi ghly m odified microstructure abO\'e the m ain bounda ry ponding to the centre of the contact zone, thin secti ons from is bright in colour due to the refl ec ti on o f li ght from the l\I[eglis and others showed the same breakdown of grains as many crack surfaces. The lateral move ment of the gra ins seen in Pl ate I3a (thin secti on 8 ta ken [i'om the central along the boundary can easil y be seen as crack li nes are in region contact zo ne). This is shown in Plate 7 a whi ch terrupted a nd offse t. illustrates tests at 50 MPa eonfinemel1l, 15 l\IPa applied devi atoric stress and a strain of about 4% . The micro structure was similar to that seen in Melanso n and others as DISCUSSION disc ussed above for 20 MPa eonfinemen t. In summary, the res ults shown in th r present thin sec As stated in Melanso n a nd others (1999), the failure or ice in tion taken fr om the co ntact zones have microstructures con compression consists of fr actures, with m ost of the load sistel1l with confining press ure ra nging from approximately being carried through hig h-press ure zo nes tha t retain their 5 MPa at the edges to about 20 MPa near the centre. Highl y integrit y, at least for a short period of time. The press ure dis damaged ice is likely to have the expected shear of approx i tribution over these zones has been obse rved to be paraboli c mately 15- 25 MPa and strains of >4%. in shape, with low confining press ures near the edges, a nd To all ow comment to be m ade on the genera l obse r hi gh pressures near the centre, Exce pt at very low speeds of vati ons represented in the thin secti ons taken from the con illleraction, a distinct, na rrow layer of highly damaged ice tact zone as compa red to res ults fr om some medium-scale has invariabl y bee n observed to form adj acent to the struc indentation tes ts, measurements of the layer ofmodiliedmi ture. Examples of this have been give n in the present paper, cros tructure due to the impact were made and a sketch was including the first examples to our kn owledge of the fo rma drawn (sec Fi g. 4a). The depth rrom the top of each thin sec ti on of this layer in a real iceberg impae l. Studies of the me tion, corresponding to the impacted surface of the contact chanics of the interaction process haw becn carried out zone, to the bo undary between the parent ice a nd the ice Uo rdaan and others, 1999), using the methods of damage \\'ith changed microstructure was noted and indicated as throry. In thi s, state variables represe nt the prior stress his points in Fig ure 4·a. Eac h point corresponds to the average tory of ice at various locati o ns, taking into account also thc depth of the bo undary and the approximate pos ition of each states of stress. In the simulati ons conducted, it was found thin sec ti on. A curve was drawn using the points as a guide that da mage associated with mierofr acturing occurred first to estimate the shape orthe boundary. As mentioned earlier, near the edgrs. then progressed in ward. Soft ening asso the boundar y was obse rved to have the greates t depth (ap ciated with press ure effects then occurred near the ce ntre prox imatel y 80 mm into the ice) near the centre of the con and adva nced towards the edge. This res ulted in a reason tact zone. Near the edges of the contact zone, the depth of abl e simulati on of layer fo rmati on, with sudden fa ilure of the bounda ry from the impacted surrace is approximately the layer ta king pl ace when the two zones meet. 50 mm. The . hape of the bounda ry is reasonably fl at in the A compa riso n of the obse n 'ati ons from the current thin sec tions a nd those from the small-scale laboratory tes ts de i 16em i sc ribed in M elanso n and others (1999) and i\legli s and others i (1999) is give n here. Fig ures have been chosen to illustrate , ~ I several points. In comparing the current thin sec ti ons and I I the laboratory tests described in l\Ielanson a nd others, the I tes ts with higher strain rates were the main focus since they i Layer 8em I represented rates ex pected in full scale. Thin secti ons from I I •• the centra l region of the impacted surface of the CO lllac t zo ne desc ribed above were compared to the tests at 20 MPa con a fining press ure, which may corres pond to the centre of the .~~ CO lllaet zone (hi gh-press ure zone). In particul ar, Pl ate 13a IMPACTED SURF'"AC E (thin section 8 taken from the ce ntral-regio n contact zo ne) Mostly Mhttvrll!' of F"lne r". was compa red to Pl ate 2b. The microstructure in Pl ate 2b is Gr-Q.,n ed o.nd Po-rent reasonably similar, indicating the fi eld tes ts I ikcly had strains Si ze GrOined Ma. t erl(ll in the 5% region. In Plate 2, a substantia l breakd O\m of grain-size can be seen with strains of > 2% . In comparing thin secti ons fr om the present paper and b the laboratory-tested ice described in M egl is and others, PARENT ICE example thin sec ti ons (i'om the edge and from near the centre of the contact zo ne a re compared to ice sampl es tes ted Fig. 4. Schematic draw ings rfjmijile qfla)'er with 1I1 0stll1 icro under conditions similar to those ex pec ted at the edge and at slntclllml change. ( a) Schematic qfprqfi.le based 011 measure the centre of a hi gh-press ure zone. Thin section I from the mel1ts rf /a)'eT depth ill thin sections. ( b) Sketch qf prrfile impacted-surface slab shown in Plate 13d compares favo ur- shape rfthe contact zone.
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middle, steep during the transition from the centre of the depicting the layer, is given in Plate 17b from M eaney (1992) contact zone and level at the edge of the contact zone. and M eaney and others (1996). This sketch shows the distinct The boundary was more pronounced away from the bounda ry away from the central region and the lack of a dis centre of the contact zone, as illustrated in Figure 4 b. The tinct boundary in the centre. The layer which extends to the definition of the boundary appears to be linked to the edges of the tes t face is a zone of pul ve ri zed fine-grained ice. amount of lateral movement of the grains. Grains near the Segments near the edges of tbe contact face which broke off centre of the co ntact zone had little lateral movement. At during the tes t are shown as spall ed areas in Plate 17b. Den the edges of the contact zo ne, the lateral moti on of grains sity measurements along the crushed layer showed th at the was significant, and offse t of many individual grains was density increased towards the centre of the crushed layer. seen above the boundary. Fine-grained material surround The substa ntiall y greater confining press ure in the centre of ing unaffected parent-size grains was observed in the layer the indentor was thought to contribute to this findin g. The both near the centre and near the edge of the contact zone. crushed layer was found to contain large ( > 25 mm), rel The centre of the contact zo ne is considered to be the ap atively unda maged particles surrounded by a m atrix of pul proximate centre of a high-press ure zone, with lower pres verized ice, suggesting a grinding action as particles slide SUITS at the edges where extrusion of fine ice particles occurs. and rotate during extrusion (Meaney, 1992). M eaney also The thin sections were compared to the results of med noted the differences produced by the use of different shapes ium-scale indentation tes ts in the Arctic O cean in 1989 (Fre of in den tors. The wedge indentors produced a higher degree derking and others, 1990), wh ere crushing and extrusion of spa lling than the fl at indentors, and the layer of crushed processes and hi gh-pressure zo nes were evident. A photo ice produced with the wedge indentor was indicative of the graph of the ice fa ce after an indentati on tes t from Frederk lower confinement present. ing a nd others (1990) is included in Plate 16. Pl ate 16a shows A section through the impacted surface after a spherical the contaet fa ce with a view of the crushed layer. Plate 16b head indentation tes t at H obson's Choice ice island in 1989 is shows a thin section taken from the area indicated by a box presented in Figure 5 for compariso n to the present set of in Plate 16a. In Plate 16 a, the shape of the layer is cl earl y thin sections. The ri gid spherical indentor used had an area shown. The central region of the layer, which is included in of 0.8 m 2 with a 1.28 m radius of curvature a nd 1 m overall the area indicated by the box, corresponds to the blue zone of diameter. Figure 5 shows the same type of crushed layer the contact face. On both sides of this central area, a layer of containing fine-grained m ateri al. A prono unced boundary opaque white ice was observed continuing to the edges of the is seen some di stance away from the centre of the contact contact face. The ice in this zo ne has been crushed a nd pul zonc. The central region contains a mixture of fin e grains veri zed into small grains. Near the centre, mos t of the large and parent-sized grains a nd ex tends some way into the ice. grains remain, but with incipient damage on the grain This is m ost likely due to the shape of the spherical indentor bo undari es and in some grains (Frederking and others, and the confi nement at the hea d of the inclentor. In disc us 1990). It was noted by Frederking and others (1990) that the sions of the indentation tests inJordaan a nd others (1990), edge of the contact zone was soft, while the centre was quite the ice within the layer is stated to have a lower density than solid even in areas where the ice had been highly pulverized. the parent ice. The sintered nature of the ice after the tes ts This finding is in agreement with the microstructure of the was discussed and the importance of pressure melting in the ice sample shown in Plate 16b. The layer of highly pulverized deformation process was noted. Obse rvations were found to ice can be see n on either side of the central region. The agree with earli er observations from drop-ball tes ts (Khey boundary between the fin e grains and the parent g rains is sin a nd Cherepanov, 1970). ve ry disti nct away from the centra l region, particul a rl y at Simila rities found in the various indentation tes ts dis the bottom right of Pl ate 16b. In the centre, there are m os tl y cussed and in the present thin secti ons are the formati on of pa rent-size grains, but the orientation of the grains is not a layer of fin e-grained materi al and the pronounced bo und ra ndom (as in undamaged ice ) and some change in micro ary towards the edges of the contact zo ne. In a ll tes ts, the structure is apparent. Res ults of the indentati on tes ts indicate centra l region showed changes in microstructure within the tha t at higher indentati on rates, which would more closel y ice but no pronounced boundary. The difference in the depth represent iceberg impacts, a relatively thin layer of crushed of cha nges in mierostructure in the centra l region was noted and damaged ice with ex trusion of fin e particles was for the various examples mentioned. In the indentation tests, observe d. The thickness of the layer was found to vary the central region was a thin band. In the present thin sec throughout the layer, but a narrow band of relatively unda tions ta ken from the contact zo ne, the central region of mi m aged ice usually in the central part of the co ntact zone was crostuctural change in the ice extends sli ghtly further into observed after each test. In the centra l part of the contact the sample. In all tests, there was evidence of extensive crack zone, measurements of local ice pressures were about three ing, extrusion of small crushed ice pa rticles and pressure times higher than the average pressure over the entire con melting. tact face (Frederking and others, 1990). Based on the observations made of the microstructural Plate 17a, ta ken from Meaney (1 992), shows a horizontal changes in the impacted-iceberg contact zone, a number of thick secti on taken from an ice face after impact with a fl at suggestions for further work are presented. It is important to rigid indentor (test denoted TFR-Ol) at Hobso n's Choice ice use iceberg ice in future small-scale testing and to conduct isla nd in 1990. M eaney (1992) stated that although the centre tests under loading and confinement conditions similar to of the horizo ntal thick secti on was damaged, a discrete those expected in iceberg- structure interaction (e.g. test crushed layer did not exist. The crushed layer show n in Pl ate durati ons of similar length to ice berg impacts). Comparisons 17 a had an average thickn ess of 57 mm and a maximum of the microstructural changes under different loading and thickness of 155 mm, and was found to increase in thickness confinement conditions have gi\"e n insights into the deform toward the perimeter of the ice face. A sketch of the zones ation m echanisms occurring in the ice. See Melanso n and observed on the test face, and the corresponding pla n view others (1999) and Megli s and others (1999) for recent labora-
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TEST 05
Parent Ice
Crushed Layer Indented Ice Interface Boundary of Crushed Layer Edge of Indentation
Fig. 5. Section acTOSS cTllshed layer after spherical-head indentation lesl, Hobson's Choice ice island, 1989 (photo taken ~) I E. Stander and used with permission).
tory test programs conducted at Memorial Uni\"Crsity. Con apprecia ted. Disc uss ions with R. E. Gagnon and S. ]. J ones tinued laboratory testing in these areas a long with further of the Institute for ?\[arine D ynamics were a lso helpru!. The full-scale testing would help to improve understanding or authors thank E. Stander {or permiss ion to use Figure 5. This the processes occurring in the ice during iccberg impacts. work was supported in part by the Natural Sciences and En gin eering Research Council and the Canada ~Newfound land OfT~ hor e Development Fund. The authors thank the CONCLUDING REMARKS rev icwers for their co nstructivc and useful comments. A few of the hi ghlights orthe obsen'ations ofthr mi crostruc ture found in the contact zone are noteworthy. A layer of ice REFERENCES with highly modified microstructure was see n in the thin secti ons taken rrom thc impacted surrace. The boundary Cracker. G. 1995. Iceberg impact experiment. C-CORE. \ ;'1("1, 20 3, 10- 12. between the ic e with modified microstructure and the par Crocker, G. B.. K. R. Croasdalc. R. F :'I1 eKenn a. G. :'II. English, J Gllzzwcll and S. Bruneau. 1997. C-COR F; iceberg impacl rrperimenl /"ha" 2,jlnal reporl. ent ice was found to be more pronounced away from the SI J ohn\ :\'Od. l\lemorial niwTsil y of i\C\\ lonndland. Ccmre 101' Cold central region orthe contact zone. Fine-grained material in Ocean Resources Engineering. (C-CORE Comran Repon 96-CI6.) the layer on either side of the central region was found to Frederking, R., I.J.Jordaan a ndJ. S. :'IlrCallum. 1990. Field ICS IS of iee in move in a lateral di rection toward the edges of th e contact dem,nion al medium scale, H obson's Choice Ire Isla nd, 1989. 111 /Af/R. zone. These findings are co nsistent with the concept of a '~)'III/}OJillm 011 Ir e, Es/)oo. Filllalld. ' ~ lIgIlJI 20-24. 1990. Proceedings. 101. 2. high-pressure zone with higher pressure in the centre and Espoo, Helsinki Cni\'crsil ), ofTechnolog)" 931 9++. Jordaan, I..J. alld 6 others. 1990. Dn'elo/lInenl '!! nf!(' ice load models. SIJohn·s. lower press ure at the edges as ice is extruded. Cracking Nlld, l\lcmo ri a l Universily of Ncwfoundlancl. Ccnlre for Cold Ocean was e\·ident throughout the thin sectio ns in \'a ri ous direc Resources Engineering C-CORE). tions. Cracking parallel to the impacted surface is tho ught J ordaan, I..J. . D. G. l\[als kc\- ilch and J. L. r..lcglis. In press. Disimegralion to be a direct res ult of the iceberg-impact process and may of ice uncle I' 1~lsI comprcss i\'e loading. 1nl. J Fracl. , 97(1 +),279-300. Kheysin, n. 'lc. a nd X V CherepanO\·. 1970. IZ\11cne ni yt" slrllklur)' I'da v be due to unloading. The findings concerning the present zone uclara I\'e rdogo Icla 0 povcrkhnosl' ledyanogo pokrm'a [Change thin sections were similar to those of previous medium-scale of ice slruClure in Ihe zone o /"impaCl ofa solid body aga insl Ihe ice cover i ndenta tion tes ts. surface]. Probl. Arl.! .. Inlark!. 34-, 79- 8+' r..I eancy, R. B. 1992. Locali zed c rushing in ice-slruclure interaction. ( r.. I.Eng Ihesis, r..lemoria l Uni\"(Tsil y ofNc\\'founclla nd.) ACKNOWLEDGEMENTS l\lcanc)" R., I..J. J ordaan andJ Xiao. 1996. Analysis o f m edium sca le icc inclenla lion leS lS. Cold Reg. Sci. Tee/1II0!.. 24 '3),279- 287. The authors would like to acknowl edge the support obtained :--kgli s, I. L.. P l\J. r../rl anson a nd 1.J Jordaan. 1999. l\licroslruCl ura l through the Nationa l Energy Board, C a nada. G. B. Crocker cha nge in ice: 11. Creep beha\'ior under Iri"xia l slress condilions. J and G. M. English of C -CORE provided valu able informa Clnciol., 45 (151), 438 148. tion [or the projec t, and their enthusiastic cooperation is l\l elanson, P. 1\1. , 1. L. l\ lcgl is. 1..J. Jordaan anclB.l\1. Stone. 1999. I\licro slructural change in ic e: l. Conslant-ci eforma li on-ratc leSlS under lriax gratefull y acknowledged. The authors acknowledge the use ial slress condilions. J Glariol., 45(151), +17.J.22. or the fa cilities at the Institute for Marine D ynami cs, Nation ~[ e m o ri a l Univcrsil )"ori'\cwfollndland.1996. Calladiall OJpho!"e Desiglljor Ire al R esearch Council of Canada. The assistance of B. M. EIIl ,irolllllenl (COf)/E) "colld alll/lInl reJiorl .. IJohn"s, I\'nd, i\J rmori a l Lni Stone, ]. F. Sweeney, 1. L. Meglis and P. M . Melanson was \'ersil ), ofNcwfo uncll and. Ocean Engineering Research CCnlre.
MSS received JI Jun e 1998 and aeee/J/ed illlevisedJorm 9 A/Jril 1999
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