Microstructural change in ice: I. Constant-deformation-rate tests under triaxial stress conditions

Abstract

Extensive da mage to ice occ urs during ice-structure interac ti o n by microc racking, recrystalli zati o n a nd melting, Th e obj ecti ve of this wo rk was to investigate this da m a ge process unde r confin ed-stress conditi o ns beli eved to bc assoc ia ted with impact zo nes th a t occ ur durin g ice-structure inte r ac ti on, "Da m age" refers to mi c ros truc tura l mod ifi cati o n th at causes d e teriorati on o f the m ee ha nica l prope rti es, Prior exp erimental work h as shown that a sm a ll a mo unt of defo rm a ti on causes pe rm a nent da mage in ice, leading to e nh a nced creep ra tes during subsequc nt load ing, To investigate thi s soft ening, fr es hwater g ra nul a r ice was defo rmed under m od era te confineme nt (20 :'IiPa ) a t -10°C, a t two rates which brac ket duc ti le a nd brittl e beh avio r (10 2 s I a nd 10 + s I). Sa mples were deform ed to different level s of ax ial strain up to 28,8% , Thin sec tio ns we re exam i ned to assess the prog ressive cha nges in microstructure, Bo th g ra in-bound a ry a nd intra-g ra nul a r c rac king began a t strains co rrespond ing to th e peak stress (1-2% ) fo r tests at both stra in ra tes. The peak stresses we re 23.4 MPa for the tests a t 10 2 S I a nd 9.8 MPa for th e tes ts a t 10 + s I, At strains of > 1-2%, d e n se elusters of intra-g ra nul a r crac ks b ega n to develop in th e samples tes ted a t the highe r ra te, At th e lower ra te, d yna mic rec rys ta lli zati on was a ppa rently the do min a nt defo rm a ti o n m eeha nism b eyo nd the pea k stress. The a\' C rage g r a in-size decreased stro ngly during th e fi rst few per ce nt stra in a nd then m a intained a rela tivel y stabl e va lue.

Microstructural change in ice: II. Creep behavior under triaxial stress conditions

“…The studies presented in this and the two companion papers (Melanson and others, 1999; Muggeridge and jordaan, 1999) are primarily aimed at addressing the first objective and using the results to guide our approach to the second. The experimental work presented in Melanson and others (1999) and this paper was not undertaken to reproduce impact conditions. Rather, the approach was to isolate two of the critical elements — high-stress conditions and the damage-formation process — and systematically characterize the mechanical and microstructural response of ice to these factors.…”

Small-scale laboratory experiments on the indentation failure of polycrystalline ice in compression: Main results and pressure distribution

“…Previous work by the present group of researchers has shown that the hpzs have a failure mechanism that does not include large fractures (see for example Meglis et al, 1999;Melanson et al, 1999;Xiao, 1997). Within hpzs, the state of stress is triaxial.…”

“…From these tests it was determined that the flow of energy into and out of the indenter is an important consideration during dynamic ice crushing (Jordaan and Timco, 1988). These early field observations also highlighted the important role of damage processes during an icestructure interaction, and guided laboratory tests (see for example Meglis et al, 1999;Melanson et al, 1999) aimed at understanding the relationship between confining pressure and damage.…”

“…Reproduction of these highly confined stress states during triaxial tests has very clearly shown that ice becomes highly recrystallized under controlled laboratory conditions, as well as under the natural conditions present during an ice-structure interaction Melanson et al, 1999). For further discussion of dynamic recrystallization, see for example, Sinha and Cai, 1992;Barrette et al, 2002;Barrette et al, 2003. Under sufficiently high hydrostatic pressures, ice will melt; see for instance Nordell, 1990.…”

“…These analyses have provided empirical information for use in design. While many aspects of the mechanics have been successfully unravelled Melanson et al, 1999;Jordaan, 2001), much remains to be understood and modelled.…”