Flexural strength and fracture toughness of sea ice

A series of mid-winter experiments were carried out on the ice in the rubble field around Tarsiut Island in the Beaufort Sea. The tests included grain structure determinations, salinity and density of the ice, small beam flexural strength and fracture toughness. Typical values for flexural strength and fracture toughness were 0.6–1.0 MPa and 100–140 kPa m^{$\text{1}\text{2}$} respectively. Both properties were dependent on brine volume and depth in the ice sheet. In comparing these results with identical tests on finegrained freshwater ice it was found that for comparable loading conditions, the strength of the sea ice was significantly lower than the strength of the freshwater ice, whereas the fracture toughness of the sea ice was higher than the fracture toughness of the freshwater ice.     

Fracture of a ridged multi-year Arctic sea ice floe

As part of the 1994 Sea Ice Mechanics Initiative experimental program, fracture experiments were carried out on an 80 m diameter ridged multi-year (MY) ice floe in the Beaufort Sea. An edge cracked, quasi-circular ridged floe was subjected to both cyclic and ramp loading sequences using a steel flat jack. Load, crack opening displacement, acoustical and seismic measurements were made during the experiments. The objective was to gain further insight into the fracture and constitutive properties of MY sea ice. Accurate predictions of the strength of MY sea ice and the forces developed during interactions between MY sea ice and floating or fixed structures are sought. Such interactions include MY ice floe collisions with offshore structures and ships. The fracture resistance of MY ice is determined to be within the range 23 < G_c < 47 J/m² for a 80 m diameter ridged MY floe. This fracture energy is similar to values obtained for the fracture of FY sea ice both in the Arctic and the Antarctic.     

Friction of sea ice on sea ice

This paper presents the results from field tests on the friction of sea ice on sea ice performed in the Barents Sea and fjords at Spitsbergen. The effects of the sliding velocity (6 mm/s to 105 mm/s), air temperatures (− 2 °C to − 20 °C), normal load (300 N to 2000 N), presence of sea water in the interface, and ice grain orientation with respect to the sliding direction on the friction coefficient were investigated. The effect of the hold time on the static friction coefficient was also studied. The roughness of the ice surface is an important parameter that determines the value of the friction coefficient. Repeated sliding over the same track led to surface polishing and decreased the kinetic friction coefficient from 0.48 to 0.05. The studies showed that the friction coefficient is independent of the velocity when sliding occurs between natural ice surfaces. As the contacting surfaces became smoother, the kinetic friction coefficient started to depend on the velocity, as predicted by existing ice friction models. Both very high (~ 0.5) and low (~ 0.05) kinetic friction coefficients were obtained in the tests performed at high (− 2 °C) and low (− 20 °C) air temperatures. The presence of sea water in the sliding interface had very little effect on the static and kinetic friction coefficients. The static friction coefficient logarithmically increased with the hold time from ~ 0.6 at 5 s to 1.26 at 960 s. The results are discussed, and the dependences are compared with existing friction models.     

Fracture toughness and cutting

An exploratory model for cutting is presented which incorporates fracture toughness as well as the commonly considered effects of plasticity and friction. The periodic load fluctuations Been in cutting force dynamometer tests are predicted, and considerations of chatter and surface finish follow. A non-dimensional group is put forward to classify different regimes of material response to machining. It leads to tentative explanations for the difficulties of cutting materials such as ceramics and brittlo polymers, and also relates to the formation of discontinuous chips. Experiments on a range of solids with widely varying toughness/strength ratios generally agree with the analysis.     

Fracture toughness of laminated steels

Lamination occurs spontaneously in the transverse direction in many commercially available steel plates, if the transverse stresses are sufficiently high. Previous investigations have indicated that lamination is often accompanied by an improvement in the fracture toughness of the plate material. In the vicinity of the crack tip, the stress concentration is so large that the bond between adjacent layers will break before crack propagation sets in. If these layers are sufficiently thin, a state of plane stress is approached near the crack tip. In the present study, the influence of layer thickness and bond strength on the fracture toughness is investigated. It is shown that lamination does improve the toughness, if certain conditions in these variables are fulfilled. This offers a possibility to build up structures with high yield stress and high fracture toughness at the same time, since the permissible defect size to prevent unstable crack growth need not be uncomfortably small.     

Fracture toughness of tooth acrylics

The hardness, fracture toughness, toughness, flexural strength and Youngs moduli of three acrylic tooth polymers were investigated. The first polymer was based on a conventional homopolymer poly(methylmethacrylate). The second was based on cross-linked poly(methylmethacrylate) with an uncross-linked poly(methylmethacrylate) coating. The third material was based on an interpenetrating polymer network (IPN) of a cross-linked and uncross-linked poly(methylmethacrylate). All three polymers had similar hardness values. The cross-linked and IPN polymers had higher fracture toughness (K_IC) and toughness (G_IC) values than the conventional homopolymer poly(methylmethacrylate) polymer and lower flexural strengths (_f). The toughness of the cross-linked and IPN polymers was higher due to crack deflection around the polymer bead structure and the polymer beads acting as crack pinning sites.     

Fracture toughness of cement-based materials

Concrete is known to be a brittle material, yet there is no acceptable material parameter to quantify the fracture toughness of concrete. With an increase in efficiency and accuracy possible with numerical methods, a need to define a criterion for crack growth thus becomes evident. In this paper, various theoretical models which describe crack growth are summarized. It is shown that a single parameter such as classical stress intensity factor cannot adequately describe fracture processes in concrete. A recently proposed two-parameter fracture model is described. Some measurements of crack profile as the crack propagates observed using laser holography are also presented. For practical reasons, it was impossible to include this invited paper in the proceedings of the RILEM Congress.     

Fracture toughness of multilayer pipes

Multilayer pipes composed of various materials improve partially the properties of a pipe system and are frequently used in service. To estimate the lifetime of these pipes the basic fracture parameters have to be measured. In the contribution a new approach to this estimation is presented. Special type of a C-shaped inhomogeneous fracture mechanics specimen machined directly from a pipe has been proposed, numerically analyzed and tested. The corresponding K values are calculated by finite-element method and fracture toughness values of polyethylene pipes material are obtained. __________ Translated from Problemy Prochnosti, No. 1, pp. 146–149, January–February, 2008.     

Rate-dependent fracture toughness of pure polycrystalline ice

A series of three-point bend tests using single edge notched testpieces of pure polycrystalline ice have been performed at three different temperatures (–20°C, –30°C and –40°C). The displacement rate was varied from 1 mm/min to 100 mm/min, producing the crack tip strain rates from about 10^–3 to 10^–1 s^–1. The results show that (a) the fracture toughness of pure polycrystalline ice given by the critical stress intensity factor (K _IC) is much lower than that measured from the J—integral under identical conditions; (b) from the determination of K _IC, the fracture toughness of pure polycrystalline ice decreases with increasing strain rate and there is good power law relationship between them; (c) from the measurement of the J—integral, a different tendency was appeared: when the crack tip strain rate exceeds a critical value of 6 × 10^–3 s^–1, the fracture toughness is almost constant but when the crack tip strain rate is less than this value, the fracture toughness increases with decreasing crack tip strain rate. Re-examination of the mechanisms of rate-dependent fracture toughness of pure polycrystalline ice shows that the effect of strain rate is related not only to the blunting of crack tips due to plasticity, creep and stress relaxation but also to the nucleation and growth of microcracks in the specimen.     

Fracture toughness of some metallic glasses

The critical stress intensity factor of Fe₄₀Ni₄₀B₂₀, Fe₃₀Cr₁₀Ni₄₀B₂₀, Ni₈₀Si₁₀B₁₀ and Ni₈₀Si₅B₁₅ metallic glass ribbons was measured. Stressing by ultrasound, a new method for preparation a sharp crack in a metallic glass, was used. Only shear rupture failures was investigated for all alloys.     

Fracture toughness of multiferroic composite materials

Fracture toughness of unidirectional ferromagnetic fiber reinforced ferroelectric matrix composite was studied based on the energy approach in a view of large scale. A half space edge crack with crack face perpendicular to the external fields was considered. The energy release rate was derived explicitly considering the magnetoelectric coupling under combined mechanical, electric and magnetic loading. Because of the magnetoelectric coupling through the interface, the fracture toughness is highly dependent on the polarization properties of the ferroelectric and ferromagnetic portions besides the volume fraction and the elastic properties of each composite.     

Fracture toughness of mung bean gels

Mung bean starch gels, of 4.5-14 wt % solid content, were stored at room temperature and at 4°C in a refrigerator and then cracked quasi-statically by driving a 40° included-angle wedge into intact specimens in order to determine their fracture toughness. The work to fracture of the gels, calculated without respect to energy loss due to viscoelasticity or to frictional effects between wedge and gel, varied from 0.5 to 22 J m^–2 and were higher for those gels stored at low temperature. For gels stored at room temperature, the effect of viscoelasticity and wedge-gel friction was examined. Hysteresis (viscoelastic energy losses) was concentrationdependent. In 8 wt % gels, it accounted for about 10% of the total work done in the wedge tests and did not depend significantly on crosshead speed. Frictional work, largely due to adhesion between the gel and the wedge, was negligible at low speeds but increased rapidly with crosshead speed. However, whether correction factors are introduced or not, the results substantiate the very low fracture toughness of gels. Between 5 and 11 wt % concentrations, the work of fracture varied linearly with gel concentration. Variation in crosshead speed from 2 to 200 mm min^–1 increased the work to fracture by a factor of two.