Fracture of notched polycarbonate under hydrostatic pressure

The brittle fracture behaviour and plastic deformation of round-notched polycarbonate bars subjected to three-point bending under hydrostatic pressure have been studied. Below a certain critical pressure, the brittle fracture initiated from an internal craze nucleated at the tip of the local plastic zone ahead of the notch rooT. The position of the nucleation of the craze receded from the tip of the notch with increasing applied pressure. When the pressure was increased over a critical value, general yielding occurred by passage of the plastic zone across the notched cross-section, that is, the brittle to ductile transition took place. A qualitative analysis of the stress distribution within the plastic zone explains that the brittle to ductile transition under hydrostatic pressure occurs when the general yield takes place before a critical stress for brittle crack propagation is reached.     

Fracture of a plasticized epoxide under superposed hydrostatic pressure

The deformation and fracture behaviour of rubber-coated and uncoated epoxy specimens has been studied under superposed hydrostatic pressures extending to 300 MN m^–2. Maximum shear stress at yield for this epoxy were about 25 MN m^–2 at atmospheric pressure and rose to about 48 MN m^–2 at 300 MN m^–2 superposed pressure. Yielding and failure of all specimens tested beyond pressures of 75 MN m^–2 took place when all the (macroscopic) principal stresses, though unequal, were compressive. Fractographic examination revealed three distinct zones of the failure surfaces at atmospheric pressure. The behaviour of all uncoated specimens and those coated and tested below 100 MN m^–2 was similar. A fracture-mechanics interpretation of failure could be applied to these tests assuming the deformation-produced first zone was the fracture initiating site. Coated samples tested beyond 100 MN m^–2 superposed pressure failed with no evidence of Zones II or III of failure; Zone I appeared to spread over the entire failure surface. An interpretation involving fluid penetration of Zone I failure nuclei, along the lines suggested by Duckett, can account for the failure stresses of the uncoated specimens but is not tenable for the coated samples. It appears that crack nucleation and (slow) growth, as opposed, perhaps, to (catastrophic) crack propagation, can take place in this polymer when all the principal stresses are compressive.     

Fracture behaviour of unmodified and rubber-modified epoxies under hydrostatic pressure

The fracture toughness and uniaxial tensile yield strengths of unmodified and CTBN-rubber-modified epoxies were measured under hydrostatic pressure. The purpose of these experiments was to learn how suppressing cavitation in rubber particles affects the deformation mechanisms and the fracture toughness of rubber-modified epoxy. It was found that the cavitation of CTBN-rubber could be suppressed at a relatively low pressure (between 30 and 38 M Pa). With cavitation suppressed, the rubber particles are unable to induce massive shearyielding in the epoxy matrix, and the fracture toughness of the rubber-modified epoxy is no higher than that of the unmodified epoxy in the pressure range studied. Unmodified epoxy shows a brittle-to-ductile transition in fracture toughness test. The reason for this transition is the postponement of the cracking process by applied pressure.Work performed while on a sabbatical leave at the University of Michigan.     

Fracture toughness diagrams of a notched body

To describe fracture toughness diagrams of notched bodies, a model of the cohesion zone near the notch root and an averaging criterion of stresses in this zone were employed. The geometric stress concentration factor and biaxiality coefficient affect greatly the shape of fracture toughness diagram. The notch root critical stress intensity factor is a decreasing function of geometric stress concentration factor. __________ Translated from Problemy Prochnosti, No. 5, pp. 142–148, September–October, 2006. Report on International Conference “Dynamics, Strength, and Life of Machines and Structures” (1–4 November 2006, Kiev, Ukraine).     

Fracture resistance of notched specimens

This paper gives a theoretical approach to the general problem of the static fracture resistance in the presence of stress concentrations. First of all, an approaching calculation model is proposed for elastic and plastic stress in the reduced section of a circumferentially notched cylindrical bar. The introduction of a normal stress criterion or of an axial strain fracture criterion on the elastic plastic interface makes it possible to explain the experimental results covering the fracture of various steels above a certain sharpness.     

The impact fracture behaviour of notched specimens of polycarbonate

The impact fracture behaviour of notched specimens of polycarbonate has been studied for a range of notch tip radii. For razor-notched specimens a simple fracture toughness analysis is appropriate, as shown by previous workers. Very blunt notches also give constant fracture toughness values, but at a much higher level, corresponding to a different mode of failure. For intermediate notch tip radii the situation is much more complex, and comparison of results for two molecular weight grades shows that the behaviour is molecular weight-dependent. Analysis of these results has been discussed either in terms of a combination of plane strain and plane stress fracture modes, or in terms of a critical stress at the root of the notch, which appears to be appropriate in certain cases.     

Equipment for creep testing under tensile stress and hydrostatic pressure

The design and operation of equipment is described that permits creep tests to be made under independently controlled tensile stresses and superimposed hydrostatic pressures at elevated temperatures. The range of usefulness and applications of the equipment are briefly indicated.     

Crystallization of glassforming melts under hydrostatic pressure and shear stress: Part I Crystallization catalysis under hydrostatic pressure: possibilities and limitations

This is the first part of a thorough study of the kinetics of melt crystallization under applied static pressure, P, and under shear stress. The thermodynamic and kinetic consequences of increased external pressure on nucleation rate, non-steady-state time lag, rate of crystal growth and overall crystallization kinetics in undercooled melts are analysed. Two types of undercooled liquids (with either positive or negative volume dilatation upon crystallization) are considered. Particular attention is given to the effect of pressure on the specific interface energy, σ, at the crystal/melt phase boundary. Using an appropriate thermodynamic model it is shown that for one-component systems, (∂σ/∂p)<0 is to be expected as a rule. Thus an additional decrease of the thermodynamic barrier of nucleation in pressurized melts is to be expected. However, it is also shown that the increase of melt viscosity with pressure in most cases reduces the effect of this decrease. Thus increased pressure has a limited effect as a nucleation catalyst. The possibilities in this respect are analysed and conditions under which static pressure may lead to enhanced crystallization are outlined. This revised version was published online in November 2006 with corrections to the Cover Date.     

The mechanism of brittle fracture in notched impact tests on polycarbonate

The impact testing of notched polycarbonate bars that are thick enough to yield in plane strain has been investigated. Shear bands occur in the plastic zone that resemble the slip line field for yielding from a circular notch. Eventually, an internal craze nucleates at the tip of the plastic zone, where the stresses are highest, and a crack forms in the thickest part of the craze. Above –15 C the stress for the craze to nucleate is a nearly constant multiple of the yield stress. It is shown that previous observations that annealing polycarbonate causes a ductile to brittle transition is a consequence of testing bars of thickness less than 5 mm.     

Transitions between crazing,fracture and yield under hydrostatic pressure

A simple model is proposed to explain transitions from crazing or fracture to shear yielding and vice-versa with increasing superposed pressure. The model is based on an estimate of the local tensile component of stress in the vicinity of a flaw, distinguishing between the different physical situations which arise depending on whether or not the pressurizing medium penetrates the flaw. The model explains naturally many observations of specimens failing in a brittle manner in nominally compressive stress fields.     

Fracture behavior of notched specimens of TiAl alloys

Fracture behavior of a two-phase TiAl alloy was investigated using notched specimens. Fracture surfaces and metallographic sections of surviving notch in double notched specimens are observed. The fracture process of notched specimens of TiAl alloys was described as that several inter-lamellar cracks initiate and extend directly from the notch root and propagate preferentially along the interfaces between lamellae and stop at various obstacles. With increasing applied load, cracks connect with each other and propagate further by translamellar cracks. The toughening mechanisms, which make the main crack difficult to propagate or cause it to be stopped, could be reducing the driving force for crack propagation. The higher toughness of near fully lamellar microstructure than that of finer duplex microstructure is attributed to the path of crack propagation. On the fracture surfaces of the finer duplex microstructure, more low-energy-spending interlamellar fracture facets are observed, which means that it is easier for crack to bypass a fine duplex lamellar grain with lamellae perpendicular to the main crack and to take a interlamellar path.     

Fracture resistance determination of freshwater ice using a chevron notched tension specimen

Crack growth resistance (R-curve behavior) measurements on large grained S1 type freshwater ice were conducted in the Ice Mechanics Research Laboratory at Clarkson University. To overcome previous difficulties in obtaining stable cracking in freshwater ice, a new crack geometry was developed. The short rod Chevron Notched Tension (CNT) specimen was found to be extremely favorable in the sense of promoting stable,stick-slip, crack growth over a large portion of the uncracked ligament. A negative fracture resistanceK _R -curve was evaluated for this ice at -16°C.     

Fracture behavior of notched unidirectional Si-Ti-C-O/BMAS composite

Experimental and modeling studies on tensile fracture behavior of notched unidirectional Si-Ti-C-O (Tyranno fiber) reinforced BMAS (barium magnesium aluminosilicate) glass matrix composite were carried out. The longitudinal crack arose at the tip of the transverse notch before overall fracture. The critical energy release rate around at initiation of the longitudinal cracking was estimated to be nearly 100 J/m² by application of the present model to the experimentally observed relation between the stress of the composite in the very early stage of longitudinal cracking and the notch size. The notched strength was higher than that predicted by the fracture mechanical criterion due to the blunting arising from the premature longitudinal cracking, but it was lower than that predicted by the net stress criterion due to the constraint effect arising from the bridging of the fibers and the spalling of the segmented matrix into the longitudinal crack.     

Deformation of a carbon-epoxy composite under hydrostatic pressure

This paper describes the behaviour of a carbon-fibre reinforced epoxy composite when deformed in compression under high hydrostatic confining pressures. The composite consisted of 36% by volume of continuous fibres of Modmur Type II embedded in Epikote 828 epoxy resin. When deformed under pressures of less than 100 MPa the composite failed by longitudinal splitting, but splitting was suppressed at higher pressures (up to 500 MPa) and failure was by kinking. The failure strength of the composite increased rapidly with increasing confining pressure, though the elastic modulus remained constant. This suggests that the pressure effects were introduced by fracture processes. Microscopical examination of the kinked structures showed that the carbon fibres in the kink bands were broken into many fairly uniform short lengths. A model for kinking in the composite is suggested which involves the buckling and fracture of the carbon fibres.List of symbols d diameter of fibre - E _f elastic modulus of fibre - E _m elastic modulus of epoxy - G _m shear modulus of epoxy - k radius of gyration of fibre section - l length of buckle in fibre - P confining pressure (= ₂ = ₃) - R radius of bent fibre - V _f volume fraction of fibres in composite - _t, _c bending strains in fibres - angle between the plane of fracture and ₁ - ₁ principal stress - ₃ confining pressure - _c strength of composite - _f strength of fibre in buckling mode - _n normal stress on a fracture plane - _m strength of epoxy matrix - shear stress - tangent slope of Mohr envelope - slope of pressure versus strength curves in Figs. 3 and 4.     

Stable homogeneous microdischarge at atmospheric pressure between a flat cathode and point anode

Conditions of stable operation of a homogeneous glow microdischarge in air at atmospheric pressure between a flat cathode and point anode are established and realized at interelectrode gap widths within ～1–30 μm and discharge currents within from ～10^?4 to 1 A.     

低温下高强钢缺口板断裂试验

为研究低温下结构钢断裂机理及抗断设防,在冷脆转换温度下(-45 ±2℃)对的个16mm厚Q345钢材缺口板进行了单向拉伸断裂试验,研究了缺口尺寸(a、2b)对结构钢断裂模式的影响.结果表明:试件断裂于刻痕心部,断口心部有一条清晰的起裂棱,宏观断面大致垂直于加载方向;试件缺口愈尖锐或缺口比率(2b/a)愈小,断裂延性愈差...     

A multichannel discharge between a stream electrolytic cathode and a metal anode at atmospheric pressure

An experimental investigation is performed of the structure and current-voltage characteristics (CVC) of a multichannel discharge (MD) between a stream electrolytic cathode (SEC) and a metal anode, as well as of the density of SEC and anode current in a wide range of values of current I = 0.02–10 A, interelectrode spacing l = 5–50 mm, and anode diameter d _a = 5–40 mm. The development of cathode spots on the SEC surface is studied. It is found that a cathode spot of SEC may have various shapes and structures (regular circle, horseshoes, filamentary spots, and so on). The CVC of a multichannel discharge between SEC and metal anode depends significantly on I, l, and d _a , as well as on the material of the anode and on the composition and concentration of electrolyte. The dependence of the density of SEC current on discharge current is nonmonotonic. It is for the first time that a bundle-like MD between SEC and metal anode and multichannel and diffuse discharges between the plasma region and stream electrolytic cathode are observed, and their characteristics are investigated.