Influence of notch root radius on ductile rupture fracture toughness evaluation with Charpy-V type specimens

The blunt notch fracture toughness of four types of carbon-manganese steel (ASTM A516 grade 70) has been determined by J-integral tests on Charpy-V type samples with different values of notch root radius, ρ. J-ρ plots, determined using specimens with a notch depth to width ratio, a/w, equal to 0.5, have shown the existence of a limiting ρ value (ρ_eff) below which applied J-intergral values at fracture initiation are constant. These ρ_eff values have been seen to depend only on second-phase particle distribution and not on their volume fraction or on the steel ferritic grain size. The procedure for deriving J-integral values at the onset of stable crack growth from J resistance curves in the case of notches has also been discussed. Experiments with Charpy specimens with a/w = 0.2 do not allow the derivation of meaningful J-ρ plots. In all cases, a ductile fracture criterion based on the constancy of the notch tip strain at rupture initiation has been proved when ρ >ρ_eff.     

Dependence of ductile fracture toughness of a weld metal on notch root radius and inclusion content

Using different fluxes of high and low basicity, two different welds have been produced with low and high inclusion content. Inclusions have been characterized in 2 and 3 dimensions, from measurements on both polished sections and on fracture surfaces. The fracture toughness is evaluated with J0integral, crack tip opening displacement (CTOD) and stretch zone width measurements. Ductile fracture satisfies the criterion of a critical fracture strain over a characteristic distance. These two critical values can be determined from the results obtained with blunt notch specimens. Characteristic distances are an order of magnitude larger than the inclusion spacing. This is attributed to the need to reach conditions of sustained crack growth for initiation of ductile tearing.

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Résumé A l'aide de flux différents de forte et faible basicité, on a produit deux soudures à faible et forte teneur inclusionnaire. Les inclusions sont caractérisées en 2 et 3 dimensions par des mesures sur coupes polies et sur les surfaces de rupture. La ténacité à la rupture est évaluée par l'intégrale J, l'ouverture critique en fond de fissure (CTOD) et la mesure de la zone d'étirement. La rupture ductile satisfait la condition qu'une déformation critique de rupture soit atteinte sur une distance caractéristique. Ces deux valeurs critiques peuvent être déterminées à partir des résultats obtenus sur éprouvettes avec rayon d'entaille. Les distances caractéristiques sont un ordre de grandeur plus grandes que les distances inter-inclusionnaires. On attribue cette différence à la nécessité de satisfaire les conditions de croissance soutenue de la fissure pour l'initiation de la rupture ductile.

     

Synthesis and characterization of polycrystalline sintered compacts of cubic boron nitride

Synthetic cubic boron nitride (CBN) compacts are an important tool material used extensively for the machining of hardened steels. The paper describes work on the synthesis and characterization of CBN compacts. A 200 tonne cubic press has been used for the generation of high pressures. Solid solutions of TiN and TiC have been used as the binder material. The CBN powder and the binder are homogenously mixed and the mixed powder is pressed in a steel die under a pressure of about 3 kbars (300 M Pa). The pellets so formed are the starting material for synthesizing the compacts. Compacts both with and without a tungsten carbide substrate have been synthesized. The best compacts are formed at 58kbar (5.8 G Pa) and 1450° C. The sintered compacts after grinding and polishing are characterized by using powder X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis. Knoop hardness measurements made on the best CBN compacts give a hardness of 3400HK _0.5.     

Shock compressibility and spallation strength of cubic modification of polycrystalline boron nitride

Methods of shock-wave physics are used to perform experimental investigations of the elastoplastic and strength properties of cubic modification of polycrystalline boron nitride. The samples are prepared by pressing to a pressure of 7–8 GPa at a temperature of 1700–1800 °C and rank just a little below diamond in hardness. The measurements are performed under conditions of the samples being acted upon by plane shock waves with the pressure behind the front of up to 85 GPa and duration of ～10^?6 s. It is demonstrated that a two-wave configuration consisting of an elastic precursor and a plastic compression wave is formed at the amplitude of shock compression of over 60 GPa. The dynamic yield strength, determined by the amplitude of elastic precursor, depends on the structure of samples and varies in the range from 31 to 49 GPa. The dynamic strength, measured under conditions of pulsed tension in expansion waves, is likewise defined by the structure of samples and, in the elastic region of deformation, lies in the range from 0.7 to 1.6 GPa.     

High-pressure sintering of cubic boron nitride

Cubic boron nitride (cBN) powder was compacted at high pressure and high temperature using a solid solution of titanium carbide and titanium nitride (TiC _x N_1–x ) as binding material in the presence of a small amount of aluminium. Different compositions of (TiC _x N_1–x ), 0<x<1 were used as binders. The weight percentage of cBN, TiC _x N_1–x and aluminium were optimized and found to be critical; any marked deviation from these optimized values deteriorated the quality of compacts. Various high-pressure sintering parameters such as pressure, temperature and sintering time, etc, were optimized for this binder. The compacts were also characterized using X-ray diffraction, scanning electron microscopy and microhardness measurements. The microstructural and X-ray diffraction observations indicated no marked changes in the compacts as the value ofx in TiC _x N_1–x was varied, but the microhardness was found to depend on the value ofx.     

Synthesis of cubic boron nitride with boron nitride powder formed from triammoniadecaborane

Cubic BN was synthesized under high temperature and pressure conditions from BN powder formed by reaction of triammoniadecaborane (TAD) with ammonia. The BN powder formed from TAD and ammonia had a low degree of ordering. The crystal lattice of the BN powder increased in regularity with increasing synthesis temperature and time for the reaction of TAD with ammonia. The conversion yield of cubic BN at 1300 °C and 6.5 GPa in the presence of AIN increased with decreasing of reaction temperature of TAD and ammonia from 1000–700 °C. Cubic BN decreased in yield with increasing reaction time of TAD and ammonia at 800 °C. BN powder pre-heat treated at 1550 °C had a crystallite size,L _c, of 22 nm, and was converted to cubic BN in a 43% yield at 1300 °C and 6.5 GPa for 10 min. The activation energy for cubic BN synthesis from BN powder-20 mol% AIN was 97 kJ mol^–1, when the starting BN was synthesized at 800 °C. The conversion yield of cubic BN from the disordered BN-20 mol% AIN was 100% after heat treatment at 1300 °C and 6.5 GPa for 20 min.     

Effect of initial notch orientation on fracture toughness in fail-safe steel

A 0.4C–2Si–1Cr–1Mo steel bar with an ultrafine-elongated grain (UFEG) structures was produced by multi-pass warm caliber rolling. The test sample was machined from the rolled bar with 0°, 45°, and 90° rotation along the rolling direction, and a static three-point bending test was conducted at ambient temperature. The toughness anisotropy on the steel with UFEG structures were studied, including the crack propagation on the basis of the microstructural features. The strength and toughness decreased with an increase in the rotation angle along the rolling direction. The toughness decreased drastically, compared to the strength. The notch orientation dependence on toughness is due to differences in the spatial distribution of weak sites such as {100} cleavage planes and boundaries of elongated grains. For the toughness design in ultrafine-grained materials, it is essential to understand the spatial distribution of these weak sites as well as the grain size.     

Influence of notch radius and R-curve behaviour on the fracture toughness evaluation of WC-Co cemented carbides

In this work the apparent fracture toughness of different grades of WC-Co cemented carbides has been evaluated by means of the single edge V-notched beam (SEVNB) method. The results are analysed, as a function of both the binder mean free path and the maximum steady-state crack shielding value, following a theoretical approach which assumes the existence of a small crack at the notch tip. The experimental findings indicate that larger defects at the notch tip and/or existence of R-curve behaviour imply higher (i.e. less restricted) values of the minimum notch radius required in SEVNB specimens for a reliable assessment of the fracture toughness of cemented carbides.     

Shock compaction of cubic boron nitride powders

C-BN powders with different grain sizes were dynamically compacted by explosive shock loading using approximate peak pressures from 33 to 77G Pa. The density and the microhardness of the resulting c-BN compacts were strongly dependent upon the grain size of the c-BN powders used as the starting materials. The best c-BN compacts, with 98% of the theoretical density and microhardness of 51.3G Pa, were obtained from the coarse c-BN powder (40 to 60m). In the compacted fine c-BN powder (2 to 4m) conversion of the c-BN to low density forms of BN at a residual temperature degraded the interparticle bonding significantly. X-ray line-broadening analysis of the compacted c-BN powders indicated that the residual lattice strain increased with the increase in grain size of the starting powder, while the crystallite size was independent of the grain size.     

Special features of the applications of cutting tools from polycrystalline cubic boron nitride with protective coatings

The results of the investigations on the efficiency of cutting tools from polycrystalline superhard materials based on cubic boron nitride with a protective coating. A hypothesis is proposed for the increase of the cutting tool life in turning hardened steel due to a decrease of the temperature in the cutting zone and a complex of physico-mechanical properties of the protective coating required for this is analyzed. A protective coating of boron nitride is considered in the amorphous state that plays a role of a solid lubricant in the zone of the tool contact with cutting chips, decreases the temperature in the contact zone through the reduction of the contact length and cutting force, ensures an increase of the tool life and reliability, especially at the stage of the run-in.     

立方氮化硼薄膜表面的XPS研究

研究立方氮化硼薄膜表面的性质对于研究立方氮化硼薄膜的成核机理和应用,具有重要的价值.本文用XPS对立方氮化硼薄膜表面进行研究,并对有关问题进行了讨论.XPS分析表明,立方氮化硼薄膜表面除了B、N外,还含有C和O.从XPS谱图计算得到含有立方相的氮化硼薄膜的N/B为0.90,较接近于氮化硼的理想化学配比11;不含立方相的氮化硼薄膜的N/B为0.86,离氮化硼的理想化学配比11较远.计算表明立方氮化硼薄膜的顶层六角相的厚度约为0.8nm.     

Effect of notch root radius on the initiation and propagation of fatigue cracks

Under the conditions of constant nominal applied stress, increasing notch root radius causes an increase in the number of cycles to initiate a fatigue crack at a notch root. An explanation of the effect is given in terms of the effective stress concentration factor of the notch. Data are presented which indicate that variations in notch root radius may cause changes in the crack growth rate during the initial stages of propagation from a notch.     

Study of influence of notch root radius on fracture behaviour of extra deep drawn steel sheets

Fracture tests are carried out on extra deep drawn steel CT specimens containing notches with different values of notch root radius (ρ= 0.07–0.75 mm). Experimental findings clearly show a critical notch root radius (ρ_c) below which the fracture toughness remains independent of ρ and above which it varies linearly with ρ. The 3D finite element analysis shows that the location of maximum stress level causing crack initiation is in the vicinity of notch tip. The maximum stress level is independent of ρ; however, its location is shifted away from notch tip along unbroken ligament length with increase in ρ.