Dynamic fracture toughness parameters for HY-80 and HY-130 steels and their weldments

Lower bound dynamic fracture toughness parameters for HY-80 and HY-130 steel and their weld metals are identified. Specific values of the parameters K_Id and K_Im obtained from direct measurements are reported together with estimates inferred from the large body of Charpy energy, nil ductility transition temperature and dynamic tear energy measurements. The emphasis is on reasonable lower bound values at 30°F, the lowest anticipated service temperature, for use in elastodynamic analyses of crack growth initiation, propagation, and arrest in ship structures. For these conditions, it has been found that the ratio K_Id/ σ_Y is approximately equal to 2 in^1/2 for HY-80 steel. For HY-130 steel and the HY-80 and HY-130 weld metals under these same conditions, K_Id/ σ_Y is approximately 1 in^1/2. Consequently, HY-80 plate appears to be substantially more resistant to fracture under dynamic loading than are the other three grades examined.     

Kic Transition-temperature behavior of A517-F Steel

Linear-elastic fracture mechanics has been widely used to obtain K_ic values on very-high-strength steels (yield strengths > 200 ksi) that generally do not exhibit a ductue-to-brittie transition in failure mode as a function of temperature. However, as the use of the K_ic test approach is extended to those steels that do exhibit a ductile-to-brittle transition, information on the K_ic transition-temperature behavior of steels is required. Therefore, to establish general relations between K_ic and Charpy test results, slow-bend K_ic fracture tests and various Charpy tests were conducted on A517-F steel at temperatures between −320 and +80°F.

The results indicated that a plane-strain K_ic température transition does exist for A517-F steel. Furthermore, this transition occurred in the same temperature range (−150 to −50°F) as the transition denned by slow-bend Charpy test results for fatigue-cracked specimens. In both the K_ic tests and the Charpy tests, the transition-temperature behavior appeared to be related to a gradual change in the microscopic fracture mechanism. The upper shelf, as denned by slow-bend Charpy tests, appeared to be a region in which K_ic values cannot be obtained, regardless of specimen geometry, because of general yielding and crack blunting.

A procedure is proposed in which the dynamic K_ic behavior of a material can be predicted from static K_ic test data by shifting the static K_ic values along the temperature axis by the same amount as the static Charpy energy values are shifted by impact testing.

In general, the results of this investigation have demonstrated that a transition in K_ic behavior of A517-F steel does exist as a fution of temperature and that that transition is independent of the K_ic to K_c stress-state transition.     

A model for predicting fatigue crack growth behaviour of a low alloy steel at low temperatures

In the present study, a model to predict the fatigue crack growth (FCG) behaviour at low temperatures is proposed for a low alloy steel (16 Mn). The experimental results indicate that fatigue ductile-brittle transition (FDBT) occurs in 16 Mn steel and the FDBT temperature (T_FDBT) is about 130 K. When T > T_FDBT, the FCG mechanism in the intermediate region is the formation of ductile striation and the FCG rates decrease with decreasing temperature. When T ≈ T_FDBT, the FCG mechanism changes into microcleavage and the fatigue fracture toughness K_fc of the steel decreases sharply. The FCG rates tend to increase as the temperature is further reduced. The test data of the FCG rates are well fitted by the formula developed by Zheng and Hirt. An approximate method to predict ΔK_th of the steel at low temperatures is proposed and then a general expression of the FCG rates is given at temperatures ranging from room temperature to T_FDBT. By means of the expressions proposed in this paper, the FCG rates at low temperatures can be predicted from the tensile properties if the endurance limit σ₋₁ and δk_th, at room temperature are known. Finally, a model for FDBT is tentatively proposed. Using this model, one can predict T_FDBT from the ductile-brittle transition curve determined from impact or slow bending tests of cracked Charpy specimens.     

Analysis of the temperature dependence of the fracture stress of sharp notched bending specimens of a structural steel with respect to intermediate transitions

The flow and fracture stresses, σ_y^g and σ_f^g (δ = crack tip displacement), of sharply notched bending specimens of a structural steel U St 37-1 are measured in the temperature range from full scale to small scale yielding. The best adaption of the experimental results for σ_f^g is obtained by a curve which exhibits an intermediate transition, i.e. which follows in a temperature range between an upper, T_tM¹ and a lower, T_tl¹, transition temperature to the curve σ_y^g(T) for the flow stress with a constant δ = δ₁. This transition corresponds to that of the slip to the twin nucleated fracture. Two analyses [3,5] according to the local fracture stress, σ_f^*, concept show that the amount and the temperature dependence of σ_f^* are somewhat different for both methods, but that both exhibit an increase of σ_f^* in the transition range. It is concluded that each transition in the nucleation mode of the fracture is connected with such a transition in the fracture stress. It may, however, become indistinct or even be covered by the scatter of the experimental points.     

Measurement of continuous damage parameter

The present paper introduces several measuring methods of continuous damage parameter derived by the classical definition D = 1 − (A_e/A_a): (1) The measuring method on the basis of D = 1 − (E'/ E); (2) The measuring method on the basis of D = 1 − (ε₁/ε₂); (3) The measuring method on the basis of D = −Δρ_o/ρ_o, (4) The measuring method on the basis of D = A_d/A_a, and comments on these measuring methods.     

A new diagram for evaluating the relationship between convective and evaporative heat transfer during the cooling of carcasses

During the cooling of a carcass, both its heat losses by convection and by evaporation are governed by the convective heat transfer coefficient c. While both phenomena occur at the same temperature difference δ_w between the surface of the carcass and the cold air, the quantity of heat flux of the former part becomes _cδ_w, and the latter part becomes the product of _c multiplied by an equivalent temperature difference δ_D which is a function of the vapour pressure differences between the wet surface and the cold air. The ratio δ_D/δ_w can be established from the diagram and the heat loss by convection and by evaporation can be calculated for any given value of _c. Consequently, the anticipated theoretical weight loss of the carcass can be calculated.     

Influence of interface modification and phase separation on damping properties of epoxy concrete

A preliminary study has been conducted to investigate the influences of interface modification and rubber phase separation on damping behaviour of epoxy concrete (EPC). The main parameters studied here are loss factor (η), i.e. damping, and dynamical modulus of elasticity (E_d). It is found that the tenacity of EPC can be improved by controlling rubber phase separation, and that the mechanical vibration can be absorbed while keeping a high enough E_d value by forming an elastic interlayer using a coupling agent and acrylonitrilebutadiene rubber with end carboxyl group. The values of η and E_d obtained in this paper are in the range of 0.0436–0.0534 and 20.97–26.72 GPa, respectively. The interface microstructure of EPC has been characterized by means of XPS and FTIR here. The results suggest that a weak Lewis acid-base reaction has taken place and chemically bonded joints have formed in the interlayer of EPC, and the rubber phase separation has been characterized by the technique of microimage analysis.     

An empirical relation between yield stress and general yield load for a charpy U-notch specimen

An empirical expression for obtaining the dynamic yield stress, σ_yd, from the general yield load, p_gy, of a Charpy U-notch specimen has been experimentally derived assuming: (i) σ_yd is the same for both Charpy U- and V-notch specimens; and (ii) the functional relations between P_GY and σ_yd for both the geometries are the same except for a dimensionless proportionality constant.     

Exploring the fracture toughness KIE and KIC of a medium-Strength steel plate using the surface-flaw test

The fracture toughness of a 30 CrMnSiA steel plate of three thicknesses (10,8 and 5 mm) and three widths (110,80 and 56 mm) has been investigated by using surface-flaw method under room temperature. It is not easy to compute the value of K_IE by the maximum applied load. But the values of K_IE and K_IC could be obtained easily, if the computation of the conditional applied load P₁₀ and P₅ based on the relative effective extension Δa/a₀ = 10% and 5% were adopted, together with the conditions of P_max/P₁₀ 1.2 and P_max/P₅ 1.3. The K_R — Δ_a curve, i.e. the resistance-curve described by the parameter K, has been plotted. The values of K_IC and K_IE are then the resistances corresponding to the real extensions of flaws of Δ/a₀ = 2 and 7%, respectively. These values so obtained are in good agreement with the computed values of K_IC and K_IE by using the conditional applied loads. The values of K_IC and K_IE so obtained are also in agreement with the value of K_IC converted from the J-integral and the effective value of K_IE computed by the maximum applied load, respectively.

An approximate relation between K_IC and K_IE has been found to be: K_IC = (0.85˜0.95)K_IE.

The requirements for the dimensions of specimens are: Thickness of plate: B 1.0(K_IC/σ_0.2)² or 1.25(K_ICσ_0.2)²]; Width of plate: 8 W/B 10, 4 W/2c 5; Effective length: l 2W.     

Growth of lithium doped silver niobate single crystals and their piezoelectric properties

Lithium doped silver niobate (Ag_1−xLi_xNbO₃, 0 < x < 0.1) is one of the candidate materials for lead-free piezoelectric materials. In this study, Ag_1−xLi_xNbO₃ single crystals were successfully grown by a slow cooling method. Crystal structure was assigned to perovskite-type orthorhombic (monoclinic) phase. Dielectric properties were measured as a function of temperature. As a result, with increasing lithium contents, the phase transition at around 60 °C was shifted to lower temperature while the phase transition at around 400 °C was shifted to higher temperature. On the basis of these peak shifts, the lithium contents in Ag_1−xLi_xNbO₃ single crystals were determined. Moreover, P–E hysteresis measurement revealed that pure silver niobate crystal was weak ferroelectrics with P_r of 0.095 μC/cm² while Ag_0.9Li_0.1NbO₃ (ALN10) crystal was normal ferroelectrics with P_r of 10.68 μC/cm². About this ALN10 crystal, polling treatment was performed and finally piezoelectric properties were measured. As a result, high electromechanical coupling coefficient k₃₁ over 70% was observed.     

Magneto-mechanical properties evolution of Fe–C alloy during precipitation process

Evolution of magneto-mechanical properties of 160 ppm Fe–C alloy due to carbides precipitation during isothermal annealing at 473 K (up to t=50×10³ s) was studied by means of classical Barkhausen noise (HBN) and mechanical Barkhausen noise (MBN) effects. The MBN was measured for the torsion mode of load with a torque motor. Also the B(H) hysteresis loop and the coercive field H_c were evaluated using a low-frequency magnetisation set. Magnetic hysteresis losses ΔW₁ were compared with the integral ΔW₂ of HBN intensity over one period of magnetisation and the integral ΔW₃ of MBN intensity over one period of the mechanical load. The internal stress distribution function and the resulting mean level of internal stress parameter σ_i were evaluated from the MBN ‘first load’ data. It was revealed that a correlated increase of ΔW₁ and ΔW₂ parameters exists. However, the relative increase of ΔW₃ is much lower than the relative increase of ΔW₁. The relationship between H_c and σ_i was found to be parabolic. This dependence explained by Néel’s model of the impact of the residual stress level on H_c. The presence of precipitates of type was confirmed by scanning electron microscopy.     

The elastically equivalent softening zone size for an elastic-softening material: II. A simple piece-wise softening law

The paper determines the elastically equivalent softening zone size R_E for an elastic-softening material when there is a semi-infinite crack in a remotely loaded infinite solid; the parameter R_E plays a central role in size effect expressions that are used to correlate the maximum loads that can be sustained by solids having different dimensions. The stress (p) versus displacement (v) softening law considered is that for which P = p_c for oc and P = ; qp_c = p_* for λδ_c< v <δ_c. Particular attention is focussed on the case where the parameters λ and q are both small. Such a softening law simulates the softening behaviour of plain concrete, where there is an initially rapid softening to a low stress value at a small displacement, followed by a long tail that is associated with a low stress. The paper shows that the behaviour of a material with such a softening law can be conveniently analysed by assuming that there is a non-zero stress intensity (K_IC) at the crack tip followed by a constant stress p_* within the softening zone; , where E_o is the reduced modulus and G_{I = λpcδc} is the contribution to the specific fracture energy arising from the initially rapid softening region. Analysis of a specific model demonstrates the viability of this approach by showing that there is consistency with the proposed size effect expressions based on R_E.     

Fracture mechanisms of ceramic coating during wear

A typical buckling phenomenon of the coating on the wear groove caused by the residual compressed stress was analyzed by the interface fracture mechanics and the buckling theory. It has been found that there is a critical thickness of coating on the wear groove for the buckling. The critical thickness can be calculated by t_b/c_d = [12(1 − v²_f)σ_R/π²E_f]1/2 (here t_b is the coating thickness, c_d the length of the interfacial crack, v_f the Poisson's ratio of the coating, σ_R the residual compressed stress in the coating, and E_f the elastic modulus of the coating).     

Some problems in the J-integral

For a blunt crack the j-integral is path dependent on contours which are very close to the crack tip even for elastic material. Using the incremental J-integral theory we introduce a new parameter J_t, characterizing the behavior of a crack tip and prove that the J-integral is almost path independent on contours whose radii are greater than several COD if σ_ij,1Δε_ij — ε_ijΔσ_ij = 0 in plastic regions for elasto-plastic material.     

Temperature Dependence of Local Structure of Rb3C60 Superconductor

EXAFS studies on Rb₃C₆₀ are made as a function of temperature from 6K to 300 K. It is found that the distance r_Rb-c and the second, third and fourth cumulant terms of Debye-Waller factors, σ(2), σ_c(3) and σ_c(4), show the anomalous behavior near T_c.     

Some physical properties of ZnO sputtered films

The secondary electron emission (SEE) yield δ of ZnO films was investigated. The films were deposited in an r.f. sputtering system using the r.f. power W, the argon pressure p, the O₂ partial pressure p_O2 and the substrate temperature T_s as parameters. Complementary measurements of the electrical resistivity and the optical absorption were performed. The ratio x of oxygen to zinc is an essential factor which determines the values of δ, and for the ZnO films.

Auger analyses showed that excess (overstoichiometric, x =1) oxygen is present in ZnO films obtained at room temperature. For x =1 the values of , the maximum SEE yield δ_m and the energy band gap E_g (determined from ) were found to be higher than those for stoichiometric ZnO (obtained at T_s200 °C). The highest values of (10⁴ Ω m), δ_m (4.4) and E_g (3.44 eV) were obtained for films with x = 1.7.     

Optical transitions near the band edge in bulk CuInxGa1−xSe2 from ellipsometric measurements

From the analysis of the variation of optical absorption coefficient with incident photon energy between 0.8 and 2.6 eV, obtained from ellipsometric data, the energy E_G of the fundamental absorption edge and E_G′ of the forbidden direct transition for CuIn_xGa_1−xSe₂ alloys are estimated. The change in E_G and the spin-orbit splitting Δ_SO=E_G′−E_G with the composition x can be represented by parabolic expression of the form E_G(x)=E_G(0)+ax+bx² and Δ_SO(x)=Δ_SO(0)+a′x+b′x², respectively. b and b′ are called “bowing parameters”. Theoretical fit gives a=0.875 eV, b=0.198 eV, a′=0.341 eV and b′=−0.431 eV. The positive sign of b and negative sign of b′ are in agreement with the theoretical prediction of Wei and Zunger [Phys. Rev. B 39 (1989) 6279].     

The static and dynamic low-temperature crack-toughness performance of seven structural steels

To properly describe the crack-toughness behavior of steels in a quantitative manner, a study was undertaken to establish the effects of strain rate and low temperature on the K_ic values of seven structural steels. Steels having room-temperature yield strengths ranging from 40 to 250 ksi-ABS-C, A302-B, HY-80, A517-F, HY-130(T), 18Ni(180), and 18Ni(250) steels-were evaluated for static and dynamic loading over the range of temperatures for which K_ic values were attainable.

The results indicate that for the ABS-C, A302-B, HY-80, and A517-F steels, an increase in strain rate of approximately six orders of magnitude caused a decrease in the K_ic values measured at the same test temperatures. No significant effect was observed for the HY-130T and 18NI(250) steels. However, the most significant effect of the increased strain rate was the increase in the threshold temperature below which plane-strain behavior occurred.

When all steels — except the 18Ni(180) maraging steel, for which insufficient valid data were obtained were compared on the basis of equivalent critical flaw-size behavior, the crack-toughness performance in terms of _ic/σ_ys for dynamic loading could be separated into three groups. The HY-80 and HY-130(T) steels were best, the ABS-C, A302-B, and A517-F steels were intermediate in performance, and the 18Ni-(250) maraging steel was the poorest. These groupings of performance prevailed over a relatively wide range of test temperatures. As a means of accounting for the differences in strain rate, the K_ic/σ_ys values for all steels investigated were plotted in terms of the rate-temperature parameter, Tln A/ε, which superimposed most of the static crack-toughness performance data into these same levels of performance. In addition, the results of the investigation substantiated the interpretation that the nil-ductility-transition temperature measured in the drop-weight test is the upper limit of dynamic plane-strain crack-toughness behavior for 1-in.-thick plates.

In general, the results of the present investigation provide a quantitative comparison of the plane-strain crack-toughness performance of 1-in.-thick plates of seven structurel steels under both static and dynamic loading conditions. Because of the increase in temperature range over which K_ic behavior occurs with increased strain rate, dynamic loading can be an especially significant factor in the performance of structural steels, particularly those having yield strengths less than approximately 140 ksi.     

Spectroscopic ellipsometry analysis of GaAs1 − xNx layers grown by molecular beam epitaxy

In this work, we present the effect of nitrogen incorporation on the dielectric function of GaAsN samples, grown by molecular beam epitaxy (MBE) followed by a rapid thermal annealing (for 90 s at 680 °C). The GaAs_1 − xN_x samples with N content up to 1.5% (x = 0.0%, 0.1%, 0.5%, 1.5%), are investigated using room temperature spectroscopic ellipsometry (SE). The optical transitions in the spectral region around 3 eV are analyzed by fitting analytical critical point line shapes to the second derivative of the dielectric function. It was found that the features associated with E₁ and E₁ + Δ₁ transitions are blue-shifted and become less sharp with increasing nitrogen incorporation, in contrast to the case of E₀ transition energy in GaAs_1 − xN_x. An increase of the split-off Δ₁ energy with nitrogen content was also obtained, in agreement to results found with MOVPE GaAs_1 − xN_x grown samples.     

On the effect of sampling volume on the microscopic cleavage fracture stress

Reported observations of an experimental variation in the microscopic fracture stress for transgranular cleavage (σ^*_f) with specimen geometry and size are quantitatively examined in terms of a weakest-link statistical model for brittle fracture, wherein failure coincides with the critical propagation of a particle microcrack into the matrix. By analysing the onset of fracture in the ‘sharp-crack’ (K_Ic) specimen, the ‘rounded-notch’ (Charpy) specimen, and the uniaxial tensile specimen, it is shown that values of σ^*_f are reduced progressively in the ‘sharp-crack’, notched and unnotched geometries and with increasing specimen size, consistent with an increase in statistical sampling volume. Quantitative predictions for the magnitude of this variation are given for a low strength steel.