Critical Fracture Toughness Research Articles

Comparative sintering study on non-conventional and commercial silicon nitride powders

The pressureless sintering and hot-pressing behaviour of three experimental and three commercial silicon nitride powders have been investigated using an MgO–Y2O3 sintering aid. Powders were characterised in terms of their bulk and surface compositions, β-Si3N4 content and morphology.Densification behaviour is assessed on the basis of linear shrinkage, green and final densities, sintering rate, phase composition and microstructure of the dense material. Mechanical properties of the densified materials are evaluated in terms of microhardness and critical fracture toughness.

Crack growth on elastic-plastic bimaterial interfaces

Finite element calculation based on finite strain theory is carried out to simulate the crack growth on bimaterial interfaces under the assumption of small scale yielding and plane strain condition. The modified Gurson's constitutive equation and the element vanish technique introduced by Tvergaard et al. are used to model the final formation of an open crack. The crack growths in homogeneous material and in bimaterials are compared. It is found from the calculation that the critical macroscopic fracture toughness for crack growth J IC is much lower in bimaterials than in homogeneous material. For bimaterial cases, the J IC of a crack between two elastic-plastic materials which have identical elastic properties with different yield strength is lower than that of a crack between an elastic-plastic material and a rigid substrate. It seems that the difference in yield strength between the dissimilar materials has more significant influence on the void nucleation and crack growth than the difference in hardening exponent.

Evaluation of fracture toughness of HSLA80 steel at high loading rates using stretch zone measurements

There is increased interest in using the stretch zone width (SZW) as a parameter for evaluation of the fracture toughness of ductile metallic materials. This approach is particularly useful in the case of fracture at high loading rates where evaluation of fracture toughness criteria such as J Ic from compliance measurements is not feasible. In the current study, specimens of HSLA80 were fractured at very high loading rates, corresponding to K larger than 10 6MPa m 1 2 s −1 using a modified Split Hopkinson Bar system. These tests were performed on compact tension specimens of 12.6 mm thickness at −30°C. Other experiments were performed using a drop weight tester on specimens of 25 mm thickness at −40, −20, 0 and 20°C. The stretch zone width was determined using scanning electron microscopy. It was found that the steel retains a high fracture toughness at −30°C, albeit lower than at 20°C. In measurement of the SZW, for the 12.6mm specimens there was no significant difference between using a nine-point approach (ASTM) and a three-point approach, while for the 25 mm specimens, the nine-point approach gave a much lower value.

Elastic-plastic crack growth on plane strain bimaterial interface

Finite element computation are carried out to simulate plane strain crack growth on a bimaterial interface under the assumption of small scale yielding. The modified Gurson constitutive equation and the element vanish technique introduced by Tvergaard et al. are used to model the final formation of an open crack. It is found from the calculation that the critical fracture toughness for crack growth is much lower in bimaterials than that in homogeneous material. The critical fracture toughness is strongly dependent on material properties of the bimaterial pair and the mixed mode of remote loads. The interface crack grows in the more compliant (lower hardening) material or in the weaker (lower yield strength) material. In Mode-I loading, the crack grows zigzag along the interface.

Micromechanics and acoustic emission analysis of the failure process of thermoplastic composites

The finite element method (FEM) and acoustic emission technique (AE) were applied to the micromechanics analysis of the failure process of composites with thermoplastic matrix materials. FEM calculations to local stress-strain distribution and the influence of very different intermediate layer properties are interpreted with regard to microscopic failure mechanisms in composite materials. The strongly differing AE behaviour of both chalk-filled Polyvinylchloride and high density polyethylene and short-glass-fibre reinforced polypropylene, polyamide, PBTP, SAN and ABS in tensile test experiments is demonstrated. Representative loading limits are derived from the nature and extent of the dominating failure mechanisms by comparison of theoretical and experimental results. The influence of critical strain, shear strength and fracture toughness properties of the modified matrix as well as the composite morphology and phase adhesion on significant deformation and failure stages is discussed. Finally some conclusions are drawn about a possible critical long-term strain of composites.

Proposal on the Method of Fixing Proof Strength for Ceramic Components

A method is proposed for determining proof strength of ceramic components based on fracture behavior. The strength for short time fracture mainly depends on the sizes of initial defects in the ceramics, but it cannot be uniquely determined by the constant fracture toughness criterion which is well known for applicability to brittle materials. For static fatigue behavior, a static fatigue limit below which cracks are arrested is observed. Both short-time fracture strength and static fatigue limit are shown to have nonlinear relationships to defect size. By considering these dependences, the method gives proof strength which is effective in designing ceramic components.

Mixed Finite Element Method for End‐Notched Flexure Test

To permit more efficient use of the experimental data in the end notched flexure (ENF) test, a mixed finite element method is used to model the macromechanical situation with a small numer of four-noded quadrilateral elements. This small number of elements is made possible by the development of two types of specialty finite elements (constant shear-strain bending element and delamination bending element) within the Hu-Washizu variational principle. With these elements it is possible to satisfy exactly the kinematic boundary conditions of the ENF test—especially at the point of zero slope. Assuming linear elastic fracture mechanics, the critical interlaminar fracture toughness (\IG\N\dI\dI\I\dc\N) is found at one critical load by calculating the difference between the strain energy at the initial crack length and that for an incremental increase in crack length. For a 32-ply, unidirectional, linear transversely isotropic laminate of varying thickness, the values for \IG\N\dI\dI\Idc\N calculated by this finite element model compared well with the results obtained from a previously developed mixed experimental and structural model.

Fracture behavior of electronic ceramics

Abstract This paper reviews the fracture behavior, e.g., the strength, fracture toughness and susceptibility to environmentally enhanced crack growth, in electronic materials such as barium titanate, lead zirconate titanate (PZT), capacitor ceramics, and the new, high temperature superconductors. All of these materials are known to exhibit critical fracture toughness, KIC , which depends on grain size, chemical composition and crystal structure. The microstructure of these electronic ceramics has a direct effect on both their strength and fracture toughness. The paraelectric to ferroelectric phase transformation in barium titanate, PZT, and related ceramics induces internal stresses into the materials which can provide an additional driving force for flaw extension. The magnitude of such stresses calculated from strength data is in good agreement with that predicted by dielectric measurements. Crack-domain interactions in the ferroelectric structure as well as crack deflection, etc. give rise to increased...

Evidence of a thin sheet toughening mechanism in Al−Fe−X alloys

A toughening mechanism, dubbed thin sheet toughening, is proposed for improving the fracture resistance (KIC and tearing modulus) of powder-metallurgy alloys of limited ductility. The basis of this approach is the recognition that internal delamination of thick-section components or cracked specimens into thin sheet ligaments in the fracture process zone leads to a drastic reduction in triaxial stresses, with the consequence of enhancing the critical fracture strain, fracture toughness (KIC orJIC), and tearing modulus. Theoretical analyses indicate that a factor of\(\sqrt 3 \), increase in theKIC value, and even greater increases in the tearing modulus are possible for idealized conditions. The predicted results are compared with experimental results of tensile,KIC, andJ tests conducted on four powder-metallurgy Al−Fe−X alloys at 25 and 316°C. The comparison reveals that thin sheet toughening is a contributor to the highKIC value observed in a state-of-the-art Al−Fe−V−Si alloy. Increasing the critical strain to fracture is also shown to be a possible method to improve the fracture toughness of Al−Fe−X alloys, independent of the thin sheet toughening effect.

Combined stress hypothesis for mixed-mode fracture toughness criterion

A new hypothesis is presented for determining the mixed-mode fracture toughness criterion of brittle materials such as rocks, cement and ceramics. The obtained criterion gives a reasonable result, which is in good agreement with published data. The data, obtained here for mixed-mode fracture toughness of flyash eement paste, confirm also the use of the hypothesis for the problem of fracture toughness, including negative K IC .

Effects of Composition and Environment on the Fracture of Fluoride Glasses

Indentation‐crack‐length procedures were used to determine the critical fracture toughness (KIC) and the sensitivity to environmentally enhanced crack growth in heavy‐metal fluoride glasses of varying chemical composition. The data show that while KIC was more or less invariant with composition, some glasses were more susceptible to subcritical crack growth than others. The results are interpreted in terms of existing crack‐growth models. A technique for predicting KIC from fundamental atomic bonding and crystal‐structure data for these glasses is discussed.

Relation between work of fracture and fracture toughness of short-fibre reinforced polymers

On the basis of micromechanical failure mechanisms acting in short-fibre reinforced polymers we derive the crack resistance curve ( R- rmcurve) as a function of crack extension. With the conditions characterizing the point of crack instability, the critical crack extension and fracture toughness, G c, are calculated. By comparing G c to the work of fracture we are able to conclude that, depending on the relevant failure characteristics, two parameters are necessary to describe the failure behaviour—the fracture toughness for crack instability or strength and the work of fracture as the energy necessary to drive the crack through the sample.

A personal computer based system to evaluate J-integral by a single specimen unloading compliance method—part II: Results on A533B class I steel and corona-5

A PC-based single specimen unloading technique was described in Part I of the paper. In this paper the applicability of the technique is demonstrated using two different materials: a nuclear grade pressure vessel material, A533B Class I steel and an α- β titanium alloy, CORONA-5. Experimental parameters investigated in the case of steel include the effect of test temperature, loading rate and neutron irradiation. Results revealed the effect of dynamic strain aging (DSA) as a drop in the critical crack initiation fracture toughness ( J q ); the minimum in J q shifts to higher temperatures at higher test speeds as predicted from DSA models. In CORONA-5, the effects of microstructure and heat treatment for two different sizes of equiaxed α morphology in a β-matrix heat treated to different yield strengths were investigated. It is found that the observed higher values of J q and tearing modulus result from a greater tortuosity of the crack path and ease of crack blunting. The largest increase in J q occurred in fine equiaxed α microstructure aged to a nominal yield strength of 896 MPa. Results reveal potential application of the low cost technique for determining fracture toughness of engineering materials.

Evaluation of Intergranular Fracture Initiation in Transition Region of Retired Steam Turbine Rotor Steel Using Small Specimens and the Acoustic Emission Technique

Abstract Fracture toughness tests were performed at 150°C in the transition region for a Cr-Mo-V steel using four different sizes of compact tension specimens and with the use of the acoustic emission (AE) technique. Two fracture toughness parameters were obtained to evaluate the intergranular mode fracture toughness: one is the critical fracture toughness of KJC calculated from Jc, the other is a fracture toughness of KJAE characterized by the onset of microscopic pop-in cracking determined by AE. The values of KJC and KJAE of the smaller specimens show a great deal of scatter. Combining the fractographic studies, the source of toughness scatter is discussed in terms of the loss of plastic constraint. The results show that the lower bound values of KJAE in the smaller specimen sizes are coincident with a valid fracture toughness, KIC(AE), of the large specimen. An accurate evaluation of the intergranular mode fracture toughness is also discussed.

MIXED MODE FRACTURE CRITERION BY COMBINED STRESS HYPOTHESIS AND ITS EXPERIMENTAL VERIFICATION

Taking account of a combined singular stress state near a crack tip, we present a new hypothesis for determining fracture toughness criterion of rock-like materials under mixed mode I and mode II loading. The combined stress hypothesis gives a reasonable KIIc/KIc which is in good agreement with published data on rock-like materials. Finally, the mixed mode fracture criterion derived by the hypothesis is tested by the measurement of KI and KII of flyash cement paste using the cracked disk specimens. The results are presented to confirm the use of this hypothesis for the problem of mixed mode fracture toughness including negative KIc.

Mechanical behavior of 18 Ni 200 grade maraging steel at cryogenic temperatures

A comprehensive study was conducted to characterize the mechanical behavior of 18 Ni 200 grade maraging steel for cryogenic applications in the National Transonic Facility. Tensile, fatigue, impact, and fracture toughness properties were determined at room and cryogenic temperatures. Experimental variables included product form, specimen orientation, and variation in metallurgical condition accomplished with a grain refining heat treatment. Room temperature tensile yield strength was approximately 200 ksi, while the yield strength at -275 F was approximately 260 ksi. Charpy V-notch energy absorption values and fracture toughness values of 18 Ni steel were found to be dependent on temperature, product form, orientation, and metallurgical condition. Fracture toughness values ranged from 80 to 113 ksi-sq rt in. at room temperature and from 67 to 90 ksi-sq rt in. at -275 F. The results of this study indicate that 18 Ni 200 grade maraging steel exceeds the strength criterion of 150 ksi minimum yield strength for use in cryogenic wind tunnel models but is marginal with respect to the 85 ksi sq rt in fracture toughness criterion at -275 F.

Subcritical crack growth in partially stabilized ZrO2(MgO)

The room temperature slow crack growth resistance in air (≈ 50% relative humidity) and in water for large cracks in MgO-partially stabilized zircoba (PSZ) improves with increase in critical fracture toughness,Klc. Ageing the as-fired PSZ at 1400° C for 8 h results in decreasing Klc from 8.5 MPa m1/2 to 6 M Pa M1/2. The ageing treatment also promotes the growth of eutectoid decomposition products on grain boundaries that is accompanied by a decrease in the dependence (A the change in Region I crack velocity with a change in the applied stress intensity. Calculated times to failures are markedly decreased in the aged as compared to the as-fired PSZ ceramic.

Fracture Mechanics Evaluation of Irradiation Embrittlement in Reactor Vessel Steels Based on the Rate Process Concept

Abstract A new evaluation procedure for the critical fracture toughness (KIc, KId) from Charpy-V impact absorbed energy is presented. The strain-rate/temperature effect on the cleavage-controlled fracture toughness transition is first explained by applying the concept of a stress-dependent rate process for fracture initiation. The expression described in terms of the Charpy-V energy ratio, Cv-us/Cv, for fracture toughness under a constant strain rate is newly derived. The feasibility of the new procedure for evaluating the irradiation embrittlement of reactor pressure vessel steel, such as SA533B-1, is here verified utilizing the data for both unirradiated and irradiated materials in the literature. Another method for the determination of coefficients in the new expression from the Cv-fitting curve is also presented.

Fracture mechanics approach to evaluation of strength in sintered silicon nitride

The evaluation of strength of ceramics is an important problem in their application to engineering components. Although fracture initiating from inherent flaws in ceramics is supposed to be analyzed through linear elastic fracture mechanics, the applicability of a plane-strain fracture toughness criterion is in dispute.

Improvements in Mechanical Properties in SiC by the Addition of TiC Particles

Silicon carbide ceramics containing up to 24.6 vol% dispersed TiC particles yielded fully dense composites by hot‐pressing at 2000°C with 1 wt% Al and 1 wt% C added. The micro‐structure consists of fine TiC particles in a fine‐grained SiC matrix. Addition of TiC particles increases the critical fracture toughness of SiC (to ∼6 MPa·m1/2 at 24.6 vol% TiC) and yields high flexure strength (≥680 MPa), with both properties increasing with increasing volume fraction of TiC. The strengths at high temperatures are also improved by the TiC additions. Observations of the fracture path indicate that the improved toughness and strength are a result of crack deflection by the TiC particles.