A fractographic criterion for subcritical crack-growth boundaries in hot-pressed alumina

The percent intergranular fracture (PIF) was measured along radii extending from fracture origins in hot-pressed alumina specimens, fractured at various loading rates and temperatures, and plotted versus estimates of stress intensity factors (K _I) at the various crack lengths. Minima in PIF occur at values ofK _I that are close to the critical stress intensity factors (K _IC) for cleavage on various crystal lattice planes in sapphire. The subcritical crack-growth boundary (K _I=K _IC of the polycrystalline material) occurs near the primary minimum in PIF suggesting that this minimum can be used as a criterion for locating this boundary. In addition, it was noted that the polycrystallineK _IC (4.2 MPa m^1/2) is very close to theK _IC for fracture on {¯1 ¯1 2 6} planes which is 4.3 MPa m^1/2. These observations suggest that critical crack growth begins when increased fracture energy can no longer be absorbed by cleavage on these planes. There is a secondary minimum atK _I>K _IC that appears to be associated with theK _IC necessary for fracture on combinations of planes selected by the fracture as alternatives to the high fracture-toughness basal plane.     

On scattering of measured values of fracture toughness parameters

The values of fracture toughness K _1C of C-Mn steels and weld metal were calculated from values of COD measured in standard specimens tested at low temperatures. The analysis of scattering of 42 values of K _1C measured in normalized C-Mn steel showed that the highest value of K _1C could be as large as 360 percent of the lowest. The factors affecting the scattering were investigated in detail and it was found that the scattering was mainly caused by the variation of locations of cleavage initiation. The local fracture stress σ was found to be the most stable parameter and combined with the minimum cleavage distance min which is determined by the triaxiality of stress reaching a critical value, it could be used to characterize the lower boundary of fracture toughness of steels. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of welding thermal cycles and cold working on fracture toughness of SN490 steel under static and dynamic loading

Abstract

The static fracture toughness K _IC and dynamic fracture toughness K _Id of SN490, its pre-strained steel, and its welding heat affected zone were measured. K _IC tests were conducted according to the ASTM standard, and K _Id tests were carried out on an instrumented Charpy impacting machine. The experimental results were used to determine the effects of welding thermal cycle and cold working. It was found that both welding heat input and cold working are harmful to the fracture toughness of SN490 steel under both static and dynamic loading. The deleterious effects are serious under static loading. The detrimental effect of welding heat input during submerged arc welding was found to be more significant than that of the 10% plastic prestrain.     

Deformation and fracture of cork in tension

Various properties related to the deformation and fracture of cork in tension were experimentally determined, including the Young's modulus, the stress and strain at fracture and the fracture toughnessK _Ic. The transverse isotropy of cork implies that there are three independent systems of mode I crack propagation andK _Ic was measured for each. The mechanisms of deformation and fracture were identified by SEM microscope observation ofin situ deformation and of the fracture surfaces and crack paths. Two fundamental mechanisms of fracture occur: crack propagation along the lateral cell walls in non-radial tension, withK _Ic = 94±16 kPam^1/2 and crack propagation by breaking the cell walls in radial tension withK _Ic=125±14 kPam^1/2. In radial tension, local fractures that do not propagate due to crack stopping were observed which lead to serrations in the tensile curves for that direction. The strain to fracture in this direction is considerably larger than in the perpendicular (non-radial) directions.     

Influence of nonmetallic inclusions on fatigue crack growth in a structural steel

A study of the fatigue behaviour of a hardened and tempered steel, at two inclusion levels, has been carried out according to the linear elastic fracture mechanics criteria. The influence of inclusions on the fatigue crack growth rate has turned out to be a function of the local stress intensity factor range,K _I, at which fracture propagates. At lowK _I values, to which are related crack growth rates less than 10^–5 mm cycle^–1, the crack growth rate in the steel with higher inclusion content is lower than in the steel with lower inclusion content. AsK _I increases, an inversion in the difference between the two rates occurs. In the dirtier steel, the higherK _I, the higher the growth rate than in the other steel. The difference between the two rates becomes nil just below the fast propagationK _Ic level. By fractographic analysis, it has been possible to find out how inclusions affect fatigue behaviour.     

Study on cleavage fracture criteria of the quasi-brittle and micro-inhomogeneous materials

On the bases of recent achievements on the micro-mechanism of cleavage this paper analyses the inherent deficiencies of the stress intensity factor K _I which is used to evaluate the fracture toughness of quasi-brittle and micro-inhomogeneous materials. The K _I parameter can uniquely determine the field intensity ahead of a crack tip in the condition of elastic and small scale yielding (SSY). However, the K _I cannot uniquely determine the critical condition triggering the cleavage fracture in a quasi-brittle and inhomogeneous steel where the cleavage fracture process is not a direct extension of the precrack but is initiated at a variable distance from the precrack tip. The variable distances of cleavage initiations invoke varied critical values of K _I. On the bases of authors' experiments, this paper analyses the physical meaning of the local fracture stress _f, its stability and the feasibility to be used as an engineering parameter for assessing the fracture toughness.     

Thickness effect on fracture in high impact polystyrene

The effect of thickness on the fracture behaviour of a high-impact polystyrene containing approximately 7% rubber is studied. For thicknesses below 10 mm plane stress ductile tearing occurs and deep edge notched tension specimens are used to obtain the specific essential work of fracture (w _e) in plane strain. Mixed mode plane strain-plane stress fracture is predominant in single-edge notched tension specimens with thicknesses above 10 mm. By assuming that the plane stress layers are given by the overall fracture toughness (K _c) a modified bimodal fracture analysis based on linear elastic fracture mechanics concepts is presented to analyse the experimental results. The plane strain fracture toughnessG _c1 (=K _c1 ² /E) is in good agreement withw _e. It is shown thatK _c1 for HIPS is larger than that of the polystyrene matrix alone due to the toughening effect of the rubber at the crack tip vicinity.     

Comparative study of the fracture toughness determination of a polymer-bonded explosive simulant

In this paper, a comparative study was carried out by using three-point bending, semi-circular bending, and flattened Brazilian disc tests to measure the plane-strain fracture toughness of a polymer-bonded explosive simulant. The K_IC magnitudes are 0.53 ± 0.03 MPa m^1/2, 0.50 ± 0.02 MPa m^1/2, and 0.53 ± 0.01 MPa m^1/2 respectively, showing good consistency in the results measured by different methods. Considering the difficulties in the machining and pre-crack fabrication of brittle explosives, the flattened Brazilian disc test is recommended as an alternative method for measuring the K_IC of PBX and other explosives. In addition, real-time microscopic observation of test samples was conducted by using a scanning electron microscope (SEM) equipped with a loading stage, and the fracture surfaces of samples were examined. The damage modes and the corresponding failure mechanisms were analyzed.     

Effect of alloying additions onK ISCC of ultrahigh strength NiSiCr steel

The effect of alloying additions viz. cobalt, molybdenum, cerium and a combination of cobalt and molybdenum, on theK _ISCC of NiSiCr steel in 3·5% NaCl aqueous solution was studied. Addition of cobalt to NiSiCr steel resulted in an increase in theK _ISCC whereas molybdenum addition decreased theK _ISCC. Cerium addition did not affect theK _ISCC while the combination of cobalt and molybdenum resulted in an increase in theK _ISCC although not as much as in the case of cobalt addition. The effect of alloying elements onK _ISCC could be attributed to their effect on the critical fracture stress and yield strength.     

DYNAMIC CRACK PROPAGATION AND CRACK ARREST INVESTIGATED WITH A NEW SPECIMEN GEOMETRY: PART II: EXPERIMENTAL STUDY ON A LOW-ALLOY FERRITIC STEEL

The novel experiment developed in our Institute to investigate crack initiation, rapid crack propagation and crack arrest with one specimen, the ring test, was applied to a ferritic HSLA pipeline steel. The maximum crack speed achieved in these experiments was between 230 m/s and 1165 m/s. The fracture toughness at crack arrest, K_Ia, was determined by a static analysis of this specific test. In all cases, it was found that K_Ia was much lower than K_Ic. The values of K_Ia decrease when the maximum crack speed increases, the results being largely scattered. The fracture toughness at crack arrest is therefore not an intrinsic parameter of the material for a given temperature. Cleavage fracture obtained under these conditions is characterized by the existence of numerous cleavage microcracks, mechanical twins and unbroken ligaments. The decrease in fracture toughness when crack speed increases is related, using the Beremin or the RKR model, to the high strain rates at the tip of a rapid propagating crack. A model which takes into account the effect of unbroken ligaments left in the wake of a propagating crack is developed to account for the large values of K_Ia which were occasionally measured.     

Evaluation of fracture toughness of interface between weld overlay and base metal in hydrocracker reactor

Abstract

The fracture toughness K _1C of Cr–Mo engineering steels and susceptibility to temper embrittlement can be assessed by several different methods. The method employed in the present research for determination of K _1C was use of Charpy V notch impact data combined with application of the Rolfe–Barsom–Novak equation in the upper shelf region of a 3Cr–1Mo steel. For this purpose, specimens from an aged hydrocracker reactor were cut at the interface region between the stainless steel weld overlay and the base metal, such that the root of the V notch was located at the interface. All specimens were de-embrittled, and some were then subjected to a step cooling operation. Hot tensile tests were additionally carried out on the material. Assessment shows that the step cooling operation increases the transition temperature by about 65–100 ° C, while K _1C in the interface region is decreased by 55%.     

Determination of valid R-curves for materials with large fracture toughness to yield strength ratios

Crack growth resistance curves are derived from a generalised theory of quasi-static crack propagation due to Gurney and Hunt. Both the subcritical and continuous cracking regions are investigated, where the fracture toughness of the material may depend on the cracking rate, the reacting environment at the crack tip and the mode of fracture. Precise conditions for stability of the spreading crack relative to chosen constraints of either a displacement- or load-controlled machine are formulated. Cracking of sheet materials with high fracture toughness and low yield stress, (e.g. (K/ _y )² > 200 mm), which do not satisfy certain size requirements, is often complicated by generalised yielding at regions remote from the crack tip. Complete R-curves for such materials cannot be established with conventional testpieces in the laboratory. The present paper adopts a new experimental technique [1] where a laboratory size reinforcement rig attached to the testpiece eliminates all irreversibilities caused by generalised yielding. Valid fracture toughness values and crack growth resistance curves are thereby determined, irrespective of the amount of elastic and plastic deformations occurring at the crack tip. Successful R-curve experiments are described for fracture in a few ductile and tough materials such as 7075-T3. and 1100-0 aluminium alloys, and a low carbon steel. Comparison is made with other published R-curves, and the influence of sheet thickness and (K _1c / _y ) ratio on the geometry of R-curves is investigated. A simple relationship for R-curves is suggested, viz.: R = R ₀ + (L) ^p , where, it seems, R ₀can be identified with the plane strain toughness (i.e. R ₀ = G _1c = K _1c ² /E(1 - v ²)1/2). A possible reason for this unexpected result is given in the paper. Useful estimates of K _lcmay thus be available from thin sheet tests.Paper presented in part at the 11th Annual Meeting of the Society of Engineering Science, November 11–13, 1974, Duke University, Durham, N.C., USA.     

Analysis of brittle fracture under the combined stresses of tension and compression

Brittle fractures occurring under biaxial stress states were analysed based on the weakest link model using the mixed mode fracture criterion. Expressions for the mixed mode fracture criterion were chosen for application to the negativeK _l region, corresponding to the compressive stress for the crack. Calculations for biaxial strength with randomly oriented constant-length cracks from the mixed mode fracture criterion were made in the region ofK _I>0 because an unstable fracture seems to occur in this region. The results indicated that the tensile stress component in the combined tension and compression stress state remains constant when the compressive component is smaller than the critical value, which is given by [1 –(K _c/K _c)²]_t derived from the mixed mode fracture criterion, (K /K _c) + (K /K _c)² = 1. Considering the statistical effects, however, calculation of the biaxial strength is modified to result in: (1) lowering the biaxial tensile strength, and in (2) a smooth transition from the constant tensile strength region to the decreasing strength region under the combined tension and compression stress. This suggests that the highK _IIc/K_Ic ratio results in the increase in the compressive strength relative to the tensile strength.     

Bruchmechanische Charakterisierung des Vergütungsstahls 30 CrNiMo 8 nach verschiedenen Anlaßbehandlungen

Fracture Mechanics Characterisation of the Q & T Steel 30 CrNiMo 8 after Different Heat Treatments The investigations show that the fracture toughness K_Ic of the Q & T steel 30 CrNiMo 8 for large structural members is more sensitive to a change of the tempering treatment than the Charpy energy A_v. For tempering temperatures between 200 and 250°C the fracture toughness K_Ic shows maximum values with K_Ic ? 87 MN/m^3/2, R_m = 1800 MN/m² and R_p0,2 = 1400 MN/m² at room temperature. With these values this steel may be an inexpensive alternative to the maraging steels, for example to 18 Ni 300 (X 2 NiCoMo 18 9 5) with K_Ic = 76 MN/m^3/2 at R_m = 2100 MN/m² and R_p0.2 = 2000 MN/m² (aged 4 h 480°C). Elastic-plastic fracture toughness values K_Ji were determined at onset of stable crack growth with specimens that fail after more or less large plastic deformations because the thickness of specimen is too small or test temperature and tempering temperature, respectively are too high. These values are essentially independent of specimen size, if some minimum size conditions are fulfilled, and they are in agreement with the linear elastic fracture toughness K_Ic. But the test of more especially large linear elastic failing specimens may clarify the uncertainty whether this procedure delivers conservative results in all cases.     

Thermodynamical approach to the brittle fracture of dry plasters

The evolution of the fracture toughness, K _lc, and fracture energy, G _lc, of set plasters was determined on notched beams as a function of sample porosity, P, and characteristic size, W. Toughness was found to decrease with decreasing crack width. For set plasters of 57.7% porosity, the lowest toughness measured was K _lc=0.13 MPa m^1/2 for a crack width of 0.2 mm. For this crack width, fracture toughness and fracture energy linearly changed with porosity: K _lc=0.5 1–1.3 P) MPa m^1/2 and G _lc= 13.47 (1–1.12 P) Jm^–2. Dense plasters were more difficult to break than porous ones. The fracture energies were affected by the velocity of the fracture propagation, which induces damaging and multicracking of the material, so that the roughly calculated chemical surface energy of set plaster was too high. After correction it was estimated to be 0.4 J m ^–2. Finally, because toughness increased with increasing sample size, it was concluded that fracture toughness and energy were not intrinsic parameters of the material. On the other hand, for our sample porosities and sizes, the reduced rupture force, F _rupt W ^–0.65 is a constant and seems to be a characteristic parameter of the mechanical resistance of set plaster beams.     

Very high cycle fatigue properties of high‐strength spring steel 60SiCrV7

The very high cycle fatigue properties of spring steel 60SiCrV7 for automotive suspension system with different hydrogen contents were studied by using ultrasonic fatigue testing and fatigue crack growth testing. The results show that the S–N curves exhibit continuous drop of fatigue lives and no obvious horizontal line exists. Similar fracture surface features were observed for all the specimens that failed mainly from internal inclusions with surrounding granular bright facet (GBF). Fatigue strength decreases remarkably with increasing hydrogen content. The applied stress intensity factor range at the periphery of GBF ΔK_GBF is approximately proportional to 1/3 power of the square of GBF area. The average values of ΔK_GBF for uncharged specimens are close to crack growth threshold ΔK_th, which indicates that ΔK_GBF could be regarded as the threshold value governing the beginning of stable fatigue crack propagation. The increase of hydrogen content tends to reduce ΔK_GBF.     

THE EFFECTS OF MICROSTRUCTURE AND FRACTURE SURFACE ROUGHNESS ON NEAR THRESHOLD FATIGUE CRACK PROPAGATION CHARACTERISTICS OF A TWO-PHASE CAST STAINLESS STEEL

In this study, the effects of stress ratio, microstructure and fracture surface roughness on the fatigue properties of a two-phase cast stainless steel were investigated. This behaviour was examined by means of the fracture mechanics approach and fractography. The fatigue crack growth rate decreased with decreasing stress ratio. The stress ratio markedly influenced the fatigue crack growth rate as ΔK approached the ΔK_th value. The roughness of the fracture surface was greater in the as-cast material than in the heat-treated material. Analysis of the crack growth data using ΔK_eff showed that the effect of R ratio could be explained but that the effect of microstructure on crack growth rate could not.     

Evaluation of a miniaturized disc test for establishing the mechanical properties of low-alloy ferritic steels

A miniaturized disc test has been used to measure load-displacement curves for a 2 1/4Cr1Mo steel, from which are derived values for the Young's modulus of elasticity, yield stress and ultimate tensile strength over the temperature range -196 to 25 °C. The miniaturized test uses disc specimens 3 mm diameter and 0.25 mm thick and a test jig that applies a load via a 1 mm diameter steel ball at a constant displacement rate. The reproducibility of the method has been determined by testing a large number of the 2 1/4Cr1Mo steel specimens at each temperature considered. Comparisons have been made between analytical and empirical methods of evaluating the tensile yield stress and ultimate tensile strength from the measured load-displacement data. In addition, consideration has been given to the total energy to fracture, K*, and the variation with temperature which confirms a transition from low-energy cleavage to high-energy ductile fracture.     

Probabilistic prediction of the crack resistance of nuclear pressure-vessel steels on the basis of a local approach. Part 2

On the basis of a new local probabilistic criterion of brittle fracture, a local criterion of ductile fracture proposed by the authors earlier, and the obtained approximate solution of the problem of stress-strain state near the crack tip, we develop a probabilistic model for the prediction of the crack resistance of pressure-vessel steels. The model enables one to predict the dependence of K _Ic on temperature for any given probability of brittle fracture and the influence of the thickness of the specimen on K _Ic. Bu using this model, we can also describe the temperature range of the brittle-ductile transition. The results of numerical calculations are compared with the experimental data for 15Kh2MFA pressure-vessel steel. It is shown that the proposed model fairly well describes the spread in the experimental data on the crack resistance of this type of steel. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 2, pp. 5–22, March–April, 1999.     

INFLUENCE OF TEMPERATURE ON FATIGUE CRACK GROWTH BEHAVIOUR OF STEELS AT ULTRASONIC FREQUENCY

Fatigue crack growth rates have been measured at 22 kHz for two types of carbon steel, between 20 and 500°C under symmetrical push-pull loading. The crack length was determined from the decrease in the resonant frequency of the specimen. For ?SN 412013 steel, an increased crack propagation resistance was observed between 250 and 300°C. The values of the constants C and n in the Paris equation, and K_a,th are dependent on temperature. The fatigue crack growth rate and C both increase with temperature, while n and K_a,th decrease with increasing temperature. Electron scanning and light microscopy have shown that intercrystalline fracture does not occur in ?SN 415313 steel at elevated temperature. Intercrystalline fracture was observed for specimens fatigued at 20°C and also in ?SN 412013 steel at temperatures of 200 and 500°C. The width of the plastic zone and the height of the surface relief around the fatigue crack increased with temperature.