Brittle crack-arrest fracture toughness in a high heat-input thick steel weld

Brittle crack-arrest fracture toughness (K_Ia) was determined as a function of the temperature in a large-scale (1 x 1 m) 50-mm thick steel base metal and its high heat-input (32 kJ/mm) weld. An impact initiates crack propagation from a notch at low temperature toward a higher temperature region where the crack stopped due to the improved fracture toughness. The relationship between the toughness and crack-arrest temperature provides the K_Ia at ?10 °C of about 100 MPa√m in the weld specimen, which is a significant decrease compared to that of the base metal (240 MPa√m). Furthermore, the path of the crack propagation was discussed in terms of the grain size, hardness, and Charpy impact energy of the localized crack region.     

Temperature effects in the fracture of PMMA

Experiments are described in which the fracture toughness,K _c, of PMMA has been determined in the temperature range –190 to + 80° C and over the crack speed range of 10^–2 to 10² mm sec^–1. Single edge notch tension was used for instability measurements but the other data were obtained using the double torsion method. In the range –80 to + 80°C the variations inK _c may be described in terms of modulus changes and a constant crack opening displacement criterion. Crack instabilities are correlated with isothermal-adiabatic transitions at the crack tip. Below –80° C there is an inverted rate dependence associated with thermal effects during post-instability crack propagation.     

A two-parameter approach to assessing notch fracture behaviour in clay/epoxy nanocomposites

Fracture toughness and other mechanical properties of epoxy are known to be affected by the addition of nanoclays. Fracture toughness has been shown by many researchers to depend on the nanocomposite structure with well-dispersed and distributed nanoparticles resulting in improvements in this property by up to 50%. Notch fracture toughness depends on the acuity of the notch as well as on the structure of the nanocomposite. In the present work, a two-parameter fracture criterion based on a critical notch stress intensity factor, K_ρ,c, and effective T-stress, T_ef, was used to study the effect of notch severity and nanoclay addition on the fracture toughness of the epoxy matrix. The results show that the average value of K_ρ,c for neat epoxy increased with increasing notch radius while the absolute value of T_ef decreased with notch radius. The addition of nanoclay to pristine epoxy reduced the average value of K_ρ,c and increased the absolute value of T_ef. The critical notch radius was found to be around 1.0 mm and the notch sensitivity was higher for neat epoxy. SEM analysis of the fractured surfaces revealed that the lower K_ρ,c for nanocomposites in both mode I and mixed mode fractures was due to early crack initiation at clay clusters or voids at the notch root.     

Notch fracture toughness of high-strength Al alloys

In this study, the notch fracture toughness (NFT) of high-strength Al alloys was examined by a non-standardized procedure. The NFT is defined as the critical notch stress-intensity factor (NSIF) K_ρ,c, which is determined by using several methods of analysis and computing. A set of specimens with different notch root radii made from overaged 7xxx alloy forging was selected. The influence of the notch radius on the fracture toughness of the material was considered. It was found that the notch radius strongly affects the fracture behavior of forged 7xxx alloy in overaged condition. The notch fracture toughness was higher than the fracture toughness of a cracked specimen and increased linearly with notch radius. The critical notch radius was related to the spacing of intermetallic (IM) particles which promote an intergranular or transgranular fracture mechanism according to their size. It appeared that ductile transgranular fracture generated by the formation of dimples around dispersoids and matrix precipitates was predominant which indicates that intense strains are limited to a much smaller zone than the coarse IM particles spacing. This double mechanism is also operate for crack propagation of ductile fatigue. The nature and morphology of IM particles exert significant effects on the rate of fatigue crack growth and fracture toughness properties.     

Fracture toughness and hydrogen embrittlement of pipes with notches

A technique for experimental determination of fracture toughness and hydrogen embrittlement of pipes made of API 5L X52 steel is described. The tests were performed using arc-shaped specimens with a notch cut out from pipes under the conditions of a three-point bend. The fracture toughness was determined in terms of the J-integral and the stress intensity factor at the notch tip. The value of K _ρ,c was established using the volumetric method based on the experimentally measured critical load and the results of the FEM calculation of the distribution of elastic-plastic stresses ahead of the notch tip, and J _ρ,c was determined using the method of separable functions. The effect of hydrogen embrittlement was studied using electrolytically prehydrogenated specimens.     

Influence of specimen size and microstructure on the fracture toughness of a martensitic stainless steel

The fracture toughness of alloy HT-9,² a martensitic stainless steel under consideration for fast reactor and fusion reactor applications, was determined from circular compact tension specimens using the multi-specimen R-curve method. Specimens with thicknesses of 11.94, 7.62 and 2.54mm and widths of 23.88 and 11.94 mm were tested to investigate the effects of specimen size on fracture toughness. The test results obtained from all specimens are in good agreement and thickness requirements for a valid J_1c test are satisfied. The experiment indicates that small specimens of HT-9 may be used for post-irradiation fracture toughness testing.Fractographic examination of the fracture surfaces reveals that fracture in HT-9 is significantly influenced by delta ferrite stringers present in the material. The fracture surface examination and crack opening displacement measurements for specimens tested at various temperatures are consistent with the temperature dependence of the J_1c results.     

The effect of initial notch shape of compact specimen on fatigue pre-cracking and fracture toughness testing

The circular notched compact specimens, along with standard specimens having straight or chevron notch are provided for fatigue and fracture toughness testings in order to study the crack observation capability during fatigue pre-cracking, skewness of the crack front, and the resulting fracture toughness K_Q. The test results indicated that circular notched specimens significantly facilitate the crack observation during fatigue testing as the cracks initiate on both surfaces of the specimen. No remarkable differences were observed on geometries of the fatigue crack front obtained and the resulting fracture toughness among these three types of specimen. The macroscopic observation of beach marks on the fracture surfaces revealed that, in the present material Ti-6Al-4V (ELI), the advance of only 1.3% of the whole crack length corresponded to the load level at which fracture toughness K_Q was evaluated.     

Fracture mechanics of short glass fibre-reinforced nylon composite

The fracture behaviour of injection-moulded short glass fibre-reinforced, thermoplastic nylon 6.6 plaques has been studied under static loading using compact tension specimens and under impact loading using single-edge notched charpy specimens. The influences of specimen position as taken from the plaque mouldings, notch direction, notch sharpness and the rate of testing on the fracture toughness of this composite system were investigated. Results indicated that the fracture toughness is highest for the cracks perpendicular to the mould fill direction and is lowest for cracks parallel to the mould fill direction. A single fracture parameter, K _c, seems to be inadequate for fracture toughness characterization. Evaluation of the fracture toughness as a function of notch sharpness indicated that for notches perpendicular to the mould fill direction the fracture toughness is not affected by the sharpness of the initial notch. However, for cracks in the mould fill direction, sharpness of the initial notch had a significant effect upon the measured value of the fracture toughness. Results also indicated, that the fracture toughness is rate insensitive over the crosshead speed ranging from 0.5–50 mm min^–1. Finally, the specimen position, as taken from plaque mouldings, had no significant effect on the measured value of the fracture toughness.     

Ermittlung des Bruchwiderstands an Oxidkeramik und Hartmetallen mit verschiedenen Methoden

Fracture Toughness Determination of Alumina and Cemented Carbide with Different Testing Methods Fracture toughness of a sintered alumina and two tungsten carbidecobalt materials was determined using four-point-bend specimens with straight through and chevron notches and with the short rod specimen. With the specimens with a straight through notch a lower K_Ic was measured for Al₂O₃ and a higher for WC-Co compared to the chevron-notched specimens. This behavior was explained by the different shapes of the crack growth resistance curves and the different critical notch radii. For Al₂O₃ a steeply rising crack growth resistance curve was measured in a controlled fracture test, for WC-Co a flatter curve was obtained. The effect of the shape of the crack growth resistance curve and of notch width on the evaluated toughness is discussed.     

Experimental analysis of the practical LBZ effects on the brittle fracture performance of cryogenic steel HAZs with respect to crack arrest toughness near fusion line

Focusing on crack arrest behavior, this study investigates the practical influence of local brittle zones (LBZs) on the brittle fracture resistance of heat-affected zones (HAZs) in advanced 9% Ni cryogenic steel welds, and discusses whether the LBZs of this steel in practice have potentially deleterious effects as previously thought, or not. By analyzing the variations in brittle crack arrest toughness (K_a) and brittle crack initiation toughness (K_c) within actual HAZ, it is found that LBZs of this steel may not be harmful in consideration of crack arrest toughness near fusion line.     

Influence of hydrogen on the crack resistance of 10Kh15N27T3V2MR steel

In high ranges of pressure and temperature, hydrogen reduces the thresholds ∆K _th, cyclic fracture toughness K _fc , and short-term static crack resistance K _c of 10Kh15N27T3V2MR austenitic dispersion-hardened steel. Conditions of plane deformation and, correspondingly, conditions of determination of the characteristic K _Ic are maintained in testing in the range 293 – 483 K of compact specimens 20 mm thick after their preliminary hydrogenation to a hydrogen content of 8 ppm. Low-cycle fatigue and sizes of the plastic zone under static crack resistance are most sensitive to hydrogen (and decrease by 80 and 70%, respectively, against those in helium), and the cyclic crack resistance parameters ΔK _th and K _fc (decrease by 25%) are least sensitive to hydrogen. The static crack resistance K _c and the percentage reduction in the cross-sectional area ψ in short-term tension are reduced by 50%.     

Anwendung des instrumentierten Kerbschlagbiegeversuchs zur Abschätzung der Rissauffangzähigkeit

Application of Charpy V‐notch testing to estimate the crack‐arrest toughness Modern structural integrity assessment relies upon fracture mechanics, thus utilizing fracture mechanical parameters describing the material fracture resistance against crack initiation and crack propagation as well as the material crack‐arrest behaviour. However, crack‐arrest fracture toughness values are usually difficult and expensive to determine. In this paper correlations are proposed for estimating the nil‐ductility temperature (T_NDT) and the crack‐arrest fracture toughness (K_Ia) from a transition temperature, based on instrumented Charpy‐V crack‐arrest load information. The transition criteria used are the 4 kN crack‐arrest force and the mean crack‐arrest fracture toughness of 100 MPa√m according to the master curve approach. Correlations between transition temperatures, T(F_a = 4 kN), T(K_Ia), and T_NDT, which were proposed for various structural steels, work very well for the 18Ch2MFA material.     

Hydrogen permeability of niobium during cathodic polarization in hydrochloric acid

Conclusions For high-strength chilled steel, the limit value of the stress intensity factor K^IIIp attained in antiplane strain during short-period fracture by shear is a characteristic that is not sensitive to the effect of an ambient medium such as water and aqueous solutions of sulfuric acid. The possible reduction of short-period bearing capacity of specimens with a crack during loading by mode III takes place only as a consequence of reorientation of the crack into the plane in which the maximum normal stresses act and is associated with reduction in fracture toughness during tear (K_1c) because of the effect of the medium.During long-term corrosion tests in antiplane strain of specimens with a crack, the final act of fracture is almost always completed by tear. In addition, preceding spontaneous fracture, subcritical crack growth is possible by both tear and shear. For certain structural states of steel and ambient media, the latter mechanism and the corresponding high rate of subcritical crack growth may become dominant at low levels of K^III0. This explains the anomalous character of curves of long-term crack resistance observed in these cases.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 13, No. 1, pp. 42–46, January–February, 1977.     

Thickness effect on the mode III fracture resistance and fracture path of rock using ENDB specimens

Study of the thickness effect in predicting the crack growth behavior and load bearing capacity of rock‐type structures is an important issue for obtaining a relation between the experimental fracture toughness of laboratory subsized samples and the real rock structures with large thickness. The fracture of rock masses or underground rock structures at deep strata may be dominantly governed by the tensile or tear crack growth mechanism. Therefore, in this research, a number of mode I and mode III fracture toughness experiments are conducted on edge notch disc bend (ENDB) specimen made of a kind of marble rock to investigate the effect of specimen thickness on the corresponding K_Ic and K_IIIc values. It is observed that the fracture toughness of both modes I and III are increased by increasing the height of the ENDB specimen. Also, the ratio of K_IIIc/K_Ic obtained from each thickness of the ENDB specimens is compared with those predicted by some fracture criteria, and it was shown that the minimum plastic radius (MPR) criterion is the main suitable criterion for investigating the fracture toughness ratio K_IIIc/K_Ic . Also, the effect of ENDB height on fracture trajectory of tested samples is assessed. It is shown that the crack grows curvilinearly in thicker ENDB samples and cannot extend along the crack front in small specimens.     

Fracture study of a polyacetal resin

Fracture experiments have been carried out with unnotched and notched tensile specimens of a polyacetal resin at room temperature at various rates of extension. An increase of approximately 13% in yield stress was observed in the unnotched tests with increased deformation rates from 1–1000 mm min^–1, whilst strain to failure was reduced from about 85% to approximately 0.05%. In all specimens, failure appeared to have taken place by initiation of a microscopic flaw upon yielding, and this flaw then slowly grew into a critical size when catastrophic fracture set in. In the slow-growth region, the mechanism of failure was by void growth whereby the lamellar texture was transformed into a fibrillar one. However, on the rapid fracture surface, there was evidence of lamellar texture being retained, but with small voids in the stacks of lamellae, In notched fracture specimens containing sharp razor cut, a fracture toughness, K _lc, of approximately 5 MPa m^1/2 was obtained between crosshead speeds of 0.5 and 50 mm min^–1. At speeds of 500 and 1000 mm min^–1, the K _lc was reduced to 4 MPa m^1/2. In the absence of a starter crack, blunt notch fracture toughness of about 6.3 MPa m^1/2 was observed at all test speeds. In specimens with a razor cut, the slow-growth region decreased as test speed increased; this can be used to construct an R-curve.     

An Investigation of the Effects of Crack Front Curvature on the Crack-Tip Opening Displacement of A707 Steel

This paper examines the effects of crack front curvature on the fracture toughness (crack-tip opening displacement) of A707 steel. Fracture mechanics specimens, in which the radii of curvature of the crack fronts are controlled in an effort to simulate potential variations in crack front profiles in fracture experiments, were produced by machining and fatigue pre-cracking. Three-point bend crack-tip opening displacements (CTOD_c) were measured in accordance with the ASTM E-1290 code. The results show that the critical CTOD_c increases with increasing crack front curvatures between 0.05 and 0.17 mm^–1. In all cases, stable crack growth and final catastrophic failure of the specimens are found to occur by transgranular ductile dimpled fracture, in which the ductile dimples are nucleated around MnS or Al₂(Mg)O₃ inclusions. The implications of the results are discussed for the measurement of critical CTOD_c in specimens with varying levels of crack front curvature.     

Strength Assessment for Butt Joints of Steel 50 by the Fracture Toughness Criteria

We propose an experimental-theoretical engineering procedure for assessing the strength of butt-welded joints using the force (K_Ic) and strain (_c) criteria of the brittle and quasibrittle fracture mechanics, respectively. For this purpose, beam specimens with square and bevel welds and an initial edge crack or notch in the weld metal are tested under three-point bending. The parameter K_Imax controlling fracture of a bent beam with an inclined (Mode I + Mode II) crack is assessed taking into account the values of the stress intensity factors K_I and K_II, and the crack inclination angle ^*. We also studied the plastic zone at the crack tip and the crack propagation kinetics depending on the weld geometry and the V-notch tip radius for butt-welded joints. The data obtained allow one to rate such joints by their strength according to the fracture toughness criteria K_Ic and _c.     

Plane strain fracture toughness of modern ferritic structural steels

Abstract

The temperature dependence of the plane strain fracture toughness of a low carbon, fine grain, ferritic steel for structural applications is investigated. The ductile–brittle transition is found to occur in the interval between 160 and 184 K. The experimental results are interpreted by an analytical model which permits calculation of the plane strain fracture toughness K _1c in the brittle domain as a function of the tensile properties and the cleavage fracture stress, making use of a piecewise approximation for the distribution of tensile stress on the crack axis and applying a deterministic fracture criterion at the stress peak. A similar criterion, which consists of equating the severest strain on the crack axis to a critical strain for cavity nucleation, provides the upper shelf fracture toughness. A relatively simple figure for predicting the transition temperature of steels in this family as a function of material properties can be obtained in this way.     

Fracture toughness measurement by cylindrical specimen with ring-shaped crack

A method for measuring the plane strain fracture toughness of metals by means of cylindrical specimen in tension with axi-symmetrical ring-shaped crack is discussed. Owing to the fact that the crack tip of such a specimen is closer to ideal plane strain state, the K_1c value measured is effective and reliable. This investigation has fairly satisfactorily solved the problems of crack prefabrication, experimental technique, data processing and requirements for specimen dimensions.In both safety evaluation and life estimation of engineering components by linear elastic fracture mechanics, it is necessary to measure the fracture resisting parameter—fracture toughness under plane strain. According to the ASTM-E399-74 standard[1], when measuring the fracture toughness K_1c values of medium and low strength steels with a standard compact tension specimen or three-point bending specimen, it is necessary to use specimens of large dimensions, great tonnage fatigue testing machine and universal testing mechine. Naturally, this presents great difficulties to the investigation and application of fracture mechanics and it is precisely for the purpose of overcoming these difficulties that we have studied the method of measuring the plane strain fracture toughness by a cylindrical specimen in tension with axi-symmetrical ring-shaped crack. This method has fairly satisfactorily solved the problems of crack prefabrication, experimental technique, data processing and requirements for specimen size. Owing to the fact that the field around the crack tip of such a specimen is closer to ideal plane strain state, the results obtained are values smaller than those by using compact tension and three-point bending specimens and are more reliable fracture resisting constants for materials in linear elastic fracture mechanics analysis. Moreover, this method is more practical and economical because no expensive large fatigue testing machine is needed and the specimen size is small.     

Effects of loading rate on the local cleavage fracture stress σf in notched specimens

Four point bending (4PB) notched specimens with different notch sizes are tested at various loading rates at a temperature of −110 °C for a C-Mn steel. An elastic-plastic finite element method (FEM) is used to determine the stress and strain distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ_f is measured. The results show that the local cleavage fracture stress σ_f does not essentially change with loading rate V and notch size. The reason for this is that the cleavage micromechanism does not change in the different specimens at various loading rates. The cleavage micromechanism involves competition of two critical events of crack propagation and crack nucleation in the high stress and strain volume ahead of notch root. The large scatter of σ_f and notch toughness are mainly caused by the different critical events in different specimens.