The use of small specimen strength ratio for measuring fracture toughness

The specimen strength ratio ($\text{R}{}_{\mathrm{s}}$), determined from small specimen tests was correlated with plane strain fracture toughness ($\text{K}{}_{\mathrm{Ic}}$) values for many heats of A533B-1 steel. A variety of loading rate and specimen size results suggest that $\text{K}{}_{\mathrm{Ic}}$ can be predicted from the small specimen strength ratio up to values of $\text{R}{}_{\mathrm{s}}$ near 2.0. Also, conservative estimates of cleavage-initiated, elastic-plastic fracture toughness can extend beyond $\text{R}{}_{\mathrm{s}}$ values of 2.0. The ASTM E399 size criterion appears to be too restrictive for the class of steel studied, and a more appropriate requirement would reduce the ASTM criterion by a factor of four.     

An evaluation of the crack growth and fracture properties of AISI 403 modified 12 Cr stainless steel

The plane strain fracture toughness, $\text{K}{}_{\mathrm{Ic}}$, and fatigue crack growth rate material properties were developed for three heats of AISI 403 modified 12 Cr stainless steel. Valid (per ASTM requirements) fracture toughness tests were conducted in the temperature range ?200°F to 175°F. In addition, both the room temperature air environment plus 520°F, 1200psi distilled water environment fatigue crack growth rate material properties are presented. Finally, a hypothetical example problem is included which demonstrates the application of fracture mechanics technology to an AISI 403 modified 12 Cr stainless steel turbine rotor.     

A comparison of the geometry dependence of several nonlinear fracture toughness parameters

A number of fracture toughness tests on compact tension specimens have been performed for the purpose of comparing several nonlinear fracture toughness methods; including the nonlinear energy ($\text{G}\text{?}{}_{\mathrm{I}}$), J-integral ($\text{J}{}_{\mathrm{I}}$), COD ($\text{G}{}_{\mathrm{\delta }}$), and linear (–$\text{G}{}_{\mathrm{I}}$) approaches. The effect of variations in specimen thickness ($\text{B}$) and width ($\text{w}$) on the fracture toughness was examined for 7075-T651, 2124-T851, 2048-T35I, and 2048-T851 aluminum alloys, Ti-6Al-4V, and 4340 steel. Fracture toughness values were evaluated at both the initiation of stable crack growth and the onset of unstable fracture (peak load).It was found that the peak load toughness values are quite geometry sensitive at thicknesses below the requirement for plane strain fracture. At the initiation of stable crack growth, the toughness values are constant over a much larger range of specimen thickness. However, the nonlinearity of the load displacement curve is quite limited at this point and the associated fracture toughness is only 30–50% of the peak-load values.     

Precracking high speed steel for fracture toughness testing

Fracture toughness testing of high speed steel, which has a high fatigue strength and low fracture toughness, is a problem because fatigue cracks are difficult, or impossible, to initiate at a maximum fatigue stress intensity of $\text{0.7 K}{}_{\mathrm{<mtext>IC</mtext>}}$, as specified. A method of initiation by the use of an electric pen and subsequent fast propagation by fatigue has been studied and a procedure developed to give accurate, reproducible values of $\text{K}{}_{\mathrm{<mtext>IC</mtext>}}$ on subsequent fracture toughness testing.     

A modified thickness criterion for fracture toughness testing

A modified criterion is developed on an empirical basis for the minimum thickness $\text{B}{}_{\min }$ of a plane strain fracture toughness test specimen: $\text{B}{}_{\mathrm{<mtext>min</mtext>}}\text{= 400}\text{K}{}_{\mathrm{Ic}}{}^2\text{Eδ}{}_{\mathrm{Y}}$ where $\text{K}{}_{\mathrm{Ic}}$ is the plane strain fracture toughness, $\text{E}$ is the Young's modulus and δ_Y is the yield stress of the material. The modified criterion is tested alongside the ASTM thickness criterion against published data on the variation of $\text{K}{}_{\mathrm{c}}$ with thickness, and shows significantly the better agreement with observed values of $\text{B}{}_{\mathrm{<mtext>min</mtext>}}$ for a wide range of materials.An attempt has been made to rationalise this criterion. The expression is considered to take into account two major factors which determine $\text{B}{}_{\mathrm{<mtext>min</mtext>}}$, the attainment of plane strain in the specimen interior ahead of the crack tip, and the role of microstructure in determining how far the quasi-plane strain fracture (square fracture) extends beyond the region of true plane strain.     

Fracture toughness and fatigue crack propagation in high strength steel from room temperature to −180°c

Fracture toughness under tensile test and fatigue test on high strength steel at temperature ranging from room temperature to ?180°C were experimentally studied. The value of fracture toughness under fatigue test is considerably tower than that obtained under tensile test.Within the range from room temperature to ?100°C the following results were obtained: the power coefficient δ of the fatigue crack propagation rate $\text{[(dc)/(dN)] = AΔK}{}^5$ is related with [(1)/($\text{T}$)] as: $\text{δ = b}{}_1\text{+ [(a}{}_1\text{)/(kT)]}$. $\text{[(dc)/(dN)]}$ shows Arrhenius type, and, however, different equation from usual stress dependent rate process equation. The trend is in good agreement with the dislocation dynamics theory of fatigue crack propagation.     

Fracture toughness and fracture energy measurements on aluminium alloys

Methods of conducting and analysing instrumented Charpy impact tests have been discussed and applied in measuring the initiation fracture toughness, $\text{K}{}_{\mathrm{Ic}}$, of two precipitation hardened Al-alloys.For full speed impact tests a method for indirectly deriving fracture load from “system stiffness” and “time to fracture” has been found to be the most suitable. In the lower speed impact tests measured fracture load has been used directly to calculate $\text{K}{}_{\mathrm{Ic}}$ In these tests an energy method superficially resembling “specific surface energy” has also been used to calculate $\text{K}{}_{\mathrm{Ic}}$.     

Fracture behaviour of dynamically loaded ship building plate material

The fracture toughness values of ship building mild steel measured over a temperature range ? 196°C to 28°C and crack tip strain rates ranging from 10^?5/sec to 10^?1/sec are examined in the light of the models recently proposed by Malkin and Tetelman. The effect of a change in inclusion morphology brought about by electroslag refining on the fracture toughness of the steel is also evaluated. It is found that the stress-induced fracture criterion ofthe model applies for the case where the ratio $\text{σ}{}^1{}_{\mathrm{f}}\text{σ}{}_{\mathrm{YS}}\text{? 3.94}$. This ratio is independent of the strain rate. In the strain induced fracture region of the model, the critical strain near the crack tip, ?_f(Rβ) is a function of the yield stress irrespective of temperature and strain rate. Electroslag refining reduces significantly the size and volume fraction of the inclusions and changes their shape from prolate ellipsoid to spherical. Apparently the electroslag refining does not improve fracture toughness significantly if the fracture toughness of the as received material measured with the major axis of the inclusions perpendicular to the crack front, is taken as a basis of comparison.     

Characterization of transition temperature behavior of HY130 steel by the JIc fracture toughness parameter

The objectives of this study were to determine whether the $\text{J-R}$ curve approach could be used to evaluate the ductile to brittle temperature performance of a high yield strength structural steel (HY130) and to demonstrate that the single specimen unloading compliance method is applicable to evaluate $\text{J}{}_{\mathrm{Ic}}$ values and $\text{J-R}$ curves for compact specimens tested at temperatures from ?192 to 150°C. The major conclusions of this work are that $\text{J}{}_{\mathrm{Ic}}$ and the complete $\text{J-R}$ curve can be obtained using the single specimen method over the above temperature range and that $\text{J}{}_{\mathrm{Ic}}$ does define a ductile to brittle transistion temperature for HY130 steel which should be more valuable for structural design than that found from Charpy $\text{V}$ or dynamic tear specimens because it is based on a fracture toughness parameter. The comparison of the ${}_{\mathrm{Ic}}$ transition temperature and that from Charpy $\text{V}$ specimens shows that the Charpy $\text{V}$ transition temperture is more conservative for the HY130 steel tested. In transitional $\text{J}{}_{\mathrm{I}}$ specimens which demonstrated a ductile crack tearing followed by a brittle failure, scanning microscope stereo pair fractography showed that the transition from ductile to brittle behavior was very gradual in comparison to the distinct crack tips obtained on ductile specimens broken in a brittle fashion at a cryogenic temperature.     

A correlation between dynamic initiation toughness and crack arrest toughness

Dynamic initiation toughness values were obtained from testing small 3-point bend specimens at ?50, ?25, 0, +23°C. Two specimen orientations were tested which showed no marked difference in critical dynamic initiation toughness. The obtained $\text{K}{}_{\mathrm{Id}}$ data were correlated with the crack arrest toughness $\text{K}{}_{\mathrm{Im}}$ and $\text{K}{}_{\mathrm{Ia}}$. The value of $\text{K}{}_{\mathrm{Id}}$ is in the range 0.93 to 1.29 times the corresponding fast fracture toughness, $\text{k}{}_{\mathrm{im}}$ while the ratio $\text{K}{}_{\mathrm{Id}}\text{/K}{}_{\mathrm{Ia}}$ varies between 1.27 and 1.60.     

The dependence of mode III fracture initiation toughness on strength and microstructure

HF1, a hypereutectoid steel which is high in silicon and manganese content, was selected to examine the dependency of Mode III fracture initiation toughness $\text{(K}{}_{\mathrm{IIIQ}}\text{)}$ on microstructure and strength. Four heat treatments of this steel were investigated and results were compared with those previously reported on a hypoeutectoid steel (4340). It was found that $\text{K}{}_{\mathrm{IIIQ}}$ is independent of strength and depends only on microstructure and other metallurgical factors. The $\text{K}{}_{\mathrm{IIIQ}}$ value of the hypereutectoid steel and for the nucrostructures examined varied between 35 and 45 MPa.√m. In the higher strength specimens, an initial Mode III crack extension was followed by Mode I crack branching.     

An investigation of the blunting line

Experimental evidence obtained on 3-point SENB specimens of carbon steel (St52-3) is used to compare ways of determining the slope of the “blunting line” in $\text{J}{}_{\mathrm{Ic}}$ testing. The results demonstrate that the slope of the blunting line obtained by the stretched zone width (SZW) method is steeper than the blunting line predicted by the ASTM method. This results in lower values for both $\text{J}{}_{\mathrm{Ic}}$ and $\text{(Δa)}{}_{\mathrm{c}}$ as determined with the SZW method, compared with those obtained by the ASTM method. The increased conservatism compared to the ASTM approach and the experimental evidence underlying it recommend the SZW method for further study. The subsequent widespread use expected for any recognized $\text{J}{}_{\mathrm{Ic}}$ determination seems to justify the additional effort and expenses associated with the SZW method.     

The effect of loading rate and temperature on fracture initiation in 1020 hot-rolled steel

Fracture initiation experiments were conducted to determine the fracture toughness of an AISI 1020 hot-rolled steel under quasi-static and dynamic loading conditions. The dynamic tests were performed on specimens consisting of notched round bars loaded in tension by means of a stress pulse, thus giving an effective value of $\text{/.K}{}_{\mathrm{I}}\text{= 2 · 10}{}^6\text{ksi √in. s}{}^{\mathrm{?1}}\text{(2.2 · 10}{}^6\text{MPa √m s}{}^{\mathrm{?1}}\text{)}$. The tests were conducted over the temperature range ?157 to 121°C, and results are compared to those obtained in quasi-static tests with like specimens. Parallel tests were conducted to determine static and dynamic flow stresses of the material over the temperature range. Finally, a comparison is made with the behavior of an AISI 1018 cold-rolled steel previously tested in the same apparatus.     

G̃c and R-curve fracture toughness values for aluminum alloys under plane stress conditions

The effect of specimen geometry and subcritical crack growth on the nonlinear energy fracture toughness, $\text{G}\text{?}{}_{\mathrm{c}}$, has been examined for thin, center-cracked sheets of 2024-T3 and 7075-T6 aluminum alloys. The procedure followed was to independently vary the specimen length, L, width, w, andd crack length-to-specimen width ratio and to determine the toughness both at the onset of subcritical crack growth and at the initiation of unstable fracture. Comparisons were also made with the R-curve toughness, G_R, evaluated at unstable fracture from which it was found that both $\text{G}\text{?}{}_{\mathrm{c}}$ and G_R displayed the same trend of change with geometrical variables, with $\text{G}\text{?}{}_{\mathrm{c}}$ consistently higher than G_R. When the nonlinear energy fracture toughness was evaluated at the onset of subcritical crack growth, it was found that the geometry dependence essentially disappeared.Scanning electron microscopic examination of some typical fracture surfaces showed that stable crack growth was accompanied by a gradual change of fracture mode from plane strain to plane stress. An analysis of possible errors in the experimental procedure showed that the scatter observed in $\text{G}\text{?}{}_{\mathrm{c}}$ values was not due to experimental errors, but apparently due to inhomogeneities in the materials. Several techniques were also introduced for the purpose of more directly incorporating crack growth into the $\text{G}\text{?}{}_{\mathrm{c}}$ determination, but it was found that they did not cause significant variation in the toughness values.     

An examination of mixed fatigue-tensile surface crack growth in rails

The fracture instability associated with alternating periods of fatigue and tensile growth of surface cracks was investigated in steel rails. Three different steels were tested. The instabilities commenced when the maximum stress intensity factor $\text{K}$ exceeded the fracture toughness $\text{K}{}_{\mathrm{IC}}$ and resulted in crack jump or total rail failure. The conditions for the establishment of fatigue-tensile crack jump and arrest are described. The load level, residual stresses, crack geometry and fracture toughness effects are analysed. The fatigue surface cracks were penetrated in both stress relieved and stress unrelieved rails. The effective stress intensity factors including the contribution of the applied load and residual stresses were calculated. For both the fatigue-tensile and tensile-fatigue transitions the stress intensity factors were almost the same with the value for the tensile-fatigue transition being slightly lower. Both calculated stress intensity factors were close to the fracture toughness $\text{K}{}_{\mathrm{IC}}$.     

On fracture toughness in the nonlinear range

A general definition of fracture toughness, designated by $\text{G}\text{?}{}_{\mathrm{c}}$, is developed which is appropriate to situations of subcritical crack growth and/or large-scale crack border plastic yield. The theoretical basis as well as comparisons with other proposed measures of fracture toughness are also discussed. A simple method is given for evaluating $\text{G}\text{?}{}_{\mathrm{c}}$ which is based on use of the load-displacement test record.     

Fracture mode and toughness vs temperature (−20–22°C) for UNS-S15500 (15-5 PH) stainless steel

An investigation was undertaken to establish a relationship between fracture mode and toughness ($\text{K}{}_{\mathrm{Ic}}$) vs temperature for UNS-S15500 (15-5 PH) in the H583dgC (H1080°F) condition. It was found that the mechanical properties such as yield strength were temperature insensitive in the range -20–22°C. Tensile rupture was by microvoid initiation and coalescence. In contrast, fracture toughness values showed a slight decrease with a temperature decrease. Over the temperature range investigated fracture was a mixed mode containing quasi-cleavage and microvoid coalescence. No single microstructural characteristic distance or model was found to correlate with the fracture toughness decrease.     

Kinetics of subcritical cracking under the chloride and sulphide environments in a low alloy steel

The effects of two aqueous environments, namely chloride and sulphide have been investigated using fracture mechanics approaches in a Ni-Cr-Mo alloy, tempered between 200–600°C temperatures after quenching. The experimental investigation included tensile and fracture toughness tests in the ambient condition, environmental tests to determine the threshold, K_ISCC and the crack growth rate values $\text{da}\text{dt}$ and fracture surface studies. An attempt has been made to substantiate the role of microstructure and the source of hydrogen on the susceptibility to failure by computing $\text{C}{}_{\mathrm{c}}\text{C}{}_{\mathrm{o}}$ ratios for the hydrogen induced cracking process. A crack growth rate expression of the type, $\text{da}\text{dt}\text{= c'(K)}{}^{\mathrm{n}}$ is proposed for Stages I and II to account for the discrepancy between the theoretically calculated and the experimental $\text{da}\text{dt}$ data. The experimental values of the constants c' and n are determined. For all the tempering conditions investigated, the H₂S environment appears to be more hostile than the NaCl medium. However, the susceptibility to both the environments is more pronounced for yield strength values greater than 1500 MPa. The $\text{K}{}_{\mathrm{If}}\text{K}{}_{\mathrm{IC}}$ ratio is bound to be less than 1 under the H₂S, and greater than 1 under the NaCl solution.     

Determination of nonlinear energy toughness values for cyclic loading applications

For several years the nonlinear energy method proposed by Liebowitz and Eftis has been examined as a failure criterion for static testing of center-cracked and compact tension specimens. Since the method appears to be valid under conditions of crack-tip plasticity, subcritical crack growth and load relaxation, tests have been conducted to ascertain the merit of this method as a failure criterion under cyclic loading conditions. The nonlinear energy toughness for cyclic loading, $\text{G}\text{?}{}_{\mathrm{fc}}$, is obtained from an envelope of the cyclic load-displacement record, which naturally imposes some restrictions on the loading program.The cyclic toughness parameter, $\text{G}\text{?}{}_{\mathrm{fc}}$, has been evaluated for thin, center-cracked sheets of 2024-T3 and 7075-T6 aluminum alloys. The specimen dimensions were held constant and the load parameters were varied so that a significant variation of the cyclic life was obtained. Both alloys exhibited a significant reduction of $\text{G}\text{?}{}_{\mathrm{fc}}$ with increasing cyclic life in a manner similar to the classical $\text{S-N}$ diagram. For example, the ratio of cyclic to static toughness, $\text{G}\text{?}{}_{\mathrm{fc}}\text{/G}{}_{\mathrm{c}}$, was found to be about 0.8 when failure occurred after approximately 150 cycles. There appeared to be a tendency for the curve to level off at this point, which suggests that these curves may represent compressed S-N curves. It is felt that this method may serve the design process by allowing the establishment of a fracture toughness parameter capable of including the effects of the entire loading history of a structure into the fracture toughness requirements.     

Fracture toughness characterization of several aluminum alloys in semi-brittle fracture

A method has recently been developed for determining a nonlinear fracture toughness parameter defined by the relation $\text{G}\text{?}{}_{\mathrm{c}}\text{=}\text{C}\text{?}\text{G}{}_{\mathrm{c}}$ where G_c is the critical elastic strain energy rate as defined by Irwin. The $\text{C}\text{?}$ term is a function of the nonlinearity of the load-displacement test record and has been evaluated using the three parameter Ramberg-Osgood approach, although other curve fitting techniques could be applied as well. The method is quite straightforward and is applicable to plane stress, plane strain and mixed mode testing although only plane stress conditions are considered in this paper. For the case of a linear load-displacement record $\text{C}\text{?}\text{→ 1}$ and $\text{G}\text{?}{}_{\mathrm{c}}$ reduces to the linear elastic result.The toughness parameter $\text{G}\text{?}{}_{\mathrm{c}}$ has been evaluated for a number of high strength aluminum alloys and compared with published G_c values for these materials. The tests were conducted on center-cracked sheets of 2014-T6, 2024-T81, 7075-T6 and 7475-T61 aluminum alloys under conditions of varying specimen geometry and displacement gage length. It was found that the values of $\text{G}\text{?}{}_{\mathrm{c}}$ obtained from displacement readings with a gage length of 2 in. generally agreed with published values of $\text{G}{}_{\mathrm{c}}\text{=}\text{K}{}_{\mathrm{c}}{}^2\text{E}$. The $\text{G}\text{?}{}_{\mathrm{c}}$ values were found to vary inversely with gage length and a/w ratios. The variation in values for $\text{G}\text{?}{}_{\mathrm{c}}$ is of the same order of magnitude as the scatter in published values for G_c. However, $\text{G}\text{?}{}_{\mathrm{c}}$ appears to be less sensitive than G_c to changes in a/w.