Stretch Zone Research Articles

Dynamic initiation toughness and stretch zone

Propagation of fracture from an existing sharptipped crack may be separated into two regimes: initial and main fractures. Initial fracture is due to crack tip blunting, and main fracture starts as the blunting process attains a critical extent [1]. Blunting is caused by plastic deformation, and a number of workers, [2, 3] have reported the existence of a "stretched zone" in a variety of steels and nonferrous alloys. If a sharp crack is caused by fatigue, the stretch zone is simply a distinct region of shear deformation observed at the crack tip between the fatigue starter crack and main fracture area of a test piece. The stretch zone at the start of the main crack extension is referred to as the critical stretch zone. Dimple or cleavage fracture can be found on the stretched surface, which is formed at an angle to the fatigue crack plane [4]. Recently, Couque et al. [5] showed that dynamic initiation toughness increases with increasing dynamic loading rate. Initiation toughness is the material resistance to the initial crack extension. High initiation toughness means there is significant crack-tip blunting, i.e. large critical stretch zone. Critical stretch zone size can be used to evaluate the critical crack tip opening displacement (CTOD), which characterizes the initiation toughness in the case of elastic-plastic fracture [6]. In the present work, formation of stretch zone at high loading rate is studied. For this purpose, fatigue-precracked three-point bend specimens are fractured at impact velocities of 15, 29 and 43 m/s. The fracture mechanisms of main crack propagation are also discussed. The tested material is the Swedish microalloyed structural steel SIS 142134 (similar to steel Grade E355, quality DD of ISO 4950/2) with carbon equivalent not exceeding 0.41%. Specimens of size 320 x 75 x 18 mm have a single notch at the midsection and they are precracked by fatigue such that the length of the notch plus the fatigue crack is one quarter of the specimen height (about 19 mm). These specimens are supported at the mid-sections, opposite to the notch, and hit at the ends by a U-shaped hammer at the three impact velocities. The fracture surfaces are examined visually, and by scanning electron microscope (SEM) with an electron beam at 90 ° to the fracture plane. Fig. 1 shows the typical distribution of the three different fracture zones (marked A, B and C) which are identified macroscopically on all fracture surfaces. Region A is a brittle flat fracture, B is a ductile flat fracture, and C is a ductile slant fracture. The areas covered by brittle flat surfaces are measured on enlarged photographs with the help of a planimeter,

JR curves of the low alloy steel 20 MnMoNi 5 5 with two different sulphur contents in oxygen-containing high temperature water at 240°C

J R curves of the low alloy steel 20 MnMoNi 5 5 with two different sulphur contents (0.003 and 0.011 wt.%) were determined at 240°C in oxygen-containing high temperature water as well as in air. The tests were performed by the single-specimen unloading compliance technique at load line displacement rates from 1 × 10 −4 down to 1 × 10 −6 mm s −1 on 20% side-grooved 2T CT specimens in an autoclave testing facility at an oxygen content of 8 ppm and a pressure of 7 MPa under quasi-stagnant flow conditions. In the case of testing in high temperature water, remarkably lower J R curves than in air at the same load line displacement rate (1 × 10 −4 mm s −1) were obtained. A decrease in the load line displacement rate as well as an increase in the sulphur content of the steel caused a reduction of the J R curves. At the fastest load line displacement rate a stretch zone could be detected fractographically on the specimens tested in air and in high temperature water and consequently J i could be determined. When testing in high temperature water, the J i value of the higher sulphur material type decreases from 45 N mm −1 in air to 3 N mm −1, much more than that of the optimized material type from 51 N mm −1 in air to 20 N mm −1 at 1 × 10 −4 mm s −1.

A Fractographic Technique for the Estimation of Initiation Fracture Toughness JIc for Ductile Materials

Abstract A single specimen fractographic technique based on critical stretch zone width measurements is suggested for the estimation of fracture toughness (JIc) for highly ductile materials. The salient feature of this technique is that it overcomes the problem of fatigue precracking and is able to predict the fracture toughness of a material using a blunt notch specimen. Fracture toughness tests on commercially pure Armco iron, nickel, and aluminum as well as Al-Mn based austenitic stainless steel and En28 steel were carried out to validate the method.

Brittle fracture initiation in low carbon steels at the ductile-brittle transition temperature region

The fracture process that determines the temperature dependence of toughness at the ductile-brittle transition was investigated for low carbon steels. It was revealed that the resistance against the stretch zone at the crack tip and the stable crack extensions is temperature independent while it depends on the carbon content. Applicability of elastic-plastic analyses of the crack tip fields was examined in terms of the relationship between J integral value normalized by the flow stress and crack opening displacement. Temperature dependence of toughness is determined primarily by the brittle fracture initiation. Locations of the initiation sites were revealed to coincide with the maximum stress triaxiality rather than the maximum tensile stress. It was suggested that the brittle fracture is caused by the deformation-induced initiation and triaxial stress assisted growth of an incipient crack.

Variation of stretch zone width with J, loading rate, temperature and pre-crack depth

The effect of J, temperature, loading rate and pre-crack depth on stretch zone width have been examined. This revealed that changes in stretch zone width with pre-critical J and temperature are larger than measurement uncertainties and thus can be determined with confidence. Changes in stretch zone width with loading rate are smaller; large changes in velocity are required if changes in stretch zone width are to be measured. No change in stretch zone width with pre-crack depth could be detected. In all cases, secondary shear cracks complicated the assessments.

破壊力学 遷移温度域の弾塑性破壊じん性に及ぼすひずみ速度の影響

Static and dynamic fracture toughness tests at three strain rates were performed on two ASTM A508Cl. 3 steels by using 1T-CT specimens and fatigue pre-cracked instrumented Charpy specimens. The KJC converted from JC of the small specimens indicated a wide scatter. When the strain rate increased, the fracture toughness transition curves shifted to a higher temperature region. An increase in strain rate reduced the scatter of KJC dramatically, especially in the high temperature region, and decreased the lower bound fracture toughness. Fractographic examination of the fractured specimen surfaces indicated that the relationships between KJC and the stable crack growth, Δa0, distance from stable crack front to trigger point, X, or distance from fatigue crack front to trigger point, Δa0+X, were expressible by a single curve, respectively. The scatter of KJC was caused by the varience of Δa0, X, and Δa0+X. With increasing strain rate, Δa0, X, and Δa0+X decreased significantly, leading to a small scatter of KJC. The KJC value at Δa0=0, X=0, and Δa0+X=0 was proposed as the lower bound fracture toughness of a steel and labeled KJCi. The shape of KJCi versus temperature curve was controlled by the critical stretch zone width, SZWc. The Weibull slope m of KJC became larger with increasing strain rate and decreasing temperature. Higher toughness data with larger stable crack extension than 100μm violated the linearity of Weibull plots. In the statistical approach to determine the lower bound fracture toughness in the transition region, much more analytical development is needed. The KJC value with 3% fracture probability coincided with the KJCi value in the low temperature region even in a high strain rate.

VALIDATION OF THE FRACTURE MECHANICS TEST METHOD EGF P1–87D (ESIS P1–90/ESIS P1–92): ANALYSIS OF AN EXPERIMENTAL ROUND ROBIN

Abstract— The elastic—plastic fracture mechanics test method EGF P1–87 D: EGF Recommendations for Determining the Fracture Resistance of Ductile Materials has been validated by an extensive round robin, covering fracture tests and their evaluation as well as scanning electron fractography. The results confirmed the suitability of the procedure, but suggested some modifications. The resulting procedure ESIS P1–90 was then further modified and its actual designation is ESIS P1–92.The present report provides comprehensive information on the determination of crack growth resistance curves and initiation values. Particular emphasis is on single specimen techniques and on determining the stretch zone width. Beyond the validation of the ESIS procedure the information given is pertinent to elastic‐plastic fracture testing in general.

STRETCH ZONE GEOMETRICAL MEASUREMENT, A PARTICULAR WAY TO MEASURE FRACTURE TOUGHNESS

AbstractStretch zone geometry (height and length) has been measured using five methods (Transmission Electron Microscopy, Retrodiffused Electron Microscopy, Nickel Plating, Roughness Measurement, Ondulation Method). Experimental investigations show that the major difficulty of these measurements is the great scatter of experimental results. However, it is seen that Transmission Electron Microscopy and Nickel Plating give sufficiently accurate results. By using these different methods, we have found a linear relationship between stretch zone geometry and the classical fracture toughness parameter JIC.Rolling direction, grain size, loading rate, yield stress and the material texture are the influential parameters determining the stretch zone geometry.

Dynamic fracture toughness at crack initiation, propagation and arrest for two pipe-line steels

The influences of temperature and loading rate on fracture toughness of two pipe-line steels at initiation, K 1 d , and at arrest, K 1 a , and on stretch zone height were measured using specific techniques. Particular attention was given to the mechanism of delamination, typical for the gas internal pressure pipe fracture.

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.

Fatigue crack propagation in dispersion-strengthened P/M aluminium alloys at room and elevated temperatures: Bray, G.H. Proc. Conf. on P/M in Aerospace and Defense Technologies, Tampa, Florida, USA, Mar. 1991

An experimental study was performed to analyze the effect of the fatigue precracking stress intensity range (ΔK) on the formation of the stretch zone, a potential measure of JIC, in AISI 4340 steel. Compact tension specimens were fatigue precracked at 62%, 100% and 147% of the ΔK limit prescribed by ASTM E813-93, then loaded to levels of J ⩽ JIC. The resulting stretch zone widths (SZW) on the post-fracture surfaces were measured using scanning electron microscopy and plotted against J calculated from load vs load-line displacement. Precracking was found to reduce the SZW formed at a given value of J in proportion to the maximum applied J during precracking. Blunting line equations were fitted to experimental data for J < 0.65 JIC, revealing a constraint factor of m = 1.08. At J > 0.65 JIC, a transition from blunting to ductile crack propagation occurred. Extrapolation of the blunting line proposed in ASTM E813 to the critical stretch zone width yielded values of JIC that were erroneously high compared with extrapolation from experimental data.

Critical Assessment of Methods for JIc Determination

Abstract Various methods for the evaluation of J-initiation toughness have been compared by conducting investigations on Mo and Cr-Mo steel at different temperatures (i.e., 30 to 460°C) by employing the single-edge-notch bending configuration. The methods investigated include ASTM Standard E 813, a method proposed by the European Group on Fracture (EGF), methods based on a stretch zone approach, and a proposed method of the Japan Society of Mechanical Engineers (JSME). The blunting line approach for toughness determination has been investigated in detail in the light of various propositions made by other investigators, and an equation for the blunting line has been proposed based on the findings from the present work as well as from literature data. A J-resistance curve based on mid-thickness crack growth combined with the stretch zone approach for the blunting line appears to provide a mose comsistent method for toughness determination.

Ductile fracture toughness of polycrystalline armco iron of varying grain size

The influence of polycrystalline grain size on the ductile fracture toughness ( J ic) of Armco iron has been studied over the grain size range of 38–1050 μm. Experimental evidence for the various stages of ductile fracture during fracture toughness testing was obtained through scanning electron metallography. J ic decreases with coarsening of grain size and follows a parabolic relation with d −1/2. The critical stretch zone width in fracture mechanics specimens decreases with increasing grain size; at 1050 μm grain size, the stretch zone is not detectable and the material fails by cleavage fracture. The cleavage fracture behaviour at room temperature has been explained in terms of a stress concentration ahead of the crack tip that reaches the level of the cleavage fracture stress. The variation of fracture toughness J ic with grain size was studied in terms of the plastic zone size. Indirect methods based on K jc and P j fail to estimate the plastic zone size correctly. Microhardness measurements were found to be quite suitable for estimation of the plastic zone size. At finer grain sizes, the plastic zone size encompasses a substantial number of grains while at the coarsest grain size (1050 μm it is confined to a single grain, a feature that corresponds with the occurrence of cleavage fracture. The present measurements on Armco iron are shown to be consistent with the J ic data of Klasen et al. [ Mater. Sci. Engng 80, 25 (1986)] on microalloyed steels once one has accounted for inclusions and the higher carbon content.

Some observations on the cyclic unloading effects of a fatigue crack

An experimental investigation has been performed on a low-carbon structural steel to determine the order of events occurring both at the fatigue crack tip and in the crack wake owing to an applied cyclic unloading sequence. The two-step loading sequence consisted of unloading a fatigue crack, grown at positive load ratios of R = 0.6 and R = 0.2, to zero load. The fatigue crack was then cycled manually at R = 0 for a period of 10 cycles whilst maintaining a constant maximum stress intensity value. Observations were made of the crack tip geometry and the fatigue crack wake by monitoring the electrical potential drop across the crack as a function of load. The resultant hysteresis effect was explained in terms of an asperity crushing model. The effect of a series of underload cycles following a single-peak overload was also investigated, and the underload cycles were shown to induce surfac cracking on the ductile stretch zone formed by the overload. The surface cracking, across the thickness of the specimen, provided evidence that a hump of plastically formed material is generated upon application of the overload cycle and this may affect the subsequent fatigue crack growth characteristics.

Comparison of methods for crack tip opening displacement toughness determination

The crack tip opening displacement (CTOD) initiation toughness has been obtained by different methods in Cr and Cr-Mo steels at 30, 200 and 400 ‡C. The crack tip stretching and grain deformation has been investigated by scanning electron microscope and optical microscopic studies and by microhardness measurements. The resistance curve approach is used employing the average and the maximum crack growth and the initiation toughness determined are with respect to 0.2 mm crack growth (δ 0.2), the stretched zone width (SZW) and also using a blunting line approach. In addition, an initiation toughness using stretched zone depth (SZD) measurement is also obtained. The various initiation toughness values have been compared and an attempt has been made to identify the realistic plane strain CTOD toughness amongst the different values. Theℐ-CTOD relationship has also been investigated.

Influence of specimen size on I–III mixed mode fracture, fracture toughness [formula omitted] and plastic dissipation with crack growth [formula omitted

The effect of thickness on the fracture toughness, plastic energy dissipation or work at the crack tip and stretch zone has been investigated with compact tension specimens. A533B-1 steel specimens with thicknesses of 25 (1TCT), 10(0.4CT), 5, 3 and 0.5 (0.2CT) mm were tested. Some did not satisfy the ASTM E813 size requirement for fracture toughness J IC . J IC was determined from a multi-specimen resistance curve. The through-thickness distribution of the plastic energy dissipation or work done within the intense strain region at the crack tip was measured using a recrystallization-etch technique. Fractographic examinations of the variation of stretch zone width and fracture modes along the specimen thickness were performed by scanning electron microscopy (SEM), where flat fracture and shear fracture regions have been distinguished. This indicated how changes in specimen thickness affect plastic deformation, fractography and the overall fracture toughness. This investigation also finds that plastic energy absorbed at the crack growth dW p/da in the mid-section of a specimen has a constant value irrespective of specimen geometry and size during the steady progagation stage of crack growth.

Fracture Toughness JIC Prediction From Super-Small Specimens (0.2CT, 0.5MM Thick) of a Martensitic Stainless Steel HT-9

The fracture toughness of alloy HT-9, a martensitic stainless steel under consideration for fusion reactor applications, was determined from 0.2CT (0.5mm thick) specimens. Specimens with thicknesses of 25 (1CT), 10 (0.4CT), 3 and 0.5 (0.2CT)mm were tested to investigate the effects of specimen size on fracture toughness. 0.2CT (0.5mm thick) specimens did not satisfy ASTM E813 size requirements for a valid JIc. Fractographic examinations of the variation of stretch zone width and fracture modes along the specimen thickness were performed by scanning electron microscopy (SEM), where flat and shear fracture regions had been distinguished. A new JIc evaluation procedure for invalid specimen size is proposed using rigid plastic analysis and shear fracture measurements with fractographic observations. Predicted JIc values were compared with the JIc values obtained from valid specimen sizes. This miniaturized specimen technique may be applicable to post-irradiation fracture toughness testing.

Dependence of ductile fracture toughness of a weld metal on notch root radius and inclusion content

Using different fluxes of high and low basicity, two different welds have been produced with low and high inclusion content. Inclusions have been characterized in 2 and 3 dimensions, from measurements on both polished sections and on fracture surfaces. The fracture toughness is evaluated with J0integral, crack tip opening displacement (CTOD) and stretch zone width measurements. Ductile fracture satisfies the criterion of a critical fracture strain over a characteristic distance. These two critical values can be determined from the results obtained with blunt notch specimens. Characteristic distances are an order of magnitude larger than the inclusion spacing. This is attributed to the need to reach conditions of sustained crack growth for initiation of ductile tearing.

Characterization of fracture toughness of high strength low alloy steel weldments using stretch zone width measurements

Most of the industrial applications involving use of high-strength low-alloy steels require good weldability. Thus it is important to characterize the properties of the welded steels, especially the heat affected zone. Attempts have been made to characterize the fracture toughness of the HAZ by the use of theJ-integral and the crack opening displacement. In the present study, the effect of addition of titanium, vanadium and niobium, as well as combinations of these, on the fracture toughness of the heat affected zone of welded steel plates is examined. Six compositions were used in this study. Three-point bending specimens as well as tensile specimens were prepared. The fracture surfaces were examined in a scanning electron microscope to determine the fracture mode as well as the extent of the stretch zone as the crack blunts. Calculation of the fracture toughness parameter,Jlc, is carried out through a quantitative stereofractographic analysis of the stretch zone at the crack tip. The results show that there is a marked increase inJlc due to the addition of the various alloying elements. Generally, the addition of niobium and titanium alone produce the highestJlc due to the extent of grain refinement that these elements produce.

ディンプルとストレッチゾーンの３次元画像定量解析

The fracture surface reconstruction technique has been applied to dimple and stretch zone patterns to elucidate the quantitative relationship between three-dimensional parameters of fracture surfaces and material properties. The followings have been made clear. Testing temperature of Charpy impact tests is closely related to the averaged ratio of surface area and the averaged depth of dimples irrespective of kinds of steels; the higher the former, the greater the latter in dimple fracture surfaces. Similarly, the greater the absorbed energy is, the greater the averaged ratio of surface area and the averaged depth of dimples become. When the height of stretch zone is high, the height can be automatically and precisely deduced without human assistance.