Stretch Zone Width Research Articles

Effects of fatigue precracking on stretch zone formation

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 J IC, 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 ⩽ J IC. 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 J IC, revealing a constraint factor of m = 1.08. At J > 0.65 J IC, 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 J IC that were erroneously high compared with extrapolation from experimental data.

A note on stretch zones in an Al-Zn-Mg alloy

The stretch zone is an intermediate fracture surface between the end of the fatigue precrack and the beginning of the crack produced by monotonic loading in a fracture toughness test. The appearance of this zone, ductile like the fatigue precrack zone, is distinguished by a more hilly relief and more elongated dimples and its width is related to the crack opening displacement (COD). Considering a blunt crack symmetric relative to the original line of the fatigue precrack, an equation relating stretch zone width (W{sub SZ}) to COD ({delta}{sub c}) can easily be established. In the present work, applying the technique to a series of peak-aged Al-Zn-Mg alloys has been attempted.

The relationship between fracture toughness, stretched zone width and mechanical properties in tensile test

The subject considered is the theoretical relationship between fracture toughness and stretched zone width ahead of a crack in an elastic-plastic material under plane-strain loading (mode 1). Distribution of the elastic-plastic equivalent strain ahead of a crack tip through the small-scale yielding region is presented as an exponential function of the distance from the crack tip and material properties. A model is proposed and discussed, which relates the fracture toughness, K lc , to the reduction of area in a tensile test, ψ, yield strength, σ 0.2 , and stretched zone length, W. The W values, calculated by means of this model, are consistent with experimental ones over the temperature range 77–223 K. It demonstrates the possibility of direct K Ic estimation through the stretched zone width.

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.

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.

Study of elastic-plastic fracture toughness determination

The elastic-plastic fracture toughness viaJ-integral and crack tip opening displacement (CTOD) has been obtained in two structural steels using several fitting equations representing the resistance curve of the material. The toughness is determined as the values corresponding to the critical stretched zone width (SZW) on theR-curves and with respect to 0.2 mm crack growth. The SZW measurements were performed by scanning electron microscopy. The various toughness values have been compared and the importance of using appropriateR-curves based on physical considerations has been pointed out. TheJ-CTOD relationship during the blunting process has been experimentally investigated from load-displacement records of the fracture test.

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.

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.

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.

A stretched zone method for ctod evaluation

where Aa=~. h is the length (or width) of the stretched zone. Conventionally, the deformed crack tip is considered to be of a semi-circular type leading to a blunting angle of 45 degrees. However, viewpoints contrary to this have been expressed [3-5]. In the present work, the blunting angle (0) has been evaluated by measurement of the stretched zone width (SZW) and the stretched zone depth (SZD) in the Mo and C"r-Mo steels and a method for CTOD initiation toughness is proposed.

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.

A comparative study of various approaches for ctod toughness evaluation

A comparative assessment of various methods proposed for CTOD toughness evaluation has been made on two structural steels. CTOD values are obtained from experimental CMOD data by direct extrapolation as given by Wells' method, BS5762 etc. On the other hand the toughness values based on R- curves (multiple specimen method) are also obtained using various proposed blunting lines. An attempt is made in the present work to find a more realistic blunting line from stretch zone width measurement. Two specific geometries, namely compact tension (CT) and three point bending are considered, besides studying the thickness effects on CTOD. The conventional methods (BS5762 etc.) are found to yield more conservative values, while the R- curve approach considered with a blunting line provides a more realistic estimation of toughness. No clear and systematic dependency of thickness could be observed. However, the CT geometry results in higher CTOD toughness than bend geometry apparently because of a lower constraint factor.

Development of a Miniaturized Specimen Technique for Fracture ToughnessJIc Measurement

AbstractThe effect of specimen thickness on fracture toughness JIc has been investigated with compact tension specimens using the multispecimen R-curve method. HT-9 and A533B-1 steel specimens with thicknesses of 25(1TCT), 10(0.4TCT), 3, and 0.5(0.2TCT) mm were tested. Some of them did not satisfy the ASTM E 813 size requirement for valid JIc. The through-thickness distribution of the plastic energy dissipation or work done within the intense strain region at the crack tip was measured by 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. A new JIc evaluation procedure for invalid specimen size is proposed using rigid plastic analysis and shear fracture measurements with recrystallization-etch or fractographic observations. Predicted JIc values are compared with the JIc values obtained from valid specimen size. This miniaturized specimen technique may be applicable to post-irradiation fracture toughness testing.

RESULTS OF A ROUND ROBIN ON STRETCH ZONE WIDTH DETERMINATION*

Abstract— ‐A round robin on experimental stretch zone width (SZW) measurement was conducted. The SZW of eight different materials showing an SZW range of 5 to 200 μm was measured by evaluating a set of micrographs. The round robin was carried out by 14 participants from different firms and institutes. The standard deviation of the experimental SZW measurement was found to be in a range of smaller than 75% of the stretch zone average SZW. For 90% of the micrographs the standard deviation of SZW remains in a range smaller than 35% of SZW. This scatter range was not significantly influenced by the experience of the participants in measuring the individual SZW, the materials tested and the magnitude of the SZW itself. The SZW results determined in this round robin match well with a theoretical SZW determination using the recently developed blunting line equation including strain hardening: SZW = 0.4 dn* J/E.