Stretch Zone Width Research Articles

Fracture toughness evaluation of ultrafine-grained nickel

The fracture behavior of ultrafine-grained Ni produced by high-pressure torsion was examined. The fracture toughness estimated for different crack propagation directions was assessed with J-Integral, stretched zone width and crack tip opening displacement measurements. The results indicate an exceptionally good combination of strength and fracture toughness.

The Role of Microfractography in Failure Analysis of Cold Forging Dies

Microfractographic features in fracture surfaces for tensile, fatigue, impact, three point bending specimens of cold forging die steels with Rockwell C scale hardness number of 52 to 68 are presented. The emphasis is placed upon the stretched zone formation ahead of the fatigue crack and the relation between the stretched zone width and fracture toughness. Fatigue crack initiation behavior of plane and notched specimens are also characterized in low cycle up to giga-cycle ranges. A couple of microfractographic analyzed results for failed actual cold forging dies are exemplified. It can be concluded that qualitative and quantitative analysis by use of microfractograpy are possible for cold forging die failures.

Establishing a methodology to predict the crack initiation load in through-wall cracked components using tensile specimen test data

Among several other definitions, the crack initiation fracture toughness ( J i) based on critical stretch zone width (SZWc), called J SZWc, is being considered as a geometry independent material property. The problem in SZWc experimental evaluation is in identifying the size of stretch zone on a blunted crack front as this requires a high degree of precision and expertise in measuring the SZW. The present study addresses finite element determination of SZWc value using tensile test data. The role of stress tri-axiality in standard fracture specimen s and the smooth round tensile specimen is also studied. Based on the ASTM-E1820 standard, the present study also showed that fracture specimen thickness greater than the specified size is to be used for numerical prediction of valid SZWc value. The numerically predicted SZWc that leads to J SZWc matches well with experimental values. Using J SZWc, the crack initiation load is also determined in circumferentially through-wall cracked (TWC) elbows and compares well with experimental results. Thus the paper establishes the methodology to predict crack initiation in cracked piping components using numerically obtained valid J SZWc from material’s tensile test data.

Numerical determination of stretch zone width (SZW) using tensile test data

There is an increasing effort to use critical stretch zone width (SZW) for the evaluation of initiation fracture toughness that is considered as geometry independent material property. The existing numerical SZW evaluation method is based on crack tip opening displacement measurement, which is evaluated using various definitions. This numerical method also does not define the critical stage for critical SZW measurement. This work attempts to establish a procedure for numerical determination of SZW, its critical value (SZWc) and initiation fracture toughness using tensile test data. The proposed methodology also tries to explain the mechanism involved in the creation of stretched zone and thus defines the stage to calculate critical SZW. Numerical analyses have also been carried out to understand the role of crack tip constraint in standard fracture specimen during the blunting process and the tensile test sample.

Effect of crack depth on fracture toughness of 20MnMoNi55 pressure vessel steel

Ductile fracture behaviour of 20MnMoNi55 steel under quasi-static loading has been studied using single-edge notched bend specimens. To understand the influence of crack depth on fracture behaviour, J– R curves were obtained from specimens pre-cracked to various a/ W in the range of 0.25–0.75, in steps of approximately 0.1. Stretch zone width dimensions were measured on the fractured surfaces of the broken specimens. The stretch zone dimensions that were determined have been used in conjunction with the experimentally derived J– R curves to obtain a value of the ductile fracture toughness parameter J SZW. The initiation toughness, J i, at the intersection of the blunting line and the power-law fit to the J– R curve, and the critical toughness, J c, following the ASTM standard, were estimated. Comparisons of J SZW with J i and J c have been made for 20MnMoNi55 steel as a function of the a/ W ratio. The J i and J c values are found to be higher at lower a/ W, i.e. under low constraint, and decrease with increasing a/ W. It is observed that J SZW is not affected by a/ W, and that its value is lower bound to J i and J c values of the material investigated.

Fractal behavior throughout stretch zone of 15-5PH steel under elastic-plastic loading conditions

The topography of the fracture surface along the stretch zone of 15-5PH steel is analyzed for a range of specimen thicknesses, providing information on the changes in elastic-plastic behavior. The relationship between the stretch zone width, as a local toughness measurement, and fractal dimension is discussed along with the restrictions imposed by the stress field evolution. Image stacks for extended depth-of-field 3D reconstructions of the stretch zone region were acquired using a reflected light microscope. Significant correlations between the stretch zone width and fractal dimension with specimen thickness were found from measurements on elevation maps, especially at the center regions. This demonstrates that the stretch zone has heterogeneous topographic behavior depending on thickness and is more homogeneous under plane strain states. Finally, a general equation is suggested to describe the stretch zone width dependence on both the fractal dimension and material thickness.

Stretched zone width and blunting line equation for determination of initiation fracture toughness in low carbon highly ductile steels

The methods of measurement of stretched zone width (SZW) have been examined and its relative insensitivity to thickness and length of pre-fatigue crack in CT specimens, encourages its use to mark the end of blunting in the J–R curve. On the basis of SZW, a mathematical equation has been determined for the slope of blunting line, also as a function of strain hardening coefficient, in the limited context of SA333 Gr. 6 steel and HSLA steel. The internal consistency of the 0.2 mm offset blunting line equation has been demonstrated by comparing the estimated and the observed initiation fracture toughness. The equation has been validated also with the results from literature. The initiation fracture toughness obtained for SA333 Gr. 6 steel following 0.2 mm offset line equation determined here compares well with the results reported in earlier investigations.

An improved method for numerical determination of SZW using FEM

The current investigation essentially comprises a number of 3D non-linear FEM simulations to study and correlate crack tip blunting with experimental results. The study attempts to establish a physically tangible procedure for numerical determination of stretch zone width (SZW). The proposed methodology explains the mechanism involved in the creation of stretched zone. This aspect was missing in its earlier method of determination as half the crack tip opening distance which in-turn determined using 45° line method. It is concluded that the proposed method of SZW determination using large deformation FEM analysis can reasonably simulate the process of blunting of the crack tip and can predict the material's SZW and J-integral values.

Determination of stretch zone width using fem

The study addresses numerical determination of stretch zone width and its critical value. The proposed method is based on the highly deformed stretch zone that is defined as stretch zone width, expected to be better than the half of the crack tip opening distance which inturn determined using 45° line method. The investigation essentially comprises a number of finite element analyses of compact tension test, using tensile test data. The proposed methodology also provides insight into the mechanism involved in the creation of stretch zone.

Microfractography in failure analysis of cold forging dies

In this paper microfractographic features in fracture surfaces for tensile, fatigue, impact and three point bending of cold forging die steels with Rockwell C scale hardness number of 52–68 is presented. The emphasis is placed upon the stretched zone formation ahead of fatigue crack and the relation between the stretched zone width and fracture toughness of these cold forging die steels. Finally it is briefly described that the quantitative analysis for cold forging die failure can be possible by measuring the stretched zone width.

バルク金属ガラスの破壊靭性と試験片表面におけるせん断帯長さ，き裂開口変位およびストレッチゾーン幅の関係

Relationship between fracture toughness and shear band length (SBL), crack tip opening displacement (CTOD) on the specimen surface and stretched zone width (SZW) on the fracture surface in Zr-, Cu- and Ti-based bulk metallic glasses (BMGs) was investigated in order to examine the mechanisms that the BMGs show a high fracture toughness. Zr50Cu40Al10 and Zr50Cu30Ni10Al10 BMGs fabricated by an ark tilt casting method, and Cu60Zr30Ti10, Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 BMGs fabricated by a high-frequency induction casting method were used. The fracture toughness test was conducted based on ASTM E399. Compact type specimen was used (1.0 mm, 2.0 mm and 2.3 mm in thickness, thickness:width=1:4). Fatigue pre-crack was induced under ΔK less than 6 MPa•m1/2. The crack length and SZW were measured on the fractographs by scanning electron microscope (SEM), and the SBL and CTOD were measured on the photographs of specimen surfaces by the SEM. Observed SBL and CTOD in each BMG showed the near values of the plastic zone size and CTOD calculated by the fracture mechanics under the plane stress condition, respectively. By the stereo-matching observation on the fracture surfaces, the crack tip inside the specimen clearly opened and a stretched zone area was shown. Average SZW in front of the fatigue crack in the Zr- and Cu-based BMGs was roughly proportional to the fracture toughness value.

Cu-Zr-Ag-Alバルク金属ガラスの平面ひずみ破壊靱性

Fracture toughness of the Cu-Zr-Ti bulk metallic glass (Former Cu-Zr BMG) under plane-strain condition could not be evaluated because the BMG with a sufficient thickness for the plane strain condition had not been fabricated. However, as the fracture morphology was similar to that in the Zr-based BMGs having the large plane-strain fracture toughness (KIC), e.g. about 50 MPa•m1/2, it was presumed that the Former Cu-Zr BMG has a large KIC. Recently, in Cu-Zr-Ag-Al BMG (New Cu-Zr BMG), a large sample of 25 mm in diameter became possible to be made. Therefore, in this study, KIC of the New Cu-Zr BMG was investigated. The New Cu-Zr BMG plate sample of Cu36Zr48Ag8Al8 was made by a copper mold casting method. Tensile strength was 1850 MPa and Young's modulus was 115 GPa. Compact type specimen with 2.3 mm in thickness and 9.2 mm in width was used. Fatigue pre-crack was induced under stress intensity factor range (ΔK) less than 6 MPa•m1/2, and the fracture toughness test was conducted at a loading rate of 1 MPa•m1/2•s-1. The new Cu-Zr BMG exhibited a KIC of 17 MPa•m1/2. The former Cu-Zr BMG that could not satisfy the plane-strain condition exhibited a fracture toughness of 62 MPa•m1/2. The stretched zone width along the inner fatigue crack front and vein pattern size on the unstable fracture surface in the new Cu-Zr BMG were smaller than those of the former Cu-Zr BMG. Therefore, it seems that the KIC of the new Cu-Zr BMG is smaller than that of the former Cu-Zr BMG.

OS0621 冷間加工SUS304鋼の破壊靭性と限界ストレッチゾーン幅(SZW_C)との相関に及ぼす試験片サイズの影響(OS06-05 材料強度の発現機構,OS06 微視構造を有する材料の変形と破壊(3))

OS0621 冷間加工SUS304鋼の破壊靭性と限界ストレッチゾーン幅(SZW_C)との相関に及ぼす試験片サイズの影響(OS06-05 材料強度の発現機構,OS06 微視構造を有する材料の変形と破壊(3))

Effects of Hydrogen on Fatigue Crack Growth and Stretch Zone of 0.08mass%C Low Carbon Steel Pipe

In order to investigate the influence of hydrogen and test frequency on fatigue crack growth rate, fatigue crack growth tests (R=0) were conducted on hydrogen charged and uncharged specimens of a 0.08 mass%C low carbon steel pipe at test frequency 0.001-10 Hz. Following the fatigue crack growth tests, the test for producing a stretch zone was carried out. Observing the morphologies of the striation and stretch zone on the fracture surface and slip bands on the specimen surface, it was revealed that uncharged specimens have no dependency of crack growth rate on test frequency and on the other hand, the crack growth rate of hydrogen charged specimens at the test frequency below 0.01 Hz is approximately 10 times faster than that of uncharged specimens. There exists an upper limit of the increase in crack growth rate of the hydrogen charged specimens at test frequency below 0.01 Hz. The degree of crack tip blunting is very different between uncharged and charged specimens. The crack tip is fully blunt and CTOD is large in uncharged specimens, while the crack tip is very sharp and CTOD is very small in charged specimens. Nevertheless, the stretch zone width of hydrogen charged specimens is approximately equal to that of uncharged specimens. Considering the increase in the crack growth rate and the morphology of the striation and stretch zone, it has been concluded that the acceleration of fatigue crack growth of hydrogen charged specimens is caused by the continuous hydrogen enhanced slip deformation ahead of crack tip (Hydrogen Enhanced Successive Fatigue Crack Growth, HESFCG).

Low Temperature Effects on the Fracture Behavior of Cold-Worked STS 304 Stainless Steel for Membrane of LNG Storage Tank

The temperature dependence of the tensile properties and the fracture toughness of the cold-worked STS 304 stainless steel have been examined in the temperature range of 293 K to 111 K. The tensile strength significantly increases with a decrease in temperature, but the 0.2% yield strength is relatively insensitive to temperature. The total elongation at 193 K abruptly decreases by 50% of that at 293 K, and it decreases slightly at 193 K to 111 K. The strain hardening exponents at low temperatures are about four times as high as that at 293K. Initiation fracture toughness (Jc) and tearing modulus (Tmat) tend to decrease with a decrease in temperature. The Jc values exhibit an inverse dependency on the effective yield strength (σflow) at all the test temperatures. Fractographic examination revealed that the critical stretch zone width (SZWc) at room temperature was about three times as large as that at 111 K. This indicates that the variation in fracture toughness according to temperature corresponds to the decrease in SZWc with decreasing temperature.

Environmental Effects on the Fracture Toughness of a Duplex Stainless Steel (UNS S31803)

Abstract The environmental effect on the mechanical properties of steels, specifically hydrogen embrittlement (HE), has been widely investigated and is one of the major factors responsible for in-service material failure. Testing methods like slow strain rate testing (SSRT) of tensile specimens have been used but the results are qualitative. This study aims at evaluating the effects of hydrogen on fracture toughness of a duplex stainless steel (UNS S31803). The effect of hydrogen on the J-integral and crack tip opening displacement (CTOD) was studied in this investigation. CTOD is a geometric measurement used to calculate the fracture toughness. Cathodically precharged specimens have been tested according to the ASTM E1820. It was seen that this standard fails to identify the initiation J-integral for these tough materials. Stretch zone width (SZW) was estimated from fractographs to estimate the initiation toughness. Results indicate that hydrogen pickup adversely affect the initiation toughness as well a...

233 炭素量0.08mass%の低炭素鋼板の疲労き裂進展とストレッチゾーンに及ぼす水素の影響(OS8-1 水素環境が金属強度特性に及ぼす影響,OS8 水素環境が金属強度特性に及ぼす影響)

Fatigue crack growth tests were conducted on hydrogen charged and uncharged specimens of a 0.08 mass%C low carbon steel sheet at test frequency 0.001 -10Hz. After fatigue crack growth tests, a stretch zone was formed in order to investigate the mechanisms of fatigue crack growth in the hydrogen environment. From the observation of striation and stretch zone on the fracture surface and slip bands on the specimen surface, it was revealed that uncharged specimens have no dependency of crack growth rate on test frequency and on the other hand, the crack growth rate of hydrogen charged specimens at the lower test frequency is approximately 10 times faster than that of uncharged specimens. There exists an upper limit of the increase in crack growth rate of the hydrogen charged specimens at test frequency below 0.01 Hz. The upper limit is almost equal to the maximum of stretch zone width which is inclined. The experimental results show that the fatigue crack growth of hydrogen charged specimen is related to the hydrogen enhanced localized plasticity.

The effect of variation of microstructure on fracture mechanics parameters of HSLA-100 steel

The variations in the FCGR curve, the J–R curve, the blunting-line slope and the stretch zone width (SZW) for systematic variation of microstructure in Cu-strengthened HSLA-100 steel has been studied. The microstructural variation in the steel has been introduced through ageing at various temperatures after an initial quenching treatment. This has resulted in progressive tempering of the as-quenched martensitic matrix, accompanied by nanoscale precipitation of coherent Cu particles that gradually coarsen and loose coherency with overageing. It was observed that although there was a systematic trend, the FCGR curves were least sensitive to microstructural changes. The variation of fracture toughness, characterized by JQ in most cases and by KQ for the microstructures displaying highest strengths, correlated well with the inverse relationship between fracture toughness and strength. A systematic trend was also observed for the pre-exponent and exponent of the power-law tearing curve (for cases in which brittle fracture was precluded), the blunting-line slope and the SZW. The effect of coherency of precipitates in restricting plastic flow, as implied from the variation of mechanical properties with ageing temperature is thought to be responsible for the effects observed.

A comparison of micro, meso and macroscale FEM analysis of ductile fracture in a CT specimen (mode I)

Several methods are available to understand the process of crack initiation and propagation in ductile materials. In an attempt to achieve an overall understanding, some of these techniques were studied using a large deformation based finite element method (FEM). In the current investigation, typical crack tip blunting prior to ductile fracture behavior of a standard (CT) specimen under mode I loading condition was simulated using FEM. An attempt was made to understand the ductile fracture by numerically determining the ductile fracture toughness at three length scales: macroscopic scale (load–displacement method), mesoscopic scale (path-integral method) and microscopic scale (stretch zone width method). In addition, the characteristic distance (lc), commonly defined as the distance between the crack tip and the void responsible for eventual coalescence with the crack tip, was also studied. Although approximate, lc assumes a special significance since it links the fracture toughness to the microscopic mechanism considered responsible for ductile fracture.

On Measuring Mixed Mode I/III Fracture Toughness of Ductile Materials Using the Critical Stretch Zone Width

Abstract Mixed mode fracture I/III toughness determination for ductile materials currently uses an empirical equation for the blunting line. A modification based on the use of experimental measurements of the critical stretch zone width (SZWc) for the blunting line is suggested. A further modification, also based on SZWc measurements, that gives a notch root independent mixed mode I/III fracture toughness from a single specimen is described. Mixed mode fracture toughness tests on Armco iron, CP titanium, mild steel, and nickel have been carried out to evaluate these methods.