Stretch Zone Research Articles

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.

Fracture toughness in a rolled AZ31 magnesium alloy

The effect of texture on fracture toughness was investigated for a wrought AZ31 magnesium alloy, which was a commercial rolled plate having strong basal texture. The value of plane-strain fracture toughness, K IC = 17.6–20.7 MPam 1/2, was obtained from the stretched zone (SZ) analysis. The value of K IC varied with the distribution of basal texture; the sample having a pre-crack normal to rolled direction was the highest value of K IC in all present samples. The crack-tip having parallel to the rolled direction was easily able to propagate and/or proceed with applied load.

Stretch-zone analysis by image processing for the evaluation of initiation fracture toughness of a HSLA steel

In this work, stretch zone width and stretch zone depth of ductile fracture surfaces of a high-strength low-alloy steel at various prestrained conditions are computed using digital image processing techniques, where the images are taken by scanning electron microscopy. Histogram equalisation is applied on the input image for enhancement of the image quality. Various edge detection filters, such as Laplacian, Sobel, and Kirsch are separately applied to the digital image to detect the edges of the stretch-zone. The images obtained after processing are binarized through thresholding for identifying the stretch zone boundaries and to compute the stretch-zone width. It is observed that the Laplacian filter produced better results compared to the other filters. The measured values of stretch-zone width and stretch-zone depth by image analysis agreed well with those obtained manually; however they showed discrepancy with the same obtained indirectly from the fracture-resistance curve. The reasons for such a discrepancy are highlighted in this paper. From the estimated stretch-zone width, initiation fracture toughness of the HSLA steel has been obtained.

Fractal analysis along stretch zone for an aluminum alloy

The topography of fracture surface along stretch zone front for Al7050 is analyzed about its fractal behavior and compared against local distributions of microstructural parameters and stretch zone height, considered here as a toughness parameter. Major influence on microscale was presented by precipitation density. Larger grains should be significant on topographic behavior at macroscale, besides the local toughness measured along stretch zone. The large scattering of fractal measurements along specimen width should limit the validity of models relating fractal values and fracture toughness. It is proposed that models based on mixed fractals must also consider some dispersion parameter instead of mean fractal measurements due to the overall complexity of fracture relief formation. It is suggested that sampling for fractal measurement must be restricted to plane strain region along fracture surface, due to smaller scattering in this region.

Initiation fracture toughness of HSLA steel through automatic measurement of stretch zone

Stretch zones are formed due to crack tip blunting in ductile materials before the initiation of a crack. Using special edge detection filters, an automatic measurement procedure based on image analysis (IA) for estimating stretch zone dimensions has been proposed. The in house developed software based on the proposed technique requires only fractographic images to automatically define stretch zone boundaries. Employing the proposed technique, stretch zone measurements were carried out on a high strength low alloy (HSLA) steel. The initiation fracture toughness of this HSLA steel has been estimated using these stretch zone dimensions. A comparison of measured stretch zone dimensions with the crack tip blunting region of the fracture resistance curve revealed that the blunting behaviour is non-linear and the data range normally considered for constructing the blunting line is inadequate to represent this non-linearity for these steels. Prediction of initiation fracture toughness (Ji) in high strength, high toughness materials therefore requires blunting line estimation from a modified data range. An upper bound blunting line data range is proposed for accurate prediction of Δacr (SZW) and Ji. The results of this study indicate that the IA technique can be used effectively for accurate determination of SZW, SZD, Ji, and JQ. It is also noted that for materials with non-linear blunting behaviour, SZW is a better parameter than SZD for predicting Ji and JQ.

Modelling ductile fracture behaviour from deformation parameters in HSLA steels

ABSTRACTIn this work, an attempt is made to model the ductile fracture behaviour of two Cu‐strengthened high strength low alloy (HSLA) steels through the understanding of their deformation behaviour. The variations in deformation behaviour are imparted by prior deformation of steels to various predetermined strains. The variations in parameters such as yield strength and true uniform elongation with prior deformation is studied and was found to be analogous to that of initiation fracture toughness determined by independent method. A unique method is used to measure the crack tip deformation characterized by stretch zone depth that also depicted a similar trend. Fracture toughness values derived from the stretch zone depth measurements were found to vary in the same fashion as the experimental values. A semiempirical relationship for obtaining ductile fracture toughness from basic deformation parameters is derived and model is demonstrated to estimate initiation ductile fracture toughness accurately.

04/00548 Simulation for steam reforming of natural gas with oxygen input in a novel membrane reformer: Chen, Z. et al. Fuel Processing Technology, 2003, 83, (1–3), 235–252.

This paper summarizes results of the work carried out on the evaluation of the susceptibility to LME (Liquid Metal Embrittlement) of the ferritic/martensitic steel T91 in contact with LBE (Lead–Bismuth Eutectic). The influence of LBE on the fracture toughness of the steel was studied using 0.5T CT specimen at 355 °C, pre-cracked by cyclic loading in the liquid metal. Tests were carried out in well-defined conditions and according to ASTM standard. It was observed that the LBE decreased the apparent fracture toughness, JIC, by more than 30%, compared to the value in air. The results are discussed based on examinations of the fracture surface evidencing LME occurrence. The stretch zone accompanying the pre-crack tip blunting was not observed in the specimens exhibiting LME. Therefore, a new fracture toughness, Jmap, determined as J integral at the maximum applied load, is proposed to be the appropriate value for fracture resistance evaluation in LBE. The Jmap can be applied for the assessment of a pre-existing LME crack stability.

Numerical simulation of viscous filament stretching flows

A numerical study on the stretching of a Newtonian fluid filament is analysed. Stretching is performed between two retracting plates, moving under constant extension rate. A semi‐implicit Taylor‐Galerkin/pressure‐correction finite element formulation is employed on variable‐structure triangular meshes. Stability and accuracy of the scheme is maintained up to large Hencky‐strain levels. A non‐uniform radius profile, minimum at the filament mid‐plane, is observed along the filament‐length at all times. We have found maintenance of a suitable mesh aspect‐ratio around the mid‐plane region (maximum stretch zone) to restrict early filament break‐up and consequently solution divergence. As such, true transient flow evolution is traced and the numerical results bear close agreement with the literature.

Backscattered electron microscopy technique enhancing stretch zone width imaging for initiation fracture toughness measurements

Backscattered and secondary electron microscopy techniques have been compared in determining critical stretch zone widths (SZW C) formed in the original and thermally embrittled microstructures of a nuclear reactor pressure vessel steel tested under Charpy impact loading conditions. It has been shown that the former technique does improve fracture surface image quality, as compared to the widely used secondary electron imaging mode, and so accurate measurements of initiation fracture toughness have been achieved over the whole range of mechanical behavior studied. Hence, a Hall–Petch dependence of the SZW C on the representative cell size of the materials tested could be identified.

Investigation of blunting line and evaluation of fracture toughness under mixed mode I/III loading in commercially pure titanium

The blunting line and fracture toughness in commercially pure titanium under mode I and mixed mode I/III loading was studied. A modified compact tension geometry was used for determining the blunting line as well as mixed mode I/III fracture toughness. The results showed that the constraint factor m in the blunting line equation under mode I loading was 1.84. Also, there was no effect of notch root radius on the slope of the blunting line. The blunting line slope under mixed mode I/III loading was found to be lower than that under mode I loading and agreed with empirical correlations. The fracture toughness under mode I loading was found to be higher for specimens with larger notch root radius. However, notch root independent fracture toughness could be obtained from blunt notch specimen tests using stretch zone width measurements. The fracture toughness was found to decrease with increasing mode III loading.

An evaluation of cast stainless steel (CF8M) fracture toughness caused by thermal aging at 430°C

Cast stainless steel may experience embrittlement when it is exposed approximately to 300°C for a long period. In the present investigation, the three classes of the thermally-aged CF8M specimen were prepared using an artificially-accelerated aging method. After the specimens were held for 300, 1800 and 3600hrs. at 430°C, respectively, the specimens were quenched in water which is at room temperature. Load versus load line displacement curves andJ- R curves were obtained using the unloading compliance method,JIC values were obtained using the ASTM E 813–87 and ASTM E 813–81 methods. In addition to these methods,JIC values were obtained using the SZW (stretch zone width) method described in JSME S 001–1981. The results of the unloading compliance method areJQ = 543.9kj/m2 for virgin materials. The values ofJIC for the degraded materials at 300, 1800 and 3600hrs. are obtained 369.25kJ/m2, 311.02kJ/2, 276.7kJ/2, respectively. The results obtained by the SZW method are compared with those obtained by the unloading compliance method. Both results are quite similar. Through the elastic-plastic fracture toughness test, it is found that the value ofJIC is decreased with an increase of the aging time.

Grain size effects on the critical stretch zone width of charpy impact specimens

The elastic-plastic fracture process from a pre-existing crack in a monotonically loaded test specimen or structure consists of several stages: crack tip blunting, plastic zone formation at crack tip, nucleation and growth of voids ahead of crack tip, coalescence of voids, i.e., crack growth, along with the development of plastic wake, and final rupture. Just before crack initiation, which corresponds to the coalescence of the original crack tip with the nearest void, crack stretching is an important stage in the fracture phenomenon [1–6]. Observed on the microscope, this particular region exhibits a typical featureless appearance, which in most cases can be distinguished from the prior precrack surface and the subsequent main fracture by ductile tearing. Since the critical stretch zone width (SZWC) is indicative of the extent of the plastic blunting of the crack tip, it offers an alternative method for determining a fracture toughness parameter that can be correlated with the critical values of energetic and geometric-based fracture mechanics parameters JIC and CTODC, respectively. The main advantage of SZWC over other fracture toughness indexes is that no load, displacement or any other operational parameters need to be known. This is particularly important in post-mortem failure assessment or when load and displacement are absent or difficult to determine, which include the case of fracture toughness determination under dynamic loading conditions, where it is not possible or at least feasible to apply the standard J-R curve approach, e.g. by compliance or potential drop measurements, in order to determine the onset of crack growth. Furthermore, it does seem interesting to have a permanent record that can be verified independently in various laboratories, in round robin programs, so long as the permanent plastic strains on the fracture surface are a record of the failure process. In this regard, the stretch zone width has been claimed to be an effective back-up technique for characterization of the fracture toughness of metals and weldments. In the present study, a thermally embrittled nuclear grade steel was impacted in a Charpy testing machine and the SZWC of the precracked specimens was measured and tentatively correlated to the representative cell size of the tested microstructures, which have been fully characterized elsewhere [7, 8]. Basically, the A508 Class 3A steel is an exceptionally high-toughness

Plastic zone development in dynamic tear-type test specimens

The J-integral is a fracture criterion, which permits measurement of the fracture toughness of a specimen where fracture occurs in the elastic–plastic regime. An understanding of the ratio of plastic zone size (radius) to the crack tip blunting (stretch zone) is required to determine the upper temperature for transition curves, where elastic–plastic fracture becomes invalid and general yielding occurs. This study endeavors to acquire this ratio using finite element techniques. The development of the plastic zone in dynamic tear (DT) specimens and non-standard three-point bending fracture test specimens was the main focus of the study. The ABAQUS finite element software was used to model the elastic–plastic behaviour of the specimens. The cracks in the specimens were induced by pressing the notch followed by fatigue cracking at 30–40% of the limit loads. The shapes of these cracks were adequately modelled in the finite element analysis. The specimens were made of 350WT steel and 304 stainless steel materials and were loaded until fixed amounts of permanent deformation were recorded. Results were obtained in the form of plots, showing the progression of the plastic zone around the crack tip. For each case, mid-point plastic deflection, stretch zone width and plastic zone radius were computed.

On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part II: influence of hydrogen trapped by inclusions

High cycle fatigue fracture surfaces of specimens in which failure was initiated at a subsurface inclusion were investigated by atomic force microscopy and by scanning electron microscopy. The surface roughness Ra increased with radial distance from the fracture origin (inclusion) under constant amplitude tension–compression fatigue, and the approximate relationship: Ra ≅ CΔK 2I holds. At the border of a fish‐eye there is a stretched zone. Dimple patterns and intergranular fracture morphologies are present outside the border of the fish‐eye. The height of the stretch zone is approximately a constant value around the periphery of the fish‐eye. If we assume that a fatigue crack grows cycle‐by‐cycle from the edge of the optically dark area (ODA) outside the inclusion at the fracture origin to the border of the fish‐eye, we can correlate the crack growth rate da/dN, stress intensity factor range ΔKI and Ra for SCM435 steel by the equation and by da/dN proportional to the parameter Ra .Integrating the crack growth rate equation, the crack propagation period Np2 consumed from the edge of the ODA to the border of the fish‐eye can be estimated for the specimens which failed at Nf > 107. Values of Np2 were estimated to be ∼1.0 × 106 for the specimens which failed at Nf ≅ 5 × 108. It follows that the fatigue life in the regime of Nf >107 is mostly spent in crack initiation and discrete crack growth inside the ODA.

Quantification of the warm prestressing effect in a shape welded 10 MnMoNi 5-5 material

This work describes investigations of the warm prestressing (WPS) effect in a 10 MnMoNi 5-5 shape welded material. Three point bend tests of precracked specimens were carried out for the following load cycles: cooling and loading to fracture without prestressing (CF), warm prestressing with loading, cooling, fracturing (LCF), and loading, unloading, cooling, fracturing (LUCF). The specimens were prestressed at room temperature (RT) and at a load level corresponding to K WPS of 70–80 MPa m 1/2, and fractured in the temperature range of −130°C to −158°C. Fracture surfaces of a number of specimens were observed in detail to measure the stretch zone width, the size and distance of cleavage origins from the blunted crack tip and to determine the character of the origins. For various loading cycles the stress distribution ahead of the crack tip and the crack tip opening displacement were numerically calculated. An increase in the apparent fracture toughness after WPS can be correlated with the geometry of the specimens, the loading cycle and, in particular, with the blunting of the original crack during prestressing. Based on the numerical results, the local fracture stress, σ f , was estimated. The local fracture stress turns out to be almost constant (2400–2500 MPa) for different specimens and load cycles. This suggests the use of σ f as a criterion to predict cleavage fracture after WPS. A statistical model is developed to predict failure loads from the calculated stress and strain fields. It is based on the Beremin concept and evaluates the incremental fracture probability, which depends not only on the maximum principal stress, but also on the equivalent plastic strain and its increment. This incremental probabilistic failure model predicts realistic fracture loads after WPS compared to the experiments, whereas the original Beremin model yields values which are too low.