Stretch Zone Width Research Articles

The effect of constraint on fracture properties of Z3CN20.09M after accelerated thermal aging

The main coolant piping material Z3CN20.09 M was heat treated at 400 °C for 18,000 h and then the fracture behavior has been studied. To understand the effect of in-plane constraint on fracture behavior, J-R curves were determined by the pre-cracked specimens with a wide range of crack depth ratios. And the stretch zone width (SZW) values were measured by experimental method and finite element method (FEM) based on fracture energy density. The comparisons of SZW value show that good agreements have been achieved among experimental tests and FEM. Moreover, to reveal and quantify the constraint effect (a/W), three-dimensional FEM was used to obtain the second terms in the Williams’ series expansion. The J-R curves, JIC and JSZW and SZW are higher at lower crack depth ratios a/W, and gradually decrease with increasing crack depth. Then, the constraint dependent fracture properties of Z3CN20.09 M were determined and could be used to predict fracture properties under other constraint conditions.

Effect of multiple heat treatments on fracture property of centrifugal casting stainless steels Z3CN20.09M with long-term thermal aging degradation

It is of great significance to investigate effect of multiple heat treatments on fracture property of centrifugal casting stainless steels Z3CN20.09M cut from pump casing with long-term thermal aging degradation for nuclear power plants to consider actual operation of nuclear power plants. Both multiple heat treatments and accelerated thermal aging experiment at the same temperature of 400 °C for different time were successively carried out on centrifugal casting stainless steels Z3CN20.09M in order to examine the metallographic modification and impact properties. Finally, an additional investigation on the related fracture properties was carried out, in which the critical initial fracture toughness Ji was determined by stretch zone width and 0.2 mm offset line methods. These results indicated that the multiple heat treatments led to the dispersed distribution of ferrite phases in austenite matrix and thus microhardness increased, but impact energy exhibited a decreasing tendency significantly. After long-term aging, the metallographic structure remained almost unchanged, but the size of ferrite phases showed a slight increasing trend because of spinodal decomposition in ferrite phases and G-phase precipitation. In addition, centrifugal casting stainless steels Z3CN20.09M with multiple heat treatments exhibited the higher microhardness, Charpy impact toughness, critical initial fracture toughness JIC (J-integral determined by 0.2 mm offset line method), and JSZW (J-integral determined by stretch zone width method) than those with primary heat treatment, while the specific number of the heat treatment had a low influence on fracture toughness.

Dynamic fracture behavior of low carbon bainitic steel after different welding thermal cycles

Dynamic fracture toughness is an important parameter to assess the fracture resistance at crack initiation although the actual crack initiation point is hard to be precisely detected. In this work, three different heat treatment microstructures (i.e., hot-rolled, welding thermal simulation coarse grained HAZ (CGHAZ) and intercritically reheated CGHAZ microstructures) were studied by instrumented Charpy impact tester. The dynamic fracture toughness was obtained based on impact load-deflection curves using various estimation methods. The dynamic fracture toughness decreased with the applied welding thermal cycles irrespective of the estimation methods. The stretch zone width proved that the fracture toughness is most acceptable when the crack initiation point is considered as the middle point between the general yield load and maximum load in the load-deflection curves.

Strength and Fracture Toughness of Hardox-400 Steel

This paper presents results of strength and fracture toughness properties of low-carbon high-strength Hardox-400 steel. Experimental tests were carried out for specimens of different thickness at wide temperature range from −100 to 20 °C. The dependences of the characteristic of material strength and fracture toughness on temperature based on experimental data are shown. Numerical calculation of the stress and strain distributions in area before crack tip using the finite element method (FEM) was done. Based on results of numerical calculation and observation of the fracture surfaces by scanning electron microscope (SEM), the critical local stress level at which brittle fracture takes place was assessed. Consideration of the levels of stress and strain in the analysis of the metal state at the tip of the crack allowed to justify the occurrence of the brittle-to-ductile fracture mechanism. On the basis of the results of stretch zone width measurements and stress components, the values of fracture toughness at the moment of crack initiation were calculated.

Method comparison for the determination of stretch zone parameters

The fracture behaviour of ductile failing materials is evaluated by parameters of yielding fracture mechanics. According to ISO 12135 the determination of fracture mechanical characteristic values involves, among other things, analysing the stretch zone width (SZW) in the scanning electron microscope (SEM). This procedure is very time-consuming and requires great experience in handling SEM images of fracture surfaces. An alternative method for determining the stretch zone parameters works with 3D elevation profiles of the fracture surface in the stretch zone area taken with a digital microscope. The fracture mechanical parameters determined using the above methods were finally compared. The Ji and δi values of scrap steels resulting from the digital determined SZW show a good agreement with analytically determined crack initiation values (Ji/BL, δi/BL).

A comparison between fracture toughness at different locations of SMAW and GTAW welded joints of primary coolant piping

Two primary coolant pipes were narrow-gap multipass circumferentially butt welded by gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW) methods separately and then subjected to micro-hardness tests to distinguish the base metal (BM), heat affected zones (HAZs), fusion zones (FZs) and weld metal (WM). Subsequently, uniaxial tensile tests were performed with a 3D DIC system to investigate the strain evolution of each area in GTAW and SMAW welded joints and to compare respective tensile properties. The fracture toughness has been investigated at the above four different locations of the SMAW and GTAW welded joints. Then the 0.2 mm offset line method and the stretch zone width method have been both employed to determine the critical initial fracture toughness Ji. The results indicate that the fusion zones (FZs) have the worst fracture toughness compared with other locations over both two types of weld joints. In addition, the GTAW welded joints have a better comprehensive performance than the SMAW welded joints.

Fracture Energy and Fracture Morphology after Three-Point Bending Test of Welded Joints Made of Cast Steel Designed for Use in Power Sector, with and without the Addition of Rare Earth Metals

The paper reports the three-point bending test results of welded joints. The joints were made of chromium-molybdenum cast steel designed for the use at elevated temperatures. TIG (tungsten inert gas) welding technique was used. The fracture energy for particular joint zones and the stretch zone width (SZW) under the notch bottom were determined in a qualitative fracture toughness assessment. Fracture surface morphology was analyzed. The stretch zone measurement indicated a qualitative relationship between its width and the values of fracture energy. The results confirmed the influence of the modification on the character of fracture and the portions of brittle and ductile fracture in particular areas of welded joints.

Temperature dependent crack initiation of 42CrMo4 steel at high loading rates

Dynamic crack initiation with crack tip loading rates K˙ of approximately 2 ‧ 106 MPa√ms− in high-strength 42CrMo4 steel was investigated. To this end, a recently developed split Hopkinson pressure bar with four-point bending was utilized. V-notched and precracked Charpy specimens were tested. The tests were performed at temperatures of –40 °C and 20 °C. The loading of the specimen was determined by analyzing the strain in the incident and transmission bars. Furthermore, strain gauges at the specimen’s surface were applied to measure the crack tip loading. High-speed photography complemented the analysis of the specimens loading and the detection of the crack initiation. Fracture surface analysis by means of scanning electron microscopy enabled the measurement of the fracture surface topography and, consequently, stretch zone height and width. Hence, the macroscopically measured dynamic crack initiation toughness was correlated with the toughness at microscopic scale. It was observed that the resistance against dynamic crack initiation decreased with decreasing temperature. Microscopically, a decrease in toughness was analogously observed. Non-metallic inclusions resulted in crack path deflection with localized shear zones. After a small stable crack extension, cleavage fracture was observed.

Effect of long-term aging on the fracture toughness of primary coolant piping material Z3CN20.09M

In this study, a series of accelerated thermal aging tests were performed on primary coolant piping materials (Z3CN20.09M) at 400 °C for 0, 2000, 5000 and 18,000 h to investigate the resulting microstructure change and fracture toughness. To further assess their fracture toughness, 0.2 mm offset line method and stretch zone width method were both employed to determine the fracture toughness. The results indicated that after long-term thermal aging, round shaped particles and characteristic morphology of spinodal decomposition were found in ferrite, thus resulting in the thermal aging embrittlement of Z3CN20.09M. Subsequently, the J-R curves and J-T curves all decreased significantly with the increasing thermal aging time. Furthermore, the fracture toughness parameters J50, JSZW(2D), JSZW(3D) and dJ/dΔa were first decreased rapidly and then slow saturated. In addition, the JSZW(2D) and JSZW(3D) values were significantly lower than JQ while the reduction of the critical crack size based on JSZW(2D) and JSZW(3D) parameters was about 30% compared with JQ, and thus JSZW was relatively close to the onset of crack initiation.

Fracture Toughness of Z3CN20.09M Cast Stainless Steel with Long-Term Thermal Aging

Accelerated thermal aging tests were performed at 400 °C for nearly 18,000 h on Z3CN20.09M cast stainless steel which was used for primary coolant pipes of nuclear power plants. A series of Charpy impact tests were conducted on Z3CN20.09M after different long-term thermal aging time. The test results indicated that the Charpy impact energy of Z3CN20.09M cast stainless steel decreased rapidly at an early stage and then almost saturated after thermal aging of 10,000 h. Furthermore, J-resistance curves were measured for CT specimens of longitudinal and circumferential pipe orientations. It showed that there was no obvious difference in the fracture characteristics of Z3CN20.09M in different sampling directions. In addition, the observed stretch zone width (SZW) revealed that the value of initiation fracture toughness J SZW was significantly lower than that of fracture toughness J IC, indicating a low actual crack initiation energy due to long-term thermal aging.

Potential drop method for online crack length measurement during fracture testing: Development of a correction procedure

Though the potential drop method is included in testing standards for creep and fatigue crack growth, it is not used in fracture toughness tests because it overestimates the crack length during the initial loading due to crack tip plastic deformation. In the present paper, a procedure is proposed for correcting for this by using the blunting line equation and stretch zone width, a physical parameter as the reference. Compact tension samples of three different thicknesses were employed. The results after correction are compared with those from the standard methods, viz., elastic unloading compliance and normalization methods.

Numerical characterization of compact pipe specimen for stretch zone width assessment

AbstractPresent work focuses on evaluating the suitability of newly proposed compact pipe (CP) fracture specimen geometry for determining critical SZW. The effect of varying the parameter of CP specimen (outer diameter, thickness, a/W ratio and straight and radial crack front) on critical SZW have been studied. It is found that SZWc values decreases slightly with the increase in outer diameter of CP specimen. In CP specimen, thickness greater than a specified value is to be used for geometry independent evaluation of SZWc. Critical SZW shows significant variation with a/W ratio, straight and radial crack front. The CP specimen and Three Point Bend (TPB) specimen also compared in terms of stress tri‐axiality.

Methodology for numerical assessment of stretch zone width under mixed mode I/III fracture

The numerical prediction of stretch zone width (SZW) under mixed mode condition is rarely reported in the literature. As in most practical situations structures experience complex loading resulting in mixed mode fracture, thus it is essential to establish a methodology for the numerical assessment of SZW under mixed mode fracture condition. Three dimensional non-linear FEM simulations were carried out to correlate numerical parameters with experimentally determined critical SZW and initiation fracture toughness. The present numerical investigation is able to explain the involved mechanism and numerically predicted SZW, critical SZW and initiation fracture toughness which compares well with experimental results.

Evaluating the effect of side grooving parameters on critical SZW

The effect of fracture specimen parameters like side grooving magnitude, its root radius and angle on the critical stretch zone width formation studied. It is found that there is maximum limit of side grooving magnitude and a minimum limit of root radius and angle which will provide geometry independent evaluation of critical SZW. It is concluded that side grooving magnitude should not be larger than 0.25 times thickness; root radius should not be less than 100μm and the side grooving angle should not be less than 70°.

Stretch zone formation in cyclic fracture of 20MnMoNi55 pressure vessel steel

Stretch zone formation during cyclic fracture in a nuclear pressure vessel steel has been examined. Cyclic fracture tests are carried out at different load ratios and incremental plastic displacements. Stretch zone features were manifested in local regions, however, their extent could be correlated to the test parameters employed. The mean stretch zone width decreased with R ratio and increased with incremental plastic displacement. Stretch zone dimension could be empirically correlated to a cyclic stretching coefficient proposed in this investigation. A comparison of cyclic fracture toughness obtained from stretch zone width and that measured experimentally was made, and the systematic difference was explained.

Evaluating a valid fracture specimen geometry for predicting geometry independent SZWc and JSZW value in SA333 Gr. 6 carbon steel material

To obtain geometry independent critical stretch zone width there is a need to understand its variation with fracture specimen thickness, initial crack size, fracture specimen geometries and the type of loading. It comes out that minimum tri-axiality quotient value on the un-cracked ligament and the plastic zone size contained within fracture specimen geometry are the two criteria to be considered while choosing the fracture specimen shape and its geometry for the evaluation of valid critical SZW value of the material. The present study also showed the possibility of further improving the minimum thickness criterion of fracture specimen.

Determination of stretch zone width and height by powerful 3D SEM imaging technology

The focus of the present study is to demonstrate a new method to determine the stretch zone height and width using a fast and easy 3D imaging tool in the scanning electron microscope. The SEM MIRA3 XMU from TESCAN Company is equipped with a unique real-time 3D imaging technology. The paper compares 2D with 3D imaging (as anaglyph pictures) of several fracture surfaces after static fracture toughness tests on austenitic and ferritic steels as well as on cast iron showing more or less well pronounced stretch zones. The 3D images give a clear view of the fracture topography, which can hardly be achieved in 2D by tilting and looking from side positions. It is shown, how 3D reconstruction and length measurement is possible with dual image photogrammetry software allowing a precise determination both of the stretch zone width and height.

Fracture Behaviour of Thin Sheet Stainless Steel

The differences in fracture behavior between the compact tension C(T) and the middle tensile M(T) specimens make structure integrity assessment uncertain. Two different types of specimens C(T) and M(T) specimens made from stainless steel have been used for fracture toughness testing at the room temperature by the principles of the ASTM 1820-05 standard procedure. Stable crack initiation and crack propagation occurred for the C(T) specimens at lower values of crack driving force than for the M(T) specimens. Crack tip opening displacement-CTOD has been directly measured on the surface of specimens by using a stereo-optical grading method. The critical crack tip opening displacement at crack initiation CTODi has been measured as a plastic Stretch Zone Width (SZW) during a post test fractographic inspection. Comparison between the CTOD-R curves of both types of specimens shows some difference between the C(T) and the M(T) specimens, but a more significant difference appeared in the crack driving force, as consequence of different constraint (triaxiality) of the C(T) versus the M(T) specimens. Therefore, the result obtained by test on laboratory C(T) specimens cannot be directly used as fracture toughness material properties in a structure integrity assessment, except as a conservative lower bound estimate.

MRT letter: Extended depth from focus reconstruction method for stretch zone measurement in 15‐5PH steel

The stretch zone width (SZW) data for 15-5PH steel CTOD specimens fractured at -150°C to + 23°C temperature were measured based on focused images and 3D maps obtained by extended depth-of-field reconstruction from light microscopy (LM) image stacks. This LM-based method, with a larger lateral resolution, seems to be as effective for quantitative analysis of SZW as scanning electron microscopy (SEM) or confocal scanning laser microscopy (CSLM), permitting to clearly identify stretch zone boundaries. Despite the worst sharpness of focused images, a robust linear correlation was established to fracture toughness (K(C) ) and SZW data for the 15-5PH steel tested specimens, measured at their center region. The method is an alternative to evaluate the boundaries of stretched zones, at a lower cost of implementation and training, since topographic data from elevation maps can be associated with reconstructed image, which summarizes the original contrast and brightness information. Finally, the extended depth-of-field method is presented here as a valuable tool for failure analysis, as a cheaper alternative to investigate rough surfaces or fracture, compared to scanning electron or confocal light microscopes.

OS0833 限界ストレッチゾーン幅を用いたオーステナイト系ステンレス鋼の破壊じん性評価

J_<IC> tests of SUS304 and SUS316L were performed according to ASTM E1820-06 with the parameter of the cold-work degree and specimen size. Specimen size, stretched zone width, tensile properties and J_Q were gathered, and then the model to estimate the similar value with J_<IC> was discussed. Regression analysis was carried out using the parameter of stretched zone and tensile properties. As the result, J_<IC> estimation expression with the small deviation within 10% was obtained.