A study on the ASTM E1921 standard in determining the fracture toughness of ferritic steels

One of the most important aims of the fracture mechanics is to determine the fracture toughness of a material. Various methods were developed for this purpose and have been still used nowadays. In the J‐integral method that is one of them, providing of a dominant linear elastic condition on the specimen is not required. However, in ferritic steels, the fracture toughness values (K_JC) obtained by the J‐integral method show some inconsistencies. Therefore, the ASTM E1921 standard was developed on ferritic steels, which are instabilities in the values of elastic or elastoplastic fracture toughness. In this study, a new method was used to determine the fracture toughness (K_IC) of ferritic steels, and it was compared with the standard. Three steels with different mechanical properties and average grain size were investigated in this study.     

ASTM E 837标准20版与08版和13版的主要差异对比

通过对ASTM E 837-08、ASTM E 837-13和ASTM E 837-20版标准进行对比分析发现：根据工件厚度等因素的不同,ASTM E 837-20对残余应力测量方法的描述最全,补全了08版和13版缺乏的中等厚度工件的测量方法。同时20版通过采用几种常用的钻孔直径、钻孔深度和工件厚度的规制化设计方案,引入孔径系数、应力深度系数和工件厚度系数,新增孔径系数计算公式、应力深度系数和工件厚度系数等计算公式等来简化计算实际孔径、实际应力深度和实际工件厚度条件下的应力释放标定常数,删减了大量繁琐的查表工作,方便了标准使用者的理解与操作。另外20版对工件厚度、钻孔钻速下限、钻孔偏心距离和测量结果适用范围的规定,与08版和13版相比,也有差异,总体是放松了要求,扩大了该标准的使用范围。总体而言,ASTM E 837-20标准的适用性和可操作性大大提高,逻辑性更加清晰。     

Gurson model parameters for ductile fracture simulation in ASTM A992 steels

This study is concerned with the modelling the ductile fracture in ASTM A992 steels using the Gurson‐Tvergaard‐Needleman (GTN) model for high stress triaxiality regime. The GTN model for ASTM A992 structural steels is calibrated from the experiments performed on axisymmetrically notched tensile specimens. The experiments are designed to obtain a range of stress triaxiality and different fracture initiation locations. The non‐uniqueness in the constitutive parameters of the GTN model is illustrated in this study. The choice of a unique set of GTN constitutive parameters is made by choosing the nucleation strain (?_N) as a material constant. The process of estimating this material specific nucleation strain is provided. All the other GTN model parameters corresponding to the material specific nucleation strain (?_N) are evaluated to best fit the experimental results. The calibrated GTN model is shown to predict the load displacement behaviour, ductility and fracture initiation locations in the notched specimens. The calibrated GTN parameters are used to successfully predict the ductility of structural components: (a) bars with a hole; (b) plate with reduced section and (c) plate with holes; that are typically found in structural engineering applications.     

A modification of ASTM E 1457 C estimation equation for compact tension specimen with a mismatched cross-weld

For the compact tension (CT) specimen with a mismatched weld in creep properties, an equivalent homogeneity model is presented based on the limit load analysis. In use of the proposed equivalent material model and existing limit load solutions, the modified experimental C^∗ integral estimation equations for the CT specimens with weld centre crack, fusion line crack and asymmetric crack were derived. Finite element (FE) analysis for C^∗ integral of CT specimens with a mismatched weld was performed by using the ABAQUS software. Results show that, compared with the η_o factor for homogeneous CT specimens, a soft under-mismatched weld will increase the non-dimensional factor η_w while a hard over-mismatched weld will decrease the η_w value. It indicates that the current C^∗ estimation equation in ASTM E1457 may underestimate C^∗ for creep soft weld but overestimate it for creep-hard weld. The discrepancies between the C^∗ from ASTM E1457 and FE analysis are sensitive to the crack location in weld, to the mismatch ratio and to the weld width. Whether for under-mismatched weld (M < 1) or for over-mismatched weld considered herein, the predicted C^∗ parameters from the presented formulae are very close to FE results and thus can be used to interpret the creep crack growth behaviour of welded specimens.     

Comparison of CTOD standards: BS 7448-Part 1 and revised ASTM E1290

Crack tip opening displacement (CTOD) has been calculated using the plastic hinge model with an assumed rotational center since the British Standards Institution (BS) standardized BS5762 in 1979. The American Society for Testing and Materials (ASTM) accepted the plastic hinge model and standardized E1290 in 1989. However, ASTM revised E1290 in 2002, and has proposed a conversion from J to CTOD. CTOD-based fracture toughness evaluation has been widely used for the defect assessment of many welded structural components, and two different CTOD calculations could lead to confusion for Fitness-for-Service. In this study, the effects of CTOD testing methodologies on CTOD values were investigated according to round robin tests conducted by the Japan Welding Engineering Society (WES), and the concept of CTOD as a fracture parameter is discussed.     

Separability property and ηpl factor in ASTM A387-Gr22 steel plate

In this work the possibility of obtaining the function G(b/W) from the variable separability property to calculate η_pl factor following the methodology proposed by Sharobeam and Landes using a deformation function H with total displacement was studied. The material employed was an ASTM A387-Gr22 steel plate and standard ASTM SE(B) and C(T) geometries were used for the test pieces with different side grooving ratios.For SE(B) specimens, the differences observed between η_pl factor values determined with total displacement, plastic displacement and that recommended in ASTM 1820-96 were not significant. For C(T) specimens, the differences observed were lower than 2% for all side grooving ratios studied. The η_pl factor values resulting from the use of Sharobeam and Landes methodology using total and plastic displacement did not seem to follow the dependence with crack length suggested by ASTM E1820-96. It was not possible within the scope of this work to find a side groove influence on the η_pl factor value for this geometry.     

Estimation of ASTM E-1921 reference temperature from Charpy tests: Charpy energy-fracture toughness correlation method

In this paper, some of the older and newer Charpy-fracture toughness correlations have been examined and new correlations have been developed for predicting the ASTM E-1921 standard reference temperature, T₀. The results have been applied to some selected new steels and compared with measured T₀, where available. It is found that the predicted reference temperature from the new procedure gives reliable and acceptably conservative engineering estimate of T₀. Predicted values of reference temperatures under dynamic conditions from these T₀ using Wallin’s strain rate shift equation agree well with measured dynamic values for a few steels giving added support to the new procedure.     

Microstructural,mechanical and electrochemical characterisation of biomaterial ASTM F745 cast by vacuum

Abstract

Countergravity low pressure casting (CLA) was performed to enhance the properties of ASTM F745 stainless steel (SS), which is usually used as biomaterial. The macro- and microstructures were compared with those obtained by the conventional process of investment casting (IC). The SS cast by CLA (SSCLA) exhibited a smaller size of solidification cell and finer dendritic microstructure. The average of its dendritic primary spacing was 110·4 μm, while for the same steel cast by IC (SSIC), it was 186·7 μm. The density of non-metallic inclusions δ_I in the SSCLA was 717 I mm^?2, being the majority of them smaller than 1·5 μm. In the case of SSIC, δ_I was 852 I mm^?2, with a size distribution of up to 8 μm. The SSCLA showed a higher breakdown potential than the SSIC, the values being 0·300 and 0·210 V(saturated calomel electrode) respectively, which means a higher resistance to suffer localised corrosion. Finely, the CLA process also allowed obtaining better mechanical properties.     

ASTM E381-01标准简介及与国内相关标准的对比

简要介绍了ASTM E381-01标准的使用范围、试样的选取与制备、侵蚀剂的配制及标准评级图的概况。与GB／T1979-2001，YB／T4002-991，YB／T4003-1991和YTB／T153-1999标准在使用中的共性和差别进行了比较。     

A direct estimate of JIc from the load versus load-line displacement record

The purpose of this study is to examine the current standard fracture toughness test procedure to determine if there could be an easier method to get a J_Ic value from the test record. The current method for determining J_Ic involves a detailed computational and construction procedure. The objective in this study is to simplify the analysis for the determination of J_Ic. The results of this study show that the load and displacement record for a fracture toughness test can be used to directly estimate a J_Q value, a provisional value for fracture toughness, J_Ic. The J value taken at the maximum load point can be used along with an adjustment factor to estimate a J_Q value. This J_Q estimate is close to the one obtained from the construction procedure of ASTM Standard E 1820. When a unit‐sized specimen is tested, that is, a specimen with a width of 50 mm and a thickness of 25 mm the maximum load point provides a direct estimate of J_Q. Other sizes require a size adjustment factor, which is simply a square root relationship between the width of the test specimen and a unit width. The proposed new method of estimating J_Q is simple in concept and requires a minimum number of calculations. It appears to produce values of J_Q which are comparable to those obtained from the ASTM E1820 construction procedure and may produce less scatter.     

A new method for determining the fracture toughness of main pipeline steels

One of the fundamental aims of fracture mechanics is to define fracture toughness K_IC of a material. Hence, the ASTM E399 standard was developed. However according to the standard, large‐sized specimens are required to determine the fracture toughness of low alloy carbon steels. ASTM E1921 standard was developed on the fracture toughness of ferritic steels. In this study, a new method was proposed to determine the fracture toughness of ferritic steels. The purpose of the present paper is to compare the results of the method with the experimental results. Two steels that are used in gas and oil main pipelines were investigated in this study.     

Evaluation of a method to characterize material inhomogeneity in ferritic steels within the ductile-to-brittle transition regime

The ASTM “Standard Test Method for Determination of Reference Temperature, T_o, for Ferritic Steels in the Transition Range,” (ASTM E1921) has been developed to characterize the onset of cleavage cracking in the ductile-to-brittle transition regime for ferritic structural steels. This standard determines T_o, the reference temperature at which the median fracture toughness of a 25 mm thick specimen is 100 MPa . Material inhomogeneity has been shown to have a strong effect on T_o, and suggestions on how to characterize and quantify the effects of inhomogeneity have been developed within a proposed annex for E1921.This study demonstrates that for small data sets neither the current E1921 outlier procedure nor the method in the proposed annex can effectively distinguish between homogeneous and inhomogeneous materials. This study also shows that the bimodal inhomogeneity analysis in the proposed annex only accurately characterizes materials if (1) at least 30 specimens are tested, and (2) the difference between the T_o values that describe each unique homogeneous population within the inhomogeneous material is greater than 20 °C, and (3) the percentage of the smallest homogeneous material population is greater than 20%.     

Evaluation of the ISO <Emphasis Type="Italic">J</Emphasis> initiation procedure using the EURO fracture toughness data set

The multiple specimen J _0.2/BL initiation fracture toughness test procedure from the ISO standard, ISO 12135:2002, is evaluated using the EURO fracture toughness data set. This standard is also compared with the ASTM standard, ASTM E 1820, multiple specimen J _Ic procedure. The EURO round robin data set was generated to evaluate the transition fracture toughness methods for steels. However, many of the tests resulted in ductile fracture behavior giving final J versus ductile crack extension points. This is the information that is measured in a multiple specimen J initiation fracture toughness test. The data set has more than 300 individual points of J versus crack extension with four different specimen sizes. It may be the largest data set of that type produced for one material. Therefore, its use to determine J initiation values can provide an important evaluation of the standard procedures. The results showed that a J _0.2/BL value could be determined from the ISO standard for three of the four specimen sizes, the smallest size did not meet the specimen size requirement on J. The construction line slopes in this method are very steep compared with the ASTM construction line slopes. This resulted in low J initiation values, about a factor of two lower than the one from the ASTM method. Of the various criteria imposed to determine a valid J _0.2/BL value, the one limiting the maximum J value was the most questionable. It had an effect of eliminating small specimen data that was identical to acceptable large specimen data.     

ASTM力学性能试验总则标准与分项试验方法标准的技术比较

对美标钢制品力学性能试验总则标准（ASTMA370—2010）与分项试验方法标准（ASTMES／E8M-2009,ASTME10—2010,ASTME18—2008b,ASTME23—2007a^ε1和ASTME384—2010^ε2等）分别从标准的适用范围、环境温度的定义、采用的单位制、对试样和试验设备的要求、尺寸测量精确度、拉伸试验的速率、不同硬度标尺间的相互转换以及试验结果的处理与修约等方面进行了系统的对比分析。结果表明：ASTM力学性能试验总则标准和与其相对应的ASTM分项试验方法标准之间存在着一定的技术差异,其中尤以拉伸试验和冲击试验方法标准与总则标准之间的差异居多,实际使用中应注意区分和选择。     

Single and multi-specimenR-curve methods forJ IC determination of toughened nylons

For characterization of the fracture resistance of materials used in the upper shelf toughness regime,J-R curves are widely considered the most promising candidates. However, there still remain problems concerning both the generation and measurement ofJ-R curves as material characterizing parameters and their application in ductile fracture analyses for failure prediction in polymeric materials. This paper reports the results of investigations conducted on two rubbertoughened nylons at room temperature. Two different methods ofJ-R curve determination are covered, namely multi-specimen and single specimen test methods. The resultingJ-R curves have also been evaluated to obtain values of the initiation toughness,J _IC, following the extrapolation and interpolation schemes prescribed by ASTM E813-81 and ASTM E813-87 test procedures, respectively. The results show that the multiple specimen unloading method and the single specimen partial unloading compliance method can be used to generate comparable crack growth resistanceJ-R curves of the toughened nylons. The value ofJ _IC for the crystalline rubber-toughened nylon was approximately twice the value obtained for the amorphous rubber-toughened nylon. The former material also exhibited a greater resistance to ductile crack growth.     

Fracture mechanics testing on specimens with low constraint--standardisation activities within ISO and ASTM

Fracture mechanics tests are traditionally designed to measure material resistance to stable or unstable crack extension using specimens that are highly constrained to plastic deformation. For a variety of reasons, structural members may be made of thin gage-materials with inherently low constraint to plastic deformation. There is currently little guidance for measuring crack extension resistance under such conditions. The international standards organisations ISO and ASTM are responding to that need, and this paper describes one aspect of their current activity.Two procedures are being developed; one based on the δ₅ crack opening displacement parameter, the other on the constant value of the crack tip opening angle, ψ_c. The measurement of δ₅ is well established and relatively simple, whereas ψ_c is more difficult to determine experimentally. Evaluations of ψ_c from finite-element analyses are currently the most accurate approach, since measurements can only be made on the exterior surfaces similar to δ₅. Questions naturally arise regarding the correspondence of surface indication with full-thickness response in the laboratory experience. Both measures of crack extension resistance are suitable for structural assessment. The δ₅ concept is applied by means of crack driving force formulae from existing assessment procedures and hence relatively easy to use. On the other hand, the CTOA concept is potentially more accurate and can be applied to cases of multiple cracks and complex structures. But its structural application requires numerical methods, which have been successful in predicting the failure of large-scale cracked structural components.     

Determining fracture toughness of polyethylene from fatigue

A newly developed fatigue method of determining the value of the J-integral at crack initiation, J _1C, was tested on single-edge-notch-tension (SENT) specimens of a high-density polyethylene (HDPE). The fatigue method is able to propagate plane-strain brittle cracks in thin specimens which more closely approximate the in-use thickness, and hence the in-use morphology, of the polymer. This newly developed fatigue method was compared to ASTM standard procedure E813 (the ASTM method), which employs thick single-edge-notch-bend (SENB) specimens. Because of the similar size and shape of the crack-tip damage zone, the J _1C values for both methods were nearly identical (J _1C=1.8 kJ m^–2, 1.4 kJ m^–2 for the ASTM method and fatigue method, respectively).     

Effect of size on plasticity and fracture toughness

The effect of thickness and width of CT specimens on the plastic zone size has been investigated. The investigation also examines the applicability of the various approaches to fracture toughness measurements such as ASTM E399 K_Ic, J_Ic, R-curve and a new procedure of K_Ic determination proposed recently, to a steel which has medium strength and which undergoes pop-in type unstable crack extension during the toughness testing. The results show that in CT specimens with a constant aspect ratio prepared from a given material and loaded to a given stress intensity factor, the plastic zone size decreases, as the specimen width increases; on the other hand, the thickness has only a comparatively small effect on the plastic zone size. The new procedure of K_Ic determination is verified with experiments on specimens with width to thickness ratio of about 50 and it gives a size independent K_Ic value in specimens whose thickness is 4.5 times less than that required by ASTM E399. The new procedure gives a meaningful value of K_Ic in a situation where all other methods such as ASTM E399 K_Ic, J_Ic and R-curve approach are observed to be inapplicable.     

ASTM布氏硬度试验标准简介

介绍了ASTM金属材料布氏硬度试验方法并与相应的国家标准进行了比较。     

Experimentally determined key curves for fracture specimens

Crack extension during fracture toughness tests of ferritic structural steels cannot be determined from measurements of unloading compliance or electric potential change when the specimen is dynamically tested. Measurements of crack extension in fracture toughness tests are also very difficult when the test temperature is high or the test environment is aggressive. To circumvent this limitation, researchers for years have been developing key curve and normalization function methods to estimate crack extension in standard elastic-plastic fracture toughness test geometries. In the key curve method (Ernst et al., 1979; Joyce et al., 1980) a load-displacement curve is measured for a so-called `source' specimen that is sub size or has a blunt notch so that the crack will not initiate during elastic-plastic loading. The load and displacement are then converted to normalized stress-strain units to obtain a key curve that can be used to predict crack extension in geometrically similar `target' specimens of same material loaded at similar loading rates and tested under similar environmental conditions. More recently Landes and coworkers (Herrera and Landes, 1990; Landes et al., 1991) proposed the normalization data reduction technique – Annex A15 of ASTM 1820 specification – that presents an alternative to the standard E1820 unloading compliance procedure. Although the normalization method works well in many cases, it has serious drawbacks: the load, displacement and crack length at the end of the test must be measured; the prescribed functional form that is fitted to the initial and final data may not be accurate for all materials; and the iterative method of inferring crack length from the combination of the data and the normalization function is complex. The compliance ratio (CR) method developed in this paper determines key curves for predicting crack extension as follows. First, a statically loaded source specimen with the unloading compliance procedure specified in ASTM 1820. Second, the so-called CR load-displacement curve is calculated for the source specimen, which is the load-displacement record that would have been obtained if the crack had not extended. Third, non-dimensionalizing the CR load by the maximum load and the displacement by the elastic displacement at the maximum load, P ^* _i/P _max and v _i/v _{el max} from the source specimen yields the adjusted key curve. Analysis of extensive data shows that the key curve is independent of notch type, initial crack length and temperature. But it is dependent on specimen size and steel type. Assuming that the key curves of the source and target specimens are one and the same, the compliance of the target specimens are calculated with a reverse application of the compliance ratio method, and the crack length is obtained using the equations in ASTM E1820. The CR Method is found to be much simpler than the normalization method described in the Annex A15 of ASTM 1820. With the compliance ratio method, Joyce et al. (2001) successfully predicted crack extension in dynamically loaded specimens using a key curve of a statically loaded specimen.