On the embrittlement of fracture toughness specimens of two high strength steels

Hydrogen embrittlement of two commercial materials, a 1$\text{1}\text{2}$ Ni-Cr-Mo steel and a 5 Cr-V-Mo steel both quenched and tempered to a nominal ilrength of 1.60 × 10³MN/m² has been studied. Frecture toughness testing was carried out on each steel in three conditions, (a) heat-treated and tested in air, (b) thermally charged with hydrogen and tested in air, and (c) uncharged and tested a an atmosphere of dry hydrogen. The effects of hydrogen pressure and loading rate on embrittlement were examined. The experimental results show that thermal charging had a greater embrittling effect on the 1$\text{1}\text{2}$ Ni-Cr-Mo steel than on the 5 Cr-V-Mo steel although the latter had a higher hydrogen concentration. In a hydrogen environment the 1$\text{1}\text{2}$ Ni-Cr-Mo steel was much more sensitive to embrittlement than the 5 Cr-V-Mo steel.     

Microstructural features affecting fracture toughness of high strength steels

The fracture toughness of quenched and tempered steels, such as AISI 4340, AISI 4130 and 300M, can be increased by 50–100% by minor changes in heat treating procedures. Certain microstructural features, particularly blocky ferrite, upper bahnte and twinned martensite plates, are deleterious to fracture toughness. Similarly, the presence of undissolved carbides and sulfide inclusions, which act as crack nuclei, can lower fracture toughness by 25–50%. Other microstructural constituents, such as lower bainte, autotempered martensite, and retained austenite can enhance fracture toughness. By controlling the amounts and distributions of the microstructural constituents, the fracture toughness values of AISI 4340, AISI 4130 and 300M can be raised to the fracture toughness level of 18Ni maraging steel at equivalent values of yield strength.     

A mechanical model to predict elastic-plastic fracture toughness in high strength materials

A mechanical model of crack initiation and propagation, which is based on the actual mechanism of ductile fracture in high strength materials, is proposed. Assuming that a crack initiates when the equivalent stress at a distance ρ from the crack tip reaches a critical value $\text{}\text{?}$gsf, an equation for predicting fracture toughness J_IC is obtained. From comparison between the predicted values and the experimental results, it is found that the distance ρ corresponds to the spacing of micro-inclusions. The temperature dependence of fracture toughness J_IC estimated according to the derived equation is given in an Arrhenius form of equation and is nearly consistent with the experimental results.     

Development of fracture toughness of ultrahigh strength low alloy steels for aircraft and aerospace applications

Abstract

Recently, ultrahigh strength low alloy steels, e.g. AISI 4340 and 300M, have been used increasingly for critical structural aircraft and aerospace applications. These steels can be employed successfully at yield strengths of ≥1400 MN m^?2 but their use has often been limited in commercial practice because of low fracture toughness compared with other types of ultrahigh strength steel. The results of studies carried out over the past two decades to improve the fracture toughness are presented. Particular emphasis is placed on improvements obtained by microstructural control via thermal and thermomechanical treatments, sulphide inclusions, and new alloying design. The major metallurgical factors controlling fracture toughness are discussed for each of these techniques.

MST/1413     

Upper shelf temperature dependence of fracture toughness for four low to intermediate strength ferritic steels

To examine the inadequacy and expose the potential consequences of utilizing transition temperature range fracture toughness values when performing fracture mechanics analyses on structures operating at upper shelf temperatures, the upper shelf fracture toughness behavior of three rotor forging steels (ASTM A471 NiCrMoV, ASTM A470 CrMoV and AISI 403 modified 12 Cr) plus an ASTM A217 $\text{2}\text{1}\text{4}\text{Cr}$-lMo cast steel was investigated. Elastic-plastic fracture toughness values were obtained via the $\text{J}{}_{\mathrm{IC}}$ resistance curve test technique. Except for the CrMoV rotor steel, upper shelf fracture toughness peaked near the temperature where zero percent brittle fracture first occurred in the Charpy V-notch impact tests after which decreases of 25–45% in toughness were realized. Fracture toughness tests were conducted up to 800°F.     

无钴高强韧钢力学性能研究

本文利用人工神经网络方法研究了化学成分和热处理条件对G50钢力学性能的影响,通过电镜观察分析了合金元素间交互作用形成的组织和第二相的作用,结果C、Cr、Mo提高钢强度,Ni、Mo、Si降低钢强度,C,Mn降低韧性,Cr,Mo,Ni提高韧性.     

Combined mode I - mode III fracture toughness of a high carbon steel

• 1.1. Brittle materials exhibit minimum energy dissipation under pure mode I loading
• 2.2. In these materials, the mode IIIcomponent essentially provides redundant plastic work and increases J_totalc.
• 3.3. This behavior contrasts with more ductile materials where combined mode I - mode III loading produces The minimum energy dissipation.

     

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.     

Strain induced fracture in low strength steels

This paper attempts to investigate the role of strain in the fracture of low strength steel in the temperature range of −196 to 28°C and at the crack tip strain rate range of 10⁻⁵ to 10°/sec. The tensile test data is suitably processed to estimate the fracture strain in the fracture toughness specimen. The micro-fractographic observations help to identify the microfracture mechanisms and relate them with the governing fracture criteria. Even for the non-microvoid coalescence mode of fracture in the alloys in the temperature region of −140 to −77°C, a significant role of plastic strain in fracture is identified. The significance of the critical strain zone for fracture is discussed and it has been compared with the process zone at fracture.     

Analytic and numerical studies on mode I and mode II fracture in elastic-plastic materials with strain gradient effects

This paper presents a study on fracture of materials at microscale (∼1 μm) by the strain gradient theory (Fleck and Hutchinson, 1993; Fleck et al., 1994). For remotely imposed classical K fields, the full-field solutions are obtained analytically or numerically for elastic and elastic-plastic materials with strain gradient effects. The analytical elastic full-field solution shows that stresses ahead of a crack tip are significantly higher than their counterparts in the classical K fields. The sizes of dominance zones for mode I and mode II near-tip asymptotic fields are 0.3l and 0.5l,while strain gradient effects are observed within land 2l to the crack tip, respectively, where l is the intrinsic material length in strain gradient theory and is on the order of microns in strain gradient plasticity (Fleck et al., 1994; Nix and Gao, 1998; Stolken and Evans, 1997). The Dugdale–Barenblatt type plasticity model is obtained to provide an estimation of plastic zone size for mode II fracture in materials with strain grain effects. The finite element method is used to investigate the small-scale-yielding solution for an elastic-power law hardening solid. It is found that the size of the dominance zone for the near-tip asymptotic field is the intrinsic material lengthl. For mode II fracture under the small-scale-yielding condition, transition from the remote classical K _IIfield to the near-tip asymptotic field in strain gradient plasticity goes through the HRR field only when K _IIis relatively large such that the plastic zone size is much larger than the intrinsic material length l. For mode I fracture under small-scale-yielding condition, however, transition from the remote classical K _I field to the near-tip asymptotic field in strain gradient plasticity does not go through the HRR field, but via a plastic zone. This revised version was published online in July 2006 with corrections to the Cover Date.     

The use of stretch zone width measurements in the determination of fracture toughness of low strength steels

The evaluation of fracture toughness from stretch zone measurements on test piece fracture surfaces using selective backscattered electron imaging in the scanning electron microscope is described. Results are compared with the initiation toughness values obtained using single specimen $\text{R}$ curve procedures.     

Relationship between fracture topography and fracture toughness of a high strength steel

The relationship between fracture topography and the plane strain fracture toughness of Comsteel En25 tempered at nine different temperatures and with crack planes in the R-L, R-C and L-R orientations has been studied. The results show that fracture toughness is qualitatively relatable to the fracture morphology. Due to the shape and alignment of the elongated MnS inclusions, fracture toughness in the L-R orientation was found to be 1.8 times those in the other two orientations. Terrace-type fracture prevailed in the R-L orientation, but this frequently was observed to change to zigzag type fracture in the R-C orientation. However, both these fracture mechanisms were absent in the L-R orientation.     

Dynamic fracture toughness of a high strength armor steel

This paper summarizes the results of a research being carried out to determine fracture behavior both in static and dynamic conditions of high strength armor steel Armox500T. In this research, notched specimens were cut to be tested in three-point bending test. Specimens were pre-cracked by flexural fatigue. Thereafter, some specimens were tested in bending up to rupture to determine the static fracture toughness K_IC. To obtain fracture toughness in dynamic conditions, a split Hopkinson bar modified to perform three-point bending tests was used. In this device, displacements and velocities of the specimen were measured, as well as the rupture time by means of fracture detection sensors, glued to the specimens. After that, a numerical simulation of the test was performed by using LS DYNA hydrocode, obtaining stresses and strain histories around the crack tip. From these results, the stress intensity factor history was derived. By using the rupture time, measured by the sensors, the value of the fracture toughness computed was unrealistic. Therefore, the use of a numerical procedure to obtain the rupture time was decided, by comparing experimental results of velocities at the transmission bar with numerical results obtained with several rupture times. With this procedure, the computation of dynamic fracture toughness was possible. The method shows that the measurement of the dynamic fracture toughness is possible without the needs of using crack sensors or strain gauges. It can be observed that fracture toughness of this steel under static and dynamic conditions is quite similar.     

Effects of crack depth on elastic-plastic fracture toughness

Short crack test specimens (a/W 0.50) are frequently employed when conventional deep crack specimens are either inappropriate or impossible to obtain, for example, in testing of particular microstructures in weldments and in-service structures containing shallow surface flaws. Values of elastic-plastic fracture toughness, here characterized by the crack tip opening displacement (CTOD), are presented for square (cross-section) three-point bend specimens with a/W ratios of 0.15 and 0.50 throughout the lower-shelf and lower-transition regions. Three dimensional, finite-element analyses are employed to correlate the measured load and crack mouth opening displacement (CMOD) values to the corresponding CTOD values, thus eliminating a major source of experimental difficulty in previous studies of shallow crack specimens. In the lower-transition region, where extensive plasticity (but no ductile crack growth) precedes brittle fracture, critical CTOD values for short crack specimens are significantly larger (factor of 2–3) than the CTOD values for deep crack specimens at identical temperatures. Short crack specimens are shown to exhibit increased toughness at the initiation of ductile tearing and decreased brittle-to-ductile transition temperatures. Numerical analyses for the two a/W ratios reveal large differences in stress fields ahead of the crack tip at identical CTOD levels which verify the experimentally observed differences in critical CTOD values. Correlations of the predicted stresses with measured critical CTOD values demonstrate the limitations of single-parameter fracture mechanics (as currently developed) to characterize the response.     

遗传算法在无钴高强韧钢设计中的应用

提出一种利用残缺实验数据建立无钴高强韧钢力学性能与钢的合金成分及热处理条件关系模型的方法,依据该方法建立了人工神经网络模型,并应用跗算法,对无钴高强韧钢进行了优化设计,实践表明所提出的方法是有效的,为研制新材料开辟了一条新途径。