Fracture Initiation Behavior Research Articles

Part I-the effects of pre-dissolved hydrogen on cleavage and grain boundary fracture initiation in metastable beta Ti-3Al-8V-6Cr-4Mo-4Zr

The effects of electrochemically pre-dissolved hydrogen on room-temperature fracture initiation in Beta-C titanium (Ti-3Al-8V-6Cr-4Mo-4Zr wt pct) have been investigated using circumferentially notched tensile specimens. Finite element-based analysis of notch stress fields was used to define relationships between the local threshold stress for crack initiation vs total internal hydrogen concentration. The as-received, solution heat treated (ST, σy.2 pct=865 MPa) and the ST + peak-aged conditions (STA, σy.2% pct=1260 MPa) were compared after defining the relationships between the fracture process zone hydrogen concentration, hydrogen-metal interactions (i.e., hydrostatic stress field occlusion, trapping, hydriding), and the resulting fracture initiation behavior of each. Solutionized + peak-aged (β+α) Beta-C fractured intergranularly above total hydrogen concentrations of ∼1000 wt ppm. (5.1 at. pct). A fracture mode consistent with cleavage occurred at ∼2100 wt ppm. (10.7 at. pct). Solutionized Beta-C resisted hydrogen-assisted cracking (e.g., did not crack intergranularly) but was not immune; cleavage cracking was provoked at ∼4000 wt ppm. (20.4 at. pct). Coldworked ST Beta-C (CW, σy.2 pct=1107 MPa) did not crack intergranularly; fracture initiation behavior was similar to the ST condition regardless of specimen orientation. This suggests that high yield strength alone does not account for the susceptibility to intergranular cracking observed in the STA β+α condition. Stroke-rate studies and X-ray diffraction investigation of H partitioning suggests that equilibrium hydriding and/or irreversible trapping does not singularly control intergranular fracture initiation of the STA condition. Fractographic evidence and finite element results show that a finite plastic zone exists prior to intergranular fracture of the STA condition. This suggests that a criterion for fracture that incorporates plastic strain and stress should be considered.

Influence of defect shape on damage evolution and fracture behavior of ductile materials

A generalized standard material model is developed for ductile materials containing aligned defects with arbitrary shape under self-similar expansion based on the work of Nguyen and Bui [Sur les materiaux elastoplastiques a ecrouissage positif et negatif. J. Mecanique 13, 321 (1974)] and Rousselier [Finite deformation constitutive relations including ductile fracture damage, in Three-dimensional Constitutive Equations and Ductile Fracture (Edited by S. Nemat-Nasser), pp. 331–355, North-Holland (1981)]. The influence of defect shape on damage evolution and instability of the material is investigated. The finite element calculation is made for notched and cracked specimens, with microvoid and microcrack damage. The result shows that the defect shape has significant influence on the fracture initiation and crack growth behavior. The present model is also applied to predict the J-resistance curve of a CT specimen for an aged duplex stainless steel, and the results are in good agreement with the experiment.

The effects of strain rate, temperature and constraint on the large scale yielding fracture initiation behaviour of a mining steel

L'article decrit la mesure de la resistance dans les conditions de fracture dynamique d'un acier minier. Les conditions de temperature, du taux de tension et de la contrainte sur le choix du materiel etaient variees. On s'est servi des modeles theoretiques qui existent deja pour faire l'etude des relations entre les proprietes micro-structurales et les valeurs mesurees de la resistance

Dynamic fracture initiation behavior of an HY-100 steel

An investigation was conducted into the effects of test temperature and loading rate on the initiation of plane strain fracture of an HY-100 steel. Fracture toughness tests were conducted using fatigue precracked round bars loaded in tension to produce a quasi-static stress intensity rate of ·K1 = 1 MPa√m/s and a dynamic rate of ·K1 = 2 × 106 MPa√m/s. Testing temperatures covered the range from -150 °C to 200 °C, which encompasses fracture initiation modes involving quasi-cleavage to fully ductile fracture. The results of toughness tests show that the lower-shelf values of fracture toughness were substantially independent of loading rate, while the dynamic values exceeded the quasi-static values by about 50 pct on the upper shelf. In analyzing these results, phenomenological fracture initiation models were adopted based on the requirement that, for fracture to occur, a critical strain or stress must be achieved over a critical distance. In separate tests, the observation of microfracture processes was investigated using fractography and anin situ scanning electron microscope (SEM) fracture technique. The layered ppearance of the fracture surfaces was found to be associated with a banded structure which generally contains many MnS inclusions, probably resulting in a reduction of the fracture toughness values.

Influence of microstructure on crack-tip micromechanics and fracture behaviors of a two-phase TiAl alloy

The tensile deformation, crack-tip micromechanics, and fracture behaviors of a two-phase (γ + α2) gamma titanium aluminide alloy, Ti-47Al-2.6Nb-2(Cr+V), heat-treated for the microstructure of either fine duplex (gamma + lamellar) or predominantly lamellar microstructure were studied in the 25 °C to 800 °C range.In situ tensile and fracture toughness tests were performed in vacuum using a high-temperature loading stage in a scanning electron microscope (SEM), while conventional tensile tests were performed in air. The results revealed strong influences of microstructure on the crack-tip deformation, quasi-static crack growth, and the fracture initiation behaviors in the alloy. Intergranular fracture and cleavage were the dominant fracture mechanisms in the duplex microstructure material, whose fracture remained brittle at temperatures up to 600 °C. In contrast, the nearly fully lamellar microstructure resulted in a relatively high crack growth resistance in the 25 °C to 800 °C range, with interface delamination, translamellar fracture, and decohesion of colony boundaries being the main fracture processes. The higher fracture resistance exhibited by the lamellar microstructure can be attributed, at least partly, to toughening by shear ligaments formed as the result of mismatched crack planes in the process zone.

Measurement and analysis of the fracture of a liquid crystal polymer

The fracture initiation behaviour of injection-moulded plaques of a liquid-crystal polymer is described. Attention is restricted to cracks in the plane of the plaques propagating along the mould fill direction. The material is both anisotropic and inhomogeneous; the latter factor is accommodated by adopting a skin-core model of the plaque structure. The inhomogeneity leads to mixed mode (Modes I and II) fracture in some cases, and both single and mixedmode testing is used. The results can be represented in terms of either stress intensity or strain energy release rate by simple mixed-mode failure criteria.

Dynamic fracture behavior of SiC whisker-reinforced aluminum alloys

This paper presents a study of dynamic fracture initiation behavior of 2124-T6 aluminum matrix composites containing 0, 5.2, and 13.2 vol pct SiC whiskers. In the experiment, an explosive charge is detonated to produce a tensile stress wave to initiate the fracture in a modified Kolsky bar (split Hopkinson bar). This stress wave loading provided a stress intensity rate, KI,, of about 2 × 106 MPa√m/s. The recorded data are then analyzed to calculate the critical dynamic stress intensity factor,KId, of the composite, and the values obtained are compared with the corresponding quasi-static values. The test temperatures in this experiment ranged from −196 °C to 100°C, within which range the fracture initiation mode was found to be mostly ductile in nature. The micromechanical processes involved in void and microcrack formation were investigated using metallographic techniques. As a general trend, experimental results show a lower toughness as the volume fraction of the SiC whisker reinforcement increases. The results also show a higher toughness under dynamic than under static loading. These results are interpreted using a simple dynamic fracture initiation model based on the basic assumption that crack extension initiates at a certain critical strain developed over some microstructurally significant distance. This model enables us to correlate tensile properties and microstructural parameters, as, for instance, the interspacing of the SiC whiskers with the plane strain fracture toughness.

The effects of tempering and test temperature on the dynamic fracture initiation behavior of an AISI 4340 VAR steel

An investigation was conducted into the effect of tempering, test temperature and loading rate on the fracture initiation behavior of an AISI 4340 VAR steel. The fracture initiation tests were conducted by stress wave loading of a prefatigued circumferential crack in a notched round bar. The geometry provides for plane strain conditions at the fracture site, and the instrumentation gives records of average stress at the fracture site and of crack opening displacement, both as functions of time. A stress intensity rate K I of about 2 × 10 6 Mpa m 1 2 s −1 is attained by this technique, and a comparison is presented with results obtained in static loading with the same geometry. Finally, the dynamic and quasi-static stress-strain behavior in shear was determined by loading a thin-walled tubular specimen in a torsional Kolsky bar. The strain rates were γ = 10 3 s −1 and γ = 10 −4 s −1. In addition, Charpy specimens of the various tempers were tested. These showed a toughness trough indicative of tempered martensite embrittlement in those specimens tempered at 350°C. However, in the fracture initiation tests with the notched round bar the embrittlement trough was observed only in tests conducted below room temperature. Extensive quantitative fractography was performed on the plane strain fracture specimens to identify the dominant mode of fracture initiation for each temper at the different rates and test temperatures.

Effect of Prestrain at Elevated Temperature on the Fracture Behavior of High Strength Steels

It is well known that fracture initiation behavior is influenced by weld thermal straining. In the present report, attention is focused on the influence of weld thermal straining on the fracture mechanism. The influence of prestrain at elevated temperature or strain-aging on the initiation and propagation behaviors of ductile and cleavage fracture in KD32 and HT80 steels is investigated. A relationship between the increase in the lowest temperature at which a tear dimple region can be observed (Ti) or the decrease in stretched zone width and the increase in hardness at the notch tip is found.

ＨＴ８０鋼溶接部不溶着切欠からのぜい性破壊発生特性

To assess the degree of embrittlement in a transformed HAZ and the possibility of additional embrittlement caused by hot straining, brittle fracture initiation behavior from a notch of poor penetration in a weldment of HT 80 steel of 31 mm thickness has been investigated by measuring critical crack opening displacement (COD) at fracture. Three points bend specimens of 15 mm thickness were extracted from butt joint with weld defect of poor penetration, and the tip of the notch was located on a fusion' line in modified single=bevel joints, or, it was located on a weld metal in modified single-Vee joints. In these butt joints the degree of hot straining was altered by changing a intensity of tensile restraint in modified Lehigh restraint test specimens.Heat input had large effect on the fracture toughness of fusion line, and reference temperature (Tr: critical COD of 0.25 mm) was markedly riled with increasing the heat input up to 42 KJ/cm. Fusion line and coarse-grained region of the HAZ was susceptible to hot straining embrittlement whereas the weld metal was not. The quantitative relation between the intensity of tensile restraint and the increase in reference temperature (ΔTr) was obtained. Small region ahead of the poor penetration in the joint of high tensile restrain received severe hot straining and showed high Tr temperature, and it was concluded that this severe hot straining was caused by restricting the free contraction of the weldments.In the as welded and hot strained conditions, all the specimens with notch of poor penetration were more brittle than the specimens with fatigue pre-crack extracted from parent material, and thermal stress relief had disadvantage effect on fracture toughness of fusion line. In the most specimens extracted from modified double-bevel joints, crack propagated along fusion line.

Effect of Mechanical Heterogeneity on Brittle Fracture Behaviors

In the welded joint, mechanical properties of weld metal and heat-affected zone are different from those of base metal. Brittle fracture initiation behaviors of the weldments will be different from those of the homogeneous and isotropic material. In the present paper, attention in focussed on brittle fracture initiation behaviors of the weldments having heterogeneity under tension parallel to the weld line.Deep notch tests of model specimens (Fig.1) having mechanical heterogeneity near crack tip were carried out. Effects of mechanical heterogeneity on deformation behaviors of crack tip and brittle fracture strength were clarified. Expending the Dugdal's idea, the relationship between the applied stress and crack opening displacement of notched specimens having mechanical heterogeneity can be calculated, and those calculated results agree well with the experimental values. The estimation of fracture stress for this heterogeneous material can be made by COD-criterion.

Initial yielding and fracture in notched sheet molybdenum

The initial yielding and fracture initiation behavior of wrought stress-relieved and recrystallized molybdenum was investigated at room temperature. The effect of notch severity and grain size on the nature of plastic flow in notched sheet tensile specimens was investigated by means of photoelastic coatings. The nature of fracture initiation in notched sheet tensile specimens was studied through the use of pre-polished and etched tandem notch specimens. Application of the Griffith-Orowan theory for crack propagation and the Cottrell criterion for crack initiation reveal that the effective surface energies for crack propagation and crack initiation are of the same order of magnitude, which is 10 3 ergs/cm 2. This finding is supported by the experimental observation that no microcracks were found. Plastic flow regions determined experimentally from the photoelastic coating studies were shown to be in good agreement, both qualitatively and quantitatively, with theoretical results predicted by elastic-plastic stress analyses. Initial yielding at the notch root was found to occur at the same nominal stress level for the given material condition, independent of the magnitude of the stress concentration factor. This nominal stress in the notch section was very nearly equal to the smooth tensile yield stress.