Notched Tensile Bars Research Articles

溶融亜鉛めっき槽用鋼板溶接材料の実機槽への適用に関する研究

This study was carried out to select suitable welding methods and welding consumables used for constructing a galvanizing bath. In this report, these were selected in terms of the elevated temperature strength, the susceptibility of liquid metal embrittlement and corrosion resistance in liquid zinc.The base metal used was a carbon steel specialized for a galvanizing bath. The shielded metal arc (SMA) and gas shield arc weldings (GMA) were used with a covered electrode (E1) and solid and flux cored wires (F1, F2 and F3).The elevated temperature strength of all deposited metals were measured at the temperature range from room temperature to 600°C.Moreover, the notched bar tensile (NBT) test, where the bottom of notch was coated by liquid zinc, was performed at the temperature of 480°C to know the susceptibility of liquid metal embrittlement. Corrosion resistance was measured at the temperature of 450°C in molten zinc of practical bath.Strength at the elevated temperature of deposited metals by GMA welding was higher than that by SMA welding. The susceptibilities of liquid metal embrittlement of deposited metals became high in the order of E1, F3, F1 and F2. The susceptibilities of F1 and F3 were almost corresponded with that of the base metal. The susceptibility of liquid metal embrittlement at liquid zinc increased with an increase in the term of C+Si+Mn. Moreover, corrosion resistances of F1 and F2 in liquid zinc were higher than those of the base metal, E1 and F3. When the value of C+Si+Mn exceeds 1.7%, corrosion resistance of weld metal decreases.The solid wire, F3 was. selected as a candidate of welding consumable used for constructing a galvanizing bath from the present results.

Effect of Stress Triaxiality and Strain Rate on Ductile Fracture Initiation in Steel

It is widely known that the critical strain to ductile crack initiation depends on the stress triaxiality. And the ductile crack may occur at the strain concentration region in the structural member unless initial crack by large deformation. Such large deformations occurs during ship crash, seismic condition, etc. And then, strain rates are usually high. But, the strain rate effect to the ductile crack initiation is not known.To study the effect on ductile crack initiation condition, the axisymmetrically notched bar tensile tests were conducted under various tensile speed for SM400B, SM490A, SM490B and SN490B steels. And critical strains to initiate ductile crack were evaluated. To estimate the stress - strain states during dynamic tensile test, the thermal mechanically coupled FE-analysis (MARC K7.2) were used. The results showed the strain rate effect on the critical strain to initiate ductile crack is small. And the results also suggested that if the load is controlled by displacement, the higher yield ratio steel is advantageous for the ductile crack initiation.

Effects of Torsional Buckling on the Cleavage Failure of Low-Alloy Steel Tension Pipe Specimens

The local approach criterion of fracture mechanics, initially developed by Beremin for brittle cleavage fracture, is applied here to A508 class 3 low-alloy ferritic steel. This criterion, based on the maximum principal stress and Weibull statistics, has previously been verified in the case of uniaxial tests. In this study, it is extended to multiaxial loading tests, that can lead to more significant levels of plastic strain, and thus permit a study of the effect of plastic strain on cleavage fracture. Uniaxial tests on axisymmetric notched tensile bars (AE2-6) were used to determine Beremin’s model parameters m and σu. The cleavage fracture behavior, described by these parameters, was then verified by multiaxial tension-torsion tests carried out on thin tubular specimens. Numerical simulations of the tension-torsion tests, by the finite element method, were also performed, taking into account the nonlinear geometrical effects and the specimen plastic buckling. The buckling critical loads were calculated and used to ascertain whether fracture was associated with the instability phenomenon. Beremin’s model is shown to correctly describe experimental data which are not affected by buckling.

Calibration of Weibull Stress Parameters Using Fracture Toughness Data

The Weibull stress model for cleavage fracture of ferritic steels requires calibration of two micromechanics parameters $$(m,\sigma _u ) $$ . Notched tensile bars, often used for such calibrations at lower-shelf temperatures, do not fracture in the transition region without extensive plasticity and prior ductile tearing. However, deep-notch bend and compact tension specimens tested in the transition region can provide toughness values under essentially small-scale yielding (SSY) conditions to support Weibull stress calibrations. We show analytically, and demonstrate numerically, that a nonuniqueness arises in the calibrated values, i.e., many pairs of $$(m,\sigma _u ) $$ provide equally good correlation of critical Weibull stress values with the distribution of measured (SSY) fracture toughness values. This work proposes a new calibration scheme to find $$(m,\sigma _u ) $$ which uses toughness values measured under both low and high constraint conditions at the crack front. The new procedure reveals a strong sensitivity to m and provides the necessary micromechanical values to conduct defect assessments of flawed structural components operating at or near the calibration temperature in the transition region. Results of a parameter study illustrate the expected values of m for a typical range of material flow properties and toughness levels. A specific calibration is carried out for a mild structural steel (ASTM A36).

Distributions of stress and plastic strain in notched tensile bars

The paper deals with a classical problem in fracture mechanics, namely the determination of stresses and plastic strains at the minimum cross-section of a notched tensile specimen. Ideas of Hill and Bridgman are combined to develop a new approximate method for solving this problem. The main aim of the paper is to introduce a damage variable in the analysis without any complicated numerical procedure. The material model proposed by Lemaitre is used to predict the evolution of the damage variable. However, any other model resulting in the incompressibility condition may be adopted with no difficulties.

Effect of void growth and shape on the initiation of ductile failure of steels

Notched tensile bars of a high strength structural steel were loaded to first void coalescence. The results show that damage curves determined by using proportionally loaded specimens are not valid for specimens under non-proportional loading. The results of the experiments with different load histories of notched tensile bars show, that a single parameter criterion like reaching a critical void volume is not sufficient to describe the initiation process of void coalescence. The stress-triaxiality at initiation and the shape of the voids exhibit a strong influence on the initiation values. Two ways to describe initiation are shown: A simple one considers a critical combination of void volume, stress-triaxiality and plastic strain at initiation. The other one uses a shape factor to describe the shape of the voids and calculates the initiation values. Both models work with an accuracy in the order of the experimental scatter.

Photo-degradation of polyethylene and polypropylene at slow strain-rate

An investigation has been conducted of the fracture behaviour of polyethylene (PE) and polypropylene (PP) in the form of notched tensile bars when deformed at a slow strain-rate (~1.3 × 10 −7 s −1) while under ultraviolet (UV) irradiation. The radiation accelerates failure with both materials, although there are differences in the mechanisms by which this occurs. In PE cracking occurs predominantly at the crack tip and is probably directly accelerated by stress-aided photo-oxidation, whereas in PP orientation of the molecules at the crack tip appears to provide some resistance to photo-oxidative chain scission. In PP failure is more influenced by cracking of the photo-degraded surface layer. As a consequence the crack opening displacement (COD) develops more rapidly in PE when exposed to UV, whereas there is far less influence of UV in the COD with PP. Pre-exposure to UV prior to the slow strain-rate test has been shown to cause failure after a much shorter test period. The effect of the direction of the UV irradiation (side-on or edge-on) was not very great, though this may be because an extended source was used and the illumination was not collimated.

Influence of Strain History on Ductile Failure of Steel

Notched tensile bars of a high strength structural steel were loaded to void coalescense. The results show that the damage curve determined using radially loaded specimens is not valid for specimens under non-radial loading. The results of the experiments with different load histories of notched tensile bars show, that a single parameter criterion like a damage parameter is not sufficient to describe the initiation process of void coalescense. The shape of the voids and the triaxiality at initiation exhibit a strong influence on the initation values. There are two ways to describe the initiation. A simple one considers a critical combination of void growth rate, triaxiality and plastic strain at initiation. The other one uses a shape factor to describe the shape of the voids and calculates the initiation values. Both models work with an accuracy in the order of the experimental scatter.

Assessment of Notches in Ceramic Components

The failure probability of notched tensile bars is calculated using the multi-axial Weibull theory. The influence exerted by the stress concentration factor, the stress gradient in the notch root, and the Weibull exponent is analyzed.

Sulfur-induced dynamic embrittlement in a low-alloy steel

The rate of crack growth by apparent intergranular decohesion of a statically loaded MnMoNiCr steel at 535–600°C in vacuum was measured using the d.c. potential-drop method. Slow-displacement-rate testing of notched tensile bars at the same temperatures in an inert atmosphere was also carried out. The steel had been air-cooled from 1300°C to simulate a weld heat-affected zone. Interrupted tests followed by low-temperature brittle fracture were used to clarify the details of the cracking process, which occurred more rapidly in some grain boundaries than in others, giving rise to oscillations in the measured crack velocity. Striations on the intergranular facets were consistent with crack growth in steps of about 0.1 μm. SEM observations indicated that the tip of the main crack had a radius on the sub-micron scale. The cracking behavior of the tensile bars was qualitatively similar to that of the CT specimens and was found to have a temperature dependence of about 55 kcal/mol. The similarity of this phenomenon to cracking in other alloy systems is pointed out. The three contending models for the brittle mode of stress-relief cracking are compared in the light of the present results; a detailed treatment of a proposed theory is given in a separate paper.

Comparison of creep rupture lifetimes of single and double notched tensile bars

Creep rupture behaviour and lifetimes have been studied for two notched bars—one having a single notch and the other a double notch—by use of the creep finite element solver, Damage XX, based on Continuum Damage Mechanics (C.D.M.). A physically-based constitutive equation which utilises two damage parameters was used to model tertiary creep behaviour. Its versatility allows a wide spectrum of material parameters to be used which simulated a range of material behaviour. All computations showed that lifetimes for the double-notch testpiece within 95% of those for the equivalent single notch, and therefore validated the double notch geometry being studied. A consequence of the calculations has been the verification of a previous suggestion that the lifetimes of materials which exhibit tertiary creep due to dislocation strain softening (suc as nickel-based superalloys) could be less affected by stress-state effects on ductility, than materials which do not exhibit such softening (such as pure metals and simple solid solution alloys).

Fatigue and fracture of metallic glass ribbon/epoxy matrix composites

Epoxy matrix composites, reinforced with a very low volume fraction (0·2%) of metallic glass ribbons (Fe 84 B 16 ; aspect ratio width thickness≈50), were fabricated as notched tensile bars with unidirectional ribbon orientation. Fracture mechanics properties (fracture toughness and fatigue crack propagation resistance) were measured under conditions of ribbon normal parallel (composite A) and perpendicular to the crack direction (composite B). Even with the very low volume fraction used, high improvements of both fracture toughness and fatigue crack propagation resistance were achieved relative to the values of the brittle epoxy resin. With respect to the fracture toughness, composite B is superior to A, whereas the opposite is observed for the fatigue crack propagation behavior. In both cases, the particular failure mechanisms have been characterized by the use of optical and scanning electron microscopy. The theoretical description of the results by modified rules of mixture leads to a qualitative understanding of possible changes in the fracture mechanics behavior due to variations in ribbon geometry and other parameters.

A local criterion for cleavage fracture of a nuclear pressure vessel steel

Experiments were performed on three heats of A508 class 3 steel in order to determine the mechanical conditions for cleavage fracture. These tests were carried out on various geometries including 4-point bend specimens and axisymmetric notched tensile bars with different notch radii which have been modelized using the finite element method. In one heat, the temperature range investigated was from 77 K to 233 K. It is shown that the cleavage resistance is increased by tensile straining. Moreover, the probability of fracture obeys the Weibull statistical distribution. All the results can be accounted for in terms of a local criterion based on Weibull theory and which takes into account the effect of plastic strain. In this criterion, the parameters which were experimentally determined are found to be temperature independent over the range 77 K to 170 K. The applicability of the approach proposed for cleavage fracture at the crack tip is also examined. It is shown that the experimental results published in the literature giving the variation of fracture toughness with temperature can be explained by the proposed criterion which predicts reasonably well both the scatter in the experimental results and theKICtemperature dependence.

The influence of hydrogen on plain carbon and low alloy steels containing strain induced cavities

Cavities were created near carbide particles in heat treated steels by various plastic pre-strains on notched tensile bars. The cavities formed preferentially in the central region of the bar. In the quenched and tempered alloy steel “void sheet” formation occurred; whereas only isolated cavities were seen in the mild steel. Small tensile specimens were then machined from the strained notched bars so that the maximum cavity density corresponded to the central region. After vacuum annealing, this region was electrolytically charged with hydrogen and showed an increase in hydrogen absorption. Hydrogen charging caused a reduction in the yield, ultimate and fracture strength and the ductility of the steels. Scanning electron micrographs of the fracture surfaces of the plain carbon steel revealed a reduction in dimple size in the central region. The severely embrittled low alloy steel showed quasi-cleavage fracture. The fracture transition from ductile to quasi-cleavage micromechanism is a function of the amount of hydrogen absorbed into the steel. Hydrogen absorption is related to the size and distribution of the pre-strain cavities.

The effect of high pressure hydrogen gas on the mechanical properties of three Ni based eutectic composites

Smooth and notched tensile bars of directionally solidified Ni-45.5W, Ni-19.7Nb-6.0Cr-2.5Al, and Ni-21.75Nb-2.55Al were tested to failure in 34.5 MN/m2 hydrogen gas and 34.5 MN/m2 helium gas. Decreases in the ultimate tensile strength, uniform elongation, and reduction in area values are found for the samples tested in hydrogen as compared to those tested in helium. The most marked and consistent changes in properties are found for the notched tensile bars. Since the presence of the hydrogen appears to have little effect on the shape of the stress strain curve at small strains, the notched bar test is the better test of hydrogen-induced damage in these materials. Hydrogen damage in the three materials is evidenced by cracking not usually found in samples tested in helium. The damage in the Ni-W eutectic is the most dramatic with extensive transverse and longitudinal cracking prior to failure observed. Additionally, the presence of hydrogen greatly weakens the reinforcement/matrix interface in each case, so that the interface becomes a preferred path for longitudinal cracking. In the other two alloys, grain boundary longitudinal cracking is also found. While no mechanisms have been conclusively established, the most likely source of the degradation is hydrogen trapping at the interfaces related to the presence of segregated trace element S or to the disordered nature of the interfacial region.

Detection of lueders' lines by means of brittle coating

Use is made of an improvement of Durelli, Kobayashi and Hofer's method1 for the detection of Lueders' lines by means of brittle coating. The precision of the method is evaluated by using Fry's etching method in comparison. The propagation of Lueders' lines by increments in notched tensile bars is shown. From a comparison with information contained in literature as regards the propagation of Lueders' lines, it results that the well-known theories of elasticity and plasticity predict the propagation of the Lueders' lines in a qualitatively correct way only in special cases. The technical importance of applying the described brittlecoating method is pointed out.

Fracture Properties of Glass Filled Polyphenylene Oxide Composites

The phenomological fracture properties of glass bead filled polyphenylene oxide composites were investigated. Yield strength and Young's modulus were obtained as a function of volume fraction of filler using standard microtensile testing techniques. Fracture toughness was measured using double edge notched tensile bars. The effects of adhesion were studied by using untreated and A-1100 silane-treated glass beads. The nature of the fracture surfaces was observed by use of an optical microscope, a scanning electron microscope and a transmission electron microscope. It was found that the fracture toughness of these materials de creased by increasing the filler content and by improving the adhesion. In general, an increase in strength and stiffness was accompanied by a decrease in toughness. The fractographic studies gave a detailed mapping of the frac ture front as it propagated through the material. Fracture occurred in two stages. The initial stage was a region of stable crack growth accompanied by, crazing, heat generation and viscous flow in the resin matrix. At a well defined condition, the cracks became un stable and catastrophic failure ensued. The fracture mechanism in the unstable region was distinctly different than in the stable region. The exact mechanism was strongly dependent on the degree of adhesion between the phases. It is felt that a reasonably clear picture has been presented of how filled polyphenylene oxide polymers fail and how some of the variables affect the resistance to castastrophic failure.