Effects of hydrostatic tension on cleavage fracture of pure polycrystalline zinc

A study was made of notch effects on the cleavage fracture of polycrystalline zinc. It was seen that the nominal fracture stress of SENB specimens was independent of notch angle. The maximum tensile stress below the notch at fracture in SENB specimens was shown to be different from the tensile stress at fracture in tensile testpieces over a temperature range from −196 to −17°C. The notch root strain at fracture was found to be the same as the uniaxial tensile fracture strain over this temperature interval. These results were interpreted as showing the cleavage fracture of polycrystalline zinc to be shear-stress or initiation controlled, as predicted by Stroh's dislocation model of cleavage.

Micromechanisms of fracture and strengthening in free-standing Pt-aluminide bond coats under tensile loading

Free-standing Pt-aluminide (PtAl) bond coats exhibit a linear stress strain response under tensile loading and undergo brittle cleavage fracture at temperatures below the brittle-to-ductile transition temperature (BDTT). Above the BDTT, these coatings show yielding and fail in a ductile manner. In this paper, the various micromechanisms affecting the tensile fracture stress (FS) below the BDTT and yield strength (YS) above the BDTT in a PtAl bond coat have been ascertained and quantified at various temperatures. The micromechanisms have been identified by carrying out microtensile testing of stand-alone PtAl coating specimens containing different levels of Pt at temperatures between room temperature and 1100 degrees C and correlation of the corresponding fracture mechanisms with the deformation substructure in the coating. An important aspect of the influence of Pt on the tensile behavior, slip characteristics, FS/YS and BDTT in the PtAl coating has also been examined. The addition of Pt enhances the FS of the coating by Pt solid solution strengthening and imparts a concomitant increase in fracture toughness and yet causes a significant increase in the BDTT of the coating. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.

The tensile fracture of mild steel

The fibrous and cleavage tensile fracture of an annealed mild steel was investigated. Round tensile specimens of two geometries, one straight and one with a circumferential notch, were pulled at temperatures between room temperature and liquid nitrogen temperature. Tensile fractures occurred at average strains from 0.02 to 0.87. The mechanism of fibrous fracture at room temperature was investigated metallographically. The stress-strain values at which fibrous and cleavage fractures are initiated were determined.

Many fine microcracks, which are associated with pearlite colonies and inclusion stringers, develop prior to fibrous fracture. The macrofracture, which leads to final separation of the tensile specimen, is initiated by the propagation of a microcrack beyond the microstructural feature with which it is associated. Thus, the fibrous fracture of mild steel does not develop by the gradual growth and coalescence of voids that are large enough to be visible in the optical microscope. When the microcracks begin to open and propagate, final fracture quickly follows. Axial cracks are a prominent feature of the macrofracture that forms in the interior of the specimen immediately before final fracture.

The Bridgman distribution of stresses is not valid in a notched tensile specimen. Fibrous and cleavage fractures occur at approximately the same value of maximum tensile stress. When the maximum tensile stress that is necessary for cleavage fracture is plotted against the corresponding maximum tensile strain, the result is an unique locus.

Fractals in fracture of bulk metallic glass

We investigate the nanoscale periodic corrugation (NPC) structures on the dynamic fracture surface of a typical tough bulk metallic glass, submitted to high-velocity plate impact and scanned by atomic force microscopy (AFM). The detrended fluctuation analysis (DFA) of the recorded AFM profiles reveals that the valley landscapes of the NPC are nearly memoryless, characterized by Hurst parameter of 0.52 and exhibiting a self-similar fractal character with the dimension of about 1.48. Our findings confirm the existence of the “quasi-cleavage” fracture underpinned by tension transformation zones (TTZs) in metallic glasses.

Elasticity, stability, and ideal strength of beta-SiC in plane-wave-based ab initio calculations

On the basis of the pseudopotential plane-wave method and the local-density-functional theory, this paper studies energetics, stress-strain relation, stability, and ideal strength of beta-SiC under various loading modes, where uniform uniaxial extension and tension and biaxial proportional extension are considered along directions [001] and [111]. The lattice constant, elastic constants, and moduli of equilibrium state are calculated and the results agree well with the experimental data. As the four SI-C bonds along directions [111], [(1) over bar 11], [11(1) over bar] and [111] are not the same under the loading along [111], internal relaxation and the corresponding internal displacements must be considered. We find that, at the beginning of loading, the effect of internal displacement through the shuffle and glide plane diminishes the difference among the four Si-C bonds lengths, but will increase the difference at the subsequent loading, which will result in a crack nucleated on the {111} shuffle plane and a subsequently cleavage fracture. Thus the corresponding theoretical strength is 50.8 GPa, which agrees well with the recent experiment value, 53.4 GPa. However, with the loading along [001], internal relaxation is not important for tetragonal symmetry. Elastic constants during the uniaxial tension along [001] are calculated. Based on the stability analysis with stiffness coefficients, we find that the spinodal and Born instabilities are triggered almost at the same strain, which agrees with the previous molecular-dynamics simulation. During biaxial proportional extension, stress and strength vary proportionally with the biaxial loading ratio at the same longitudinal strain.

The Flow Theory of Mechanism-Based Strain Gradient Plasticity

The flow theory of mechanism-based strain gradient (MSG) plasticity is established in this paper following the same multiscale, hierarchical framework for the deformation theory of MSG plasticity in order to connect with the Taylor model in dislocation mechanics. We have used the flow theory of MSG plasticity to study micro-indentation hardness experiments. The difference between deformation and flow theories is vanishingly small, and both agree well with experimental hardness data. We have also used the flow theory of MSG plasticity to investigate stress fields around a stationary mode-I crack tip as well as around a steady state, quasi-statically growing crack tip. At a distance to crack tip much larger than dislocation spacings such that continuum plasticity still applies, the stress level around a stationary crack tip in MSG plasticity is significantly higher than that in classical plasticity. The same conclusion is also established for a steady state, quasi-statically growing crack tip, though only the flow theory can be used because of unloading during crack propagation. This significant stress increase due to strain gradient effect provides a means to explain the experimentally observed cleavage fracture in ductile materials [J. Mater. Res. 9 (1994) 1734, Scripta Metall. Mater. 31 (1994) 1037; Interface Sci. 3(1996) 169].

Deformation and Failure of Polymer Bonded Explosives under Diametric Compression Test

The tensile deformation and failure of polymer bonded explosives (PBXs), a particulate composite, is studied in this paper. Two HMX-based PBXs with different binder were selected for study. A diametric compression test, in which a disc-shaped specimen is loaded diametrically, was chosen to generate tensile failure in the materials. The quasi-static tensile properties and the tensile creep properties were studied by using conventional displacement transducers to measure the lateral strain along the horizontal diameter. The whole-field in-plane creep deformation was measured by using the technique of high resolution moire´ interferometry. Real time microscopic examination was conducted to monitor the process of deformation and failure of PBXs by using a scanning electron microscope equipped with a loading stage. A manifold method (MM) was used to simulate the deformation and failure of PBX samples under the diametric compression test, including the crack initiation, crack propagation and final cleavage fracture. The mechanisms of deformation and failure of PBXs under diametric compression were analyzed. The diametric compression test and the techniques developed in this research have proven to be applicable to the study of tensile properties of PBXs.

Preparation and mechanical properties of a bulk icosahedral quasicrystalline Ti45Zr35Ni17Cu3 alloy

A bulk Ti45Zr35Ni17Cu3 alloy, which consisted of the icosahedral quasicrystalline phase, was prepared by mechanical alloying(MA) and subsequent pulse discharge sintering. Ti45Zr35Ni17Cu3 amorphous powders (with particle size < 50 mu m) were obtained after mechanical alloying for more than 150 h from the mixture of the elemental powder. The transformation temperature range from amorphous phase to the quasicrystalline phase was from 400 K to 900 K. The mechanical properties of the bulk quasicrystalline alloy have been examined at room temperature. The Vickers hardness and compressive fracture strength were 620 +/- 40 and 1030 +/- 60 MPa, respectively. The bulk quasicrystalline alloy exhibited the elastic deformation by the compressive test. The fracture mode was brittle cleavage fracture.

Microstructures and properties of melt-spun and as-cast Mg-20Gd binary alloy

Mg-20Gd(%, mass fraction) samples were prepared using melt-spinning and copper mold casting techniques. Microstructures and properties of the Mg-20Gd were investigated. Results show that the melt-spun ribbon is mainly composed of supersaturated alpha-Mg solid solution phase and the as-east ingot mainly contains alpha-Mg solid solution and Mg5Gd phase. The differential scanning calorimeter (DSC) curve of the ribbon exhibits a small exothermic peak in the temperature range from 630 to 680 K, which indicates that the ribbon contains a metastable phase (amorphous). Tensile strength at room temperature of the melt-spun ribbon and as-cast specimen are 308 and 254 MPa, respectively. The elongations of the two samples are less than 2%. The fracture surfaces demonstrate that the fracture mode of the as-cast Mg-20Gd is a typical cleavage fracture and that of the melt-spun sample is a combination of brittle fracture and ductile fracture.

Stress corrosion cracking of AISI 321 stainless steel in acidic chloride solution

The stress corrosion cracking (SCC) of LambdaISI 321 stainless steel in acidic chloride solution was studied by slow strain rate (SSR) technique and fracture mechanics method. The fractured surface was characterized by cleavage fracture. In order to clarify the SCC mechanism, the effects of inhibitor KI on SCC behaviour were also included in this paper. A study showed that the inhibition effects of KI on SCC were mainly attributed to the anodic reaction of the corrosion process. The results of strain distribution in front of the crack tip of the fatigue pre-cracked plate specimens in air, in the blank solution (acidic chloride solution without inhibitor KI) and in the solution added with KI measured by speckle interferometry (SPI) support the unified mechanism of SCC and corrosion fatigue cracking (CFC).

Nitride-strengthened reduced activation ferritic/martensitic steels

Two nitride-strengthened reduced activation ferritic/martensitic (RAFM) steels with different Mn contents were investigated. The experimental steels were designed based on the chemical composition of Eurofer 97 steel but the C content was reduced to an extremely low level. Microstructure observation and hardness tests showed that the steel with low Mn content (0.47 wt.%) could not obtain a full martensitic microstructure due to the inevitable δ-ferrite independent of cooling rate after soaking. This steel showed similar room temperature strength and higher strength at 600 °C, but lower impact toughness, compared with Eurofer 97 steel. Fractography of the Charpy impact specimen revealed that the low room temperature toughness should be related to the Ta-rich inclusions initiating the cleavage fracture. The larger amount of V-rich nitrides and more dissolved Cr in the matrix could be responsible for the strength being similar to Eurofer 97 steel. In the second steel developed from the first steel by increasing the Mn content from 0.47 wt.% to 3.73 wt.%, a microstructure of full martensite could be obtained.

Effect of Carbon Reduction on the Toughness of 9CrWVTaN Steels

Nitride-strengthened, reduced activation, martensitic steel is anticipated to have higher creep strength because of the remarkable thermal stability of nitrides. Two nitride-strengthened, reduced activation martensitic steels with different carbon contents were prepared to investigate the microstructure and mechanical property changes with decreasing carbon content. It has been found that both steels had the microstructure of full martensite with fine nitrides dispersed homogeneously in the matrix and displayed extremely high strength but poor toughness. Compared with the steel with low carbon content (0.005 pct in wt pct), the steel with high carbon content (0.012 pct in wt pct) had not only the higher strength but also the higher impact toughness and grain coarsening temperature, which was related to the carbon content. On the one hand, carbon reduction led to Ta-rich inclusions; on the other hand, the grain grew larger when normalized at high temperature because of the absence of Ta carbonitrides, which would decrease impact toughness. The complicated Al2O3 inclusions in the two steels have been revealed to be responsible for the initiated cleavage fracture by acting as the critical cracks.

Effects Of The Flash Welding Process On Mechanical And Microstructural Properties Of Structural Steel Joints Assessed Using Dest

ASTM A529 carbon¿manganese steel angle specimens were joined by flash butt welding and the effects of varying process parameter settings on the resulting welds were investigated. The weld metal and heat affected zones were examined and tested using tensile testing, ultrasonic scanning, Rockwell hardness testing, optical microscopy, and scanning electron microscopy with energy dispersive spectroscopy in order to quantify the effect of process variables on weld quality. Statistical analysis of experimental tensile and ultrasonic scanning data highlighted the sensitivity of weld strength and the presence of weld zone inclusions and interfacial defects to the process factors of upset current, flashing time duration, and upset dimension. Subsequent microstructural analysis revealed various phases within the weld and heat affected zone, including acicular ferrite, Widmanstätten or side-plate ferrite, and grain boundary ferrite. Inspection of the fracture surfaces of multiple tensile specimens, with scanning electron microscopy, displayed evidence of brittle cleavage fracture within the weld zone for certain factor combinations. Test results also indicated that hardness was increased in the weld zone for all specimens, which can be attributed to the extensive deformation of the upset operation. The significance of weld process factor levels on microstructure, fracture characteristics, and weld zone strength was analyzed. The relationships between significant flash welding process variables and weld quality metrics as applied to ASTM A529-Grade 50 steel angle were formalized in empirical process models.

Estudo experimental do comportamento à fratura frágil em aços estruturais ferríticos e aplicações à determinação da curva mestra.

Este trabalho apresenta uma investigação experimental sobre o comportamento à fratura frágil de aços estruturais ferríticos, ASTM A285 Gr C e ASTM A515 Gr 65. Os resultados deste trabalho ampliam a base de dados de propriedades mecânicas utilizadas nas análises de integridade de estruturas pressurizadas tais como vasos de pressão e tanques de armazenamento construídos com esta classe de material. O trabalho tem por objetivo também avaliar a aplicabilidade de corpos de prova de dimensões reduzidas, PCVN, na determinação da temperatura de referência, T0, por meio da metodologia da Curva Mestra, a qual define a dependência da tenacidade à fratura do material em função da temperatura. Os ensaios de tenacidade à fratura foram conduzidos utilizando-se corpos de prova solicitados em flexão três pontos com geometria SE(B), PCVN e PCVN com entalhe lateral, extraídos de chapas laminadas. Os resultados dos ensaios foram obtidos em termos de integral J no momento da instabilidade, denotados por Jc. Dados adicionais de resistência à tração e de Impacto Charpy convencional também foram obtidos para caracterizar o comportamento mecânico dos aços utilizados. Os resultados mostraram uma forte influência da geometria dos corpos de prova sobre os valores de Jc, evidenciada pela grande variação nos valores de tenacidade à fratura.