Different Mechanisms Of Fracture Research Articles

Wear Resistance, Patterns of Wear and Plastic Properties of Cr,Mo-(Cr,Mo,)N-(Cr,Mo,Al)N Composite Coating with a Nanolayer Structure

This paper discusses the results of studies focused on the wear resistance, patterns of wear and plastic properties of Cr,Mo-(Cr,Mo,)N-(Cr,Mo,Al)N coating, containing 20 at.% Mo. The coating had a nanolayer structure with a modulation period λ = 50 nm. The studies revealed the hardness, fracture resistance in scratch testing, as well as elemental and phase composition of the coating. The studies of the tool life of carbide cutting tools with the Cr,Mo-(Cr,Mo,)N-(Cr,Mo,Al)N coating proved their longer tool life compared to that of uncoated tools and tools with the reference Cr-(Cr,Al)N coating of equal thickness and equal content of aluminum (Al). The studies included the comparison of the tools coated with Cr,Mo-(Cr,Mo,)N-(Cr,Mo,Al)N and Cr-(Cr,Al)N. The experiments focused on the specific features of the coating nanostructure and were conducted using a transmission electron microscope (TEM), revealing the different mechanisms of fracture. The penetration of particles of the material being machined between nanolayers of the coating results in interlayer delamination. When exposed to a moving flow of the material being machined, plastic deformation (bending) of the coating nanolayers occurs. The diffusion of iron into the coating (up to 200 nm) and diffusion of Cr and Mo into the cut material to a depth of up to 250 nm are observed. The presented information can help in the design of metal cutting tools and the choice of coatings for them.

Fracture Mechanisms of S355 Steel-Experimental Research, FEM Simulation and SEM Observation.

In this study, the fracture mechanisms of S355 ferritic steel were analyzed. In order to obtain different mechanisms of fracture (completely brittle, mixed brittle and ductile or completely ductile), tests were carried out over a temperature range of −120 to +20 °C. Our experimental research was supplemented with scanning electron microscopy (SEM) observations of the specimens’ fracture surfaces. Modeling and load simulations of specimens were performed using the finite element method (FEM) in the ABAQUS program, and accurate calibration of the true stress–strain material dependence was made. In addition, the development of mechanical fields before the crack tip of the cracking process in the steel was analyzed. The distributions of stresses and strains in the local area before the crack front were determined for specimens fractured according to different mechanisms. Finally, the conditions and characteristic values of stresses and strains which caused different mechanisms of fracture—fully brittle, mixed brittle and ductile or fully ductile—were determined.

Tensile strength of Fe–Ni and Mg–Al nanocomposites: Molecular dynamic simulations

In this work, molecular dynamic simulations of the tensile strength of Fe–Ni and Mg–Al nanocomposites in the conditions of high-rate uniaxial tension were carried out. Two different mechanisms of fracture were identified. In the case of nickel inclusion in iron matrix, the fracture begins on the interface between the inclusion and the matrix, a formed void penetrates both into the inclusion and into the matrix; presence of inclusion reduces the tensile strength. In the case of aluminum inclusion in magnesium matrix, fracture takes place into magnesium matrix and does not touch the inclusion; presence of inclusion has practically no effect on the tensile strength. Molecular dynamic simulations were carried out in a wide range of strain rates and temperatures.

Mechanical characterization of Ti(C,N)-based cermets fabricated through different colloidal processing routes

In this work the influence of different colloidal processing routes on the mechanical behavior of several Ti(C,N)-based cermets has been studied. The materials were designed with a Ti(C,N) content ranging between 80 and 85 vol %, in a Fe-Ni alloy matrix (with and without carbon addition). Three processing techniques were investigated: (i) Slip Casting (SC), (ii) Slip Casting + Cold Isostatic Pressing (SC+CIP), and (iii) Spray-Dry + Uniaxial Pressing (SDP). Biaxial strength distributions were determined in all samples using the ball-on-three-balls (B3B) method on disc-shaped specimens. Results were interpreted in the framework of Weibull theory. The characteristic strength ranged between 1090 MPa and 1870 MPa. A fractographic analysis performed on selected specimens showed different critical flaws, depending on the processing route, related to the composition and porosity level in each case. Single Edge V-Notched Beam (SEVNB) specimens fabricated with SDP were tested under 4-point-bending, in order to identify the effect of Fe-Ni content and carbon addition on their fracture resistance. Detailed fractographic analyses revealed slightly different mechanisms of fracture associated with the Fe-Ni and carbon content. Fracture toughness values ranging between ∼12 and ∼14 MPa m1/2 were measured. Based upon these findings the SDP route is proposed to improve both the strength and toughness of Ti(C,N) cermets, which can be enhanced by the addition of carbon during the colloidal processing.

Fracture Behavior of Refractory Chromium-Base Alloy Kh65NVFT

The structure, mechanical properties and fracture behavior of refractory alloy Kh65NVFT are studied. The causes and different mechanisms of fracture are determined as a function of temperature correlated to the cold-shortness threshold.

Fracture behavior and environmental stress cracking resistance (ESCR) of HIPS/PE blends and the effect of compatibilization on their properties

Attempts have been made to study the fracture behavior and environmental stress cracking resistance (ESCR) of HIPS/PE blends. The effect of compatibilization on their properties was also studied. EWF tests were conducted to measure the essential specific work of fracture ( w e ) and non-essential specific work of fracture ( βw p ). The ESCR of the samples was investigated using a special modified tensile creep test under an aggressive environment (sunflower oil). It was found that EWF methodology could be applied to uncompatibilized and compatibilized HIPS/PE blends as well as HIPS. The essential specific work of fracture of compatibilized HIPS/PE blends was higher than uncompatibilized HIPS/PE blends and pure HIPS, while its non-essential work of fracture was higher than uncompatibilized blends and lower than pure HIPS. The results also showed that the ESCR of HIPS decreases with incorporation of PE, but an effective compatibilization of this blend increases its ESCR even higher than pure HIPS. The different properties of compatibilized and uncompatibilized blends and HIPS, in EWF and ESCR tests, were attributed to the different mechanisms of fracture in these materials. The different mechanisms of fracture were justified using morphological studies performed on fracture surfaces of each sample. SEM images showed that there is a reasonable correlation between mechanisms of fracture and microstructure of the samples.

Investigation of wear products of high-nitrogen manganese steels

The wear rate of hydrogenated specimens of high-nitrogen cold-worked manganese steels is five times larger than that of nonhydrogenated ones. In the absence of hydrogenation, the size of the wear products ranges from 25 to 40 μm at P = 400 N and from 40 to 100 μm at P = 500 N. For hydrogenated specimens, the size of the wear products is above 350 μm under a load of 250 N and ranges from 600 to 1000 μm at P = 400 N. The morphology of the wear products demonstrates an excellent microrelief, which indicates that fracture occurs by different mechanisms of fracture during the formation of a particle under friction conditions. On the products, we detected dimples in which, probably, particles containing σ-type intermetallics, carbides, and nitrides, which cause the initiation of cracks under both sliding friction and rolling friction, spalled.

Manifestation of Hydrogen and Low-Temperature Brittleness in Near-Threshold Cyclic Crack Resistance of Materials

We experimentally show that the realization of conditions of plane deformation at the tip of a fatigue crack is not sufficient for guaranteeing the unique dependence of the crack growth rate on the range of the stress intensity factor, which is explained by the effect of crack closure. We describe advantages and disadvantages of the effective range of the stress intensity factor as a parameter that determines the mechanical conditions for the propagation of a fatigue crack. We analyze the phenomenon of positive influence of strengthening factors (a decrease in the temperature of testing and hydrogenation) on the cyclic crack resistance of materials in a low-amplitude range of loading determined with regard for the effect of crack closure. The decrease in the crack growth rate and the increase in fatigue thresholds are intensified as the level of loading decreases and the ductility of materials increases. Differences in the influence of strengthening factors in low- and high-amplitude ranges of loading are explained by different mechanisms of fracture controlled by the shearing strength and the tensile strength, respectively. We give several examples of the mechanical behavior of materials that show the inversion of the influence of hydrogen on the resistance to fracture: fatigue fracture of smooth steel specimens in gaseous hydrogen, high-temperature corrosion fatigue of preliminary hydrogenated titanium alloys, and the influence of hydrogenation on the wear resistance of structural steels in the process of friction and cavitation and on the parameters of cutting of a tool steel.

Analysis of the Ductility on Short Fiber Reinforced Polypropylene Plates

The incorporation of fibers improved the mechanical properties of the thermoplastic matrix and also modified impact and fracture behavior. An experimental study of dynamic fracture behavior of short fiber reinforced poly-propylene was carried out in an instrumented impact pendulum on notched izod specimens and the variation of the ductility in different points of a rectangular injected plate was studied. The different mechanisms of fracture were analyzed following the information given by force-time and energy-time curves obtained from experiments performed on the neat matrix and on its glass fibers composites.

Environmental stress cracking of poly(acrylonitrile‐butadiene‐styrene)

AbstractThe environmental stress cracking (ESC) of acrylonitrile‐butadiene‐styrene (ABS) copolymer caused by a non‐ionic surfactant (poly‐oxyethylene alkylether) was studied by constant‐load tensile creep tests and edge crack tension (ECT) tests. The fracture surfaces were investigated by a scanning electron microscope (SEM) and the morphology of the crack tip was investigated by a transmission electron microscope (TEM). It was found that the results of the creep tests performed in the non‐ionic surfactant were very different from those performed in air. SEM images of the fracture surfaces showed that there were three different mechanisms of fracture and that specimens had a tendency to rupture by ESC when the stress was small. The results of the ECT tests and the TEM images showed that the change in the mechanism of the fracture was attributable to the change of morphology at the crack tip.

Rubber-modified polystyrene compositions

The physical and mechanical properties of polystyrene compositions, prepared by blending high impact polystyrene (HIPS) with different types of elastomer, are studied. The results obtained show a considerable increase in HIPS impact strength when modified with isoprene and butadienestyrene elastomers. This increase in impact strength is due to factors such as lowTg and the flexibility of macromolecules of the elastomer phase and adhesion at the interphase boundary. An optimal elastomer concentration of 12.5 parts per hundred of resin is observed. A multi-directional influence of the elastomer additive on the impact strength and on the tensile and flexural strength has been found, due to different mechanisms of fracture.

Effect of Origin Flaw Characteristics on Glass Strength

Glass breakage experiments were made which indicate two different mechanisms of fracture. The breaking process is dependent on the widths as well as the depth of the origin flaws. When the fracture is of the large, open type, the reciprocal of the breaking stress is a linear function of the depth of flaw; when the flaw is minute, the reciprocal of the breaking stress is a linear function of the square root of fracture depth. A pronounced strength increase was observed in cuts formed under various liquids and aged for various lengths of time. This increase took place about one hour after cutting and was interpreted as a rehealing phenomenon. Cuts made in air did not show the rapid strength increase. Cycle fatigue tests were also made on laths of glass and it was observed that there was an initial decrease in strength followed by an increase. This was interpreted as rehealing effect in the minute type of flaw.