Formation Of Surface Cracks Research Articles

Surface deformation, crack formation, and acoustic velocity changes in pyrophyllite under polyaxial loading : Spetzler, H A; Sobolev, G A; Sondergeld, C H J Geophys Res, V86, NB2, 10 Feb 1981, P1070–1080

Surface deformation, crack formation, and acoustic velocity changes in pyrophyllite under polyaxial loading : Spetzler, H A; Sobolev, G A; Sondergeld, C H J Geophys Res, V86, NB2, 10 Feb 1981, P1070–1080

The sequential observation of the pitting process in discs

The sequence of events leading to the formation of pits on the surfaces of discs was followed using optical microscopy and scanning electron microscopy. It was found that, under the experimental conditions used, the formation of surface cracks was preceded by two stages of topographical change of the disc surface. Experimental evidence for these two stages of development is presented together with a hypothesis of their appearance and role in the process of pitting.

Surface deformation, crack formation, and acoustic velocity changes in pyrophyllite under polyaxial loading

Jacketed cubes of pyrophyllite (31.6 mm on an edge) with variable water content were stressed monotonically to failure under polyaxial compression (σ1 > σ2 = 2σ3). The maximum and minimum principal stresses, σ1, and σ3, were applied with pistons, and the intermediate stress σ2 with a transparent fluid. An optical window in the pressure vessel allowed in situ measurements of the σ2 face deformation by optical holography. In addition, the more conventional techniques of stress, strain, and elastic wave velocity measurements as well as optical and electron microscopy were used to study the formation and propagation of fractures. The strength ( σ1 − σ3)f of the samples increased by about 50% as σ2 was increased from 5 MPa to 100 MPa. Air dry samples were stronger than water‐soaked samples by about ∼20%. Increasing the strain rate from 2 × 10−8 to 10−6 s−1 at σ2 = 5 MPa and 30 MPa increased the strength by about 10%. At σ2 = 100 MPa this trend was reversed, and samples that were deformed at the low strain rate were stronger by approximately 5%. The crack morphology of recovered samples was studied by optical and scanning electron microscopy. Subparallel sets of fractures and en échelon fracture patterns ahead of the macrofracture were easily visible with the unaided eye when σ2 was 5 MPa or 30 MPa. However, at σ2 = 100 MPa these fracture patterns were only visible under the microscope, and the cracks appeared much thinner. The holographic observations of the σ2 face revealed the following: as σ1 was increased, broad bulges formed in a crosslike pattern along the lines of the maximum shear stress. Macrofracture initiation, which occurred in a corner, was preceded by concentrated surface deformation. As the macrofracture propagated across the sample, it deviated from the direction of the maximum shear stress. Ahead of the tip of the macrofracture and migrating with it was a pronounced bulge. In response to monotonically increasing σ1, the displacement in the σ2 direction of a point adjacent to the eventual crack plane went through a local maximum that was followed by a local minimum as the crack passed. The results of elastic wave velocity measurements in the σ2 direction were very sensitive to the spatial relationship of the macrofracture and the elastic wave travel path. However, in general, above ∼50% of (σ1 − σ3)f the velocities decreased. As the migrating bulge approached an elastic wave travel path, the velocity decrease became more pronounced. The velocity increased again as the bulge passed.

The ductility of continuously-cast steel near the melting point—hot tearing

At temperatures near the melting point steels fail in a brittle manner. This brittle failure can lead to the formation of surface and internal cracks in continuously cast steel, during casting, if the steel is subject to a tensile strain. In this investigation the nature of the brittle failure has been considered for a wide range of continuously-cast steels, by examining the strength and ductility of the steels as a function of temperature, composition, and cast structure. The results show that for steels containing 0.05 to 0.12 pct C, brittle failure is due to incipient melting at grain boundaries at temperatures between approximately 40°C below the solidus and the solidus. The incipient melting is ascribed to solute or residual segregation, at the grain boundaries following extensive boundary migration. For steel containing approximately 0.16 pct C, with increasing test temperature brittle failure starts 70°C below the solidus. For steels containing 0.25 to 1.0 pct C brittle failure starts 40°C below the solidus over the entire carbon range. Failure due to melting alone occurs interdendritically at temperatures above the solidus. In general the melting or ductile-brittle transition temperatures are independent of the initial cast structure, or large increases in the solute or residual levels, other than carbon.

Fretting of AISI 9310 Steel and Selected Fretting-Resistant Surface Treatments

Fretting wear experiments were conducted with uncoated AISI 9310 mating surfaces and with combinations incorporating a selected coating to one of the mating surfaces. Wear measurements and SEM observations indicated that surface fatigue, as made evident by spallation and surface crack formation, is an important mechanism in promoting fretting wear to uncoated 9310. Increasing humidity resulted in accelerated fretting and a very noticeable difference in nature of the fretting debris. Of the coatings evaluated, aluminum bronze with a polyester additive was most effective at reducing wear and minimizing fretting damage to the mating uncoated surface by means of a self-lubricating film that developed on the fretting surfaces. Chromium plate performed as an effective protective coating, itself resting fretting and not accelerating damage to the uncoated surface. Presented at the 32nd Annual Meeting in Montreal, Quebec, Canada, May 9–12, 1977

The effect of changing the rolling direction on the rolling contact fatigue lives of annealed and case-hardened steel rollers

The effects of changing the rolling direction and of repeated loading on the rolling contact fatigue lives of annealed 0.45% carbon steel rollers and case-hardened nickel-chromium steel rollers under conditions of sliding rolling contact were studied. The influence of plastic flow in the subsurface layer on the rolling fatigue life was examined. The increase in the rolling fatigue life of an annealed steel roller due to a change in the rolling direction was significant, especially when the rolling direction was changed just before the formation of macroscopic surface cracks and pits. The effect with case-hardened steel rollers was negligible. The varying effects of changing the rolling direction on the rolling fatigue life were due to differences in work-hardening and the extent of plastic flow in the rollers.

One means of formation of axisymmetric surface cracks in cylindrical samples

One means of formation of axisymmetric surface cracks in cylindrical samples

Formation of surface cracks on 4Kh13 steel and methods of preventing them

1. The primary reason for surface cracks in 4Kh13 steel after quenching is the existence of a decarburized layer. The depth of the cracks depends on the depth of decarburization. 2. Heating to quenching temperature in a neutral salt bath or in a furnace with a protective atmosphere removes the possibility of crack formation. 3. Immediate tempering after quenching, which prevents surface cracks, is necessary when there is a decarburized layer.

Effect of residual austenite on crack formation

1. We confirmed the fact that residual austenite in quenched steel decreases the susceptibility to the formation of deep longitudinal cracks and surface cracks induced by decarburization. 2. When the concentration of residual austenite in high-carbon steel is 30–40% the steel samples with a cross section up to 75×75 mm have no tendency to form deep longitudinal cracks or surface cracks induced by decarburization.

Formation of surface cracks as the result of quenching machine parts made of steel No. 45

Formation of surface cracks as the result of quenching machine parts made of steel No. 45

The geometry of surface cracks

The question of the cracking of thin surface layers or coatings arises in a number of different applications. For example, it occurs in enamel and other protective coatings due to stresses set up by cooling and is also observed in the stress crazing of plastics. Cracking o f surface coatings is also important in connection with thermal shock. An analogous form of surface cracks appears in prolonged dry weather in the dried-out mud at the bot tom of lakes and reservoirs. The cracking generally occurs in the form of irregular honeycombs or polygon patterns. The general form of such patterns may be explained by application of the principle of least work, and an analysis of the geometry of such cracks is given below. It is assumed that the material is homogeneous and is in the form of a plane surface; during cooling, the tension is assumed uniform and increases progressively until it exceeds the fracture strength. The rectilinear surface cracks which develop form polygons which cover the surface or part of the surface affected. If the internal tension T per unit length at any instant is assumed to be constant in every direction at each point, then, assuming the rupture to take place for a unit depth, we consider a resulting rectilinear crack, such as AB, shown in fig. 1. Suppose that under tension it has opened uniformly by an amount RQ, so that on either side of the crack there is a corresponding displacement of the material perpendicular to the axis of the crack. The width varies from one crack to another, but is assumed to be uniform for each individual crack.