Formation Of Subsurface Cracks Research Articles

Dry sliding of Cu–15 wt%Ni–8 wt%Sn bronze: Wear behaviour and microstructures

Dry sliding wear of Cu–15Ni–8Sn (in wt%) bronze against a stainless steel 440C counter surface is investigated using a pin-on-disc tester in air and flowing Ar gas. The microstructure of the debris, the worn surface and the subsurface of the pin have been characterized using X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy. The debris is found to comprise of particles of Fe 2O 3 and CuO phases, in addition to bronze particles. EDS analysis of the debris revealed that bronze picked up Fe and Cr elements from the stainless steel while the Fe 2O 3 particles picked up Cu, Ni and Sn elements from the pin, attesting the mechanical mixing of chemical species during the wear process. Cross-section SEM of the worn pin revealed a highly deformed subsurface layer, capped by a thin layer separating the outer surface and the deformed layer. Cross-sectional TEM investigations of the capped layer revealed it to be a set of mechanically mixed layers (MML) comprising nanograins with varying grain size and composition. SEM micrographs also revealed the formation of subsurface cracks in the vicinity of the interface between the deformed subsurface layer and the capping layer. Wear processes are discussed in the light of these microstructural observations, combined with wear rate and friction coefficient measurements.

High-Cycle Fatigue Crack Initiation and Growth in TIMETAL LCB

This investigation has examined the influence of grain boundary α contiguity on the high-cycle fatigue behavior of aged TIMETAL LCB, with the fatigue performance being evaluated under tension-tension loading conditions atR=0.1 in laboratory air and 25 Hz. Fractographic analysis indicated that fatigue initiation, independent of processing history, involved subsurface crack formation. Serial section studies also indicated that crack initiation occurred at the interface between the triple-point α and the aged β matrix. Further back-scattered electron microscopy examination of the aged microstructures indicates that the observed differences in high-cycle fatigue behavior can be understood by considering the effect of processing history on the connectivity of grain boundary α, with decreased connectivity being associated with enhanced fatigue performance.

Chamber wall materials response to pulsed ions at power-plant level fluences

Candidate dry-wall materials for the reactor chambers of future laser-driven Inertial Fusion Energy (IFE) power plants have been exposed to ion pulses from RHEPP-1, located at Sandia National Laboratories. These pulses simulate the MeV-level ion pulses with fluences of up to 20 J/cm 2 that can be expected to impinge on the first wall of such future plants. Various forms of tungsten and tungsten alloy were subjected to up to 1600 pulses, usually while being heated to 600 °C. Other metals were exposed as well. Thresholds for roughening and material removal, and evolution of surface morphology were measured and compared with code predictions for materials response. Powder-metallurgy (PM) tungsten is observed to undergo surface roughening and subsurface crack formation that evolves over hundreds of pulses, and which can occur both below and above the melt threshold. This roughening is worse than for other metals, and worse than for either tungsten alloyed with rhenium (W25Re), or for CVD and single-crystal forms of tungsten. Carbon, particularly the form used in composite material, appears to suffer material loss well below its sublimation point. Some engineered materials were also investigated. It appears that some modification to PM tungsten is required for its successful use in a reactor environment.

Wear resistance of Cu–Ni–Mo austempered ductile iron

A series of ductile iron samples alloyed with 0.66% Cu, 1.02% Ni, and 0.26% Mo were austempered at 315 and 370 °C for 5–240 min and then tested for wear strength. A block-on-ring wear testing machine was used for this purpose. The wear samples were tested under a load of 45 N and a displacement speed of 2.40 m/s. The experimental outcome indicates that the wear properties of the austempered ductile iron (ADI) are strongly influenced by the exhibited microstructure. In particular, optimal wear properties were found in samples austempered at 370 and 315 °C for 90 and 120 min, respectively. These heat treatment times are long enough to promote the development of a relatively high volume fraction of high carbon retained austenite concomitant with ferrite and a fine dispersion of carbides. After wear testing, scanning electron microscopy (SEM) observations on the wear samples did not show any evidence of a transformation-induced-plasticity (TRIP). Hence, the experimental evidence suggests that the dominant wear mechanism was delamination associated with sub-surface crack formation and final wear particle debris removal.

Subsurface Indentation Damage and Mechanical Characterization of α‐Sialon Ceramics

Two hot‐pressed sintered α‐sialon samples of differing microstructures, but identical chemical composition, were evaluated first, in terms of indentation hardness and modulus, by depth‐sensing indentation (DSI) tests on planes parallel and normal to the hot‐pressed surface. The surface and subsurface cracks created under the DSI tests have also been investigated in relation to the effect of microstructure. Subsequently, Vickers indentation tests were conducted to explore the deformation and fracture characteristics in the two samples. The effect of microstructure and grain orientation on the development of different types of cracks, in particular subsurface cracks, was revealed and analyzed. Additionally, it suggested that the focused ion beam (FIB) miller is a preferred tool, in comparison to the conventional cross‐sectioning techniques, for examining subsurface crack formation and structural characteristics.

Laser inertial fusion dry-wall materials response to pulsed ions at power-plant level fluences

Pulses of MeV-level ions with fluences of up to 20 J/cm 2 can be expected to impinge on the first-wall of future laser-driven Inertial Fusion Energy (IFE) power plants. To simulate the effect of these ions, we have exposed candidate dry-wall materials to ion pulses from RHEPP-1, located at Sandia National Laboratories. Various forms of tungsten and tungsten alloy were exposed to up to 1000 pulses, with some samples heated to 600 °C. Thresholds for roughening and material removal, and evolution of surface morphology were measured and compared with code predictions for materials response. Tungsten is observed to undergo surface roughening and subsurface crack formation that evolves over hundreds of pulses, and which can occur both below and above the melt threshold. Heating and Re-alloying mitigate, but do not eliminate, these apparently thermomechanically-caused effects. Use of a 3-D geometry, and/or use of the tungsten in thin-film form may offer improved survivability compared to bulk tungsten.

The role of heat treatment on wear behavior of powder metallurgy low alloy steels

The influence of manufacturing parameters such as compacting pressure, heat treatment and microstructure on the static and dynamic properties of Fe–1.75 Ni–1.5 Cu–0.5 Mo–0.6 C was investigated. Wear and fatigue tests were carried out on reciprocate wearing and tension compression mode, respectively. The results show that delamination wear mechanism of sintered steel is similar to that of conventional wrought or cast materials. But, in sintered material, open porosity acts as sites of generation and collection of wear debris or formation of subsurface cracks. Wear rate is decreased and fatigue strength is increased by heat treatment.

X-ray microtomography (microCT) of the progression of sulfate attack of cement paste

High-resolution X-ray computed tomography (i.e., microCT or microtomography) was used to study the sulfate attack of cylinders of Type I cement paste cast with water–cement (w/c) ratios of 0.45, 0.50 and 0.60. Damage levels in samples exposed to a Na 2SO 4 solution with 10,000 ppm sulfate ion concentration were qualitatively rated from 0 ( no damage) to 4 ( extreme damage) based upon visual examination of the samples' exteriors and microtomography of the samples' interiors. The greater the w/c ratio, the more rapid the onset of sulfate damage. The corners of the cylinders appeared to be particularly susceptible to spalling, and damage may have continued into the cement paste by formation of subsurface cracks.

Contact damage in porcelain/Pd-alloy bilayers

An analysis is made of contact damage in brittle coatings on metal substrates, using a case study of a dental porcelain coating of thickness between 0.1 and 1 mm fused onto a Pd alloy base, with spherical indenter of radii 2.38 and 3.98 mm. At large coating thicknesses (>300 μm), the first damage takes the form of surface-initiated transverse cone cracks outside the contact. At small coating thicknesses (<300 μm), the first damage occurs as yield in the substrate, with attendant formation of subsurface transverse median cracks in the coating. At high loads and thin coatings, both forms of transverse cracking occur, along with subsequent delamination of the ceramic/metal interface, signalling impending failure. Conditions for avoiding such transverse cracking are considered in terms of minimum coating thicknesses and maximum sustainable contact loads. General implications concerning the design of brittle coating systems for optimum damage resistance are considered, with special reference to dental crowns.

Experimental simulation of subsurface crack formation in continuous casting

With increasing casting speed or/and the addition of B, P or S as microalloying elements, assurance of crack-free products becomes more difficult.

812 ビタミンE添加超高分量ポリエチレンの耐疲労特性と生体内変化(OS-3 バイオ・マイクロ精密機器のトライボロジー)

Flaking-type wear, so-called delamination, is often observed in polyethylene joint components. It is thought that crack formation and propagation at grain boundaries were accelerated by gamma-irradiation and other factors such as material processing. We previously had reported that gamma-irradiation increased subsurface crack area of UHMWPE under multi-direction sliding fatigue testing and hardened grain boundaries of UHMWPE. And vitamin E addition into UHMWPE suppressed these effects of gamma-irradiation. In this study, unidirectional and multi-directional sliding fatigue testings were carried out for virgin, gamma-irradiated, vitamin E added and gamma-irradiated UHMWPE specimens. The produced cracks were observed by Scanning Acoustic Tomography, and surface roughness(Ra) of loaded area was measured. Rising of surface roughness were observed just before the delamination on gamma-irradiated UHMWPE. It is possible that the delamination destruction depend not only on subsurface crack formation and propagation, but also on crack formation at surface.

The effect of test parameters on alumina wear under low contact stress

A flat pin-on-disc wear test method in which a very low contact stress is applied was used to evaluate alumina wear behaviour. Wear tests were performed using alumina pin and disc with grain size of 8 μm in dry sliding conditions. The normal load, the sliding velocity and sliding distance were varied during the tests as follows: Load- varied between 5 N to 13 N, velocity from 0.5 ms −1 to 2.5 ms −1 and distance was up to 15.3 km. Two main wear regimes were identified, mild and severe wear respectively, with a transition region in between. A microabrasive mechanism was found to be responsible for the nucleation of cracks and initiation of wear. In the mild wear region, plastic deformation and tribo-film formation were the dominant mechanisms, whereas transgranular fracture and sub-surface crack formation were found to be the dominant mechanisms in the severe wear region.

Prevention of fatigue cracks in ultrahigh molecular weight polyethylene joint components by the addition of vitamin E.

Flaking-type wear, so-called delamination, is often observed in polyethylene joint components. This is thought to occur partly due to crack formation and propagation at grain boundaries. This study examined the effect of vitamin E on the crack formation and/or propagation in UHMWPE by using 2-dimensional sliding fatigue testing and micro indenter testing. An in vitro sliding fatigue test was performed under two simplified articulating movements, and the cracks produced were observed by scanning acoustic tomography (SAT). Gamma-irradiated ultrahigh molecular weight polyethylene (UHMWPE) specimens demonstrated a smaller area of accumulated cracks as compared to virgin specimens, when the loading movement was reciprocated on a single linear locus. However, four out of five gamma-irradiated UHMWPE specimens exhibited severe flaking-like destruction under the complicated sliding condition, suggesting that gamma irradiation accelerated crack propagation under multidirectional loading. All the gamma-irradiated vitamin-E-containing specimens demonstrated no subsurface crack formation and no flaking-like destruction. Results using micro indenter testing showed that the dynamic hardness at grain boundary was higher than that in grain, and was increased by gamma irradiation. This hardening at grain boundary was reduced by adding vitamin E. It is possible that the presence of vitamin E prevents crack propagation partly due to reduced hardness at grain boundaries. The gamma-irradiated vitamin-E-containing UHMWPE is a promising material to prevent flaking-like destruction of polyethylene joint components.

The sliding wear behaviour of reactively hot pressed nickel aluminides

The sliding wear behaviour of reactively hot pressed nickel aluminides has been studied. The influence of load and intermetallic stoichiometry on wear rate has been considered using a block-on-ring test method with 440C as the counterface material. The results shows that wear rate increases linearly with load and decreases as the nickel content increases. Stoichiometric NiAl exhibits wear rates similar to 440C, with material removal proceeding by the formation of sub-surface voids and cracks and the generation of plate-like wear debris typical of a delamination mechanism. The higher nickel containing materials such as Ni-40% Al, undergo severe deformation with the formation of an ultra-fine grained sub-surface region. The hardness of this region exceeds 750 Hv. Metal transfer from the 440C counterface is followed by oxidation of the transferred film to form a FeO rich layer resulting in mild wear and an order of magnitude reduction in wear rate compared with NiAl and 440C.

Microbial pollution of well water in southeastern Minnesota

Abstract Groundwater contamination is a growing problem in southeastern Minnesota. This region is characterized by karst topography which is responsible for the formation of sinkholes, subsurface cracks, and underground rivers. These features enhance transportation of surface contaminants into groundwater. The present study was conducted to determine the presence of coliforms, fecal coliforms and coliphages in private rural wells situated in this region. Another purpose was to study the occurrence of drug resistance in bacteria isolated from ground‐water. Well water from 18 sites was tested monthly for a period of 16 months. Seventeen of 18 sites sampled showed detectable levels of indicator bacteria. A total of 161 samples were tested for the presence of coliphages. Of these, 13 samples from 7 sites were found positive. On two occasions, coliphages were isolated from samples in which coliforms were undetectable. Water from 10 sites yielded drug resistant indicator bacteria. Twenty‐five of 38 (65.8%) total coliforms and 9 of 27 (33.3%) fecal coliforms tested were found to carry drug resistance.

Elastic-plastic analysis of indentation damage: cyclic loading of copper

This is the second in a two-paper series examining the elastic-plastic deformation of a metallic target material under repeated impact. This paper examines the effect of statically cyclic loading on copper and numerical calculations have been performed up to ten cycles. The extent of the loading phase in each cycle is specified by a constant external work done by the indenter. The deformed configuration of the target material, the locus of material flow, and elastic-plastic boundaries due to cyclic loading are presented. There is a saturation of the stress field near the bottom of the indented crater after the first few cycles of loading. A residual tensile stress field in the direction normal to the target surface exists underneath the indenter in every cycle, which is responsible for the formation of subsurface layer cracks. Results of the coefficient of restitution obtained from the analysis and experiments are also presented.

Investigation of rolling contact fatigue damage of a case-carburized low alloy steel

Experimental investigations were carried out on case-carburized En 36 A steel to study contact fatigue behaviour and pit formation. The effects of case depth, specimen hardness, different combinations of hardness of the specimen and mating roller and the influence of grinding operations before or after heat treatment were assessed. Optical and electron microscope studies revealed the formation of subsurface cracks which propagate because of the alternating shear stresses in the subsurface and which then turn towards the surface where the final failure occurs by ductile and then brittle fracture. Surface cracks initiated by micropits were also observed in some specimens.

The delamination theory of wear

A new theory for wear of metals is considered. The theory is based on the behavior of dislocations at the surface, sub-surface crack and void formation, and subsequent joining of cracks by shear deformation of the surface. The proposed theory predicts qualitatively that the wear particle shape is likely to be thin flake-like sheets and that the surface layer can undergo large plastic deformation. It also predicts a number of experimentally observed phenomena such as the difference in wear particle sizes and the dependence of fretting wear rate on displacement amplitude. All theoretical predictions are supported by experimental evidences. A wear equation is developed based on the proposed theory.