Presence Of Dimples Research Articles

Numerical investigation of laminar jet arrays impinging onto dimpled target surface

The flow and heat transfer of three-by-eight laminar jet arrays impinging onto a dimpled surface with Reynolds number 100 in steady state have been investigated numerically in the present study. Grid independence was confirmed and good agreement was reached through comparison between the results obtained from the literature and the present work. The effect of jet-to-plate spacing (H/d = 1–10) dimple diameter (Dd/d = 2–4) and the dimple relative depth (δ/Dd = 0.10–0.30) on the flow and heat transfer characteristics of laminar impinging jet arrays was explored numerically. The average Nusselt number decreases with increasing jet-to-plate spacing, dimple relative depth, and dimple diameter. The jet flow in the downstream jets is deflected by crossflow, which makes low-velocity fluid appear in the corresponding dimples and the area is enlarged. Thus, the presence of dimples causes overall heat transfer deterioration in the laminar impinging jet arrays.

Cyclic creep behaviour under tension–tension loading cycles with hold time of modified 9Cr–1Mo steel

Cyclic creep behaviour of modified 9Cr–1Mo steel was investigated by a series of cyclic creep (CC) tests at 600°C, which were performed under controlled tension–tension loading cycles with the magnitude of stress ranges in a constant stress ratio (R = 0·1). Hold time was applied for a 10 min hold at the maximum stress (σmax) and minimum stress (σmin). The CC properties were compared with the static creep (SC) using Norton’s power law, Larson–Miller plot, and Monkman–Grant relation, and the microstructure was examined. For the test conditions employed in the present investigation, retardation in the CC behaviour in terms of a lower creep rate and longer rupture time compared to those in the SC was obtained. The retardation was ascribed to the effects associated with anelastic recovery during the 10 min hold time at the minimum load of the cyclic loading. The creep rupture ductility decreased with a general decrease in stress, and there was no difference in the creep ductility between the CC and SC. The steel displayed a transgranular fracture characterised by the presence of dimples resulting from micro-void coalescence. Carbide precipitation was more coarsened with increasing in exposure time in the CC tests.

Surface Texturing of Vitamin E Blended UHMWPE for Reduction of Wear

Ultra high molecular weight polyethylene (UHMWPE) has been the most commonly used bearing material in arti cial hip and knee replacements. But wear and wear debris of UHMWPE has been the most important problem which cause aseptic loosening and revision surgery of these replacements. For enhangement of wear resistance, new generation UHMWPE materials have been developed and di erent surface modi cation techniques have been tried. The objective of this study is to determine the e ect of surface dimples on wear behavior of conventional UHMWPE and vitamin E blended UHMWPE (VE-UHMWPE) in ultra-pure water lubrication conditions. Pin-on-disc tribotester was used for measurement of friction coe cient and wear. 0.5 mm in diameter dimples were machined on the surfaces of UHMWPE and VE-UHMWPE discs. CoCrMo pins were used as counter surface. Results were evaluated in terms of materials and presence of dimples.

Solder joint reliability of Sn–0·7Cu and Sn–0·3Ag–0·7Cu lead-free solder alloys solidified on copper substrates with different surface roughnesses

The bond shear test was used to assess the integrity of Sn–0·7Cu and Sn–0·3Ag–0·7Cu lead-free solder alloy drops solidified on copper substrates with smooth and rough surface finishes. Solder alloys solidified on smooth substrates required higher shear force compared to that on rough substrates. Sn–0·3Ag–0·7Cu alloy required higher shear energy than Sn–0·7Cu alloy. Solder alloys solidified on smooth substrate surfaces exhibited complete ductile failure. On rough copper surfaces, solder alloys showed a transition ridge characterized by sheared intermetallic compounds (IMCs) and the presence of dimples. The peak shear strength decreased with increase in contact area of the solder bond on the substrate. Smooth surface and the presence of minor amount of Ag in the solder alloy enhance the integrity of the solder joint.

Effect of cryogenic treatment on the tensile behaviour of En 52 and 21-4N valve steels at room and elevated temperatures

This experimental study investigates the effects of cryogenic treatment on the tensile behaviour of En 52 and 21-4N valve steels at room and elevated temperatures. The materials are subjected to shallow cryogenic treatment (SCT) at 193K and deep cryogenic treatment (DCT) at 85K and the tensile behaviour is compared with that of the conventional heat treatment (CHT). The high temperature tensile test is conducted at 673K (400°C) and 923K (650°C) for the En 52 and 21-4N valve steels respectively. The ultimate tensile strength of the En 52 and 21-4N DCT samples show an enhancement of 7.87% and 6.76% respectively, over the CHT samples tested at the elevated temperature. The average yield strength of the En 52 DCT samples has an improvement 11% than that of the CHT samples when tested at room and elevated temperatures. The deep cryogenic treatment conducted at the optimized condition shows 7.84% improvement in the tensile strength for the En 52 valve steel and 11.87% improvement for the 21-4N valve steel when compared to the strength of the samples without the cryogenic treatment. A scanning electron microscopic analysis of the fracture surface indicates the presence of dimples and microvoid coalescence on the grain facets and interfaces of the cryo-treated specimens. The fracture surface of the deep cryo-treated 21-4N valve steel specimen shows a complete intergranular fracture with deep secondary cracks between the grains. On comparing the results of the percentage elongation, the cryo-treated samples show a smaller reduction in the elongation than that of the CHT samples. It is concluded that the precipitation of fine secondary carbide through cryogenic treatment is the reason for the improved strength and the reduction in elongation.

Numerical investigation of an axi-symmetric laminar jet impinging on a dimpled surface under uniform heat flux using a finite element method

The purpose of this study is to numerically investigate heat transfer and flow field in a semi-confined axi-symmetric laminar air jet impinging on a concave surface, or dimple, with uniform heat flux. A commercial software package relying on the finite element method was used for the simulation, and mesh convergence was examined in order to minimize computational cost. Jet impingement on a flat plate was used as a baseline reference case, and flat plate results were validated against previously published experimental data with good agreement. The effects of various parameters involved in dimple impingement -such as Reynolds number (Re) between 100–1,400; jet-to-plate spacing (H/Dj) ranging from 2 to 6 jet diameters; dimple depths (d/Dd) of 0.1, 0.15, and 0.2; and the ratio of jet diameter and dimple projected diameter (Dj/Dd) from 0.25 to 1—were all studied. Comparisons show that heat transfer reduction occurs in the presence of dimples because of the larger impingement area, which results in less momentum flux. The dimple curvature lifts the post-impinging fluid and creates a backflow, instead of allowing it to maintain contact with the surface, as is the case with flat plate impingement.

Experimental investigation of impingement heat transfer on a flat and dimpled plate with different crossflow schemes

A nine-by-nine jet array impinging on a flat and dimpled plate at Reynolds numbers from 15,000 to 35,000 has been studied by the transient liquid crystal method. The distance between the impingement plate and target plate is adjusted to be 3, 4 and 5 jet diameters. Three jet-induced crossflow schemes, referred as minimum, medium and maximum crossflow correspondingly, have been measured. The local air jet temperature is measured at several positions on the impingement plate to account for an appropriate reference temperature of the heat transfer coefficient. The heat transfer results of the dimpled plate are compared with those of the flat plate. The best heat transfer performance is obtained with the minimum crossflow and narrow jet-to-plate spacing no matter on a flat or dimpled plate. The presence of dimples on the target plate produce higher heat transfer coefficients than the flat plate for maximum and minimum crossflow.

High strain rate deformation of Ti–15Mo–5Zr–3Al alloy over wide temperature range

This study employs a split Hopkinson pressure bar to investigate the deformation behaviour of Ti–15Mo–5Zr–3Al alloy under strain rates of 8 × 102 to 8 × 103 s−1 at temperatures ranging from 25 to 900u C. The mechanical properties and fracture features of the alloy are found to be significantly dependent on both the strain rate and the temperature. The flow stress increases with increasing strain rate, but decreases with increasing temperature. For a constant temperature, the work hardening rate, maximum stress, strain at maximum stress and fracture strain all increase with increasing strain rate. For a given strain rate, the mechanical properties of Ti–15Mo–5Zr–3Al alloy exhibit their lowest values at a temperature of ∼700°C. As the strain rate increases, the strain rate sensitivity increases, but the activation volume decreases. However, as the temperature increases, the strain rate sensitivity decreases and the activation volume increases. For the current strain rate and temperature conditions, the activation energy decreases with increasing flow stress. Finally, optical microscopy (OM) and scanning electron microscopy (SEM) observations reveal that the alloy specimens fracture primarily as the result of the formation of adiabatic shear bands. The presence of dimples on the fracture surfaces is indicative of a ductile failure mode. The density of the dimples reflects the toughness of the alloy specimen and is found to vary directly as a function of the strain rate and the temperature.

Mechanical Properties and Fracture Surface Morphologies in Unnotched Specimens of Rubber-PMMA Composites

Correlations between mechanical properties and microscopic features were investigated using unnotched specimens of rubber-PMMA composites in very low to medium range of cross head speeds. It is found that: (1) a trapezoid-shaped smooth region and fish scale-like texture with bands in rough region correlates with brittle failure in pure PMMA, while a quarter circle-shaped smooth region and hackle-like texture, and the presence of dimples and/or voids correlate with ductile failure in rubber-PMMA composites; (2) decrease in degree of roughness in rubber-PMMA composites can be correlated with decrease in Young’s modulus; (3) decrease in size of the smooth region with increasing speed can be correlated with decrease in modulus of toughness; (4) larger smooth region in rubber-PMMA composites containing more rubber correlates with higher modulus of toughness.

The effect of tempering temperature on the mechanical properties and fracture morphology of a NiCrMoV steel

The present study focuses on the mechanical properties and the fracture morphology of a NiCrMoV steel with a specific composition. All specimens were austenitized at 870 °C for 1 h, followed by oil quenching, and then tempered at temperatures in the range of 200–600 °C. The results of tensile testing indicated that the yield strength (YS) and ultimate tensile strength (UTS) decreased with increased tempering temperature. However, UTS decreased at a higher rate compared with that for the YS. Fractography of the Charpy impact specimens did not show cleavage and/or quasi-cleavage. The presence of dimples was obvious at the fracture surface of these specimens, indicating a ductile fracture mechanism. The Charpy V-Notch (CVN) impact energy was improved by increasing the tempering temperature without any evidence of tempered martensite embrittlement (TME). The lack of TME was attributed to the chemical composition, namely the Mn, Si and Mo content of the steel investigated.

Effect of cryogenic treatment on tensile behavior of case carburized steel-815M17

The crown wheel and pinion represent the most highly stressed parts of a heavy vehicle; these are typically made of 815M17 steel. The reasons for the frequent failure of these components are due to tooth bending impact, wear and fatigue. The modern processes employed to produce these as high, durable components include cryogenic treatment as well as conventional heat treatment. It helps to convert retained austenite into martensite as well as promote carbide precipitation. This paper deals with the influence of cryogenic treatment on the tensile behavior of case carburized steel 815M17. The impetus for studying the tensile properties of gear steels is to ensure that steels used in gears have sufficient tensile strength to prevent failure when gears are subjected to tensile or fatigue loads, and to provide basic design information on the strength of 815M17 steel. A comparative study on the effects of deep cryogenic treatment (DCT), shallow cryogenic treatment (SCT) and conventional heat treatment (CHT) was made by means of tension testing. This test was conducted as per ASTM standard designation E 8M. The present results confirm that the tensile behavior is marginally reduced after cryogenic treatment (i.e. both shallow and deep cryogenic treatment) for 815M17 when compared with conventional heat treatment. Scanning electron microscopic (SEM) analysis of the fracture surface indicates the presence of dimples and flat fracture regions are more common in SCT specimens than for CHT and DCT-processed material.

DNS of low Reynolds number turbulent flows in dimpled channels

Direct numerical simulation (DNS) is performed to study turbulent flows over dimpled surfaces in a channel. Results on mean field and second-order quantities are obtained. ‘Horseshoe’ vortices can be observed in the dimples of sparse arrays. As inter-dimple separation is reduced, the ‘feet’ of the horseshoe vortices are gradually lifted off the dimple surface, and the resulting flow structures in the cavities become flattened and stretched to become something akin to two-dimensional separation bubbles. At the higher dimple density, the stream traces near the surface also develop a distinct formation similar to what had been observed in earlier Reynolds-averaged Navier–Stokes (RANS) simulations (Isaev, S.A., Leont'ev, A.I. and Baranov, P.A., 2000, Technical Physics Letters, 26, 15; Lin, Y.L., Shih, T.I.-P. and Chyu, M.K., 1999, ASME paper, 99-GT-263; Lin, Y.L. Shih, T.I.-P., 2001, International Journal of Transfer Phenomena, 3, 1). Regions of high turbulence intensity are found above the downstream half of the dimples and along their side edges. These regions coincide with the locations of vortex shedding found in the experiments of Ligrani et al. (2001, Physics of Fluids, 13, 3442) and the locations of vorticity concentrations observed in Park et al. (2004, Numerical Heat Transfer, Part A (Applications), 45(1), 1) and Won and Ligrani (2004, Numerical Heat Transfer, Part A (Applications), 46(6), 549). For a fixed mean pressure gradient, it is observed that the flow rates through the channels are reduced by the presence of dimples. This indicates that the dimpled channels we have studied so far have larger drag than flat-wall channels. Computed friction coefficients for dimpled channels also confirmed the conclusion.

Microstructure and mechanical properties of two ultrahigh boron steels

An investigation has been made of the tensile behaviour between 20 and 600°C of two ultrahigh boron steels (Fe–2·2B and Fe–4·9B), consolidated by hot isostatic pressing at temperatures ranging from 700 to 1100°C. Tensile tests showed plastic deformation only in the Fe–2·2B alloy. A decrease in yield and ultimate tensile stresses occurred when the consolidation temperature was increased. This was accompanied by an increase in the elongation to failure. This alloy satisfies the Hall–Petch relation for all testing temperatures. The slope of the yield stress versus d−1/2 curve (d is grain size) decreases as the temperature increases, indicating that the mechanism controlling plastic deformation becomes independent of grain size at high testing temperatures. The fracture mode observed was brittle at room temperature and ductile, shown by the presence of dimples, at temperatures above 400°C.MST/2050

Congenital symmetrical weakness of the upper limbs resembling brachial plexus palsy: A possible sequel of drug toxicity in first trimester of pregnancy?

We report a 14-month-old girl with a symmetrical paralysis from birth, limited to the upper limbs and resembling a severe, complete bilateral brachial plexus palsy. The presence of dimples over the wrists, shoulders and scapulae and abnormal palmar dermatoglyphics suggested an early prenatal onset. Previous reports and the course of the disease in our case suggest this sporadic condition is not progressive. Although no definitive causative factor has been identified in previously reported cases, the affection in our case is possibly related to Debendox (Bendectin) and nitrofurantoin taken in early pregnancy for nausea and renal tract infection, respectively.

On micromechanisms of fatigue crack growth in the 8090 T651 aluminium-lithium alloy

Fatigue crack growth tests under constant-amplitude loading conditions in air and in vacuum have been carried out in the moderate growth rate regime. The fracture surfaces were examined at low andhigh magnifications in a scanning electron microscope to identify the different fractographic features. An etchant that attacks the basal crystallographic planes preferentially was used to elucidate the cracking crystallography. The results obtained show that large facets lying in or close to {111} planes are found in air at low R ratio and in vacuum at both the R ratios studied. These facets are formed by micro-void coalescence as shown by the presence of dimples. The other fractographic features identified are pseudointergranular facets, the formation of herringbone patterns and the formation of pseudo-striations. These features are found only in air. Different mechanisms are proposed to explain the observed cracking modes.

Ionic interactions and anion binding in the gramicidin channel. An electrostatic calculation

An electrostatic calculation is performed in order to examine basic features of interactions between ions (including anions) within the gramicidin channel. The calculation focuses on the effect of image forces. The substitute charge method is used for the calculation of image-force energies. Good arrangements of fictitious charges and contour points are described. Errors of calculated image-force energies are estimated not to greatly exceed 0.1%. The cases assumed are (i) the effective radius r of the channel is between 2·5 and 3·5 Å, (ii) the binding site with the highest affinity is between 1 and 3·5 Å from the channel end ( outer site), and (iii) the dimple at the channel mouth is 0–5 Å in depth. The induced energy of an ion placed at the outer site increases (and hence the affinity of the outer site decreases) with the increase in the depth of dimples, whereas the barrier height for translocation between the outer sites decreases in the presence of deeper dimples. The interactional energy between two monovalent cations placed at the outer sites is relatively small in the absence of dimples. It is large, however, in the presence of deeper dimples if the outer sites are 2·5 or 3·5 Å from the ends; but, it is still relatively small even in the presence of dimples if the sites are 1 Å from the ends. The interactional energy is very unfavorable for simultaneous occupancy by three cations. It is suggested that an ion pair may be formed at the channel mouth. Deeper positions of the outer site, smaller values of r, and deeper dimples favor the formation of the ion pair. In the presence of 5 Å dimples, the binding constant of an anion (Cl −) for a cation (Na +) which has already been bound at an outer site (with no second cation at the opposite mouth) is estimated to be 0·4–8 molal −1 if local interactions between the bound anion and the channel wall is negligibly small. The anion binding constant increases in the presence of a cation (or an ion pair) at the opposite mouth. It decreases markedly in the absence of dimples. The interactional energy is considerably unfavorable for the binding of a third cation even if the presence of an ion pair is postulated. It is still large in the presence of an ion pair at each of the mouths.