On the Effects of Subgrade Erosion on the Contact Pressure Distribution under Rigid Surface Structures

The performance of rigid surface structures such as concrete pavements and slabs-on-grade supported by a deteriorated subgrade and experiencing local contact loss is investigated experimentally and numerically in this study. A laboratory setup has been designed to facilitate the simulation of subsurface erosion and measure the changes in contact pressure at selected locations under a slab-on-grade supported on granular material. The presence of erosion voids under a slab-on-grade can lead to rapid increase in the contact pressure in the immediate vicinity of the void in addition to an increase in tensile stresses at the outermost fibers of the slab. This preliminary study suggests that efforts to detect and arrest the growth of erosion voids under slabs-on-grade should be made before the voids reach the size where significant loss of support develops and the tensile strength of the slab material is exceeded.     

Utilizing Advanced Characterization Tools to Prevent Reflective Cracking

To prevent premature failures of rehabilitated concrete pavements, transportation authorities need tools to characterize the prerehab pavement condition of its load carrying capacity, and to determine the resistance of the overlay material to underlying crack/joint movements. Two quantitative methods, the rolling dynamic deflectometer (RDD) and overlay tester (OT), along with field performance data were employed in rehabilitation studies involving reflective cracks. The RDD is able to continuously assess vertical differential movements at joints/cracks that represent the potential for reflective cracks on existing pavements. The OT has the ability to determine the resistance of the overlay material to underlying crack/joint movements. The RDD W1?W3 deflections were used to determine areas that have a high potential for reflective cracking due to poor load transfer across joints and cracks. This paper documents results from the RDD and OT on the following five rehabilitation projects: (1) SH225; (2) US96; (3) SH12; (4) SH342; and (5) IH35W. Based on the available test results from these five projects, it was observed that the W1?W3 threshold values of 5.5 mils (0.140 mm) for exposed concrete pavement and 6.5 mils (0.165 mm) for composite pavement with existing hot mix asphalt overlay and an OT threshold value of 700 cycles correlated well with the field performance. Ignoring either of these critical factors may lead to premature reflective cracking.     

Roadway Heaving Caused by High Organic Matter

A forensic study was conducted to investigate the premature pavement failure of heaving and cracking on the north bound lane of SH6 and to determine (1) the causes of the heaving and cracking; (2) the severity and extent of the problem; and (3) a prevention strategy. Ground penetration radar (GPR), falling weight deflectometer (FWD), dynamic cone penetrometer (DCP), and soil boring and laboratory tests were conducted. Soil maps provided by the Natural Resource Conservation Service (NRCS) and the electrical resistivity tomography (ERT) were utilized to locate areas that may have similar problems for the ongoing project. It was concluded that the heaving was caused by high organic content in soils. The low pH of the lime treated subgrade layer indicated that the lime stabilization was ineffective. This was due to the high concentration of organic matter. FWD and DCP results indicated that the heaved/cracked areas are losing structural load support. Approximately 84% of the bumps/dips detected by the profiler were also detected by the GPR. Based on the GPR results, it was estimated that about 1.2?miles of the roadway may have potential heaving in the future. Although it is not a standard practice to determine the organic content of soil for new construction, it is critical to determine the organic matter through soil boring and laboratory testing in the suspicious areas. It was found that the soil maps provided by the NRCS yielded a reasonable estimate, and can be used as a screening tool. All five locations (O1 to O5) identified by ERT were verified by boring and laboratory tests to have high organic content (1.9–3.3%). Boring results indicated that ERT was able to map the soil strata and could differentiate between sandy and clay soil types. Although ERT was able to identify the anomalies with high organic contents, and the results were confirmed by boring and laboratory testing, additional work is needed to refine the procedure.     

CSP热轧过程中夹杂物周围孔洞的演变

借助于动态显式有限元软件ABAQUS/Explicit,模拟了CSP热轧过程中,板坯厚度方向上不同位置处直径为50μm的Al2O3夹杂物周围孔洞的形成和演变过程。结果表明,对于不同位置处的Al2O3夹杂物,在它们沿轧制方向的前、后部位都形成了孔洞,但前孔洞面积大于后孔洞面积;夹杂物越靠近板坯表面,形成的孔洞越大;热轧过程中,孔洞的演变是一个"愈合—长大—愈合"的动态过程;随着道次的增加,夹杂物前、后孔洞在轧制方向上的投影长度变化与面积变化趋势相一致,而与孔洞尖端夹角的变化趋势恰好相反;孔洞在热轧过程中发生转动,导致前孔洞尖端朝着带钢表面扩展。模拟结果有助于揭示夹杂物周围孔洞与表面缺陷的关系,并为实际生产提供理论指导。     

Evaluation of Full-Depth Asphalt Pavement Construction Using X-Ray Computed Tomography and Ground Penetrating Radar

In the past few years, a number of full-depth or perpetual pavements have been designed and constructed in the State of Texas. A study was conducted to examine the quality of the compaction of the thick asphalt layers within these pavements using advanced forensic tools such as X-ray computed tomography (X-ray CT) and ground penetrating radar (GPR). The GPR is a nondestructive tool for evaluating the uniformity of density in pavements at highway speed. X-ray CT is a laboratory tool that is used to conduct detailed analysis of air void distribution and uniformity in asphalt pavement cores. This paper presents the results of analyzing one of the perpetual pavements constructed in State Highway 114 (SH-114). In this project, two different structural asphalt pavement sections were placed, one included a 1?in. (25.4 mm) stone filled (SF) Superpave mix and the other included a traditional dense graded Type B material. The dense graded Type B material was found to be uniformly compacted. However, major compaction problems were identified with the coarse SF Superpave mix. The poor compaction and associated high percent air vsoids were found to permit moisture infiltration, which could potentially lead to rapid pavement deterioration. The analysis showed very good agreement between the GPR and X-ray CT results and demonstrated the efficiency of using GPR and X-ray CT in the evaluation of asphalt pavement compaction.     

Model Tests and Analyses of Bearing Capacity of Strip Footing on Stiff Ground with Voids

A series of 1G loading tests under the plane-strain condition were conducted on stiff ground with continuous square voids with the view of shallow foundation on calcareous sediment rocks, which contain voids because of their susceptibility to water dissolution. Detailed experimental observation revealed three types of failure modes for a single void: bearing failure without void failure, bearing failure with void failure, and void failure without bearing failure, depending on the location of the void as well as the size of the void. Upper-bound calculations were presented to interpret the changes of bearing capacity observed because of the existence of a void.     

Probabilistic Capacity Models for Corroding Posttensioning Strands Calibrated Using Laboratory Results

The presence of air voids, moisture, and chlorides inside tendons or ducts was cited as a reason for the early age strand corrosion and failure in the Mid-bay, Sunshine Skyway, and Niles Channel posttensioned (PT) bridges in Florida, United States. Although rare, these incidents call for frequent inspection and structural reliability assessment of PT bridges exposed to moisture and chlorides. This paper develops and presents probabilistic strand capacity models that are needed to assess the structural reliability of such PT bridges and recommends a time frequency of inspection. A total of 384 strand test specimens were exposed to various void, moisture, and chloride concentration conditions for 12 and 21 months; the remaining tension capacities were then determined. Using this experimental data and a Bayesian approach, six probabilistic capacity models were developed based on the void type. The mean absolute percentage errors of these models are less than 4%, indicating that reasonably accurate prediction of the strand capacity is possible, when void, aggressive moisture, and chloride conditions are present.     

Hydraulic Conductivity of MSW in Landfills

This paper presents a laboratory investigation of hydraulic conductivity of municipal solid waste (MSW) in landfills and provides a comparative assessment of measured hydraulic conductivity values with those reported in the literature based on laboratory and field studies. A series of laboratory tests was conducted using shredded fresh and landfilled MSW from the Orchard Hills landfill (Illinois, United States) using two different small-scale and large-scale rigid-wall permeameters and a small-scale triaxial permeameter. Fresh waste was collected from the working phase, while the landfilled waste was exhumed from a borehole in a landfill cell subjected to leachate recirculation for approximately 1.5 years. The hydraulic conductivity tests conducted on fresh MSW using small-scale rigid-wall permeameter resulted in a range of hydraulic conductivity 2.8×10?3–11.8×10?3?cm/s with dry unit weight varied in a narrow range between 3.9–5.1?kN/m3. The landfilled MSW tested using the same permeameter produced results between 0.6×10?3–3.0×10?3?cm/s for 4.5–5.5?kN/m3 dry unit weights. The hydraulic conductivity obtained from large-scale rigid-wall permeameter tests decreased with the increase in normal stress for both fresh and landfilled waste. The hydraulic conductivity for fresh MSW ranged from 0.2 cm/s for 4.1?kN/m3 dry unit weight (under zero vertical stress) and then decreased to 4.9×10?5?cm/s for 13.3?kN/m3 dry unit weight (under the maximum applied normal stress of 276 kPa). The hydraulic conductivity of the landfilled MSW decreased from 0.2 cm/s to 7.8×10?5?cm/s when the dry unit weight increased from 3.2 to 9.6?kN/m3. The results clearly demonstrated that the hydraulic conductivity of MSW can be significantly influenced by vertical stress and it is mainly attributed to the increase in density leading to low void ratio. In small-scale triaxial permeameter, when the confining pressure was increased from 69 to 276 kPa the hydraulic conductivity decreased from approximately 10?4?to?10?6?cm/s, which is much lower than those determined from rigid-wall permeameter tests. The published field MSW hydraulic conductivities are found to be higher than the laboratory results. Landfilled MSW possesses lower hydraulic conductivity than fresh MSW due to increased finer particles resulting from degradation. The decreasing hydraulic conductivity with increasing dry unit weight is expressed by an exponential decay function.     

Pavement Structures Damage Caused by Hurricane Katrina Flooding

In September of 2005, Hurricane Katrina devastated New Orleans and caused sustained flooding. Limited pre- and postflooding tests indicated that the pavement structures tested were adversely impacted by the flood water. Consequently, the Louisiana Dept. of Transportation and Development hired an independent contractor to structurally test approximately 383 km (238?mi) of the region’s federally aided urban highway system both inside and outside of the flooding area. Falling weight deflectometer (FWD) tests were performed every 161 m (0.1?mi) over each selected roadway, along with other field tests. The FWD data were imported into a geographical information system and plotted against a USGS geo-referenced map. Comparative analyses were made possible through the use of extensive flood maps made available through NOAA and FEMA. This arrangement made it possible to classify spatially and graphically all test points on the basis of flooding versus nonflooding, short flooding duration versus longer flooding duration, shallow flooding versus deep flooding, and thin pavements versus thick pavements. Three pavement types, asphaltic concrete, Portland cement concrete, and composite, were considered in this analysis. The statistical inference about the difference in the means of compared data groups was conducted with 95% confidence.     

大型内球面镗削专用组合刀具的设计与应用

阐述一种根据曲柄滑块机构和合成运动原理进行设计的、在镗床上进行大尺寸内球面加工的镗削专用组合刀具,利用这种特制的专用组合刀具在镗床上加工Φ5.5 m×115 m特大型回转窑的关键零件——内球面轴承座,解决了轴承座尺寸大、内球面加工精度要求高,加工难度大的技术难题。这种特制的镗削专用组合刀具的研发成功,对利用普通镗床进行内球面加工提供了一种新的途径。     

Void formation in INCONEL MA-754 by high temperature oxidation

Subsurface void formation in oxide dispersion strengthened MA-754 caused by high temperature oxidation was investigated at temperatures of 1100, 1150, and 1200 °C for times of 1, 10, 50, and 100 hours. Material exposed at 1200 °C was examined using microprobe, SEM, and optical microscopy techniques. After exposure in air at 1200 °C for 100 hours, chromium depletion by as much as 10 wt pct was observed near the surface, and voids of various sizes up to 15 μm in diameter were found to depths of 300 μm. The fraction of voids increases with exposure time and, with the exception of anomalous values near the surface, decreases with depth. The maximum area fraction of voids observed was approximately 8 pct. Correlation of the void area fraction profile with the measured chromium depletion through a diffusion analysis shows that void formation is due to vacancy injection. Similar void formation in Ni-Cr alloys without oxide dispersions suggests that void formation is not dependent upon the presence of oxide dispersions. The diffusion coefficient for chromium in MA-754 at 1200 °C was computed from microprobe data to be 4 × 10^-10 cm² per second.     

Comparison of Water Distribution Uniformities between Increased-Discharge and Continuous-Flow Irrigations in Blocked-End Furrows

A comparative analysis of the distribution uniformities of infiltrated water depth in blocked end furrows between three irrigations with increased-discharge (IDI) and three irrigations with constant flow rate (CFI) is presented. The goal is to evaluate IDI irrigation contrasted to CFI irrigation, which is the most commonly used in underdeveloped countries. The average distribution uniformities of IDI irrigation is 9% higher than that of CFI irrigation. Therefore, IDI irrigation is a viable alternative of efficient irrigation in blocked end furrows. Field assays were performed in 290?m?long×0.75?m?wide, 0.6% slope furrows in clay loam.     

Method for Calculating the Edge Moisture Variation Distance

A three-dimensional moisture diffusion and volume change model was developed for determining the distribution of soil suction and the associated volume changes in an expansive soil mass under a covered area (e.g., slab foundations or pavements) with respect to time. The model was used to determine a relationship among the edge moisture variation distance, em, the amplitude of surface suction change, U0, and the diffusion coefficient of the soil, α. The edge moisture variation distance calculation is based on the change in soil suction beneath the covered area as a result of climate changes. The edge moisture variation distance is considered to be the distance between the edge of the covered area and the point beneath the covered area where suction change is no more than 0.12 kPa (0.1 pF). Comparison between the predicted values of edge moisture variation distance by using the proposed relationship to that measured in the field or predicted by other methods is also presented.     

Hydraulic Conductivity of Bentonite Slurry Mixed Sands

The hydraulic conductivity (k) of specimens from columns containing initially dry sands mixed with bentonite slurries was measured. The mixed specimens represented a range in void ratios (0.672 ≤ e ≤ 3.94) and bentonite contents (0.61% ≤ BC ≤ 7.65%, by dry weight). The measured k values, which ranged from 2.4×10?7?cm/s to 6.8×10?4?cm/s, correlated poorly with the total void ratio (e) of the specimens, due to the complicating effect of the bentonite in the sand-bentonite slurry mixtures. However, the measured k values correlated better with the void ratio of the bentonite (eb), which is consistent with the results of previous studies involving permeation of compacted bentonite and sand-bentonite specimens, even though the range in values of eb in this study (42.5 ≤ eb ≤ 127) was much higher than that previously reported. The relatively large range in eb values for the sand-bentonite slurry mixtures was also consistent with the relatively large range in measured k values, which are about one to seven orders of magnitude higher than values of k commonly reported for compacted sand-bentonite mixtures, despite similar bentonite contents. In terms of bentonite content, addition of more than 3% bentonite via slurry injection and mixing with the sands was successful in reducing the k of the unmixed sands (9.4×10?3?cm/s ≤ k ≤ 5.4×10?2?cm/s) by as much as four orders of magnitude to values less than 1.0×10?6?cm/s.     

Theory of high temperature intercrystalline fracture under static or fatigue loads,with or without irradiation damage

Hull and Rimmer’s theory of high temperature creep fracture by grain boundary void growth and Skelton’s theory of high temperature fatigue fracture have been extended. The present theory includes the effect on void growth of volume diffusion and of vacancy generation by irradiation damage or plastic deformation within the grain interiors. The growth rate of cylindrical as well as spherical voids is calculated. We find that the presence of volume diffusion or of vacancy generation within the grain does not increase very much the growth rate of spherical voids but under certain conditions the growth rate of cylindrical voids is increased markedly. Unlike Skelton, we find that vacancy production within the grains does not decrease the critical radius for the growth of a grain boundary void in push-pull fatigue. (Likewise intra-granular vacancy production does not decrease the critical radius of grain boundary voids in creep.) Since in push-pull fatigue void growth is observed in the case of radii smaller than the critical value it is concluded that the stress concentrations that are produced by grain boundary sliding (according to the theory of Raj, Ashby, and Gifkins) are responsible for this growth. Whether voids aided by stress concentrations can continue to grow beyond a certain size depends critically upon the “roughness” of the grain boundary. It is strongly recommended that grain boundary internal friction peak studies be carried out in connection with void growth and high temperature fracture experiments. In particular, grain boundary voids are more likely to grow in push-pull fatigue (or in creep) if the value of the grain boundary sliding activation energy is close to that of volume self diffusion rather than if the sliding activation energy is close to the grain boundary self diffusion activation energy.     

On void nucleation and growth in metal interconnect lines under electromigration conditions

Electromigration failure in rigidly passivated metal interconnect lines is studied with particular reference to the vacancy supersaturations and hydrostatic stresses that can be developed at blocking grain boundaries under electromigration conditions. It is shown that the high stresses needed for homogeneous void nucleation to occur are probably too high to be developed by electromigration and that failure is more likely to involve the growth of pre-existing voids. We also show that the amount of void growth that can occur at a blocking grain boundary by electromigration of vacancies down the adjoining grain boundaries is small relative to the dimensions of the line unless the adjoining grain boundaries are continuous in a very long section of the line. This suggests that other mechanisms of void growth are responsible for electromigration failure. An analysis of the electromigration of small pre-existing voids shows that above a critical size, large voids migrate faster than smaller ones. This leads to a catastrophic process in which large voids can catch up with and coalesce with smaller ones, growing in size and migrating more rapidly as they do so. We conclude that the migration and coalescence of preexisting voids is a more likely mechanism of electromigration failure. This paper is based on a presentation made in the “G. Marshall Pound Memorial Symposium on the Kinetics of Phase Transformations” presented as part of the 1990 fall meeting of TMS, October 8–12, 1990, in Detroit, MI, under the auspices of the ASM/MSD Phase Transformations Committee.     

CFD Predictions and Experimental Comparisons of Pressure Drop Effects of Turning Vanes in 90° Duct Elbows

This paper presents new results for numerical predictions of air flow and pressure distribution in two commonly used elbows: (1) 90° mitered duct elbows with turning vanes having 0.05 m radius, 0.038 m vane spacing and (2) 90° mitered duct elbows without turning vanes, in 0.2×0.2?m (8?in.×8?in.) duct cross section using the STAR-CD computational fluid dynamics (CFD) code. A k-ε turbulence model for high Reynolds number and k-ε Chen model were used for that purpose for comparative purposes. The simulation used 13 different Reynolds numbers chosen between the range of 1×105 and 2×106. To validate the CFD results, the results of two experimental papers using guided vanes were compared with simulated vane runs under the same condition. The first experimental study used a 0.6×0.6?m (24?in.×24?in.) square elbow with 0.05 m radius, 0.038 m vane spacing and air velocities at 2.54 m/s (500 fpm) and 25.4 m/s (5,000 fpm), the second experiment used a 0.81×0.2?m (32?in.×8?in.) rectangular elbow geometry with 0.05 m radius, 0.038 m vane spacing with air velocities from 10.16 m/s (2,000 fpm) to 13.97 m/s (2,750 fpm). For Reynolds numbers (1.00–2.00)×105 the pressure drop difference between vaned and unvaned elbows was found to be 35 Pa as compared to 145 Pa. The simulations also agreed reasonably well with published experimental results. For the 0.6×0.6?m (24?in.×24?in.) square elbow and 0.81×0.2?m (32?in.×8?in.) rectangular elbow with vanes, the difference in pressure drop was 3.9 and 4.1% respectively and indicates that CFD models can be used for predictive purposes in this important HVAC applications area.     

The effect of time dependent void density on grain boundary creep fracture—i. continuous void coalescence

When voids in a creeping solid are non-uniformly distributed over a grain boundary facet void coalescence occurs simultaneously with void growth. The effect of such coalescence on subsequent grain boundary void growth can be analysed assuming a random (i.e. Poisson) initial void distribution. A simple relationship between the fraction of voids remaining on a grain boundary facet and their area fraction is found. This relationship is independent of the mechanism by which the voids grow. When voids grow by surface diffusion or by power-law creep very little effect of coalescence on the overall rate of damage accumulation is found. However, when void growth is controlled by grain boundary diffusion, the rate of void growth is decreased dramatically during coalescence, and effectively stops. Final fracture occurs by creep controlled void growth. The model predicts a non-linear stress dependence for this process, and an activation energy different from that for boundary diffusion alone. The model is compared with creep fracture data for pure metals implanted with water vapour bubbles. Good agreement with the model is found in two cases, while in a third case void growth appears to have been controlled by surface diffusion.     

A rationalization of tensile ductility and fracture in Alpha-Beta Ti-Mn alloys

Void nucleation and growth were studied in equiaxed α-gb Ti-Mn alloys, containing 1.8, 3.9, 5.8, and 8.0 wt pct Mn, in an attempt to understand why ductility tended to remain relatively constant, as both yield strength and fracture strength increased with increasing Mn content. In addition, measurements of volume fraction of voids,/,, and average void diameter,d _v, in the minimum neck section were made to determine whether catastrophic void coalescence was a possible mechanism of fracture. The values of_v andd _v at fracture were obtained by linear extrapolation of log_v andd _v vs true strain. These data indicated that the average distance between voids in the minimum neck section for Ti-3.9 Mn which contained the largest/, was most likely too large to permit catastrophic void coalescence to take place. These results supported earlier^1,2,3 and current observations that fracture occurred, during tensile straining, when a critical relationship was reached between void length and the fracture stress corrected for necking, σ_fc. It was shown that the changes in this critical relationship with both microconstituent size and volume fraction of phases were balanced by a change in void growth rate to critical size, with the result that strain to fracture remained nearly constant.     

Ductile fracture by the growth and coalescence of microvoids of non-uniform size and spacing

A two-dimensional plane strain model of void growth and coalescence in a rigid/plastic solid, containing void sizes and spacings which can be highly non-uniform, is developed to investigate the effects of non-uniform distributions of void-nucleating particles on the ductility of a metal. The theoretical void-growth strains to ductile fracture for a wide variation in void diameters and spacings show that, for a given volume fraction of voids, the minimum ductile-fracture strain occurs when the voids are of uniform size and spacing. For the same volume fraction of voids, greatly increased ductility is likely to be achieved when the void sizes and spacings are highly non-uniform and the sub-cell volume fractions are also non-uniform.