Large fracture toughness boron-epoxy composites

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76716/1/AIAA-1975-773-197.pd

Hydrodynamic lubrication in cold rolling

Particular attention has been paid to the effects of roll flattening and to the inlet zone of pressure build-up in the determination of lubricant film thickness in cold rolling. It is shown that under present-day practical conditions, the thicknesses of the lubricant films relative to surface roughnesses are insufficient to maintain full fluid film lubrication.Although the mathematical model predicts a "speed effect" for rolling (plots of rolling load against speed looking like a Sommerfeld diagram for a journal bearing), the speeds involved are much faster than present commercial rates. Thus speed effects in the literature must have been caused by a change over from boundary to mixed lubrication and lubricant puddle entrapment in surface microcrevices. The non-dimensional form of the solutions shows that laboratory experiments rarely approach full-scale mill conditions, thus reflecting the notoriously difficult problem of evaluating commercial metal-working lubricants.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22428/1/0000878.pd

Intermittent bonding for high toughness/ high strength composites

High strength and high toughness are usually mutually exclusive in brittle filament/brittle matrix composites. The high tensile strength characteristic of strong interfacial filament/matrix bonding can, however, be combined with the high fracture toughness of weak interfacial bonding, when the filaments are arranged to have alternate sections of high and low shear stress (and low and high toughness). Such weak and strong areas can be achieved by appropriate intermittent coating of the fibres. The strong regions ensure that the filament strength is picked up; weak areas randomly in the path of running cracks serve to blunt them by the Cook/Gordon mechanism which, in turn, produces long pull-out lengths with an associated large contribution to toughness. Boron-epoxy composites of volume fraction 0.20 to 0.25 have been made in this way which have fracture toughnesses of over 200 kJ m −2 , whilst retaining rule of mixtures tensile strengths (∼ 650 MN m −2 ). At the volume fractions used, this apparently represents K IC values greater than 100 MN m −3/2 .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44665/1/10853_2005_Article_BF01163077.pd

Determination of Ground Water Associated with Lignite Mining in Arid Climate,

Ground water and surface water create a range of problems in lignite mining utilizing surface mining methods. In order to create a safe and economic mining environment, it is essential to carry out mining after dewatering the rock mass surrounding the lignite mines by advance dewatering techniques. This paper briefly describes the ground water regimes including pressure gradients associated with various lignite deposits together with the practical examples of some important lignite deposits in the world. An effective method of controlling ground water in multi-aquifer environment in lignite deposits is to carry out rock mass dewatering using borehole pumps. This approach will help reducing the inflow rates of ground water to the mining excavation and also increase the effective strength of the overburden strata, thus, increasing the slope stability of the mining excavations. The main theme of this paper is to present a case history analysis of Thar lignite deposit in Sindh, Pakistan which has lignite reserves of some 193 billion tonnes. The paper presents a proposed method of dewatering the Thar prospect together with an assessment of the quality of aquifer water which can be used to improve the quality of life of people inhabiting in the Thar Desert area of Sindh, Pakistan. Water samples from three aquifers were collected from nine different locations and were analyzed in the laboratory for evaluating their physical and chemical characteristics. The test results indicated that the aquifer water can be classified as (sodium+ potassium) -chloride type water with a TDS range of 1000 to 20,000 mg/L. Consequently, this ground water is classified as brackish (saline water) requiring treatment before it can be utilised for domestic or industrial consumptions. It should be noted that this ground water does not contain heavy metals and toxic metals including arsenic, mercury and lead or cyanide. However, results indicate that groundwater from a few locations contained traces of silver (<4oppb) Owithozinc0<0.1ppm

The laws of similitude and crack propagation

The mechanics of cracking follow the laws of similitude in an odd sense. As a result, crack load-external displacement-crack extension data are not usually non-dimensionalized. It follows that a new "group" should be used (analogous to the Reynolds, Froude or Cauchy numbers) when scaling ship ice-breaking resistance from tests of models in ice-towing-tanks.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22303/1/0000747.pd

The incorporation of work hardening and redundant work in rod-drawing analyses

With few exceptions, metal deformation analyses employ a constant yield stress (rigid, perfectly plastic metal) which ignores strain hardening, or a "mean" yield stress which attempts to accommodate strain hardening in a simplified manner. Since strain hardening is of interest here, little reference will be made to a rigid, plastic type of behavior.The first part of this paper demonstrates that the use of a mean yield stress underestimates the working loads (or stresses) needed to draw metal through conical dies as compared to those loads predicted by more "exact" analyses. In this context "exact" refers to those solutions obtained by incorporating the strain hardening relationship in the governing "force balance" differential equations prior to the integration of the said equations. It is shown, however, that the error introduced by the use of a mean yield stress is no more than some 8 per cent for conditions that typify actual practice. Since analyses of other metal-working processes, such as rolling and extrusion, employ the same sort of differential equation, it is felt that these results are applicable there also.The second part of this paper shows that redundant work in rod drawing may be approached either from considerations of the mechanical properties that result after the metal is drawn or from considerations of the stress necessary to draw down the rod. Contrary to what is implied in the literature, it is shown that these two approaches lead to different interpretations of the "redundant work factor". Relationships are given between the two for metals that are assumed to strain harden in certain simple ways.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33226/1/0000616.pd

Contribution on "Stability of cracking" by C. Gurney and Y. W. Mai

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22399/1/0000849.pd

Effects of rate, temperature and absorption of organic solvents on the fracture of plain and glass-filled polystyrene

The rate/temperature dependent fracture behaviour of plain and glass-filled polystyrene has been investigated over the crack speed ( a ) range of 10 −6 to 10 −2 m sec −1 and in the temperature ( T ) range of 296 to 363 K. The K c (a, T) relationships obtained, where K c is the stress intensity factor at fracture, are shown to follow those given by the Williams/Marshall relaxation crack growth model and the toughness-biased rate theory. Crack propagation in both materials is shown to be controlled by a β -relaxation molecular process associated with crazing. Crack instabilities observed in plain polystyrene are analysed successfully in terms of isothermal-adiabatic transitions at the crack tip. Fracture initiation experiments are also conducted in which the effects of organic liquids on the fracture resistances of both plain/glass-filled polystyrene have been determined. Good correlations between K i 2 ( K i being the crack initiation stress intensity factor) and δ s , solvent solubility parameter, of various liquid environments have been obtained, which give a minimum K i 2 value at δ s ≈ δ p , where δ p is the solubility parameter of the polymer. For a given temperature, liquid environment and crack speed, the glass-filled polystyrene is shown to possess greater resistances to crack propagation than plain polystyrene.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44672/1/10853_2005_Article_BF01209454.pd

Determination of valid R -curves for materials with large fracture toughness to yield strength ratios

Crack growth resistance curves are derived from a generalised theory of quasi-static crack propagation due to Gurney and Hunt. Both the subcritical and continuous cracking regions are investigated, where the fracture toughness of the material may depend on the cracking rate, the reacting environment at the crack tip and the mode of fracture. Precise conditions for stability of the spreading crack relative to chosen constraints of either a displacement- or load-controlled machine are formulated. Cracking of sheet materials with high fracture toughness and low yield stress, (e.g. ( K /σ y ) 2 > 200 mm), which do not satisfy certain size requirements, is often complicated by generalised yielding at regions remote from the crack tip. Complete R -curves for such materials cannot be established with conventional testpieces in the laboratory. The present paper adopts a new experimental technique [1] where a laboratory size reinforcement rig attached to the testpiece eliminates all irreversibilities caused by generalised yielding. Valid fracture toughness values and crack growth resistance curves are thereby determined, irrespective of the amount of elastic and plastic deformations occurring at the crack tip. Successful R -curve experiments are described for fracture in a few ductile and tough materials such as 7075-T3. and 1100-0 aluminium alloys, and a low carbon steel. Comparison is made with other published R -curves, and the influence of sheet thickness and ( K 1 c /σ y ) ratio on the geometry of R -curves is investigated. A simple relationship for R -curves is suggested, viz. : R = R 0 + (Δ L ) p , where, it seems, R 0 can be identified with the plane strain toughness (i.e. R 0 = G 1c = K 1c 2 / E (1 - v 2 )1/2). A possible reason for this unexpected result is given in the paper. Useful estimates of K lc may thus be available from thin sheet tests.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42760/1/10704_2004_Article_BF00032834.pd

A yield criterion for anisotropic and pressure dependent solids such as oriented polymers

The anisotropic yield criterion first posed by Hill has been modified to account for differences in tensile and compressive yield strengths in a given direction; additionally, the influence of hydrostatic pressure on yielding is also considered. Predictions using this new criterion are compared with published experimental results involving oriented polymers and excellent agreement is found. It is suggested that this criterion is more correct on fundamental grounds than those put forth in earlier publications.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44783/1/10853_2004_Article_BF00754900.pd