Stanley Backer

Effect of inclining angle, bundling and surface treatment on synthetic fibre pull-out from a cement matrix

Abstract In an experimental investigation of synthetic fibre pull-out at an angle, it was generally observed that the force and energy of fibre pull-out increase with the inclination angle, but such increases are limited by the strength of the cement matrix at high angles due to matrix spalling. Studies were also conducted on the effects of fibre bundling and surface treatment on the pull-out behaviour. It is suggested that for effective use of the reinforcing fibres, fibre bundling should be minimized and the fibre/matrix bond property should be controlled.

A MICROMECHANICAL MODEL OF TENSION- SOFTENING AND BRIDGING TOUGHENING OF SHORT RANDOM FIBER REINFORCED BRITTLE MATRIX COMPOSITES

Abstract A micromechanical model has been formulated for the post-cracking behavior of a brittle matrix composite reinforced with randomly distributed short fibers. This model incorporates the mechanics of pull-out of fibers which are inclined at an angle to the matrix crack plane and which undergo slip-weakening or slip-hardening during the pull-out process. In addition, the random location and orientation of fibers are accounted for. Comparisons of model predictions of post-cracking tension-softening behavior with experimental data appear to support the validity of the model. The model is used to examine the effects of fiber length, snubbing friction coefficient and interfacial bond behavior on composite post-cracking tensile properties. The scaling of the bridging fracture toughening with material parameters is discussed.

Some Principles of Nonwoven Fabrics1

The authors compare the mechanical behavior of woven and nonwoven fabrics. They outline the principal mechanisms by which woven cloths deform and recover in manufacture and in end usage. They attempt to formulate a parallel sequence of events in the distortion process for nonwovens and to show how these actions influence the ability of nonwovens to fit smoothty on three-dimensionally curved surfaces, to hang and drape freely, and to recover from bends and tensile extension. Some results of a research program recently conducted at M.I.T. on nonwovens are presented. This study is related to the directional tensile behavior of nonwoven fabrics and demonstrates how knowledge of fiber properties and web structure can be used effectively to predict web behavior. Fiber behavior measured on the Instron provide the material constants necessary for insertion in derived analytical expressions , for nonwoven fabric stiffness and contraction ratios. Predicted nonwoven behavior shows reasonable agreement with experimental measurements on the particular materials studied.

Modelling of fibre pull-out from a cement matrix

Abstract Theoretical analyses of fibre pull-out from a matrix reported in the literature are briefly reviewed. The effects of Poissons ratio, elastic-frictional bond strengths, and bond strength variation with slippage distance on the pull-out relation are discussed. A theoretical model motivated by observations of fibre surface abrasion is developed to predict the pull-out force versus displacement relationship. The model takes into consideration the variation of the frictional fibre-matrix bond strength with fibre slippage distance. Good agreement is achieved between model predicted pull-out behaviour and experimental pull-out curves for nylon and polypropylene fibres. For these synthetic fibres, the bond strength increases with the slippage distance during the process of pull-out. The model also predicts reasonably well the pull-out behaviour of steel fibres for which the bond strength decreases with the slippage distance.

The Relationship Between the Structural Geometry of a Textile Fabric and Its Physical Properties I: Literature Review

This is the first in a series of papers to be issued by the Quartermaster Corps discussing the importance of fabric geometry in the development of more utilitarian and serviceable textile materials. It is a literature review which shows the relationship between the structural char acteristics of fabrics and various functional characteristics, such as breaking strength and elongation, tear-resistance, thermal insulation, abrasion-resistance, and gas permeability.

An experimental study of synthetic fibre reinforced cementitious composites

Fibre reinforcement is one of the effective ways of improving the properties of concrete. However, current studios on fibre -reinforced concrete (FRC) have focused mainly on reinforcements with steel and glass fibres. Thin paper reports on an experimental programme on the properties of various synthetic fibre reinforced cementitious composites and the properties of the reinforcing fibres. Acrylic, polyester, and aramid fibres were tested in uniaxial tension, both in their original state as we!! as after ageing in nerO*nL Samples of these fibres were found to lose varying amounts of strength with time, depending on the ageing temperature. Two different test methods were used to measure the fibre-cement interfacial bond strength. The tensile properties of concrete reinforced with acrylic, nylon, and aramid fibres, in the form of random distribution or unioxial alignment, were studied by means of three different tests: compact tension, flexural, and splitting tensile tests. The properties of concrete, particularly that of apparent ductility, were found to be greatly improved by the inclusion of such fibre reinforcement.

The Mechanics of Bent Yarns

This paper analyzes the idealized geometry of a bent yarn. Expressions are derived for the cal culation of local fiber tensile strain, average strain in the helix half loop, motion of the fibers during bending of the yarn, changes in local helix angles, and fiber curvatures at the bend. The relationships between the geometric quantities of a bent yarn are presented graphically in dimen sionless parameters, and conversions to the usual textile terms are provided. Attention is given to the practical applications of this analytical approach to textile problems.

The Relationship Between the Structural Geometry of a Textile Fabric and Its Physical Properties: Part IV: Interstice Geometry and Air Permeability

This paper is a study of the geometry of fabric interstices as related to air flow through cloth structures. It assumes that textile yarns act as flexible, inextensible, circular cylinders. Modes of thread interlacing are classified, and minimum horizontal pore areas for each type of intermesh are calculated. Minimum pore areas are then related to the air permeabilities of both open- and close-weave fabrics. Considerable attention is devoted to the elements of fabric structure which complicate geometric calculations of pore-sectional areas. These include crimp balance and yarn flattening and ballooning. Further attention is given to the relative importance of interfiber spacings and to the interaction between yarn and fabric structures. Finally, this study emphasizes the significance of the primary and secondary structural features of woven fabrics in the design of materials which must satisfy air-permeability requirements.

Mechanistic Role of Yarn and Fabric Structure in Determining Tear Resistance of Woven Cloth Part I: Understanding Tongue Tear

The goal of this study is to acquire an understanding of the manner in which cotton yam properties (including structure) contribute to the tear resistance of woven fabrics as measured by the tongue tear test. Our approach to this overall goal is a three step process. First, we examined samples during and after failure to identify the important phenomena that contribute to tearing strength, termed the subcomponents of tear. Next, we designed tests to observe the isolated contributions of pull-in (of yams into the del zone), pull-ahead (of yams towards the untorn fabric), and yam tenacity. Finally, in Part II of this paper, we will describe a model based on a mechanical spring analogy that will allow us to predict load-displacement responses of woven fabrics being torn.

Mechanical Properties of Fabrics Woven from Yarns Produced by Different Spinning Technologies: Yarn Failure as a Function of Gauge Length

In a study of yam strength translation into woven fabric behavior, experiments were conducted to establish the effect of test gauge length on yarn properties. Yams produced on each of the three major spinning systems were tensile tested at varying gauge lengths. Yam strength data were fit to two-parameter Weibull distributions and corresponding shape and scale parameters were determined. Strength increased as gauge lengths decreased, a trend indicated by the weakest-link theory. At very short gauge lengths, however, the data deviated from prediction based on the weakest-link theory, thus suggesting a change in the yam failure mechanism, as one would expect when the gauge length approximates the staple length. More direct evidence of such a change is provided in SEM photomicrographs of tensile failures of long versus short gauge test specimens. Combined fiber slippage/pullout and breakage prevailed at longer gauges, whereas a greater extent of fiber breakage with less slippage occurred at shorter gauge lengths. The balance between fiber slippage and fiber breakage varied with yarn structure as produced on different spinning systems. Finally, tensile tests were con ducted on plain and twill weave fabrics woven from yams produced on the different spinning systems. The resultant fabric tenacities approximated corresponding yarn tenacities only for the shortest gauge lengths.