Lokeswarappa R. Dharani

Fade and wear characteristics of a glass-fiber-reinforced phenolic friction material

Abstract The fade and wear characteristics of a glass-fiber-reinforced friction material were studied using a Chase friction material testing machine. At low counterface temperatures, the friction material showed relatively high friction in the range 0.4–0.5. During fade tests, the coefficient of friction dropped to about 0.18 at 343 °C. Re-conditioning the wear surface at the end of a fade test altered the frictional behavior during a subsequent fade test. The wear tests showed that the specific weight loss per unit load and sliding distance decreases with increasing applied load and speed, but increases with increasing bulk drum temperature. At high temperatures, thermochemical degradation and fiber pull-out appear to contribute to higher specific wear rate. The worn surfaces of the specimens were observed by scanning electron microscopy and analyzed by energy-dispersive X-ray analysis. The results were consistent with low friction coefficients due to film formation on the worn surfaces of glass fiber at high temperatures. This film could be removed at lower temperatures by either sliding (application) or by sanding.

Hybrid phenolic friction composites containing Kevlar® pulp Part 1. Enhancement of friction and wear performance

The friction and wear characteristics of control (without Kevlar® pulp) and hybrid (with Kevlar® pulp) phenolic composites containing milled E-glass or steel were determined at various counterface speeds and temperatures using a Chase friction tester. In general, Kevlar® pulp significantly improved the wear resistance and decreased the coefficient of friction for both types of hybrid composites. Kevlar® pulp also imparted excellent frictional stability at high speeds in steel-fiber composites and significantly reduced higher frequency ( > 5 kHz) noise at high speeds in both steel and glass-fiber composites. The stabilization of the coefficient of friction and reduction of noise was not due to the reduction of the coefficient of friction because it also occurred at constant frictional force. The addition of Kevlar® pulp to a steel-fiber-containing formulation significantly improved its overall performance.

Mathematical modeling of damage in unidirectional composites

Abstract Solutions are developed for the two-dimensional region containing unidirectional fibers embedded in an elastic matrix with an initial flaw in the form of a transverse notch, a rectangular cut-out, and a circular hole. Subsequent damage due to the presence of the flaw is generated by remote stresses acting parallel to the fibers. This work is an extension of the paper by Goree and Gross [1] in which the flaw was taken in the form of a notch (crack) and the subsequent damage, due to loading, consisted of longitudinal matrix yielding and splitting at the end of the notch. The present study accounts for longitudinal matrix damage as in [1] and, in addition, includes transverse matrix and fiber damage in the vicinity of the flaw for the above three initial shapes. The fibers are taken as linearly elastic, the matrix material as elasticperfectly plastic and the classical shear-lag stress displacement assumptions are used. An ultimate stress failure criterion is used for both the fibers and the matrix; simple tension for the fibers and shear failure for the matrix. For ductile matrix composites (boron/aluminum) the present results indicate that both longitudinal matrix yielding and transverse notch extension must be included in order for the model to agree with experimental results. Interestingly, the extent of the transverse damage region at failure is shown to be approximately constant, independent of the initial flaw shape or length. Very little difference is found between the results for the three types of initial damage, i.e. the notch, rectangular cut-out and circular hole. In all cases, the presence of additional damage changes the nature of the stress distribution in the unbroken fibers. For the original Hedgepeth[2] problem of a notched laminate the stresses decay as the square root of the distance from the notch tip. Inclusion of longitudinal or transverse damage significantly reduces the maximum stress concentration in the unbroken fibers and gives a much more uniform stress state. It is shown that this behavior cannot be accounted for by introducing an effective notch length or crack tip damage zone with a square root behavior.

Load, speed and temperature sensitivities of a carbon-fiber-reinforced phenolic friction material

Abstract A model friction material was formulated with a cashew-modified phenolic resin, short carbon fiber, phenolic particles, barytes and steel fiber. The friction, wear and fade characteristics of this material were determined using a Chase friction material testing machine. The coefficient of friction was found to vary between 0.2 and 0.5 with lower values associated with higher loads, speeds and drum temperatures and vice versa. Conditioning the specimens with several fade-recovery test cycles resulted in steady friction during subsequent fade tests followed by excellent recovery characteristics. The specific wear rate per unit load and sliding distance decreased with increasing loads, but increased with increasing drum speeds and temperatures due to thermal degradation of the resin. The carbon-fiber-reinforced friction material showed lower specific wear rates than that of a milled-glass-fiber-based friction material at low speeds and temperatures over a wide load range.

Micromechanics characterization of sublaminate damage

A micromechanics analytical model is developed for characterizing the fracture behaviour of a fibre reinforced composite laminate containing a transverse matrix crack and longitudinal debonding along 0/90 interface. Both the matrix and the fibres are considered as linear elastic. A consistent shear lag theory is used to represent the stress-displacement relations. The governing equations, a set of differential-difference equations, are solved satisfying the boundary conditions appropriate to the damage configuration by making use of an eigenvalue technique. The properties of the constituents appear in the model explicitly. Displacements and stresses in the fibres and the matrix are obtained, and the growth of damage is investigated by using the point stress criterion. The investigation includes fibre stress distribution in zero degree plies, transverse crack and debonding intitiation as functions of laminate geometry, and the effect of fibre breaks in the zero degree ply on damage growth. The predicted damage growth patterns and the corresponding critical strains agree with the finite element and experimental results.

Modelling fracture in laminated architectural glass subject to low velocity impact

Standard finite element wave propagation codes are useful for determining stresses caused by colliding bodies; however, their applicability to brittle materials is limited because an accurate treatment of the fracture process is difficult to model. This paper presents a method that allows traditional wave propagation codes to model low velocity, small missile impact in laminated architectural glass such as that which occurs in severe windstorms. Specifically, a method is developed to model typical fractures that occur when laminated glass is impacted by windborne debris. Computational results of concern to architectural glazing designers are presented.

Stresses in laminated glass subject to low velocity impact

Finite element analysis is used to study small, low velocity missile impact of laminated architectural glass. The impact situation models that commonly observed during severe windstorms in which small, hard missiles impact laminated glass windows in large buildings. Architectural laminated glass is typically made of two soda-lime glass plies separated by a clear, sticky, polyvinyl butyral (PVB) interlayer. In order to increase the damage tolerance of laminated glass windows, various geometric and material parameters are investigated to determine their effect in minimizing stress wave propagation through the three-layer system to the critical inside ply. Parameters investigated include the thickness of each layer of the system and the viscoelastic properties of the PVB interlayer.

Hybrid phenolic friction composites containing Kevlar® pulp Part II—wear surface characteristics

Abstract The wear surfaces of control (without Kevlar®) and hybrid (with Kevlar®) phenolic composites were analyzed using a scanning electron microscope and energy dispersive X-rays to determine the influence of Kevlar® pulp on the wear surface morphologies at various sliding conditions. At a high sliding speed of 11.2 m s −1 , the wear surface of a control glass fiber composite showed the formation of a nearly continuous and homogeneous film comprised of elements from the counterface and the composite itself. This film did not appear to form in a glass-Kevlar® hybrid composite at a similar operating speed. At 11.2 m s −1 , a control steel fiber composite was found to wear due to a predominantly abrasive mechanism, while this mechanism was diminished in a steel-Kevlar® hybrid composite.

Damage probability in laminated glass subjected to low velocity small missile impacts

The probability of damage at the impact site in the outer glass ply of laminated glass units subjected to low velocity small missile impacts is investigated. A dynamic, non-linear finite element analysis is applied to compute the stress response on impacts. Based on the cumulative damage theory, a damage factor is introduced and related to Weibulls distribution of probability to characterize the probability of damage. In conjunction with the finite element analysis, controlled experiments are conducted to determine the material constants appearing in the damage model and Weibulls distribution of probability. A parametric study involving impact velocity, glass ply thickness and interlayer thickness is presented.

Effect of fiber coating and interfacial debonding on crack growth in fiber-reinforced composites

Abstract The problem of a crack impinging upon an interface between dissimilar materials is investigated using a Consistent Shear-Lag (COSL) model. The purpose is to determine whether the crack will penetrate the interface or be deflected into it. This is important in fiber-reinforced composites since fracture resistance can be enhanced when deflection is favored over penetration. To determine the mode of crack growth, the energy release rates associated with each mode (penetration and deflection) are calculated and the ratio of energy release rates is compared to the corresponding ratio of toughnesses. Results are given for fiber-reinforced composite materials. Since many practical composites are produced using coated fibers, the energy release rate ratio is given for various types of coatings and varying coating thicknesses. It is possible to infer that coating thickness has a small effect, but that the best toughness can be achieved from the thinnest possible coating. Since realistic composite materials often contain inherent flaws such as debonding at the fiber-matrix interface, the previous formulation is modified to include a crack impinging upon an initially debonded interface. It is shown that proximity of the crack tip to the debond is very important in predicting the mode of crack growth.