Search Results

Substantial improvements have been made in the accuracy of component fatigue life predictions through the incorporation of cyclic stress-strain concepts in fatigue analysis procedures. This paper describes recent advances in material characterization, complex history analysis and notch analysis, and their application in computer-aided fatigue analysis procedures. A simplified strain-based approach, which offers conceptual and computational advantages, is then described and shown to be useful in many ground vehicle problems. Finally, several applications of these techniques in engineering design practice are presented.

Dynamometer tests of a production disc brake provided new information on asbestos fiber emissions during break-in, normal use, and high temperature use conditions. Both ambient air and brake cooling air were sampled isokinetically, using 0.45 μm filters. Examination of test and background filters required a clarification process to maximize fiber detectability, the use of transmission electron microscopy (at 40,000X) for detection, and electron diffraction for positive identification of asbestos fibers. Most of the lining asbestos was found to be converted to a nonfibrous material by the high flash temperatures of the braking surface. Less than 0.02% of the lining wear was released as asbestos fibers. The concentration of asbestos fibers in the urban atmosphere, due to brake usage, was conservatively estimated at less than 0.07 X 10-9 g/m3. Based on this upper bound, the use of brakes was judged to be not significant as a source of atmospheric asbestos.

Several resin-bonded friction materials were used to establish the interrelationship between the resin-cure state and the functional properties. Pyrolytic gas chromatography (PGC) was used to measure the cure state of the resin and the Friction Assessment Screening Test (FAST) was used to characterize the friction and wear behavior of the materials. A phenolic resin, an oil-modified phenolic resin, and two different cashew resins were used as binders for simple resin-asbestos composites as used in brake linings. Both the PGC and functional properties of these materials showed systematic variations with changes in cure conditions of the resin. For all resins studied, a linear or bilinear relationship was found to exist between the PGC measured cure state of the resin and wear as measured by the FAST machine. A chemical kinetic model was successfully applied to relate both sample wear and a characteristic PGC peak of the resin to a unique function of cure time and temperature.

Applying hydrodynamic lubrication theory for porous bearings and boundary lubrication theory, this paper presents a method of analyzing the performance of a water-lubricated sleeve type porous bushing in an automotive water pump design. Relations of bearing load capacity versus shaft speed have been obtained and compared for sintered iron-graphite bushings (a cermet material developed by the Ford Motor Co.), sintered iron or sintered bronze bushings, and steel bushings. The load capacity was computed, based on a minimum allowable film thickness during hydrodynamic operation, and on a maximum allowable temperature during boundary lubrication operation. The results show that sintered iron-graphite bushings are superior to sintered iron or sintered bronze bushings, as well as steel bushings, in this application, due to the lower coefficient of friction.

A modified Ausform process has been developed which improves the fatigue properties of spring steels. In brief, the process combines metal deformation with heat treatment. The fatigue resistance of SAE 5150 and 1052 steels is greatly improved by this treatment. The amount of deformation directly influences the fatigue resistance; and with more than 50% deformation, the fatigue life is improved by 700% over that of SAE 5160 spring steel. For a 100,000 cycle minimum life, both maximum stress and stress range can be increased by 30,000 psi over that of conventionally heat treated SAE 5160 steel. Superior fatigue properties have been obtained in sections with thicknesses of 0.200-0.500 in. Surface treatments such as sandblasting, shot peening, and glass bead peening are effective in prolonging fatigue life; glass bead peening was by far the most effective. Modified Ausformed steels display an unusual fracture behavior which is beneficial in fatigue and notch toughness.

A study was made of the responses of pyrolytic gas chromatography (PGC) and the friction assessment screening test (FAST) to variations in curing conditions for a liquid, oil-modified, phenolic resin system. Both PGC, which characterizes the organic resin, and FAST, which characterizes the friction and wear properties, show systematic variations with changes in cure time and temperature. A linear relationship exists between the area of one PGC peak (phenol) and the wear as determined by the FAST procedure. A chemical kinetic model is postulated which relates the concentration of phenol produced on pyrolysis to a function of cure time and temperature. An index of cure is introduced which defines the curing conditions in terms of a single parameter. The PGC phenol peak area, FAST friction, and FAST wear correlate well with this index of cure.

A compact new laboratory friction and wear test machine has been developed. Test procedures have been established for this machine in a constant output (that is, constant friction force) mode of operation. These procedures have been shown to be particularly well suited for quality control of brake lining materials. The test, designated Friction Assessment Screening Test (FAST), has been shown to yield highly reproducible results which correlate well with vehicle performance. The results are highly sensitive to those variations in brake lining properties which are most significant in brake performance.