Stiffnesses compression

Because of differences in stiffness, compression forces wilt be neglected in cross members. 

Materials selection process can be depicted in terms of Figure 1.40. Materials selection involves many factors that have to be optimized for a particular application. The foremost consideration is the cost of the material and its applicability in the environmental conditions so that integrity can be maintained during the lifetime of the equipment. When the material of construction is metallic in nature, the chemical composition and the mechanical properties of the metal are significant. Some of the important mechanical properties are hardness, creep, fatigue, stiffness, compression, shear, impact, tensile strength and wear. 

Schertmann (1970,1978) developed a procedure for estimating footing settlements on sand using cone penetration test (CPT) data. This CPT method uses cone-tip penetration resistance, as a measure of the in situ stiffness (compressibility) soils. Schertmann s method is expressed as following ... 

The relationship between torsional stiffness, aspect ratio and compression ratio and the fatigue life of rubber bushings for military vehicle track systems is discussed. Results are presented of tests conducted on traditional and prototype bushing designs using a laboratory endurance simulation machine, and which suggest that the endurance life is more sensitive to changes in torsional stiffness (shear strain) than radial stiffness (compressive stress). 5 refs. 

The addition of an inorganic filler to a glassy polymer resin reduces curing shrinkage and increases stiffness, compressive strength, and abrasive wear resistance. These desirable effects are often (but not always) accompanied by detrimental effects on tensile strength, fracture toughness, brittleness, etc. 

Years of development have led to a standardized system for objective evaluation of fabric hand (129). This, the Kawabata evaluation system (KES), consists of four basic testing machines a tensile and shear tester, a bending tester, a compression tester, and a surface tester for measuring friction and surface roughness. To complete the evaluation, fabric weight and thickness are determined. The measurements result in 16 different hand parameters or characteristic values, which have been correlated to appraisals of fabric hand by panels of experts (121). Translation formulas have also been developed based on required levels of each hand property for specific end uses (129). The properties include stiffness, smoothness, and fullness levels as well as the total hand value. In more recent years, abundant research has been documented concerning hand assessment (130—133). 

These values are determined by experiment. It is, however, by no means a trivial task to measure the lamina compressive and shear strengths (52,53). Also the failure of the first ply of a laminate does not necessarily coincide with the maximum load that the laminate can sustain. In many practical composite laminates first-ply failure may be accompanied by a very small reduction in the laminate stiffness. Local ply-level failures can reduce the stress-raising effects of notches and enhance fatigue performance (54). 

The PAI resin is utilized for small, simple parts. Because of its stiff melt flow, the resin is available in compression moldable powder. Grades include neat, filled, and glass-reinforced resins it is also available in solution for use as an enamel. Because of high prices ( 33—44/kg) the market is limited, estimated at 150—300 tons for the mol ding resin. Resins competing with PAI include PEI, polyethersulfones, and thermoset resins. 

The air spring effect results from adiabatic expansion and compression of the air in the actuator casing, Niirnericallv, the small perturbation value for air spring stiffness in Newtons/rneter is given bv Eq, (8-107),... 

The elastomer compression coupling provides both tuning and damp ing to the system. In some cases, the two functions interact, that is, the stiffness K or damping C may be a function of the other. The elastomers are torsionally softer than the metal-metal resilients, but will introduce higher levels of damping into the system. 

The topic of materials with different strengths and stiffnesses in tension than in compression will not be covered further in much depth (except to report different strengths) because research on such materials is still in its infancy. However, the topic is very important for the general class of composite materials, if not fiber-reinforced laminated composites. Ambartsumyan and his associates first reported research on this topic in 1965 [2-9]. A few Americans have also investigated some aspects of the mechanics of these materials (see Jones [2-10], Bert [2-11], and Bert and Reddy [2-12]). 

S. A. Ambartsumyan, The Axisymmetric Problem of a Circular Cylindrical Shell Made of Material with Different Stiffness in Tension etnd Compression, Izvesttya Akademii Nauk SSSR Mekhanika, No. 4, 1965, pp. 77-85 English translation N69-11070, STAR. 

The mechanics of materials approach to the micromechanics of material stiffnesses is discussed in Section 3.2. There, simple approximations to the engineering constants E., E2, arid orthotropic material are introduced. In Section 3.3, the elasticity approach to the micromechanics of material stiffnesses is addressed. Bounding techniques, exact solutions, the concept of contiguity, and the Halpin-Tsai approximate equations are all examined. Next, the various approaches to prediction of stiffness are compared in Section 3.4 with experimental data for both particulate composite materials and fiber-reinforced composite materials. Parallel to the study of the micromechanics of material stiffnesses is the micromechanics of material strengths which is introduced in Section 3.5. There, mechanics of materials predictions of tensile and compressive strengths are described. 

Strengths and stiffnesses of the laminae are the same in tension as in compression. 

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