Load Shear Strength

The data obtained using the test method reviewed should be reported as direct shear strength. These data can only be compared to data determined by the same direct-shear methods. This test cannot be used to develop shear S-S curves or determine a shear modulus, because bending or compression rather than pure shear transfer a considerable portion of the load. The test results depend on the susceptibility of the material to the sharpness of load faces. 

It is important to note material such as certain plastics, concrete, or wood that are weak in either tension or compression will also be basically weak in shear. For example, concrete is weak in shear because of its lack of strength in tension. Reinforced bars in the concrete are incorporated to prevent diagonal tension cracking and strengthen concrete beams. Similar action occurs with RPs using fiber filament structures. 

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The radioautographic work suggests another model illustrated in Fig. XII-11. The load is supported over area A, with metal contacts of shear strength s over a portion of the area ctA and film-film contacts of shear strength Sf over the rest of the area. In analogy to Eq. XII-9, one can write the total frictional force, F as... 

Adhesives are used to transfer loads and are typically designed with much higher tensile and shear strengths than sealants. The most important rating of an adhesive ia many appHcatioas is the determiaatioa of how much load it can handle. Some sealants are used as adhesives and some adhesives as sealants and thus arises the occasional blurring of their roles. If the material s primary function is the exclusion of wiad, water, dirt, etc, it is a sealant. 

Plastic Forming. A plastic ceramic body deforms iaelastically without mpture under a compressive load that produces a shear stress ia excess of the shear strength of the body. Plastic forming processes (38,40—42,54—57) iavolve elastic—plastic behavior, whereby measurable elastic respoase occurs before and after plastic yielding. At pressures above the shear strength, the body deforms plastically by shear flow. 

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). 

Here, [L is the coefficient of internal friction, ( ) is the internal angle of friction, andc is the shear strength of the powder in the absence of any applied normal load. The yield locus of a powder may be determined from a shear cell, which typically consists of a cell composed of an upper and lower ring. The normal load is applied to the powder vertically while shear stresses are measured while the lower half of the cell is either translated or rotated [Carson Marinelli, loc. cit.]. Over-... 

That this is not always the case should be expected. In fact, if it was not for heterogeneous localization of some flow phenomena, it would be very diflicult to initiate secondary explosives, or to effect shock-induced chemical reactions in solids. Heterogeneous shear deformation in metals has also been invoked as an explanation for a reduction in shear strength in shock compression as compared to quasi-isentropic loading. We present here a brief discussion of some aspects of heterogeneous deformation in shock-loaded solids. 

Rapid loading - shear stresses set up in impaet may be aeeompanied by high normal stresses whieh exeeed the eleavage strength of the material... 

We know that the eoeffieient of variation, C, of the applied torque is approximately 0.1, and that the final loading stress variable will have a similar level of variation beeause the dimensional variables have a very small varianee eontribution in eompar-ison. We also know the ultimate shear strength parameters of the weak link material, therefore substituting in equation 4.91 and rearranging to set the right-hand side to zero gives ... 

Table 1 contains the metal-to-metal engineering property requirements for Boeing Material Specification (BMS) 5-101, a structural film adhesive for metal to metal and honeycomb sandwich use in areas with normal temperature exposure. The requirements are dominated by shear strength tests. Shear strength is the most critical engineering property for structural adhesives, at least for the simplistic joint analysis that is commonly used for metal-to-metal secondary structure on commercial aircraft. Adhesive Joints are purposefully loaded primarily in shear as opposed to tension or peel modes as adhesives are typically stronger in shear than in Mode I (load normal to the plane of the bond) loading. 

Shear strength is measured via a simple single overlap shear specimen of standard dimensions (Fig. 9). In contrast to its simple appearance, the forces in a thin-adherend shear specimen can be quite complex due to the inherent offset loading of the specimen and subsequent bending in the substrates. The single overlap shear test is anything but a pure shear test, but the configuration is easy to manufacture, simple to test and is firmly entrenched in the industry as a primary examination technique for materials qualifications, inspection and process control. 

But, there is no need to rely on hugonium. The theory and practice of the deformation of solids under other, less intense, loadings are well developed and show that the fluidlike flow of shock deformation is the expected consequence of the motion of defects in response to applied shear stresses that exceed the shear strength of solids. In most shock loadings, the shear stresses are well in excess of that shear strength and there is certainly ample theory and experiment to qualitatively identify overall features of the defect genera-... 

As loading stresses approach or exceed the shear strength of a solid, inelastic effects are to be expected, and details of the behavior have been readily observed with modern, time-resolving measurement techniques. There are many observations of these behaviors. 

Fig. 2.10. Certain high strength solids with low thermal conductivity show a loss or reduction of shear strength when loaded above the Hugoniot elastic limit. The idealized behavior of such solids upon loading is shown here. The complex, heterogeneous nature of such yield phenomena probably results in processes that are far from thermodynamic equilibrium.

Most comparisons of a failure criterion with failure data will be for the glass-epoxy data shown in Figure 2-36 as a function of off-axis angle 0 for both tension and compression loading [2-21]. The tension data are denoted by solid circles, and the compression data by solid squares. The tension data were obtained by use of dog-bone-shaped specimens, whereas the compression data were obtained by use of specimens with uniform rectangular cross sections. The shear strength for this glass-epoxy is 8 ksi (55 MPa) instead of the 6 ksi (41 MPa) in Table 2-3. 

For this, failure interfacial shear strength (t) is obtained by dividing the maximum load P, by interfacial area A. 

As fibers are crushed in the composite, their strength increases. So long as the fibers interact with each other they can stand a load. When, however, the fiber length becomes so small that the shear stresses between the fiber and the matrix become about as low as the shear strength at the interphase the crushing process will stop and fiber fragments will be pulled out of the matrix. 

A shaft subject to torque is generally considered to have failed when the strength of the material in shear is exceeded. For a torsional load the shear strength used in design should be the published value or one half the tensile strength, whichever is less. The maximum shear stress on a shaft in torsion is given by the following equation ... 

The specimen is mounted in a punch-type shear fixture, and the punch (1 in. diameter) is pushed down at a rate of 0.05 in./min until the moving portion of the sample clears the stationary portion. Shear strength is calculated as the force per area sheared. Shear strength is particularly important in film and sheet products where failures from this type of load may occur (Fig. 2-21). This property can be used for comparison with other materials and for determination of the forces needed for punching openings (holes, etc.). 

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