Presence Of Tensile Stress Research Articles

Theoretical and experimental evaluation of the Iosipescu shear test

Use of the Iosipescu shear test for measurement of shear properties of unidirectional laminae has been studied both analytically and experimentally. The intralaminar shear strength and shear stiffness of glass-reinforced polyester material have been measured using specimens with two different fibre orientations. Acoustic emission has been monitored and a fractographic study carried out. A finite element analysis has been conducted to evaluate the stress distribution within the specimen, assuming isotropic and orthotropic elastic properties of the material. There is a complicated stress distribution in the specimen, particularly in the vicinity of each notch root, depending on the elastic properties of the composite. The shear stress region in the specimen gauge section is almost uniform, though small normal compressive stresses exist. The experimental results have shown that the measured shear modulus does not depend on reinforcement orientation. However, it has been observed that different failure modes occur in each case. This results in a change in apparent shear strength of the composite with fibre orientation. Some explanations of these differences have been found in a detailed analysis of the local stresses at the roots of the notches. It is considered that the presence of tensile stresses in this area is primarily responsible for the apparent reduction in the shear strength.

Evaluating the strength of parts of brittle materials by the acoustic emission method

1. Under conditions of radial compression and bending of porcelain and cement specimens the stress σg at which during unloading continuous acoustic emission changes to discrete correlates with the ultimate strength σult in repeated loading. 2. The coefficients of the regression equation σult = A + B · σg do not change their values in converting from radial compression to bending, which makes it possible to assume them to be constant for a material in the presence of tensile stresses. 3. For porcelain A=0 and B=1.96, and for the cement specimens A=5.6 MPa and B=0.48. The difference in A from zero limits the minimum values of ultimate strength which may be determined with the acoustic emission method using the stress σg. With B<1 the maximum measurable value of the ultimate strength is limited by the condition σult < A/(1 − B). Apparently this limitation is the result of the fact that with an increase in the ultimate strength σult the role of microcracking in the fracture process increases. 4. Regardless of the values of A and B the acoustic emission of microcracking reduces the strength of the correlation relationship between σult and σg and narrows the range of measurable values of σult. These negative effects may be decreased by repeated preliminary loading both in determination of the relationship of σult to σg and in inspection of a part. 5. In compression of the cement specimens the compressive strength correlates with the stress at which occurs the maximum in intensity 004E of acoustic emission preceding the sharp increase in 004E before failure. However, such a correlation relationship is not observed in repeated loading and, consequently, may not be used with an unknown prehistory of the inspected part.

Jointed blocks of peridotite xenoliths in basalts and mantle dynamics

In considering the formation of fracture in the upper mantle of the Earth, several authors1,2 have pointed out that the major problem in the formation of cracks is the fact that tensile stresses in the upper mantle region, particularly in the lithospheric plate, are too small compared with the hydrostatic overburden pressure. From theoretical considerations, Anderson2 demonstrated that cracks can nucleate in the lithospheric plates by the accumulation of low density fluid at the base of the plate in the presence of tensile stresses parallel to the plate surface. However, there has been no direct geological observation that supports this conclusion. This letter reports geological observations of planar features in ultramafic inclusions which may have important implications on the existence of fracture in an ambient upper mantle condition.

STRESS FIELD AND FRACTURING OF SOLID PROPELLANT DURING ROCKET FIRING

T occurrence of crazing, cracking, or fracturing in viscoelastic .materials has been attributed to its strong association with the presence of tensile stress.2 In the problem of studying fracturing of solid propellant during rocket firing, the determination of the stress distribution in the solid propellant must be made as a prerequisite. However, because of the development of the anisotropic properties of the solid propellant as a result of microscopic nonhomogeneity in its constituents, the stress analysis must be based upon anisotropic viscoelastic considerations. Using general indicial notations and summation convention over repeated diagonal indices, the anisotropic constitutive equations in linear infinitesimal viscoelasticity may be deduced from invariant considerations. Under certain restrictions the stress tensor cr(x,l) at time t can be related to the history of the infinitesimal strain tensor aj(x,t — T) where 0 < r < <», i.e., -£ (1)

AN ANATOMICAL STUDY OF THE MECHANICS, PATHOLOGY, AND HEALING OF FRACTURE OF THE FEMORAL NECK

Dissection and roentgenographic analysis of the upper portion of the shaft and neck of the femur revealed the presence of a highly independent laminar, bony system, from ten to twelve centimeters long, which begins from two to four centimeters below the lesser trochanter and ends in fanlike fashion at the cartilaginous plate of the head. This proved to be the internal weight-bearing system of the proximal portion of the femur. This system undlergoes slow resorption in its distal portion after middle age, but never disappears; and on the roentgenogram it can be seen throughout life from its distal to its proximal end. Microscopically, it consists of a great number of laminae and bone cells, and a rich endosteal lining. It resembles the spongy laminae of the iliac bone,—hence the speed with which it is capable of new-bone formation. Mechanically, the internal weight-bearing system is a compression system, and during weighit-bearing it is to be founod almost in the vertical plane. The presence of tensile stresses is doubtful. Preceding adduction fractures, the internal weight-bearing system is resorbed and replaced by cortical bone in its entire distal portion. Its proximal end remains spongy, with laminar character. The separation occurs at the site where the spongy proximal portion begins and the distal cortical portion ends. In abduction fractures, the internal weight-bearing system suffers only trauma, but otherwise it is healthy and strong. This explains the rapid healing of such fractures. On the basis of the anatomy and pathology of the femoral neck, two types of fracture may be distinguished: (1) compression fractures, comprising the abduction, intermedial, and adduction fractures; and (2) subcapital separations, represented by most fractures of the aged. The prognosis of the compression fractures is good, if the fragments are kept in good apposition andl fixation in plaster, in Russell traction, or by nailing. For the treatment of the adduction fractures, a method with a perforated (sieve) sublay graft has been developed.