Loading in tension

Creep tests require careful temperature control. Typically, a specimen is loaded in tension or compression, usually at constant load, inside a furnace which is maintained at a constant temperature, T. The extension is measured as a function of time. Figure 17.4 shows a typical set of results from such a test. Metals, polymers and ceramics all show creep curves of this general shape. 

By calculating the plastic work done in each process, determine whether the bolt passing through the plate in Fig. A1.2 will fail, when loaded in tension, by yielding of the shaft or shearing-off of the head. (Assume no work-hardening.)... 

Natural ice has large grains typically 10 mm or more (it is like a casting which has solidified very slowly). Then this equation gives a tensile fracture strength of 1 MPa -precisely the observed strength. So when ice is loaded in tension, it creeps when the stresses are less than 1 MPa the strength is limited to a maximum of 1 MPa by fast fracture. 

Just obtaining these cylinders does not setde the design. The manufacturer must verify that no cylinder interferences exist and that the rod loading in tension and compression is satisfactory. This design detail is handled by the manufacturer. The final design agreement should be by the manufacturer, as he should be responsible for the final quoted performance of the unit. 

Creep-test specimens may be loaded in tension or flexure (to a lesser degree in compression) in a constant temperature environment. With the load kept constant, deflection or strain is recorded at regular intervals of hours, days, weeks, months, or years. Generally, results are obtained at three or more stress levels. 

Methods employed to determine the impact resistance of plastics include pendulum methods (Izod, Charpy, tensile impact, falling dart, Gardner, Dynatup, etc.) and instrumented techniques. In the case of the Izod test, what is measured is the energy required to break a test specimen transversely struck (the test can be done either with the specimen notched or unnotched). The tensile impact test has a bar loaded in tension and the striking force tends to elongate the bar (Chapter 5, Impact Strength). 

It is useful to get preliminary learning on the mechanical properties of materials under simple static tension. Members of engineering structures are often subjected to steady axial loads in tension. Moreover, the response of materials subjected to other types of loading also can often be explained or predicted on the basis of knowledge of their behaviour under simple tension. In addition, such behaviour is usually quite easy to study experimentally. 

Fig. 4. Distribution of surface strain measured along a specimen of oriented S. B. S. copolymer loaded in tension (From Odell and Keller38))...

The residual stresses have a large influence on the properties of a structural body, e.g., a correct design of the composition gradient can generate compressive stresses at those locations which are loaded in tension during application. Compressive stresses at the surface can also have a beneficial effect on the tribological properties of the component (Novak et al., 2005). 

Tendons and ligaments are made up of units termed fascicles that are bound into functional units by a sheath termed epitendineum (Figure 3.22). Individual fascicles are composed of rows of fibroblasts that alternate with bundles of collagen fibrils parallel to the tendon axis. These structures are normally loaded in tension to maintain joint stability. 

Figure 9.6. Diagram illustrating the pretension present in the superficial zone of articular cartilage. Normal articular cartilage shown at the top is loaded in tension across the surface like a drumhead that is pulled taut over a drum. When a piece of cartilage is cut from the surface, it curls as a result of release of this tension, as shown in the lower diagram. The presence of tension in the superficial zone makes articular cartilage behave like a drumhead, allowing compressive forces applied to the surface at specific points to be distributed across the surface to lower local stresses. The presence of tension on the chondrocytes in the superficial layer may be important to limit inflammation and support reparative processes by stimulating mechanochemical transduction.

Here the solution of Dugdale will be followed since it clearly shows a significant feature of the model. A flat elliptical hole of length 2c is considered in an infinite plate loaded in tension by a stress a remote from and normal to the ellipse. The ends of the ellipse terminate in small plastic zotees whose boundaries are under uniform internal pressure stresses (see Fig. 2.2). For these internal stresses static equilibrium is achievai by imposing equal and compressive opposite stresses ct,.. This... 

The expressions for the strain energy release rate associated with local delaminations growing from the tips of angle ply matrix cracks in orthotropic laminates loaded in tension are presented. Strain energy release rate and the laminate residual stiffness properties are predicted as functions of matrix crack density and delamination length. 

Our discussion of the kinetic theory of fracture in Section 1 has already indicated the manner in which applied stress can bring about a net accumulation of nwlecular breakages in a jxrlymeric solid. Since the stress is continuous throughout a specimen loaded in tension, these breakages are distributed throughout the material and can be detected by ESR in terms of a volume concentration of free radicals. 

The crack-driving force G may be estimated from energy considerations. Consider an arbitrarily shaped body containing a crack, with area A, loaded in tension by a force P applied in a direction perpendicular to the crack plane as illustrated in Fig. 2.6. For simplicity, the body is assumed to be pinned at the opposite end. Under load, the stresses in the body will be elastic, except in a small zone near the crack tip i.e., in the crack-tip plastic zone). If the zone of plastic deformation is small relative to the size of the crack and the dimensions of the body, a linear elastic analysis may be justihed as being a good approximation. The stressed body, then, may be characterized by an elastic strain energy function U that depends on the load P and the crack area A i.e., U = U(P, A)), and the elastic constants of the material. 

Figure 2.6. A body containing a crack of area A loaded in tension.

Figure 11.23 Cross section of ceramic part loaded in tension as shown. The heavy lines denote flaws.

This mechanism is similar to the one occurring during liquid-phase sintering, where the dissolution of crystalline material into the glassy phase occurs at the interfaces loaded in compression and their reprecipitation on interfaces loaded in tension. The rate-limiting step in this case can be either the dissolution kinetics or transport through the boundary phase, whichever is slower. This topic was discussed in some detail in Chap. 10, and will not be repeated here. 

If the specimen in Prob. 11.7 was loaded in tension under a stress of 10 MPa and all cracks but the surface crack shown on the left-hand side were ignored, calculate the lifetime for the part. Assume = 15 and A — 0.34 m/s. State all other assumptions. 

For comparison and reference, stress MS studies on different polymers with different forms of mechanical deformation using different mechanical degradation techniques are summarized in Table II. The major portion of the work has been done on relatively simple polymers, PMMA and PS, by loading in tension. Most of the work was performed by scientists at the A. F. Ioffe Physicotechnical Institute of the Russian Academy of Sciences in Leningrad. 

Most stress corrosion service failures occur from the influence of residual stresses. These arise during component manufacture, and plant assembly, including welding. In laboratory work the stress is usually applied externally since it is then much easier to control and measure. The oldest and simplest test is to measure how long a specimen takes to break — the time-to-failure, tf. Typical specimens might be loaded in tension and surrounded by the corrodent or bent into a U, clamped and then immersed. Typically, the value t varies as shown in Figure 1. Two points can be made. First, the tf changes less markedly at stresses above the 0.1X proof stress than below it. Secondly, it is not always clear whether a threshold stress exists below which stress corrosion failures will not occur. This is of obvious importcuice but it has proved a... 

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