Stresses in bent specimens

This is a well-known original ESC test developed by Bell Labs in the late 1940s. Ten rectangular-shaped specimens are cut from a molded plaque prepared using standard methods. A controlled notch is cut horizontally across each specimen, which serves as a crack initiation point. The specimens are bent and inserted into a C -shaped bracket, creating a stress in the specimen. A diagram of this test method is shown in Fig. 10.73. The specimens and bracket are inserted into a tube filled with Igepal solution. The tube is then placed into a heated environment... 

Bend-beam specimens provide a quantitative estimate of the maximum stress in the outer fibers of the bent beam when stressed below the yield stress. Experimental standards for bent beam specimen are given in ASTM G 39-99 [16]. In this test, both the specimen... 

Fig. 9.4 Bend beams specimen used in constant-deformation test [16]. Reprinted, with permission, from ASTM G 39-99 Standard Practice for Preparation and Use of Bent-Beam Stress-Corrosion Test Specimens copyright ASTM International, West Conshohocken, PA.

A double beam specimen, shown in Fig. 9.4d, consists of two flat strips bent against each other over a centrally located spacer [10,16]. This specimen provides an uniform longitudinal stress in the convex space between the hnes of contact with the spacer. The stress decreases linearly to zero at the ends of the specimen. The elastic stress in the convex between the lines of contact is estimated hy Eq. (9.8). 

Flex life n. Informally, the number of bending-reversal cycles causing a part to fail in a particular service. Most specifically, the number of cycles to failure of a test specimen repeatedly bent in a prescribed manner. The ASTM test for plastics is D 671. The specimen, molded or cut from sheet, is subjected to load reversal at 30 Hz at a predetermined level of outer-fiber stress until it either fails or the test is discontinued. By setting up different stresses for successive specimens, one can develop a graph of stress at failure versus number of cycles to failure (usually plotted on semi-logarithmic coordinates), i.e., the flex-life curve of fatigue curve. 

Considerable attention has been devoted to the interaction between mechanical loading and corrosion phenomena. Corrosion fatigue has been investigated with sinusoidal loading [26] and with force patterns more representative of actual in vivo stresses [27]. Both specimens of simple geometry [28] and actual prosthetic devices [29] have been tested. SCC has been studied using U-bend [30], stress ring [37], bent beam [37], and fracture mechanics [32] type... 

In the constant-strain method, the specimen is stretched or bent to a fixed position at the start of the test. The most common shape of the specimens used for constant-strain testing is the U-beud, hairpin, or horseshoe type. A bolt is placed through holes in the legs of the specimen, and it is loaded by tightening a nut on the bolt. In some cases, the stress may be reduced during the test as a result of creep. In the constant-load test the specimen is supported horizontally at each end... 

Fig. 3.33 Bent-beam test results in aerated distilled water. These specimens were exposed to the environment at a stress of 70% of yield (after Setterlund, Materials Protection, 4 No. 12,...

Flexural strength The strength of a material in bending, expressed as the stress on the outermost fibers of a bent test specimen, at the instant of failure. Dimension N/mm2 (MPa). 

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... 

Brittle failure was exhibited by all materials, which indicated that at temperatures down to 76°K the stresses imposed upon these materials by the bending are sufficient to crack them and cause loss of adhesion. The important consideration here, however, is not whether a sealant passes or fails this test, but how it reacts to the stress. The test is most severe and it is doubtful that any existing sealant, without considerable reinforcement, will show enough strength and flexibility to be bent under these conditions and not break. But the mode of failure does suggest that some polymers are not as brittle as others and have promise as a basis for further modifications. Photographs of representative bend specimens, after test, are shown in Fig. 2. 

ESC resistance. In the Bell Telephone test, a razor cut is made parallel to the length of rectangular 38 x 13 x 3 mm specimen. It is bent into a U shape, with maximum surface tensile strain of about 12% and placed in concentrated surface active agent at 50 °C. The time until cracks appear, at the corner between the cut and the tensile surface, is measured. The cut does not create a stress concentration, because it is parallel to the bending stresses, but the two free surfaces may assist the crack opening process. 

To verify this postulate, two CPNC specimens were prepared, one sheared dynamically between parallel plates (disks) at >= 100 rad/s and y = 40% for 15 min, and another just inserted into the rheometer, melted, but not sheared. To determine the clay orientation, the specimens were microtomed close to the disk border (maximum shear strain) in the planar and perpendicular directions and then observed under the high-resolution transmission electron microscope [Perrin, 2002]. In the first specimen the well-dispersed clay platelets (see Figure 16.8) were found to be oriented perpendicular to the stress direction, while in the second, unsheared specimen, the exfoliated, often bent platelets were randomly oriented. 

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