Coupons Cracking

Crack densities were calculated for all of the coupons. Saturation (rupture) crack densities were taken from failed coupons. Due to the varying stress field, crack density measurements need to be handled carefully. For the notched coupons, crack densities were calculated from regions 5 mm from the center of the notch as well as far-field ( 5 mm < x < 12.5 mm). A summary of the results are shown in Table I. 

Coupon tests involved a number of metallurgies and were done to evaluate precipitator-plate alloys. Test stainless steel plates failed, not only because of pitting but also because stress-corrosion cracks developed. 

This transitory behavior was observed to arise from all the weathering agents considered in this study except ozone. Instead, test coupons exposed to ozone exhibited an initial decline in the crosslink density of the silicone with the formation of surface cracks, which were difficult to distinguish with the naked eye. With continued exposure to ozone, however, the material would begin to crosslink. We proposed that ozone s greatest affinity... 

The weep point or strain-to-first-crack in a wall for filament-wound pipe constructed using isophthalic plastic is currently found to be not less than 0.009 in./in. This has been repeatedly demonstrated by careful coupon testing and burst testing of pipes with strain gauge instrumentation attached. 

Due to the large thermal expansion mismatch between barium chromate and the sealing glass or FSS (e.g., AISI446) [213], the extensive formation of barium chromate has been shown to result in crack initiation and growth between the sealing glass and alloy coupons. 

Figure 4. Polarized transmitted light photomicrograph at x 70 of a typical single bare (water-sized) E-glass fiber embedded in a coupon of DER 383/DACH epoxy matrix. The specimen was strained in a direction parallel to the fiber axis until the fiber started to fracture in the coupon, showing the region around the break taken directly after the fiber fractured. There is a small matrix crack that extends outward from the fiber into the matrix.

The increase in ILSS for the epoxy-sized fibers over the bare fibers is 12.4%, approximately 50% of the increase observed in the interfacial shear strength as measured by ITS testing. Changes in the failure mode at the fiber-matrix interface may account for the differences. The sized fibers produced large matrix cracks that grew quickly to catastrophic size under load. This would tend to limit the increase in composite shear properties if at every fiber break in the tensile surface of the coupon a matrix crack was created. The presence of these matrix cracks... 

A typical rack employed for installation of specimens in pilot plants is shown in Fig. 6. Both corrosion coupons, 2 X 1 X 0.35 in. thick, and bend specimens intended to determine stress-corrosion cracking susceptibility, are included in the installation for aqueous corrosion testing. Specimens are separated by high density alumina spacers to eliminate electrochemical effects. During exposure, the racks are welded to existing components in the pilot plant equipment. 

The techniques used for monitoring corrosion in a process plant are (i) visual inspection (ii) weight loss coupons (iii) electrical resistance probes (iv) measurement of corrosion potential (v) linear polarization measurement (vi) hydrogen probes (vii) thickness measurement and crack detection (viii) visual inspection (ix) sentinel holes. 

Alloys that are ductile in nature are used to form a U-shape specimen in order to avoid mechanical cracking of the sample. They are made from sheet strips or machined from bars, plates, weldments, or castings. Rectangular sheet coupons can be used to fabricate bend specimens, as shown in Fig. 9.3. The stress conditions in most of the cases are not known, because ranges of stresses are present in a single specimen. These types of... 

RTR pipe designers also use a S-S curve but instead of a yield point, they use the point of first crack (empirical weep point) as shown in Figure 8.11. Either the ASTM hydrostatic or coupon test determines it. The weep point is the point at which the RTR matrix (resin) becomes excessively strained so that minute fractures begin to appear in the structural wall. At this point it is probable that in time even a more... 

At the undulation of warp and fill threads, delaminations develop in the interior, being initiated at intersections of the transverse and longitudinal cracks, where local tensile stresses are provided by the transverse warp threads. As soon as delaminations occur, the stiffness of a test coupon decreases considerably and enters stage III, that is, the last 50% of fatigue life, producing final failure. 

Lap shear and crack-extension coupons were prepared for these systems using their respective selected cure-cycles and subjected to environmental exposure and test. 

K/95% relative humidity for up to 2000 hours using lap shear (Figs. 15 and 16) and crack extension coupons (Fig. 17). The coupons were tested (lap shear) or measured (crack extension) after 1000 and 2000 hours exposure. Both PPQ and LARC-TPI do not show any shear strength degradation when... 

To further establish the durability of PPQ and LARC-TPI adhesive bonds both systems were exposed to various fuels and fluids using lap shear and crack extension coupons. The... 

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