Specimen curvature

Figure 7.42 Almen test configuration to measure the specimen curvature, (a) Mounting block and (b) a thin foil before (1) and after grit blasting (2). The convex curvature will be reduced by tensile stresses induced during coating (3) (SAE Handbook, 1977).

Although the Rockwell test is intended to be used on flat parallel-sided specimens, its use can be extended to rounded surfaces by using a curvature correction factor. Compound surfaces such as gear teeth can be tested but the results must be corrected for curvature. 

Figure 12.13 illustrates severe damage suffered by a component of a cooling tower water pump. The jagged, undercut, spongelike metal loss characteristic of cavitation damage is apparent in Fig. 12.14. All damage occurred along the inner curvature of the specimen.

Typically specimens for reflectivity measurements are prepared on flat, smooth, rigid substrates. For example, these substrates can be polished fused silica, quartz, or silicon. It is important, however, that the substrates be thick to avoid distonions of the specimen when mounted in the reflectometer. Any curvature or bowing will increase the divergence of the incident beam and result in a deterioration of the resolution. 

For a thicker laminate than in Figure 6-26, the critical length is longer and the curvatures are smaller. For example, for a [04/904]-,-laminate, the critical L is 71 mm. Moreover, what was a circular cylindrical specimen at 50 mm for a [02/902lx laminate becomes a saddle-shaped specimen [6-38]. 

Figure 6.9 Curvature of a screw dislocation near a free surface a. screw dislocation that has moved about halfway thru the specimen and is emerging from the surface, b. by becoming curved the screw dislocation reduces its length and hence its energy.

Plastic deformation is the permanent change in shape of a specimen due to applied stress. The onset of plastic deformation is seen as curvature in the stress—strain curve. Plastic deformation is important because it allows pharmaceutical excipients and drugs to establish large true areas of contact during compaction that can remain on decompression. In this way, good, intact, tablets can be prepared. 

If the specimen crystal is curved, there will be a range of positions where the diffraction conditions are satisfied even for a plane wave. The rocking curve is broadened. It is simple to reduce the effect of curvature by reducing the collimator aperture. For semiconductor crystals it is good practice never to mn rocking curves with a collimator size above 1 mm, and 0.5 mm is preferable. Curved specimens are common if a mismatched epilayer forms coherently on a substrate, then the substrate will bow to reduce the elastic strain. The effect is geometric and independent of the diffraction geometiy. Table 2.1 illustrates this effect. 

The curvature itself can be quite easily measured by translating the specimen a distance x in its plane along a diameter (a circular wafer is assumed), repeating the measurement and noting the shift in the absolute position of the Bragg peak. Then, again,... 

However, if the dispersion is small, as in the case, for example, of GaAs and InP crystals mixed in the 004 reflections as reference and specimen, the rocking ciuve simply broadens. Then to a reasonable approximation, one can simulate this by adding a curvature equal to the angrrlar separation of the K 1 and K 2 Bragg angles. 

Figure 5.2 A (normalised) comparison between experimental and simulated rocking curves. InP specimen 004 reflection, CuK radiation, (a) Comparison without incorporation of curvature, (b) comparison with incorporation of curvature of 10 arc seconds, x-axis scale in arc seconds...

Fig. 3,34. Edge delamination test (EDT) (a) specimen configuration (b) typical stress-strain curve (c) outward curvature along specimen edges. After Whitney and Knight (1985).

To account for small thickness variations between different specimens, the dimensionless curvature rj = 2h kc is used. Note that rj depends directly on material properties through Yc and E and on the nonmechanical loading through (el — e2). 

The dimensionless curvature is used to preclude the influence of small thickness variations between specimens. 

Based on the results of the characterization studies, the viscoelastic residual moments of the cross-ply specimens were predicted by the viscoelastic analysis using Equation 8.39 together with the curvature-moment relations in Equation 8.50 for the intermittent cure specimens. 

The MRC cycle calls for a 182°C cure temperature. The effect of cure temperature on residual stress was investigated by curing specimens at four other cure temperatures (171, 165, 160, and 149°C) while holding the dwell time (4 hours) constant. In Figure 8.18 the dimensionless curvature for these specimens is plotted versus the cure temperature. The curvature is reduced as the cure temperature is decreased with significant reduction in curvature obtained for dwell temperatures of 165°C or less. The final curvature as predicted by the viscoelastic process model is overlaid with the experimental data in Figure 8.18 and is shown to capture the trend. 

To ascertain the effect of increased transverse modulus after postcure, nine [90]8 254 mm x 152.4 mm postcured specimens were mechanically tested to obtain the transverse modulus after postcure. Once this data was obtained the increase in curvature due... 

Figure 8.19 Dimensionless curvature (a) and weight loss (b) after postcure of partially cured specimens...

Mechanical testing of the three-step cure specimens indicated that no sacrifice in properties resulted from the modification of the process cycle. The retainment of mechanical properties (transverse strength and modulus) coupled with the reduction in dimensionless curvature for the three-step cure cycles investigated provides another suitable cure cycle modification for reduction of residual stresses in composite materials. Overall processing time has not been increased beyond that specified in the MRC cycle. Thus, with no increase in process time and comparable mechanical properties, the residual stresses have been reduced by more than 20 percent in comparison to the MRC cycle baseline data. 

The operation of a transmission scanning acoustic microscope requires the lenses to be set up so that they are accurately confocal. This requires holders that can be moved relative to each other along three axes, with rather fine adjustment, and that are rigid to better than a wavelength even when a specimen is vibrating between them. The separation must first be set. If ro is the radius of curvature of each lens, ciq the aperture radius, and n the refractive index, then the focal planes of the two lenses will coincide when the separation between their front surfaces is... 

Relaxation times for water filling the pores of an NaX specimen have been fitted to a model with the following assumptions (a) coupling, as above, of molecular diffusion and rotation (b) the median jump time r is governed by a free volume law (allows the curvature in the plots of jump rate, (3r) x vs. 10S/T in Figure 5), and (c) a broad distribution of correlation times (allows a better fit to the data, accounts for an apparent two-phase behavior in T2 (31, 39), and is reasonable in terms of the previous discussion of Pi(f) and r). 

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