Optimum specimen thickness

There is an optimum specimen thickness for the transmission method, because the diffracted beams will be very weak or entirely absent if the specimen is either too thin (insufficient volume of diffracting material) or too thick (excessive absorption). As will be shown in Sec. 9-8, the specimen thickness which produces the maximum diffracted intensity is given by l/ i, where /t is the linear absorption coefficient of the specimen. Inspection of Eq. (1-10) shows that this condition can also be stated as follows a transmission specimen is of optimum thickness when the intensity of the beam transmitted through the specimen is 1 /e, or about j, of the intensity of the incident beam. Normally this optimum thickness is of the order of a few thousandths of an inch (0.1 mm). There is one way, however, in which a partial transmission pattern can be obtained from a thick specimen and that is by diffraction from an edge (Fig. 6-13). Only the upper half of the pattern is recorded on the film, but that is all that is necessary in many applications. The same technique has also been used in some Debye-Scherrer cameras. 

The conditions for optimum specimen thickness in transmission and infinite thickness in reflection are such that the same specimen can serve for both methods. The penalty for exceeding the optimum thickness is not severe a thickness of double the optimum value for transmission at a = 0 reduces the diffracted intensity by only 26 percent (Problem 9-8). 

The width of the lap shear specimen is generally 1 in. The recommended length of overlap, for metal substrates of 0.064-in thickness, is 0.5 + 0.05 in however, it is recommended that the overlap length be chosen so that the yield point of the substrate is not exceeded. In lap shear specimens, an optimum adhesive thickness exists. For maximum bond strengths, the optimum thickness varies with adhesives of different moduli (from about 2 mils for high-modulus adhesives to about 6 mils for low-modulus adhesives).5... 

Based on the intensity ratio of white lines, a new experimental approach has been demonstrated for mapping the valence states of Co and Mn in oxides using EFTEM. Resulting L3/L2 images are almost independent of either the specimen thickness (provided f/A < 0.8) or the diffraction effects, and are reliable for mapping the distribution of cation valences. An optimum spatial resolution of... 

The primary dimensional requirement on a polymer sample is that it be sufficiently thin. (It is possible to obtain reflection spectra of polymers [Robinson and Price (187, 188)], in which case thin specimens are not required, but the use of this technique has thus far not proven to be as fruitful as transmission spectra, and we will not consider it here.) In the NaCl prism region (roughly 650 to 3500 cm-1) specimens as thin as 0.002 mm may be required in order to avoid essentially 100% absorption at some band peaks. The average thickness required in this region for most bands is usually about 0.02 mm. Thicknesses about ten times larger are optimum for frequencies above 3500 cm 1 (the overtone and combination region) and below 650 cm-1 (the far infrared region). Samples areas down to 1 by 3 mm are usable [Wood (247)], and even smaller if a microspectrometer is employed [Blout (76)]. 

Consider the diffraction geometry for a = 0 in the transmission method for determining preferred orientation and for a = 90° in the reflection method. Let rjnf be the infinite thickness required in the reflection method, and assume ti is that thickness which would diffract 99 percent of the intensity diffracted by a specimen of truly infinite thickness. Let opt be the optimum thickness for the transmission method. 

Figure 4.7. Schematic illustration of typical load-displacement records from finite-thick specimens used in plane strain fracture toughness measnrement by using the pop-in concept (a) a very thick specimen, (b) a very thin specimen, and (c) a specimen with optimum thickness (Boyle, Sullivan, and Krafft [3]).

However, not all preformed layers were equal in performance. The optimum thickness of the preformed layers was between 1 and 2.3 pm. Figure 11.27 shows the progress of oxide formation in the autoclave of preformed oxide layers of various thicknesses in addition to a control specimen that lacks a preformed layer [65]. 

The optimum size of a biological sample for X-ray diffraction is about 0.5-1.0 mm thick. A smaller sample will give rise to weaker scattering while a larger sample will absorb too much of the scattered radiation. Samples can be kept hydrated by placing them in thin-walled glass or quartz capillaries or in a thin specimen chamber with plastic or berylium windows. 

Each resin fiber combination has one or more optimum cure techniques, depending on the proposed environment. The prepreg manufacturer supplies a time-temperature cycle that may have been used to manufacture the test specimens employed in generating preliminary mechanical and physical properties. Frequently, it has been necessary to modify the preliminary cure cycle because of part configuration (thickness) or because of production economics. There are several developments that help to inject science into the otherwise hit-or-miss, time-consuming procedure of cure cycle optimization. They are... 

Sample preparation. All ingredients were mixed in an open two-roll laboratory mill at room temperature. The rotors operated at a speed ratio of 1 1.4. Rubber compounds were vulcanized in an electrically heated hydraulic press. The compounds were cured into 0.25 mm thick films at 150 °C for 15 min according to the optimum cure time ( 90) derived from the curing curves previously determined by means of a Rubber Process Analyzer (RPA2000 Alpha Technologies). Rectangularshaped specimens were mechanically cut out from the film samples. 

Testing with the Hopkinson bar has been applied also to nonconventional cases, to assess the material response in special conditions. For instance, Martinez et al. (1998) have studied the behavior of a confined adhesive, i.e., when it cannot freely expand laterally under compression. The interest for such a case is related to the use of an adhesive to join, in an armored panel, the front ceramic layer to the metal backing plate. To this aim, the adhesive specimen in the Hopkinson bar tests has been surrounded by a confinement tube. The study, carried out on several types of adhesives, has evidenced an optimum value of the adhesive thickness in the armor, in order to ensure enough strength to contain the ceramic fragments and the minimum... 

Maximmn shear strength is achieved at an optimnm hne energy value. For this study, the optimum line energy for 1 mm thick specimens was fonnd to be approximately 0.3 J/mm for the laser and beam profile used in this stndy. 

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