Specimen grip

Auger specimens 10 x 2 x 1 mm were heat-treated and then notched to control the location of fracture. Two of these notched samples at a time were loaded into specimen grips such that they could be broken independently under UHV and then examined by a SAM instrument. Scanning electron micrographs allowed different grain boundaries and transgranular cleavage areas to be selected for analysis. 

When a specimen is gripped, the stress must be transferred from the grip to the specimen. This transfer takes place across the specimen-grip interface by a shear mechanism. Furthermore the stress must diffuse across the entire specimen cross section, before we reach the situations described by the theoretical solutions. We have already discussed the theoretical background to this problem in the previous chapter here we shall be concerned with the practical problems it imposes on real measurements. For isotropic materials it is usual to adopt the convenient rule of thumb that specimens should have an aspect ratio of at least 10. We feel it is useful to illustrate the effect of aspect ratio on modulus for isotropic materials. In Fig. 1 we present the modulus , measured as the... 

Fig. 2.47 View of DMA specimen holder, red arrow indicates specimen and blue arrow indicates environmental chamber that closes over the specimen grips during testing...

An experimental uniaxial stress-strain relationship, determined in tension, would provide the necessary design information for ceramics and hard materials Young s modulus, Poisson s ratio and the tensile fracture strength. However, tensUe data for brittle materials are often unreliable, due to parasitic bending stresses associated with dimensional inaccuracy in machining tensile dog-bone specimens and misalignment of the specimen grips. 

The specimen is carefully placed in the specimen attachment device (specimen gripping system) and is tested until fracture, after a short period for stabilisation of the test temperature ( 0.5°C). 

The specimen grip system was 100 mesh stainless steel screen wrapped around 2.5 cm of each end before clamping between two stainless steel plates (0.6 cm thick) cross-serrated with 10 lines/cm. After assembly in an alignment jig, three pairs of 10-32 screws, torqued to 2.3 to 4.5 N-m (20 to 40 in-lbs), were used to clamp the grip onto the specimen. Holes in the steel plates allowed a load pin attachment to clevises on each pull-rod. The test frame design made it possible to immerse the entire system in a dewar of liquid helium. 

Spacer Bar — The horizontal spacer bar, which extends from the fece plate to the end of the tensiometer, supports the loading rod, the specimen grips, and the link plate between the quasi-fixed grip and the load cell. 

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