A new push‐pull sample design for microscale mode 1 fracture toughness measurements under uniaxial tension

Abstract

A sample geometry is proposed for performing microscale tensile experiments based on a push‐pull design. It allows measuring mode 1 fracture toughness under uniform far‐field loading. Finite element simulations were performed to determine the geometry factor, which was nearly constant for Young's moduli spanning 2 orders of magnitude. It was further verified that mode 1 stress intensity factor KI is nearly constant over the width of the tension rods and an order of magnitude higher than KII and KIII. Notched samples with different a/w ratios were prepared in (100)‐oriented Si by a combination of reactive ion etching and focused ion beam milling. The mode 1 fracture toughness KI,q was constant with a/w and in average 1.02 ± 0.06 MPa√m in good agreement with existing literature. The geometry was characterized and experimentally validated and may be used for fracture toughness measurements of all material classes. It is especially interesting when a uniaxial, homogeneous stress field is desired, if crack tip plasticity is important, or when positioning of the indenter is difficult.

The brittle-ductile transition of tungsten single crystals at the micro-scale

“…Firstly, KI is even at a bending displacement of up to 1 µm more than one order of magnitude larger than KII and KIII. A similar relation was found for a recently presented geometry for fracture toughness measurements under uniaxial tension [73]. Secondly, the off-plane shearing (mode III) is in the first approximation zero, because of the symmetric loading condition.…”

Small-Scale Mechanical Testing

“…The toughness values obtained from small-scale experiments in various geometries, including clamped beams, cantilevers, pillar splitting, double cantilevers loaded by compression loading, and wedge loading, have been validated against bulk experiments, yielding confidence that small-scale experiments and FIB precracking are reliable (29) (Figure 17). Two studies argued (174,175) that fatigue precracking in compression was necessary to obtain reliable toughness data and that FIB-milled notches overestimated the numbers. A topic that has attracted recent interest is the temperature-dependent ductile-brittle transition in silicon, which is similarly size sensitive ( 14) as for grain size in steel, due to the stress concentrating effects of slip bands in the Cottrell model.…”