Effect of Lateral Cracks on Fracture Toughness Determined by the Surface‐Crack‐in‐Flexure Method

Abstract

The surface-crack-in-flexure (SCF) method uses a Knoop indenter to create small, semielliptical surface precracks in beam specimens. Lateral cracks may interfere with the primary median crack and cause errors of up to 10% in determination of fracture toughness, particularly for materials for which the fracture toughness is ϳ3 MPa⅐m 1/2 or less. Although the residual-stress-damage zone is ground or polished away by hand by removing 4.5-5 times the indentation depth, this amount may not be sufficient to completely remove the lateral cracks in low-fracture-toughness materials. A series of tests were conducted on sintered alpha silicon carbide with different amounts of material removed after indentation. Once the lateral cracks were fully removed, the SCF results concurred with single-edged-precracked-beam and chevron-notchedbeam data collected in accordance with ASTM Designation C1421. A simple remedy for the SCF method is to examine the outer ground surface for remnants of lateral cracks before fracture and to remove more material if necessary.

Fracture Toughness of Advanced Structural Ceramics: Applying ASTM C1421

“…(Lateral cracks were present on the tensile surface of these specimens.) In fact, the SC value of 2.81 ± 0.05 MPa√m in Table is in excellent agreement with previously reported values of 2.89 ± 0.03 MPa√m and 2.86 ± 0.03 MPa√m that were determined before the effect of lateral cracks was observed and quantified. The K Isc value with a 12% reduction would be 2.47 MPa√m which is in excellent agreement with the K Ipb value.…”

“…The PB value in Table is in good agreement with the values of 2.54 and 2.58 MPa√m listed in the standard. However, the SC value is higher, probably due to the presence of lateral cracks that have been shown to overestimate apparent toughness values by as much as 12% . (Lateral cracks were present on the tensile surface of these specimens.)…”

Handbook of SiC properties for fuel performance modeling

“…The fracture toughness of SiC has been widely investigated by various test techniques: micro-and nano-indentation [32,71,76,78,87,[90][91][92]94,110,112,113,117,118], surface crack in flexure (SCF) [29,71,72,77,82,105,109,112,[119][120][121][122][123], double cantilever beam (DCB) [117], double torsion (DT) [75,106,117,124], single edge notched beam (SENB) [54,58,88,117,120,121,[125][126][127][128][129][130][131][132], Chevron notched beam (CNB) [121], and fractography [120]. The representative values determined by each test are sum...…”

Charakterisierung der Kratzanfälligkeit von Gläsern im Bauwesen

“…Die Initiierung und Ausbreitung des lateralen Risssystems ist wesentlich von der Ausprägung der Verdichtungszone abhängig (Cook & Roach, 1986). Erreichen die Risse die Glasoberfläche, kommt es typischerweise zu muschelförmigen Abplatzungen, die auch als Chipping bezeichnet werden (Srinivasan & Scattergood, 1987;Chiu et al, 1998;Quinn & Salem, 2002;Le Houérou et al, 2003 (Peter, 1964;Lawn & Wilshaw, 1975;Lawn & Fuller, 1975;Lawn & Swain, 1975;Cook & Pharr, 1990;Lawn, 1993). 3.8a-b).…”