Accurate measurement of strength and fracture toughness for miniature-size thick diamond-film samples by three-point bending at constant loading rate

Fracture strength and fracture toughness (K-CI) of miniature-size thick freestanding diamond film samples were accurately measured by three-point bending at a constant loading rate. A specially designed testing rig, capable of achieving a constant loading rate as small as < 0.5 N s(-1) was used, which is beneficial for accurate measurement of the fracture strength and toughness for brittle materials like diamond films. A novel method was employed to nucleate and accurately control the length of the pre-crack. A specially designed loading fixture was used to ensure that the diamond film sample was tested in a near-perfect vertical position, as required by the ASTM E399 standards. Effects of sample dimensions on the measurement of fracture toughness and fracture strength are discussed; it was shown that miniature-size freestanding diamond film samples can be used for the accurate determination of fracture strength and fracture toughness. However, for accurate measurement of fracture toughness, the use of pre-cracked diamond film samples is absolutely necessary. The effect of loading configuration on the measurement of fracture strength is discussed in detail. In addition to the frequently reported two loading configurations (with the nucleation and growth surface in tension, respectively), a third loading configuration with the edge surface in tension is also discussed. The Weibel modulus was also found to be strongly dependent on the loading configuration. The significance of these findings is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.