Fracture toughness characterization

Sokolov M A, Nanstad R K and Miller M K (2004), Fracture toughness characterization of a highly embrittled RPV weld, pp. 123-137 in Effects of Radiation on Materials, AS>Tyi STP1447, M L Grossbeck, ed., ASTM International, West Conshohocken, PA. 

Fracture toughness characterization has been discussed throughout this article from different view points. It requires well-defined specimens and procedures, which have already been indicated. For convenience this information is here summarized again. 

New ceramic material specifications may incorporate fracture toughness characterization. One example is the new ASTM Standard F 2094 -01 for silicon nitride ball bearings. Three grades of bearing balls are specified in this standard and fracture... 

Lu ML, Chiou KC, Chang FC. Fracture toughness characterization of a PC/ABS blend under different strain rates by various J-integral methods. Polym Eng Sci 1996 September 36(18) 2289-95. 

Practure toughness is another way to characterize the strength of a material. It measures how well a material resists crack propagation and is expressed as the stress needed to enlarge a crack of a specific size. The room temperature fracture toughness of clear, vitreous sihca is approximately 0.75 - 0.80 MPa-mT2 (87,163). 

Recently siloxane-imide copolymers have received specific attention due to various unique properties displayed by these materials which include fracture toughness, enhanced adhesion, improved dielectric properties, increased solubility, and excellent atomic oxygen resistance 1S3). The first report on the synthesis of poly(siloxane-imides) appeared in 1966, where PMDA (pyromellitic dianhydride) was reacted with an amine-terminated siloxane dimer and subsequently imidized 166>. Two years later, Greber 167) reported the synthesis of a series of poly(siloxane-imide) and poly(siloxane-ester-imide) copolymers using different siloxane backbones. However no physical characterization data were reported. 

In the matrix of PLA/ polycaprilactone (PCL)/OMMT nano-composites, the silicate layers of the organoclay were intercalated and randomly distributed (Zhenyang et at, 2007). The PLA/PCL blend significantly improved the tensile and other mechanical properties by addition of OMMT. Thermal stability of PLA/PCL blends was also explicitly improved when the OMMT content is less than 5%wt. Preparation of PLA/thermoplastic starch/MMT nano-composites have been investigated and the products have been characterized using X-Ray diffraction, transmission electron microscopy and tensile measurements. The results show improvement in the tensile and modulus, and reduction in fracture toughness (Arroyo et ah, 2010). 

Many variables used and phenomena described by fracture mechanics concepts depend on the history of loading (its rate, form and/or duration) and on the (physical and chemical) environment. Especially time-sensitive are the level of stored and dissipated energy, also in the region away from the crack tip (far held), the stress distribution in a cracked visco-elastic body, the development of a sub-critical defect into a stress-concentrating crack and the assessment of the effective size of it, especially in the presence of microyield. The role of time in the execution and analysis of impact and fatigue experiments as well as in dynamic fracture is rather evident. To take care of the specihcities of time-dependent, non-linearly deforming materials and of the evident effects of sample plasticity different criteria for crack instability and/or toughness characterization have been developed and appropriate corrections introduced into Eq. 3, which will be discussed in most contributions of this special Double Volume (Vol. 187 and 188). 

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