On the Fracture Toughness of Advanced Materials

2009

S E On the Fracture Toughness of Advanced Materials A R C H By Maximilien E. Launey, and Robert O. Ritchie* N E W S Few engineering materials are limited by their strength; rather they are limited by their resistance to fracture or fracture toughness. It is not by accident that most critical structures, such as bridges, ships, nuclear pressure vessels and so forth, are manufactured from materials that are comparatively low in strength but high in toughness. Indeed, in many classes of materials, strength and toughness are almost mutually exclusive. From a fracture-mechanics perspective, the ability of a microstructure to develop toughening mechanisms acting either ahead or behind the crack tip can result in resistance-curve (R-curve) behavior where the fracture resistance actually increases with crack extension; the implication here is that toughness is often developed primarily during crack growth and not for crack initiation. Biological materials are perfect examples of this; moreo...

WIT transactions on engineering sciences, 1970

The account for non-singular terms of the near-crack-tip stress concentration makes it possible to explain experimentally observed dependence of the fracture toughness upon the crack length. Formulae for the stress intensity factor and the leading non-singular terms for an arbitrarily loaded edge crack are obtained in closed form. The influence of remote boundaries is discussed. Two examples of fitting the theoretical curves to the experimental data on the size effect in fracture toughness are presented.

Engineering Fracture Mechanics, 2012

Stress intensity factor Energy release rate CTOD CTOA J-integral J-R curve K-R curve ASTM standard a b s t r a c t

This research work is an atomic theory of fracture and quantization of Kic Fracture toughness. Especially in ceramics. It shows the atomic level aspects of fracture process from the stress intensity factor or fracture toughness, KIc. The crystalline structure, the atomic positions and lattice points, and how nanomaterials show atomic level fracture process as well as nanoceramics exhibit Quantization of fracture toughness and other nanomaterials show higher stress intensity factor, KIc than microsize equivalents of those nanomaterials. This is a deep treatment of the fracture process with a survey of present status of fracture, the application of the fundamentals of fracture toughness for the atomic theory of fracture, the data evidence for confirmation of the theory and some extension for its applications in biomaterials, electronic materials and cutting tools for manufacturing. This is a rigorous and clear treatment of the atomic theory of fracture.

International Journal of Fracture, 1986

Dynamic fracture toughness versus crack velocity relations of Homalite-100, polycarbonate, hardened 4340 steel and reaction bonded silicon nitride are reviewed and discrepancies with published data and their probable causes are discussed. Data scatter in published data are attributed in part to the observed fluctuations in crack velocities. The results reaffirmed our previous conclusion that the dynamic fracture toughness versus crack velocity relation is specimen dependent and that the dynamic crack arrest stress intensity factor is not a unique material property.

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiation via transgranular cleavage and crack initiation and subsequent growth via microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulus TR and the nondimensionalized crack initiation fracture toughness Ji~ is described and shown to yield a good fit to experimental toughness data for a wide range of steels.

European Journal of Mechanics - A/Solids, 2003

A heterogeneous brittle material characterized by a random field of local toughness K c (x) can be represented by an equivalent homogeneous medium of toughness, K eff . Homogenization refers to a process of estimating K eff from the local field K c (x). An approach based on a perturbative expansion of the stress intensity factor along a rough crack front shows the occurrence of different regimes depending on the correlation length of the local toughness field in the direction of crack propagation. A "weak pinning" regime takes place for long correlation lengths, where the effective toughness is the average of the local toughness. For shorter correlation lengths, a transition to "strong pinning" occurs leading to a much higher effective toughness, and characterized by a propagation regime consisting in jumps between pinning configurations.

Strength of Materials, 1987

Dental Materials, 2010

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e63-e77 a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . i n t l . e l s e v i e r h e a l t h . c o m / j o u r n a l s / d e m a Energy release rate a b s t r a c t Dental adhesives are usually tested in shear or tension even though neither of these approaches measures the local stress triggering failure. Because the stress level varies extensively over the bonded surface, it seems as a fracture mechanics approach would be more appropriate.

Journal of the American Ceramic Society, 2008

The mechanical behavior of four rare earth (RE)-Mg-doped Si 3 N 4 ceramics (RE 5 La, Lu, Y, Yb) with varying grainboundary adhesion has been examined with emphasis on materials containing La and Lu (which represent the extremes of RE ionic radius). Fracture-resistance curves (R-curves) for all ceramics rose very steeply initially, giving them exceptional strength and relative insensitivity to flaw size. The highest strength was seen in the Lu-doped material, which may be explained by its steeper initial R-curve; the highest ''apparent'' toughness (for fracture from millimeter-scale micronotches) was seen in the lowest strength La-doped material, which may be explained by its slowly rising R-curve at longer crack lengths. Excellent agreement was found between the predicted strengths from R-curves and the actual strengths for failures originating from natural flaws, a result attributed to careful estimation of the early part of the R-curve by deducing the intrinsic toughness, K 0 , and the fact that this portion of the R-curve is relatively insensitive to sample geometry. Finally, it was found that RE elements with relatively large ionic radius (e.g., La) tended to result in lower grain-boundary adhesion. This implies that there is a small window of optimal grain-boundary adhesion which can lead to the fastest rising R-curves (for short cracks) and the highest strengths. The importance of this work is that it reinforces the notion that factors which contribute to the early part of the R-curve are critical for the design of ceramic microstructures with both high strength and high toughness.