Fracture toughness

About: Fracture toughness is a research topic. Over the lifetime, 39642 publications have been published within this topic receiving 854338 citations.

A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct Crack Measurements

Abstract: The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this flrst part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elasticlplastic indentation frac- ture mechanics. It is thereby asserted that the key to the radial crack response lies In the residual component of the contact fkld. This residual term has important implications concerning the crack evolution, including the possibility of postindentation slow growth under environment-sensitive conditions. Frac- tographic observations of cracks in selected "reference" mater- Ys are used to determine the magnitude of this effect and to investigate other potential complications associated with de- partures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely hpie in procedure and economic in its use of material.

Mechanical behavior of selective laser melted 316L stainless steel

Abstract: The tensile, fracture, and fatigue crack growth properties of 316L stainless steel (SS) produced using the selective laser melting (SLM) technique were evaluated and compared with those of conventionally manufactured (CM) austenitic SSs. For SLM, both single melt (SM) and checker board (CB) laser scanning strategies were employed, so as to examine the effect of scanning strategy on the mechanical properties. The experimental results show that the SLM alloys' yield strength is significantly higher than that of CM 316L SS, a result of the substantial refinement in the microstructure. In contrast, only a marginal improvement in the ultimate tensile strength and a marked reduction ductility, which are a result of the loss of work hardening ability, are attributed to the absence of stress induced martensitic transformation common in CM austenitic SSs. In spite of these, the fracture toughness, which ranges between 63 and 87 MPa m 0.5 , of the SLM alloys is good, which is a result of the mesostructure induced crack tortuousity. The SLM process was found to marginally reduce the threshold stress intensity factor range for fatigue crack growth initiation and enhance the Paris exponent within the steady state crack growth regime. Both tensile and toughness properties were found to be anisotropic in nature. SLM with CB scanning strategy improves both these properties. All these observations on the mechanical properties are rationalized by recourse to micro- and meso-structures seen these alloys.

Residual stress effects in sharp contact cracking

Abstract: A study is made of residual stress effects in the mechanics of median fracture in sharp indenter contact. Starting with a simplistic treatment of the elastic-plastic indentation field, the problem is conveniently resolved into two separable parts, involving reversible (elastic) and irreversible (residual) components. The assumption of geometrical similarity in the residual field about the deformation zone, later backed up by stress birefringence measurements, leads to a stress intensity factor for median crack propagation containing the elastic and residual parts as the sum of two terms. The resulting formulation for equilibrium fracture shows some differences in the crack response during the loading and unloading half-cycles. By imposing certain stress states on the specimen surface during indentation the residual component of the field may actually cause the median crack to continue in downward extension as the indenter is withdrawn, a response which is especially amenable to experimental investigation. Direct observations of median crack evolution in soda-lime glass confirm this and other essential predictions of the fracture mechanics theory. The contribution of the residual component to the crack growth is found to be by no means secondary in importance to that of the elastic component.

Engineered interfaces in fiber reinforced composites

Abstract: Characterisation of Interface Properties . Introduction. Theories of adhesion and types of bonding. Physio-chemical characterization of interfaces. Measurements of Interface/Interlaminar Properties . Introduction. The mechanical properties of fiber-matrix interfaces. Interlaminar/intralaminar properties. Interlaminar fracture toughness. Micromechanics of Stress Transfer Across the Interface . Introduction. Fiber fragmentation test. Fiber pull-out. Fiber push-out. Cyclic loading in fiber pull-out and fiber push-out. Surface Treatments of Fibers and Effects on Composite Properties . Introduction. Glass fibers and silane coupling agents. Carbon fibers. Polymeric fibers. Inorganic fibers. Interface Mechanics and Fracture Toughness Theories . Interface-related fracture toughness theories. Toughness theories for short and randomly oriented fiber composites. Fracture toughness maps. Crack-interface interaction. Improvement of Transverse Fracture Toughness with Interface Control . Introduction. Fiber coating and intermittent bonding concept-experimental studies. Improvement of Interlaminar Fracture Toughness with Interface Control . Introduction. Effects of matrix materials on interlaminar fracture resistance. Delamination resisters. Three-dimensional textile composites concept. References. Appendices. List of Symbols an Abbreviations.

Failure mechanisms of polymer interfaces reinforced with block copolymers

Abstract: The fracture toughness (characterized by the critical energy release rate G c ) of interfaces between polystyren (PS) and poly(2-vinylpyridine) (PVP) reinforced with poly(styrene-b-2-vinylpyridine) was measured with a double cantilever beam test geometry. The effect of the PVP block degree of polymerization and the areal density of block copolymer chains at the interface on the measured G c and on the fracture mechanisms was investigated quantitatively.