Photochemical crack healing in cinnamate-based polymers

Four kinds of cinnamate-type monomers were synthesized as healing agents. Photoirradiation of the monomers gave cyclobutane-containing crosslinked polymers via [2+2] cycloaddition. Cyclobutane cleavage upon cracking of the crosslinked polymers and re-cycloaddition of the cracked polymers were investigated by FT-IR spectroscopy. Photochemical crack healing was demonstrated by measurement of flexural strength of crosslinked, cracked, and healed polymers. It was observed that microcracks with width of 200 nm to 2 microm were healed by photoirradiation.     

Fracture mechanics approach to fatigue crack initiation from deep notches

Fracture mechanics approach is applied to fatigue crack initiation at the tips of deep, blunt notches including those with very small notch-tip radius. The theoretical relations between the stress intensity range ΔK_ρ and the notch-tip radius ρ for a fixed life for crack initiation were derived based on the models of dislocation-dipole accumulation and blocked slip-band. Those are approximated by a simpler equation: $\text{ΔK}{}_{\mathrm{\rho }}\text{ΔK}{}_{\mathrm{o}}\text{= (1 + ρ/ρ0)}\text{1}\text{2}$ where ΔK₀ and ρ0 are material constants which are related to the fatigue strength of smooth specimens Δρ0 as $\text{Δρ0 =}\text{2ΔK}{}_0\text{(πρ0)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. The results of experiments done with bluntly notched compact tension specimens of a structural low-carbon steel agree with the above relation between $\text{ΔK}{}_{\mathrm{gr}}\text{ΔK}{}_{\mathrm{o}}$ and ρ/ρ_o. The method to predict ΔK_o, ρ_o and Δρ_o from the fatigue data of cracked and smooth specimens is proposed.     

Fracture mechanics analysis of a crack in a residual stress field

The standard definition of the J integral leads to a path dependent value in the presence of a residual stress field, and this gives rise to numerical difficulties in numerical modelling of fracture problems when residual stresses are significant. In this work, a path independent J definition for a crack in a residual stress field is obtained. A number of crack geometries containing residual stresses have been analysed using the finite element method and the results demonstrate that the modified J shows good path-independence which is maintained under a combination of residual stress and mechanical loading. It is also shown that the modified J is equivalent to the stress intensity factor, K, under small scale yielding conditions and provides the intensity of the near crack tip stresses under elastic-plastic conditions. The paper also discusses two issues linked to the numerical modelling of residual stress crack problems-the introduction of a residual stress field into a finite element model and the introduction of a crack into a residual stress field.     

Fracture mechanics

Book Review

Fracture mechanicsT.L. Anderson, CRC press, inc., Boca Raton, FL     

Fracture mechanics analysis of an anti-plane crack in gradient elastic sandwich composite structures

International Journal of Mechanics and Materials in Design - The strain gradient elasticity theory is applied to the solution of a mode III crack in an elastic layer sandwiched by two elastic...     

Fracture mechanics analysis of thermally tempered glass plate: fracture induced by an embedded crack

This paper presents a fracture mechanics analysis of a thermally tempered glass plate. The fracture is induced by an embedded penny-shaped crack. The analysis shows that the existence of a penny-shaped crack will reduce the strength of tempered glass. The impact and fatigue resistance of the glass is related to the position and size of the penny-shaped crack. When the tempering intensity reached to a certain level, thermally tempered glass with a penny-shaped crack could experience spontaneous fracture. The damage of a central crack on glass is more severe than a surface crack. With surface compression, thermal tempering will increase the critical applied stress of the glass if the surface penny-shaped crack size is in the range of 0 < a/d < 0.27, where a is the crack size, d is the half thickness of the glass plate. For a small surface crack with the size of a/d 0.09, the tempering can hinder its extension. However, if there is a central penny-shaped crack, the critical applied stress of the tempered glass will decrease with the intensity of tempering.*Author for correspondence. (E-mail: lejun@mail.sic.ac.cn)     

Applicability of classical isotropic fracture mechanics specimens to wood crack propagation studies

Different specimen types (double cantilever beam: DCB, compact tension: CT, single edge notched in tension: SENT) have been numerically studied (with special finite elements) for ten wood species and for cracks situated in a material plane of symmetry (the crack is denoted xy with x the direction of the normal to the crack plane and y the direction of propagation). Single-edge notched speciments used in RL, TL directions appear to be insensitive to the orthotropic properties of the material. So, calibration known for isotropic materials can be used in stress intensity factor calculations. In transverse directions (LT, LR), such specimens could be used with a double height. Small differences, but depending on the species tested, have also been obtained in TR and RT directions. Calibration of CT specimens of an isotropic material cannot be used even in TL and RL directions. A calibration acceptable for all the studied species is proposed. A different calibration is also given in the TR direction. It is suggested, however, not to use specimens in the RT direction without a calibration specific for the wood species tested. DCB specimens could be used in RL, TL and TR directions with a calibration which is a function of elastic moduli. This calibration is obtained from analytical calculations. In other directions, these specimens do not offer any experimental interest.     

Fracture mechanics of surface fatigue crack growth by ductile striation space measurement in notched Waspaloy

Ductile striation space (DSS), a parameter to predict actual cracks in both direction of length and depth, is proposed for the surface fatigue crack behaviors on notched Waspaloy. Three different lengths (1, 2 and 4 mm) of artificial notches are formed as the initial surface crack for an applied maximum stress of 1,103 MPa at the stress ratio R of 0.05. These notches are similar with the appearance of the surface cracks found from the survey of compressor disk. The results show that, all initial crack sites in the depth direction started from the multiple origination sites. The DSS parameter was clearly confirmed, and it also proves the high effectiveness of the measurement in the range of the stress intensity factors for acquiring the crack growth rate on the fractured surface. The surface cracks on Waspaloy at room temperature in an atmosphere perfectly follow the relation of ΔK versus da/dN and db/dN, even though there are, respectively, earlier and later timing differences on the initiation of cracks for the notch sizes of 1 and 4 mm. The results of ΔK versus da/dN and db/dN relations show a similar slope for three different kinds of notches.     

Fracture mechanics and notch sensitivity

A diagram valid for the analysis of the fatigue limit of cracks and notches centred in an infinite plate was recently proposed by the authors of the present work with the aim to make explicit the bridging at the fatigue limit between defect sensitivity (correlated to the length parameter a₀, according to El Haddad–Topper–Smith's definition) and notch sensitivity (correlated to a*, where a* is a particular notch depth corresponding to the intersection between the ΔK_th and Δσ₀/K_t curves). The expression being valid, defect sensitivity and notch sensitivity were seen as two sides of the same medal. Such a diagram is now extended to finite size components by simply introducing the shape factor α commonly used in fracture mechanics. The obtained critical defect size is termed a_D, which is a material and geometry dependent parameter, in order to distinguish it from a₀, which is a material parameter. As a consequence the critical notch depth a_N is introduced, such that . This results in the proposal of a ‘universal’ diagram able to summarize experimental data related to different materials, geometry and loading conditions. The diagram, the validity of which is checked by means of several results available in the literature, is applied both to the interpretation of the scale effect and to the surface finishing effect.     

Fracture mechanics of polymers. Critical evaluation for linear elastic behaviour at high speed testing

Over the last few years considerable effort has been made to obtain reliable stress intensity factor and strain energy release rate (K _IC andG _IC) data on polymeric materials. Experience has shown that a valuable method to minimize viscoelastic losses and plastic deformation is to work at high speed. However, some problems remain, for this kind of experimental method, which have to be solved before standard methods can be defined. On the basis of measurements done with 3-point bending geometry on poly(vinylchloride) (PVC) and poly(propylene) (PP) at room temperature and over a large range of notch depths, the present work demonstrates that the linearity of experimental data both in the load (F) and in the energy (U) against (2BW ²/3LYa) and (BW) plot is not a critical test for linear elastic behaviour, so that (K _IC andG _IC) values can be affected by large errors. Only the knowledge of the experimental curves, which can be obtained by means of instrumented pendula in optimized test conditions, allows a critical test to be applied for linear elastic behaviour, based on the comparison between experimental and predicted data for load, displacement and energy. These tests show that the linear elastic fracture mechanics, LEFM, criterion is satisfied for PVC, but not for PP. This conclusion is further supported by the morphologies of the fracture surfaces.     

Fracture mechanics of plastic-fiber composites

The concept of fracture mechanics is introduced to characterize the toughness of fiber-reinforced composites which should be distinguished from tensile strength. A material may have a high tensile strength but a low toughness meaning that it has a low resistance to crack extension. Depending on the analytical model used, the same experimental data may report different fracture toughness values. In general, the combination of crack propagation in directions parallel and perpendicular to the fibers makes the composite problem very difficult to analyze.Composites with well-aligned fibers are more susceptible to crack propagation parallel than perpendicular to the fibers and can be analyzed with a reasonable degree of accuracy by using the existing K_c or S_c-theory in fracture mechanics. The K_c-theory is used primarily in laboratory testing where the load is aligned normal to the crack plane whereas the S_c-theory is not subjected to such a restriction. The K_c or S_c values for several composites are reported and applied to solve various example problems illustrating the advantage of fiber reinforcement.     

Fracture mechanics of ion-nitrided steel

Materials Science - The results show that ion nitriding has a positive effect on the cracking resistance of the examined tool steel. The critical thickness of the layer, above which nitriding...     

Fracture mechanics of ion-nitrided steel

Conclusions The results show that ion nitriding has a positive effect on the cracking resistance of the examined tool steel. The critical thickness of the layer, above which nitriding reduces K_lc, was determined.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 2, pp. 13–16, March–April, 1992.