Effects of dynamic strain ageing on fatigue crack propagation and threshold behaviour in a Ni-base superalloy

In the temperature range 200-400 degree C the Ni-base superalloy, N901, develops marked dynamic strain ageing effects in its tensile behavior. These include inverse strain rate sensitivity, especially in UTS values, strongly serrated stress-strain curves and a heavily sheared failure mode at the higher test-temperatures. As for steels these effects seem to be due to interactions between the dislocations and the interstitial carbon atoms present. The results of tensile and fatigue threshold tests carried out between 20 degree C and 420 degree C are reported and the fatigue behavior is discussed in terms of the effects of surface roughness induced closure, temperature and strain aging interactions.

Effect of strain rate on the high-temperature low-cycle fatigue properties of a nimonic PE-16 superalloy

Strain-rate effects on the low-cycle fatigue (LCF) behavior of a NIMONIC PE-16 superalloy have been evaluated in the temperature range of 523 to 923 K. Total-strain-controlled fatigue tests were per-formed at a strain amplitude of +/-0.6 pct on samples possessing two different prior microstructures: microstructure A, in the solution-annealed condition (free of gamma' and carbides); and microstructure B, in a double-aged condition with gamma' of 18-nm diameter and M23C6 carbides. The cyclic stress response behavior of the alloy was found to depend on the prior microstructure, testing temperature, and strain rate. A softening regime was found to be associated with shearing of ordered gamma' that were either formed during testing or present in the prior microstructure. Various manifestations of dynamic strain aging (DSA) included negative strain rate-stress response, serrations on the stress-strain hysteresis loops, and increased work-hardening rate. The calculated activation energy matched well with that for self-diffusion of Al and Ti in the matrix. Fatigue life increased with an increase in strain rate from 3 x 10(-5) to 3 x 10(-3) s-1, but decreased with further increases in strain rate. At 723 and 823 K and low strain rates, DSA influenced the deformation and fracture behavior of the alloy. Dynamic strain aging increased the strain localization in planar slip bands, and impingement of these bands caused internal grain-boundary cracks and reduced fatigue life. However, at 923 K and low strain rates, fatigue crack initiation and propagation were accelerated by high-temperature oxidation, and the reduced fatigue life was attributed to oxidation-fatigue interaction. Fatigue life was maximum at the intermediate strain rates, where strain localization was lower. Strain localization as a function of strain rate and temperature was quantified by optical and scanning electron microscopy and correlated with fatigue life.

Flow Instabilities and fracture in Ti-6Al-4V deformed in compression at 298 K to 673 K

Uniaxial compression tests were conducted on Ti-6Al-4V specimens in the strain-rate range df 0.001 to 1 s(-1) and temperature range of 298 to 673 K. The stress-strain curves exhibited a peak flow stress followed by flow softening. Up to 523 K, the specimens cracked catastrophically after the flow softening started. Adiabatic shear banding was observed in this regime. The fracture surface exhibited both mode I and II fracture features. The state of stress existing in a compression test specimen when bulging occurs is responsible for this fracture. The instabilities observed in the present tests are classified as ''geometric'' in nature and are state-of-stress dependant, unlike the ''intrinsic'' instabilities, which are dependant on the dynamic constitutive behavior of the material.

Critical stress intensity factors for cracked hollow pipes under transient thermal loads

Finite element analyses of a long hollow cylinder having an axisymmetric circumferential internal edge crack, subjected to convective cooling on the inner surface are performed. The transient thermal stress intensity factor is estimated using a domain version of the J-integral method. The effect of the thickness of the cylinder, crack length, and heat transfer coefficient on the stress intensity factor history are studied. The variations of critical normalized stress intensity factor with crack length-to-thickness ratio for different parameters are presented. The results show that if a small inner surface crack begins to grow, its stress intensity factor will increase with increase in crack length, reach a maximum, and then begin to drop. Based on the results, a fracture-based design methodology for cracked hollow pipes under transient thermal loads is discussed.

Fracture analysis of concrete using singular fractal functions with lattice beam network and confirmation with acoustic emission study

In the present study singular fractal functions (SFF) were used to generate stress-strain plots for quasibrittle material like concrete and cement mortar and subsequently stress-strain plot of cement mortar obtained using SFF was used for modeling fracture process in concrete. The fracture surface of concrete is rough and irregular. The fracture surface of concrete is affected by the concrete's microstructure that is influenced by water cement ratio, grade of cement and type of aggregate 11-41. Also the macrostructural properties such as the size and shape of the specimen, the initial notch length and the rate of loading contribute to the shape of the fracture surface of concrete. It is known that concrete is a heterogeneous and quasi-brittle material containing micro-defects and its mechanical properties strongly relate to the presence of micro-pores and micro-cracks in concrete 11-41. The damage in concrete is believed to be mainly due to initiation and development of micro-defects with irregularity and fractal characteristics. However, repeated observations at various magnifications also reveal a variety of additional structures that fall between the `micro' and the `macro' and have not yet been described satisfactorily in a systematic manner [1-11,15-17]. The concept of singular fractal functions by Mosolov was used to generate stress-strain plot of cement concrete, cement mortar and subsequently the stress-strain plot of cement mortar was used in two-dimensional lattice model [28]. A two-dimensional lattice model was used to study concrete fracture by considering softening of matrix (cement mortar). The results obtained from simulations with lattice model show softening behavior of concrete and fairly agrees with the experimental results. The number of fractured elements are compared with the acoustic emission (AE) hits. The trend in the cumulative fractured beam elements in the lattice fracture simulation reasonably reflected the trend in the recorded AE measurements. In other words, the pattern in which AE hits were distributed around the notch has the same trend as that of the fractured elements around the notch which is in support of lattice model. (C) 2011 Elsevier Ltd. All rights reserved.

Analysis of strip electric saturation model of crack problem in piezoelectric materials

This paper presents a fully anisotropic analysis of strip electric saturation model proposed by Gao et al. (1997) (Gao, H.J., Zhang, T.Y., Tong, P., 1997. Local and global energy release rates for an electrically yielded crack in a piezoelectric ceramic. J. Mech. Phys. Solids, 45, 491-510) for piezoelectric materials. The relationship between the size of the strip saturation zone ahead of a crack tip and the applied electric displacement field is established. It is revealed that the critical fracture stresses for a crack perpendicular to the poling axis is linearly decreased with the increase of the positive applied electric field and increases linearly with the increase of the negative applied electric field. For a crack parallel to the poring axis, the failure stress is not effected by the parallel applied electric field. In order to analyse the existed experimental results, the stress fields ahead of the tip of an elliptic notch in an infinite piezoelectric solid are calculated. The critical maximum stress criterion is adopted for determining the fracture stresses under different remote electric displacement fields. The present analysis indicates that the crack initiation and propagation from the tip of a sharp elliptic notch could be aided or impeded by an electric displacement field depending on the field direction. The fracture stress predicted by the present analysis is consistent with the experimental data given by Park and Sun (1995) (Park, S., Sun, C.T., 1995. Fracture criteria for piezoelectric materials. J. Am. Ceram. Soc 78, 1475-1480).

Torsional impact response of a penny-shaped interface crack in bonded materials with a graded material interlayer

In this paper, the dynamic response of a penny-shaped interface crack in bonded dissimilar homogeneous half-spaces is studied. It is assumed that the two materials are bonded together with such a inhomogeneous interlayer that makes the elastic modulus in the direction perpendicular to the crack surface is continuous throughout the space. The crack surfaces art assumed to be subjected to torsional impact loading. Laplace and Hankel integral transforms are applied combining with a dislocation density,function to reduce the mixed boundary value problem into a singular integral equation with a generalized Cauchy kernel in Laplace domain. By solving the singular integral equation numerically, and using a numerical Laplace inversion technique, the dynamic stress intensity factors art obtained. The influences of material properties and interlayer thickness on the dynamic stress intensity factor are investigated.

Relationship between strain localization and catastrophic rupture

In order to explore a prior warning to catastrophic rupture of heterogeneous media, like rocks, the present study investigates the relationship between surface strain localization and catastrophic rupture. Instrumented observations on the evolution of surface strain field and the catastrophic rupture of a rock under uniaxial compression were carried out. It is found that the evolution of surface strain field displays two phases: at the early stage, the strain field keeps nearly uniform with weak fluctuations increasing slowly; but at the stage prior to catastrophic rupture, a certain accelerating localization develops and a localized zone emerges. Based on the measurements, an analysis was performed with local mean-field approximation. More importantly, it is found that the scale of localized zone is closely related to the catastrophic rupture strain and the rupture strain can be calculated in accord with the local-mean-field model satisfactorily. This provides a possible clue to the forecast of catastrophic rupture. (c) 2007 Elsevier Ltd. All rights reserved.

Asymptotic solution of mode-iii crack in damaged softening materials

In this paper, a constitutive model of elasticity coupled with damage suggested by Lemaitre et al, [1] is used. The macroscopic stress-strain response of the model includes two stages: strain hardening and strain softening. The basic equation is derived for the anti-plane shear problem. Several lowest order asymptotic solutions are obtained, and assembled for the crack-tip fields.

Fatigue crack-growth rate in ferrite martensite dual-phase steel

<p style="border: 0px; font-size: 13px; margin: 0px 0px 9px; padding: 0px; vertical-align: baseline; word-spacing: -0.15ex; text-align: justify; color: #2e2e2e; font-family: 'Arial Unicode MS', 'Arial Unicode', Arial, 'URW Gothic L', Helvetica, Tahoma, sans-serif; line-height: 20px">An empirical study is made on the fatigue crack growth rate in ferrite-martensite dual-phase (FMDP) steel. Particular attention is given to the effect of ferrite content in the range of 24.2% to 41.5% where good fatigue resistance was found at 33.8%. Variations in ferrite content did not affect the crack growth rate&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><a style="color: #0156aa; border: 0px; margin: 0px; padding: 0px; vertical-align: baseline" title="View the MathML source" class="mathImg"><img style="border: 0px; margin: 0px; padding: 0px; vertical-align: bottom; display: inline; max-width: 600px" class="imgLazyJSB" src="http://ars.els-cdn.com/content/image/1-s2.0-016784429190028I-si1.gif" border="0" alt="View the MathML source" title="View the MathML source" width="53" height="18" /></a></span>when plotted against the effective stress intensity factor range&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><a style="color: #0156aa; border: 0px; margin: 0px; padding: 0px; vertical-align: baseline" title="View the MathML source" class="mathImg"><img style="border: 0px; margin: 0px; padding: 0px; vertical-align: bottom; display: inline; max-width: 600px" class="imgLazyJSB" src="http://ars.els-cdn.com/content/image/1-s2.0-016784429190028I-si2.gif" border="0" alt="View the MathML source" title="View the MathML source" width="39" height="14" /></a></span>&nbsp;which was assumed to follow a linear relation with the crack tip stress intensity factor range&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><span style="border: 0px; font-size: 14px; margin: 0px; padding: 0px; vertical-align: baseline; font-family: STIXGeneral, STIXGeneral-Bold, STIXGeneral-BoldItalic, STIXGeneral-Italic, STIXIntegralsDisplay, STIXIntegralsDisplay-Bold, STIXIntegralsSmall, STIXIntegralsSmall-Bold, STIXIntegralsUp, STIXIntegralsUp-Bold, STIXIntegralsUpDisplay, STIXIntegralsUpDisplay-Bold, STIXIntegralsUpSmall, STIXIntegralsUpSmall-Bold, STIXNonUnicode, STIXNonUnicode-Bold, STIXNonUnicode-BoldItalic, STIXNonUnicode-Italic, STIXSize1Symbols, STIXSize1Symbols-Bold, STIXSize2Symbols, STIXSize2Symbols-Bold, STIXSize3Symbols, STIXSize3Symbols-Bold, STIXSize4Symbols, STIXSize4Symbols-Bold, STIXSize5Symbols, STIXVariants, STIXVariants-Bold; cursor: pointer; letter-spacing: 0.15em" class="formulatext stixSupport mathImg">&Delta;K</span></span>. A high&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><a style="color: #0156aa; border: 0px; margin: 0px; padding: 0px; vertical-align: baseline" title="View the MathML source" class="mathImg"><img style="border: 0px; margin: 0px; padding: 0px; vertical-align: bottom; display: inline; max-width: 600px" class="imgLazyJSB" src="http://ars.els-cdn.com/content/image/1-s2.0-016784429190028I-si4.gif" border="0" alt="View the MathML source" title="View the MathML source" width="39" height="14" /></a></span>&nbsp;corresponds to uniformly distributed small size ferrite and martensite. No other appreciable correlation could be ralated to the microstructure morphology of the FMDP steel. The closure stress intensity factor&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><a style="color: #0156aa; border: 0px; margin: 0px; padding: 0px; vertical-align: baseline" title="View the MathML source" class="mathImg"><img style="border: 0px; margin: 0px; padding: 0px; vertical-align: bottom; display: inline; max-width: 600px" class="imgLazyJSB" src="http://ars.els-cdn.com/content/image/1-s2.0-016784429190028I-si5.gif" border="0" alt="View the MathML source" title="View the MathML source" width="21" height="13" /></a></span>, however, is affected by the ferrite content with&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><a style="color: #0156aa; border: 0px; margin: 0px; padding: 0px; vertical-align: baseline" title="View the MathML source" class="mathImg"><img style="border: 0px; margin: 0px; padding: 0px; vertical-align: bottom; display: inline; max-width: 600px" class="imgLazyJSB" src="http://ars.els-cdn.com/content/image/1-s2.0-016784429190028I-si6.gif" border="0" alt="View the MathML source" title="View the MathML source" width="64" height="17" /></a></span>&nbsp;reaching a maximum value of 0.7. In general, crack growth followed the interphase between the martensite and ferrite.</p><p style="border: 0px; font-size: 13px; margin: 0px 0px 9px; padding: 0px; vertical-align: baseline; word-spacing: -0.15ex; text-align: justify; color: #2e2e2e; font-family: 'Arial Unicode MS', 'Arial Unicode', Arial, 'URW Gothic L', Helvetica, Tahoma, sans-serif; line-height: 20px">Dividing the fatigue crack growth process into Stage I and II where the former would be highly sensitive to changes in&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><span style="border: 0px; font-size: 14px; margin: 0px; padding: 0px; vertical-align: baseline; font-family: STIXGeneral, STIXGeneral-Bold, STIXGeneral-BoldItalic, STIXGeneral-Italic, STIXIntegralsDisplay, STIXIntegralsDisplay-Bold, STIXIntegralsSmall, STIXIntegralsSmall-Bold, STIXIntegralsUp, STIXIntegralsUp-Bold, STIXIntegralsUpDisplay, STIXIntegralsUpDisplay-Bold, STIXIntegralsUpSmall, STIXIntegralsUpSmall-Bold, STIXNonUnicode, STIXNonUnicode-Bold, STIXNonUnicode-BoldItalic, STIXNonUnicode-Italic, STIXSize1Symbols, STIXSize1Symbols-Bold, STIXSize2Symbols, STIXSize2Symbols-Bold, STIXSize3Symbols, STIXSize3Symbols-Bold, STIXSize4Symbols, STIXSize4Symbols-Bold, STIXSize5Symbols, STIXVariants, STIXVariants-Bold; cursor: pointer; letter-spacing: 0.15em" class="formulatext stixSupport mathImg">&Delta;K</span></span>&nbsp;and the latter would increase with&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><span style="border: 0px; font-size: 14px; margin: 0px; padding: 0px; vertical-align: baseline; font-family: STIXGeneral, STIXGeneral-Bold, STIXGeneral-BoldItalic, STIXGeneral-Italic, STIXIntegralsDisplay, STIXIntegralsDisplay-Bold, STIXIntegralsSmall, STIXIntegralsSmall-Bold, STIXIntegralsUp, STIXIntegralsUp-Bold, STIXIntegralsUpDisplay, STIXIntegralsUpDisplay-Bold, STIXIntegralsUpSmall, STIXIntegralsUpSmall-Bold, STIXNonUnicode, STIXNonUnicode-Bold, STIXNonUnicode-BoldItalic, STIXNonUnicode-Italic, STIXSize1Symbols, STIXSize1Symbols-Bold, STIXSize2Symbols, STIXSize2Symbols-Bold, STIXSize3Symbols, STIXSize3Symbols-Bold, STIXSize4Symbols, STIXSize4Symbols-Bold, STIXSize5Symbols, STIXVariants, STIXVariants-Bold; cursor: pointer; letter-spacing: 0.15em" class="formulatext stixSupport mathImg">&Delta;K</span></span>&nbsp;depending on the&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><a style="color: #0156aa; border: 0px; margin: 0px; padding: 0px; vertical-align: baseline" title="View the MathML source" class="mathImg"><img style="border: 0px; margin: 0px; padding: 0px; vertical-align: bottom; display: inline; max-width: 600px" class="imgLazyJSB" src="http://ars.els-cdn.com/content/image/1-s2.0-016784429190028I-si9.gif" border="0" alt="View the MathML source" title="View the MathML source" width="115" height="18" /></a></span>&nbsp;ratio. The same data when correlated with the strain energy density factor range&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><span style="border: 0px; font-size: 14px; margin: 0px; padding: 0px; vertical-align: baseline; font-family: STIXGeneral, STIXGeneral-Bold, STIXGeneral-BoldItalic, STIXGeneral-Italic, STIXIntegralsDisplay, STIXIntegralsDisplay-Bold, STIXIntegralsSmall, STIXIntegralsSmall-Bold, STIXIntegralsUp, STIXIntegralsUp-Bold, STIXIntegralsUpDisplay, STIXIntegralsUpDisplay-Bold, STIXIntegralsUpSmall, STIXIntegralsUpSmall-Bold, STIXNonUnicode, STIXNonUnicode-Bold, STIXNonUnicode-BoldItalic, STIXNonUnicode-Italic, STIXSize1Symbols, STIXSize1Symbols-Bold, STIXSize2Symbols, STIXSize2Symbols-Bold, STIXSize3Symbols, STIXSize3Symbols-Bold, STIXSize4Symbols, STIXSize4Symbols-Bold, STIXSize5Symbols, STIXVariants, STIXVariants-Bold; cursor: pointer; letter-spacing: 0.15em" class="formulatext stixSupport mathImg">&Delta;S</span></span>&nbsp;showed negligible dependence on mean stress or&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><span style="border: 0px; font-size: 14px; margin: 0px; padding: 0px; vertical-align: baseline; font-family: STIXGeneral, STIXGeneral-Bold, STIXGeneral-BoldItalic, STIXGeneral-Italic, STIXIntegralsDisplay, STIXIntegralsDisplay-Bold, STIXIntegralsSmall, STIXIntegralsSmall-Bold, STIXIntegralsUp, STIXIntegralsUp-Bold, STIXIntegralsUpDisplay, STIXIntegralsUpDisplay-Bold, STIXIntegralsUpSmall, STIXIntegralsUpSmall-Bold, STIXNonUnicode, STIXNonUnicode-Bold, STIXNonUnicode-BoldItalic, STIXNonUnicode-Italic, STIXSize1Symbols, STIXSize1Symbols-Bold, STIXSize2Symbols, STIXSize2Symbols-Bold, STIXSize3Symbols, STIXSize3Symbols-Bold, STIXSize4Symbols, STIXSize4Symbols-Bold, STIXSize5Symbols, STIXVariants, STIXVariants-Bold; cursor: pointer; letter-spacing: 0.15em" class="formulatext stixSupport mathImg">R</span></span>&nbsp;ratio for Stage I crack growth. A parameter &alpha; involving the ratio of ultimate stress to yield stress, percent reduction of area and&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><span style="border: 0px; font-size: 14px; margin: 0px; padding: 0px; vertical-align: baseline; font-family: STIXGeneral, STIXGeneral-Bold, STIXGeneral-BoldItalic, STIXGeneral-Italic, STIXIntegralsDisplay, STIXIntegralsDisplay-Bold, STIXIntegralsSmall, STIXIntegralsSmall-Bold, STIXIntegralsUp, STIXIntegralsUp-Bold, STIXIntegralsUpDisplay, STIXIntegralsUpDisplay-Bold, STIXIntegralsUpSmall, STIXIntegralsUpSmall-Bold, STIXNonUnicode, STIXNonUnicode-Bold, STIXNonUnicode-BoldItalic, STIXNonUnicode-Italic, STIXSize1Symbols, STIXSize1Symbols-Bold, STIXSize2Symbols, STIXSize2Symbols-Bold, STIXSize3Symbols, STIXSize3Symbols-Bold, STIXSize4Symbols, STIXSize4Symbols-Bold, STIXSize5Symbols, STIXVariants, STIXVariants-Bold; cursor: pointer; letter-spacing: 0.15em" class="formulatext stixSupport mathImg">R</span></span>&nbsp;is introduced for Stage II crack growth so that the&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><a style="color: #0156aa; border: 0px; margin: 0px; padding: 0px; vertical-align: baseline" title="View the MathML source" class="mathImg"><img style="border: 0px; margin: 0px; padding: 0px; vertical-align: bottom; display: inline; max-width: 600px" class="imgLazyJSB" src="http://ars.els-cdn.com/content/image/1-s2.0-016784429190028I-si13.gif" border="0" alt="View the MathML source" title="View the MathML source" width="53" height="18" /></a></span>&nbsp;data for different&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><span style="border: 0px; font-size: 14px; margin: 0px; padding: 0px; vertical-align: baseline; font-family: STIXGeneral, STIXGeneral-Bold, STIXGeneral-BoldItalic, STIXGeneral-Italic, STIXIntegralsDisplay, STIXIntegralsDisplay-Bold, STIXIntegralsSmall, STIXIntegralsSmall-Bold, STIXIntegralsUp, STIXIntegralsUp-Bold, STIXIntegralsUpDisplay, STIXIntegralsUpDisplay-Bold, STIXIntegralsUpSmall, STIXIntegralsUpSmall-Bold, STIXNonUnicode, STIXNonUnicode-Bold, STIXNonUnicode-BoldItalic, STIXNonUnicode-Italic, STIXSize1Symbols, STIXSize1Symbols-Bold, STIXSize2Symbols, STIXSize2Symbols-Bold, STIXSize3Symbols, STIXSize3Symbols-Bold, STIXSize4Symbols, STIXSize4Symbols-Bold, STIXSize5Symbols, STIXVariants, STIXVariants-Bold; cursor: pointer; letter-spacing: 0.15em" class="formulatext stixSupport mathImg">R</span></span>&nbsp;would collapse onto a single curve with a narrow scatter band when plotted against&nbsp;<span style="border: 0px; margin: 0px; padding: 0px; vertical-align: baseline; position: relative" class="mathmlsrc"><span style="border: 0px; font-size: 14px; margin: 0px; padding: 0px; vertical-align: baseline; font-family: STIXGeneral, STIXGeneral-Bold, STIXGeneral-BoldItalic, STIXGeneral-Italic, STIXIntegralsDisplay, STIXIntegralsDisplay-Bold, STIXIntegralsSmall, STIXIntegralsSmall-Bold, STIXIntegralsUp, STIXIntegralsUp-Bold, STIXIntegralsUpDisplay, STIXIntegralsUpDisplay-Bold, STIXIntegralsUpSmall, STIXIntegralsUpSmall-Bold, STIXNonUnicode, STIXNonUnicode-Bold, STIXNonUnicode-BoldItalic, STIXNonUnicode-Italic, STIXSize1Symbols, STIXSize1Symbols-Bold, STIXSize2Symbols, STIXSize2Symbols-Bold, STIXSize3Symbols, STIXSize3Symbols-Bold, STIXSize4Symbols, STIXSize4Symbols-Bold, STIXSize5Symbols, STIXVariants, STIXVariants-Bold; cursor: pointer; letter-spacing: 0.15em" class="formulatext stixSupport mathImg">&alpha;&Delta;S</span></span>.</p>

Study on cracked plates, shells and 3 dimensional bodies

<span style="color: #2e2e2e; font-family: 'Arial Unicode MS', 'Arial Unicode', Arial, 'URW Gothic L', Helvetica, Tahoma, sans-serif; font-size: 13px; line-height: 20px; text-align: justify; word-spacing: -1px">This paper presents a summary of the authors' recent work in following areas: (1) The stress-strain fields at crack tip in Reissner's plate. (2) The calculations of the stress intensity factors in finite size plates. (3) The stress-strain fields at crack tip in Reissner's shell. (4) The calculations of the stress intensity factors and bulging coefficients in finite size spherical shells. (5) The stress-strain fields along crack tip in three dimensional body with surface crack. (6) The calculation of stress intensity factors in a plate with surface crack.</span>

Mixed-mode fracture mechanisms near the fatigue threshold of aisi-316 stainless-steel

<span style="font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">Near threshold, mixed mode (I and II), fatigue crack growth occurs mainly by two mechanisms, coplanar (or shear) mode and branch (or tensile) mode. For a constant ratio of &Delta;</span><em style="margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: baseline; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">K</em><sub style="margin: 0px; padding: 0px; border: 0px; outline: 0px; font-size: 0.8em; white-space: nowrap; line-height: 0.7em; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif">I</sub><span style="font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">/&Delta;</span><em style="margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: baseline; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">K</em><sub style="margin: 0px; padding: 0px; border: 0px; outline: 0px; font-size: 0.8em; white-space: nowrap; line-height: 0.7em; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif">II</sub><span style="font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">&nbsp;the shear mode growth shows a self-arrest character and it would only start again when &Delta;</span><em style="margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: baseline; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">K</em><sub style="margin: 0px; padding: 0px; border: 0px; outline: 0px; font-size: 0.8em; white-space: nowrap; line-height: 0.7em; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif">I</sub><span style="font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">&nbsp;and &Delta;</span><em style="margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: baseline; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">K</em><sub style="margin: 0px; padding: 0px; border: 0px; outline: 0px; font-size: 0.8em; white-space: nowrap; line-height: 0.7em; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif">II</sub><span style="font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">&nbsp;are increased. Both shear crack growth and the early stages of tensile crack growth, are of a crystallographic nature; the fatigue crack proceeds along slip planes or grain boundaries. The appearance of the fracture surfaces suggest that the mechanism of crack extension is by developing slip band microcracks which join up to form a macrocrack. This process is thought to be assisted by the nature of the plastic deformation within the reversed plastic zone where high back stresses are set up by dislocation pile-ups against grain boundaries. The interaction of the crack tip stress field with that of the dislocation pile-ups leads to the formation of slip band microcracks and subsequent crack extension. The change from shear mode to tensile mode growth probably occurs when the maximum tensile stress and the microcrack density in the maximum tensile plane direction attain critical values.</span>

Discontinuous yielding and fracture initiation near a notch in mild steel

<p>This thesis presents the results of an experimental investigation of the initiation of brittle fracture and the nature of discontinuous yielding in small plastic enclaves in an annealed mild steel. Upper and lower yield stress data have been obtained from unnotched specimens and nominal fracture stress data have been obtained from specimens of two scale factors and two grain sizes over a range of nominal stress rates from 10^2 to 10^7 lb/in.^2 sec at -111°F and -200°F. The size and shape of plastic enclaves near the notches were revealed by an etch technique. </p> <p>A stress analysis utilizing slip-line field theory in the plastic region has been developed for the notched specimen geometry employed in this investigation. The yield stress of the material in the plastic enclaves near the notch root has been correlated with the lower yield stress measured on unnotched specimens through a consideration of the plastic boundary velocity under dynamic loading. A maximum tensile stress of about 122,000 lb/in.^2 at the instant of fracture initiation was calculated with the aid of the stress analysis for the large scale specimens of ASTM grain size 8 1/4.</p> <p>The plastic strain state adjacent to a plastic-elastic interface has been shown to cause the maximum shear stress to have a larger value on the elastic than the plastic side of the interface. This characteristic of dis continuous yielding is instrumental in causing the plastic boundaries to be nearly parallel to the slip-line field where the plastic strain is of the order of the Lüder's strain.</p>

Modeling mode i fracture of bitumen films

The fracture behavior of thin films of bitumen in double cantilever beam (DCB) specimens was investigated over a wide range of temperature and loading rate conditions using finite-element analysis. The model includes a phenomenological model for the mechanical behavior of bitumen, implemented into a special-purpose finite-element user material subroutine, combined with a cohesive zone model (CZM) for simulating the fracture process. The finite-element model is validated against experimental results from laboratory tests of DCB specimens by comparing measured and predicted load-line deflection histories and fracture energy release rates. Computer simulation results agreed well with experimental data of DCB joints containing bitumen films in terms of peak stress, fracture toughness, and stress-strain history response. The predicted "normalized toughness," G=2h, was found to increase in a power-law manner with effective temperaturecompensated strain rate in the ductile region as previously observed experimentally. In the brittle regime, G=2h is virtually constant. The model successfully captured the ductile and brittle failure behavior of bitumen films in opening mode (tension) for stable crack growth conditions. © 2013 American Society of Civil Engineers.

Fracture mechanics of idealised bituminous mixes

© 2014 Taylor & Francis. The durability of asphalt pavements is strongly impaired by cracks, caused primarily by traffic loads and environmental effects. In this work, fracture behaviour of idealised asphalt mixes is investigated. Experiments on idealised asphalt mixes under pure-tension mode (mode I cracking) were performed and fracture parameters were evaluated. In these three-point bend fracture tests, the test variables were temperature and load rate. The test data were stored in an asphalt materials database and special-purpose tools were implemented to analyse and handle the laboratory data automatically. Fracture mechanism maps were constructed, showing the conditions associated with ductile, brittle and ductile-brittle transition regimes of behaviour. The mechanism maps show the failure response of the material in terms of the stress intensity factor, strain energy release rate and J-integral as a function of the temperature-compensated crack mouth opening strain rate. Fracture behaviour of asphalt mix specimens was simulated by cohesive zone model in conjunction with a novel material constitutive model for asphalt mixes. The finite element model agrees well with the experimental results and provides insights into fracture response of the notched asphalt mix beam specimens.