Critical Crack Tip Opening Displacement Research Articles

Effects of Ni and Mn addition on critical crack tip opening displacement (CTOD) of weld-simulated heat-affected zones of three high-strength low-alloy (HSLA) steels

In order to understand and improve fracture toughness of heat affected zones (HAZs) of high-strength low alloy (HSLA) steels, complex microstructures including quasi-polygonal ferrite (QPF), acicular ferrite (AF), granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituent should be identified, quantified, and then correlated with critical crack tip opening displacement (CTOD). In this study, microscopic analysis methods were achieved for identification and quantitation of microstructures in the HAZs of three HSLA steels. The coarse-grained HAZ (CGHAZ) consisted of AF, GB, and BF together with a small amount of MA, while the inter-critically heated HAZ (ICHAZ) consisted of QPF, GB, and MA. In the CGHAZ, Ni promoted the formation of AF, while it prevented the formation of GB, and the addition of Ni resulted in very high critical CTOD. In the CGHAZ, both Ni and Mn promoted the formation of AF and prevented the formation of GB, while Ni was more effective than Mn. Thus, the addition of Ni resulted in very high critical CTOD. In the ICHAZ, both Ni and Mn promoted the formation MA. However, in the high-Ni-containing steel, a number of MAs were densified along Ni-segregated bands, and thus readily provided void initiation sites. This played an important role in reducing the mean free path for coalescence of voids and crack propagation, which easily led to the serious deterioration of critical CTOD.

Numerical and Experimental Study on Critical Crack Tip Opening Dis-placement of X80 Pipeline Steel

High strength pipelines is widely used in the long distance transportation of oil and gas resources. Fracture toughness is one significant material property for these high strength pipeline steels. In this study, the critical crack tip opening displacement (CTOD) of X80 pipeline steel was studied both experimentally and numerically. 3 point bending tests were conducted for 6 specimens with 2 thicknesses. Numerical model using the extended finite element method was also established, with maximum principle strain criterion used for damage initiation. Results show that, the critical CTOD decreases with the increases of the specimen thickness, and to give reasonable simulation results, larger maximum principle strain criterion should be used in the numerical model. This study can be referenced for the performance evaluation and quality assurance for X80 steel gas pipelines.DOI: http://dx.doi.org/10.5755/j01.mech.23.2.14535

Plastic deformation behavior in SEB specimens with various crack length to width ratios

The authors have already proposed a new crack tip opening displacement (CTOD) calculation formula based on the plastic hinge model considering the unique crack tip blunting behavior due to strain hardening of the material. This formula is limited to the standard length-to-width ratio (a0/W) range of between 0.45 and 0.55. However, CTOD calculations are necessary for various other a0/W conditions when the critical CTOD for a specific microstructure must be evaluated in a welded joint. ISO15653, which prescribes CTOD test methods for welds, covers a wide range of a0/W between 0.10 and 0.7 by applying a J-integral based CTOD derived from ASTM E1290 for a0/W from 0.10 to 0.45 and a plastic hinge based CTOD for a0/W from 0.45 to 0.70. This procedure leads to a discontinuity in the evaluated CTOD at a0/W=0.45. This problem results from the fact that the J-integral based CTOD is inconsistent with the conventional plastic hinge based CTOD, especially in low yield-to-tensile ratio materials. Thus, a single CTOD calculation method for the wide range of a0/W is required for rational evaluation. In this study, the plastic deformation behavior of SEB specimens was analytically examined in the a0/W range from 0.05 to 0.70, and the capacity of the plastic hinge based CTOD was discussed with the aim of establishing a unified CTOD calculation formula for SEB specimens with the wide range of a0/W.

Implementation of the Double-Edge-Notched Tension Test for Asphalt Cement Acceptance

This paper focuses on an investigation of the implementation of the double-edge-notched tension protocol (DENT, Ministry of Transportation of Ontario LS-299 and American Association of State and Highway Transportation Officials TP 113-15) for the acceptance of asphalt cement. Since 2012, the DENT test has been implemented with associated criteria for the acceptance of modified binders used in provincial and municipal paving contracts. A total of 21 binders from different contracts were collected for this study and tested according to the LS-299 DENT protocol to determine essential and plastic works of failure (we and βwp, measures of strength and toughness), and an approximate critical crack tip opening displacement (CTOD, a measure of strain tolerance). It was found that nearly all samples were able to reach the CTOD as specified in the contract but that a significant number contained recycled engine oil bottoms, with anticipated long term durability and moisture sensitivity problems. Polymer contents were inconsistent across different grades and performance differences between tank and recovered binders were found to be considerable and unpredictable. Hence, it is imperative that, at an early opportunity, recovered binders are tested to assure minimum performance criteria are being met in terms of CTOD as well as we and βwp, and that a ban be implemented on the use of deleterious additives. The implementation of the DENT test will allow user agencies to reduce detrimental fatigue cracking distress in thin asphalt pavements.

Determination of the Fracture Toughness Parameters of Cement Composite Using Specimens with Rounded Notch

The fracture mechanics parameters are used in different applications to formulae the classic criteria of failure as well as in computational models to analyze the concrete structures and other brittle materials. The parameters are determined using the specimens with notches. The study for assessing the influence of the notch curvature in specimen on fracture mechanics parameters, e.g. critical stress intensity factor, critical crack tip opening displacement and fracture energy, was performed on cement mortar as a typical quasi-brittle material. The U-shaped notches with a depth of 30 mm were formed during molding the specimens. The radius of notch curvature was changed in the range from 0.15 mm to 30.0 mm. The results obtained demonstrated that the notch curvature radius has a significant effect on the level of maximum stress and the parameters describing the cement composite resistance to cracking. Therefore, the experimental determination of the fracture characteristics requires a strict definition of the test conditions.DOI: http://dx.doi.org/10.5755/j01.sace.15.2.15376

Effect of recycled engine oil bottoms on the ductile failure properties of straight and polymer-modified asphalt cements

This paper presents a study on the effects of recycled engine oil bottoms (REOB) on ductile failure properties of asphalt cements. Binders were tested according to regular AASHTO M320 as well as double-edge-notched tension (DENT) test protocols. The DENT test determines essential work of failure (we, a measure of strength and toughness) and critical crack tip opening displacement (CTOD, a measure of strain tolerance). REOB was found to increase the AASHTO M320 grade span but worsen we and/or CTOD. Straight and polymer-modified binders were studied, with and without REOB and reclaimed asphalt pavement (RAP) binder, and significant decreases in we and CTOD were found, pointing to reduced cracking performance.

Austempered Ductile Iron (ADI) Alternative Material for High-Performance Applications

Austempered ductile iron (ADI) grades are standardized, and the requirements of current international standards (EN 1564-12/ASTM A897-15) are given terms of conventional mechanical properties, such as hardness and tensile strength. Nevertheless, these properties do not show the real potential of the ADI grades. In order to promote the use of ADI parts in place of other materials, this work proposes a comparison between GJS-900-8 and GJS-1200-3 grades, both in terms of conventional and advanced mechanical properties, employing stress intensity factors and critical CTODs (Crack Tip Opening Displacement). This study is completed with mechanical fatigue testing, so that it can be shown that the service life of ADI parts is comparable to that given by other heavier and more expensive options.

Characterisation of Fracture and HAC Resistance of an Individual Microstructural Constituent with Micro-Cantilever Testing

This paper focuses on the application of miniaturized fracture tests to evaluate the fracture and hydrogen assisted cracking (HAC) resistance of a selected microstructural constituent (acicular ferrite, AF) which only occurs in microscopic material volumes. Site-specific Focused Ion Beam (FIB) micro-machining was used to fabricate sharply notched micro-cantilevers into a region fully constituting of AF. The micro-cantilevers were subsequently tested under uncharged and hydrogen charged conditions with a nanoindenter. The load displacement curves were recorded and analysed with a simplified plastic hinge model for the uncharged specimen, as AF demonstrated an essentially ductile behaviour. The simplified model assisted with FE simulations provided values of the critical plastic crack tip opening displacement (CTOD). A value of the conditional fracture toughness was thereby determined as 12.1 MPa m1/2. With LEFM, a threshold stress intensity factor, Kth, to initiate hydrogen crack propagation in AF was found to range between 1.56 MPa m1/2 and 4.36 MPa m1/2. All these values were significantly below the corresponding values reported for various ferrous alloys in standard macro-tests. This finding indicates that the fracture and HAC resistance at the micro-scale could be very different than at the macro-scale as not all fracture toughening mechanisms may be activated at this scale level.

Fracture Properties of Polystyrene Aggregate Concrete after Exposure to High Temperatures.

This paper mainly reports an experimental investigation on the residual mechanical and fracture properties of polystyrene aggregate concrete (PAC) after exposure to high temperatures up to 800 degrees Celsius. The fracture properties namely, the critical stress intensity factor (), the critical crack tip opening displacement (CTODC) for the Two-Parameter Model, and the fracture energy (GF) for the Fictitious Crack Model were examined using the three-point bending notched beam test, according to the RILEM recommendations. The effects of polystyrene aggregate (PA) content and temperature levels on the fracture and mechanical properties of concrete were investigated. The results showed that the mechanical properties of PAC significantly decreased with increase in temperature level and the extent of which depended on the PA content in the mixture. However, at a very high temperature of 800 °C, all samples showed 80 percent reduction in modulus of elasticity compared to room temperature, regardless of the level of PA content. Fracture properties of control concrete (C) and PAC were influenced by temperature in a similar manner. Increasing temperature from 25 °C to 500 °C caused almost 50% reduction of the fracture energy for all samples while 30% increase in fracture energy was occurred when the temperature increased from 500 °C to 800 °C. It was found that adding more PA content in the mixture lead to a more ductile behaviour of concrete.

Experimental and computational investigation of the effect of phase transformation on fracture parameters of an SMA

A comprehensive, multi-method experimental characterization of fracture is conducted on shape memory alloy NiTi that exhibits superelasticity due to austenite-to-martensite stress induced phase transformation. This characterization includes (i) load-based measurement of critical stress intensity factor (Kmax) using ASTM standard E399, (ii) measurement of crack tip opening displacement (CTOD) per ASTM standard E1290, (iii) the digital image correlation (DIC) characterization of the transformation zone as well as the displacement-field based measurement of Kmax from the DIC data. Samples have also been tested at T = 100 °C to suppress the martensitic transformation to investigate transformation toughening. The experimental investigation is complemented with finite element (FE) analysis that uses Auricchio–Taylor–Lubliner constitutive model. A direct observation with DIC revealed a small scale transformation (K-dominance). Kmax of the transforming material is higher than that of the transformation-suppressed material tested at 100 °C, suggesting transformation toughening. At 100 °C, the material becomes quite brittle with a very small crack-tip plastic zone when the transformation mechanism is blocked. By measures of critical CTOD, the gap widens even more between the superelastic and transformation-suppressed cases, particularly because of the side effect that, in this very interesting material, material modulus increases with temperature. Evaluating the transformation zone from the DIC strains with reference to the uniaxial stress–strain curve, an equivalent strain form is proposed in conjunction with the plane stress FE prediction.

Influence of RAP sources and proportions on fracture and low temperature cracking performance of polymer modified binder

The present study evaluates effects of reclaimed asphalt pavement (RAP) obtained from two different sources on fracture and low temperature cracking resistance of a polymer modified binder (PMB). Two different RAPs, one from urban highway (RAP-A) and other from rural highway (RAP-S) were collected. The binder extracted from RAP-A and RAP-S had different viscosity, rheological and chemical characteristics. The binder extracted from RAP-A was found to have higher viscosity and stiffer nature compared to RAP-S. The PMB binder was blended with different proportions (i.e., 0, 15, 25 and 40% by weight of binder) of RAP-A and RAP-S. The double edge notched tension (DENT) test was conducted to evaluate fracture cracking potential of PMB binder containing different percentages of both the RAPs. Further, low temperature cracking potential of RAPs blended binders was measured using bending beam rheometer (BBR) test. The results from the DENT test showed the second peak in the load-displacement curve for the control PMB disappeared with addition of RAPs, indicating damage to polymer interlinking. Inclusion of RAP-A (urban highway source) in PMB resulted in a smaller critical crack tip opening displacement (CTOD) value compared to RAP-S (rural highway source). The statistical analyses showed that lower percentage of RAPs (i.e., 15% RAP-A and 15% RAP-S) had similar effect on CTOD value of PMB, however, at higher percentage of RAPs (25% and 40%), both RAPs found to behave differently. The BBR test showed that addition of RAPs resulted in increase in stiffness and decrease in m-value, indicating poor low temperature cracking potential of PMB. The addition of RAP-A resulted in lower m-value compared to RAP-S. Based on the DENT and the BBR tests, it can be concluded that the PMB binder with RAP-A could be more fracture susceptible compared to binder with addition of RAP-S. The low temperature performance grade (PG) of PMB did not change significantly with addition of even 40% of both the RAPs, which is contrary to what is being reported in literature. The findings presented in the paper showed that RAP source and its proportions can have different impact on performance of mixes, thus assuming that nature of RAP binders remains same irrespective of change in a source may lead to premature failure of a pavement.

Effects of film thickness on critical crack tip opening displacement in single-crystalline and polycrystalline submicron Cu films

We performed in-situ field emission scanning electron microscopy fracture toughness tests on single-crystalline and polycrystalline submicron-thick Cu films. In all specimens, the notch root became blunt, and a crack was initiated from the blunted notch root. The critical crack tip opening displacement (CTOD) for crack initiation, δi, was thickness dependent, where δi decreased with a decrease in the thickness. The critical CTOD normalized by the thickness, δi/B, values of the films were similar (δi/B=1.4–1.9), irrespective of the film thickness and microstructure. This suggested that the local fractures of the submicron Cu films were similar.

Portland cement mortar nanocomposites at low carbon nanotube and carbon nanofiber content: A fracture mechanics experimental study

A thorough fracture mechanics characterization of Portland cement mortars reinforced with multi wall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) took place. The critical values of stress intensity factor, KICS; strain energy release rate, GICS; crack tip opening displacement, CTODc; and critical crack length, ac of 3, 7, and 28 days Portland cement mortars, reinforced with well dispersed carbon nanotubes and carbon nanofibers were experimentally determined. Prismatic notched specimens of neat mortars and mortars reinforced with 0.1 wt.% CNFs, and 0.1 and 0.2 wt.% MWCNTs were subjected to a three point closed loop bending test, using the crack mouth opening displacement, CMOD, as the feedback signal. The fracture parameters of the nanoreinforced mortars were then determined using the two parameter fracture model. The excellent reinforcing and toughening efficiency of MWCNTs and CNFs is demonstrated by a significant improvement in the critical stress intensity factor/fracture toughness (128.6%), critical strain energy release rate (154.9%), and critical crack tip opening displacement (39%). These results allow us to conclude that the MWCNTs and CNFs beneficially alter the nanostructure of the mortar matrix, resulting to a significant enhancement of all fracture and mechanical properties and provide the material, with the ability of performing multiple structural functions.

Effect of Prefabricated Crack Length on Fracture Toughness and Fracture Energy of Fly Ash Concrete Reinforced by Nano-SiO2 and Fibers

This paper summarizes the test data obtained from an experimental investigation of nanoparticles and steel fiber-reinforced concrete. Tests were conducted to investigate the effect of prefabricated crack length on determining fracture parameters of concrete composite. The results reveal that most of the fracture parameters and the fracture relational curves of the concrete composite are influenced greatly by the prefabricated crack length of beam specimens, while the prefabricated crack length has little effect on the critical crack tip opening displacement. There is a tendency for effective crack length and the critical crack mouth opening displacement to increase with the increase in prefabricated crack length, while the initial fracture toughness, unstable fracture toughness and fracture energy gradually decrease. Besides, when the prefabricated crack length decreases from 50 to 20 mm, these relational curves become much larger, and the nonlinear stage of the curves become longer and longer, and the descent stage of the curves become flatter and flatter.

Fracture parameters of site-cast dam and sieved concrete

A conversion relationship between the fracture parameters of site-cast dam concrete and that of sieved concrete is presented. The concrete mixture produced by the mixing tower system on the construction site of a super-high arch dam was used to cast and mould different sized specimens. The maximum aggregate size for the dam concrete specimens was 150 mm and that for the sieved concrete specimens was 40 mm. The fracture parameters of the dam concrete and sieved concrete, including fracture energy, effective fracture toughness, critical crack tip opening displacement and characteristic length, were determined. In addition, the effect of dam concrete and sieved concrete specimen size on fracture behaviour was analysed. It was found, regardless of the dam concrete or sieved concrete specimen, that when the ratio of the ligament height to the maximum aggregate size was equal to or greater than 6·0, stable values of the fracture parameters were obtained. Based on the test results, formulae for calculating the fracture parameters for large dam-concrete specimens were derived from those for small sieved-concrete specimens.

Forward and Inverse Analysis of Concrete Fracture Using the Disk-Shaped Compact Tension Test

Abstract A new concrete fracture geometry is presented, which can quantify multiple fracture properties from a single specimen test. The disk-shaped compact tension (DCT) geometry allows specimens to be fabricated from laboratory cylinders or field cores. The DCT fracture test characterizes the concrete's critical stress intensity factor, KIC, critical crack-tip opening displacement, CTODc, and initial fracture energy, Gf, as well as the specimen-dependent total fracture energy, GF. The DCT-based fracture properties have the same experimental variation as the single-edge notched beam test. The experimentally derived fracture parameters were implemented into a cohesive zone model, which enabled estimation of concrete tensile strength from field-extracted cores.

Application of Weibull Stress Criterion to Brittle Fracture Assessment of Heat-Affected Zone-Notched Welds With Residual Stress

A constraint loss correction procedure using the Weibull stress criterion is specified in ISO 27306. However, this standard is applicable only to structural steel components with defects, not to welded joints. Therefore, we propose a method for estimating the brittle fracture limit of a weld with a notch in the heat-affected zone (HAZ) and residual stress based on the Weibull stress criterion. Three-point bending (3PB) tests and wide-plate (WP) tension tests of HAZ-notched welds made of 780-MPa class high-strength steel were conducted at −40 °C. The minimum critical crack tip opening displacement (CTOD) of the WP specimen fracturing at the coarse-grained region of the HAZ (CGHAZ) was approximately four times that of the 3PB specimen. Then, the effects of specimen geometry, residual stress, crack-front shape, and HAZ microstructure classification on the Weibull stress were investigated by using a finite element analysis (FEA). The results of these analyses showed that the specimen geometry, the welding residual stress, and HAZ microstructure affect the Weibull stress of HAZ-notched welds as crack driving force. Based on above results, the CTOD–Weibull stress curves for 3PB and WP specimens fracturing at CGHAZ were calculated by using an FEA. It was confirmed that the brittle fracture limit of an HAZ-notched weld with residual stress could be predicted from the Weibull stress criterion because predicted critical CTOD of WP specimens obtained by Weibull stress included experimental critical CTOD of WP specimens.

Prediction of Fracture Parameters of Concrete by Relevance Vector Machine

This article employs Relevance Vector Machine (RVM) for prediction of two fracture parameters {Critical Stress Intensity Factorand Critical Crack Tip Opening Displacement (CTODc)} of concrete. Input variables of the RVM model are water-cement ratio (), maximum size of aggregate () and characteristic strength (). The developed RVM gives equations for prediction of and CTODc. A comparative study has been presented between the developed RVM and the ANN models. A sensitivity analysis has been done to determine the effect of each input parameter on the predicted and CTODc. This article shows that the developed RVM can be used as an efficient tool for determination of and CTODc.

Flexural fracture response of a novel iron carbonate matrix – Glass fiber composite and its comparison to Portland cement-based composites

This paper explores the fracture properties of a novel and sustainable glass-fiber reinforced composite, the matrix for which is formed through the aqueous, anoxic, room-temperature carbonation of (waste) metallic iron powder along with other minor ingredients. A comparison of the properties of this binder with Ordinary Portland Cement pastes, which constitutes one of the most common and economic ceramic matrices is also provided. The iron-based binder system exhibits fracture parameters (fracture toughness, KICS and critical crack tip opening displacement, CTODC, determined using two parameter fracture model, TPFM) that are significantly higher when compared to those of the OPC systems in both the unreinforced and glass fiber reinforced states. The beneficial influence of the unreacted metallic iron particles of large aspect ratio, on the fracture parameters of iron-based binders are elucidated. The strain energy release rates show trends that are in line with the fracture parameters from TPFM. The elastic and inelastic components of strain energy release rate are separated in an effort to capture the fundamental toughening mechanisms in these systems. The fracture parameters determined using a non-contact, digital image correlation technique are found to relate well to those obtained from TPFM.

Minimum specimen size for fracture parameters of site-casting dam concrete

This paper presents the minimum specimen size for experimental determination of the fracture parameters of site-casting dam concrete. Five series of wedge-splitting specimens for dam concrete with maximum aggregate size of 150mm were tested, casting in the site of an actual super-high arch dam with its mixing tower system. The complete load-crack mouth opening displacement and load-crack tip opening displacement curves of the specimens were acquired. The fracture parameters, including fracture energy, effective fracture toughness, and critical crack tip opening displacement, were also obtained. Furthermore, the effect of specimen size on fracture parameters was discussed, and it was found that the minimum specimen size for determining the independent fracture parameters of the site-casting dam concrete should satisfy that ligament length is six times larger than the maximum aggregate size.