J-resistance curve and ductile tearing of a mild steel

The onset of ductile tearing at room temperature of mild steel BS 15 was studied using side grooved compact tension specimens. The approach to this problem was divided conveniently into two basic parts: first, identification and evaluation of the toughness at initiation of crack extension, and second, assessment and characterization of the subsequent crack growth behaviour. The critical value of the J integral for crack initiation, J_c was obtained using two different techniques: the multispecimen method and a single specimen compliance test. It was found that the latter could be used with much larger unloading than originally proposed. This has the advantage of greater accuracy in the determination of the compliance, and consequently in the evaluation of crack extension. In the second part of the work, resistance curves were obtained applying two different approaches. The resistance curves obtained following the more exact method were used for the determination of the tearing modulus T of the material, and the values thus derived, compared with a selection of other steels.     

Some observations on the tearing of ductile materials

A relationship between tearing force and tensile properties is deduced by analogy with notched tensile failure and proves to be quite accurate in predicting the tearing forces of a wide range of materials. It is found that the elasticity modulus is a significant factor in the relationship and its role is investigated. A model of the tearing process is presented which seems to be a more accurate representation of the physical situation for many materials than those previously suggested. A simple analysis of the model leads to the relationship which is found to be experimentally reliable.     

Out-of-plane stretching and tearing fracture in ductile sheet materials

Pre-cracked ductile steel sheets are fractured by combined in-plane streching and bending. The deformation mimics the mode of fracture when plating is dented and torn as in ship grounding. Fracture toughness is determined for this mode of tearing. Values are greater than those obtained with DENT testpieces on the same material because of the different mode of crack opening. This revised version was published online in August 2006 with corrections to the Cover Date.     

Engineering assessment of ductile tearing in uniaxial and biaxial tension

To decide between clearly different approaches for engineering assessment of plane stress tearing, we performed uniaxial and biaxial tensile tests on middle-cracked specimens at various in-plane constraint states. Attention was focused on determination of the crack tip opening spacing δ_n, crack tip opening angle ψ_c, crack mouth opening angle α_s, energy dissipation rate R, and specific work of fracture A_s. Our experimental results demonstrate that the values of δ_n, ψ_c, α_s, R, and A_s for a high-strength low-hardening aluminium alloy all depend on the specimen geometry, its size, and the load biaxiality ratio. However, assessment of ductile tearing by interconnected characteristic quantities α_s and A_s is more preferable over the use of δ_n, ψ_c, and R values for a number of basically important reasons discussed in this paper.     

A transferability model for brittle fracture including constraint and ductile tearing effects: a probabilistic approach

This study describes a computational framework to quantify the influence of constraint loss and ductile tearing on the cleavage fracture process, as reflected by the pronounced effects on macroscopic toughness (J _c , _c). Our approach adopts the Weibull stress _w as a suitable near-tip parameter to describe the coupling of remote loading with a micromechanics model incorporating the statistics of microcracks (weakest link philosophy). Unstable crack propagation (cleavage) occurs at a critical value of _w which may be attained prior to, or following, some amount of stable, ductile crack extension. A central feature of our framework focuses on the realistic numerical modeling of ductile crack growth using the computational cell methodology to define the evolution of near-tip stress fields during crack extension. Under increased remote loading (J), development of the Weibull stress reflects the potentially strong variations of near-tip stress fields due to the interacting effects of constraint loss and ductile crack extension. Computational results are discussed for well-contained plasticity, where the near-tip fields for a stationary and a growing crack are generated with a modified boundary layer (MBL) formulation (in the form of different levels of applied T-stress). These analyses demonstrate clearly the dependence of _w on crack-tip stress triaxiality and crack growth. The paper concludes with an application of the micromechanics model to predict the measured geometry and ductile tearing effects on the cleavage fracture toughness J _c of an HSLA steel. Here, we employ the concept of the Dodds-Anderson scaling model, but replace their original local criterion based on the equivalence of near-tip stressed volumes by attainment of a critical value of the Weibull stress. For this application, the proposed approach successfully predicts the combined effects of loss of constraint and crack growth on measured J _c -values.     

The essential work of fracture for tearing of ductile metals

A simple energy balance analysis is presented for the tearing of ductile sheet metals using the trousers test. It is shown that the specific essential work of fracture (w _e ) for tearing can be estimated by extrapolating the straight line relationship between the tearing force per unit thickness and the trousers leg width to zero leg width. There are two contributions to the specific essential work of fracture: one is due to the localised plastic shearing work in a zone contiguous with the torn edges (w _e1 ) and the other is the final out-of-plane tearing work (w _e2 ).

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Résumé En utilisant le Trousers Test, on présente une analyse simple d'équilibre énergétique dans le cas du déchirement de feuilles métalliques ductiles. On montre qu'il est possible d'estimer le travail spécifique de rupture en déchirement w _e en extrapolant la relation linéaire qui lie la force de déchirement par unité d'épaisseur et la largeur de l'échantillon caractéristique entre une valeur 0 et une valeur donnée. Deux éléments contribuent au travail spécifique essentiel de rupture, l'un est dû au travail de cisaillement plastique local dans une zone contigue aux bords de l'arrachement (w _e1 ) et l'autre est le travail de déchirement final hors du plan de la feuille (w _e2 ).

     

Analysis of ductile tearing using a local approach to fracture

In this paper, the Rice and Tracey (RT) model based on growth of cavities is verified in order to describe the ductile tearing in stainless steel 12Ni6Cr and aluminium alloy. Experimentally, central crack panel and compact tension specimens were tested. The comparison of the experimental results with the classical RT model shows that this model cannot make a good prediction using J −Δ a curve. On the basis of fitting procedure and numerical simulations, we have proposed a modification of this model by introducing corrective coefficients to improve the classical RT model. The proposed model, although very simple, is able to approximate with good accuracy the experimental results and that the approximation is, as expected, better than using the RT approach directly.     

Assessment of effect of ductile tearing on cleavage failure probability in ductile to brittle transition region

The fracture behaviour of ferritic and ferritic martensitic steels in ductile to brittle transition (DBT) region has been extensively studied in recent years and a probabilistic approach of master curve method is generally used to describe the fracture toughness of BCC steels in DBT region as a function of temperature. The assessment of cleavage failure probability however is still untouched in the upper region of ductile to brittle transition, although various extensions of master curve approach and various local approaches has been explored. Additionally the geometry and loading in tension and bending also adds up to the difficulties when cleavage failure is assisted with prior ductile tearing. In this work the cleavage fracture is investigated in upper region of DBT and a modified master curve approach is presented which can satisfactorily describe the fracture toughness as a function of temperature as well as amount of ductile tearing preceded by cleavage.     

J andG canalysis of the tearing of a highly ductile polymer

The use of a conventionalJ analysis to describe the ductile tearing of thin low density polyethylene sheet is described. This is a measure of the total energy required to cause fracture. The use of the current energy release rate to determine the local dissipation rate is then given and it is shown that an initiation (plane strain) and reasonably constant propagation (plane stress) values are obtained. Input energy of system - A Area of specimen - B Thickness of specimen - W Width of specimen - a Crack length - J J-integral - G _c Energy release rate - P Load - U Energy - C Compliance - M Constraint factor - _y Yield stress - u Displacement - Dimensionless factor     

Assessment of stable tearing on fatigue fracture surfaces

Fatigue fracture surfaces in a range of structural metals sometimes exhibit bands, visible as dull crescent-shaped regions which contrast to the “bright” background fatigue surface. Such bands are believed to be created within a single load cycle under either constant-amplitude or variable-amplitude load conditions. Microscopically, crack advance by tearing has similarity to the final unstable fracture of the component, with the difference that the tearing is stable: it arrests after a certain distance of crack front advance, which is then followed by further fatigue crack growth. Multiple stable tearing bands, separated by regions created by fatigue crack growth, may be visible on a single fracture surface, to the extent that they may even make up the majority of the fracture surface area.Post-failure analysis of fracture surfaces often relies on quantitative fractography to relate various crack-front progression markings to specific loads in the load history, in order to estimate the crack growth history in the component. Matching this data to predicted crack growth, however, is complicated by the fact that stable tearing is not included in fatigue predictive models, and so the presence of significant tearing can greatly complicate the derivation of a crack growth history which can be matched to a crack growth model. The post-failure analysis of fracture surfaces is particularly difficult in cases where the load history record is poor, and that process has been observed to be much more difficult when significant tearing is present. Several empirical models and concepts have been proposed to assist with post-failure analysis of tearing, but as yet we do not have a good understanding of the parameters which influence both the onset and the arrest of the tearing, and the factors which control crack shape change as tearing progresses. As a result, the interpretation of tearing on service fracture surfaces remains difficult.This paper reviews the existing empirical models for tearing analysis, and describes a series of tests which produced tearing in an aircraft aluminium alloy, with the aim of developing a better understanding of these fracture surface features. The analysis of the tearing shapes indicates that one of the empirical models may have broad value as an engineering analysis tool, since it correlates reasonably well with tearing characteristics in these tests. The study reveals that the stress intensity factor is one of the key controlling parameters in tearing onset and arrest, although simple methods for estimation of relevant stress intensity values are problematic because of the key role of crack front curvature. The main conclusions relate to the notable differences between tearing under constant-amplitude and variable-amplitude loading. The constant-amplitude conditions confer some resistance to stable tearing, and this is believed to result from the high loading prior to tear onset; several mechanisms could be involved. This research is part of a program which is developing improved analytical and prognostic models for stable tearing.     

Low-cycle fatigue of austempered ductile irons

The effects of austempering temperature and isothermal transformation time on the low-cycle fatigue (LCF) behaviour in ductile irons have been studied. The fracture surfaces were observed by a scanning electron microscope in order to understand the fracture mechanism of LCF. From the results, it can be concluded that the best LCF behaviour is for the irons austenitized at 950 °C and there is very good cyclic stability at the lower strain amplitude irrespective of the austempering condition. However, there is a little cyclic softening at higher strain amplitudes for all the austempering conditions. Under a larger strain amplitude, the best LCF behaviour is for the specimen that has undergone austempering at a higher temperature, but under a smaller strain amplitude, the best LCF behaviour is for the specimen austempered at 350 °C.     

Corrosion fatigue of austempered ductile iron

Corrosion fatigue (CF) behavior has been investigated for an austempered ductile iron (ADI) by conducting systematic fatigue tests at 20 Hz, including both high-cycle fatigue (HCF, S-N curves) and fatigue crack growth (FCG, da/dN-K curves), in air, lubrication oil and several aqueous environments. Results showed the HCF resistance of ADI was dramatically reduced by the given aqueous media, in particular, to a greater extent with a decrease in pH value. However, the given room-temperature aqueous solutions did not exert significantly detrimental effects on the Stage II crack growth compared with an atmospheric environment but an increase in solution temperature caused enhanced Stage II crack growth. Among the given variables of the bulk environment, pH had the greatest influence on HCF response while temperature had the most influence on the FCG of long cracks. In addition, SAE 10W40 lubrication oil provided an inert environment to remove the corrosive effect and enhance the CF resistance of ADI. The overall comparisons indicated the environmental effects would generate more influence on Stage I cracking than on Stage II cracking for the given ADI.     

Micromechanical analysis of mechanical heterogeneity effect on the ductile tearing of weldments

The objective of this study is determination of the effect of mechanical heterogeneity on ductile crack initiation and propagation in weldments using micromechanical approach. Welded single-edge notched bend (SENB) specimens were experimentally and numerically analysed. Material properties of welded joint zones were estimated using a combined experimental and numerical procedure; strains on a smooth tensile specimen were determined using ARAMIS stereometric measuring system in order to obtain true stress – true strain curves. High-strength low-alloyed steel was used as base metal, in quenched and tempered condition. J–R curves and crack growth initiation values of fracture mechanics parameter were experimentally and numerically obtained for specimens with a pre-crack in the heat-affected zone (HAZ) and weld metal (WM). The complete Gurson model (CGM) was used in prediction of J–R curves and crack growth initiation. It is shown that the resistance to crack initiation and growth can be predicted using micromechanical analysis, and that the results are significantly affected by mechanical heterogeneity of the weldment.     

The re-characterisation of complex defects: Part I: Fatigue and ductile tearing

Defect assessment codes idealise complex defects as simple shapes which are amenable to analysis in a process known as re-characterisation. The present work examines the re-characterisation of complex defects which extend by fatigue, ductile tearing or cleavage. A family of representative defects were analysed numerically, while a related experimental programme investigated defect interaction and failure. Part I of the paper focuses on fatigue and ductile tearing. Part II examines cleavage. The numerical and experimental results are discussed within the context of the re-characterisation procedures described in BS 7910 (Guidance on methods for assessing the acceptability of flaws in metallic structures. London, UK: British Standard Institution; 1999 [Chapter 7]) and R6/4 (Assessment of the integrity of structures containing defects. Gloucester: British Energy Generation Ltd.; 2001 [Revision 4, Chapters I and II.3]).The level of conservatism of the re-characterisation procedures for fatigue and ductile tearing are discussed. A possible non-conservatism of the re-characterisation for cleavage is discussed in Part II, within the framework of constraint based statistical fracture mechanics.     

The Sandia Fracture Challenge: blind round robin predictions of ductile tearing

Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments.