Crack Tearing Research Articles

Mixed-Mode I/II Crack Extension in Thin Sheet Metal

Real-time displacement in stable crack tearing was measured by digital image correlation technique. Quantitative analyses were provided on mixed-mode Ⅰ/Ⅱ fracture and critical CTOA for recycled thin sheet metals. It is shown that the fracture of recycled thin sheet metals is tough due to the large plastic deformation, and mixed-mode crack is easier to extend than mode Ⅰ crack. The critical CTOA at stable crack tearing is constant and can be used as a fracture criterion for the thin sheet metals.

Predictive models for stable tearing crack growth due to overloading in fatigue

ABSTRACTStable tearing is a recurring process in which fatigue crack growth is interspersed by substantial jumps of crack growth, commonly at the central cross‐section of the component while the crack front nearer the surface lags behind. These tearing bands have been observed to start very early during fatigue life and can make up the majority of the fatigue fracture surface. This paper presents the development of predictive models, in which the tongue‐shaped stable tearing band is first idealised as trapezoidal shape, and then two alternative fracture criteria are formulated with the aid of the finite element (FE) method and the Forsyth stable tearing concept. Parametric solutions of the stress intensity factor at the front of the trapezoidal crack front are obtained using the FE method. Comparisons between the model predictions and experimental results indicate that both models produce satisfactory prediction of the stable tearing crack jump length in aluminium alloy coupons of varying cross‐sectional thickness.

Improved Incremental J-Integral Equations for Determining Crack Growth Resistance Curves

The J-integral resistance curve is the most important material properties in fracture mechanics that is often used for structural integrity assessment. ASTM E1820 is a commonly accepted fracture toughness test standard for measuring the critical value of J-integral at the onset of ductile fracture and J-R curve during ductile crack tearing. The recommended test procedure is the elastic unloading compliance method. For a stationary crack, the J-integral is simply calculated from the area under the load-displacement record using the η-factor equation. For a growing crack, the J-integral is calculated using the incremental equation proposed by Ernst et al. (1981, “Estimations on J-integral and Tearing Modulus T From a Single Specimen Test Record,” Fracture Mechanics: Thirteenth Conference, ASTM STP 743, pp. 476–502) to consider the crack growth correction. For the purpose of obtaining accurate J-integral values, ASTM E1820 requires small and uniform crack growth increments in a J-R curve test. In order to allow larger crack growth increments in an unloading compliance test, an improved J-integral estimation is needed. Based on the numerical integration techniques of forward rectangular, backward rectangular, and trapezoidal rules, three incremental J-integral equations are developed. It demonstrates that the current ASTM E1820 procedure is similar to the forward rectangular result, and the existing Garwood equation is similar to the backward rectangular result. The trapezoidal result has a higher accuracy than the other two, and thus it is proposed as a new formula to increase the accuracy of a J-R curve when a larger crack growth increment is used in testing. An analytic approach is then developed and used to evaluate the accuracy of the proposed incremental equations using single-edge bending and compact tension specimens for different hardening materials. It is followed by an experimental evaluation using actual fracture test data for HY80 steel. The results show that the proposed incremental J-integral equations can obtain much improved results of J-R curves for larger crack growth increments and are more accurate than the present ASTM E1820 equation.

Mechanics and mechanisms of tear resistance of woven fabrics

Modeling and calculating strain energy are among the most important steps in evaluating the mechanics and mechanical of the tearing process for woven textile materials. In this paper, the parameters controlling the mechanisms of crack propagation in woven textiles have been investigated. A new model, that describes the relationship between tearing energy and fabrics’ properties, has been established and validated. Several plain and twill woven fabrics made of cotton, polyester/cotton blend and polyester are examined to evaluate the proposed model using tongue tear and tensile central crack tear tests. By studying the variation of the critical strain energy with the yarn breaking force and slippage force (FYB/FS) ratio, the effect of the crack propagation mechanisms of the fabric tearing behavior was analyzed. This model provides very useful information on tearing mechanism of woven textiles and is a useful tool for the in-depth study of the tearing phenomena of nonwoven woven textiles and for the development of new models involving the tearing process. The results showed that experimental data are consistent with theoretical values. This good correlation suggests that the proposed model is able to accurately describe the behavior of the textiles in tearing tests.

Semi-quantitative predictions of hot tearing and cold cracking in aluminum DC casting using numerical process simulator

Cracking is one of the most critical defects that may occur during aluminum direct-chill (DC) casting. There are two types of cracking typical of DC casting: hot tearing and cold cracking. To study and predict such defects, currently we are using a process simulator, ALSIM. ALSIM is able to provide semi-quantitative predictions of hot tearing and cold cracking susceptibility. In this work, we performed benchmark tests using predictions of both types of cracks and experimental results of DC casting trials. The trials series resulted in billets with hot tearing as well as cold cracking. The model was also used to study the influence of several casting variables such as casting speed and inlet geometry with respect to the cracking susceptibility in the ingots. In this work, we found that the sump geometry was changed by the feeding scheme, which played an important role in hot tear occurrence. Moreover, increasing the casting speed also increased the hot tear and cold crack susceptibility. In addition, from the result of simulation, we also observed a phenomenon that supported the hypotheses of connection between hot tearing and cold cracking.

Noncontact optical measurement of CTOA and CTOD for interface crack in DCB test

The crack-tip opening angle (CTOA) and the crack-tip opening displacement (CTOD) are used as fracture parameters for characterizing the resistance of a material to fracture and predicting the instability of crack propagating fracture process. Digital image correlation method (DIC) was used to measure the displacement fields in a double cantilever beam (DCB) test with an initial interfacial crack. An apparent crack-tip was determined using the y-displacement along two parallel lines of the two crack flanks, which in turn enables the CTOA and CTOD to be evaluated. The experimental results revealed that the extension process for interface crack is divided into stable and unstable crack extension phases which changed alternately in the DCB test. The crack growth rate (CGR) varied from one stable phase to another, but was a constant value within each stable phase. The CGR, however, increased gradually over time within the unstable phases. The CTOA and CTOD did not increase synchronously in the loading process: the CTOA begins to increase at the first stable crack growth phase, whereas, the CTOD increased rapidly with the CTOA until entered into the second phase of unstable crack propagation. The above crack propagation behaviour on the bi-material corresponds to the interface crack transition, which is transformed from the tearing crack Mode I gradually into the mixed Mode I/II crack form.

Modeling of mixed-mode crack growth in ductile thin sheets under combined in-plane and out-of-plane loading

This paper describes a modeling approach for analyzing mixed-mode crack growth events in ductile thin-sheet materials under large deformation and combined in-plane and out-of-plane loading conditions. The remote mixed-mode I/III loading leads to local mixed-mode I/II/III fields near the crack front. Making use of full-field surface deformation measurements, finite element models of mixed-mode I/III stable tearing events in thin-sheet specimens have been developed. Model predictions have been compared with experimental measurements (a) just prior to initial crack growth and (b) during stable tearing crack growth. Analyses of curvilinear crack growth events are carried out using a nodal release option or a local re-meshing option and using a generalized CTOD parameter with experimentally measured critical CTOD values. Results of this study suggest that the modeling approach can be employed to numerically re-construct experimental crack growth events in thin plate specimens. This offers a viable means of analyzing and understanding the mixed-mode crack growth events and provides a tool for further investigations of 3D crack front fields (which are otherwise unavailable experimentally) and for the study of fracture criteria for stable tearing events.

Crack Tip Opening Displacement of X100 SAWL Pipe

The fracture toughness of different zones of X100 SAWL pipe was studied by three-point bend test method under different temperature. At the same temperature, the crack propagation resistance and tear modulus of X100 SAWL pipe distributed as: base metal>HAZ>weld bead, and the distribution of the CTOD value of X100 SAWL pipe is :base metal >weld bead >HAZ. Further investigation was conducted to analyze the decrease of fracture toughness (especially low temperature fracture toughness) and the occurrence of local unstable phenomenon in HAZ specimens.

Tear resistance of woven textiles – Criterion and mechanisms

The resistance to tearing of textiles is an important characteristic of materials, especially for fabrics used in personal protective equipment. In this project, work has been done to develop a criterion for textile structures based on the tearing energy. Tongue tear and tensile central crack tear tests were performed on plain and twill woven fabrics made of cotton, polyester/cotton blend and polyester. The tearing energy was computed from the variation of the work as a function of the tear crack surface area. Results show that for the two types of studied weaves, the tearing energy G is independent of the sample configuration. This tearing energy criterion was used to study the effect of fabrics linear density, yarn density and weave on their tearing performance. It also provided evidence of the occurrence of an additional phenomenon in the tongue tear mechanism, i.e. the sliding of transverse yarns in addition to the already reported sliding of longitudinal yarns. This tearing energy criterion appears as a valuable tool for the in-depth study of the tearing resistance of woven textiles.

Measuring Mechanical Behavior of Steel During Solidification: Modeling the SSCC Test

The submerged split chill contraction (SSCC) test can measure forces in a solidifying steel shell under controlled conditions that match those of commercial casting processes. A computational model of this test is developed and applied to increase understanding of the thermal-mechanical behavior during the initial solidification of steel. Determining the stress profile is difficult because of the complicated geometry of the experimental apparatus and the nonuniform temperature and strength across the shell. The two-dimensional axisymmetric elastic-viscoplastic finite-element model of the SSCC test features different mechanical properties and constitutive equations for delta-ferrite and austenite that are functions of both the temperature and the strain rate. The model successfully matches measurements of (1) temperature history, (2) shell thickness, (3) solidification force, and (4) failure location. In addition, the model reveals the stress and strain profiles through the shell and explains what the experiment is actually measuring. In addition to the strength of the shell, the measured force is governed by the strength of the junction between the upper and lower test pieces and depends on friction at the shell–cylinder interface. The SSCC test and the validated model together are a powerful analysis tool for mechanical behavior, hot tear crack formation, and other phenomena in solidification processes such as continuous casting.

COMPARISON OF SINE VERSUS PULSE WAVEFORM EFFECTS ON FATIGUE CRACK GROWTH BEHAVIOR OF NR, SBR, AND BR COMPOUNDS

Abstract Fatigue crack growth experiments on carbon black-filled rubber compounds have been carried out to evaluate the influence of testing conditions over different compound formulations. Investigations on the influence of waveform, data acquisition, and compound formulation have been performed on strip-tensile specimens reproducing the mode I of crack opening. The response of three different compound formulations (based on either natural rubber, butadiene rubber, or styrene-butadiene rubber) to the application of two different waveforms, pulse and sine, has been analyzed, showing significant differences in fatigue behavior and ranking of the various compounds. Compared to the sinusoidal waveform, the use of a pulse waveform provided an improved correlation of the tearing energy with the crack propagation speed. This difference was particularly evident in the case of natural rubber and butadiene rubber, while it resulted negligible in the case of styrene-butadiene rubber. Such a different behavior could be attributed to differences in macromolecular chains orientation. Fine-tuning of video acquisition parameters provided an accurate observation of the crack growth process, as confirmed by the low standard deviation of the estimated tearing energy and crack growth rate.

Characterization of the fracture behavior of NBR and FKM grade elastomers for oilfield applications

The monotonic fracture behaviour of two elastomer compounds (HNBR and FKM) was characterized in this study. These elastomeric materials are frequently used in many oilfield applications (e.g., seals and hoses) and are exposed to a complex combination of mechanical, thermal and environmental loads. Furthermore, rapid gas decompression failure of these elastomeric materials was observed under real service conditions. A phenomenon termed rapid gas decompression (RGD) damage occurs when elastomer seals exposed to high gas pressure fail in a brittle manner upon the sudden release of the gas pressure. Hence, to support both material development and design efforts, it is of prime theoretical and practical importance to characterize the fracture behaviour of these materials. Fracture mechanics tests using a faint waist pure shear specimen (FWPS) configuration were performed over a wide loading rate range. First, global force and displacement values were measured and single parameter fracture mechanics values in terms of peak tearing energy, T p values were calculated. Furthermore, the crack growth process and the local crack tip deformation were characterized by non-contact optical devices. The digital image correlation technique used allows for the determination of displacement and full-field strain up to a high strain and crack growth rate. Based on above measurements, crack growth resistance curves in terms of tearing energy ( T − Δ c) and crack tip opening angle ( CTOA − Δ c) functions were derived. While a continuous crack growth was observed for HNBR at all loading rates, a discontinuous crack extension was observed for FKM. Finally a phenomenological model was deduced for describing the rapid monotonic crack extension process for these elastomeric compounds.

Safety Assessment Procedure for a Cracked Square Hollow Section (SHS) Y-joint

In this paper, an accurate crack model of a square hollow section (SHS) welded Y-joint is proposed, and a high quality finite element mesh is generated. Based on this numerical model, the load-displacement curves and the stress intensity factors (SIFs) along the crack front are calculated. It is shown that the highest value of the stress intensity factor is always located at the chord corner. From the full-scale static strength test carried out on the cracked SHS Y-joint, it was found that ductile crack tearing initiated at the crack front parallel to the chord side wall where the fracture toughness is smaller, and not from the crack front around the corner where the SIF is the highest. Using the plastic collapse load obtained via the twice elastic compliance technique and the fracture toughness obtained from the Charpy V-notch and CTOD tests, the assessment points of Kr and Lr are calculated and plotted on the BS7910 (2005) Level 2A Failure Assessment Diagram (FAD) curve, thus demonstrating its usage. It shows that the standard curve gives a conservative assessment for the cracked SHS Y-joint subjected to brace end axial loads.

Assessment of Segregation in Hot Tear Cracks

In solidified slabs different types of segregations such as crystal segregation (micro segregation), centre segregation (macro segregation) and hot tear segregation (HTS) may occur. The present paper examines the segregation behaviour of different elements in hot tear cracks depending on the carbon content. The aim of this work is to determine the segregation factors in the hot tear cracks filled with residual melt and compare with micro and macro segregation. Within the scope of this examination, a microanalytical assessment was made of eight slab samples with different steel grades each showing different types of hot tear cracks that had been healed up by an inflow of residual melt. The hot tear cracks are located outside the primary dendrites in the dendritic interstices, parallel or transversal to the direction of casting. Segregation in the cracks healed up by residual melt depends on the carbon content and will become more pronounced as the carbon content increases. The intensity of segregation for the various elements in the hot tear cracks (hot tear crack segregation) is between that of crystal segregation and centre segregation. The thickness of the segregated zone in the hot tear crack area is 30 ‐ 50 μm. Apart from an enrichment of the alloying elements manganese, silicon and chromium, the healed up hot tear cracks also contain secondary precipitates of sulphides and niobium‐titanium‐carbonitrides. Towards the slab centre, the latter can cause, among other things, the development of niobium‐titanium‐carbonitrides (Nb1‐x, Tix)(C1‐y, Ny) networks in the primary dendritic interstices.

Three-Dimensional Crack Growth in Ductile Materials: Effect of Stress Constraint on Crack Tunneling

Crack tunneling is a crack growth feature often seen in stable tearing crack growth tests on specimens made of ductile materials and containing through-thickness cracks with initially straight crack fronts. As a specimen is loaded monotonically, the midsection of the crack front will advance first, which will be followed by crack growth along the rest of the crack front, leading to the formation of a thumbnail shaped crack-front profile. From the viewpoint of fracture mechanics, crack tunneling will occur if the operating fracture criterion is met first in the midsection of the crack front, which may be due to a higher fracture driving force and∕or a lower fracture toughness in the midsection. A proper understanding of this fracture behavior is important to the development of a three-dimensional fracture criterion for general stable tearing crack growth in ductile materials. In this paper, the phenomenon of crack tunneling during stable tearing crack growth in a single-edge crack specimen is investigated by considering the effect of stress constraint on the fracture toughness. Crack growth in the specimen under nominally Mode I loading conditions is considered. In this case, crack tunneling occurs while the initially flat crack surface (which is normal to the specimen’s lateral surfaces) evolves into a final slanted fracture surface. A mixed-mode crack tip opening displacement (CTOD) fracture criterion and a custom three-dimensional (3D) fracture simulation code, CRACK3D, are used to analyze the crack tunneling event (but not crack slanting) in the specimen. Results of this investigation suggest that the critical CTOD value (which is the fracture toughness) has a clear dependence on the crack-front stress constraint Am (the constraint measure in this work is the stress triaxiality, which is the ratio of the mean normal stress to the von Mises effective stress). For simplicity, this dependence can be approximated by a straight line within the range of stress constraint values found, with the toughness decreasing as the constraint increases. It is found that crack tunneling in this case is mainly the result of a higher stress constraint (hence a lower fracture toughness) in the midsection of the crack front. Details of the crack growth simulation and other findings of this study will also be presented.

Tensile deformation and fracture behaviors of high purity polycrystalline zinc

The damage and fracture behaviors of high purity polycrystalline zinc with two grain sizes during tension were investigated experimentally at different strain rates. It was found that specimens with coarse grains (∼1 mm) showed serrated flow behavior and failed in intermittent brittle cleavage fracture, while specimens with fine grains (∼70 μm) showed no cleavage crack initiation before necking even at high strain rate. It was observed that the fracture process of the fine-grained specimen was highly related to strain rates. With the strain rate increasing, the damage mechanism transformed from formation of tearing cracks along interfaces (including grain boundaries, twin boundaries and kink band boundaries) and cavity coalescence into abrupt quasi-cleavage fracture. Based on the observation, the inter-crystalline fracture of zinc was investigated, and the damage and fracture behaviors of polycrystalline zinc during tension at room temperature were discussed in general.

Elastic–plastic J and COD estimation schemes for 90° elbow with throughwall circumferential crack at intrados under in-plane opening moment

Leak-before-break (LBB) assessment of primary heat transport piping of nuclear reactors involves detailed fracture assessment of pipes and elbows with postulated throughwall cracks. Fracture assessment requires the calculation of elastic–plastic J-integral and crack opening displacement (COD)1 for these piping components. Analytical estimation schemes to evaluate elastic–plastic J-integral and COD simplify the calculations. These types of estimation schemes are available for pipes with various crack configurations subjected to different types of loading. However, such schemes for elbow (or pipe bend), which is one of the important components for LBB analyses, is very meager. Recently, elastic–plastic J and COD estimation scheme has been developed for throughwall circumferentially cracked elbow subjected to closing bending moment. However, it is well known that the elbow deformation characteristics are distinctly different for closing and opening bending modes because the ovalisation patterns of elbow cross section are different under these two modes. Development of elastic–plastic J and COD estimation scheme for an elbow with throughwall circumferential crack at intrados subjected to opening bending moment forms the objective of the present paper. Experimental validation of proposed J-estimation scheme has been provided by comparing the crack initiation, unstable ductile tearing loads and crack extension at instability with the test data. The COD estimation scheme has been validated by comparing the COD of test data with the predictions of the proposed scheme.

Three-dimensional finite element simulations of mixed-mode stable tearing crack growth experiments

Mixed-mode stable tearing crack growth events in Arcan plate specimens made of aluminum alloy 2024-T3 are simulated using three-dimensional (3D) finite element methods. A modeling/simulation procedure utilizing a mixed-mode CTOD fracture criterion and the custom 3D crack growth simulation software, CRACK3D, with an automatic local re-meshing option is demonstrated. Simulation predictions of the load–crack extension curve and the in-plane curvilinear crack growth path are compared with experimental measurements for various mixed-mode loading cases. Issues such as the effects of near-tip finite element size and crack extension increment size on simulation predictions are investigated.

Application of prestressed transparent composite beams in fracture mechanics

This work presents the mixed-mode I/II/III version of the prestressed end-notched flexure system for the general delamination characterization of composite materials. The novel fracture mechanical configuration combines the traditional mode-I double-cantilever beam and the mode-II end-notched flexure specimens with the mode-III modified split-cantilever beam. First, a steel roller with a given diameter - which should not exceed the critical crack opening - is inserted to the delamination front, this fixes the mode-I part of the total energy release rate. Second, the prestressed specimen is put into the special rig of the MSCB specimen, and with the help of a prestressing screw the mode-III part of the total energy release rate is also fixed. Third, the prestressed specimen is put into a simple three-point bending setup and the mode-II part of the total energy release rate is increased up to fracture initiation. Using this method, i.e. varying the crack opening displacement by the steel roller and the crack te aring displacement by the MSCB rig the fracture surface in the GI-GII-GIII space can be obtained. To demonstrate the applicability and limitations of the novel system experiments on glass-fiber reinforced polyester specimens were performed including separated mode-I, mode-II, mode-III, mixed-mode I/II, II/III, I/III and I/II/III fracture tests. To reduce the measured data a previously validated improved beam theory scheme is applied. Finally, the surface of the fracture criterion is constructed by the generalization of the traditional criterion by Williams.

放電被覆処理による押出ダイスの表面改質と性能評価

The electro-spark deposition (ESD) processing method is a kind of the electric discharge machining method. It is a method of generating not the processing on a liquid inside but the spark discharge in the atmosphere, and stiffening and coating the surface of the metallic substrate with the deposition of the electrode material. In this research, the surface modification on the extrusion die bearing by the ESD processing method is tried, and the performance is evaluated. In addition, it is investigated the correlation between the abrasion of the electrical discharge coating layer and the surface properties of hot-extruded 5056 high strength aluminum alloy. As a result, it has been understood that the ESD processing of the super-hard alloy whose WC, TiC, and Co are principal ingredients is effective in the extension at the tool life and the prevention of the tearing (crack) of the extrusion material.