Crack Arrest Phenomena Research Articles

Influence of Different Types of Small-Size Defects on Propagation of Macro-cracks in Brittle Materials

The presence of defects in the structure requires noticeable attention and understanding of fracture mechanisms in brittle materials has to be established. Defects in the form of holes, macro- and micro-cracks are the main interest of this paper. This work investigates the dual role of holes and micro-crack arrays on toughening and degradation mechanisms in concrete structures. An ordinary state-based peridynamics (PD) model is utilized to analyse the fracture problem at the micro-level. The application of PD shows its advantage in crack-hole, macro- and micro-crack interaction problems since PD can accurately predict the contribution of defects on structural behaviour. The study of the three-point bending problem with five types of holes existing in the structure showed the crack arrest phenomena at the hole boundary and the “attraction” of the crack to propagate towards the hole. For the study of the macro- and micro-cracks interaction problem, various cases of the micro-crack distribution and inclination angles are considered and validated with analytical studies. The PD quasi-static simulations show good agreement with analytical solutions. Moreover, PD dynamic solutions show the capability of PD to capture complex crack propagation paths. It is observed that the presence of micro-cracks and holes ahead of the main crack can suppress its further propagation as well as have an influence on the crack propagation direction. The numerical results demonstrate the efficiency of the PD modelling of multiple crack interaction problems.

Dynamic crack arrest analysis by ordinary state-based peridynamics

Dynamic fracture analysis for the crack arrest phenomenon is performed by ordinary state-based peridynamics formulation and discretization employing transition bond concept. Double cantilever beam specimen is chosen for our numerical evidence purpose. The analysis consists of two main phases namely, generation and application (prediction) phases. In the generation phase, the dynamic stress intensity factors of propagating and arrested cracks are estimated by the present formulation for given crack path histories, and a good agreement is achieved. As for the application phase, dynamic stress intensity factors as well as total crack lengths after crack arrests are in good agreement with the experiments. Moreover, the influence of transition bond concept on the crack arrest behavior is investigated and it is found that the transition bond is very efficient in the simulation of the crack arrest problem such that premature arrests of cracks are observed without transition bond cases.

A multi-scale strategy to predict fretting-fatigue endurance of overhead conductors

Abstract Overhead conductors enduring aeolian vibrations are subjected to fretting fatigue damage. To predict the fretting fatigue cracking risk along the conductor-clamp assembly involving a multitude of crossed-strands contacts, a multi-scale analysis is considered. First, a global model provides the spatial distribution of normal, tangential and fatigue loadings for every contact. “Specific” loading cases were simulated using a local FEM model involving a Crossland fatigue stress analysis and a critical distance approach to correct the stress-gradient effects. Mono-contact fretting fatigue experiments were then performed reproducing the exact same loading cases. The comparison between numerical predictions and experimental results showed that a very good prediction of crack-nucleation is achieved using this strategy. However, total failures were not systematically observed. It is believed to be related to the manufacturing process. Wire drawing induces a longitudinal microstructure which promotes cracks with a low diving angle, rather than cracks propagating directly into the bulk. 3D-surface-crack simulations confirm a crack arrest phenomenon induced by the contact-induced compressive stress field.

Determining role of microstructure on crack arrest and propagation phenomenon in low-carbon microalloyed steel

The relationship between microstructure and crack arrest characteristics of a microalloyed steel with a yield strength of 460 MPa was elucidated in this study with particular focus on the effect of the microstructure on the propagation of brittle cracks. The study emphasized that the acicular ferrite (AF) content could be increased to 41% by lowering the final rolling temperature and final cooling temperature, which reduces the average effective grain size (EGS) to as small as ~4.1 μm and increases the high-angle grain boundaries (HAGB) proportion to as high as ~48.8%. The presence of AF in large numbers could reduce the ductile-to-brittle transition temperature to approximately −87 °C through refinement of the microstructure. The presence of HAGB between adjacent grains could significantly arrest cracks by changing the propagation direction of brittle cracks and was a decisive factor in determining the characteristics of brittle fracture. Moreover, the fracture roughness varied directly with the proportion of the HAGB and inversely with the average EGS. Martensite/austenite islands with low content (0.92%) and small size (~1.5 μm) reduced the stress concentration significantly at the crack tip. The excellent crack arrestability was attributed to AF, which minimized the expansion of cleavage facets by the split nail effect ahead of the growth direction, so that the cleavage facets were divided into two or more branches, considerably reducing the stress concentration and retarding brittle crack growth during crack propagation at low ambient temperature.

Crack propagation and arrest modeling in peridynamics

Dynamic fracture and crack propagation phenomena are unstable, and especially brittle fracture of relatively thick plates in ship structures for large container ships has been remarked recently. Numerical simulation is convenient and powerful method for designing and evaluating the safety of structures. The recently developed ordinary state-based peridynamics (OSPD) is further enhanced to study elastodynamic propagating crack based on the dynamic stress intensity factors (DSIFs). Dynamic fracture analysis for simulating crack arrest phenomenon is performed by OSPD is discretization employing transition bond concept. Double-cantilever-beam specimen is chosen as the numerical model. In application phase, dynamic stress intensity factors as well as total crack lengths after crack arrests are in good agreement with the experiments and reference solution.

Study of the effect of loading rates on crack propagation velocity and rock fracture toughness using cracked tunnel specimens

In the process of tunnel blasting excavation, radial cracks could be induced. To investigate the effect of loading rates on crack propagation velocity and rock initiation toughness, a new cracked tunnel specimen was proposed in this paper, and drop weight impact experiments were conducted. The crack propagation velocity and crack initiation time were measured by using crack propagation gauges (CPGs), and the measuring results were applied in the determination of initiation toughness. In order to validate the effectiveness of the cracked tunnel specimens and to predict the test results, finite difference numerical models were established by using AUTODYN code. The simulation results of crack propagation speeds and delayed fracture time generally agree with the experimental results, and crack arrest phenomena occur during crack propagation. The dynamic stress intensity factors (SIFs) were calculated by using ABAQUS code, and the initiation toughness under different loading rates was obtained by using experimental-numerical method. The studying results show that crack propagation speeds and initiation toughness increase with loading rates, but as the loading rate is larger than a certain value, the crack speeds tend toward a stable value; Delayed fracture time decreases with the increase of loading rates, and as the loading rate is larger than a certain value, it tends toward a stable value.

Study of mode I crack dynamic propagation behaviour and rock dynamic fracture toughness by using SCT specimens

AbstractTo study crack dynamic propagation behaviour and rock dynamic fracture toughness, a single cleavage triangle (SCT) specimen was proposed in this paper. By using these specimens and a drop‐weight test system, impact experiments were conducted, and the crack propagation velocity and the fracture time were measured by using crack propagation gauges. To examine the effectiveness of the SCT specimen and to predict the test results, finite difference numerical models were established by using AUTODYN code, and the simulation results showed that the crack propagation path agrees with the test results, and crack arrest phenomena could happen. Meanwhile, by using these numerical models, the crack dynamic propagation mechanism was investigated. Finite element code ABAQUS was applied in the calculation of crack dynamic stress intensity factors (SIFs) based on specimen dimension and the loading curves measured, and the curves of crack dynamic SIFs versus time were obtained. The fracture toughness (including initiation toughness and propagation toughness) was determined according to the fracture time and crack speeds measured by crack propagation gauges. The results show that the SCT specimen is applicable to the study of crack dynamic propagation behaviour and fracture toughness, and in the process of crack propagation, the propagation toughness decreases with crack propagation velocity, and the crack arrest phenomena could happen. The critical SIF of an arrest crack (or arrest toughness) was higher than the crack propagation toughness but was lower than the initiation toughness.

Damage evaluation of laminated composite material using a new acoustic emission Lamb-based and finite element techniques

In this paper, a very promising procedure is proposed to evaluate delamination using Acoustic Emission (AE) technique in composite laminates. First, a new procedure was developed to decompose the fundamental Lamb wave modes in small size specimens. The damage mechanisms in End Notched Flexure (ENF) in woven and unidirectional specimens were then discriminated using Fuzzy Clustering Method (FCM). Afterwards, the crack-arrest phenomenon was examined in each specimen. After that, experimental and Cohesive Zone Modeling (CZM) techniques were conducted to characterize the delamination using ENF specimens. The results showed how, it is possible to successfully decrease the effect of propagating media such as attenuation of AE signals using the new proposed methodology. As a final point, the results of this study could lead to efficiently distinguishing different damages in laminated composite using AE Lamb-based technique.

Investigation of softwood fracture criteria at the mesoscopic scale

In order to improve the knowledge of softwood mechanical behaviour and particularly fracture mechanisms, investigation at small scales is needed. For that purpose, new numerical tools based on the real wood morphology in the transverse plane are specifically used. A mode I fracture criterion applicable to cracks oriented in the transverse plane in softwood is then studied at the mesoscopic scale, i.e. the annual ring scale. Numerical investigation combined with digital image correlation help to obtain wood fracture parameters in the annual ring. These parameters give additional knowledge and understanding of wood fracture according to local specificities such as crack tip position in an annual ring and notch orientation in relation to natural wood orientation and loading orientation. Mesoscopic fracture criteria can help our understanding of secondary crack appearance and crack arrest phenomena. Mixed mode study and coupling between mode I and mode II of fracture will be investigated further. Mixed mode fracture criteria can then help to better understand crack bifurcation phenomenon.

Characterization of Crack Arrest Phenomena in a Modified 9Cr-1Mo Steel

Abstract Crack arrest toughness (K1A) is a measure of the material’s ability to stop a propagating cleavage crack. However, experimental evaluation of K1A from impact tests of small size specimens is often difficult due to the uncertainties involved in the determination of the two necessary parameters, namely, the crack arrest load (Parrest) and the crack arrest length (aarrest). In this study with Mod.9Cr-1Mo steel in normalized and tempered and in subsequent cold worked conditions, these two parameters and thus the K1A have been determined from instrumented impact tests carried out in the ductile-brittle transition temperature regime. While the Parrest has been directly determined from the load-displacement traces, the aarrest was inferred using an analytical technique, namely, the “key curve” method. The crack arrest phenomenon has been attributed to the cumulative effect of the blocking of the cleavage crack at the microstructural boundaries with high degree of misorientations as identified by scanning electron microscope and electron back scattered diffraction studies. The effect of cold work on the K1A has been found to be insignificant.

Dynamic modeling of cleavage crack propagation and arrest with a local approach

The assessment of reactor pressure vessels, usually based on crack initiation, could be completed using the crack arrest concept. The methodology classically recommended in safety codes and standards does not take into account the dynamic effects which are very important in crack arrest phenomena. To overcome this difficulty, a local crack arrest criterion has been recently proposed by some of the authors, for cleavage crack propagating in a low alloy bainitic steel: the crack propagates since the largest principal stress reaches a critical stress that depends on temperature. This study deals with the modeling of crack arrest experiments performed on A533-C1.B type steel with this criterion. Crack arrest was obtained on CT under isothermal conditions at –150°C and –125°C. The crack arrest criterion was identified with 2D computations. The application of this criterion shows that the crack propagation versus time leads to a good prediction of the crack speed and of the crack length.

Monotonic and cyclic short beam shear response of 3D woven composites

Monotonic, multi-step and cyclic short beam shear tests were conducted on 2D and 3D woven composites. The test results were used to determine the effect of z-yarns on the inter-laminar shear strength as well as the multi-loading behavior. The presence of z-yarns was found to affect not only the inter-laminar shear strength of the composite but also the behavior of the composite beyond the elastic limit. Microscopic examination of the damaged specimens revealed large delamination cracks in 2D woven composites while delamination cracks were hindered by z-yarns in 3D composites. This crack arrest phenomena resulted in a reduction in inter-laminar crack lengths and a higher distribution of the micro-cracks throughout the 3D composite. The multi-step and cyclic loading tests are found to be useful in the monitoring of specimen behavior during short beam shear testing. The induced damage was quantified in terms of the loss of strength and stiffness during each loading cycle. It was found that while the 2D composites have higher damage resistance, the 3D composites have a higher damage tolerance.

Dynamic propagation phenomena of multiple delaminations in composite structures

Abstract A general model is proposed to predict dynamic propagation phenomena of interfacial cracks in laminated composite structures subjected to multiple delaminations. The model is based on a combined approach developed in the framework of Fracture Mechanics and moving mesh methodology. The former is utilized to predict the crack growth, whereas the latter defines the way to take into account the geometry changes on the basis of the invoked fracture parameters. Consistent with Fracture Mechanics, crack propagation depends on the energy release rate and its mode components, which are calculated by means of the decomposition methodology of the J-integral expression. The geometry variation, produced by the crack advance, is taken into account by means of a moving mesh strategy based on the Arbitrary Lagrangian–Eulerian (ALE) formulation. The crack tip motion is governed by prescribed speeds, which are expressed as function of a mixed mode crack criterion based on the energy release rate and its mode ratio. The coupling characters of the governing equations of the proposed model arising from the Fracture Mechanics and the moving mesh methodology are discussed. Comparisons with experimental results are reported to validate the proposed modeling. Moreover, a parametric study for single and multiple delamination problems, is developed to investigate some features regarding the crack tip behavior, such as dependence of the solution on the loading rates, the laminate geometry and the crack arrest phenomena.

Stress intensity solutions of thermal fatigue induced cracks in a thin plate hot spot using LEFM and finite element analysis

The chief aim of this research is to marry elegant LEFM methods, published material data, and a commercially available FEA code together to represent complex crack growth and arrest phenomena in a thin plate with residual viscoplastic stresses. Additionally, representing the variation in these stress intensities across a thermal gradient to include ultimate fatigue crack arrest is of specific interest. To the authors’ knowledge, modeling fracture mechanics due to these hot spots with viscoplastic residual stresses using LEFM methods on this type structure has not been achieved. With these stress intensities, predictions are to be made regarding not only the FCG rate but the probable radial arrest distance of a nominal crack from the hot spot center.

Nonpropagating incipient cracks from sharp notches under fatigue

Growing incipient cracks from sharp notches can stop completely even when the remote applied stress amplitude remains constant. As the crack lengthens, the crack-tip driving force, the stress intensity factor, should increase for a constant remote applied stress, yet, arrest occurs. For this reason, the fatigue stress concentration factor K f differs from the conventional stress concentration factor K t for sharp notches. The arrest has been attributed to increased crack growth resistance as an incipient crack lengthens. The increased resistance has been related to plasticity-induced crack closure, which builds up with crack-wake plasticity during crack growth. In this paper we present a different concept based on our unified approach that considers the existence of two fatigue thresholds rather than one. It is shown that nonpropagation of incipient crack occurs when the total stress intensity factor falls below the long crack growth threshold value in terms of K max. Total stress intensity results from both the remote applied stress and the local internal stress. Even when the remote applied load is constant, the local internal stress can decrease sharply as the incipient crack grows away from the notch tip. Notch-tip plasticity contributes to the changes in the internal stresses. Using extensive data from the literature, it is shown that it is the decreasing crack-tip driving force with length that causes the crack arrest phenomenon. Crack growth under tension–tension, tension–compression, and compression–compression fatigue are all accounted for on the basis of gradient in the internal stresses.

Durability assessment of composite repairs bonded to aircraft structures

The durability or long-term performance of composite repairs of aircraft skin grade aluminum with simulated flaws was evaluated. Mechanical tension/tension fatigue and thermal cyclic aging were used, in addition to monotonic tensile loading, to assess the durability of the repairs. Thermal wave imaging was employed to inspect the initial quality of the repairs and to quantify the size of the interfacial disbond associated with the mechanical fatigue and the combined mechanical fatigue and thermal cyclic aging. Boron/epoxy patches with different numbers of plies were installed on aluminum substrates with simulated damage in the form of side-cracks with stop-drill holes. It was found that a patch with more than 4 plies resulted in the residual strength of the pre-cracked substrate to that of the uncracked aluminum. A considerable increase in the fatigue lifetime and a decrease in the crack growth rate were also observed as the number of plies increased. Both the energy release rate and the hysteresis (damage) energy for substrates repaired with 4- and 6-ply patches showed a slight decrease, indicative of the crack arrest nature of the boron/epoxy patches. This crack arrest phenomenon was captured using thermal wave imaging. The size of the interfacial disbond was found to taper down as the crack propagated from the stop-drill holes. Thermal cyclic aging according to ASTM D 1183-70 reduced the fatigue lifetime of the repaired structures due to the deterioration of the interface. Thermal wave imaging also showed that the pre-cracked structures repaired with 4-ply patches exhibited more disbond than those with 6-ply patches after being exposed to the same number of thermal cycles and fatigue-tested until complete separation of the aluminum substrate. Boron/epoxy repairs with 6 plies proved to be very effective in retarding multi-site damage emanating from rivet holes. More than an order of magnitude increase in the fatigue lifetime between the unrepaired and repaired structures was found. Thermal wave imaging also showed clearly that the propagation of multi-site damage from rivet holes was associated with interfacial disbond between the boron/ epoxy doublers and the aluminum substrate. The integrity of the boron/epoxy doubler installed over rivets was evaluated using thermal wave imaging. It was found that the doubler was intact after exposing the joint to severe fatigue loading which resulted in failure of the substrates, rather than disbond or delamination of the repair.

Fatigue crack growth at arbitrary angles to bimaterial interfaces

The fracture characteristics of monotonically loaded cracks located parallel to, in the vicinity of, and at arbitrary angles to the interface between two elastic solids have the subjects of considerable theoretical research in the literature. The present experimental work on steel-steel bimaterials clearly demonstrates that, for essentially the same elastic properties, the plastic mismatch between the constituent layers of bilayer composite can dictate whether a fatigue crack approaching the interface at various angles continues to advance through the interface or arrests prior to penetrating the interface. The former crack arrest phenomenon occurs when the fatigue crack approaches the interface from the plastically weaker material, whereas continued crack growth results when the crack approaches the interface from the plastically stronger material. The observed experimental trends are qualitatively consistent with the numerical predictions of crack-tip shielding or amplification from the interaction of the near-tip plastic zone with an interface oriented normal to the plane of the crack. When the fatigue crack is initiated parallel to the interface, it deviates from the strongly bonded interface to advance in one of the two steels. This trend appears to experimentally confirm predictions of the possible evolution of strong mode mixity ahead of interface cracks subjectedmore » to cyclic tensile loading.« less

Observation of individual bond breaking events in precursors, cascades, etc. in the onset and progression of fracture

Experimental measurements of fracture have been made using a wide bandwidth (10 MHz) transducer and a system with a relatively large bond size, with the result that one is able to observe individual bond breaking events. The data provide a direct test of ‘‘fuse’’ models [L. de Arcangelis, S. Redner, and H. J. Herrmann, J. Phys. Lett. (Paris) 46, L585 (1985)] and scaling theories of brittle fracture. Measurements have been made with uniform tensile stress and bending stress, with the latter corresponding to a phase transition in the presence of an external field. For samples of the same width, the observations are quite consistent, showing the effects of precursors, onset, progression, stress waves, crack arrest, and other interesting phenomena. A significant difference in precursors for the uniform and bending stress fields is observed, perhaps a result of the dimensionality of the maximum stress field. [Work supported by NSF Grant No. DMR-9306791 and by ONR.]

A K1 Calibration for a Modified Single Edge Notched Tension Specimen

Abstract Using global and boundary collocation techniques, a single edge notched tension (SE(T)) specimen, modified by introduction of a semi-circular cutout in front of the crack, was developed and characterized. This specimen was found to produce fixed-grip K1 values two times greater than is possible with a conventional SE(T) specimen of the same size. This new specimen could be used to investigate upper transition crack arrest phenomena with smaller specimens and testing machine capacities than have been possible previously.

Crack Propagation Modes in Particulates: A Study Approaching Reality

Cracked composite plates with spherical glass inclusions bonded inside a flexible matrix made of an epoxy polymer were subjected to impact. The effect of the inclusions on an edge crack propagating inside the plate was investigated, as well as the interaction of two skew-parallel edge cracks propagating in the same composite plates. It was found that gen erally the inclusions repel the cracks in both cases studied, and they are responsible for the bifurcations of the propagating cracks. A crack arrest phenomenon also appeared at the neighborhood of inclusions, which was followed by a subsequent emission backwards of Rayleigh waves. Moreover, the skew-parallel cracks always attracted each other, although the inclusions provoked the cracks to deviate from their initial paths. For the study of crack propagation inside these composite plates the optical method of caustics was used for the evaluation of the crack velocities, as well as the instantaneous values of the dyanmic stress-intensity factor Kr(t) and K rr(t) at the crack tips. Interesting results were derived concerning the variation of the velocities and SIFs of the cracks inside particulate- filled plates.