Crack Jumps Research Articles

A machine learning model for predicting progressive crack extension based on experimental data obtained using DCPD measurement technique

A series of low cycle fatigue loading experiments were performed, using Inconel 718 (IN718) superalloy, to produce time history data using Direct Current Potential Drop (DCPD) approach at a range of temperatures and it was used to train a Bidirectional Long-Short Term Memory Neural Network (BiLSTM) model. The experimental investigations were conducted to understand the statistical nature of crack jumps during the fatigue loading. The BiLSTM model was trained on high sampling rate experimental data from crack initiation up through the Paris regime. For a single sample, the trained machine learning model was able to predict future crack events based on prior crack jumps in the time history. The model was able to predict the progressive crack extension at intermediate temperatures and stress intensities that lie between experimental conditions. The BiLSTM model demonstrated the potential to be used as a tool for future investigation into fundamental mechanisms such as high-temperature oxidation at the crack tips.

Evaluation of the interlaminar fracture toughness on composite materials using DCB test on symmetric and unsymmetric configurations

A procedure to measure the interlaminar fracture toughness of a composite material is proposed in this work. This property is evaluated using the DCB test, the new formulation developed being applicable to small thicknesses and nonsymmetric configurations. The GC evaluation proposed in this paper does not require to know the evolution of the crack length with the load. This fact facilitates the performance of the test and allows complex situations (crack jumps, tests at high temperature,…) to be taken under consideration. An experimental campaign has been carried out on four different configurations, three symmetric (2 + 2, 4 + 4 and 8 + 8 layers) and one nonsymmetric (4 + 8 layers). The results show that, when they are appropriately treated, the values obtained are independent of the number of layers of the adherents and even of the symmetry of the joint, validating the use of the DCB test on nonstandardized coupons.

Radiation response of the overlay cladding from the decommissioned WWER-440 Greifswald Unit 4 reactor pressure vessel

The Results of tensile and crack extension testing conducted on irradiated austenitic overlay cladding material are presented. The specimens were machined from three trepans sampled from the decommissioned WWER-440/V-230 reactor pressure vessel of the nuclear power plant Greifswald Unit 4. Crack extension curves were measured with Charpy size SE(B) specimens using the unloading compliance technique according to ASTM E1820-11 at different temperatures. Provisional crack initiation fracture toughness values JQ and KJQ are determined. The highest KJQ values were found in the temperature range from 20 to 80°C. A significant scatter was observed in the initiation values. This is due to the inhomogeneous structure of the welded overlay cladding. During the loading the crack moves through regions with different tearing strength, thus the J–Δa values show discontinuities caused by fast crack propagation or crack jumps. The comparison of the measured KJQ values and conservatively estimated stress intensity factors at an assumed surface crack shows that the cladding would remain intact during pressurized thermal shock transient.

ATOMISTIC VIEWS OF DYNAMICAL FRACTURE INSTABILITIES IN SILICON: MOLECULAR DYNAMICS STUDIES

This study investigates the crystallographic orientations most widely known to exhibit fractures in silicon, such as those on the (111) plane cracks travelling along the [Formula: see text] direction ([Formula: see text] cracks). The (111) crack plane is believed to be the most stable fracture plane. However, fracture instabilities caused by brittle crack jumps remain on (111) the crack plane in a discontinuous manner. In this study, molecular dynamics simulations were performed to investigate the atomistic-level studies of fracture properties under a uniaxial tensile load (mode I load) in the (111) [Formula: see text] Si system. Our simulation results suggest that the formation of untypical-membered Si atomic rings in the vicinity of the crack tip, which can be induced by atomic stress near the crack tip, has an important role in the behavior of crack propagation instabilities. The presence of untypical-membered Si atomic rings acts as a self-protecting mechanism that contribute in maintaining the crack on the (111) fracture plane. Notably, our simulations also presented that the situation when a seven- Si atomic ring moves away from the crack tip associated with a sudden jump of crack speed can be regarded as the origin of the peculiar speed advancement behavior of the [Formula: see text] systems. Moreover, several of our simulation results are in agreement with related experimental measurements.

On mechanism of fatigue fracture in interstitial free steel sheet

The mechanism of fatigue fracture in an interstitial free steel sheet has been studied. The process can be divided into four regimes:(i) the fatigue crack initiates on the specimen surface, from the mesocracks along the grain boundaries in stage I(ii) propagates mostly in an opening mode through grain boundaries in stage II(iii) propagates through microscopic striations and transverse intergranular cracking briefly in stage III(iv) the crack path changes from flat to slant along with through thickness necking and it propagates to failure through discrete crack jumps in stage IV. The crack jumps are associated with crack progression marks (CPMs), the spacing of which increases exponentially from few micrometres to few hundred micrometres with crack length.

Subcritical crack growth law and its consequences for lifetime statistics and size effect of quasibrittle structures

For brittle failures, the probability distribution of structural strength and lifetime are known to be Weibullian, in which case the knowledge of the mean and standard deviation suffices to determine the loading or time corresponding to a tolerable failure probability such as 10−6. Unfortunately, this is not so for quasibrittle structures, characterized by material inhomogeneities that are not negligible compared with the structure size (as is typical, e.g. for concrete, fibre composites, tough ceramics, rocks and sea ice). For such structures, the distribution of structural strength was shown to vary from almost Gaussian to Weibullian as a function of structure size (and also shape). Here we predict the size dependence of the distribution type for the lifetime of quasibrittle structures. To derive the lifetime statistics from the strength statistics, the subcritical crack growth law is requisite. This empirical law is shown to be justified by fracture mechanics of random crack jumps in the atomic lattice and the condition of equality of the energy dissipation rates calculated on the nano-scale and the macro-scale. The size effect on the lifetime is found to be much stronger than that on the structural strength. The theory is shown to match the experimentally observed systematic deviations of lifetime histograms from the Weibull distribution.

Slow crack growth: Models and experiments

.The properties of slow crack growth in brittle materials are analyzed both theoretically and experimentally. We propose a model based on a thermally activated rupture process. Considering a 2D spring network submitted to an external load and to thermal noise, we show that a preexisting crack in the network may slowly grow because of stress fluctuations. An analytical solution is found for the evolution of the crack length as a function of time, the time to rupture and the statistics of the crack jumps. These theoretical predictions are verified by studying experimentally the subcritical growth of a single crack in thin sheets of paper. A good agreement between the theoretical predictions and the experimental results is found. In particular, our model suggests that the statistical stress fluctuations trigger rupture events at a nanometric scale corresponding to the diameter of cellulose microfibrils.

Acoustic Emissions in Fracturing Sea Ice Plate Simulated by Particle System

In-plane dynamic fracture jumps and acoustic emissions in sea ice floes of different sizes are simulated by the particle system. Fracture patterns and time sequences similar to those observed in experiments are achieved. For the given spatial location of hydrophone, the acoustic signals from crack jumps in ice are calculated in the frequency domain by a modified Farmer and Xie's acoustic model for ice plate floating in shallow sea. The acoustic pressure-time histories are synthesized by the Fourier inverse transform. The calculated acoustic signals resemble the recorded signals. Their overall character is found to depend on the plate size. This is a size effect that is manifested in the calculated root-mean-square history of the acoustic pressure at hydrophone. Differences among the acoustic records for different fracture lengths are found. They offer the possibility of making inferences on the fracture characteristics of sea ice from the acoustic records.

Scanning Force Microscopy as a Tool for Fracture Studies

AbstractDynamic simulations of the fracture toughness as a function of the orientation and tem- perature were carried out and compared with experimental results obtained by in-situ loading pre-cracked NiAl single crystals inside a scanning force microscope (SFM). In order to compare the simulations with the experiments the problem of the short crack with dislocations was solved for general loading and arbitrary slip line directions. The stress and strain field obtained could be directly connected to FEM calculations which allowed the examination of the stability of micro cracks at notches. The effect of different fracture conditions for biaxial loading were studied in detail.The dynamic simulation yielded predictions of K1C, slip line length and dislocation distri- butions as a function of loading rate, temperature and orientation. These predictions were tested by in-situ loading NiAl single crystals inside a SFM at various temperatures. The local COD, slip line length and apparent dislocation distribution at the surface were measured as a function of the applied load and the temperature. The experiments clearly demonstrated that dislocations emit from the crack tip before unstable crack jumps occur. The local COD could be directly related to the number of dislocations emitted from the crack tip. With increasing temperature the number of dislocations and the local COD increased before unstable crack jumps or final fracture occurred.

Delamination Failure of a Single Yarn Glass Fiber Composite

When the constraint of the fill yarns was removed from a woven fiber composite, delamination followed an intrayarn pathway. Despite extensive fiber-matrix debonding during crack growth, resin toughness translated directly to composite toughness. This suggests that damage from intrayarn cracking contributes significantly to toughness of woven fiber composites even though the crack is expected to follow an interply pathway. Effects of fiber undulation were isolated from the effects of yarn interlacing by studying specimens a single yarn in width. In the double cantilever beam (DCB) test, intrayarn crack growth caused the crack to follow the undulating pathway of the fibers. The orientation of the crack changed as the crack moved along the undulating fibers; thus, the extent of mode II loading relative to mode I loading increased as the fibers deviated from the plane normal to the loading direction. Because composite toughness increases with the degree of mode II loading, the variation in toughness induced a characteristic stop-start crack growth mechanism. The toughness of the matrix was varied by changing the soft segment content of the urethane-methacrylate resin. Soft segment content affected the crack initiation toughness more than crack arrest toughness. Consequently, longer crack jumps were observed with the tougher resins.

Method of accelerated evaluation of Klscc under stress corrosion cracking

The method of accelerated evaluation of the threshold value of stress intensity factor K lscc in materials under stress corrosion cracking by registration of the crack jumps is proposed. It is based on the model of subcritical jump-like propagation of mode 1 crack as a source of acoustic signals, when material is subjected to the corrosive environment and durable static loading simultaneous effect. The method was experimentally verified during evaluation of K lscc of 9HF steel in NaCl water solution and gaseous hydrogen using the up-to-date equipment for recording the acoustic signals.

Interlaminar and intralaminar fracture modes in 0/90 cross-ply glass/epoxy laminate

Delamination of a cross-ply 0/90 glass fibre-reinforced composite laminate with an epoxy-phenol matrix was studied using a double cantilever beam test. Fracture toughness was determined by measurement of bend angle of the cantilever beams. Results obtained with this method were in agreement with those from conventional compliance and area methods. Two different fracture modes were observed: interlaminar and intralaminar. In the interlaminar fracture mode, crack jumps in the space between two neighbouring 0° and 90° plies were observed. With the interlaminar fracture mode, during crack initiation G Ic decreased with crack length. Intralaminar fracture mode consisted of the gradual growth of a crack through a 0° ply. Fibres bridging the opposite sides of the crack were observed in this case, and fracture toughness G Ic did not change with crack length. G Ic (420 J m −2) at intralaminar fracture mode was approximately twice that at interlaminar fracture mode (220 J m −2). The difference in fracture toughness was explained by the dissipation of energy by fibres bridging the opposite sides of the crack at intralaminar fracture mode.

UNSTABLE FATIGUE CRACK PROPAGATION AND FATIGUE FRACTURE TOUGHNESS OF STEELS

AbstractThis paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high‐pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions.It has been shown that steels in an embrittled state, caused primarily by thermal treatment and low‐temperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature.An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour.

Analysis of Brittle Intergranular Crack Propagation During Stress Corrosion

AbstractCrack growth rates during intergranular stress corrosion, in materials such as Fe in Ca(NO3)2 at +750 mV (SCE), appear to exceed rates which can occur by dissolution and ion transport within the crack. Brittle crack jumps induced by a corrosion or stress corrosion process, and which magnify the dissolution supported crack velocity by a factor of up to 103, could explain this behavior. Hydrogen can induce these very fast crack growth rates, but these rates have been obtained at electrochemical potentials where H reduction is not thermodynamically possible. Other possible mechanisms for this accelerated crack growth include: 1) intergranularcorrosion altered tearing modulus, 2) grain boundary dealloying, or 3) adsorption induced brittle fracture. Examination of each of these mechanisms suggests that intergranular corrosion could reduce the tearing modulus through changes in the crack shape. However, crack opening analysis clearly shows that the crack tip is shielded from the applied stress intensity and that an embrittlement process which reduces the interface toughness is need for brittle intergranular crack jumps to occur. Further research is in progress to identify the source of this embrittlement.

Acoustic emission from integranular subcritical crack growth

Acoustic emission (AE) has been measured during subcritical crack growth of Fe, Ni, 304 SS, a tool steel, and an Fe−Si single crystal. Tests were conducted using compact tension type samples in a variety of environments. Iron with two levels of S and P grain boundary concentrations was tested in Ca(NO3)2 at +750mV; Ni with P segregation was tested in 1N H2SO4 at +900 mV; two heats of 304 SS were tested in Na2S2O3 at open circuit potentials; Fe and Ni with S-enriched grain boundaries were tested in 1N H2SO4 at cathodic potentials; and RDS tool steel and an [001] oriented Fe−Si single crystal were tested in gaseous hydrogen at a pressure of 1 atm. The number of AE events/mm2 of crack extension increased with an increase in transgranular fracture for all of these test conditions with the exception of the Fe−Si single crystal. In this case, crack extension was 100 pct transgranular, but the number of events/mm2 was negligible. It was concluded that the short, discontinuous crack jumps were not detectable with the AE monitoring equipment. Likewise, for the intergranular subcritical crack growth, it was concluded that crack extension was either anodic dissolution with no accompanying AE or short, discontinuous crack jumps with nondetectable AE. It also was postulated that the transgranular fracture surfaces accompanying the intergranular subcritical crack growth resulted from ligaments which fractured behind the advancing crack front.

The fractography of stress corrosion cracking of 310 stainless steel: Crystallographic aspects and the influence of the stress intensity factor

The microfractography of transgranular stress corrosion cracking (TSCC) of annealed 310 stainless steel tested in boiling MgCl 2 solution was studied in relation to the behaviour of crack velocity as a function of the stress intensity factor K. The observations indicated that TSCC occurred by a series of discontinuous cleavage-like crack jumps of approximately 0.3 μm in the near-threshold region and 1.0 μm in stage II of the crack velocity curve. Cracking occurred predominantly on {100} planes, with a preferred 〈110〉 crack propagation direction. Striation-like markings were often present perpendicular to the 〈110〉 crack propagation direction. The manner in which some of these markings, which correspond to slip traces, interact with river lines also clearly indicates correspondence to crack arrest sites. Fan-shaped patterns of river lines were found predominantly at intermediate K values and their presence is interpreted with respect to the relative ease of crack propagation across grain boundaries compared with that in the newly entered grains. The TSCC crystallography identified here and in previous studies on face-centred cubic (f.c.c.) metals is discussed with respect to different propagation mechanisms which have been proposed to account for crystallographic TSCC and with respect to the probable arrest mechanism for the crack jumps. The crystallographic orientations and fractographic features produced by these crack jumps are shown to be consistent particularly with cracking produced by dislocation emission in the immediate vicinity of the propagating crack tip.

Interference optical measurements during continuous and discontinuous crack propagation in fatigue loaded polymers

Continuous and discontinuous fatigue crack propagation behaviour have been investigated in PMMA and PVC, respectively, using the optical interference method. Thus craze zone sizes at the crack tip have been measured as functions of cycle number and relative stress intensity factor during crack propagation and between crack jumps. The results obtained give new and improved information on the material behaviour in the micro-region at the crack tip and also lead to modifications of a previously proposed model on crack and craze propagation behaviour during fatigue.

Crack growth for the double slip plane crack model-I. The R-curve

The double slip plane (DSP) crack model of Weertman, Lin and Thomson has been used to obtain crack growth equations for the mode II or III crack under a monotonically increasing stress (the R-curve) in this paper. [In a companion paper the growth under cyclic stress (the Paris fatigue crack growth equation) is determined.] The success of the analysis depends upon the fact that the DSP crack closely approximates a Bilby-Cottrell-Swinden (BCS) crack when the slip zone is large compared with the slip plane to crack plane spacing. Consequently the dislocation distribution on the slip planes approximates the BCS crack one ahead of the crack tip. Behind the crack tip a result fortuitous for the analysis is found that the gradient of the dislocation density on a slip plane is proportional to the shear stress on the slip plane. The results obtained are: if the friction stress on a slip plane is constant the crack never propagates catastrophically. Instead crack extension occurs which is proportional to K 2 where rK is the stress intensity factor. Were the surface energy of a solid to be suddenly reduced, as it might be by the sudden introduction of an active environment, the distance the tip of a stressed stationary crack jumps is proportional to K. The distance jumped, for a large drop in surface energy, is smaller than the crack advance that occurs if the active environment were always present and the crack is monotonically loaded to the same value of K. If work hardening of the friction stress takes place stable crack growth takes place up to a critical K c value. The R-curve equation is given by an integral which is simple in form but requires a numerical integration or series expansion. The critical K c value is proportional to the critical K cb stress intensity factor of a Griffith crack raised to the power ( m + 1 )/2 m where m is the power exponent of the simulated plastic stress-strain curve. We believe that this paper demonstrates the double slip plane crack model is the first crack model since the Griffith crack model and its variants that can give an explicit fracture equation starting from first principles.

Fracture toughness of structural alloys under cyclic loading. Communication 1

1. The fracture toughness characteristics under cyclic loading (Kfc1 and Kfck for cyclically softening steels are considerably lower than the fracture toughness characteristic under static loading (KIc). the characteristic Kfck is close to the fracture toughness characteristic for dynamic loading (KDc). No significant difference is seen between KIc and Kfc1=Kfck, for cyclically stable and cyclically hardening materials. 2. The regime of cyclic loading — and especially the initial stress intensity factor Ko-has a substantial effect on the value of the stress intensity factor corresponding to the first crack jump. 3. Using a generalization of empirical data as a basis, we proposed a model of unstable crack propagation during cyclic loading which takes into account basic empirically established laws of this process and permits determination of the ratios of the quantities KIc, KDc, and Kfck for different classes of materials and loading regimes. We also proposed basic relations connecting the size of the fatigue-damage region, the number of load cycles until a crack jump, the fracture toughness Kfci, and the sizes of the crack jumps.

Fracture toughness of structural alloys under cyclic loading. Communication 2

1. We experimentally confirmed the need to allow for the change in the parameters of the equation of state of structural alloys during cyclic loading (primarily the change in σplc) in evaluating fracture toughness characteristics for cyclic loading. 2. A correlation was established between the size of the fatigue-damage zone calculated from the proposed relation and values of σplc and crack jump size found for the investigated materials. 3. We established the dependence of equation of state characteristics σo.2c and σplc and fracture toughness characteristic Kfc1 for cyclic loading on the value of the ratio σu/σo.2 for structural steels. This dependence, in the absence of direct empirical data on the effect of load cyclicity on equation-of-state parameters, makes it possible to evaluate σo.2c, σplc, and Kfc1 for prescribed cycling regimes from mechanical characteristics of the steels determined in the static loading of smooth specimens. 4. We demonstrated and experimentally confirmed the possibility of using the proposed model to construct the relations Kfc1=f(Nf), determine the size and number of crack jumps during cyclic loading before final fracture, and establish the relationship between fracture toughness characteristics for static and dynamic loading (KIc and KDc), as well as for cyclic loading (Kfc1 and Kfck).