Crack Propagation Phenomena Research Articles

Stress threshold identification of progressive fracturing in Bukit Timah granite under uniaxial and triaxial stress conditions

This paper reports the results of an extensive study of the deformation and fracturing of Bukit Timah granite in Singapore under uniaxial and triaxial loading conditions. In the investigation, the stress fracturing thresholds (crack initiation, crack coalescence and crack damage) were determined using stress–strain and stiffness curves and the acoustic emission (AE) detection technique. Crack initiation was found to be best determined by the volumetric strain curve in both uniaxial and triaxial compression tests. The subjectivity of determining the crack closure region and these stress thresholds can be overcome and the results validated by the AE technique by simply observing the cumulative curve of the AE count plotted against deviator stress, which showed four distinctive zones of fracturing. Furthermore, AE detection in the loading stage was found to be a powerful method for determining the phenomena of stable and unstable crack propagation, which are helpful in predicting the rockburst phenomenon. Finally, a comprehensive petrographical analysis using a petrological microscope and a scanning electron microscope was performed to identify the mode of cracking and the characterisation of the cracking pattern.

Numerical simulation of crack curving and branching in brittle materials under dynamic loads using the extended non-ordinary state-based peridynamics

Abstract In this paper, the stress-based failure criteria are implemented into the extended non-ordinary state-based peridynamics (NOSB-PD). When the mean stresses between the interacting material points satisfy the stress-based failure criteria, the breakage of bonds between the interacting material points occurs. The phenomena of crack curving and branching in brittle materials subjected to dynamic loads are investigated using the proposed method. A benchmark example and the Kalthoff-Winkler experiment are firstly simulated to prove the ability and accuracy of the proposed numerical method. Then, one single crack curving and branching in brittle materials subjected to dynamic biaxial loads are also simulated. The effects of geometric and loading conditions on crack curving and branching are studied. The present numerical results are in good agreement with the previous experimental and numerical results. Finally, the phenomena of multiple crack propagation, branching and coalescence under biaxial dynamic loads are also investigated. The effects of non-overlapping length on crack propagation, branching and coalescence under biaxial dynamic loads are analyzed.

Experimental analysis of bonding strength in shape rolling of Al–Cu bimetallic circular pipes into square tubes

In the present research, the intermetallic layer situation and bond strength of square tubes produced by shape rolling of Al–Cu bimetallic pipes has been investigated. The explosive welding process was used for the production of bimetallic circular pipes. The macro- and microscopic features of explosive-welded pipes and shape-rolled specimens at various stages were experimentally measured by using shear and hardness testing and optical metallography. Moreover, scanning electron microscopy was used for fractography of the fracture surfaces after the last stage of forming. The experimental results showed that the shear strength variation is dependent on the intermetallic layer thickness and its continuity. Also, continuity in Al–Cu interfaces that have a thick intermetallic layer intensifies the microcrack propagation, while due to discontinuity in the thin intermetallic layer, the retardation of catastrophic crack propagation was observed in the early passes of the shape-rolling process. However, the results of the present study showed that as the passes proceed in the rolling process, the crack propagation phenomenon, which is the cause of a decrease in shear strength, is expected.

Acoustic sources from damage propagation in Ti grade 5

In this paper fatigue test results are presented for titanium grade 5 specimens subjected to uniaxial cyclic loads. The material behavior of titanium was studied by comparing the Acoustic Emission (AE) detection technique with the Infrared Thermography (IT). The AE technique consists on attaching a couple of piezoelectric sensors on the surface of the sample in order to allow real-time recording of acoustic activity occurring in the material during the test and to localize the acoustic source, based on the assumption that part of the acoustic activity depends on the crack propagation process. All typical data of AE were collected during the tests and some of them were properly post-processed by using filters or derivative functions in order to better understand the crack propagation phenomenon. At the same time, thermographic analysis was also carried out during the experiments by continuous monitoring of surface temperature of the sample. Results of the fatigue behavior of the analyzed material beside the acoustic emission track and the thermal images of the sample are analyzed and critically discussed, in order to assess the capability of each technique in predicting the imminent failure of material.

Aspect ratio evolution associated with surface cracks in sheets subjected to fatigue

This paper presents a fracture mechanics approach to the phenomenon of fatigue crack propagation in geometries such as plates and sheets. The aim is to analyze the crack path (aspect ratio evolution) in a wide range of engineering materials (characterized by different values of the m-parameter of the Paris law) and subjected to two kinds of cyclic loading (tension and bending). To this end, a numerical procedure was designed and implemented starting from a discretization of the crack front (defined as an ellipse) and considering a sort of crack advance on the basis of the Paris law, governed by the stress intensity factor (SIF) obtained by Newman and Raju. Numerical results suggest that surface cracks in sheets tend to grow following a preferential cracking path associated with a very shallow initial flaw with quasi-circular front shape. The rise of the Paris coefficient m leads to a faster convergence between the different crack paths (i.e. quicker approach of the diverse curves) during fatigue propagation starting from the distinct initial crack aspect ratios. In addition, better convergence is also achieved for bending loading (when compare with the case of tension).

Phase-field modeling of brittle and ductile fracture in shells with isogeometric NURBS-based solid-shell elements

In this paper, we investigate fracture in shells with a phase-field modeling approach. The shell model is based on solid-shell kinematics with small rotations and displacements and is discretized using quadratic Non-Uniform Rational B-Spline basis functions. Membrane and shear locking is alleviated through the Assumed Natural Strain approach. The solid-shell formulation is combined with a brittle phase-field model for elastic materials, as well as with a ductile fracture model for elasto-plastic materials exhibiting J2 plasticity with isotropic hardening. Several examples demonstrate the ability of the proposed framework to capture crack initiation, propagation, merging and branching phenomena as well as crack bulging effects in shells under different states of loading.

Analysis of crack propagation in stainless steel by comparing acoustic emissions and infrared thermography data

Abstract The aim of this work is to provide, in a non-destructive way, information about the structural integrity of materials subjected to a sinusoidal fatigue loading. In particular, the crack propagation was studied in AISI 430F/1 by using Acoustic Emission (AE) and Infrared Thermography (IT) at the same time. The main advantages of both approaches refer to the possibility to monitor the whole history of the specimen; in fact, differently by ultrasound inspection, no scanning is required and moreover the measurement is simplified by the fact that access from just one side of the specimen is sufficient. For AE technique two sensors were placed on the surface of the specimen in order to detect the elastic waves emitted by the material under stress and due to the activation of inner defects. By analyzing the overall acoustic signals it is possible to correlate a part of them directly to the crack propagation phenomenon. At the same time a thermocamera was placed in front of the specimen to live monitor the variation of surface temperature. Above all, the rise of temperature around the crack tip was observed in order to evaluate the advancing of the crack during the test. Finally a comparison between the two techniques was carried out aiming to assess the capability of each approach in following the evolution of the damage process.

Low-velocity Impact and Flexural Properties of Thermoplastic Polyurethane/Woven Glass Fabric Composite Laminates

Composite laminates based on a commercial thermoplastic polyurethane (TPU) grade and reinforced by woven glass fibers were prepared by film stacking and compression molding procedures and subjected to static and dynamic mechanical tests, at ambient and low temperatures. The results were interpreted taking into account that preliminary analysis of TPU based laminates showed a fiber volume fraction equal to 53%, 56% and 48% for plates involving 10, 16 and 22 lamina, respectively. Flexural tests, performed so far only at room temperature (RT), revealed that the intermediate thickness laminates offer the highest performances, probably for their higher volumetric fraction of reinforcement. Impact results collected, instead, also at low temperatures (-25°C and -50°C) indicated that no delamination is verified all over the considered range of impact temperatures and thickness of plates. On the contrary crack propagation phenomena are enhanced by increasing the thickness of impacted samples and/or reducing the impact temperature.

Crack analysis of pre-cracked brittle specimens under biaxial compression

Crack propagation process in rock like specimens has been studied experimentally and numerically considering both; a single crack and double cracks in the center of specimens. The rock-like specimens are specially prepared from Portland Pozzolana Cement (PPC), fine sands and water. These precracked cylindrical specimens are experimentally tested under compressive loading. Effect of confining pressure on the crack propagation path of specimens with single crack is explained first. In these experiments, it has been observed that the crack propagation path is diverted from its rout due to lateral confinement compared to that of the uniaxially loaded pre-cracked experiments. The same experiments are numerically simulated by an indirect boundary element method using higher order displacement discontinuity elements. Effect of lateral stress on the crack propagation of specimens containing single and double cracks have been studied and compared with the experimental results showing a good agreement between them. Effect of confinement on the cracks coalescence in the specimens containing two cracks is also investigated. It has been observed that wing cracks are produced at the first stage of compression and start their propagation toward the direction of uniaxial compressive loading in the absence of confinement. In the presence of confining pressure the propagating wings are diverted related to the uniaxially loaded cracks and showing a mixed mode crack propagation phenomenon due to both induced tension and shear. It is also observed that the crack coalescence path is affected a little bit (due to confinement) compared to that of wing cracks.

Out of phase thermal mechanical fatigue investigation of a directionally solidified superalloy DZ125

Out of phase (OP) thermal mechanical fatigue (TMF) behavior of a directionally solidified (DS) superalloy DZ125 was experimentally and numerically studied. Two different temperature conditions, which are 500–1000°C and 400–900°C, were considered in the present research. Stress and strain responses as well as fatigue life results were presented and discussed. Scanning electron microscope (SEM) and metallographic analysis were used to study the damage mechanism. An oxidation assisted crack initiation and propagation phenomenon were found to explain the shorted life under TMF cycles. In order to characterize the stress and strain deformations under TMF loadings, a modified Chaboche’s constitutive model was applied. Additionally, the TMF life of the material was modeled and predicted by Neu–Sehitoglu damage law with high accuracy.

Effect of laser peening with different energies on fatigue fracture evolution of 6061-T6 aluminum alloy

To deeply understand the effect of laser peening (LP) with different laser pulse energies on 6061-T6 aluminum alloy, the fatigue fracture morphologies evolution process at various fatigue crack growth (FCG) stages and the corresponding strengthen mechanism were investigated. At the initial stage of FCG, more fatigue micro-cliffs were found after LP, while the fatigue striation spacing simultaneously reduced. A “stop-continue” phenomenon of crack propagation was discovered for laser peened samples. The fatigue striation spacing at the middle stage of FCG increased significantly while compared with that at the initial stage, in addition, the fatigue striation spacing decreased with an increase in laser pulse energy. Fracture morphologies in transition region of laser peened samples exhibit a mixing fracture characteristic of striations and dimples. The laser peened sample with laser pulse energy of 7J presents more circuitous growing paths. Due to the complex stress state induced by LP, dimples with different sizes appeared in the final fracture region.

Experiments and numerical simulations of low‐velocity impact of sandwich composite panels

This article investigates the response of composite sandwich panel with Nomex honeycomb core subjected to low‐velocity impact and compression after impact (CAI) by using the methods of experiments and numerical simulations. Low‐velocity impact of sandwich panels at five energy levels is carried out to research the damage resistance and tolerance. A failure model based on Hashin failure criterion is implemented to model the intralaminar damage behavior of laminated plies in the numerical simulation. The cohesive zone model is used to simulate the delamination damage between adjacent laminated plies. The honeycomb core behavior is defined as an elastic–plastic material. Good agreements, in terms of contact‐force histories, damage shapes, and indentation depths of the sandwich panels, are observed between the experimental and numerical results. During CAI analysis, the damaged panels present a phenomenon of quick crack propagation from impact indentation location to each unloaded side after the structural strength reached. It is found that the in‐plane compressive strength of damaged sandwich panels is almost 25–35% reduction than that of undamaged panels. POLYM. COMPOS., 38:646–656, 2017. © 2015 Society of Plastics Engineers

Examples of applications of the peridynamic theory to the solution of static equilibrium problems

AbstractPeridynamics is a recently proposed continuum theory based on a non local approach and formulated with integral equations. The theory is suitable for dealing with crack propagation in solid materials. The original peridynamic formulation regarded dynamic problems and was adapted to the static case mainly using a relaxation method by introducing a substantial amount of numerical damping in the time integration. In the present work the implementation of the theory within an implicit code for static crack propagation phenomena based on the Newton-Raphson method is presented and applied to several examples of static crack propagation equilibrium problems. Results obtained with the newly developed procedure are presented for various structural configurations, with different boundary and load conditions and quantitatively compared to published data.

GS0208-217 二種のTWIP鋼における微小き裂進展挙動の比較研究 : Mh-C相互作用による疲労限創出の可能性

The high cycle fatigue properties of Fe-30Mn-3Si-3Al and Fe-22Mn-0.5C twinning-induced plasticity (TWIP) steels were evaluated by rotary bending fatigue testing and a replica technique. More specifically, small fatigue crack propagation and non-propagating fatigue crack phenomenon were comparatively studied in terms of occurrence of dynamic strain aging. In fact, dynamic strain aging occurs in the Fe-22Mn-0.5C TWIP steel because of the presence of Interstitial-substitutional (I-S) atom interactions. Probably due to the occurrence of dynamic strain aging, only the Fe-22Mn-0.5C TWIP steel showed non-propagating fatigue crack phenomenon at the fatigue limit. In contrast, a small fatigue crack of the Fe-30Mn-3Si-3Al TWIP steel continuously propagated even at the fatigue limit which was defined that the specimen does not fracture until 10^7 cycles.

Crack Propagation in Compression and Mounted Arrestors

Strengthening of masonry walls is a key aspect during building retrofitting operations having conservation goals. “Reinforced repointing technique” involves the application of materials having high tensile strength such as steel bars, steel textile sheets or composites thin pultruded laminae with cracks arrestors function, to reduce the vulnerability of masonry structures against in-plane and out-of-plane actions. In this paper, a three-dimensional numerical model is developed to evaluate the efficiency of reinforcing steel bars facing crack propagation phenomenon under increasing compressive loads.

Crack propagation in flat panels stiffened by bonded pads

This paper presents the results of an experimental and numerical investigation about crack growth in metallic flat panels stiffened by means of multilayered bonded pads.An experimental program was performed to assess the fatigue crack growth and the residual strength of a number of selected configurations of stiffened panels, within the framework of a collaboration between Piaggio Aero Industries (PAI) and the University of Pisa.In the paper, the test procedures and equipments used for the experimental test campaign are described and the main results of the fatigue crack growth and of the residual strength tests are reported and analysed.Finite Element analyses were performed to further investigate the crack propagation phenomena. The numerical model was validated by comparing the strains measured, at different crack lengths, by a set of strain gauges bonded on the tested panels, with those provided by FEM.Two possible scenarios are accounted for: crack propagates only in the skin, while pads remain un-cracked, or crack propagates simultaneously in the skin and in the pad layers.The values of the stress intensity factor numerically evaluated well compare with the values obtained from the experiments.

Phase-Field Modeling of Fracture in Ferroelectric Materials

This paper presents a family of phase-field models for the coupled simulation of the microstructure formation and evolution, and the nucleation and propagation of cracks in single and polycrystalline ferroelectric materials. The first objective is to introduce a phase-field model for ferroelectric single crystals. The model naturally couples two existing energetic phase-field approaches for brittle fracture and ferroelectric domain formation and evolution. Simulations show the interactions between the microstructure and the crack under mechanical and electromechanical loadings. Another objective of this paper is to encode different crack face boundary conditions into the phase-field framework since these conditions strongly affect the fracture behavior of ferroelectrics. The smeared imposition of these conditions are discussed and the results are compared with that of sharp crack models to validate the proposed approaches. Simulations show the effects of different conditions and electromechanical loadings on the crack propagation. In a third step, the model is modified by introducing a crack non-interpenetration condition in the variational approach to fracture accounting for the asymmetric behavior in tension and compression. The modified model makes it possible to explain anisotropic crack growth in ferroelectrics under the Vickers indentation loading. This model is also employed for the fracture analysis of multilayer ferroelectric actuators, which shows the potential of the model for future applications. The coupled phase-field model is also extended to polycrystals by introducing realistic polycrystalline microstructures in the model. Inter- and trans-granular crack propagation modes are observed in the simulations. Finally, and for completeness, the phase-field theory is extended to the simulation of the propagation of conducting cracks under purely electrical loading and to the three-dimensional simulation of crack propagation in ferroelectric single crystals. Salient features of the crack propagation phenomenon predicted by the simulations of this paper are directly compared with experimental observations.

The effects of temperature and vacancies on dynamics of crack in graphene sheet

Crack propagation in a defected graphene sheet is investigated at finite temperature using molecular dynamics simulation. The effects of several initial cracks, temperature and different percentage of vacancies are considered. It is found that i) the critical load, which is a criteria for crack propagation, is larger when the load is applied on the zigzag direction, ii) the critical load decreases with increasing temperature, iii) a hole in the center of the sheet and the presence of randomly distributed vacancies reduce the critical load giving different crack propagation trajectory. Our new results would help to understand the crack propagation phenomena in defected graphene at finite temperature.

Strength failure and crack coalescence behavior of sandstone containing single pre-cut fissure under coupled stress, fluid flow and changing chemical environment

In order to study the strength failure and crack coalescence characteristics of cracked rocks, uniaxial compression experiments were conducted on cylindrical sandstone specimens, sampled from Longyou Grottoes of Zhejiang Province, China, with a single pre-cut crack soaking in different chemical solutions. Based on the results of uniaxial compressive test under different chemical solutions and velocities of flow, the effect of strength and deformation characteristics and main modes of crack coalescence for cracked rocks under chemical corrosion were analyzed. The results show that the pH value and velocity of the chemical solutions both have great influence on the sandstone sample’s uniaxial compressive strength and deformation characteristics. Cracked sandstone samples are tension-destructed under uniaxial compression, and the crack propagation directions are consistent with the loading direction. The phenomena of crack initiation, propagation and coalescence of sandstone are well observed. Four different crack types are identified based on the crack propagation mechanism by analyzing the ultimate failure modes of sandstone containing a single pre-cut fissure. The failure process of specimen in air is similar with the specimen under chemical solutions, however, the initial time of crack occuring in specimen under chemical solutions is generally earlier than that in the natural specimen, and the crack propagation and coalescence process of specimen under chemical solutions are longer than those of the natural specimen due to softening of structure of rock caused by hydro-chemical action. Immersion velocity of flow and chemical solutions does not have influence on the ultimate modes of crack coalescence.

Testing Study and Mechanics Analysis on Fatigue Life of MSD Structure

Multiple site damage (MSD) usually occurs in the form of some small fatigue cracks at several sites within aging aircraft structures. It is very important to study the fatigue life of this kind of structure. This paper used testing method to observe the phenomenon of crack propagation, to analysis the result of the crack propagation. It provides testing reference for the study on fatigue life of MSD structures. According to the analysis of fatigue life of typical structures, a series of valuable conclusions are obtained.