Crack Growth Phenomena Research Articles

Fracture of polymer-like networks with hybrid bond strengths

The design and functionality of polymeric materials hinge on failure resistance. While molecular-level details drive crack evolution in polymer networks, the connection between individual chain scission and bulk failure remains unclear and difficult to probe. In this work, we systematically study the fracture mechanics of polymer-like networks with hybrid bond strengths. We reveal that varying the ratio of strong and weak strands within otherwise identical networks gives a non-monotonic relationship between intrinsic fracture energy and strong strand fraction. Networks with some weak strands can counterintuitively outperform those with exclusively strong strands. Experiments on poly(ethylene glycol) gels and architected polymer-like lattices together with simulations unveil these properties. We show through computational visualization that strand type concentrations impact crack growth patterns and fracture energy trends. Cracks propagate through weak layers at low strong strand fractions. Aggregate clusters deflect or pin cracks at similar concentrations of strong and weak strands. Cracks blunt due to dispersed weak strand failure at high strong strand fractions. The sacrificial weak strands can notably deconcentrate stress near the crack tip, which toughens by delaying crack advancement. The interplay between concentration and clustering of strand types in networks with hybrid bond strengths, combined with crack growth phenomena and nonlocal energy release, provides insights into unusual fracture characteristics. Results shed light on fracture in polymer networks and percolated lattices.

Effect of stress ratio and welding residual stresses on the fatigue crack growth behaviour of Weldox-700 steel using LGDM

In this work, numerical simulations of high-cycle fatigue and fatigue crack growth have been performed using localizing gradient damage methodology (LGDM) for base and welded weldox-700 steel. LGDM models crack growth phenomenon by avoiding any non-physical widening of damage bands. The effect of stress ratio is included in the strain-based damage evolution in the present work. Accordingly, the generalized expressions of damage parameters have been derived. Experimental fatigue tests are performed on the base material and the welded joints. In order to numerically investigate the welded joints, residual stresses developed during welding are obtained through sequentially coupled thermo-mechanical finite element analysis. Subsequently, the fatigue-life and crack-propagation are simulated by applying the residual stresses as a pre-existing field in the LGDM framework, which uses full coupling between deformations and non-local equivalent strains in the finite element analysis. The simulated results and experimental fatigue data are found to be in good agreement.

Characteristics of crack growth in brittle solids with the effects of material heterogeneity and multi-crack interaction

AbstractDespite the extensive research on crack propagation in brittle solids, numerous unexplored problems still necessitate in-depth study. In this work, we focus on numerical modeling of multi-crack growth, aiming to explore the effect of material heterogeneity and multi-crack interaction on this process. To do this, an improved singular-finite element method (singular-FEM) is proposed with incorporation of heterogeneity and crack interaction. An efficient algorithm is proposed for simulating multi-crack propagation and interaction. Stress singularity near crack tip is reproduced by the singular elements. The singular-FEM is convenient and cost-effective, as the zone far away from crack tips is directly discretized using linear elements, in contrast to the quadratic or transition elements utilized in traditional FEM. Next, the proposed method is validated through benchmark study. Numerical results demonstrate that the superiority of the singular-FEM, which combines the merits of low cost and high accuracy. Then, the mechanics of crack growth are explored in more complex scenarios, accounting for the effects of crack interaction, loading condition and heterogeneity on crack trajectory, stress field and energy release rate. The findings reveal that the combined effect of heterogeneity and crack interaction plays a critical role in the phenomenon of crack growth, and the proposed method is capable of effectively modeling the process.

Relation of craze to crack length during slow crack growth phenomena in high‐density polyethylene

AbstractThe craze‐crack mechanism occurring in high‐density polyethylene (HDPE) causing slow crack growth and environmental stress cracking is investigated in detail with respect to the relation of crack length and the related craze zone. This is essential for the understanding of the resulting features of the formed fracture surface and their interpretation in the context of the transition from crack propagation to ductile shear deformation. It turns out that an already formed craze zone does not inevitably result in formation of a propagating crack, but could also undergo ductile failure. For the examination, the full notch creep test (FNCT) was employed with a subsequent advanced fracture surface analysis that was performed using various imaging techniques: light microscopy, laser scanning microscopy, scanning electron microscopy, and X‐ray micro computed tomography scan. FNCT specimens were progressively damaged for increasing durations under standard test conditions applying Arkopal, the standard surfactant solution, and biodiesel as test media were used to analyze the stepwise growth of cracks and crazes. From considerations based on well‐established fracture mechanics approaches, a theoretical correlation between the length of the actual crack and the length of the preceding craze zone was established that could be evidenced and affirmed by FNCT fracture surface analysis. Moreover, the yield strength of a HDPE material exposed to a certain medium as detected by a classic tensile test was found to be the crucial value of true stress to induce the transition from crack propagation due to the craze‐crack mechanism to shear deformation during FNCT measurements.Highlights Progress of crack formation in high‐density polyethylene is analyzed by different imaging techniques Determined growth rates depend on distinction between craze zone and crack The ratio of the present crack to the anteceding craze zone is validated theoretically The transition from crack propagation to ductile shear deformation is identified An already formed craze zone may still fail by ductile mechanisms

Investigating the fracture toughness of the self compacting concrete using ENDB samples by changing the aggregate size and percent of steel fiber

In reality, concrete structures are normally under various loadings, and results of different studies have shown that cracks in these structures and their materials, due to their nature as well as the loading type, do not develop along the crack plane (pure mode I); rather, they expand under mixed modes, making the crack growth studies under these modes a very important issue. In the crack growth phenomenon, the fracture toughness is a very effective parameter usually calculated by ENDB samples because they are easy to handle. In this study, several samples were made by changing the maximum aggregates size (dmax = 9.5, 12.5 & 19 mm) and the amount of hooked-end steel fibers (SF = 0.1, 0.3 & 0.5%), and tested under different loading modes (pure/mixed modes I and III) using the strain control jack device. According to the results, the lowest fracture toughness belonged to pure mode III, aggregates with dmax = 12.5 mm performed better in the self-compacting concrete reinforced with steel fiber, Also, the results show that the increasing trend of steel fibers does not have a positive effect on the fracture toughness performance.

A robust staggered localizing gradient enhanced isotropic damage model for failure prediction in heterogeneous materials

In the present work, a robust and efficient staggered localizing gradient damage model (LGDM) with a simple isotropic damage variable is adopted to predict crack growth phenomena in bi-material and fiber reinforced composites under mixed-mode loading. We first consider a series of three-point bend experiments conducted by Lee and Krishnaswamy (2000) on bi-material specimens of PMMA/Al6061, where the staggered LGDM is shown to capture correct crack profiles under different boundary conditions. It is observed that the numerical crack paths are well compared with the experimental crack paths. Next, a fiber reinforced composite with a central fiber is considered where the effect of change in fiber sizes on the overall structural responses is investigated. It is seen that the increasing fiber size results in reduced load carrying capacity of fiber reinforced composites. Finally, we consider a fiber reinforced composite with fibers located on opposite edges of the specimen. Here, a parametric study on the strength ratio of the matrix phase is conducted to investigate their effect on crack initiation, propagation, and orientation. These simulations highlight the predictive capability of the present model in terms of structural responses as well as sharp crack profiles.

Time-dependent subcritical crack growth and its mechanism in Ni-based single crystal superalloys at 500 °C

The subcritical crack growth behavior under sustained loading and its microscopic mechanism in <100> oriented single crystal (SC) Ni-based superalloys subjected to a relatively low temperature condition (500 °C) are critically investigated. An evident time-dependent crack growth (TDCG) phenomenon is observed even at such a temperature condition, but its rate (da/dt) is significantly small and weakly dependent on the crack length, i.e. the stress intensity (K) value. The cross-sectional observation of a crack tip region by electron microscopy (SEM/TEM), together with the conventional fractography, reveals the following points: the macroscopic crack path shows pronounced branching influenced by the {100} planes; most dislocations in the γ’ phase near the crack tip are rectilinearly aligned and locked on the {100} planes; the crack tip dislocations provide the microscopically preferable oxidation paths. These points strongly suggest that the dislocation-mediated micro-oxidation process plays a major role in the observed anisotropic TDCG mechanism that eventually leads to the anomalous da/dt - K property.

Improved XFEM for 3D interfacial crack modeling

The conventional extended finite element method (XFEM) usually suffers from the crack tip enrichment for removing the blending elements. This may be inconvenient when treating the three dimensional (3D) and curved crack modeling since the crack tip enrichment is usually defined in the local coordinate system depending on the crack front. To avoid these shortcomings of the XFEM, an improved enrichment scheme is proposed for the 3D interfacial crack modeling. The proposed enrichment scheme removes the crack tip enrichment and two weight functions are introduced to capture the strong and weak discontinuities in the element including the crack front. The new enrichment scheme has advantages for the 3D curved interfacial cracks since all the enrichment functions are defined in the global system and it is unnecessary to introduce additional coordinate systems. Several benchmark problems including the mixed mode stress intensity factors (SIFs), anti-plane load and curved cracks are analyzed by the proposed method with the local refinement technology. Numerical results demonstrate that the proposed method can obtain accurate 3D energy release rate and SIFs. In addition, the proposed method enhances the applications of XFEM and has the potential abilities to the multiple-fields coupling and interfacial crack growth phenomena due to the fact that compatible enrichment functions are utilized.

The computational study of crack growth process in defected polymethyl methacrylate/hydroxyapatite composite beam: peridynamic simulation

The presence of cracks in structural materials can be attributed to a wide array of factors. Gaining a comprehensive understanding of the underlying influences on crack formation in solid structures holds significant implications for the development of efficient industrial equipment. In the present computational investigation, we employ the peridynamic method to characterize the crack growth process in polymethyl methacrylate/hydroxyapatite beams. This particle-based simulation encompasses two fundamental steps. Initially, a modeled composite is brought to equilibrium under standard conditions, followed by an in-depth analysis of the crack growth phenomenon after subjecting the target structure to impact tests at three distinct velocities. The present investigation aims to quantitatively assess a range of physical parameters, such as crack length, repulsive force, relative bond length, potential energy, and damage criterion, to provide a comprehensive characterization of the crack growth process in polymethyl methacrylate/hydroxyapatite-based samples. The computational tool utilized for conducting the peridynamic simulations is the Large Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The findings from the peridynamic analysis demonstrate a clear association between the ratio of hydroxyapatite and the resulting crack length in the sample, which consequently leads to heightened brittleness and a decrease in mechanical strength. The crack length ratio reaches a measurement of 9.09 mm when the beam under consideration contains 15% hydroxyapatite. The observed behavior can be ascribed to a reduction in the attractive forces acting between the constituent particles present in the sample. The numerical findings derived from this investigation possess significant implications for the development of mechanically robust composites in practical contexts.

Unveiling thereinforcement potentiality of MWCNTs architecture towards the improvement of microstructural vis-a-vis mechanical and thermo-mechanical properties of pressureless sintered MgAl2O4 spinel ceramic composite

The study presented in this work focuses on the preparation of composites made of multi-walled carbon nanotubes (MWCNTs) and magnesium aluminate spinel through a pressureless sintering technique. The effects of sintering temperature and MWCNTs content on the densification behavior and mechanical properties of the composite were investigated in detail. The results revealed that the composite exhibited improved densification behavior, enhanced mechanical performance and decent control at the microstructural refinement with increasing MWCNTs content up to 0.75 wt%. The superior mechanical properties obtained in case of sintered composite were attributed to the synergetic toughening mechanisms involving the fiber pull out, cracks bridging and cracks deflection effects of reinforcing phases, observed as a resultant effect of strong interfaces with the matrix phase. The crystalline structural arrangement obtained from the XRD and Raman spectral analysis of the composites structure also satisfies the attainment of the promising reinforcement potential of the nanotubular structure. Most importantly, the study also exemplified the subsidiary interference of a surface decorative coating on the MWCNTs structure with the MgAl-binary oxide network. The strategic grafting of the oxide protective shell and the consequent development of a robust interfacial bridging network between the matrix and reinforcement phase regulated the energy dissipation characteristic for fiber debonding. This, in turn, restricted the crack growth phenomenon during mechanical loading, and thereby ultimately contributing to the enhanced fracture toughness, reduced thermal expansion co-efficient and augmented thermo-mechanical performance of the evolved spinel-based composite structure

Heat treatment effects on near threshold region for AISI 4340 steels

Abstract The microstructure effect is critical in the near-threshold region in terms of fatigue crack propagation. Despite numerous studies on the crack growth phenomenon in the literature, there is still no comprehensive understanding of the mechanism behind it. The fatigue crack growth mechanism occurs in the plastic zone region, which is quite small in size; the order is regarded as microstructural units, particularly at low stress intensities. Microstructural differences caused by heat treatment methods are frequently attributed to changes in monotonic and yield strength, resulting in differences in plastic zone size. The driving force required for crack growth under alternating loading is proportional to the plastic zone size ahead of the crack tip. When the microstructure is modified using isothermal transformations, the stress intensity near the threshold and corresponding crack propagation rates were found to be affected by stress ratio, material yield strength, particle size distribution, and impurity segregation. The crack growth threshold ΔK 0 is discovered to be inversely related to steel strength, and a relationship between ΔK 0 and cyclic yield stress is established. In the scope of this paper, annealed and tempered conditions were investigated to assess near threshold behavior for AISI 4340 steel. Effect of microstructure will be detailed around low stress intensities via performed crack growth tests.

A reconfigurable dynamic Bayesian network for digital twin modeling of structures with multiple damage modes

Dynamic Bayesian networks (DBNs) are commonly employed for structural digital twin modeling. At present, most researches only consider single damage mode tracking. It is not sufficient for a reusable spacecraft as various damage modes may occur during its service life. A reconfigurable DBN method is proposed in this paper. The structure of the DBN can be updated dynamically to describe the interactions between different damages. Two common damages (fatigue and bolt loosening) for a spacecraft structure are considered in a numerical example. The results show that the reconfigurable DBN can accurately predict the acceleration phenomenon of crack growth caused by bolt loosening while the DBN with time-invariant structure cannot, even with enough updates. The definition of interaction coefficients makes the reconfigurable DBN easy to track multiple damages and be extended to more complex problems. The method also has a good physical interpretability as the reconfiguration of DBN corresponds to a specific mechanism. Satisfactory predictions do not require precise knowledge of reconfiguration conditions, making the method more practical.

Experimental investigation of crack opening loads in an aircraft load spectrum

The concept of plasticity-induced fatigue crack closure is indispensable in explaining many fatigue crack growth phenomena; and it is now widely applied in advanced fatigue life evaluation procedures. This concept has been extensively investigated experimentally over the past sixty years using different methods. However, most of these investigations are related to constant amplitude loading or simple load cases such as single or multiple overloads or block loading. In contrast, there are currently no experimental studies investigating crack closure under variable amplitude loading or load spectra, which are more realistic scenarios. This can be attributed to the many difficulties associated with the evaluation of crack opening loads for large numbers of fatigue cycles, including sensor sensitivity, signal noise, data limits, etc. Recently, the authors proposed an advanced piezoelectric sensor for cycle-by-cycle crack opening load measurements, which addresses the above limitations. In this work, the piezoelectric sensor is utilised to investigate crack closure under a typical transport aircraft load spectrum. In particular, it is demonstrated that crack opening load levels can change significantly from cycle-to-cycle, and these levels are typically larger than for constant amplitude loading at the same R-ratio. The relative density plot of crack opening load levels reveals three distinct regions, which can be associated with certain load sequences. The outcomes of this research are important for the development and validation of spectrum compression algorithms as well as advanced fatigue life evaluation procedures.

The application of digital image correlation (DIC) in fatigue experimentation: A review

AbstractIn recent years, the use of digital image correlation (DIC) in fatigue experiments has become widespread. It is estimated that ~1000 published works exist that outline fatigue experiments in which DIC is employed for displacement and strain measurement. Of these, ~900 were published in the last 10 years. DIC is a noncontact method that uses a series of digital images to calculate full‐field strains on the surface of an object, planer or curved. Typical commercial DIC systems compute strains at resolutions high enough to trace hysteresis loops in metals. Properly operated open‐source systems can do the same. The DIC method is applied not only on optically based digital images but also on digital images from ultra‐high resolution (ultra‐HR) microscopes like a scanning electron microscope (SEM) or on volumetric images from computed tomography (CT) scans. In fatigue analysis, DIC provides much more information than that of an extensometer. Full‐field strains from DIC can be acquired at different scales (i.e., microscale, macroscale, and nanoscale) and can be related to items such as microstructural features, interacting surfaces (e.g., fretting), fatigue crack growth phenomenon, and distinct forms of energy. Because fatigue is a highly complex, strain‐induced process, the DIC method is and will be an important tool for current and future research in fatigue. This review begins with an overview of the history and fundamentals of DIC including an evaluation of the overall performance and accuracy of the method. Publications selected for review are then presented and discussed. Remarks about the present state‐of‐the‐art and an outlook for future work to be done are then provided.

A multiscale model to predict fatigue crack growth behavior of carbon nanofiber/epoxy nanocomposites

In this study, we developed a multiscale model for predicting the fatigue crack growth behavior of carbon nanofiber (CNF)/epoxy nanocomposites. Here, we focus on considering the effect of experimentally observed microscopic toughness mechanisms (CNF debonding, subsequent plastic nanovoid growth, and pull-out of CNFs) to predict the fatigue crack growth behavior of CNF/epoxy nanocomposites. The predictions of the multiscale model exhibited a high degree of conformity with experimental data. The results of this study are expected to elucidate the complex phenomenon of fatigue crack growth in nanocomposites with randomly distributed CNFs and provide highly accurate predictions.

An efficient implementation of a phase field model for fatigue crack growth

Recently, phase field modeling of fatigue fracture has gained a lot of attention from many researches and studies, since the fatigue damage of structures is a crucial issue in mechanical design. Differing from traditional phase field fracture models, our approach considers not only the elastic strain energy and crack surface energy, additionally, we introduce a fatigue energy contribution into the regularized energy density function caused by cyclic load. Comparing to other type of fracture phenomenon, fatigue damage occurs only after a large number of load cycles. It requires a large computing effort in a computer simulation. Furthermore, the choice of the cycle number increment is usually determined by a compromise between simulation time and accuracy. In this work, we propose an efficient phase field method for cyclic fatigue propagation that only requires moderate computational cost without sacrificing accuracy. We divide the entire fatigue fracture simulation into three stages and apply different cycle number increments in each damage stage. The basic concept of the algorithm is to associate the cycle number increment with the damage increment of each simulation iteration. Numerical examples show that our method can effectively predict the phenomenon of fatigue crack growth and reproduce fracture patterns.

Nano-mechanics based crack growth characterization of concrete under fatigue loading

Developments of scaling laws are crucially important for modeling an engineering phenomenon. Based on the type of physical problems scaling laws have been developed in conjunction with the concept of self-similarity. Scale effect should take into account when the size of an object reduces to extremely small-scale level. Therefore, scaling/power laws and similarity concepts have been considered to be important in nano mechanics in the recent times. In the case of quasi-brittle materials like concrete, crack growth phenomenon can considered as a multi-scale problem comprising of atomistic separation, nano scale level coalesce to form micro and subsequently, major crack. Therefore, it is necessary to have a clear understanding of cracking phenomenon in concrete at different length scales under the action of repetitive loading cycles.In this work, an attempt has been made out to understand the micro-fracture scale effect on the crack growth rate in concrete material under the action of fatigue loading. A theoretical model has been developed based on atomic fracture mechanics theory. Using the concept, the activation energy controls the random movement of the nano-crack front in the subcritical environment of micro-crack growth within the cyclic fracture process zone of quasi-brittle material. Kramer’s formula has been used for the prediction of net frequency of the forward crack front jumps by assuming, fatigue crack propagation rate is governed by thermally activated breakage of interatomic bonds. A multi-scale transition approach has been adopted from micro to the macro using scaling law considering energy dissipation in each cycle to grow a macro-crack is equal to the sum of the energy dissipations associated with the propagation of all the active micro-cracks inside the cyclic fracture process zone.

Fatigue crack propagation for an aircraft compressor under input data variability

The quantification of all the uncertainty sources affecting the phenomenon of fatigue crack-growth is essential to enhance competitiveness in designing more high-performance products, e.g. aircraft engines. This work reports crack propagation simulations for a first-stage compressor of a turbo-fan engine, by considering the impact on life prediction accuracy given by geometrical tolerances, material variability and actual rotational speed. Crack-growth simulations were developed by Finite Element Method (FEM) considering the variability of the fillet radius between blades and disk, so as to derive the distribution of Stress Intensity Factors (SIF) along the crack size as a function of both fillet radius and rotational speed. Consequently, a NASGRO formulation, calibrated on Ti6Al4V test data, was implemented in a MATLAB routine in a stochastic way so as to derive the residual life predictions for the component. Statistical evaluations were made and the contributes of all the considered uncertainty sources were compared and classified.

Transient delamination growth in GFRP laminates with fibre bridging under variable amplitude loading in G-control

To predict fatigue-driven delamination growth in real structures, it is inevitable to understand delamination growth under real load spectra, also known as in-service loads, which most commonly are variable amplitude load patterns. In this work, delamination growth in glass fibre-reinforced polymer laminates subjected to two-level block amplitude loading is investigated using a pure moment loaded DCB test configuration for G-controlled cyclic testing. The variable amplitude load patterns consist of single- and periodic repeated load amplitude changes. The method utilises a digital image-based technique that allows for precise tracking of delamination fronts in translucent materials. The work characterises transient crack growth phenomena following load amplitude changes with emphasis on the actual fatigue crack growth rate. The load amplitude changes may increase the crack growth rate by factors of 5–6 in comparison to the crack growth rate in constant amplitude base-line tests. Periodic repeated load amplitude changes particularly increase the fatigue crack growth. For example, after just 10 periodic repeated low- and high-load blocks, the delamination has extended more than a factor of 2 times the delamination extension predicted from a non-interaction model based on constant amplitude data. The effect of the frequency of load amplitude changes on the fatigue crack growth is also investigated. Frequent load amplitude changes are proven to increase the fatigue crack growth significantly because of the transient crack growth responses following the load amplitude changes.

Panel flutter analysis of cracked functionally graded plates in yawed supersonic flow with thermal effects

In the present paper, an isogeometric analysis formulation along with the Nitsche technique is implemented based on the first order shear deformation plate theory to investigate the aero-thermo-elastic panel flutter behavior of a functionally graded (FG) plate containing cracks in the supersonic flow field. A FG material is considered with temperature-dependent mechanical properties distributed in the plate thickness according to the power law. The temperature distribution in the plate thickness is calculated through solution of the one-dimensional steady-state conductive heat transfer equation while the surface temperatures are kept fixed. The physical contact and the crack growth phenomena are overlooked in the present research. The panel flutter behavior is investigated under combination of crack orientation, crack length, thermal conditions, flow direction, FG volume fraction index and boundary conditions. The accuracy and quality of the present isogeometric formulation is shown in comparison with those available in literature.