Critical Stress Intensity Factor Research Articles

Urea‐glutaraldehyde‐formaldehyde resin with low formaldehyde emission and high flexibility: Structure and toughening mechanism

AbstractUrea‐formaldehyde (UF) resin with excellent intrinsic flame retardancy, high strength, and low cost has been widely used as adhesives, coatings as well as molding compounds, and it is a challenge to prepare UF resin with combined properties of high toughness/strength and low formaldehyde emissions. In this work, glutaraldehyde was introduced into the synthesis system of UF resin to partially replace formaldehyde, and urea‐glutaraldehyde‐formaldehyde (UGF) copolycondensation resin was prepared. It was found that glutaraldehyde participated in additional/condensation reactions of UF resin, and the crosslinking reaction of UGF resin was hindered with higher curing activation energy than that of neat UF resin. Due to the controllable curing kinetics and introduction of long methylene chains, UGF resin presented relatively low crosslinking density, and under external force, it underwent distinct yielding before fracture and many yield folds appeared on the fractured surface, showing high toughness and strength. Compared with neat UF resin, the tensile strength, elongation at break, impact strength, and critical stress intensity factor (KIC) of UGF resin increased by 26%, 42.30%, 14.6%, and 30%, respectively. Meanwhile, the free formaldehyde emission for UGF resin decreased by 47.5%, meeting the requirement of E0 grade. Such developed eco‐friendly UGF resin exhibited promising application potentials.

Optimization of stone matrix asphalt mixture design based on fracture mechanics approach

Low-temperature cracking is one of the most common pavement distresses that occurred in a short period of service life and is known as premature distress. Stone matrix asphalt (SMA) with gap gradation, which has high strength and rutting performance, should also perform well against low-temperature cracking. So in this research, the criteria of SMA mixture design based on the National Cooperative Highway Research Program (NCHRP) 9–8 were optimized in a way so that SMA has the best performance against low-temperature cracking. Therefore, the optimization process was carried out using the response surface method (RSM) considering the combined effect of compaction energy (Ndesign) and binder content as mixture design factors. The fracture evaluation was done based on stress and energy. So critical stress intensity factor (KIC) and critical fracture energy (Gf) were used as fracture evaluation parameters. After conducting the tests and analyzing the results, it was found that 6.09 % binder with Ndesign = 78 leads to a design with a desirability of 0.869, which passed all the NCHRP 9–8 criteria of the mixture design in line with the maximum fracture performance.

Analysis of the resistance to crack propagation in SENT test specimens printed in ABS using parallel or crossed filaments between layers

Additive manufacturing techniques continue to develop and cover all industrial fields. However, the performances of aspect and mechanical behavior of the parts obtained by this process remain to be mastered and are still the subject of current research works. Among these performances, the one corresponding to the resistance to the propagation of cracks. In order to improve this very interesting property in various industrial fields, it is desirable to master the understanding of crack propagation in this type of structure obtained by 3D printing. The objective of this paper is to analyze and understand the effect of the adopted raster angle on the crack propagation in SENT specimens obtained by FDM in ABS (Acrylonitrile-Butadiene-Styrene). Two approaches were developed: one is experimental to determine the critical stress intensity factor KIC and the other is numerical to predict the possible paths of crack propagation.

Investigation of Woven Orientation of Natural Fiberby Finite Element Method

Natural fiber is recognized for having a lower density than synthetic fiber, as well as being cheaper, more plentiful, and most significantly, renewable. In Malaysia, the focus on waste fiber somehow produces an abundance of bio waste in the form of leaves, mainly composted or burned, thus wasting the good potential of fiber sources. The burning of the leaves will lead to environmental pollution problems. Many researchers have investigated the potential of this waste and have demanded the development of new materials with enhanced strength, stiffness, density, and cost-effectiveness. Due to the various applications of natural fibers, especially bamboo and kenaf, in several industries, they may be exposed to cyclic load based on their preferred applications. Thus, this study implemented a finite element method by using ANSYS to investigate the characteristics of the notch at crack tipopening angle for bamboo and kenaf woven fiber under quasi static loading as well as the deformation, strain, stress, and mode I stress intensity factor (SIF) criteria. This simulation focuses on the three types of woven orientations, which include 0°/90°, 30°/-60°, and 45°/-45°. The result shows that Kenaf fiber with a 0°/90° orientation showed good results in terms of strength compared to bamboo fiber, and among the three woven orientations, the 0°/90° orientation showed the highest stress intensity factor followed by 30°/-60° and 45°/-45° for both bamboo and kenaf fiber.

Optimization of Composition of Nanofiber Concrete in Terms of Fracture Toughness by Matrix Modifiсation

Concrete is a quasi-brittle building material that has low tensile strength. The process of its destruction under loading is inhomogeneous, due to the nature of the concrete structure mass, consisting of components with different physical and mechanical properties. Gradual deformation and destruction can be characterized as a process of formation and development of microcracks. The presence of different-sized components in concrete makes it possible to consider its structure as a multi-level system. In this system, each level is a matrix with its own structural inclusions, which play both a structure-forming role and the role of stress concentrators under the action of mechanical loads. The critical stress intensity factor is a good indicator of the crack resistance (fracture toughness) of a material. Nanoconcrete, from the point of view of a multilevel system, is a concrete composite with crack propagation inhibitors at the level of the cementing substance (carbon nanotubes are consi-dered as inhibitors). The presence of fiber fibers at subsequent scale levels allows us to consider concrete as a composite with multi-level dispersed reinforcement (nanofiber concrete). The paper discusses the change of concrete fracture toughness indicator (crack resistance) with dispersed reinforcement of the matrix at different structural levels. The presented for normal separation of notched cubes under eccentric compression with the determination of the stress intensity factor for concrete modified with carbon nanotubes acting as crack propagation inhibitors at the level of cementing substance (nanoconcrete), as well as for nanofiber concrete with dispersed reinforcement at the level of fine-grained concrete. Based on experimental studies by non-equilibrium methods of fracture mechanics, compositions of nanofiber-reinforced concrete of maximum crack resistance (fracture toughness) with different fiber concentrations and several types of matrices modified with nanocarbon additives are proposed in the paper.

Basis for calculation the residual mechanical properties of corroding bars

AbstractCorrosion of steel reinforcement is known to affect the residual bearing capacity of the structure. The loss of metal supposes a decrease of the sectional area of the reinforcing bar, either homogeneously distributed or concentrated in some zones. The morphology of the corroded area is a critical issue because while the uniform loss is influencing the steel strength, when it is localized also the ductility is reduced due the local stress concentration. There are previous publications showing the impact of corrosion on some mechanical aspects of the steel as on the ductility, the strength or the elastic modulus, but still we lack of a comprehensive framework of all types of corrosion. In this work a classification is made of the types of damages for better assessing their consequences. Further, through application of basic classical mechanics are given expressions to calculate the “corrosion factor” related to the yield strength, the ultimate strength, the elastic modulus and the elongation (ductility). Finally, through fracture mechanics and an energetic approach are given the expressions for the critical stress intensity factor and the “apparent” residual section “loss” of the corroded steel bars.

Anisotropic fracture in nacre-like alumina

Fracture in nacre-like alumina is studied experimentally and numerically with alumina platelets main alignment direction perpendicular (along-configuration) to the one commonly tested (across-configuration) in the literature. In along-configuration, brittle failure is noted after crack initiation with no macroscopic crack deviation, unlike what is observed in across-configuration. However, NLA exhibits in along configuration initiation critical stress intensity factor KIc = 5.3 ± 0.5 MPa m1/2, in the same order of magnitude as in across-configuration. The increase in KIc compared to monolithic alumina can be explained by a phenomenon of crack initiation containment at the interface between platelets. Unstable crack propagation occurs just after initiation, mainly following the platelet interfaces which results in crenelated fracture surfaces with crack decentering with respect to the initial notch tip.

Effect of the Mixing Technique of Graphene Nanoplatelets and Graphene Nanofibers on Fracture Toughness of Epoxy Based Nanocomposites and Composites.

In this work, the effect of different mixing techniques on thermal and mechanical properties of graphene nanoplatelets (GNPs) and graphene nanofibers (GANFs) loaded epoxy nanocomposites was investigated. Three dispersion methods were employed: a high shear rate (HSR), ultrasonication (US) and the fluidized bed method (FBM). The optical microscopy has revealed that the most suitable dispersion, in terms of homogeneity and cluster size, is achieved by implementing the US and FBM techniques, leading to nanocomposites with the largest increase of glass transition temperature, as supported by the DMA analysis data. The fracture toughness results show a general increase of both the critical stress intensity factor (KIC) and the critical strain energy release rate (GIC), likely due to the homogeneity and the low scale dispersion of the carbonaceous nanostructures. Based on the nanocomposite fracture toughness improvements and also assuming a potential large scale up production of the nanocomposite matrix, a single mixing technique, namely the FBM, was employed to manufacture the carbon fiber reinforced composite (CFRC). This method has resulted in being less time-consuming and is potentially most suitable for the high volume industrial production. The CFRCs were characterized in terms of tensile, flexural and interlaminar fracture toughness properties and the results were analyzed and discussed.

Evaluation of dispersion reinforcement in concrete in terms of strength and crack resistance

Introduction. The purpose of the study was to evaluate the effectiveness of dispersed concrete reinforcement in terms of strength and crack resistance.Materials and methods. When conducting research, test methods were used, regulated by the provisions of national standards, equipment specially designed for this study was used.Results. The results of testing fiber-reinforced concretes made using low-modulus and high-modulus fibers are presented.Discussion and conclusions. As a result of the analysis of the data presented, it is shown that in order to assess the effectiveness and the correct choice of reinforcing fibers, it is not enough to characterise fiber-reinforced concrete only by tensile strength in bending. For a more accurate assessment, it is proposed to use crack resistance characteristics, for example, the critical stress intensity factor, or to supplement the description of the advantages of dispersed reinforcement with the features of the behavior of fiber-reinforced concrete under load.

Evaluation of fracture toughness properties of polymer concrete composite using deep learning approach

AbstractUsing artificial intelligence‐based methods in predicting material properties, in addition to high accuracy, saves time and money. This paper models and predicts the fracture toughness properties of polymer concrete (PC) composites using the deep learning method. After preparing a database consisting of 209 experimental data from 19 relevant studies, the effect of seven important variables on critical stress intensity factor (KIc) and crack tip opening displacement (CTOD) is considered. Then, the deep neural network (DNN) model is developed and trained using the prepared database. The accuracy of the DNN model is examined by implementing four statistical criteria, MSE, R2, RMSE, and MAE. Finally, the sensitivity of the KIc and CTOD to each input variable is evaluated using a partial dependence plot (PDP) analysis. While aggregate size, nanofiller content, and a/R ratio have the most positive effect on KIc, aggregates and nanofiller content have the most positive influence on CTOD.

Effect of crack length on fracture behavior of particleboard

The effect of crack length on the fracture behavior of particleboard was investigated using the single-edge-notched bending (SENB) test method under mode I loading. The initial slope (kinit), critical stress intensity factor (KIC), specific fracture energy (Gf), and brittleness number were calculated for five different crack length/specimen width (a/W) ratios varying from 0.1 to 0.9 at intervals of 0.2. The results show that the fracture properties were significantly higher for specimens with an a/W ratio of 0.1 than for the others. However, for the critical stress intensity factor and specific fracture energy, there were no significant differences among the a/W ratios of 0.3, 0.5, and 0.7 where the crack tip was placed in the core layer of the particleboard. In general, as the a/W ratio decreased, the stiffness of the material increased, and the specimens with an a/W ratio of 0.1 showed brittle behavior. However, there was no statistically significant difference between a/W ratios of 0.5 and 0.7.

Investigation on the mixed mode fracture toughness of freshwater ice using the semi-circular bend method

The fracture toughness of ice is of great importance to understand the fracture behaviour of ice impact and blasting. In this study, the mixed mode (I + II) fracture toughness and mode II fracture toughness of granular freshwater ice were first investigated using the semi-circular bend method. A series of fracture tests were carried out to investigate the effects of the grain size (0.8 mm to 4.5 mm) and notch inclination angle (0° to 75°) on the fracture toughness under mixed mode loading conditions. The experimental results indicate that the mode II and mixed mode fracture toughness values slowly decrease in general as the grain size increases. The fracture initiation angles and critical stress intensity factors of the SCB ice specimens were predicted using the generalized maximum tangential stress (GMTS) criteria and conventional maximum tangential Stress (MTS). The GMTS criterion provides a better understanding for mixed mode fracture toughness of ice specimens with various grain sizes, particularly for mode II dominated loading conditions. The grain size of ice directly affects the size of the fracture process zone, which in turn affects the variation law of mixed mode fracture toughness. The strain field and displacement field were initially analysed based on digital image correlation. The results indicate that the variation in the mixed mode crack was complex, eventually exhibiting an asymmetrical distribution of the displacement field.

L-Arginine as Bio-Based Curing Agent for Epoxy Resins: Temperature-Dependence of Mechanical Properties.

The precise characterization of new bio-based thermosets is imperative for the correct assessment of their potential as matrix material in fiber-reinforced polymer composites. Therefore, the mechanical properties of diglycidyl ether of bisphenol a (DGEBA) cured with l-arginine were investigated to determine whether the bio-based thermoset possesses the required mechanical properties for application as a matrix material. The cured thermoset is called Argopox. The mixture of amino acid and epoxy resin was prepared via three-roll milling and cured in the presence of an urea-based accelerator. The tensile, compression, flexural and toughness properties of Argopox were characterized at T=−40 ∘C, 22 ∘C and 80 ∘C to determine the temperature-dependence of the thermoset’s mechanical properties in its service temperature range. The glass transition temperature Tg was analyzed via dynamic mechanical analysis (DMA) and is approximately 119 ∘C. The tensile, compression and flexural strength at 22 ∘C are about 56 MPa, 98 MPa and 85 MPa, respectively. The critical stress intensity factor KIC and fracture energy GIC at 22 ∘C are roughly 1.1 MPam0.5 and 510 Jm−, respectively. Consequently, Argopox possesses mechanical properties that reach performance levels similar to that of materials which are already used as matrix for fiber reinforced composites.

Interaction between crack and grain boundary in magnesium

In polycrystalline materials, grain boundaries (GBs) play an important role in resisting crack growth. In this study, the interaction of a crack on basal plane with GBs in magnesium was investigated by molecular statics simulations. Three types of GBs with different mis-orientation angles were considered, i.e., [21¯1¯0] asymmetric tilt GB, (011¯0) twist GB and [0001] asymmetric tilt GB. Simulation results show that, for [21¯1¯0] asymmetric tilt GBs, the crack always penetrates through the GBs but the crack growth path in neighboring grain changes from (0001) basal plane to (01¯11) pyramidal plane as the tilt mis-orientation angle increases. For (011¯0) twist GBs, the crack propagation mechanism changes from trans-granular fracture into inter-granular fracture with the increasing twist mis-orientation angle. For [0001] asymmetric tilt GBs, trans-granular fracture happens at small and high tilt mis-orientation angles, while inter-granular fracture occurs at intermedium mis-orientation angles. The critical stress intensity factor (SIF) for crack propagation across or along GB is correlated with the corresponding GB energy. Trans-granular fracture occurs when the GB energy is low, and the critical SIF also increases with the increasing GB energy. For high energy GBs, inter-granular fracture occurs and the critical SIF changes hardly with the increasing GB energy. The current work provides insights into understanding the behavior of cracks and ductility enhancement in magnesium.

Microstructures and properties of a high strength, toughness, and corrosion resistance Al-Zn-Mg-Cu alloy under an over-aging state

• Highest fracture toughness and critical stress intensity factor in Al-Zn-Mg-Cu alloy. • The excellent mechanical properties were attributed to the novel heat treatment. • Deformed structures without recrystallization retained after aging. • Pre-aging treatment introduced G.P. zones, leading to fine age hardening precipitates. • The size of the GBPs and the width of PFZs were significantly reduced. This work studied an Al-Zn-Mg-Cu alloy with excellent comprehensive properties using a novel heat treatment. The result shows that the fracture toughness ( K IC ) value of 65.2 ± 2.0 MPa∙m 1/2 and critical stress intensity factor ( K ISCC ) of 37.9 ± 1.7 MPa∙m 1/2 can be achieved. Each of which is the highest value reported. The fatigue strength with this heat treatment exceeded 180 MPa at the stress ratio of −1 ( σ -1 ), which was much higher than the other Al-Zn-Mg-Cu alloys with similar tensile strength. This superior combination of mechanical properties and fracture toughness for the studied alloy is greatly attributed to a significant fraction of deformed grain structures and dense age hardening precipitates evolved from existing G.P. zones.

Effect of reclaimed asphalt pavement (RAP) on fracture properties of stone matrix asphalt (SMA) at low temperature

In this research, the fracture properties and low temperature cracking of stone matrix asphalt)SMA(mixture containing reclaimed asphalt pavement)RAP(were evaluated. To achieve appropriate gradation of SMA with a nominal maximum aggregate size (NMAS) of 12.5 mm, 30 % dense graded RAP was used. Evaluation of fracture properties was done based on stress and energy approaches. To evaluate the fracture properties of the SMA mixture containing 30 % RAP and compare it with the non-RAP mixture, semicircular bending (SCB) test was performed at −10 °C. Air void in the mixture (Va), loading rate, and combined mode parameter (Me) was considered as research factors. The results showed that the critical stress intensity factor (KIC) of mixtures containing 30 % RAP was higher than the non-RAP mixture. In both types of mixtures, with increasing Me, KIC first decreased and then increased. In pure mode I (Me = 1) with increasing air void, KIC decreased. Regardless of the effect of RAP, in all fracture loading modes with increasing loading rate and air void, the fracture energy (Gf) decreased. Therefore, the presence of RAP did not affect the energy absorption of the mixture at low temperatures. The presence of RAP and coarse aggregate skeleton structure causes the brittle fracture of the mixture at low temperatures. The effect of RAP on the crack path only in samples with low air voids and high loading rate caused the crack path through coarse aggregates with smaller size and mastic to show a more share compared to non-RAP samples.

Stress corrosion characteristics of electron beam welded titanium alloys joints in NaCl solution

The Ti6321 titanium alloy has stress corrosion sensitivity in 3.5% NaCl solution, and the sample with cracks will fracture in a short time. The aim of this paper is to reveal the key factors affecting the stress corrosion resistance and clarify the stress corrosion mechanism of different regions of the joint by studying the crack propagation and stress corrosion critical stress intensity factor of the different regions of titanium alloy welded joint. Stress corrosion cracking, fracture and crack propagation path of electron beam welded titanium alloy joint were analyzed, characteristics of microstructure, texture and electrochemical were studied. The results indicated that the stress corrosion intensity factor KISCC of the weld metal is >76.50 MPa • m 1 / 2 , and the maximum KISCC value of heat affected zone is 51.62 MPa • m 1/2 , so KISCC of weld metal is better than heat affected zone. The crack growth rate of weld metal is slightly higher than that of heat affected zone. The fractured surface of weld metal has no obvious stress corrosion characterization, while heat affected zone has. The propagation path of weld metal is more zigzag than that of heat affected zone. The results reveal that microstructure, texture and electrochemical properties are the main reasons for stress corrosion. • The stress corrosion of titanium alloy electron beam welded joint in different regions has been studied. • The stress corrosion resistance of weld metal is significantly higher than heat affected zone, which is related to composition, microstructure and microtexture. • The stress corrosion crack mechanism of titanium alloy welded joint is the combined action of crack propagation and potential corrosion under stress.

Mixed-mode dynamic fracture behavior of a rubber toughened epoxy adhesive under stress wave loading

This work seeks to measure mixed-mode fracture parameters for a dynamically growing crack in a core shell rubber toughened epoxy adhesive (EPONTM 828 with PARALOID™ EXL-2691J particles). Digital image correlation technique coupled with ultrahigh-speed photography and a hybrid finite element-based post-processing approach are used to extract fracture parameters. The dynamic loading is accomplished by using a striker to induce stress waves in an incident bar in contact with a free-standing semi-circular specimen. A companion finite element analysis is performed for investigating suitable experimental parameters and configurations for implementing the approach and determining specific experimental parameters (crack length, crack angle, and impact velocity) necessary to achieve the desired experimental conditions (mode mixity at crack initiation and strain rate). The critical fracture parameters are used to generate mixed mode fracture envelope and fracture toughness-mode mixity relationship for the material. Further investigation is carried out to evaluate various fracture criteria for predicting initial crack kink angles. The measured dynamic fracture quantities are also compared with the quasi-static counterparts to assess strain rate-dependence of the adhesive. The results reveal a decrease in critical stress intensity factors of approximately 10 % and 30 % for the mode I and mode II states, respectively, with increasing strain rate. Post-initiation, the dynamic crack propagates at a near-constant mode I stress intensity factor of ∼1.4 MPa-√m. Under dynamic conditions, the critical, effective stress intensity factors are relatively constant with respect to mode mixity. This is in contrast to quasi-static results where the effective values increased with respect to mode mixity. Experimentally measured crack kink angles agree well with those predicted by the maximum energy release rate criteria and the maximum tangential stress criteria, particularly under mode I dominant conditions.

LEFM is agnostic to geometrical nonlinearities arising at atomistic crack tips

Various fields such as mechanical engineering, materials science, etc., have seen a widespread use of linear elastic fracture mechanics (LEFM) at the continuum scale. LEFM is also routinely applied to the atomic scale. However, its applicability at this scale remains less well studied, with most studies focusing on non-linear elastic effects. Using a harmonic “snapping spring” nearest-neighbor potential which provides the closest match to LEFM on a discrete lattice, we show that the discrete nature of an atomic lattice leads to deviations from the LEFM displacement field during energy minimization. We propose that these deviations can be ascribed to geometrical nonlinearities since the material does not have a nonlinear elastic response prior to bond breaking. We demonstrate that crack advance and the critical stress intensity factor in an incremental loading scenario is governed by the collectively loaded region, and can not be determined analytically from the properties (max. elongation, max. sustained force, etc.) of the stressed crack tip bond alone.

Experimental Study on Size Effect and Fracture Properties of Polypropylene Fiber Reinforced Lightweight Aggregate Concrete

In this experimental research, the effects of polypropylene fibers on size effect and fracture properties of lightweight aggregate concrete were studied. Two methods, including size effect method and work of fracture method, were used to investigate and analyze the size effect and fracture properties on different sizes of notched beams. The polypropylene fiber contents were 0.5%, 0.75%, and 1% by volume fraction. The obtained results revealed that increasing the polypropylene fibers in lightweight aggregate concrete relatively decreased the dependence of strength on the size effect parameter, while the addition of polypropylene fibers exhibited significant influence on decreasing the size dependency of ductility and fracture energy in lightweight aggregate concrete. Moreover, the increase of polypropylene fibers improved the total fracture energy (GF), initial fracture energy (Gf), characteristic length (lch), length of fracture process zone (cf), and critical stress intensity factor (KIc) of lightweight aggregate concrete in both size effect method and work of fracture method. This increase was more significant in work of fracture method because of considering the post-peak behavior. The size effect method was suitable and accurate for plain lightweight aggregate concrete. The GF ⁄ Gf ratio increased from 2.88 in plain lightweight aggregate concrete to 12.26 in polypropylene fiber reinforced lightweight aggregate concrete.