Critical Stress Intensity Factor KIC Research Articles

Research on Crack Propagation of Nitrate Ester Plasticized Polyether Propellant: Experiments and Simulation.

To understand the fracture properties of the nitrate ester plasticized polyether (NEPE) propellant, single-edge notched tension (SENT) tests were carried out at room temperature (20 °C) under different tensile rates (10-500 mm/min). The mechanical response, crack morphology, evolution path, and crack propagation velocity during the fracture process were studied using a combination of a drawing machine and a high-speed camera. The mode I critical stress intensity factor KIc was calculated to analyze the tensile fracture toughness of the NEPE propellant, and a criterion related to KIc was proposed as a means of determining whether the solid rocket motors can normally work. The experimental results demonstrated that the NEPE propellant exhibited blunting fracture phenomena during crack propagation, resulting in fluctuating crack propagation velocity. The fracture toughness of the NEPE propellant exhibited clear rate dependence. When the tensile rate increased from 10 mm/min to 500 mm/min, the magnitude of the critical stress intensity factor increased by 62.3%. Moreover, numerical studies based on bond-based peridynamic (BBPD) were performed by modeling the fracture process of the NEPE propellant, including the crack propagation speed and the load-displacement curve of the NEPE propellant. The simulation results were then compared with the experiments.

CaSiO3-HAp Metal-Reinforced Biocomposite Ceramics for Bone Tissue Engineering.

Reconstructive and regenerative bone surgery is based on the use of high-tech biocompatible implants needed to restore the functions of the musculoskeletal system of patients. Ti6Al4V is one of the most widely used titanium alloys for a variety of applications where low density and excellent corrosion resistance are required, including biomechanical applications (implants and prostheses). Calcium silicate or wollastonite (CaSiO3) and calcium hydroxyapatite (HAp) is a bioceramic material used in biomedicine due to its bioactive properties, which can potentially be used for bone repair. In this regard, the research investigates the possibility of using spark plasma sintering technology to obtain new CaSiO3-HAp biocomposite ceramics reinforced with a Ti6Al4V titanium alloy matrix obtained by additive manufacturing. The phase and elemental compositions, structure, and morphology of the initial CaSiO3-HAp powder and its ceramic metal biocomposite were studied by X-ray fluorescence, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Brunauer-Emmett-Teller analysis methods. The spark plasma sintering technology was shown to be efficient for the consolidation of CaSiO3-HAp powder in volume with a Ti6Al4V reinforcing matrix to obtain a ceramic metal biocomposite of an integral form. Vickers microhardness values were determined for the alloy and bioceramics (~500 and 560 HV, respectively), as well as for their interface area (~640 HV). An assessment of the critical stress intensity factor KIc (crack resistance) was performed. The research result is new and represents a prospect for the creation of high-tech implant products for regenerative bone surgery.

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.

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.

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.

Fracture Performance of Cementitious Composites Based on Quaternary Blended Cements

This study presents test results and in-depth discussion regarding the measurement of the fracture mechanics parameters of new concrete composites based on quaternary blended cements (QBC). A composition of the two most commonly used mineral additives, i.e., fly ash (FA) and silica fume (SF), in combination with nanosilica (nS), has been proposed as a partial replacement for ordinary Portland cement (OPC) binder. Four series of concrete were made, one of which was the reference concrete (REF) and the remaining three were QBC. During the research, the main mechanical parameters of compressive strength (fcm) and splitting tensile strength (fctm), as well as fracture mechanics parameters and the critical stress intensity factor , along with critical crack-tip opening displacements (CTODc) were investigated. Based on the tests, it was found that the total addition of siliceous materials, i.e., SF + nS without FA, increases the strength and fracture parameters of concrete by approximately 40%. On the other hand, supplementing the composition of the binder with SF and nS with 5% of FA additive causes an increase in all mechanical parameters by approximately 10%, whereas an increase by another 10% in the FA content in the concrete mix causes a significant decrease in all the analyzed factors by 10%, compared to the composite with the addition of silica modifiers only.

Toughness arising from inherent strength of polymers

This study carries out spatial-resolved optical birefringence observations to quantify, for mode I (tensile opening) loading, the stress intensification at crack tip of brittle and ductile glassy polymers (polymethyl methacrylate—PMMA, polyethylene terephthalate—PET) as well as one type of elastomer: (ethylene propylene diene monomer—EPDM). We measure the stress buildup in a precut specimen by correlating retardation with the corresponding tensile stress. Given the adequate spatial resolution under 10μm and natural bluntness of intentional through-cuts, we are able to show that (a) during drawing at different stages up to the onset of fracture in precut PMMA and EPDM, the local stress saturates, namely, ceases to increase as r−1/2 upon approaching the cut tip (with r reaching rss in a range of 0.05–0.15 mm), (b) tip stress σtip, i.e., the tensile stress in the stress saturation zone (r≤rss), linearly grows with (operationally defined) stress intensity factor KI until fracture, reaching a level below the breaking stress σb observed of uncut specimen. Thus, the inherent strength σF(inh) under plane strain, taken to be the tip stress at fracture is only comparable to σb. Moreover, a characteristic length P, involved in the observed linearity between KI and σtip, i.e., in KI=σtipP1/2, is found to be comparable to 2πrss. Here rss appears to depend on the tip sharpness, which may be characterized by a radius of curvature ρtip. Thus, toughness given by the critical stress intensity factor KIc is determined by the product of σF(inh) and ρtip1/2, and the critical energy release rate GIc is given by the product of specific work of fracture wF = [σF(inh)]2/2E and ρtip.

Methods for Accuracy Increasing of Solid Brittle Materials Fracture Toughness Determining

Method for determining of the fracture toughness of brittle materials by indentation is described. The critical stress intensity factor KIC quantifies the fracture toughness. Methods were developed and applied to improve the accuracy of KIC determination due to atomic force microscopy and nanoindentation. It is necessary to accurately determine parameters and dimensions of the indentations and cracks formed around them in order to determine the KIC . Instead of classical optical and scanning electron microscopy an alternative high-resolution method of atomic force microscopy was proposed as an imaging method.Three methods of visualization were compared. Two types of crack opening were considered: along the width without vertical displacement of the material and along the height without opening along the width. Due to lack of contact with the surface of the samples under study, the methods of optical and scanning electron microscopy do not detect cracks with a height opening of less than 100 nm (for optical) and less than 40–50 nm (for scanning electron microscopy). Cracks with opening in width are determined within their resolution. Optical and scanning electron microscopy cannot provide accurate visualization of the deformation area and emerging cracks when applying small loads (less than 1.0 N). The use of atomic force microscopy leads to an increase in accuracy of determining of the length of the indent diagonal up to 9.0 % and of determining of the crack length up to 100 % compared to optical microscopy and up to 67 % compared to scanning electron microscopy. The method of atomic force microscopy due to spatial three-dimensional visualization and high accuracy (XY ± 0.2 nm, Z ± 0.03 nm) expands the possibilities of using indentation with low loads.A method was proposed for accuracy increasing of KIC determination by measuring of microhardness from a nanoindenter. It was established that nanoindentation leads to an increase in the accuracy of KIC determination by 16–23 % and eliminates the formation of microcracks in the indentation.

Study of the destruction of the chassis main cross member made of VT22 alloy

The reasons for the destruction of the chassis main cross member made of alloy VT22 are considered and analyzed in bench test conditions. The chemical composition, mechanical properties, as well as macro- and microstructure of the material were studied. The tests of the cross-arm material for crack resistance and low-cycle fatigue (LCF) with the determination of the durability were carried out. The results of analysis proved that material meets the declared performance characteristics. A fractographic study of the traverse fracture showed that the fracture occurred from several foci according to the fatigue mechanism. The length of the longest fatigue crack was 1.7 mm and the critical stress intensity factor KIc was thus attained. Proceeding from the dimensions of the part at the site of fracture, the maximum crack length and the value of the critical stress intensity factor obtained experimentally KIc = 56.5 MPa • m1/2, we have calculated the nominal tensile stress at the moment of fracture. The calculated value of the nominal stresses is 1022 MPa, which is comparable to the yield strength of the material (1100 MPa). A high level of tensile stresses in the loading cycle is considered the most probable reason for the destruction of the chassis main cross member in the conditions of bench tests.

Fracture toughness of ABS additively manufactured by FDM process

Purpose: The purpose on this article is to study the failure of FDM printed ABS by exhibiting an exhaustive crack growth analysis mainly based on raster angle parameter. Design/methodology/approach: Two approaches have been developed in this study; On one hand, mechanical experiments were carried out to determine the critical stress intensity factor KIC. On the other hand, numerical analysis was used to predict the paths within the part as well as the crack propagation. Findings: This work has clearly shown the effect of raster angle on the damage mechanism of the ABS printed by FDM. Indeed, for the combination 1 (0°/90°), the structure presents an important stiffness and a high degree of stress distribution symmetry with respect to the notch. Moreover, the crack propagation is regular and straight, and the damage surfaces are on the same plane. However, for the combination 2 (-45°/45°), the structure is less resistant with an asymmetrical stress distribution according to two different planes. Research limitations/implications: In order to present an exhaustive study, we focused on the effect of two raster angles (including 0°/90°, -45°/45°) on the ABS crack propagation, additively manufactured. This study is still in progress for other raster angles, and will be developed from a design of experiments (DoE) design that incorporates all relevant factors. To highlight more the cracking mechanisms, microscopic observations will be developed in more depth. Practical implications: Our analysis can be used as a decision aid in the design of FDM parts. Indeed, we can choose the raster angle that would ensure the desired crack propagation resistance for a functional part. Originality/value: In this article, we have analyzed the mechanism of damage and crack propagation. This topic represents a new orientation for many research papers. For our study, we accompanied our experimental approach with an original numerical approach. In this numerical approach, we were able to mesh distinctly raster by raster for all layers.

Matrix Composite Coatings Deposited on AISI 4715 Steel by Powder Plasma-Transferred Arc Welding. Part 3. Comparison of the Brittle Fracture Resistance of Wear-Resistant Composite Layers Surfaced Using the PPTAW Method.

This article is the last of a series of publications included in the MDPI special edition entitled “Innovative Technologies and Materials for the Production of Mechanical, Thermal and Corrosion Wear-Resistant Surface Layers and Coatings”. Powder plasma-transferred arc welding (PPTAW) was used to surface metal matrix composite (MMC) layers using a mixture of cobalt (Co3) and nickel (Ni3) alloy powders. These powders contained different proportions and types of hard reinforcing phases in the form of ceramic carbides (TiC and WC-W2C), titanium diboride (TiB2), and of tungsten-coated synthetic polycrystalline diamond (PD-W). The resistance of the composite layers to cracking under the influence of dynamic loading was determined using Charpy hammer impact tests. The results showed that the various interactions between the ceramic particles and the metal matrix significantly affected the formation process and porosity of the composite surfacing welds on the AISI 4715 low-alloy structural steel substrate. They also affected the distribution and proportion of reinforcing-phase particles in the matrix. The size, shape, and type of the ceramic reinforcement particles and the surfacing weld density significantly impacted the brittleness of the padded MMC layer. The fracture toughness increased upon decreasing the particle size of the hard reinforcing phase in the nickel alloy matrix and upon increasing the composite density. The calculated mean critical stress intensity factor KIc of the steel samples with deposited layers of cobalt alloy reinforced with TiC and PD-W particles was 4.3 MPa⋅ higher than that of the nickel alloy reinforced with TiC and WC-W2C particles.

Study of the influence of nanoparticles on the behavior of composite materials

The objective of this work is to study the influence of additives of multi-walled carbon nanotubes (MWCNT) and silica nano-powder (SiO2) on the mechanical behavior of epoxy resin. Different volume fractions of MWCNT and SiO2were added. Mechanical characterization by tensile, three-point bending and Charpy tests were carried out. The experimental results show an increase in the mechanical performance of the mixtures (MWCNT ​+ ​epoxy), (SiO2 ​+ ​epoxy) up to 2% of additive. Beyond this value, a degradation of performance was observed. The addition of MWCNT gives better results when compared to the addition of SiO2. KIC-KCV correlations were made using empirical formulas to estimate the critical stress intensity factor KIC from the impact energy of the Charpy test.Unfortunately, this estimation does not provided a promising results, but other optimization methods were used to fit these empirical models to the behavior of our nanocomposites for which a good estimate was obtained after fitting these empirical models for SiO2.

The use of AFM in assessing the crack resistance of silicon wafers of various orientations

Crack resistance of silicon wafers plays a vital role in development of MEMS technologies containing beam elements. In the present research, this characteristic was determined using the Vickers tip indentation method. The critical stress intensity factor KIC and fracture energy GIC of silicon wafers of (100), (110), and (111) crystallographic orientations were evaluated. The measurements were supplemented by imaging of indents using atomic force microscopy (AFM). The correlation of these parameters with the specific surface energy, Young’s modulus E and microhardness H was conducted. The values of E and H were evaluated by nanoindentation. The dependences of KIC and GIC on the load of silicon wafers of (100), (110), and (111) orientations were obtained.

Railway Rail Material Quality Tests

The article presents the obligatory qualitative tests of the material of railway rails, performed for the railway industry and having the status of qualifying tests, to which railway rails are subjected. They include tests of fracture mechanics (determination of the critical stress intensity factor KIC, determination of the fatigue crack development velocity da / dN), determination of stresses in rail feet and fatigue tests. The article presents the results of tests of standard-gauge rails type 60E1, rolled from R260 steel by selected European producers, and an analysis of the results was carried out based on the guidelines of the PN-EN 13674-1 + A1: 2017-07 standard. Keywords: quality tests of rails, KIC coefficient, propagation da/dN, stress

Mode I fracture toughness determination in Cu/W nano-multilayers on polymer substrate by SEM - Digital Image Correlation

Nanostructured metallic multilayers with carefully designed mechanical and functional properties are omnipresent in cutting edge technological applications. To ensure the mechanical integrity of such coatings, the Mode I critical Stress Intensity Factor KIC is used to quantify their fracture toughness in order to avoid material failure by appropriate design. In this article, we present a novel approach for the KIC determination of thin and ultrathin films on compliant substrate, based on micro-displacement field analysis using Digital Image Correlation within SEM. Using this method, KIC of a Cu/W nano-multilayer with a total coating thickness of 240 nm was determined as KIC=4.8±0.05MPam, showing excellent agreement with the values published for comparable systems in the literature. To verify the validity of the chosen approach, two independent finite element simulations were employed, thus revealing the role and effect of the compliant substrate on the stress and displacement fields arising around the crack tip in thin films.

Mechanical Behavior of Mode I Fractured Concrete Repaired by Polymethyl Methacrylate (PMMA)

This paper attempts to investigate the fracture toughness of the cracked concrete repaired by polymethyl methacrylate (PMMA) - a thermo-plastic polymer resin. A single-edged notch beam in three-point bending test was conducted to examine the performance of cracked concrete beam after reparation. Three concrete crack widths of 1 mm, 5 mm, 10 mm were designated to be repaired and examed. The fracture properties include critical stress intensity factor KIC by Jenq and Shah and fracture energy GF by Hillerburg, were evaluated for the concrete before cracking and after reparation. Four sand contents of 0%, 10%, 20% and 40% by volume were used in the PMMA mortars. From the testing results, it appears that the load capacity and fracture toughness of repaired concrete increased with the increasing sand content. And, the PMMA mortar made repaired concrete better strength and fracture toughness as well.

Static Crack Resistance of Heat-Resistant KhN43MBTYu Nickel-Chromium Alloy in Gaseous Hydrogen

The effect of hydrogen at a pressure of up to 35 MPa and a content of up to 29 ppm on the strength, ductility, short-term and long-term static crack resistance of four KhN43MBTYu (EP-915VD) alloy modifications with different heat treatment modes and chemical composition has been studied. It has been found that the critical stress intensity factor KIc in the presence of hydrogen, just as the ductility characteristics of smooth specimens, depends on the deformation rate, reaching minimum values at rates of less than 0.1 mm/min. The fracture toughness decreases under the action of hydrogen by a factor of 2.5, and the plane strain state occurs at a much smaller specimen thickness. An optimal combination of high strength, ductility, short- and long-term static crack resistance in air and hydrogen has been achieved in a fine-grained alloy with low carbon and sulfur content. Based on the results of long-term static crack resistance tests at the predetermined maximum fatigue test duration of 300 h, the invariant characteristics of crack resistance, the threshold values of stress intensity factor in hydrogen, have been determined, which vary from 23 to 48 MPa ∙ m1 / 2 depending on the alloy heat treatment mode.

A Fractal Model of Cracking of Cement Matrix Composites

The modern methods of materials (including cement matrix materials) design and testing impose the application of an approach appropriate to materials engineering. A quantitative description of the association between the properties of these materials and their structure is a necessity. What remains the scientific aim, however, is the clarification and description of the occurring phenomena by means of models mapping their actual behavior in the closest way possible. The article presents a cracking fractal model based on tests on the morphology of concrete fracture surfaces. The recorded fractal nature of the cracking of cement matrix materials enabled fractal geometry in the model development to be applied. Owing to the application of statistical analysis, together with an extensive base of data on the profile lines separated out of the real fracture surfaces of concrete, it was possible to develop a cracking fractal model. Not only does this model satisfy the condition of the equality of the fractal dimension of the real and model profile lines, it also offers the possibility of introducing an order to the apparently chaotic phenomena, such as the cracking process. An advantage and novelty of the model is that unlike the other authors’ proposals, there is a possibility of reaching an infinitely large number of solutions for model profile lines, which approximates the model to the real-life scenario. The results of fractal tests were supplemented with strength measurements, identifying concrete’s compressive and fracture toughness (determining the critical stress intensity factor KIcS). A connection between the fractal dimension and the investigated properties of concrete was demonstrated. A higher fractal dimension was observed in the profile lines separated out of the fracture surfaces of concretes of higher water–cement ratio. The advantages of the model include the simplicity and applicability in model studies on other materials of the cement matrix.

Optimization of Composite Process Parameters on Fracture Toughness of Glass Fiber Reinforced Plastics

The objective of the work was to investigate statistical analysis of fracture toughness of the glass fiber reinforced composites at different stacking sequences, strain rate and crack length. The glass reinforced Vinylester composites prepared by hand lay-up technique with various stacking sequences such as 0/90, 45/-45 and chopped strand orientations. 3-point bending for fracture toughness test for various notch depth ratio’s as per linear elastic fracture mechanics concept was conducted. Further the initial notch depth method was adopted to find the critical stress intensity factor KIC for the given notch depth ratio. Design of experiment using the Taguchi L9 array was formulated to understand the influence of parameters on fracture toughness. ANOVA was performed to find out the influencing weightage factor for fracture toughness. Regression analysis was used to develop a mathematical model that gave the contribution of individual parameters on responses. The fracture surface was investigated using scanning electron microscopy (SEM).The DOE response surface methodology showed the stacking sequence of [45/-45/0/90/Chopped]3s, with strain rate of 0.5 mm/min and without notch having the highest fracture toughness of 81.36 Mpa . ANOVA was carried out to know the statistical significance of factors influencing fracture toughness, Results showed that notch depth ratio was the most influencing factor with 68.67 % and stacking sequence with 24%. Accuracy of developed regression model was 90.61 % (R2), which indicating the confidence level of regression.

Development of experimental methodology on fracture toughness of microscale polymer resins under in situ observation

The purpose of this study is to develop a method on fracture toughness test for single-digit micrometer scale polymeric materials, including shape design of the specimen and introduction of precrack. Acrylonitrile butadiene styrene (ABS) polymer resin is employed as test material for the development. The specimens are carefully manufactured by means of the focused ion beam (FIB), and a precracks are successfully introduced at the notch root by the asymmetric loading. After then, the specimens are reshaped, and the mode I load is applied for the evaluation of fracture toughness. The stress field around the crack tip is calculated by the finite element analysis, and the critical stress intensity factor KIC is found to be KIC = 0.145 to 0.175 MPa .