Crack Resistance Curves Research Articles

Investigation of the Influence of Different Filler Contents of Wine Pomace in PBS on Fracture Mechanics Properties

Biobased polybutylene succinate (PBS) represents a promising alternative to petrochemical-based polymers. The use of this biopolymeris limited in many areas by its low resilience against environmental influences. With the help of bio-based stabilizers the thermo-oxidative degradation process can be slowed down. Suitable stabilizing additives are natural antioxidants present in plant extracts with a high flavonoid content, which can be found in grapes, wine and wine by-products. PBS was modified with two different bio-stabilizers based on wine grape pomace. The highest filler content tested was 20 wt.-%. In addition to improving stability, the additives also impact the polymer’s mechanics. The influence of these functional fillers on the fracture mechanical properties was examined in a quasi-static test. The crack growth was recorded using integrated video monitoring. Based on the results, the corresponding crack resistance curve and tearing modulus were determined depending on filler type and content. Additional optical analysis was used to correlate fracture mechanics and structure. The two bio-stabilizers based on red (RWP) and white wine pomace (WWP) differs distinctly in terms of their influence on fracture mechanical properties. The Influence of RWPon the fracture toughness is significantly higher than that of WWP. Especially at lower filler contents with RWP, there is a strong increase in the fracture mechanics parameter tearing modulus (TJ) and an increase in the slope of the R-curve. With 5 wt.% RWP DOM the TJ is 13.64 x 102, whereas with WWP Silv a value of only 6.39 x 102 can be achieved. This difference is also reflected in the increase in the R-curves. With 5 wt.% a slope of the fitted R-curve of 265.59 (RWP DOM) and 121.02 (WWP Silv) could be determined with the first derivative. In the optical analysis it was noticeable that the RWP particles were more homogeneously dispersed in the matrix while the WWP filler tended to agglomerate. The inhomogeneous distribution and strong agglomeration tendency can be attributed to a higher sugar content of WWP and a higher particle size distribution. The top cut (D97) of WWP Silv is 62.37 ± 0.05 µm and that of RWP DOM is 51.97 ± 0.09 µm.

Extracting ductile cast iron microstructure parameters from fracture surfaces: A deep learning based instance segmentation approach

This study investigates the deep-learning based microstructural analysis from SEM images of ductile cast iron fracture surfaces. A Mask R-CNN model was trained, achieving 70% precision and 75% recall in graphite particle detection. Combined with a fracture surface reconstruction using the.4-quadrant backscattered electron signal, key parameters, including the particle size, shape and distance were extracted accurately. Compared to micrograph analysis, following probabilistic simulations showed the impact of the higher microstructural variance for the fracture surfaces on crack initiation, leading to higher scatter and elevated crack resistance curves. This highlights the potential of deep-learning based analysis for comprehensive microstructural characterization.

Evaluating the Suitability of the Gurson—Tvergaard—Needleman Plasticity Model for Crack Resistance Curve Analysis

Evaluating the Suitability of the Gurson—Tvergaard—Needleman Plasticity Model for Crack Resistance Curve Analysis

Numerical investigation on the crack resistance curve of thermally treated quartz‐diorite rock under mixed mode I + II loading

AbstractTo investigate the effects of thermal treatment and loading conditions on the crack initiation and propagation characteristics of quartz‐diorite, mixed mode I + II fracture tests were conducted by using three‐point bending (TPB) specimens. The numerical model, established on ABAQUS, couples the initial fracture toughness criterion and cohesive zone model to predict the crack initiation and propagation. The effects of thermal treatment and loading conditions on the crack propagation process, crack resistance curve, fracture path, and some fracture parameters at peak load have been analyzed and discussed. The results demonstrate that the thermal treatment has an obvious influence on the crack resistance curve of the specimen. The crack of the specimen will be dominated by the mode I fracture component with crack extension. The numerical results also show that the temperature and loading condition affect the crack length and fracture toughness at peak load.

Comparative analysis of unloading compliance and normalization methods for fracture toughness assessment in 316L stainless steel at cryogenic temperatures

In the present study, the impact of test temperature, specimen geometry, and evaluation methodologies on the fracture toughness of 316L stainless steel was evaluated. Tensile and fracture mechanics tests were conducted utilizing a cryostat across a range of temperatures from ambient conditions to liquid hydrogen environments. The analysis of crack length prediction and resistance curve was performed using two fracture toughness evaluation methods: the unloading compliance method and the normalization method. The difference in fracture toughness by specimen thickness using the normalization method was within acceptable tolerances for all temperatures. However, fracture toughness evaluation using the unloading compliance method showed apparent negative crack growth at all temperatures, causing differences in J-integrals between specimen thicknesses and discrepancies between physical and estimated crack extensions. The application of the offset technique to the unloading compliance method significantly reduced these differences and made the J-integral of the normalization method and the unloading compliance method agree well. The substantial underestimation in the correction of the rotation angle at ambient temperatures was confirmed through real-time imaging of the crack tip with the unloading compliance method, which contributed to the large deviations between estimated and measured crack extensions.

Machine learning-based determination of Mode II translaminar fracture toughness of composite laminates from simple V-notched shear tests

This paper presents a novel method for measuring the translaminar crack resistance curve of composite laminates under Mode II shear loading. A machine learning (ML)-based approach is utilized to extract the inapparent information of the crack resistance curve from the translaminar shear strength measurements obtained from simple V-notched shear tests. The entire campaign is built on the framework of the Finite Fracture Mechanics (FFM) combined with Finite Element Method (FEM). Special emphasis is made on the nonlinear mechanical behavior of composites under shear stress since the original FFM models are designed for quasi-brittle materials. With the well-trained recurrent neural network model, the Mode II R-curve of composite laminate can be obtained with un-notched and V-notched shear strength values as inputs. Experiments were conducted on carbon fiber-reinforced composites to validate the accuracy of the R-curve obtained by the proposed approach and that by the traditional compact shear test. The successful implementation of the method suggests a more convenient and low-cost way of obtaining this important damage-related parameter for composites.

Influence of deformation, microstructure, and temperature on the fracture resistance of technically pure tungsten

The fracture resistance of technically pure W with pre-deformed and recrystallized microstructures was tested in a temperature range of ambient to 700 °C. To account for the impact of microstructure anisotropy, samples with different specimen orientations were investigated. Room temperature tests were carried out in a scanning electron microscope and at elevated temperatures in a vacuum furnace. Both, microstructure and testing temperature significantly influence the fracture resistance of W and a strong dependence of the crack propagation resistance on the crack plane orientation was found. Pre-deformation of the testing material increases the fracture resistance in terms of the crack initiation toughness but shortens the measurable crack resistance curve (R-curve). Testing at elevated temperatures results in higher initiation toughness for both states, pre-deformed and recrystallized; however, only the recrystallized structure shows an R-curve behavior for the used sample geometry.

Characterization of fracture behavior under impact-like loading conditions of elastomeric grades for high-pressure gas applications

Rapid gas decompression (RGD) is a major concern for elastomeric sealing components in cyclic gas exposure conditions as it generates cracks and can even cause total mechanical damage threatening the sealing integrity. Therefore, to analyze the fracture behavior of elastomeric grades under impact-like loading conditions, a test methodology was developed using a single edge notch in tension (SENT) specimen. To gain more insights into the behavior of differently formulated four nitrile butadiene rubber (NBR) grades (carbon black (CB) or Silica-filled, with different curing characteristics), tests were conducted at a wide range of temperatures, −20, 23, and 80 °C and under a wide range of loading rates (0.2–5 m/s). The single fracture parameters, strain energy release rate (G) at stable crack initiation and G at maximum load, as well as multiple fracture parameters from crack resistance curves (R-curves), G–Δa and crack growth rate vs crack extension curves, da/dt–Δa, were derived. All the grades showed distinct temperature and loading rate sensitivity on fracture behavior, however, the effect was more pronounced in sulfur-cured grades. Higher cross-link density, delivered a lower resistance to fracture and a uniform higher crack growth rate, conversely higher filler loading improved the fracture resistance. Within the tested conditions, the component-level instrumented tests on RGD failure revealed a similar ranking to the tested grades and it emphasizes the importance and relevance of intrinsic fracture properties. The generated information on single fracture parameters and crack growth kinetics is vital in phenomenological modeling attempts of the actual failure of components as well.

On the predictive capabilities of non‐local models for ductile crack propagation under different levels of stress triaxiality

AbstractDuctile materials are used in many applications such as hydrogen storage and transport, energy plants and additively manufactured components. High safety standards are vital for such applications, which underline the necessity of thoroughly investigating ductile failure to ensure safety and increase components efficiency. Ductile failure is mainly prompted by the evolution of the so‐called ductile damage, characterized by the nucleation, growth and coalescence of microvoids due to plastic deformation. Moreover, the plastic zones formed at the crack tip of ductile materials exhibit high sensitivity to the stress triaxiality level, which in turn distinctly depends on the geometry of the considered component. The quantification of the stress triaxiality at the crack tip is therefore essential to better understand and predict ductile crack propagation and failure. For that reason, a non‐local ductile damage model is employed in this work to simulate the ductile crack propagation under different stress triaxiality conditions. Different geometries are considered, such as constrained geometries of notched bending specimens and unconstrained geometries of center cracked tension specimens, which characterize the different triaxiality levels. To address the effects of thickness and initial crack length, three‐dimensional geometries are simulated, which account for the out‐of‐plane crack‐tip constraints. Finally, to evaluate the prediction quality of the simulations, corresponding experiments have been carried out and direct comparisons are conducted, with respect to the crack length, ductile crack propagation and resistance curves.

A multiscale nonlinear fracture model for staggered composites to reveal the toughening effect of process zone

The attainment of both strength and toughness is a vital requirement for most structural materials. Unfortunately, these properties are generally mutually exclusive in synthetic materials. Natural structural materials defeat the conflict of strength versus toughness, and provide design motifs for the high-performance materials. However, there still lacks a deep understanding of the toughening mechanisms in bioinspired composites. This paper presents a microstructure-based nonlinear fracture mechanics model for staggered composites, to investigate the toughening effect of the nonlinear deformation in the process zone. Firstly, a nonlinear stress–strain relationship of staggered composites is derived, to capture the plastic deformation of the matrix. Then, a multiscale nonlinear fracture mechanics model incorporating the effect of crack bridging and process zone is established based on the above constitutive relation and HRR theory. Finally, the model is used to reveal the influence of geometric and constitutive parameters. The results demonstrate that the process zone leads to a tremendous contribution to the overall toughness. And the crack resistance curve is rising with crack extension due to the increase of the process zone. What is more, there still exists an optimal balance between toughness, stiffness, and strength in biological materials. These results can provide guidelines for the design of high-performance composites.

Longitudinal failure mechanisms and crack resistance curves of unidirectional thermoplastic composites

This paper presents a study on the longitudinal fracture toughness, and associated crack resistance curves, of thermoplastic-based composite materials. Double-edge notched (DEN) specimens loaded in tension and compression are used to relate the size-effect laws with the crack resistance curves of unidirectional carbon-fibre reinforced PEKK. The results are corroborated with SEM and fractographic images to provide a better overview of the main failure mechanisms involved during the failure process. Moreover, tensile tests interrupted at 90% of the estimated failure load are CT-scanned to highlight the early stages of the crack propagation. This investigation confirms the suitability of the size-effect method for characterizing thermoplastic composites and it stands out as a first qualitative and quantitative analysis of their characteristic intra-laminar failure mechanisms.

Quasi‐static fracture characterization of fiber metal laminate with crack resistance curve based on δ5

AbstractFiber metal laminate (FML) is a hybrid composite typical in aircraft industry. It is necessary to effectively characterize the quasi‐static fracture performance of FML, considering the importance of damage tolerance design of aircraft structures and the complicated fracture mechanism of FML. In present investigation, glass fiber reinforced aluminum laminates (Glare) with middle cracks were tested under quasi‐static tensile load. The experimental crack resistance curve (R curve) based on the special crack opening displacement at original crack tip (δ5) was used for revealing the superior crack resistance of Glare laminate to aluminum sheet, which expressed in crack extension aspect was the extended critical crack length. This was thought to be benefit from the contributions of glass fiber layers and fiber/metal delamination, by performing the progressive damage analysis. Since δ5 can embody the elastic–plastic response of the crack but without need to interactively correct for crack tip plasticity. The δ5‐R curve‐based macro and meso fracture models were proposed to accurately predict the residual strength and stable crack extension of Glare laminate, which gave further evidence for the effectiveness of δ5‐R curve in characterizing the quasi‐static fracture performance of FML.

Characterization of Ductile Crack Propagation by Fractal Energy Dissipation Rate

Because of its geometry dependence and loss of physical meaning, the incremental crack resistance curve cannot characterize ductile fractures with large crack extensions and plastic deformations. Therefore, the energy dissipation rate R is employed to overcome these deficiencies, even though specimen size effects still exist. In the study, considering the fractal crack path and concomitant plastic dissipation in the fractal domain, a scale-invariant energy dissipation rate, γp*, is proposed in the context of renormalization group theory. Some experiments in the literature have validated this approach. The fitted fractal energy dissipation rate is independent of the specimen size and initial crack length; moreover, as the specimen size increases, progressive fractality vanishing is found consistently with geometrical multifractality.

Effects of microstructure anisotropy on dynamic fracture behaviors of a selective laser melting nickel-based superalloy

In the study, the crack initiation and propagation behaviors of Ni-based GH3536 manufactured by selective laser melting (SLM) are quantified by the absorbed impact energy and the dynamic crack resistance curves. It is found that the SLM fabricated alloy exhibited slightly lower strength but much higher dynamic fracture toughness along the building direction, compared to those along transverse directions and the hot-rolled counterpart. The microstructure and defects of the SLM GH3536 alloywere characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), electron back-scattered diffraction (EBSD) and micro-computer tomography (μ-CT). Furthermore, the correlation between microstructure anisotropy and dynamic fracture behaviors are explored by the KAM, fractography and crack trajectories analyses. It is concluded that the anisotropy related to dynamic fracture mechanism of GH3536 alloy manufactured by SLM can be attributed to the ‘weakest link’ interface due to the laser melted pool geometry, the grain boundary or dendrite, which is strongly dependent on the relative orientation difference between the building direction and the crack plane.

Estimating the mode I through-thickness intralaminar [formula omitted]-curve of unidirectional carbon fibre-reinforced polymers using a micromechanics framework combined with the size effect method

A three-dimensional micromechanics framework is developed to estimate the mode I through-thickness intralaminar crack resistance curve of unidirectional carbon fibre-reinforced polymers. Finite element models of geometrically-scaled single edge notch tension specimens were generated. These were modelled following a combined micro-/meso-scale approach, where the region at the vicinity of the crack tip describes the microstructure of the material, while the regions far from the crack tip represent the mesoscopic linear-elastic behaviour of the composite. This work presents a novel methodology to estimate fracture properties of composite materials by combining computational micromechanics with the size effect method. The size effect law of the material, and consequently the crack resistance curve, are estimated through the numerically calculated peak stresses. In-depth parametric analyses, which are hard to conduct empirically, are undertaken, allowing for quantitative and qualitative comparisons to be successfully made with experimental and numerical observations taken from literature.

Analytical investigation on the unstable fracture toughness of fine-grained quartz-diorite rock considering the size effect

In this paper, an analytical method was introduced to investigate the size effect on the unstable fracture toughness of fine-grained quartz-diorite. The analytical method can predict the axial load, crack mouth opening displacement, and crack resistance curve of the three-point bend specimen when the elastic modulus, tensile strength, fracture energy, and initial fracture toughness are given. Mode I fracture tests were conducted on the three-point bend specimen with five different depths to acquire these parameters. And, the effectiveness of the analytical method was validated by the experimental results. The analytical method is then utilized to compute the unstable fracture toughness of three-point bend specimens with different sizes and normalized initial crack lengths. The results reveal that unstable fracture toughness is a size-dependent parameter that first increases and then remains unchanged with the increase of the specimen size. The nominal stress follows Bažant’s size effect law and the unstable fracture toughness will converge to the size-independent mode I fracture toughness obtained by Bažant’s size effect model with the increase of specimen depth. Furthermore, for the specimen with a small depth, unstable fracture toughness decreases with the increase in normalized initial crack length, which further illustrates the boundary effect on the unstable fracture toughness of rock specimens.

Mechanical behaviour of W particulate-reinforced Cu composites: Fracture toughness and R-curves

One of the most critical technology challenges regarding the European demonstrational fusion reactor (DEMO) project is to develop a heat-exhaust system (divertor) that fulfils the required structural design criteria at the heat flux loads and irradiation dose level foreseen for the DEMO divertor, which is predicted to be an order of magnitude higher in the DEMO divertor compared to the ITER divertor [1]. To address the material issues posed by this substantial gap in nuclear loads, a dedicated technology R&D work to develop advanced composite materials for the high-heat-flux and heat-exhaust systems is being carried out in the framework of Eurofusion work package "Divertor".These advanced materials are mostly tungsten-copper composites aiming to achieve high-temperature strength and toughness and reduce thermal stresses [2]. However, the limitations of the testing environment and the actual grade of development of these materials make it necessary the evaluation of their fracture properties out of the standard. With this aim, a general analytical model to obtain the crack resistance curve (R-curve) for non-standard samples and with any span-to-depth ratio has been developed.The case study addresses the three-point-bending fracture of three W-Cu (15, 30 and 40 wt.%Cu) metal matrix composites for heat sink applications in future fusion devices. The preliminary experiments show predominant rising R-curves, though their shape and fracture energy slopes depend strongly on the Cu-content and temperature. Thus, it is evident that the energy of the high Cu-content samples is higher than that of the low Cu-content ones, and this difference can be quantified in terms of initial fracture energies and microstructure of the fracture surfaces. Though intense degradation is observed at the highest temperature tested, the mechanical properties at operational temperatures, i.e. below approximately 350 °C, remain relatively high and even better than W/Cu composites reported previously.

Effect of heat treatment on the strength and fracture resistance of a laser powder bed fusion-processed 18Ni-300 maraging steel

Engineering materials processed using additive manufacturing (AM) techniques such as laser powder bed fusion (LPBF) often exhibit unique microstructures and defects that must be controlled to obtain peak performance in mechanical properties. Towards this end, we here investigate how three heat-treatments (direct aging, solution treatment + aging, and thermal cycling + aging) impact microstructure evolution and corresponding mechanical performance of LPBF-processed 18Ni-300 maraging steel. Specifically, we focus on how strength and fracture toughness measured in two orthogonal directions correlate with phase transformation kinetics as well as micro- and meso-structural features and compare findings with results from as-built and conventionally processed material. Aging results in the formation of Ni–Ti and Mo-rich nano-precipitates that enhance strength but reduce both ductility and fracture resistance when compared to as-built material. With the solution treatment, however, AM-characteristics such as cellular sub-structures and melt-pool boundaries dissolve almost entirely and as a result the damage-tolerance capacity of the material degrades and becomes comparable to cast material. Thermal cycling, on the other hand, results in the formation of reverted austenite thereby enabling the transformation-induced plasticity (TRIP) effect. This leads to extensive gains in ductility that are accompanied by only moderate reductions in strength while fracture toughness is improved significantly. While anisotropy in the obtained data from both the tensile tests and the crack resistance curve (R-curve) measurements is noticeable, it is low relative to many LPBF-processed alloys due to the strong bonding characteristics between the individual layers.

The rising crack resistance curve behavior and mechanism of La2O3 doped zirconia toughened alumina composites prepared via vat photopolymerization based 3D printing

In this study, we combined liquid precursor infiltration of high introduction amounts of bi-additives (20 wt%) and vat photopolymerization-based 3D printing to fabricate zirconia toughened alumina, and the infiltration systems consist of the four following systems Zr4+/La3+, Zr4+/Ce4+, Zr4+/Gd3+, and Zr4+/Er3+. The sample immersed with Zr4+/La3+ shows intense peaks of m-ZrO2 phase compared to the other samples while a new phase of rod-like LaAl11O18 grain occurs in the Zr4+/La3+ immersed sample, the existence of which could be confirmed by XRD and EDS. The fracture toughness of the Zr4+/Ce4+, Zr4+/Gd3+, and Zr4+/Er3+samples remained basically unchanged versus the crack size, with the fracture toughness between 4 MPa m1/2 and 5 MPa m1/2; while the measured fracture toughness values for the Zr4+/La3+ system show a rising crack resistance curve behavior, with the toughness enhanced from 6.37 MPa m1/2 to 8 MPa m1/2. The obvious enhancement of the toughness could be attributed to thermal expansion misfit and rod-like LaAl11O18 in the Zr4+/La3+ system. The toughening mechanism and the effect of residual stresses on the fracture mode are discussed on the basis of theoretical calculation and analysis. The rising crack resistance curve behavior could provide a new toughening strategy in ceramic 3D printing.

Research on the effect of thermal treatment on the crack resistance curve of marble using notched semi-circular bend specimen

The crack resistance curve is widely used to describe the complete fracture process of quasi-brittle materials. In the paper, the crack resistance curves of thermally treated marble were tested through the notched semi-circular bend specimens. The acoustic emission (AE) and digital image correlation (DIC) techniques were used to monitor the fracture behaviors of the specimens during the test. The AE results indicated that the crack will initiate from the notch when the axial load is less than the peak load. And, when the axial load reaches the peak load, the DIC results demonstrated that there exists a large fracture process zone (FPZ) in front of the crack notch. The effect crack length was calculated by the compliance method, which is coincided with the DIC results. The amplitude of the crack resistance curves decreases with the increase of treatment temperature, which suggested that the fracture resistance of the specimen declines due to thermal treatment. Based on the K-resistance curves, the initial fracture toughness, unstable fracture toughness, and fracture toughness when FPZ is fully formed have been determined for the thermally treated specimens. The results showed that these parameters both decrease in the increase of treatment temperature. Moreover, The FPZ length first increases and then decreases with the increase of normalized crack length and the critical and maximum non-dimensional FPZ length tends to increase with the increase in treatment temperature.