Fracture Mechanics Tests Research Articles

Crack penetration versus deflection in extrusion-based additive manufacturing – Impact of nozzle temperature and morphology

Two different modes of fracture propagation can occur when a crack encounters a weak interface in a fused filament fabricated (FFF) part: the crack either deflects into the interface, or penetrates the subsequent layers. The objective of this work is to verify the suitability of an energy- and a strength-based criterion for predicting which failure mode will occur in FFF printed parts. Four different materials, glycol-modified poly(ethylene terephthalate), polylactide acid and two different poly(methyl methacrylate) grades were examined. Fracture mechanical tests were performed on single edge-notched bending specimens for the energy-based approach and tensile tests performed on dumbbell specimens for the strength-based approach. Additionally, porosity measurements and thermal analysis were carried out to provide structural information. The energy-based approach proved unreliable for failure mode prediction. Potential problems include failure to meet the requirements of linear elastic fracture mechanics and issues with notch design. The strength-based approach, in contrast, correctly predicted the crack path for all tested materials and seems a promising candidate for failure mode prediction in FFF materials.

Environment-Induced Crack Initiation and Early Stages of Crack Growth in Aluminum Alloys, Part 2

A second group of ‘state-of-the-art’ contributions [1-4] by invited researchers on the ‘Environment-Induced Crack Initiation and Early Stages of Crack Growth in Aluminum Alloys’ appear in this issue, complementing the papers published in the January 2023 issue of Corrosion [5-8]. The second section, presented herein, includes an overview of the experimental study of the initiation of environment-induced cracking (EIC) in aluminum alloys conducted since the 1950’s [1], showing crack initiation typically dominates the total life experienced during both service-life for structural applications and for smooth tensile test specimens subjected to conventional standard EIC testing. The review also reveals, although research conducted over the last 30 years almost exclusively focus on EIC propagation, there are notable exceptions worthy of re-evaluation and further study using modern in-situ advanced experimental techniques, paying particular attention to the roles of creep, crack-arrest and surface conditions during EIC initiation, paying particular attention the immediate sub-surface layers. The three additional papers in this collection are experimental studies. One investigates the significant differences in EIC initiation observed from fatigue pre-cracks and corrosion-induced fissures under controlled electrochemical conditions in a sensitized AA5456-H116 fracture mechanics test specimen exposed to a marine environment [2]. The final two papers involve testing conducted on high Zn-content Al-Zn-Mg-Cu 7xxx series alloys [3, 4]: one involves two commercial thick pate commercial high Zn content new-generation alloys [3], AA7449 and AA7085 in the T7651 temper and provides a detailed description of EIC initiation and its transition to long-crack growth during exposure to water vapor at 70 °C. Information gleaned from in-situ time-lapse, optical imaging over large areas has enabled the exact initiation sites to be identified and investigated with high-resolution fractographic studies. In the final paper [4] based on the deformation and fracture behavior of a high Zn-content Al-Zn-Mg-Cu 7xxx alloy observed in situ using synchrotron radiation X-ray tomography, it is proposed the dispersion of Mn-based second-phase particles may offer a novel approach for preventing hydrogen embrittlement in 7xxx series aluminum alloys.

Bridging macro to micro-scale fatigue crack growth by advanced fracture mechanical testing on the meso-scale

Fatigue and fracture mechanics testing is strictly regulated by standards that specify a minimum specimen size. However, when component sizes or material volume decrease below this limit, test methods need to be adapted for small scales. In fatigue crack growth studies, experiments become complex and challenging. Thus, there is lack of systematic scale-bridging studies from meso- to micro-scales on ductile materials. This study outlines a method to bridge this scale gap by performing meso-scale studies. Cantilver bending specimens with a cross section of 50x50µm2 and a lever arm length of 200µm of nanocrystalline nickel, as a model material, are milled by Xe plasma focused ion beam technique and tested following ASTM E647 standard. The gathered da/dN-curves are highly reproducible. The threshold stress intensity factor ΔKth is obtained by step-wise load reduction and discussed in context of crack closure. So, not only the gap between macro and micro-scaled testing was bridged, but also a pre-cracking step was shown to be essential for obtaining reliable data, and a new method to determine the intrinsic threshold stress intensity factor was introduced.

Experimental Study of Crack Propagation through Cladded 316L/S355 Steel Produced by the Hot-Roll Bonding Process

A hot-rolled clad plate made of 316L stainless steel and S355 carbon steel was produced in a real industrial environment via GMAW welding, vacuuming, preheating to 1240 °C, and hot-roll bonding on four stand plate mills in nine passes with reduction rates between 22–26% per pass. The clad plates were solution annealed to achieve the usual delivery condition. Ultrasound testing showed a bonding rate of 100%. Metallographic examination revealed predominantly low concentrations of oxides at the interface. The tensile strength of the bond reached 155 MPa. The shear strength always satisfied the requirements of ASTM A264. It was observed that the concentration of oxide particles influenced the shear strength strongly. The lowest measured value was 172 MPa. Specimens notched in the base material and specimens notched in the clad layer were used in the Charpy impact test. In every case, only the notched layer broke, while the other layer was only bent. The absorbed energies were always higher than those of each material alone. Fracture mechanic tests gave similar results. The crack could not propagate from the notched layer into the other one, regardless of the material of the notched layer. Instead, delamination occurred. Due to delamination, K1c could not be determined, so KQ was given instead.

Effect of multi-scale reinforcement on fracture property of ultra-high performance concrete

This paper aims to achieve multi-scale crack control of concrete by using graphene oxide (GO) and macroscopic steel fiber in combination, and to reveal the multi-scale collaborative reinforcement mechanism of GO and steel fiber. A three-point bending fracture mechanics test was employed to analyze the macroscopic fracture properties of ultra-high performance concrete (UHPC) reinforced by GO, and a single fiber pull-out test was utilized to assess the impact of GO on the fiber–matrix interface properties. The results showed that 0.04% GO reinforced UHPC had the best fracture property, and steel fiber and GO showed good synergistic crack control effect. GO directly acted on controlling the cracking of concrete matrix at the nanoscale, thus improving the fracture energy of UHPC matrix by 14% from 73.08 N/mm to 83.39 N/mm. This micro-scale matrix reinforcement provided by GO further improved the pull-out property of steel fibers, and the pull-out energy dissipation of straight steel fibers and hook-end fibers was increased by 43.7% and 31.5%. A micromechanical prediction model for fracture energy of straight steel fiber UHPC was established, which could construct the quantitative relationship between UHPC fracture energy and micromechanical parameters. The model showed that the initial fiber–matrix bond strength (τ0) and slip hardening coefficient (k0) had a powerful influence on the UHPC fracture energy.

Bioinspired fracture toughness enhancement of a fully bio-based epoxy resin

In this work, the concept of fracture toughness improvement via spatial variation of mechanical properties is applied to a composite with 100% bio-based carbon content based on epoxidized linseed oil. By proper selection of the hardener, either citric acid or sebacic acid, the mechanical properties were adjusted in a way that a bio-composite exhibits a stiff-soft-stiff layer architecture. Samples with two different thicknesses of soft interlayer (approx. 0.1 mm and 1.3 mm) were subsequently analyzed regarding their final performance. Specimen characteristics, owing to the curing and manufacturing process, were analyzed by means of local Fourier-transform infrared spectroscopy and differential scanning calorimetry. Fracture mechanics tests were performed to verify if the soft interlayer acts as a crack arrester. The results propose a high chemical compatibility between the used epoxy resins. Embedding a soft thin interlayer into a stiff resin matrix led to an increase in fracture toughness of 13 times, compared to the pure stiff resin. An increase in interlayer thickness led to a further increase in fracture toughness of 24 times. However, the stiffness decreased by 44% and 67%, respectively.

Assessment and evaluation of Chitosan-Metamizole nanoparticles for the fracture healing and analgesic effect: Preclinical study in rat model.

To assess and evaluate Chitosan-Metamizole nanoparticles for fracture healing and analgesic potential, nanoparticles were formulated using the ionotropic gelation method. The nanoparticles were evaluated for particle size, zeta potential, polydispersity index, loading efficiency, surface characteristics and drug release properties. The analgesic activity was determined in carrageenan-induced arthritic male Wister rats. Further fracture healing potency, mechanical testing, radiographic examination and bone histology of the femur were studied. The drug loading efficiency of 11.38%-17.45%, particle size of 140-220nm, and zeta potential of 19.12-23.14mV were observed with a spherical, smooth appearance. Nanoparticles showed sustained release behaviour over a longer period. Nearly 4-fold inhibition of oedema was observed in animals treated with nanoparticles with excellent fracture healing potential. The femurs treated with nanoparticles required greater force to fracture. Nanoparticles significantly improved the strength and healing process. Histopathological studies showed the potential of nanoparticles in the healing process. The study confirmed the potential of nanoparticles in fracture healing and enhancement of analgesic activity.

Effect of heating–water cooling cycle treatment on the pore structure and shear fracture characteristics of granite

In hot dry rock (HDR) geothermal energy extraction systems, jointed rock masses are often subjected to heating–water cooling (H-WC) cycles. In this process, HDR is at risk of failure and instability due to the change in pore structure and fracture characteristics. To identify the shear fracture characteristics of rocks after H-WC cycle treatment. First, shear fracture mechanical tests were carried out on granite specimens treated with H-WC cycles by using the short core compression (SCC) test method. The experimental results show that cyclic H-WC treatment leads to a decrease in fracture toughness values. It is especially noteworthy that the weakening effect of fracture toughness decreases with increasing cycles. Second, the NMR analysis results show that the porosity of granite samples increases with the number of H-WC cycles. At 400℃ and 600℃, the pore size distribution changes under H-WC. The slope of the fitting curve between fracture toughness and porosity indicates that the higher the temperature is, the lower the sensitivity of fracture toughness to porosity. Then, acoustic emission characteristics in the mechanical experimental process were analysed by using the b-value analysis method. The results indicated that the b-value of acoustic emission decreased with increasing cycles at 200 °C, so the crack size and energy release increased; however, at T = 400 °C and 600 °C, the b-value increased with an increasing number of cycles, so the crack size and energy release decreased. Finally, the fracture plane morphology of the failure specimens was obtained by a 3D optical scanner. The calculation results of the fractal dimension DF indicate that the cyclic H-WC treatment enhanced the fracture plane roughness, and the average increase in fractal dimension DF increases with increasing temperature, also indicating that the increasing effect of fracture plane roughness with increasing temperature becomes increasingly significant. At the same time, the fracture toughness under cyclic H-WC action was negatively correlated with the fracture plane roughness of granite.

Large-Scale Tungsten Fibre-Reinforced Tungsten and Its Mechanical Properties

Tungsten-fibre-reinforced tungsten composites (Wf/W) have been in development to overcome the inherent brittleness of tungsten as one of the most promising candidates for the first wall and divertor armour material in a future fusion power plant. As the development of Wf/W continues, the fracture toughness of the composite is one of the main design drivers. In this contribution, the efforts on size upscaling of Wf/W based on Chemical Vapour Deposition (CVD) are shown together with fracture mechanical tests of two different size samples of Wf/W produced by CVD. Three-point bending tests according to American Society for Testing and Materials (ASTM) Norm E399 for brittle materials were used to obtain a first estimation of the toughness. A provisional fracture toughness value of up to 346MPam1/2 was calculated for the as-fabricated material. As the material does not show a brittle fracture in the as-fabricated state, the J-Integral approach based on the ASTM E1820 was additionally applied. A maximum value of the J-integral of 41kJ/m2 (134.8MPam1/2) was determined for the largest samples. Post mortem investigations were employed to detail the active mechanisms and crack propagation.

Effect of Mounting Orientation on Testing Equipment on Elastic Compliance of Clamped SE(T) Specimens: an exploration

The accurate evaluation of fracture mechanics properties is key to the safety of operation of high responsibility structures such as nuclear reactor components. This paper evaluates the impact of mounting orientation on the test machine of clamped SE(T) (SE(T)c) specimens on its compliance. The elastic unloading compliance is a commonly used technique to measure the crack depth of specimens within fracture mechanics tests. Inaccurate measurements affect reliability of resulting measured properties. Two mounting orientations are evaluated in this paper together with two SE(T)c specimens with different width-to-thickness ratios (W/B=2 and W/B=4). Conclusions show that machine stiffness is significantly different in both orientations, and such value has the potential to affect crack depth prediction. On the other hand, the analysis scope of this work was unable to detect significant differences between both orientations, probably because loading was limited to maintain specimen integrity. Further investigation is necessary to precisely adress the impact of this effect.

Controlled surface topography of nanofilm by local strain modulation in mechanical transfer process

The mechanical transfer of nanofilms has been successfully established based on interfacial delamination behaviors. However, controlling the formation of surface cracks and wrinkles in nanofilms during mechanical transfer processes remains a challenge. Here, we demonstrate the controlled surface topography of nanofilms using local strain in the mechanical transfer process. A single cantilever beam (SCB) fracture mechanics test is performed using a specimen composed of an elastomer receiver–Au nanofilm–Si donor structure to investigate the variation in the surface topography of a transferred nanofilm with respect to the receiver thickness, loading angle, and loading rate. The results indicate that owing to the reduction of the tensile strain at the crack tip, the crack density in the nanofilm decreases when a thick elastomer, high loading angle, and low loading rate are applied during the transfer process. Furthermore, beyond the reduction in the tensile strain, the use of a rigid receiver generates a compressive strain at the crack tip, resulting in wrinkles in the nanofilm without cracks. The mechanisms of surface cracking and wrinkling are elucidated by analyzing the strain distribution at the crack tip via numerical simulation. We believe that this study can potentially contribute to advancements in mechanical transfer technology.

Effect of the manufacturing parameters on the tensile and fracture properties of FDM 3D-printed PLA specimens

Fused Deposition Modeling (FDM), also known as material extrusion, is currently one of the most popular Additive Manufacturing (AM) technology on the market. In this study, two commercial FDM printers (Prusa i3 MK3 and WN400 3D Platform) were used for layer-by-layer manufacturing of polylactic acid (PLA) Dog Bone (DB) and Single Edge Notched Bend (SENB) specimens, aimed at investigating the influence of the manufacturing parameters on the tensile and fracture properties of PLA elements obtained by FDM technology. The effects of growing direction (horizontal and vertical), building orientation (0°, 45° and 90°), printer type, layer thickness (0.15 and 0.40 mm), specimen thickness (4 and 10 mm) and filament color (purple, white, black, gray, red, orange) are discussed in detail. Tensile tests were performed on DB specimens, while fracture mechanics tests on SENB specimens. Both the tensile and fracture properties of FDM 3D-printed PLA specimens have been found to be dependent on the investigated manufacturing parameters. From the microstructural analyses of the SENB fracture surfaces, it has been observed that the fracture mechanisms and crack propagation is a step-wise process. Finally, the material properties charts (Young’s modulus and mode I fracture toughness versus tensile strength) are plotted.

A technical approach toward pre-crack generation and its effect on the fracture behavior of polymeric materials

Regarding the urgent need for a proper pre-crack generation method in polymeric materials, this study addressed the design and fabrication of a broaching machining apparatus. The performance of this device was examined by preparing polyvinyl chloride (PVC) samples with high sensitivity to the crack-tip radius. Pre-cracks generated by broaching and slotting and then examined by SEM analysis. The fracture parameters were determined by the essential work of fracture (EWF) method. Studies indicated that the energy required for crack growth was on average 24 % lower in the broached samples, suggesting a higher stress concentration in the crack-tip by the broaching approach. The numerical analysis also revealed the more coincidence of the modeled energy values with the broaching method results, further highlighting the higher resemblance of the cracks made by this method to the ideal cracks. Interestingly, the radius of the plastic region in the slotting method was three times larger than in the broaching method, indicating the sharper tip of the pre-cracks in the broaching method. The application of the broaching method for pre-crack generation can lead to more accurate results in the mechanical fracture tests.

Crack length estimations for small-scale fracture experiments via image processing techniques

Accurate knowledge of the current crack length is crucial to evaluate fracture mechanical tests. At the sub-micron to micron scale, the crack length is directly accessible via observation during in-situ experiments in electron microscopes, or indirectly via calculation from sample stiffness. In the current work, image processing techniques were used to introduce a semi-automatic technique to measure crack lengths at the micron scale from image sequences. The technique utilizes manually defined filters and searches for contours near the previous crack tip locating the new one according to the previous position. To demonstrate validity and capability, three micron-sized notched cantilevers were prepared for bending experiments and tested in-situ by partial unloading. Comparison of crack lengths determined by the proposed method, manual measurement and sample stiffness revealed a reasonable agreement, while occasional deviations allow further insights into the crack behaviour. Thus, our new approach enables more in-depth investigation of small-scale fracture processes.Graphical abstract

Strain rate sensitivity and toughness behaviour of ductile iron under specific loading

An efficient assessment of the mechanical properties of ductile iron (DI) under dynamic loading is usually missing. To address this gap, a plasticity model for DI grade EN-GJS-500-14 was suggested. Fracture mechanics (FM) and Charpy impact (CI) tests, high-speed and impact tensile tests as well as tensile tests at variable temperature and stress states were carried out on DIs with silicon contents from 2.10 to 3.64 wt-%. Toughness-related results indicate a difference in the transition temperatures of 100–140 K between CI and FM tests. The reason for this is assumed to be a pronounced strain rate sensitivity, which could be confirmed in the study. The plasticity model was able to predict toughness properties in CI tests.

Determination of the yield strength and the energy distribution in the milling of Inconel 718 considering fracture toughness

This article presents an analytical method to determine the yield strength and the energy distribution in the milling process of nickel alloy Inconel 718 by incorporating the fracture toughness, the plastic deformation and the friction effect. To study the geometry and mechanics in the oblique cutting process, mathematical analyses are done to derive the angles related to the friction (λ, λn), the shear flow (δ, ϕn) and the chip flow (β). Geometrical model for the serrated chip formation is established to derive the equations that are needed for calculating the fundamental quantities such as forces, velocities and shear strains. Then, the workpiece material's cutting yield strength and fracture toughness are predicted, and efforts also focus on characterizing and classifying the geometrical relationships among the uncut chip thickness, the actual feed rate and the instantaneous radial depth of cut. Equations to calculate the work in relation to the plastic flow, friction and material fracture are established, and subsequently, energy distribution for the serrated chip formation is evaluated from the cutting data. The proposed method has the advantages of convenience, feasibility and generality, compared with the standard tensile tests and the standard fracture mechanical tests, which are usually used to measure the material properties under static loadings or at a range of temperatures and loading rates. A series of milling tests have verified the method.

In-situ X-ray tomographic imaging and controlled steering of microcracks in 3D nanopatterned structures

An experimental approach to control the fracture behavior of 3D nanopatterned structures in real time and to describe the microcrack propagation in solids quantitatively is presented. The three-dimensional details of the complicated failure mechanism are unveiled with high resolution using a method that integrates a micro-scale fracture mechanics test into a nano X-ray computed tomography system, to allow in-situ 3D imaging of the kinetics of damage mechanisms in integrated circuits. With the unique combination of a miniaturized micro-mechanical experiment and high-resolution X-ray imaging, the critical energy release rate at the crack tip of materials is determined quantitatively in sub-100 nm dimension, which allows to reveal scale-dependent mechanical properties. The ability of controlled microcrack steering in engineered materials and structures into regions with high fracture toughness is demonstrated. This unique characterization capability promises broad applications for design and manufacturing of robust microchips in future technology nodes, and it is applicable to the study of a broad variety of 3D nanostructured material systems.

A unified fracture criterion considering stress state dependent transition of failure mechanisms in bcc steels at –196 °C

The fracture properties of a high-strength steel with a body-centered cubic (bcc) crystal structure have been characterized at –196 °C by performing tensile tests with different specimen geometries, three-point bending tests using Charpy specimens, and fracture mechanics tests, covering a broad range of stress states under quasi-static conditions. Both strength and ductility of the bcc steel are significantly increased when the temperature is decreased from room temperature to –196 °C. Enormous plasticity occurs in the material during tensile tests using various specimens at –196 °C, while macroscopic brittle fracture takes place in high triaxiality scenarios. A stress state dependence of ductile to brittle transition properties is observed, as the failure mechanisms at –196 °C change from cleavage fracture to shear failure with decreasing stress triaxiality. A unified stress-state-dependent fracture criterion, which considers the transition of failure mechanisms, is proposed to describe the fracture properties of similar bcc materials at cryogenic temperatures. The threshold triaxiality at which the transition of failure mechanisms takes place is a material property that is determined by the strain hardening capacity and fracture strength. In addition, a probabilistic formulation relying on the extreme value distribution has been incorporated into the model to render the statistical nature of cleavage fracture.

A coupled mix-mode cohesive law based on a cylindrical potential function

A novel mixed-mode cohesive law derived from a potential function is presented. The potential function is formulated using physical parameters that can be extracted from any fracture mechanics test capable of providing R-curves in terms of the J-integral as a function of the normal and tangential end-openings. The proposed cohesive law is able to describe the fracture behaviour of composites with large fracture process zones, including fibre bridging. As such, it is capable of describing both the crack tip, as well as the bridging region. An important aspect of the formulation is that the shape of the mixed-mode cohesive laws are derived and not assumed. The mixed-mode cohesive law was tested using synthetic data emulating mixed-mode fracture mechanics tests. The cohesive tractions extracted from the method exhibited characteristics which were not seeded on the model such as negative normal tractions under pure mode shear loading and non-zero shear loading under pure normal mode loading.

Comprehensive Fracture Model of Reservoir Ice Layers in the Northeastern Cold Region of China

Meteorological and hydrological changes have an important influence on the ice formation mechanism and the detailed structure of ice materials in cold reservoirs, and directly determine the mechanical properties of ice materials. Based on long-term meteorological and hydrological monitoring data, and detailed structural evolution analysis of ice materials, combined with fracture mechanics and energy methods, a comprehensive fracture model of ice materials in cold regions is established. At the same time, the fracture mechanics test results and simulation results of ice materials are compared, and the model is finally optimized accurately to provide theoretical support for the study of the mechanical mechanism of ice materials.