Fracture Toughness Of Specimens Research Articles

A phase-field fatigue fracture model considering the thickness effect

Modern aero-engine turbine structures often have variable-thickness geometries (e.g., turbine structures with complex cooling systems) that pose a significant challenge to crack analysis. The phase-field fracture method is a numerical method that has recently received considerable attention. However, phase-field fatigue fracture models that consider the effect of thickness are still lacking. To address this problem, this paper proposes a modification of the fracture toughness in the phase-field fatigue model by introducing a relationship function between material fracture toughness and thickness (Gc-B curve). We use reference experimental data to fit the Gc-B curve of GH4169 and predict the fracture toughness of specimens with other thicknesses. The predictions of the proposed model agree well with the experimental results reported in the literature. This achievement contributes significantly to the structural safety design and life prediction of aero-engine components.

Experimental evaluation of fracture toughness of basalt macro fiber reinforced high performance lightweight aggregate concrete

High performance lightweight aggregate concrete has become a popular topic for the development of green concrete due to the incorporation of various industrial wastes to substitute aggregate. However, its brittleness has gradually emerged as an issue with the increasing of compressive strength. To address this concern, the authors in this paper investigated the influence of basalt macro fiber (BMF) on the compressive and splitting tensile strengths of lightweight aggregate concrete at different fiber volume contents and lengths. Moreover, the three-point bending test on notched beams, with the help of acoustic emission and digital image correlation, was also carried out to evaluate the fracture toughness of basalt macro fiber reinforced lightweight aggregate concrete. Finally, the softening constitutive model was determined in the present study. The results showed that the addition of BMF increased the compressive strength of lightweight aggregate concrete and the maximum increase reached 9.03%. As for splitting tensile strength, the maximum increase was 39.88%. In addition, it was observed that the incorporation of basalt macro fiber significantly improved the initial cracking toughness and unstable fracture toughness of plain specimen. The fracture energy of BMF reinforced lightweight aggregate concrete was also increased, and the largest improvement was seen in BMF-0.5–22.5 group, 8.5 times that of plain specimen.

Degradation of SMA-13 low-temperature performance under coupled dry and wet-salt corrosion

To investigate the low-temperature performance of polyester fiber-modified asphalt mixtures under the influence of dry-wet-salt corrosion coupling, six different polyester fiber content levels (0%, 0.3%, 0.35%, 0.4%, 0.45%, and 0.5% by weight of the mixture) were chosen for preparing SMA-13 asphalt mixture specimens. The Ultrasonic non-destructive testing, semi-circular bending tests, and scanning electron microscopy (SEM) were employed to evaluate the low-temperature performance of these mixtures under dry-wet-salt corrosion conditions with varying cycle numbers (0, 2, 4, 6, 8 cycles). The results of ultrasonic non-destructive testing showed that the wave velocity exhibited a trend of initial increase followed by a subsequent decrease with the increase in polyester fiber content. It reached a maximum value of 4.06 cm/us at a fiber content of 0.4%. The semi-circular bending tests (SCB) revealed that the fracture toughness of the polyester fiber asphalt mixture decreased as the number of dry-wet cycles increased. The impact of salt-dry-wet coupling > water-dry-wet coupling. Notably, after undergoing 4 cycles of water-dry-wet exposure, the overall fluctuation in specimen fracture toughness was minimal, indicating that results after four cycles could be suitable for evaluating the long-term degradation performance of asphalt mixtures under extended water-dry-wet cycling. SEM test results demonstrated that a polyester fiber content of 0.4% led to the formation of a strong fibrous network structure within the asphalt mixture. This significantly reduced moisture and salt intrusion during the dry-wet cycling process, enhancing the low-temperature crack resistance of the specimens.

Hygrothermal effects on translaminar fracture toughness of quasi-isotropic laminates with different stacking sequences

Hygrothermal effects on the translaminar fracture toughness of the carbon/epoxy composite with two different stacking sequences at 120 °C were studied and the failure mechanisms are explained. The eccentrically loaded single-edge-notch tension (ESET) experiments showed that two different quasi-isotropic stacking sequences ([90/45/0/-45]4s and [45/90/-45/0]4s) have negligible difference in translaminar fracture toughness. It was found that the fracture toughness of conditioned specimens at hot temperature wet (HTW) condition increased by 75 % compared to that at room temperature dry (RTD). The average saturated damage height at HTW increased by 48 % compared with that at RTD according to ex situ X-ray computed tomography (CT) observation. The larger saturated damage height observed in the 0° plies of the HTW specimens results in the higher translaminar fracture toughness.

Effect of final temperature and heating rate on the mechanical and optical properties of a zirconia veneering ceramic

PurposeTo evaluate the effect of firing temperature and heating rate on the volumetric shrinkage, translucency, flexural strength, hardness, and fracture toughness of a zirconia veneering ceramic. Material and methodsZirconia veneering ceramic specimens (N = 45) with varying final temperatures (730 °C, 750 °C, and 770 °C) and heating rates (70 °C/min, 55 °C/min, and 40 °C/min) were fabricated (n = 5). Each specimen's shrinkage, translucency, flexural strength, hardness, and fracture toughness were determined. Two-way analysis of variance, Scheffé test, and Pearson's correlation analysis were used to evaluate data (α = 0.05). ResultsThe shrinkage (44.9 ± 3.1–47.5 ± 1.6 vol%) and flexural strength (74.1 ± 17.4–107.0 ± 27.1 MPa) were not affected by tested parameters (P ≥ 0.288). The interaction between the main factors affected the translucency, hardness, and fracture toughness of the specimens (P ≤ 0.007). Specimens with 770 °C final temperature and 70 °C/min heating rate had the lowest (21.8 ± 3.2 %) translucency (P ≤ 0.039). The hardness ranged between 4.98 ± 0.51 GPa (730 °C; 70 °C/min) and 5.60 ± 0.37 GPa (770 °C; 70 °C/min). Fracture toughness ranged between 0.54 ± 0.04 MPa√m and 0.67 ± 0.08 MPa√m with the highest values for specimens fired at 730 °C with 70 °C/min (P ≤ 0.001). There was a positive correlation between translucency and hardness (r = 0.335, P = 0.012), and a negative correlation between fracture toughness and all parameters other than shrinkage (translucency: r = −0.693/P < 0.001, flexural strength: r = −0.258/P = 0.046, hardness: r = −0.457/P < 0.001). ConclusionsHeating rate and final temperature should be considered while fabricating veneered zirconia restorations with tested ceramic as they affected the translucency, hardness, and fracture toughness.

Edge notched disc test for evaluation of mode-I fracture toughness of brittle material

In the current investigation, the fracture toughness of plexiglass specimens was accurately measured employing Edge Notched Disc (END) type specimens. Besides, the Notched Brazilian Discs (NBD) were used in order to validate the results of the conducted END experiments. Moreover, a numerical analysis was carried out on the END tests to verify the accuracy of the measured fracture toughness values. In the first stage, the splitting tensile test was used to accurately calibrate the micro-scale properties of the Flat Joint (FJ) model. In the second phase, the numerical simulation of END and NBD models was created, in which the notch lengths in END and NBD were set to 5, 15, 25, 40 and 10, 20, 30, 40, 50, 60 in millimeters, respectively. According to the obtained results, the considered FJ model is capable of making an accurate determination of the potential crack growth path and crack initiation stress like the experimentally obtained results for plexiglass specimens. It was also derived that lengthening the notch could not have a considerable effect on the fracture toughness of the models. The Tensile stress distribution on the failure surface of END test could be the main cause of the lower fracture toughness levels of END test. Besides, the same fracture extension patterns were seen from the experimental investigations and numerical simulations.

Constraint loss correction: Enhancing transferability of fracture toughness of Eurofer97 compact tension specimens between specimen sizes

This paper proposes and implements a constraint loss correction as a supplement to the ASTM correction factor for the master curve method (ASTM-E1921) for Eurofer97 reduced activation steel in order to enhance the transferability of results between compact tension (CT) specimen sizes. To facilitate the acquisition and scalability of reliable fracture toughness data from one specimen size to another and to apply the Master-Curve method with confidence when using small specimens, the difference in the structure of the crack tip stress field between large and small specimens was modeled. The stress intensity factors and critical volumes/stresses of two specimen sizes (1 T and 0.18 T) were calculated with 3D finite element simulations run over a large displacement range and at a temperature range of −120 to −80 °C and the results were compared with real data from a database of toughness tests of Eurofer97 steel. The K values from a miniaturized specimen without or with ASTM correction and with the proposed combined correction with constraint loss factor (which is variable over the entire loading range) were compared with simulated CT-1 T stress intensity values. It was found that this new correction factor was accurate up to a target toughness of 200 MPa.m1/2, which represents a major improvement on the current state of the art.

Effect of aging on the low-temperature performance of fiber-reinforced asphalt mixtures

To study the effect of aging on the low-temperature crack resistance of fiber asphalt mixes, three kinds of fibers (lignin fiber, polyester fiber, and basalt fiber) were dry blended into AC-13 asphalt mixes to prepare unaged, asphalt aged (that is, the original asphalt material is subjected to aging treatment), short-term aged, and long-term aged fiber reinforced asphalt mixes. Semicircular bending (SCB) tests and scanning electron microscopy tests were used to analyze and study the effects of aging time and fiber type on the low-temperature properties of asphalt mixtures at an ambient temperature of −5 °C. The SCB test results indicated that: with the deepening of aging, the fracture toughness and fracture energy of the fiber asphalt mixture specimens will tend to decrease, and the aging of the original asphalt has a greater impact on the specimens without fiber; there was great variability in the effect of lignin fibers on the crack resistance of the mix, but it could be a good promoter in unaged asphalt mixes; polyester fiber is the material of choice for improving low-temperature crack resistance in long-term aging pavements; the incorporation of basalt fiber greatly enhances the crack resistance of asphalt mixes and is an excellent material for improving aging asphalt mixes. It is clearly seen by the scanning electron microscope test that with the deepening of aging, the adhesion of asphalt and aggregate gradually deteriorates, and even more so, there is the phenomenon of fine aggregate flaking, and the fiber cannot play an excellent role in reinforcement and crack resistance.

Numerical investigation on spall fracture in metallic materials due to laser shock peening via phase field approach to fracture

Laser shock peening (LSP) is an effective surface reinforcement technique for metallic materials. However, when LSP is applied to a thin specimen, an undesirable result, spall fracture, may occur. In this work, the spall fracture generated in multiple-shock LSP processes is investigated via the phase field approach to fracture, and is validated with the experimental result. In particular, the relations between the formation of spall fracture and the shock number, the shock wave pressure, the specimen thickness and the fracture toughness of specimen are studied. The results indicate that the formation of spall fracture in multiple-shock processes is an accumulative process, and is mainly influenced by the shock wave pressure and the specimen thickness, and multiple shocks will complicate the interaction between the tensile wave tail and the compressive wave front of the shock wave, leading to an enlargement of the spall fracture region.

A new technique to measure the dynamic fracture toughness of solids

We propose and assess a new experimental technique to measure the fracture toughness of engineering materials and its sensitivity to strain rate. The proposed method is based on a ring expansion technique and it overcomes the limitations of current dynamic fracture tests, as it is not affected by transient stress wave propagation during loading and it results in spatially uniform remote stress and strain fields prior to fracture; the method is also suitable to achieve remote strain rates well in excess of 1000 s−1. We demonstrate the technique by measuring the plane-stress Mode I fracture toughness of PMMA specimens at remote strain rates ranging from 10−3 s−1 to 102 s−1. The experiments show an increase of the toughness of the material with increasing strain rate.

Study on the effects of microstructures on the fracture behavior of an Al-Cu-Mg alloy

In this study, a special fracture delamination in L-T fracture toughness specimens of an Al-Cu-Mg alloy was researched by applying three-dimensional optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) methods. The results indicate that the fracture delamination is contributed by the cracks extending a long distance in the T-direction and extending internally along the L-direction, which can result in a lower fracture toughness value. The fracture delamination is determined by the level of grain flattening and the distribution of the second phases. The flat grains result in a tendency for cracks to extend in the T-direction. At the same time, the unevenly distributed Al2Cu and Al2CuMg phases lead to cracks extending in the L-direction, which eventually causes the formation of delamination fractures. The reduction in grain size can also contribute to weakening the fracture delamination to a certain extent.

An integrated experimental-computational framework to assess the influence of microstructure and material properties on fracture toughness in clinical specimens of human femoral cortical bone

Microstructural and compositional changes that occur due to aging, pathological conditions, or pharmacological treatments alter cortical bone fracture resistance. However, the relative importance of these changes to the fracture resistance of cortical bone has not been quantified in detail. In this technical note, we developed an integrated experimental-computational framework utilizing human femoral cortical bone biopsies to advance the understanding of how fracture resistance of cortical bone is modulated due to modifications in its microstructure and material properties. Four human biopsy samples from individuals with varying fragility fracture history and osteoporosis treatment status were converted to finite element models incorporating specimen-specific material properties and were analyzed using fracture mechanics-based modeling. The results showed that cement line density and osteonal volume had a significant effect on crack volume. The removal of cement lines substantially increased the crack volume in the osteons and interstitial bone, representing straight crack growth, compared to models with cement lines due to the lack of crack deflection in the models without cement lines. Crack volume in the osteons and interstitial bone increased when mean elastic modulus and ultimate strength increased and mean fracture toughness decreased. Crack volume in the osteons and interstitial bone was reduced when material property heterogeneity was incorporated in the models. Although both the microstructure and the heterogeneity of the material properties of the cortical bone independently increased the fracture toughness, the relative contribution of the microstructure was more significant. The integrated experimental-computational framework developed here can identify the most critical microscale features of cortical bone modulated by pathological processes or pharmacological treatments that drive changes in fracture resistance and improve our understanding of the relative influence of microstructure and material properties on fracture resistance of cortical bone.

Pressure reduction strategy based on a new CTOD model considering room temperature creep for X80 pipeline steel

The growing demand for energy consumption has led to the rapid development of high strength and high toughness pipeline steel technology, with fracture toughness being a crucial factor in evaluating the mechanical properties of pipeline steel. Despite experimental studies demonstrating that metallic materials, including pipeline steel, undergo creep at room temperature, the effects of room temperature creep (RTC) on pipeline steel fracture properties are still relatively unexplained. This study analyzes the impact of RTC on the fracture toughness of X80pipeline steel, using the X80pipeline steel base material as the research object. Firstly, the time-dependent change in crack notch opening displacement is added to the calculation formula for fracture toughness of three-point bending specimens, considering the effect of RTC. The test data is fitted to a power-law equation of the form, where k=0.01423 and m=-0.89651, with the improved formula indicating an incremental CTOD of approximately 0.0098 mm after 36 h of RTC for X80 pipeline steel. Secondly, a time-dependent CTOD model is proposed based on the ductile defect growth model, with the plastic part derived from the D-B model and the creep part obtained by fitting the test data. The study proposes a CTOD criterion for RTC and a pressure reduction strategy based on this criterion, providing a reference for quantitatively assessing the fracture toughness of pipeline steel under RTC conditions.

Influence of hybrid nano/micro particles on the mechanical performance of cross-ply carbon fibre fabric reinforced epoxy polymer composite materials

This study presents the flexural strength, compressive strength, and impact energy behaviour of graphene nanoplates and fly ash microparticle-loaded cement-based CFRP composites manufactured by the vacuum-assisted resin infusion method (VARIM). The composite structures consist of a fixed ratio of graphene nanoplates (2%), fly ash (2%), silica (2%), cement (2%), and sand (4%) particles filled into cross-plied carbon fibre-reinforced epoxy resin polymer composite beams and columns. Composites fabricated by hand lay-up methods because of infiltration problems. A three-point bending test through the width was used to measure the flexural strength of the graphene nanoplate (GnP)-filled CFRP composite beam. The low-velocity Izod impact and Charpy impact tests through the thickness were used to determine Charpy impact energy (3.46 J), Izod impact energy (2.5 J), and the dynamic fracture toughness of notch specimens under Charpy (28.5 KJ/m2) and Izod impact (25.0 KJ/m2). Also, the compressive test method was used to measure the compressive strength of hybrid particles and short glass fibre-reinforced epoxy resin composite square and circular columns. The results show compressive strength and flexural strength. Izod impact energy, Charpy impact energy, and dynamic fracture toughness of hybrid nano/microparticle-filled fibre composites have higher values than virgin fibre composites because of the influence of graphene nanoparticles and the perfect interface bonding between two dissimilar molecules of nano and microparticles, which improve the fracture toughness and absorb impact energy. Overall, the results show that molecules of nano/microparticle-filled carbon fibre and glass fibre-reinforced epoxy resin composites can be used in seismic wave resistance because of their improved mechanical properties compared to virgin fibre composites. In addition, SEM micrographs clearly show that nano- and microparticles are resistant to crack propagation and the debonding of matrix fibres.

Mode-III fracture toughness measurements of foam-cored sandwich composites using a constrained Shear-Torsion-Bending specimen

This work presents a constrained version of the Shear-Torsion-Bending (STB) test fixture, specially developed for testing of the mode-III face/core fracture toughness of sandwich specimens. Compared to the traditional STB test fixture, the free end of the specimen is now constrained between two support rollers, restraining the twisting deformation of the specimen resulting from the mode-III shear forces. The constrained STB test fixture was applied for mode-III fracture characterization of sandwich specimens with longitudinal slits and consisting of E-glass/epoxy face sheets and Divinycell PVC foam cores. Three density classes of the foam material were considered and tested, that is, H45, H100, and H160. Furthermore, two core thicknesses were tested for the H160 cored specimens. The experimental results from the constrained STB test method show that stable crack growth is obtained without kinking into the core. However, post-mortem investigations showed that longitudinal 45° tension cracks in the core just below the face/core interface occurred, before interface crack opening. Consequently, an apparent mode-III fracture toughness was computed using a three-dimensional Finite Element (FE) model in conjunction with the Crack Surface Displacement Extrapolation (CSDE) mode-mixity method and an optical Digital Image Correlation (DIC) based crack propagation method. Finally, based on the experimental observations presented in this paper, a general discussion of the existence of pure mode-III face/core interface propagation in foam-cored sandwich composites in relation to core and face/core bond strength is included.

Aggregate size effects on fracture behavior of concrete SCB specimens

Fracture in concrete and rock-like materials is the result of applying external loads, initiation, and propagation of the cracks, and finally, creating the failure surface in the material. The most common type of fractures in solid materials are usually formed under combined tensile/shear loading conditions. The stress intensity factors KI and KII, corresponding to opening and sliding modes (I and II), are the main fracture parameters, and their critical values (i.e., fracture toughness) show the material's resistance against the initiation and propagation of cracks. In this paper, the fracture toughness of concrete specimens is determined by considering different aggregate sizes and using the three-point bending test on semi-circular bend (SCB) specimen. The fracture toughness parameters have been determined by different fracture criteria and compared with the values obtained in the laboratory on concrete specimens. The effect of aggregate size on fracture parameters and crack propagation path has been evaluated. To predict thecrack initiation angle and fracture toughness values, themodified maximum tangential stress (MMTS) and extended maximum tangential strain (EMTSN) criteria along with conventional fracture criteriawere used. The results showed, that with the increase in aggregate size, the fracture toughness value under mode I, II, and mixed-mode (I-II) has increased. The laboratory results showed that specimens with aggregate sizes of 0–19 mm have the highest fracture load, and specimens with the aggregate sizes of 0–5 mm have the lowest fracture load

Experimental study and hybrid optimization of material extrusion process parameters for enhancement of fracture resistance of biodegradable nanocomposites

Among the additive manufacturing processes, the material extrusion (ME) is the most popular and affordable technique in production of a wide range of products, from prototypes to the final products, for different industrial sectors. The most popular raw material in this process is the biodegradable polylactic acid (PLA) polymer that offers ease of production in relatively low price and wide applications in synthesis of medical proteases. However, the mechanical strength of fabricated samples and their resistance against the fracture loadings, which are critical for components synthesized for medical applications, are strongly dependent on the ME process parameters and the composition of the base material. Therefore, this study is aimed to address both the strengthening mechanisms of inorganic CaCO3 nanoadditives in the PLA matrix and the influence of ME process parameters on the fracture resistance of fabricated samples. To this aim, the PLA/CaCO3 nanocomposite filaments have been produced by mix-blending/extrusion technique and were employed to fabricate tensile and fracture test samples in ME process under different sets of parameters, including printing speed, layer thickness, filling ratio and printing pattern, determined by Taguchi L27 orthogonal array. The fracture experiments were conducted on a uniaxial tensile test machine using a specially designed fixture that makes the application of mixed-mode loading on fracture samples possible. The results of fracture toughness of specimens were employed as input data to model the response of fabricated samples and to determine the ME parameter levels and nanoparticle concentrations for maximum fracture resistance of fabricated samples based on a hybrid algorithm of artificial neural network and ant colony optimization (ANN/ACO). Differential scanning calorimetry was used to characterize the thermal features (i.e. glass transition, crystallization and melting temperatures) as well as the degree of crystallization of ME samples. Scanning electron microscopy of fracture surfaces were employed to study the influence of ME parameters and CaCO3 nanoadditives on the characteristics of fracture of 3D printed specimens under various modes of loading. Overall, the results showed that the effects of ME parameters on the fracture behavior of 3D printed samples are highly interconnected and, in the case of a semicrystalline polymer, are highly dependent on the physical characteristics of the fabrication process, including the heat transfer and crystallization rate of the matrix.

Effects of freeze–thaw cycles on mode I fracture toughness of adhesively bonded CFRP joints

The effects of freeze–thaw (FT) cycles on the mode I fracture toughness of adhesively bonded carbon fiber reinforced plastic joints were studied. Fracture toughness specimens were immersed in water and exposed to thermal cycles consisting of thawing (+60 °C) and freezing (−50 °C). Two types of fracture toughness specimen were prepared: those without mechanical loading (non-stressed) and those with mechanical loading using a wedge (stressed) during the environmental conditioning. Fracture toughness of the non-stressed specimens gradually decreased with increasing number of FT cycles, whereas that of the stressed specimens first increased and then gradually decreased. Fracture mechanisms are discussed on the basis of the results of fracture toughness tests, the tensile properties of bulk adhesives, and fracture morphologies.

Mode I fracture behavior of heterogeneous granite: Insights from grain-based FDEM modelling

The failure of brittle rock has a significant influence on engineering stability. The purpose of this study is to understand the progressive fracture mechanism of heterogeneous granite from a mesoscopic perspective. In the present work, the fracture process zone (FPZ) model is used to characterize the constitutive relationship of rock cracking in the combined finite-discrete element method (FDEM). Moreover, the breakable grain-based model (bGBM) is established to study the fracture behavior of granite via the semi-circular bending (SCB) tests. To rebuild the meso-structure of granite specimens, an algorithm that considers the mesoscopic grain geometry characteristics and grain contact mechanical properties is utilized. Then, three classic examples, namely, a uniaxial compressive test, a Brazilian splitting test and a semi-circular bending test, are investigated with the numerical model. A good agreement of the load–displacement curves and crack propagation paths can be found between the simulation and laboratory tests. The meso-heterogeneity of granite cause the macroscopic fracture surface of the slit tip to be nearly upright, with local roughness and zigzag. The slit length has a significant effect on the damage distribution range of the SCB specimen, and the peak load and mode I fracture toughness decreases with increasing slit length and supporting span. The accurate measurement of the mode I fracture toughness of SCB specimens needs to consider the FPZ, and the average increment is 39% and 41% under different slit length and supporting span conditions, respectively.

Development of a Unified Specimen for Adhesive Characterisation-Part 1: Numerical Study on the Mode I (mDCB) and II (ELS) Fracture Components.

Adhesives are increasingly being employed in industrial applications as a replacement for traditional mechanical joining methods, since they enable improvements in the strength-to-weight ratio and lower the cost of the overall structures. This has led to a need for adhesive mechanical characterisation techniques that can provide the data needed to build advanced numerical models, allowing structural designers to expedite the adhesive selection process and grant precise optimisation of bonded connection performance. However, mechanically mapping the behaviour of an adhesive involves numerous different standards resulting in a complex network of various specimens, testing procedures and data reduction methods that concern techniques which are exceedingly complex, time-consuming, and expensive. As such, and to address this problem, a novel fully integrated experimental characterisation tool is being developed to significantly reduce all the issues associated with adhesive characterisation. In this work, a numerical optimisation of the unified specimen's fracture toughness components, comprising the combined mode I (modified double cantilever beam) and II (end-loaded split) test, was performed. This was achieved by computing the desired behaviour as a function of the apparatus' and specimens' geometries, through several dimensional parameters, and by testing different adhesives, widening the range of applications of this tool. In the end, a custom data reduction scheme was deduced and set of design guidelines was defined.