Fracture Toughness Of Materials Research Articles

Comparison of Mechanical Properties of Bulk NiAl-Re-Al2O3 Intermetallic Material Manufactured by Laser Powder Bed Fusion and Hot Pressing

AbstractThe low fracture toughness of NiAl at room temperature is one of the critical issues limiting its application in aircraft engines. It has been previously shown that a small addition of rhenium and alumina significantly improves the fracture toughness of hot-pressed NiAl. In this work, NiAl with an admixture of rhenium and alumina was produced by laser powder bed fusion additive technology (LPBF). The purpose was to compare the fracture toughness, bending strength, and microhardness of the NiAl-Re-Al2O3 material produced by LPBF and hot pressing (HP). Our results show that the LPBF material has lower fracture toughness and bending strength compared to its hot-pressed equivalent. Microcracks generated by thermal stresses during the LPBF process were the primary cause of this behavior. To improve the LPBF material, a post-processing by HP was applied. However, the fracture toughness of the (LPBF + HP) material remained at 50% of the KIC of the HP material. This study supports hot pressing as a suitable processing method for NiAl with rhenium and alumina additions. However, a hybrid approach combining LPBF and HP proved to be highly effective on the raw NiAl powder, resulting in superior fracture toughness of the final material compared to that consolidated by singular HP.

Static and dynamic fracture toughness of graphite materials with varying grain sizes

Graphite materials play critical roles as moderators, reflectors, and core structural components in high-temperature gas-cooled nuclear reactors. During the reactor operation, graphite materials may experience a variety of loads, including thermal, radiation, fatigue, and dynamic loads, potentially leading to crack initiation and propagation. Therefore, it is imperative to investigate their fracture properties. Despite this, there remains a paucity of comprehensive studies on the fracture toughness of graphite materials with varying grain sizes, particularly concerning dynamic fracture toughness. This study addresses this gap by employing a digital-image-correlation-based virtual extensometer to analyze crack propagation in graphite materials of different grain sizes, enabling precise measurement of crack propagation length and fracture toughness. Findings reveal that static fracture toughness increases with larger grain sizes, while under dynamic loading, smaller grain sizes exhibit greater fracture toughness. Additionally, dynamic fracture toughness shows a near-linear increase with impact speed. Scanning electron microscopy analysis of fracture surface morphology highlights the impact of grain size and impact speed on fracture toughness. Nuclear graphite specimens with larger grain sizes have more irregular grain and pore distributions, enhancing crack deflection and propagation resistance, thereby increasing fracture toughness. The observed loading rate dependence of dynamic fracture toughness is attributed to a gradual transition from intergranular to transgranular fracture modes with increasing impact speed.

Compact-tension-shear specimen for orthotropic materials in fracture toughness testing

Conducting Compact-Tension-Shear (CTS) tests emerges as a promising research methodology to determine the fracture toughness of orthotropic materials in mixed-mode I&II. However, the absence of pertinent fracture parameter solutions incorporating material orthotropy impedes the further application and standardization of CTS testing. Therefore, this study performed a systematic two-dimensional finite element analysis (FEA) on orthotropic CTS specimens to evaluate critical fracture parameters, including the normalized stress intensity factors (YI, YII) and compliance (u). First, the impacts of three commonly cited boundary conditions (BCs) of CTS models were investigated, revealing that the simplified or equivalent methods developed for isotropic CTS models are no longer applicable. Subsequently, the suitable CTS models were used to analysis the coupling effects of material orthotropy (λ, ρ), crack length ratios (a/W), and loading angles (β). Findings indicated that YI or u no longer exhibit a monotonic relationship with respect to a/W or β under strong orthotropy. The impact of λ and ρ on fracture parameters is equally significant across different geometries and loading conditions. Further, the impact of orthotropy on YII is relatively small, while the more affected YI may have a difference compared to isotropic results of over 80%. Finally, CTS tests were carried out on the unidirectional carbon fiber-reinforced epoxy resin composite measuring the fracture toughness in mixed-modes I&II. The obtained fracture parameter solutions were applied and validated during the process. The acquired results are poised to contribute to the advancement of fracture toughness testing standardization for orthotropic materials under mixed-mode loading.

Characterization of hydrogen embrittlement sensitivity of 18CrNiMo7-6 alloy steel surface-modified layer based on scratch method

PurposeThe purpose of this paper is to assess the hydrogen embrittlement sensitivity of carbon gradient heterostructure materials and to verify the reliability of the scratch method.Design/methodology/approachThe surface-modified layer of 18CrNiMo7-6 alloy steel was delaminated to study its hydrogen embrittlement characteristics via hydrogen permeation, electrochemical hydrogen charging and scratch experiments.FindingsThe results showed that the diffusion coefficients of hydrogen in the surface and matrix layers are 3.28 × 10−7 and 16.67 × 10−7 cm2/s, respectively. The diffusible-hydrogen concentration of the material increases with increasing hydrogen-charging current density. For a given hydrogen-charging current density, the diffusible-hydrogen concentration gradually decreases with increasing depth in the surface-modified layer. Fracture toughness decreases with increasing diffusible-hydrogen concentration, so the susceptibility to hydrogen embrittlement decreases with increasing depth in the surface-modified layer.Originality/valueThe reliability of the scratch method in evaluating the fracture toughness of the surface-modified layer material is verified. An empirical formula is given for fracture toughness as a function of diffused-hydrogen concentration.

Microstructure and fracture toughness of 850°C quenched and aged Zr-2.5Nb alloy

The Zr-2.5Nb alloy pressure tubes manufactured by Water Quenched and Aged (WQA) fabrication route has reportedly exhibited superior resistance against in-reactor dimensional changes. WQA route comprises three consecutive thermo-mechanical treatments viz., solution heat treatment (SHT), cold working and vacuum aging, which govern the final microstructure and therefore fracture toughness of the pressure tube material. This work describes the influence of SHT soaking duration, degree of cold working and aging temperature on the microstructure and fracture toughness of WQA Zr-2.5Nb alloy. WQA pressure tubes are expected to have better resistance to in-reactor dimensional changes. However, if proper care of hydrogen absorption during thermo-mechanical processing is not taken, though resistance to creep will improve expectedly but fracture toughness at ambient temperature will deteriorate.

A finite crack growth energy release rate for interlaminar fracture analysis of composite material at different temperatures

The interlaminar fracture toughness of unidirectional fiber reinforced polymer composite material is temperature and crack growth history dependence. The non-constant interlaminar fracture toughness challenges the wide application of the linear elastic fracture mechanics-based approach for analyzing the interlaminar crack growth in composite material and structures. In addition, a quantitative relationship between the ductility of the matrix and interlaminar fracture toughness of the fiber reinforced composite is still highly desirable. In this paper, both pure resin and composite double cantilever beam (DCB) are tested at different temperatures. A finite crack growth energy release rate is used to analyze the interlaminar crack growth behavior of composite material at different temperatures. The plasticity of the composite DCB is modeled by Hill’s anisotropic plasticity, with the properties determined from multiscale analyses. It is found that a single constant finite growth energy release rate can well predict the interlaminar mode I crack growth of composite material at different temperatures. The roles of the ductility of matrix, thickness of the DCB, and crack growth history on the interlaminar mode I fracture toughness of composite material are quantitatively determined by using the present method.

Laboratory Investigations on Fracture Toughness of Self-compacting Concrete Containing Recycled Materials

This article investigates the fracture toughness in self-compacting concrete containing different percentages of recycled concrete materials using edge-notched disc bend (ENDB) samples. For this reason, 0, 25, 50, 75 and 100 percent of recycled aggregate were used instead of natural aggregate in the mixing designs. To obtain the fracture toughness, ENDB samples were used under loading of mixed mode I/III and pure modes I and III. Cubic samples were also used to obtain compressive strength. The results show that the compressive strength of the samples has a direct relationship with the fracture toughness and the reverse ratio with the percentage of recycled concrete materials. Also, samples containing 25% recycled materials have the highest compressive strength and fracture toughness. Furthermore, as the recycled percentage of the samples increases, the performance of samples against shear forces and compressive strength is reduced. Keywords: Fracture toughness, Mixed mode I/III, Recycled materials, Self-compacting concrete, ENDB sample

Mechanical properties modeling of cathode materials for lithium-ion batteries based on bond valence model

To clarify electrochemical-mechanical relation of cathode materials for lithium-ion batteries, a series of empirical mechanical property models based on the bond valence model are applied to estimate the hardness, bulk, shear, and Young’s moduli, Poisson’s ratio and fracture toughness of typical cathode materials, such as LiCoO2, LiNi1-x-yCoxMnyO2, LiNi0.80Co0.15Al0.05O2, LiMn2O4, and LiFePO4. The calculated results are in agreement with experimental values. The mechanical properties have been predicted for low Ni compounds LiNixCo1–2xMnxO2 (0 ≤ x ≤ 0.5), high Ni compounds LiNi0.8Mn0.2-xCoxO2 (x = 0, 0.1, 0.2), and free Co compounds LiNixMn1-xCo0O2 (x = 0.5, 0.6, 0.8). For LiCoO2 during lithiation and delithiation processes, the hardness and bulk modulus of LixCoO2 decrease nearly linearly with the decrease of Li concentration. The effects of Al doping on the mechanical properties of LiAlxCo1-xO2 are also revealed from the insights into the chemical bonds. The models are powerful for extensively predicting and tuning electrochemical-mechanical properties of cathode materials.

Fracture toughness testing of metallic materials based on scratch tests

As a crucial parameter for determining a material's resistance to crack propagation, the convenient characterization of fracture toughness has attracted much attention. The scratch test is an alternate method for determining fracture toughness because it does not require notches or pre-existing cracks and only requires a minimal sample volume. In this study, the product of strength and elongation of materials is incorporated into a modified model based on the energy size effect law to calculate the fracture toughness of metallic materials using scratch tests. To validate the proposed model, scratch tests of DP800, 18CrNiMo7-6, 4140, and 4340 are performed to characterize their fracture toughness. The fracture toughness of these four metallic materials determined from scratch tests utilizing the proposed model, Akono's linear elastic fracture mechanics (LEFM) model, Akono's microscopic energetic size effect law (MESEL) model, Hubler's MESEL model, and Liu's MESEL model are compared with the results from a compact tensile test or single edge notched beam test. Our results indicate that our modified approach is applicable to metallic materials. The proposed model could be reduced to the linear elastic fracture mechanics model. This study offers an efficient way of characterizing the fracture toughness of materials using the scratch method.

The effect of resin properties on toughness translation in interleaved composites

Interleaving is a well-recognized method for enhancing the interlaminar fracture toughness of composite materials. However, theoretical toughness is often not realized since cracks tend to propagate through regions of lower toughness. While most studies have focused on the effect of interleaf properties, e.g., thickness and interleaf resin properties, on composite interlaminar fracture toughness, the effect of matrix resin properties on interleaved composite toughness has been overlooked. Recently, we hypothesized that there is a relationship between toughness translation and the ratio of matrix to interleaf resin toughness. In this work, we use additive manufacturing to test this hypothesis with a range of resins in interleaved composite. Toughness is quantified via mode I delamination resistance. Our results confirm our hypothesis that the ratio of matrix to interleaf resin fracture toughness, i.e., the ratio of [Formula: see text], is directly correlated to the degree of toughness translation. More specifically, a ratio of approximately 0.5 is required for effective interleaving; below 0.5, the toughness translation is significantly below the theoretical value. This approach has important implications for the design and fabrication of tough composites via choice of resins.

Effect of constraint on creep fracture toughness of a Cr-Mo-V steel

The creep fracture toughness of materials is required for the assessment of defective components at high-temperatures. Existing evidence indicates that creep crack-tip constraint state has a great influence on the fracture behavior of materials. In this work, the effect of constraint on creep fracture toughness was explored by conducting a series of creep crack growth tests on specimens with various thicknesses and loading modes. Results showed that the curves of creep fracture toughness with creep crack initiation time on the logarithmic scale for different groups of specimens were almost parallel with one another. Through the characterization of crack-tip constraint level for different groups of specimens in terms of a load-independent creep fracture constraint parameter, R*, the creep fracture toughness was found to increase with the decrease in the crack-tip constraint level. Moreover, evident effect of specimen geometry on creep fracture toughness was observed. Of all types of specimens, the CCT2 and CT10-SG exhibited the highest and lowest values of Kcmat at the same ti, respectively. In addition, the Kcmat was found to increase following a specimen group order of CT10-SG, CT10, CT5, SENT5, CT2, SENT2 and CCT2 at a given ti. Finally, a constraint-dependent creep fracture toughness equation was established for the Cr-Mo-V steel, which is expected for practical applications to improve accuracy of life assessment methods. Follow-up work on life assessment methods incorporating the constraint parameter R* is still necessary.

Quantification of pre-strain effect on ductile fracture and application to irradiation embrittlement problems

This paper proposes a numerical method to predict fracture resistance of pre-strained materials, which is further applied to predict fracture resistance of irradiation-embrittled materials. The proposed method is based on a multi-axial fracture strain damage model using the fracture strain locus and critical damage. The key point is that the pre-strain effect on the critical damage is predicted using the pre-strain effect on plastic strain energy density of tensile data. Application to published experimental data for pre-strained SUS316 and SM490A shows that the predicted fracture toughness results agree well with experimental data. Based on published experimental results that the pre-strain effect on tensile and fracture toughness properties is similar to the irradiation effect, the proposed model is further applied to predict fracture toughness of irradiation-embrittled SUS316 material. Although only numerical simulation results are available, they show that maximum loads for irradiated specimens are higher than those for un-irradiated specimens. Furthermore, the effect of irradiation gradient along the crack growth direction is not so significant.

Predicting failure in injection-moulded short-fibre subcomponents under varied environmental conditions through fracture mechanics

Injection-moulded short-fibre composites are lightweight materials suitable for high-volume applications; however, current simulation methods (based on failure initiation criteria) to design components using these materials cannot yet accurately predict failure. This work presents a methodology to predict failure of injection-moulded short-glass-fibre reinforced thermoplastic (IM-SFRP) composite subcomponents, based on experimentally measured properties. The material's fracture toughness was characterised by Compact Tension tests for different fibre orientations and environmental conditions. These fracture toughnesses were used as the input for cohesive zone modelling in Finite Element simulations of subcomponents representative of automotive applications, coupled with fibre orientation fields predicted by an injection-moulding process simulation. These coupled simulations presented excellent agreement with the experimental results for subcomponents both in terms of (i) the peak load (highlighting the importance of accounting for the finite fracture toughness of the material to accurately predict the ultimate failure of the subcomponents), and (ii) the pre- and post-peak sequence of failure events (verified using fractographic analyses). This work also verified the applicability of temperature-moisture equivalence, not only for material characterisation using coupons including the material's fracture toughness, but also for the mechanical response of subcomponents until final failure. The methodology demonstrated in this paper contributes to designing safer and more efficient damage-tolerant IM-SFRP components.

Effect of thermocycling on surface topography and fracture toughness of milled and additively manufactured denture base materials: an in-vitro study

BackgroundStudies investigating thermocycling effect on surface topography and fracture toughness of resins used in digitally manufactured denture bases are few. The study aimed to assess the impact of thermocycling on surface topography and fracture toughness of materials used for digitally manufactured denture bases.MethodsWater sorption, solubility, hardness, surface roughness, and fracture toughness of both three-dimensional (3D)-printed and computer-aided design, computer-aided manufacturing (CAD-CAM) milled specimens (n = 50) were assessed both prior to and following 2000 thermocycles, simulating 2 years of clinical aging. Surface hardness (n = 10) was measured using a Vickers hardness testing machine, surface roughness (n = 10) was determined by a contact profilometer, and fracture toughness (n = 20) was measured using the 3-point bend test, then studying the fractured surfaces was done via a scanning electron microscope (SEM). Prior to and following thermocycling, water sorption and solubility (n = 10) were assessed. Normally distributed data was tested using two-way repeated ANOVA and two-way ANOVA, while Mann Whitney U test and the Wilcoxon signed ranks test were used to analyze data that was not normally distributed (α < 0.05).ResultsFollowing thermocycling, Vickers hardness and fracture toughness of both groups declined, with a significant reduction in values of the 3D-printed resin (P < .001). The 3D-printed denture base resins had a rougher surface following thermocycling with a significant difference (P < .001). The sorption and solubility of water of both materials were not affected by thermocycling.ConclusionsBefore and after thermocycling, milled specimens had lower surface roughness and a greater degree of hardness and fracture toughness than 3D-printed specimens. Thermocycling lowered hardness and fracture toughness, and increased surface roughness in both groups, but had no effect on water sorption and solubility.

Fracture toughness of two-dimensional materials dominated by edge energy anisotropy

Two-dimensional materials (2DMs) are prone to brittle failure under load but a recent experiment has demonstrated intrinsic toughening in hexagonal boron nitride (h-BN), which calls for a general understanding of fracture toughness in 2DMs. Using atomistic calculations combined with a developed size-dependent extrapolation method, we show that 2DMs with strong anisotropy of edge energy favor bifurcated cracks for intrinsic toughening as in h-BN, while those with weak edge energy anisotropy exhibit split cracks for brittle failure as in graphene. The interplay between chemical bond strength and fracture energy of bifurcated crack tips leads to alternate deflections and, thus, rough crack edges as observed in the previous experiment. We further develop a robust descriptor for identifying 2DMs exhibiting similar fracture behavior to that in h-BN. More interestingly, we reach a physically interpretable formula capable of quantitatively determining the toughness of 2DMs based on easily accessible intrinsic features of elements. These findings lay a solid foundation for nanodevice applications where controlled toughness is required.

Higher order structures and toughening mechanisms for simultaneously polymerized polymethacrylate/polyurethane

AbstractAmorphous polymer/crystalline polymer blends can be prepared via the simultaneous polymerization of polymethacrylate/polyurethane combinations. The relationship between higher order structures and fracture mechanisms in these blends must be uncovered to elucidate the source of the increased fracture toughness of such materials. The present work involves the production of blended polymethacrylate/polyurethane and assess the internal structures of these specimens using optical and electron microscopy. These observations reveal the presence of both spherulites and elastomeric phases. The spherulites consisting of the polyurethane and are several micrometers in diameter whereas the phase‐separated polyurethane elastomeric domains are approximately 100 nm in size. Multiple cracks, crack bridging and plastic deformation around the precrack tips of loaded specimens are evidently responsible for the increased toughness of these blends. The former two phenomena are attributed to the presence of spherulites while the plastic deformation of the methacrylate matrix is ascribed to cavitation of the polyurethane elastomeric phases in response to loading.

Using the Point Method to estimate failure loads in 3D printed graphene-reinforced PLA notched plates

This work estimates failure loads in Fused Filament Fabrication (FFF) printed graphene-reinforced PLA (polylactic acid) plates containing different types of stress risers. With this aim, firstly, several notched plates are tested and conducted to fracture. Then, linear elastic Finite Element (FE) analyses are completed to define the corresponding stress profiles and, finally, the Point Method (PM) is applied to establish the failure criterion. This approach asserts that fracture conditions are achieved when the stress level equates the inherent strength (σ0) at a distance from the notch tip equal to L/2, so both parameters (related to each other through the material fracture toughness, Kmat) have been defined beforehand. The estimations of fracture loads obtained following this approach agree with the experimental results. Thus, the present work demonstrates the accuracy of the PM to estimate failure loads in this 3D printed material.

A four-point bending technique for characterizing the interface fracture toughness between soft thin films and stiff substrates

Interface fracture toughness, also called work of adhesion or adhesion energy, is a quantity characterizing the resistance of an interface between two adhered solids against interfacial delamination. Determining the interface fracture toughness should be of great value to the studies on the related adhesion and delamination problems. Four-point bending test is an experimental approach that was originally used to measure the flexural stiffness and fracture toughness of monolithic materials. It was also used to characterize the fracture toughness of the interface when a bi-layered notched composite beam specimen is adopted (Int. J. Frac. 1989; 40: 235). However, this method does not work very well when a thin and soft material is encountered because the interface delamination cannot be triggered easily. To address this problem, in this paper, we revise the configuration of the four-point bending specimen from bi-layer to tri-layer by imposing an additional stiffer layer on the top of the thin and soft layer. The analytical solution to the energy release rate of the preexisting interfacial crack is revisited. Finite element analysis is carried out to assess the applicability and limitations of the analytical solution. With the modified four-point bending specimen, the interface fracture toughness between an epoxy coating and a steel substrate is successfully measured from the critical load that leads to the delamination of the preexisting interfacial crack.

Experimental investigations on tensile strength and fracture toughness of a polyoxymethylene fiber reinforced concrete

The crack resistance performance of concrete materials, characterized by tensile strength and fracture toughness, is vital for the durability and structural functionality of concrete components. In this study, polyoxymethylene (POM) fibers were added into concrete to enhance the crack resistance. Firstly, three-point-bending (3-p-b) tests and splitting tension tests were performed on notched beam and cubic specimens of plain concrete and POM fiber reinforced concrete. Then, considering the heterogeneity of concrete, the 3-p-b and splitting tension test results were analyzed to reveal the failure behaviors of plain concrete and POM fiber reinforced concrete materials. It was found that the addition of POM fibers was beneficial for improving the fracture resistance of concrete. The Boundary Effect Model (BEM) was used to estimate the tensile strength and fracture toughness of plain concrete and POM fiber reinforced concrete materials from the 3-p-b test results, which correlated well with those obtained from the splitting tension tests and the conventional method. Moreover, the influence of POM fiber length and volume fraction on fracture resistance was evaluated. Findings revealed that increasing fiber length and volume fraction effectively enhanced the tensile strength and fracture toughness of concrete material.

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.