Lower Fracture Toughness Values Research Articles

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.

Evaluation of Tight Sandstone Mechanical Properties and Fracability: An Experimental Study of Reservoir Sand−Stones from Lufeng Sag, Pearl River Mouth Basin, Northern South China Sea

Reservoir rocks of the Pearl River Mouth Basin’s Lufeng Sag have low porosity (average porosity 12.6%) and low permeability (average permeability 16.5 mD), requiring hydraulic fracturing to obtain economic production of oil and gas. To contribute to the understanding of these reservoirs, and to promote successful production in the region, we analyzed the mechanical properties of tight sandstone. Moreover, we introduced the shear/tensile strength factor, in combination with the fracture toughness and horizontal stress difference coefficient, as an innovative approach to characterize the ease of forming a complex fracture network after reservoir fracturing. Based on this, we established a fracability evaluation model suitable for offshore low-permeability sandstone reservoirs by an analytic hierarchy process from the perspective of whether the reservoir can form an effective transformation volume and complex fracture network after fracturing. The results indicate that the primary minerals of the target reservoir are quartz and clay minerals, and the natural fractures are not developed. The mechanical properties exhibit a high Young’s modulus (ranging from 30.4 to 34.4 GPa) and high compressive strength (with cohesion between 41 and 45 MPa and an angle of internal friction between 31.0 and 33.5°). The relatively low tensile strength and fracture toughness values are conducive to fracture initiation and extension during the fracturing process. Through the fracability evaluation model constructed in this paper, the depth interval at 4155.1–4172.1 m is identified as a high-quality fractured layer. The results of this study not only provide theoretical guidance for target well and formation selection in the Lufeng Sag, but also have important practical implications for increasing oil and gas production from tight sandstone reservoirs.

Observation of the Coupling Current During Stress Corrosion Cracking of Aluminum Alloy 5083

A unique fracture mechanics device was created to allow for simultaneously measuring the load and localized electrochemical potential gradients in AA5083 bend bars. For the first time, periodic Scanning Vibrating Probe (SVP) maps were collected in order to obtain a full view of the coupling current, (which can be deduced from measured potential gradients) that emanates from an anodic crack tip. As a result of mapping the current density over a wide surface area compared to the area of the notch and crack tip, clarification is given on whether or not the external surfaces of an alloy or any other metal that experiences Stress Corrosion Cracking (SCC) play a role in crack growth. The SVP maps clearly show the anodic and cathodic potential gradients of a growing crack (resulting from SCC) in an AA5083 fracture mechanics specimen while the stress intensity increases. The notch and crack-tip remained anodic throughout most of the obtained scans for sensitized and as-received specimens. Furthermore, a coupling effect is observed on sensitized specimens even at low fracture toughness values in sensitized specimens under load for longer periods of time (> 1 week). These higher resolution maps captured during the crack growth process give more insight to the SCC phenomenon and the electrochemical mechanisms of crack growth in AA5083 and other alloys.

Microstructural influences on fatigue threshold behavior and fracture toughness of an additively manufactured γ-titanium aluminide

The fracture and fatigue behavior of an additively manufactured γ-TiAl alloy (BMBF3) produced by selective electron beam melting and further subjected to two different heat treatment conditions was investigated. As one of the heat-treatments led to a pronounced layer structure, the experiments were also performed with different specimen orientations to account for possible anisotropy effects. In order to characterize the frequently found peculiarities of the local fracture process in this material class, such as shear ligament bridging, static and cyclic R-curves were recorded. In addition, also fatigue crack growth curves were measured. The results were critically compared with the properties of a selected heat treatment condition of an established TNM alloy manufactured by a conventional processing route. The investigations show that large α2/γ-colonies lead to high fracture toughness, but have almost no positive effect on the long-crack fatigue threshold. On the other hand, duplex microstructures with small α2/γ-colonies and thick lamellae favor high long-crack thresholds, but low fracture toughness. Microstructures with high fractions of globular γ-phase exhibited both, low fracture toughness and low long-crack fatigue threshold values. In comparison to the established TNM reference alloy, the additively manufactured counterparts can compete especially when the evolving anisotropy of the properties is taken into account for component design.

Characterization and Analysis of Micromechanical Properties on DNTF and CL-20 Explosive Crystals

To study the crystal mechanical properties of 3,4-dinitrofurazanofuroxan (DNTF) and hexanitrohexaazaisowurtzitane (CL-20) deeply, the crystals of DNTF and CL-20 were prepared by the solvent evaporation method. The crystal micromechanical loading procedure was characterized by the nanoindentation method, and then obtained the mechanical parameters. In addition, the crystal fracture behaviors were investigated with scanning probe microscopy (SPM). The results show that the hardness for DNTF and CL-20 was 0.57 GPa and 0.84 GPa, and the elastic modulus was 10.34 GPa and 20.30 GPa, respectively. CL-20 obviously exhibits a higher hardness, elastic modulus and local energy-dissipation and a smaller elastic recovery ability of crystals than those of DNTF. CL-20 crystals are more prone to cracking and have a lower fracture toughness value than DNTF. Compared to DNTF crystals, CL-20 is a kind of brittle material with higher modulus, hardness and sensitivity than that of DNTF, making the ignition response more likely to happen.

Strain rate effect of mode II interlaminar fracture toughness on the impact response of a thermoplastic PEEK composite

Recent advancements in composite production and processing are making thermoplastics a viable option in a wider array of aerospace applications. In particular, Carbon Fibre Reinforced Plastics (CFRP) with thermoplastic resin are believed to have better damage tolerance properties than thermosets. However, few studies have been conducted regarding the numerical modelling of the behaviour of such materials submitted to low energy impacts. Here the Discrete Ply Model (DPM), that predicts the failure of laminated composites with the help of cohesive elements, is used to compare thermosetting and thermoplastics impact damage tolerances. The DPM is improved to take into account the strain rate effect of the fracture toughness (FT) in mode II of interlaminar interfaces. First, the End Notched Flexure (ENF) test that induces unstable crack growth is used both to experimentally measure the value of FT in mode II for high speed crack growth and to identify the strain rate effect used in the model. Second, the DPM is then used to simulate impact tests for various stacking sequences ([452, −452, 02, 902]2S, [02, 452, 902, −452]2S, [02, 302, 902, −302]2S and [902, −452, 02, 452]2S) and impact energy levels (10, 20 and 30 J). Good correlations with experiment are observed in terms of force/displacement curves and delaminated areas. The numerical model correctly describes the asymmetry of the delaminated interfaces and the propagation of groups of interfaces located near the mid-thickness of the laminated plates. Finally, the damage associated with a 30 J impact is compared for the carbon/PEEK of this study and classical carbon/epoxy plates using numerical simulations (DPM). No significant difference has been found. The results corroborate those obtained in previous studies showing the relatively low value of FT in mode II, using an ENF test and infrared thermography (IRT). This article therefore questions the apparent superiority of carbon/PEEK laminated composites over carbon/epoxy laminated composites in terms of impact damage tolerance.

Fracture Toughness of Hollow Glass Microsphere-Filled Iron Matrix Syntactic Foams.

In this study, iron-based metal matrix syntactic foam (MMSF) containing hollow glass microspheres as filler was investigated with respect to notch sensitivity aspects. The MMSF was produced by means of metal powder injection molding. The notch sensitivity was studied via (i) elastic-plastic fracture mechanics measurements (determination of R-curves based on three-point bending tests) and (ii) Charpy impact tests. In both cases, the samples were machined with two different (U- and V-shaped) notch geometries. The critical J-integral value was determined for both notch types, which resulted in lower fracture toughness values in the case of the V-shaped notches and thus notch sensitivity of the material. This finding can be connected to the characteristics of the deformation zone and the associated stress concentration at the tip of the machined notches. The results were confirmed by Charpy impact tests showing ~30% higher impact energy in the case of the U-shaped notch. The failure modes were investigated by means of scanning electron microscopy. In contrast to the bulk material, the MMSF showed brittle fracture behavior.

Effect of strontium-containing compounds on bone grafts

Biomaterials are biocompatible materials that are used to repair bone defects caused by trauma and bone infections, or bones injured in wars and accidents. Calcium phosphates (CaPs), which have a bioinert structure due to high corrosion resistance and being stable in the body, are the most used biomaterial and considered to be bioactive materials because they are perceived by the body as its tissue and have an accelerating healing effect. CaPs are used extensively for hard- and soft-tissue applications in medicine because of their elemental composition, high level of similarity to the minerals in bone and teeth, and high biocompatibility within the body. However, due to low fracture toughness values or poor mechanical properties of CaPs, different additives like strontium (Sr) and its compounds are added into them to improve their properties. Especially in recent years, it has become an important purpose and field of study to produce more economical alternative CaPs-based, as well as Sr- and its compounds-added biocompatible bone grafts and implant materials having enhanced microstructure and mechanical properties. Sr and its compounds that improve the mechanical properties of CaP compounds have been indicated as an impressive stimulant for bone repair and new bone formation. Sr2+ ion (Sr ranelate), which is used as a drug, especially in osteoporosis disorders, reveals biocompatibility and also increases bone fracture repair. This review explains the literature on the morphological and mechanical properties of CaP-based Sr compounds, which have been added into various CaP-based bone grafts or used as implant coating on bone grafts. Also, biocompatibility studies on bone grafts containing Sr are explained, and it is aimed to contribute to the studies done in this area.

Flexural strength, fracture toughness, three-body wear, and Martens parameters of pressable lithium-X-silicate ceramics

ObjectivesTo test and compare five pressable lithium-X-silicate-ceramics on their mechanical and wear properties. MethodsSpecimens were pressed and prepared from: i. Amber Press (AP), ii. Celtra Press (CP), iii. Initial LiSi Press (IL), iv. Livento Press (LP), and v. IPS e.max Press (IE). Four-point flexural strength (FS), SEVNB fracture toughness (KIC), three-body wear (3BW), Martens hardness (HM) and indentation modulus (EIT) were measured. For CP, FS and HM were measured with and without additional Power Firing. Each subgroup contained 15 specimens. Data were analyzed using Kolmogorov–Smirnov, one-way ANOVA followed by Scheffé test, Kruskal–Wallis-H-, Mann–Whitney-U-, and Spearman–Rho-test (p < 0.05). The Weibull modulus was calculated using the maximum likelihood estimation method. ResultsAP and CP presented higher FS than IL. LP presented the highest Weibull modulus. CP showed lower KIC values than AP, and AP was not significant compared to LP and IE. The most 3BW material loss was observed for CP. CP revealed higher HM values than the remaining ceramics. IL presented lower EIT compared to AP and CP. The following correlations were observed between the test parameters: 3BW with FS (r = 0.279, p = 0.015), with HM (r = −0.378, p = 0.001), and with EIT (r = −0.344, p = 0.004); EIT with FS (r = 0.203, p = 0.028); and HM with FT (r = −0.223, p = 0.027) and EIT (r = 0.884, p < 0.001). No correlations were observed between FS and KIC (r = 0.046; p = 0.346). SignificanceAP followed by LP showed the highest and IL followed by CP the lowest properties tested. Power Firing of CP improved the flexural strength. Ceramics with high flexural strength and Martens parameters showed lower wear. Materials with high Martens hardness presented lower fracture toughness values and ones with high indentation modulus showed high flexural strength.

Fracture Toughness of Silicon Nitride Measured by the Surface Crack in Flexure (SCF) Test Method

The surface crack in flexure (SCF) is a method for the evaluation of the fracture toughness of advanced ceramics. Conventionally is practiced by using a Knoop indenter to make a very small pre-crack. Removal on indent and the plastically deformed zone is required before the fracture test. The purpose of this removal is to eliminate residual stresses under the Knoop impression and to obtain a semi elliptical pre-crack shape. In this work fracture toughness values by the SCF method are compared with those measured using the SEVNB (single edge V-notched beam) method. The material chosen for this purpose was gas pressured sintered silicon nitride (Si3N4) containing 3wt. % Al2O3 and 3 wt. % Y2O3 (SL200B, Ceram Tec, Plochingen, Germany). The varied parameters the amount of material removed from the surface. Short specimens with size 3 x 4 x >25 mm were prepared for this purpose. The fracture toughness of specimens with a surface removal in the range suggested from ASTM C 1421 were found to agree with the results obtained from SEVNB. A surface removal below the recommendation resulted in low values of fracture toughness. Increasing the amount of surface removal moderately was found to still fit with results obtained from SEVNB. Surface removal of much more from the recommended amount leads to failure from natural flaws.

Microstructural dependence of the fracture toughness of metallic thin films: A bulge test and atomistic simulation study on single-crystalline and polycrystalline silver films

The microstructure contribution to the very low fracture toughness of freestanding metallic thin films was investigated by bulge fracture tests on 200-nm-thick {100} single-crystalline and polycrystalline silver films. The single-crystalline films exhibited a significantly lower fracture toughness value (KIC= 0.88 MPa m1/2) than their polycrystalline counterparts (KIC= 1.45 MPa m1/2), which was rationalized by the observation of an unusual crack initiation behavior—characterized by twinning in front of the notch tip—during in situ testing in the atomic force microscope. Twinning was also observed as a dominant deformation mechanism in atomistic simulations. This twinning tendency is explained by comparing the resolved shear stresses acting on the leading partial dislocation and the full dislocation, which allows to develop a size- and orientation-dependent twinning criterion. The fracture toughness of polycrystalline samples was found to be higher because of the energy dissipation associated with full dislocation plasticity and because of crack meandering along grain boundaries.

ANALYSIS OF RING CRACKS IN CERAMIC ROLLING ELEMENTS USING THE BOUNDARY ELEMENT METHOD

Ceramic materials have been increasingly used in bearing systems for over a dozen years. This is due to the specific properties of ceramic materials, such as high hardness, corrosion resistance, the possibility of use in aggressive chemical environments, as well as due to the lower specific weight as compared to steel materials. However, the use of ceramic materials imposes many limitations. The main disadvantages include surface cracks and a low fracture toughness value. The paper presents a numerical analysis of crack propagation in silicon nitride balls. The directions of propagation were analysed for the cracks that are most commonly found on the surface of the commercially available ceramic balls. The directions were analysed along the crack front and considering the location of the crack in relation to the contact point of the balls in the rolling contact. The numerical calculations are based on a three-dimensional model of the ring crack. Numerical calculations were carried out using the boundary element method. Numerical solutions were compared with the results of experimental research.

Microstructures and mechanical characterizations of high-performance nacre-inspired Al/Al2O3 composites

Herein, we adopted a simple way to fabricate nacre-inspired “brick-and-mortar”-structured composites with freeze-cast alumina and pure Al. The ceramic layer thickness (2.5–16.6 μm) and alumina volume fraction (8–48%, resulting from the volume shrinkage of the scaffolds after sintering) were controlled by varying the slurry concentration (5, 10, 15 and 20 vol%). The Al/8 vol% Al2O3 composite, with a relatively thick metal layer (24.8 ± 5.7 μm) and low density (2.78 g/cm3), exhibited a bending strength of 319 ± 13 MPa and fracture toughness of 138 ± 7 MPa m1/2. Compared to the Al/8 vol% Al2O3 composite, the Al/20–48 vol% Al2O3 composites had higher bending strengths (332–368 MPa) but lower fracture toughness values (87–41 MPa m1/2). The relationships between the cracking modes and structural features in the composites were discussed to account for the strengthening and toughening mechanisms. This work provides new ideas and solutions concerning the design and fabrication of nacre-mimetic metal-ceramic composites.

Effect of Au additions on the structure and properties of (Fe0.5Co0.5)71Nb4Si5B20 bulk metallic glass

ABSTRACTThe effect of addition of gold (Au) on the structure and mechanical properties of (Fe0.5Co0.5)71Nb4Si5B20 BMG alloy was studied. Addition of Au formed crystals in the BMG alloy. Gradual increase in the amount of Au added to the BMG alloy resulted in an increase in the amount of crystallinity and hardness of the alloy. Hardness values in excess of 1100 HV were found in all the alloys with a maximum value of 1245 HV. The thermal transition temperatures were also found to increase with an increasing amount of Au. Corner cracks were produced in the alloy with 1.5 at.-% Au content showing a low indentation fracture toughness value of 2.98 MPa-m0.5.

Experimental Investigation of the Size Effect of the Mode I Static Fracture Toughness of Limestone

To study the size effect of the fracture toughness of notched semicircular bend (NSCB) specimens, the dimensionless energy release rate equation of the NSCB specimen was deduced on the basis of the Bažant energy release rate. The influence of the crack length and the specimen size on the fracture toughness was analyzed. The Bažant scale equation was obtained using the International Union of Laboratories and Experts in Construction Materials, Systems, and Structures (RILEM) method. Finally, the Bažant equation was used to analyze the fracture toughness of an NSCB specimen with a radius of 75 mm, and the degree of variation was predicted. The results show that a longer fracture is correlated with a lower fracture toughness value for the same sample size and that a larger specimen radius is correlated with a higher fracture toughness value for the same crack length. The obtained Bažant equation correctly reflects the scale law of the fracture toughness of the NSCB specimen and provides highly accurate predictions of the fracture toughness of large specimens, with an error of not more than 3%. The results obtained in this study provide a new reference method and theoretical basis for the future testing work.

Fracture-Toughness and Fatigue Crack Growth Evaluation in the Transversal Direction of the Longitudinal Weld of an API X52 Steel Pipeline

Abstract In this paper, the fracture-toughness and fatigue crack growth rates of the deposited seam weld on the longitudinal pipeline API 5L X52 were evaluated. The study was performed with nonstandard curved specimens, which were machined in the transversal direction of the seam weld with a crack-initiating notch aligned perpendicular to the direction of the deposited seam weld metal. The fractographic analysis showed that the low fracture-toughness of the weld can be attributed to the the existence of elements capable of nucleating brittle fracture and the fact that the intrinsic porosity in the transversal direction provided a favorable crack path for separating the fracture plane, resulting in low fracture-toughness values. Finally, the resistance decrease with respect to the fatigue crack growth was attributed to the small grain size because of the effect of roughness-induced crack closure as well as the interaction of the crack and the inherent porosity.

Comparison of Mode II Fracture Toughness Test Methods for Wood and Wood-Based Composites

Abstract Currently, there is no standard test methodology for the evaluation of Mode II fracture toughness, GII, for wood and wood composite materials. This article measured the Mode II fracture toughness of solid wood, laminated veneer lumber (LVL), and oriented strand lumber (OSL) using the three-end notched flexure (three-ENF) test and the compact shear (CS) test. Fracture toughness values from the three-ENF test were similar for the solid wood and LVL material, demonstrating similar failures of specimens. The OSL materials had significantly lower fracture toughness values and exhibited exceedingly brittle failure. The CS test was not considered to be a practical method for measurement of Mode II fracture toughness because of the lack of stable crack propagation.

The Effect of Indentation Load, Tilted Angle, and the Amount of Surface Removal to the Fracture Toughness of Silicon Nitride Measured by the Surface Crack in Flexure (SCF) Test Method

The surface crack in flexure (SCF) is a method for the evaluation of the fracture toughness of advanced ceramics. Conventionally is practiced by using a Knoop indenter to make a very small precrack. Removal on indent and the plastically deformed zone is required before the fracture test. The purpose of this removal is to eliminate residual stresses under the Knoop impression and to obtain a semi elliptical precrack shape. In this work the influence of the variation of several experimental parameters is investigated. Fracture toughness values by the SCF method are compared with those measured using the SEVNB (single edge V-notched beam) method. The material chosen for this purpose was gas pressured sintered silicon nitride (Si3N4) containing 3wt.%Al2O3 and 3 wt.%Y2O3 (SL200B, Ceram Tec, Plochingen, Germany). The varied parameters were indentation load, orientation of the indentation crack with respect to the bending axis and the amount of material removed from the surface. Additional investigations were performed to determine the crack geometry for various indentation loads. A procedure was developed to facilitate the location and measurement of the precrack size on the fracture surfaces. The effect of measurement accuracy of the results is evaluated. The fracture toughness of specimens with a surface removal in the range suggested from ASTM C 1421 were found to agree with the results obtained from SEVNB. A surface removal below the recommendation resulted in low values of fracture toughness. Increasing the amount of surface removal moderately was found to still fit with results obtained from SEVNB. Surface removal of much more from the recommended amount leads to failure from natural flaws. Observations from serial sectioning revealed that precracks obtained from HK20 and HK30 have an irregular precrack shape with large lateral cracks. It is suggested to use HK5 and HK10 to produce semi-elliptical surface cracks.

An experimental and theoretical comparison of CCNBD and CCNSCB specimens for determining mode I fracture toughness of rocks

AbstractThe cracked chevron notched Brazilian disc (CCNBD) specimen, suggested by the International Society for Rock Mechanics for testing mode I fracture toughness of rocks, usually yields rather conservative toughness measurements, and the reasons have not been fully explored. In this study, the CCNBD method is compared with the cracked chevron notched semicircular bending (CCNSCB) method in the fracture process zone (FPZ) and its influence on the fracture toughness measurement. Theoretical analysis reveals that the FPZ is longer in the CCNBD specimen than in the CCNSCB specimen using a relatively large support span, the toughness measurement using the former is affected more seriously by the presence of FPZ, and thus the CCNBD method is usually, more or less, conservative compared with the CCNSCB method. These inferences are further validated by experimental results, which indicate that the CCNBD test indeed produces much lower fracture toughness values and even the results of 75‐mm radius CCNBD specimens are still lower than those of 25‐mm radius CCNSCB specimens. Consequently, due to smaller FPZ, the CCNSCB specimen with a relatively large span is more likely to produce comparably accurate or representative toughness value, and it may be more suitable than the CCNBD specimen for the engineering applications that require more representative or less conservative fracture toughness.

Mechanical energy dissipation in natural ceramic composites.

Ceramics and glasses, in their monolithic forms, typically exhibit low fracture toughness values, but rigid natural marine ceramic and glass composites have shown remarkable resistance to mechanical failure. This has been observed in load-extension behavior by recognizing that the total area under the curve, notably the part beyond the yield point, often conveys substantial capacity to carry mechanical load. The mechanisms underlying the latter observations are proposed as defining factors for toughness that provide resistance to failure, or capability to dissipate energy, rather than fracture toughness. Such behavior is exhibited in the spicules of glass sponges and in mollusk shells. There are a number of similarities in the manner in which energy dissipation takes place in both sponges and mollusks. It was observed that crack diversion, a new form of crack bridging, creation of new surface area, and other important energy-dissipating mechanisms occur and aid in "toughening". Crack tolerance, key to energy dissipation in these natural composite materials, is assisted by promoting energy distribution over large volumes of loaded specimens by minor components of organic constituents that also serve important roles as adhesives. Viscoelastic deformation was a notable characteristic of the organic component. Some of these energy-dissipating modes and characteristics were found to be quite different from the toughening mechanisms that are utilized for more conventional structural composites. Complementary to those mechanisms found in rigid natural ceramic/organic composites, layered architectures and very thin organic layers played major roles in energy dissipation in these structures. It has been demonstrated in rigid natural marine composites that not only architecture, but also the mechanical behavior of the individual constituents, the nature of the interfaces, and interfacial bonding play important roles in energy dissipation. Additionally, the controlling effects of thin organic layers have been observed in other natural ceramic composite structures, such as teeth and bones, indicating that a variety of similar energy dissipating mechanisms in natural ceramic composites may operate as means to resist failure.