Plane Strain Fracture Toughness Research Articles

Experimental investigation on the fatigue and fracture properties of a fine pearlitic rail steel

This study reports an experimental investigation of the fatigue and fracture resistance of R350HT, a heat-treated pearlitic rail steel with refined microstructure used in rails. Monotonic tensile, rotating bending, linear elastic plane strain fracture toughness, and fatigue crack growth rate tests are presented. The results are used to outline the basic properties and are corroborated by fractographic investigation. This enables the identification of the dominant type of fracture. Regarding fatigue and fracture resistance, the investigated material shows similar properties as other pearlitic rail steels, such as R260. At room temperature, the dominating fracture is of brittle cleavage type, showing some ductile regions associated with pro-eutectoid.

An approach to coupling the gas flow dynamics and thermodynamics of hydrogen solubility in L485ME steel grade for estimation of fracture toughness

The paper presents the problem of coupling the gas flow dynamics in pipelines with the thermodynamics of hydrogen solubility in steel for the estimation of the fracture toughness. In particular, the influence of hydrogen blended natural gas transmission on hydrogen solubility and, consequently, on fracture toughness is investigated with a focus on the L485ME low-alloy steel grade. Hydraulic simulations are conducted to obtain the pressure and temperature conditions in the pipeline. The hydrogen content is calculated from Sievert’s law and, as a consequence, the fracture toughness of the base metal and heat-affected zone is estimated. Experimental data is used to define hydrogen-assisted crack size propagation in steel as well as to a plane strain fracture toughness. The simulations are conducted for a real natural gas transmission system and compared against the threshold stress intensity factor. The results showed that the computed fracture toughness for the heat-affected zone significantly decreases for all natural gas and hydrogen blends. The applied methodology allows for identification of the hydrogen-induced embrittlement susceptibility of pipelines constructed from thermomechanically rolled tubes worldwide most commonly used for gas transmission networks in the last few decades.

Fatigue crack growth properties of PMMA material used in underwater transparent structures

ABSTRACT Polymethyl methacrylate (PMMA) is an ideal material used in underwater transparent structures such as submersible windows and the fully-transparent pressure chamber of the sightseeing submersibles. The failure of PMMA structures usually come from small crack-like defects. In this paper, the fatigue crack growth experiments were carried out on PMMA materials with different thicknesses (B = 3, 6, 12 mm) and stress ratios (R = 0.1, 0.3) as well as plane strain fracture toughness test for unstable crack propagation conditions. Thickness effect and dwell time effect are experimentally investigated in addition to load ratio effect. The applicability of an improved crack growth rate model on PMMA material is validated by test results. The application of this model for prediction of crack growth rates can greatly save experimental time under different working conditions and can be applied in fatigue assessment of full-transparent structures under random loading.

Fracture parameters of woven jute fibre reinforced axial flow fan blade material: an experimental investigation and FEA analysis

Airflow required for the heat and mass transfer in industrial applications is often provided by axial flow fans. Glass fibre reinforced polymers (GFRP) are most often used to make these fan blades. The usage of glass fibres in composite materials is growing. By 2025, the global market for glass fibres will be worth 16.5 billion pounds. However, non-biodegradable composite reinforcement has major ecological and human health concerns, especially when reaching the end of its operational lifespan. Until now, industries have shown a strong inclination to use GFRP without adequately contemplating the potential consequences associated with their disposal. The economic viability of glass fibre recycling is questionable. However, natural fibres like jute and fibres extracted from vegetable plants are of much popular among academics and researchers due to their sustainability and ease of use. So, they’re suitable for making polymer composites. The amount of natural fibres that may be added to GFRP composites without materially altering their mechanical characteristics is a crucial factor in improving their decreased embodied emissions and biodegradability after service life. In this work the material represents an 18-ft axial flow fan blade material made of GFRP. This study used the term denotes the specific material utilised in the fabrication of 18-feet axial flow fan blades for industrial purposes. By substituting one layer of with woven jute and changing its position in it, the materials through are produced. All the materials are tested for fracture parameters like plane strain fracture toughness and Critical strain energy release rate as per ASTM D-5045. The results were validated by using finite element models implemented in a commercially available software ANSYS R19.2. The optimal layup sequence for fan blade material with single-layer woven jute reinforcement was found, which preserves fracture properties, also improves biodegradability after service life.

High-strength and high-toughness austenitic stainless steels based on type 316LN at 4.2 K

High-strength and high-toughness prototype steel forgings, N2, N3, and N4, were developed based on type 316LN as a cryogenic structural material. By extrapolating the proportional relationship between carbon plus nitrogen content (C + N) and 0.2 % proof stress (σ0.2) in the range of approximately 0.10–0.25 mass% C + N content, the C + N content was successfully designed to achieve the target σ0.2. To suppress the degradation of the plane-strain fracture toughness (KIC(J)) while increasing the nitrogen solid-solution content, the alloying level was designed by considering the stability of the austenite phase and stacking fault energy. The steel forgings showed a good balance of strength and toughness at 4.2 K, i.e., N2 (σ0.2 = 1142 MPa, KIC(J) = 228 MPa√m), N3 (σ0.2 = 1226 MPa, KIC(J) = 219 MPa√m) and N4 (σ0.2 = 1296 MPa, KIC(J) = 215 MPa√m). The σ0.2 and KIC(J) of ITER-grade 316LN (316LNL, 316LNM, and 316LNH) thick plates with low interstitial (C + N) content of 0.13–0.17 mass% were also evaluated at 4.2 K. Then, the effects of nitrogen content on strength and fracture toughness were successfully clarified in the full range of interstitials concentrations. An alloy design with higher Ni, Mn, and Mo contents in the chemical composition range of 316LN would be desirable for improving the fracture toughness by increasing σ0.2 due to the higher nitrogen content.

Effect of bimodal microstructure on the tensile properties and fracture toughness of extruded Mg-9Gd-4Y-0.5Zr alloy

The effects of bimodal microstructure on the tensile properties and fracture toughness of Mg-9Gd-4Y-0.5Zr alloys were investigated. The Mg-9Gd-4Y-0.5Zr alloy was extruded at different temperatures to produce samples with bimodal and fully dynamic recrystallized (DRXed) grain structures. The results show that the bimodal grain structure exhibits a better combination of yield strength (YS of 277 MPa) and plane strain fracture toughness (KIc = 21.5 MPa m1/2), compared to the fully DRXed microstructure (YS of 207 MPa, KIc = 16.3 MPa m1/2). The fracture mechanisms for the bimodal and fully DRXed microstructures are the ductile fracture mode and quasi-cleavage fracture mode, respectively. Both hetero-deformation induced (HDI) strengthening effect and strong basal texture can contribute to high tensile strength in the bimodal grain structured sample. As compared to the sample with fully DRXed microstructure, the stronger strain hardening capacity and larger hardened region around the crack tip in the bimodal grain structured sample can bring about the improvement in the fracture toughness. The tortuous crack propagation path and the hindrance of propagating crack by the un-DRXed grains result in larger fracture resistance and more energy dissipation. The increase of geometrically necessary dislocation (GND) density and the activation of non-basal dislocations also have positive effects on the fracture toughness of the samples with bimodal grain structure. The fracture toughness is effectively improved via intrinsic and extrinsic toughening mechanisms associated with the bimodal microstructure.

The Tensile Properties and Fracture Toughness of a Cast Mg-9Gd-4Y-0.5Zr Alloy

Low fracture toughness has been a major barrier for the structural applications of cast Mg-Gd-Y-Zr alloys. In this work, the tensile properties and fracture toughness of a direct-chill-cast Mg-9Gd-4Y-0.5Zr (VW94K) alloy were investigated in different conditions, including its as-cast and as-homogenized states. The results show that the tensile properties of the as-cast VW94K alloy are greatly improved after the homogenization treatment due to the strengthening of the solid solution. The plane strain fracture toughness values KIc of the as-cast and as-homogenized VW94K alloys are 10.6 ± 0.5 and 13.8 ± 0.6 MPa·m1/2, respectively, i.e., an improvement of 30.2% in KIc is achieved via the dissolution of the Mg24(Gd, Y)5 eutectic phases. The initiation and propagation of microcracks in an interrupted fracture test are observed via an optical microscope (OM) and scanning electron microscope (SEM). The fracture surfaces of the failed samples after the fracture toughness tests are examined via an SEM. The electron backscatter diffraction (EBSD) technique is adopted to determine the failure mechanism. The results show that the microcracks are initiated and propagated across the Mg24(Gd, Y)5 eutectic compounds in the as-cast VW94K alloy. The propagation of the main cracks exhibits an intergranular fracture pattern and the whole crack propagation path displays a zigzag style. The microcracks in the as-homogenized alloy are initiated and propagated along the basal plane of the grains. The main crack in the as-homogenized alloy shows a more tortuous fracture characteristic and a trans-granular crack propagation behavior, leading to the improvement of the fracture toughness.

Exceptional fracture toughness in a high-strength Mg alloy with the synergetic effects of bimodal structure, LPSO, and nanoprecipitates

Addressing the strength-toughness trade-off issue has been highly desirable for high strength Mg alloys. Herein, an Mg-9Gd-4Y-1Zn-0.5Zr (wt %) alloy is designed, and a yield strength of 430 MPa and plane-strain fracture toughness KIc of 23.1 MPa·m1/2 are achieved. Intrinsically, significant stress dissipation is induced by interactions between the tensile twinning and the kinking of the lamellar γ' phase, leading to increased KIc. Owing to the residual stress release, the fracture resistance is remarkably improved by the non-basal 〈a〉 and 〈c + a〉 dislocations activated in the fine dynamic recrystallized (DRXed) grains. Extrinsically, the microcracks initiated along the interfaces between lamellar γ' phase and α-Mg matrix facilitate the reduction of local crack-driving forces, resulting in an enhanced KIc. Moreover, the underlying toughening mechanisms were revealed, including the crack propagation path deflection caused by coarse deformed grains and microcrack shielding by the long-period stacking ordered (LPSO) and lamellar γ' phases.

Development of a low-carbon carbide-free nanostructured bainitic steel with extremely high strength and toughness

High-carbon carbide-free nano-bainitic steels show extremely high strength but relatively poor fracture toughness and welding prospects. Therefore, an attempt has been made to develop a strong and damage tolerant low-carbon bainitic steel in the current study by taking recourse to two-step austempering at 350 °C for 20 min and 250 °C for 7 h (20min-TS) and the properties have been compared with another block prepared using a single step transformation for 3 h at 350 °C (3hr-SS). Two stage heat-treatment leads to lath thickness refinement in addition to block refinement as compared to a single step transformation. Moreover, a NW misorientation developed in 20min-TS as compared to KS in a single-stage transformation. An average bainitic lath thickness below 100 nm in a steel having 0.26 wt% carbon has been obtained in 20min-TS which in turn results in an ultimate tensile strength of 1.5 GPa, percentage elongation of 18.5, impact energy of 31 J and plane-strain fracture toughness of 82 MPam−1/2. This is an around two and a half times increase in toughness without significantly compromising on the strength when compared to nanostructured bainitic steels produced in a similar duration of austempering for high carbon steels.

Toughing epoxy nanocomposites with graphene-encapsulated liquid metal framework

Multifunctional epoxy nanocomposites have long been pursued as the crucial component of future aerial vehicles, while their practical implementation is unfortunately restricted by the poor toughness and incapability of chemical activity derived from highly cross-linked networks. Here, inspired by “brittle–ductile” laminated architecture, a stress-induced graphene-encapsulated liquid metal framework was developed to enhance the fracture toughness of multifunctional epoxy nanocomposites and their electromagnetic interference shielding, outdoor deicing and crosslinking behaviors were discussed. The resulting “brittle-ductile” laminated structure enables effective crack deflection and unique fracture surface; the plane strain fracture toughness (KIC) of epoxy nanocomposites is 2.34 times higher than that of pure epoxy resin. Moreover, the reinforced epoxy nanocomposites exhibited high electromagnetic interference shielding effectiveness of 48.24 dB at X-band, about 20 times greater than that of the pure epoxy (∼2.4 dB), and its efficient solar de-icing capability enable spontaneously melting surface ice/frost within 3 min. The dimensionless Cure Index (CI) was used for qualitative analysis of epoxy crosslinking in the presence of graphene-encapsulated liquid metal framework performed by nonisothermal differential scanning calorimetry (DSC). This work shows for the first time to the best of our knowledge graphene encapsulated liquid metal framework as reinforced fillers in epoxy nanocomposite achieve a unification of high toughness, electromagnetic shielding capabilities, and solar deicing properties. Present work thus provides an alternative of constructing high-toughness multifunctional epoxy nanocomposites for aerial vehicles application.

Fracture toughness and martensitic transformation in type 316LN austenitic stainless steel extra-thick plates at 4.2 K

The plane strain fracture toughness of type 316LN nitrogen-strengthened austenitic stainless steels depends on their stability at 4.2 K, which is characterized by the Md30 index. Deformation microstructures and martensite that developed at the crack tip of type 316LN steel extra-thick (535 mm) plates at 4.2 K were evaluated. Two kinds of type 316LN steel, LF and HF, represented low and high fracture toughness, respectively. The Md30 of the plates correlated well with their respective fracture toughness values, where a higher Md30 indicated lower fracture toughness. The plastic strains developed in austenite grains at the crack tip for the HF were higher and more homogeneous and extensive than those of the LF. The volume of α′-martensite detected in the HF was lower than that of the LF. Planar slip bands with the primary slip system and α′-martensite were dominant in the LF. In the HF, however, microslip bands with multiple slip systems, deformation twins, and α′-martensite formed. For the LF, extensive martensite leaves with multiple variants were formed at the crack tip. The martensite leaves grew along the high stress field at crack tip developed by the crack extension. For the HF, α′-martensite platelets were dispersed along {111} slip bands. Shear bands containing martensite were favored by low plastic strains and constituted a smaller fraction of their total density. Therefore, low plastic strains were responsible for an increase of α′-martensite formation at the crack tip. An alloy design having a higher Ni, Mn and Mo, and lower N content in its chemical composition would be favorable for type 316LN steel to provide a higher fracture toughness owing to higher stacking fault energy.

Effect of prior austenite grain size on the tensile properties and fracture toughness of nano-structured bainite

Prior austenite grain size (PAGS) can significantly impact the bainitic lath thickness formed during austempering of high carbon steels. This study examines the effect of two different PAGS on the microstructure development and in turn the tensile properties and fracture toughness of nano-structured bainitic steels. Two PAGS have been obtained by austenitizing at 900 °C and 1000 °C respectively but the bainite fraction has been maintained to be around the same range by austempering at 250 °C. Less amount of blocky austenite is formed when the PAGS is larger. It is also observed that with an increase in the PAGS, the thickness of bainitic lath and retained austenite (RA) decreases and the tensile strength, tensile ductility and the plane strain fracture toughness is enhanced. The fraction of RA transformed to martensite is less for larger PAGS during a tensile test while showing greater ductility owing to the greater stability of finer RA films. • Higher austenitization temperature results in higher prior austenite grain size (PAGS). • Higher PAGS leads to finer, morphology of bainite and more amount of film type austenite. • Refinement of bainitic lath thickness leads to enhancement of strength, ductility and plane strain fracture toughness. • Bainitic ferrite block size reduces with an increase in PAGS. • Fractography features correspond to bainitic ferrite block size.

An investigation on the plane-strain fracture toughness of a water atomized 4130 low-alloy steel processed by laser powder bed fusion

A water atomized 4130 steel powder was processed by Laser powder bed fusion and investigated both in as-built condition and after quench and tempering thermal treatment. Analyses were focused on the different microstructures developed and on steel fracture behavior in terms of tensile fracture elongation, Charpy impact properties, and linear elastic fracture toughness. Comparisons were also drawn by testing a reference 4130 steel fabricated from a gas atomized powder. The slightly higher oxygen content found in the water atomized powder led to the formation of finely dispersed nano-size oxide particles in the steel matrix. It was found that these inclusions have a minor effect on the tensile properties, but a significant influence on the impact toughness response. The fracture toughness tests showed that the orientation leading to propagation of cracks along the inter-layer planes represented the most critical situation, and the steel toughness could be significantly improved after the quench and tempering treatment owing to the achievement of a more homogeneous microstructure. The results suggest that the investigated water atomized low-alloy steel powder feedstock can be considered as a suitable and cheaper alternative for structural parts produced by additive manufacturing, which could replace the more popular gas atomized steel grades.

Rate dependent fracture behavior of highly cross-linked epoxy resin

The fracture behavior of highly cross-linked epoxy resin under plane strain conditions for crack tip strain rates spanning over four orders of magnitude (10-4 −10-1 s−1) has been examined. The plane strain fracture toughness initially decreases with increasing crack tip strain rate (∊̇tip) and saturates at ∼ 0.8 MPa-m−0.5 after ∊̇tip ∼ 0.02 s−1 is attained. For all the fracture specimens, a fracture process zone (FPZ) forms ahead of the sharp pre-crack prior to catastrophic failure due to the intensified stresses ahead of the crack tip. This FPZ region consists of crazes that govern the micro mechanism of fracture and this was explicitly proved using in-situ testing in a scanning electron microscope. For low strain rates, crazing in the FPZ is expected to be dominated by chain disentanglements, with craze openings more than 200 nm, whereas, chain-scission is expected to dominate at higher strain rates, with lower craze openings of ∼ 150 nm.

Effect of Fe-Rich Phases on Fracture Toughness of High Strength Al-Zn-Mg-Cu Alloy

In order to analyze the effect of insoluble Fe-rich phases on the fracture toughness of high-strength Al-Zn-Mg-Cu alloys, plane-strain fracture toughness test, scanning electron microscopy, finite element analysis and quantitative modeling were used to investigate the fracture toughness of three commercial high-strength Al-Zn-Mg-Cu alloys. The results show that the fracture mode of the three experimental alloys is strongly related to the Fe-rich phases, which are prone to initiate intergranular fracture of the experimental alloys. Finite element analysis proved that the larger the shape factor of the Fe-rich phases, the more sensitive it is to external stress and more prone to secondary cracking, which is consistent with the fracture morphology observation. Quantitative analysis shows that the size, volume fraction and morphology of the Fe-rich phases have a greater influence on the fracture toughness of the high-strength Al-Zn-Mg-Cu alloy. The fracture toughness of the alloy will be improved when the shape factor of the Fe-rich phases is small and close to circular and the volume fraction of the Fe-rich phases is small. The fracture toughness value of the Al-Zn-Mg-Cu alloys can be better predicted when the deviation of the shape and distribution of the Fe-rich phases is considered.

364 A Novel Preparation of Bone Cement for Vertebroplasty: Can Strontium Folate Improve the Mechanical and Biological Properties of Calcium Phosphate Cement?

Abstract Aim Bone cements currently used for vertebral body augmentation procedures lack biological properties that allow them to osseointegrate. Strontium folate (SrFo) has been shown to increase the viability of human osteoblast (HOb) cells. The potential of a novel preparation of calcium phosphate cement (CPC) with added SrFo is explored to investigate whether SrFo can increase its biocompatibility. Method Brushite-forming CPCs with 0% (negative control) and 5% and 10% weight-weight additions of SrR (positive control) and SrFo were formulated. Initial (Ti) and final (Tf) setting times were measured. Mechanical analyses included ultimate compressive strength and plane strain fracture toughness. Topography and distribution analyses occurred during materials characterisation and in vitro analysis using scanning electron microscopy (SEM). Results There was no significant difference in the Ti between the CPCs, but significantly greater Tf for all SrFo-CPCs. There was no significant difference in the compressive strength between the CPCs, but a significantly decreased fracture toughness in all SrFo-CPCs. SEM revealed non-homogeneous structures in all SrFo-CPCs. Whilst the pH of the HOb cell culture medium remained the same for all CPCs, the proportion of dead cells was significantly higher than that of alive cells. Conclusions The addition of SrFo did not produce any significant differences in the material characteristics of the cements, nor the in vitro properties, except for a prolonged setting time. Future studies should seek to determine the osteogenic nature of SrFo by altering the formulation of the CPCs to a non-toxic preparation, such as apatite-forming cements which are known to have superior mechanical properties.

Effect of Strain Rate and Temperature on Tensile and Fracture Performance of AA2050-T84 Alloy.

AA2050-T84 alloy is widely used in primary structures of modern transport aircraft. AA2050-T84 is established as a low-density aluminum alloy with improved Young’s modulus, less anisotropy, and temperature-dependent mechanical properties. During flights, loading rate and temperature variation in aircraft engine subsequent parts are commonly observed. The present work focuses on the effect of loading rate and temperature on tensile and fracture properties of the 50 mm thick (2-inch) AA2050-T84 alloy plate. Quasi-static strain rates of 0.01, 0.1, and 1 s−1 at −20 °C, 24 °C and 200 °C are considered. Tensile test results revealed the sensitivity of mechanical properties towards strain rate variations for considered temperatures. The key tensile properties, yield, and ultimate tensile stresses were positive strain rate dependent. However, Young’s modulus and elongation showed negative strain rate dependency. Experimental fracture toughness tests exhibited the lower Plane Strain Fracture Toughness (KIC) at −20 °C compared to 24 °C. Elastic numerical fracture analysis revealed that the crack driving and constraint parameters are positive strain rate dependent and maximum at −20 °C, if plotted and analyzed over the stress ratio. The current results concerning strain rates and temperatures will help in understanding the performance-related issues of AA2050-T84 alloy reported in aircraft applications.

Crack path direction in plane-strain fracture toughness assessment tests of quasi-brittle PLA polymer and ductile PLA-X composite

Plane-strain fracture toughness is one of the main parameters in linear elastic fracture mechanics and its purpose is to show the material capability to withstand load while having a defect. Main validity aspect for such an assessment is to provide a wide enough crack front to enable plane-strain condition. Nonetheless, in FDM (Fused Deposition Modeling), due to structural anisotropy caused by polymer material properties and Additive Manufacturing (AM) process parameters, more validity aspects must be met. During the plane-strain fracture toughness test a crack must follow a straight line from initiation up to the point of structural failure. Plane-strain fracture toughness assessment is conducted according to the ASTM D5045-14 standard for testing of polymer materials. Tests are performed on SENB (Single Edge Notched Bend) specimens, made from two similar polymer materials: quasi-brittle PLA and ductile PLA-X composite. Specimens are manufactured with four different AM process parameters, i.e., layer height, infill density, printing orientation and one specimen batch was dried before testing.

The Theory of Critical Distances to perform the static assessment of 3D-printed concrete weakened by manufacturing defects and cracks

The Theory of Critical Distances groups together a number of approaches postulating that, in cracked/notched materials subjected to static loading, breakage takes place as soon as a critical length-dependent effective stress exceeds the material tensile strength. The characteristic length used by the Theory of Critical Distances is a material property that can directly be estimated from the ultimate tensile strength and the plane strain fracture toughness. In the present investigation, based on a large number of bespoke experimental results, it is demonstrated that the Theory of Critical Distances is successful also in quantifying the detrimental effect of cracks and manufacturing defects in 3D-printed concrete subjected to Mode I static loading.

Plane-strain fracture toughness of thin additively manufactured maraging steel samples

This study aimed to evaluate the aging effects on mechanical properties of 18Ni (300) maraging steel processed by additive manufacturing powder bed fusion and subjected to different aging conditions. The aging temperatures were 470 °C, 510 °C, and 530 °C, with exposure times ranging from 0.33 to 96 h, followed by air cooling. Despite the layer-by-layer material consolidation mechanism, weak preferred orientation was found in the as-built condition; moreover, only martensite and austenite were found in the X-ray diffraction results. This alloy showed a steep increase in hardness, compressive strength, and tensile strength, while the work-hardening ability decreased with aging. Despite using thin single-edge bending samples (5 mm), the plane-strain fracture toughness ( K IC ) size requirement was met, and valid K IC results were 49 ~ 64 MPa m . This result could help design new additive manufacturing applications of high-strength steel, such as thin-walled, lattice structures, and reduced cross-section parts projected to minimize weight in different industries.