Notch Sensitivity Research Articles

Fatigue strength and notch sensitivity of high-manganese flake graphite cast iron

To increase the recovery rate of high-tensile-strength steel (HTSS) scrap, the effect of manganese (0.5% and 2.0% Mn) on the fatigue properties of several flake graphite cast iron (FGI) samples is investigated, along with the thickness and notch sensitivity of high-manganese (Hi-Mn) FGIs. The results reveal an increase in the pearlite fraction, an increase in the crystallization of Type-A graphite, and a decrease in the interlamellar spacing owing to the addition of Mn, which results in a significant increase in fatigue strength. Moreover, Mn does not worsen the notch sensitivity of the Hi-Mn FGIs. However, Hi-Mn FGIs have a higher thickness sensitivity because a longer cooling time results in wider interlamellar spacing and increased graphite length. Overall, HTSS can be recycled as a raw material for Hi-Mn FGIs without removing Mn, and Hi-Mn FGIs can be used for thinner parts with high-fatigue-strength requirements.

Influence of reduced carbon content on microstructure and mechanical behavior of Inconel 718 prepared by laser powder bed fusion

• Successful production of a reduced carbon Inconel 718 alloy via SLM. • Significant changes in the precipitation behavior of NbC and γ’’ are achieved. • Reductions in NbC content and finer γ’’ precipitation verified By Thermocalc. • Reduced γ’’ size results in improved tensile strengths at both 25 °C and 650 °C. Inconel 718 powder with reduced carbon content was used to manufacture coupons via laser powder bed fusion (LPBF) to study the influence of carbon content on microstructures and mechanical properties of HIP'ed and heat-treated material. The carbon concentration in this material was kept deliberately low (0.01 wt%) relative to a typical commercial powder composition (0.04 wt%) of IN718 in order to minimize the precipitation of primary carbides during post-processing heat treatments, which have been observed to be deleterious to elevated temperature mechanical properties such as elongation, stress rupture ductility, and notch sensitivity. The final microstructure of the lean C alloy is shown to have a significantly reduced NbC population relative to the baseline material. Furthermore, other microstructural variations such as an increased δ phase population and decreased γ’’ particle size were observed. Empirical observations agreed with microstructural simulations calculated by the ThermoCalc/TC-PRISMA precipitation module. These changes to the microstructure resulted in a 10% increase in the yield strength at both 25 and 650 °C in the lean C alloy, bringing the strength of LPBF-printed material closer to that of wrought IN718. Overall, the reduction in carbon content in the raw material has significant effects beyond limiting the pre-solution treatment precipitation of NbC, primarily by freeing Nb for formation of δ and γ’’ phases.

Notch sensitivity and short cracks tolerance in a super 13Cr stainless steel under sulfide stress corrosion cracking conditions

Abstract Notch sensitivity under sulfide stress corrosion cracking (SSC) conditions is quantified in this work, considering its tolerance to short cracks that start at notch tips and become non-propagating after growing for a short while. The considered material is an UNS S41426 super 13Cr5Ni2Mo martensitic stainless steel, frequently used in pipelines to transport aggressive fluids in offshore applications. Tolerance to short SCC cracks depends on the notch size and shape, on the stress gradient ahead of its tip, and on basic material resistances to crack initiation S SSC and growth K ISSC inside the aggressive environment. The proposed notch-sensitivity model can be a powerful and economical alternative design tool to substitute traditional pass/non-pass criteria normally used to choose materials for SSC and similar environmentally assisted cracking (EAC) service conditions. This model is validated by testing the steel into an aqueous solution of 100 g/L of Cl−, pH 4.0, 25 kPa of H2S, and 75 kPa of CO2, at 23 ± 2 °C. After measuring S SSC (461 ± 23 MPa) and K ISSC (36.9 ± 0.6 MPa√m), short crack tolerance predictions are verified on notched specimens loaded under a peak stress at their tips σ max = 0.95⋅S Y >> S SSC, where S Y = 826 MPa is the yield strength of the tested steel.

The Effect of Primary Laves Phase on Notch Sensitivity in Thermo-span Superalloy

The Effect of Primary Laves Phase on Notch Sensitivity in Thermo-span Superalloy

A network mechanics method to study the mechanism of the large-deformation fracture of elastomers

This work presents a network mechanics method to reveal the fracture mechanism of the inhomogeneous polymer network of elastomers. The polymer network in elastomers is abstracted as a network model composed of cross-linkers, polymer chains, and volume elements. The hyperelastic deformation of the network model is characterized using the total energy composed by the free energy of all polymer chains and the volumetric deformation energy of all volume elements. The fracture behavior of the network model is described by the fracture criterion of polymer chains. By comparing the large-deformation fracture simulations of homogeneous and inhomogeneous network models, we find that the network inhomogeneity is one important fundamental cause of the ductile fracture and the low notch sensitivity of elastomers. We also find that the widely used Lake–Thomas model underestimates the intrinsic fracture energy of elastomers, because the intrinsic fracture energy of an elastomer network includes the elastic energy stored not only in the broken polymer chains, but also in the newly created dangling chains on a crack surface. Our simulations show that the intrinsic fracture energy of a four-armed elastomer network is about three times of that predicted by the Lake–Thomas model. This result agrees quite well with the previous experimental results.

Double network self-healing hydrogel based on hydrophobic association and ionic bond for formation plugging

Self-healing hydrogels have attracted tremendous attention in the field of oil and gas drilling and production engineering because of their excellent self-healing performance after physical damage. In this study, a series of double network self-healing (DN SA ) hydrogels based on hydrophobic association and ionic bond were prepared for plugging pores and fractures in formations in oil and gas drilling and production engineering. The mechanical, rheological, and self-healing properties of the DN SA hydrogels were investigated. Results revealed that the DN SA hydrogels exhibited excellent mechanical properties with a tensile stress of 0.67 MPa and toughness of 7069 kJ/cm 3 owing to the synergistic effect of the double network. In addition, the DN SA hydrogels exhibited excellent compression resistance, notch insensitivity, and self-healing properties. The DN SA -2 hydrogel was granulated and made into gel particles with different particle sizes and used as a plugging agent. The self-healing mechanism of DN SA -2 hydrogel particles in fractures was explored, and it's plugging effect on fractures of different widths and porous media of different permeabilities were investigated. Experimental results revealed that the plugging capacity of the DN SA -2 hydrogel particles for a fracture with width of 5 mm and a porous medium with a permeability of 30 μm 2 was 3.45 and 4.21 MPa, respectively, which is significantly higher than those of commonly used plugging agents in the oilfield. The DN SA hydrogels with excellent mechanical and self-healing properties prepared in this study will provide a new approach for applying hydrogels in oil and gas drilling and production engineering.

Notch-insensitive, underwater adhesive, and self-healing ionic skins

Ionic skins (I-skins) mimic the sensing function of natural skin based on ionic conduction have essential applications in wearable sensors for human motion detections. However, the fragile, low electrical sensitivity, lack of adhesive and self-healing performances still limits their practical applications and service life. Herein, we reported an ionic skin which was fabricated by the ionic liquid, acrylic acid and Fe 3+ through the one-pot method. The polyacrylic acid network is cross-linked by intermolecular hydrogen bonds and the complexation of Fe 3+ , where the ionic liquid can bind through reversible electrostatic interactions. Such design endows the resultant hydrogel with superior tensile properties (tensile strength 341 kPa, strain at break 2042%), good notch insensitivity, air/underwater dual adhesive property for the surface of various solid materials (including glass, iron, copper and aluminum) and excellent healability (self-healing efficiency as high as 80.5% at room temperature). In addition, the I-skin exhibits ultra-high electrical sensitivity (GF = 13.12) and an extensive strain monitoring range (5%–300%). Not only can it be used as expression and human motion detector, but also can realize “information programming”. The rich application scenarios and functions prove its potential to be used as wearable sensors. • Physical cross-linking hydrogel prepared by one-pot method. • Not-insensitive, air/underwater adhesive. • Mechanical/electrical dual self-healing. • Wearable strain sensors for human motion detection. • Electrical signal conversion through Morse code.

Off-axis tensile performance of notched resin-infused thermoplastic 3D fibre-reinforced composites

This study presents a comparison of off-axis tensile performance for notched (open-hole) and unnotched (no-hole) 3D fibre reinforced composites (FRC) specimens having two different types of matrices. The two matrix systems compared are, a novel infusible thermoplastic (Elium) resin and conventional thermoset (epoxy). Three different configurations, (one unnotched and two notched) were tested for each 3D-FRC. The resulting notched net strength, gross strength, failure strains, notch sensitivity and energy absorbed by each configuration were evaluated and compared. Additionally, 2D digital image correlation (DIC) was used to evaluate full-field strain distribution in each case. The results elucidate that thermoplastic 3D-FRCs are notch insensitive irrespective of the notch size and possess higher failure strains (around 30 percent in the cases investigated) and energy absorption (around 33 percent in the cases investigated). In contrast, thermoset 3D-FRC appeared to be notch sensitive, as the notched size increased, and they failed at lower axial strains (up to 60 percent reduction compared to unnotched specimens for the size investigated). Thus, resin-infused thermoplastic off-axis configurations are effective for composite joint applications, particularly in notch-insensitive designs, requiring higher energy absorption and failure strains.

Notched tensile strength of long discontinuous glass fiber reinforced nylon composite

In this study, the hole size effect in the open hole tension (OHT) of discontinuous glass fiber-polyamide 6 organosheet composite material was studied. Organosheet used in this study is a novel long discontinuous fiber-reinforced polymer composite with long glass fiber and in-situ polymerized polyamide 6 (nylon) matrix. A progressive failure analysis (PFA) using finite-element model was proposed to predict OHT strength using measured fiber orientation distribution. The generated virtual ogranosheet samples were used to conduct OHT Monte Carlo simulations to predict variability in strength. The notch sensitivity was observed experimentally and analyzed using PFA model, which captured interactions between various failure modes. The experimental and PFA results of OHT strength were used for calibrating analytical models, Point Stress Criterion (PSC) and Modified Point Stress Criterion (MPSC). Both, experimental and modeling approaches, yielded nearly identical analytical curves, indicating that virtual testing of organosheet can be used for OHT characterization of organosheet. The results show that the notched strength of organosheet with the diameter of 2 mm was at the same level as strength of unnotched organosheet. The notch insensitivity was attributed to the increased plasticity in the matrix due to ductility in nylon.

Tough Hydrogels with Isotropic and Unprecedented Crack Propagation Resistance

AbstractMuscles and some tough hydrogels can maintain perfect mechanical properties after millions of loading cycles owing to the anisotropic microstructures inside them. However, applications of intrinsic anisotropic microstructures in biological tissues and tough hydrogels are limited by the poor mechanical performance in the perpendicular direction relative to the alignment direction. Here, a universal strategy is proposed for developing hydrogels with unprecedented isotropic crack propagation resistance only depending on the interpenetrating entanglements of polymer chains (polyacrylamide (PAAM) or poly‐(1‐acrylanmido‐2‐methylpropanesulfonic acid) (PAMPS)) in deformable polymeric microspheres (PAMPS or PAAM). The deformable interpenetrating network in microspheres can transform the hydrogel from isotropic to anisotropic instantaneously in any load direction, and effectively alleviate the stress concentration at the crack tip, dissipate energy, and eliminate notch sensitivity. The best isotropic hydrogel displays an ultimate strain of 5300%, toughness of 18.9 MJ m–3, fracture energy of 157 kJ m–2, and fatigue threshold of 4.2 kJ m–2. Furthermore, the mechanical strength of hydrogels can be simply tuned by solvent replacement. The strategy presented here can be expanded to prepare other isotropic hydrogels with super tear‐resistant and anti‐fatigue properties, based on a wide variety of deformable microspheres and matrix polymers.

Development and evaluation of the sandwich open-hole compression test

An open-hole compression test method has been developed to assess the notch sensitivity of sandwich composites under compression loading. This test method, in the process of being standardized by ASTM International, utilizes end loading as well as knife-edge side supports to provide out-of-plane restraint to specimen buckling. Finite element analyses focused on identifying a sandwich specimen geometry that minimized finite-width and finite-length effects on the stress concentration produced by the centrally-located through-hole. Geometric variables included the specimen width-to-hole diameter (w/D) and length-to-hole diameter (l/D) ratios. The sandwich configurations investigated consisted of a Nomex honeycomb core and carbon/epoxy facesheet laminates with a range of material orthotropy ratios. Numerical results were used to identify a candidate sandwich specimen geometry and optimal strain gage placements for use in specimen alignment. A series of mechanical tests were performed to evaluate the proposed specimen design, the proposed test fixture, and the recommended test procedure. The first set of experiments were performed using specimens with different hole diameters but the same specimen width and length. A second set of experiments used specimens with different lengths but with the same specimen width and open-hole. In addition, the use of specimen end potting of the core region was investigated to prevent facesheet separation and end brooming.

Mechanisms of notch insensitivity in long-fiber discontinuous, prepreg platelet compression molded composites

The present treatise offers an investigation into the operative mechanisms of stress redistribution and notch insensitivity of the prepreg-based, long-fiber discontinuous composites with stochastic fiber orientation state manufactured via a compression molding process. Open-hole tensile behavior is examined to determine the source of notch insensitivity. Both experimental and simulation approaches reveal that local damage mechanisms result in stress redistribution and corresponding reduction in notch sensitivity as measured by the ratio of the notched to un-notched tensile strengths. The analysis showed that open-hole coupon fracture may occur in locations at or away from the hole, depending on the details of the stochastic meso-morphology and the notch size. It was also demonstrated that as the notch diameter is increased, the probability of fracture at the notch is increased. The experimentally observed notch insensitivity of the prepreg platelet composite was reproduced in the numerical simulations.

Influence of the ratio between specimen thickness and grain size on the fatigue and tensile properties of plain and notched aluminium plate specimens

The influence of the ratio between specimen thickness (t) and grain size (D) on the tensile and fatigue properties of plain and notched aluminium plate specimens has been studied. Grain sizes ranging from 66 μm to 9.74 mm have been prepared and used in the tests. The observed decrease on tensile properties when D is comparable, or even exceeds, t can be accounted for by adding an additional negative exponential term to the Hall–Petch equation. An exponential decrease has also been found in endurance limits as grain size approaches plate thickness in plain specimens. In notched specimens, however, increasing grain size has two opposing effects, namely: a decrease in the plain endurance limit associated with the decrease in thickness relative to grain size and an increase in notched fatigue strengths brought about by a decrease in notch sensitivity.

Shear and Tensile Behaviors of Fiber-Reinforced Resin Matrix Composites Printed by the FDM Technology

Resin/fiber composites were prepared by the FDM printing technology. The effects of arrangements, types (carbon, glass, and Kevlar), and volume fraction of fibers on the shear and tensile properties of resin 3D-printed composites are investigated in this paper. The experimental results show that the addition of continuous fibers increases the shear strength and tensile strength of FDM-3D-printed composites, but the strength will not keep increasing with an increase in the fiber content. As the fiber content increases, the print quality decreases, and the porosity between the fibers increases. The enhancement degree of the shear stress of specimens by different fiber types can be classified as follows: glass fiber > carbon fiber > Kevlar fiber. Notch sensitivity is reduced when the 90° arrangement of fibers is added, while the addition of 0° arranged fibers will improve the notch sensitivity of the sample. The research results of this paper have an important guiding significance for selecting fiber types and arrangement mode of notched components in engineering applications.

Ultra‐stretchable, self‐healable, and reprocessable ionic conductive hydrogels enabled by dual dynamic networks

AbstractThe pursuit of stretchable ionic conductive hydrogels for skin‐inspired sensors is highly attractive but challenging due to the difficulty in fabricating an ionic conductive hydrogel with combined characteristics of high stretchability, notch insensitivity, self‐healability, and unique reprocessability. Herein, a dual‐dynamic‐network ionic conductive hydrogel (DICH) with both hydrophobic association network and metal coordination network is one‐pot synthesized. Benefiting from the formation of the highly dynamic double networks, the resultant DICH exhibited large stretchability (>2800%) and excellent fracture toughness (4820 kJ m−3). The DICH also demonstrated autonomous healability, notch insensitivity, and unique remoldability because of the formation of the active reversible interactions including hydrophobic association and metal coordination. Due to the integration of high stretchability and favorable ionic conductivity of the DICH containing substantial amounts of electrolytic ions, stretchable ionic strain sensors could be assembled, displaying high sensitivity (gauge factor of 2.6), fast response time (180 ms), and wide response range (0.5%–600% strain). The ionic sensors could also maintain excellent strain‐sensing abilities even after being cut/self‐healed or reprocessed. Wearable DICH resistive‐type strain sensors could be assembled, demonstrating high performance in detecting and distinguishing both large and subtle motions of a human body.

Mechanical characterization of 3D printed continuous carbon fiber reinforced thermoplastic composites

Three dimensional (3D) printing of continuous fiber reinforced composites has attracted great interest in various industrial applications, due to convenient complex geometry manufacturability and material savings. Understanding the mechanical properties of 3D printed composites is required for design and simulation. Hence, this paper aims to carry out a comprehensive study on tension, compression, bend and impact properties as well as failure behavior of 3D printed continuous carbon fiber reinforced thermoplastic (CCFRT) composites. The results show that the print fiber filament fill pattern significantly affect the mechanical properties of CCFRTs. Low strain rate has influence on failure mode and micro failure mechanism, but possesses no strong effect on the tensile elastic modulus and strength. Based on the data from longitudinal (0°) and transverse (90°) specimens, the mechanical properties of cross (0°/90°) specimens for three-point bend and unnotched Charpy impact can be predicted by the rule of mixing. However, it is not suitable for the notched specimens. The notch reduced the impact strength of cross specimens by 61.04%, indicating significant notch sensitivity. This study could provide some insights for testing and applying 3D printed continuous fiber composites.

The atomistic mechanism of notch sensitivity on the deformation mode in metallic glasses

In previous studies, the influence of flaws on macro-scale and nano-scale metallic glasses (MGs) has always been a topic of great interest, with important applications in engineering. Moreover, through experiments and simulations, many researchers draw contradictory conclusions of the notch sensitivity, including the notch strengthening, weakening, and insensitivity in different MGs. In order to better understand its atomic mechanism, classical molecular dynamics simulations have been performed on the nano-scale notched MGs in the present work to investigate the deformation modes under the uniaxial tension. According to the results, after setting notches, the MG samples exhibit a significant change from the shear to necking failure, which indicates a notch strengthening phenomenon. At the same time, the evolution of atomic structures suggests that the introduction of notches will increase the degree of shear localization and further disturb the structural order of MGs. The atomic displacement, which is different from shear strain, can also be used as an index to visually describe the deformation behavior of MGs and shows the high notch sensitivity.

High Mechanical Properties of Stretching Oriented Poly(butylene succinate) with Two-Step Chain Extension.

The structure, morphology, fracture toughness and flaw sensitivity length scale of chain-extended poly(butylene succinate) with various pre-stretch ratios were studied. PBS modification adopted from a multifunctional, commercially available chain-extension containing nine epoxy groups (ADR9) as the first step chain extension and hydroxyl addition modified dioxazoline (BOZ) as the second step. Time-temperature superposition (TTS) studies show that the viscosity increased sharply and the degree of molecular branching increased. Fourier transform infrared spectroscopy (FT-IR) confirm successful chain extension reactions. The orientation of the polymer in the pre-stretch state is such that spherulites deformation along the stretching direction was observed by polarized light optical microscopy (PLOM). The fracture toughness of sample (λfix = 5) is Γ ≈ 106 J m-2 and its critical flaw sensitivity length scale is Γ/Wc ≈ 0.01 m, approximately 5 times higher than PBS without chain-extension (Γ ≈ 2 × 105 J m-2 and Γ/Wc ≈ 0.002 m, respectively). The notch sensitivity of chain-extended PBS is significantly reduced, which is due to the orientation of spherulites more effectively preventing crack propagation. The principle can be generalized to other high toughness material systems.

Deformation and failure of pseudo-ductile quasi-isotropic all-carbon hybrid FRPS with an open hole under tension

An experimental study has been carried out to investigate the elastic and pseudo-ductile deformation processes of carbon fibre reinforced plastics, containing low elongation (LE) and high elongation (HE) layers of standard thickness. The behaviour of unidirectional homogeneous LE- and HE-laminates, hybrid [HE/LE/HE] and quasi-isotropic composites, with sublaminates sequence [0/45/90/-45]s and [0/60/-60]s were studied up to complete tensile failure. Tests carried out on standard prismatic specimens and specimens with a central hole. A clip-on extensometer and digital image correlation (DIC) were used to distinguish the stable elastic deformation, the microdamage accumulation in the LE-layers, and the delamination of LE and HE layers. For notched specimens, acoustic emission signals revealed the accumulation of scattered microdamage and the localisation of the pseudo-ductility around the hole. The quasi-isotropic hybrid laminates were almost notch insensitive, with the notch sensitivity factors in the open hole tension test for pseudo-yield and ultimate stress below 1.15.

Improvement of traction-traction fatigue properties of A100 steel plate-hole-structure by double shot peening

To reduce the stress concentration and improve fatigue resistance of A100 steel plate-hole-structure, double shot peening (DSP) of the primary shot peening with strong intensity iron shots and the secondary shot peening with weak intensity ceramic shots is carried on material surface and hole and the effect of surface integrity on fatigue properties is investigated. The results show that the surface of hole presents severe surface plastic deformation (SSPD) with increasing DSP intensity. A high intensity compressive residual stress (CRS) distributed in deep layer is generated. Meanwhile, the compressive residual stress is much more stable during the fatigue process, which is beneficial to improve fatigue properties by inhibiting fatigue crack initiation and early propagation. In addition, due to the reduced stress concentration at the edge of hole and higher compressive residual stress, the fatigue resistance of A100 steel plate-hole-structure is remarkably improved. The average fatigue strength of DSP-treated specimens with medium intensity primary shot peening was at least improved by 59.6%, and the fatigue notch sensitivity was reduced by 61%. However, it is also discovered the secondary shot peening cannot repair the rough surface and surface damage caused by excessive primary shot peening, which compromise the improvement of fatigue resistance.