Single Edge Notch Research Articles

Effect of different parameters on mode I fracture toughness of resin samples manufactured by DLP

Abstract Additive manufacturing is spreading rapidly in almost all industries, from household to advanced engineering. Components produced by Digital Light Processing (DLP) are not comprehensively characterized, but exceed the capabilities of many AM processes. Its advantages include the ability to produce highly complex designs, superior precision, fast printing and lower operating costs. The present paper investigates the mode I fracture toughness (KIC) of UV-sensitive resin specimens obtained through DLP process. Single edge notch bending (SENB) specimen were 3D printed and tested according to the ASTM D5045 standard. The influence of printing orientation-PO (0°, 45° and 90°), resin color-RC (white, black and transparent) and corrosive environment-CE (air, water and saline solution) was studied. It was observed that all the studied parameters have some effect on the fracture properties. The highest KIC values were obtained for 45°-PO, white-RC (1.88 MPa•m0.5) and saline solution-CE (2.24 MPa•m0.5). However, the greatest influence is highlighted by RC (∼92%), while CE (∼32%) shows a minimal effect. The fracture surface of the investigated samples is influenced by the printing parameters.

Rupture mechanics of blood clots: Influence of fibrin network structure on the rupture resistance

Embolization is a leading cause of mortality, yet we know little about clot rupture mechanics. Fibrin provides the main structural and mechanical stability to blood clots. Previous studies have shown that altering the concentration of coagulation activators (thrombin or tissue factor (TF)) has a significant impact on fibrin structure and viscoelastic properties, but their effects on rupture properties are mostly unknown. Toughness, which corresponds to the ability to resist rupture, is independent of viscoelastic properties. We used varying TF concentrations to alter the structure and toughness of human plasma clots. We performed single-edge notch rupture tests to examine fibrin toughness under a constant strain rate and we assessed viscoelastic mechanics using rheology. We utilized fluorescent confocal and scanning electron microscopy (SEM) to quantify the fibrin network structure under varying TF concentrations. Our results revealed that increased TF concentration resulted in increased number of fibrin fibers with a reduction in network pore size, thinner and shorter fibrin fibers. Increasing TF concentration yielded a maximum toughness at mid-TF concentration, such that fibrin diameter and number of fibers underlie a complex role in influencing the rupture resistance of blood clots, resulting in a nonmonotonic relationship between TF and toughness. A simple mechanical model, built on our findings from our Fluctuating Spring (FS) computational model, adopted to estimate the fracture toughness (critical energy release rate) as a function of TF predicts trends that are in good agreement with experiments. The differences in mechanical responses point to the importance of studying the structure-function relationships of fibrin networks, which may be predictive of the tendency for embolization. Statement of significanceFibrin, a naturally occurring biomaterial, is the main mechanical and structural scaffold of blood clots that provides the necessary strength and stability to the clot, ensuring effective stemming of bleeding. The rupture of blood clots can result in the blockage of downstream vessels thereby blocking blood flow and oxygen supply. The fibrin network structure has been shown to influence the viscoelastic mechanical properties of clots, but has not been explored for fracture mechanics. Here, we modulate the fibrin network structure by varying the concentration of Tissue Factor (TF). Interestingly, the association between TF concentration and maximum toughness of the clots is non-monotonic. The variations in mechanical responses highlight the importance of studying the structure-function relationships of fibrin networks, as these may predict the tendency for embolization.

The Influence of the Rolling Direction on the Mechanical Properties of the Al-Alloy EN AW-5454-D

A complementary characterisation of the Al-alloy EN AW-5454 was carried out, intended for obtaining the laser hybrid welding parameters of subassemblies in the automotive industry. The investigation included a microstructural examination and the determination of the alloy’s properties using several analytical methods (HV5 hardness measurement, tensile test, Charpy impact toughness, fracture mechanics analysis). Samples were prepared in the longitudinal and transverse directions of a cold-rolled sheet of EN AW-5454 with thicknesses of 3.5 mm and 4 mm. The measured hardness on the thinner sheet was 5% higher than on the thicker sheet. The tensile and yield strength were nominal, while the elongations were smaller by 2.2–3.2% for the longitudinal samples and by 2.7–13.7% for the transverse samples. The smaller deviations from the nominal values are for the thinner sheet metal. A precise topographical analysis showed the brittle fractures of the samples. The Charpy impact toughness results on the thicker plate showed a 20% greater work needed to break it in the longitudinal direction than in the transverse direction. With the thinner sheet metal, 40% greater work was needed. SEM (scanning electron microscope) analysis has shown that the intermetallic Al6(Mn,Fe) particles in the longitudinal samples were mostly intact, with evidence of tough areas on the upper part of the fracture, indicating a better toughness than the specimens in the transverse direction. More crushed intermetallic particles were observed at the fractures of the transverse samples, and their distribution appeared to be more oriented in the direction of rolling. Fracture mechanics SENB (single edge notch bending) tests and their analysis showed that the resistance of the material to crack propagation in the longitudinal sample was about 50% greater than that in the transverse sample. SEM analysis of the fractures showed that the state of the intermetallic particles in the fracture mechanics testing and the fracture mechanism differed from the one in the Charpy fractures.

The effect of semi-curing on neat resin mode I fracture properties

The sensitivity of fracture toughness to the cure path of a high-performance hot infusion epoxy resin (EP2410) is assessed in this study through single-edge notch bend testing of neat resin specimens. The specimens were prepared using a two-stage semi-curing process. In the initial stage, the partial degree of cure and the semi-cure temperature were varied. In the second stage, samples were heated to 180 °C for a secondary post-cure. These samples were compared to two baseline samples produced using the manufacturer's recommended cure cycle of 180 °C, and a modified isothermal cure at a lower cure temperature of 140 °C. The results show that the semi-curing path has no statistically significant influence on the mode I fracture toughness of the material studied here. Sample similarity was confirmed through SEM examination of the fracture surface, which showed minimal variation in surface topography. These findings open new possibilities to use this type of material for novel manufacturing processes involving semi-curing steps.

Influence of processing parameters on the fracture behaviour of 316L SS printed by laser powder bed fusion

This study explored the relationship between process parameters and fracture behavior in 316L stainless steel printed by laser powder bed fusion. Fracture testing was conducted according to ASTM E1820 for single edge notch bending, and elastic mechanical properties were determined using ultrasonic surface wave analysis. Five test sets were considered in a vertical building configuration using five different volumetric energies belonging to conduction mode. The critical fracture toughness was calculated and discussed along with the plastic deformations at the crack tip. The study found that local plastic deformation for single edge notch bending was influenced by powder bed fusion process parameters. A correlation was observed between energy density, fracture toughness, and the dimensions of the fracture process zone. The R-curves showed different fracture behaviors depending on the energy density. The energy required to grow a crack was associated with larger plastic zones, resulting in fracture toughness values ranging from 43 (43 J mm−3) to 427 kJ/m2 (68 J mm−3). Results are discussed in terms of porosity and strain hardening capacity depending on the manufacturing conditions.

Carbon-Aramid Fiber/Epoxy Hybrid Composite Laminates with the Presence of Defect: An Experimental Study

Abstract Carbon-Aramid fiber-reinforced epoxy has been used extensively in the aerospace and automobile industries. The combination of high-strength carbon fiber and the high toughness of aramid fiber is believed to be beneficial to the structural behavior of composites. In the current study, Aramid fiber was sandwiched between carbon fiber layers to maintain high strength and toughness simultaneously. The behavior of the laminate with the presence of an open hole and single-edge notch was investigated. For justification, the response of the hybrid laminate was compared with two other laminates, one is made totally from carbon fiber-reinforced epoxy (CFRP) and the other is made from aramid fiber-reinforced epoxy (AFRP). The effect of an open hole was assessed by a tension test, while the single-edge notch effect was evaluated by the flexural test. Tensile and flexural tests were also performed on the regular samples. As per the current results, the notch sensitivity of hybrid laminate was found to be less than that of CFRP laminate. The CFRP laminate failure type was dominated by delamination. AFRP composite laminate failure was dominated by fiber breakage and crack propagation through the matrix. The hybrid composite laminates were dominated by fiber breakage of the AFRP laminates and delamination of CFRP outer layers. The flexural modulus of hybrid laminate resulted in the greatest value, followed by CFRP and AFRP. The hybrid laminate’s fracture toughness is significantly higher than that of CFRP but lower than that of AFRP.

Comparison of Mode I fracture toughness of an elastic and an elastoplastic methyl methacrylate polymer as measured by SENB and DCDC tests

An elastic and an elastoplastic methyl methacrylate polymer, both in the glassy state, have been submitted to mode I fracture characterization with two different tests, the single edge notch bending (SENB) and the double cleavage drilled compression (DCDC) tests. The elastic material toughness was simply assessed by the linear elastic fracture mechanics analysis. For the elastoplastic material, the J-integral was computed when the strain fields were available. Moreover, numerical analyses were carried out using finite element simulations with a phase-field damage approach, providing comparison when the J-integral had been calculated and estimate otherwise, of the critical energy release rate. For each material, as well as for each test, the values of the critical energy release rate that have been estimated here, are in good agreement with the literature. More interestingly, for the same material, both tests provide with very different estimates of this material parameter, leaving engineers uncertain on which test to choose to assess the material toughness in crack opening mode.

Automated crack extension measurement method for fracture and fatigue analysis using digital image correlation

A novel Automated Crack Extension Measurement (ACEM) method based on combining decorrelation in strain field and a threshold near maximum of full-field strain obtained from digital image correlation, is developed in this work. A manual method for measuring crack extension from images of the specimen surface and ImageJ, was utilized to validate the accuracy of the proposed method. Initially, ACEM method was validated for crack extension under mode-I fracture of single edge notch specimens with different initial notch sizes. A satisfactory agreement was obtained from this analysis along with limits on the different parameter values used in ACEM. The method was then applied to low-cycle fatigue specimens with different initial notch lengths. Comparison of crack extension between ACEM and manual method reveals satisfactory agreement reinforcing the fidelity of the developed method for fracture and fatigue analysis. Fracture resistance and fatigue crack growth rate curves were also obtained for the different specimens.

Experimental evaluation of the short and long fatigue crack growth rate of S355 structural steel offshore monopile weldments in air and synthetic seawater

Welded steel structures used in the offshore wind industry are exposed to harsh marine environments, which can result in corrosion-induced fatigue damage. Of particular concern is the heat affected zone (HAZ) of welded joints, a region known for its altered microstructure and mechanical properties, which can significantly influence the initiation and propagation of fatigue cracks. This study investigates the short and long fatigue crack growth rates, and the effect of seawater exposure, for the HAZ in S355 steel weldments. Single-edge notch bend (SENB) specimens are used, with a shallow notch in the HAZ. A series of specimens is immersed in synthetic seawater that is continuously circulated at a controlled temperature to assess the synergistic effects of corrosion and fatigue. The experimental method integrates a novel application of front face strain compliance for monitoring short cracks, alongside an extended back-face strain compliance approach for monitoring long crack propagation. It is concluded that the short fatigue crack growth rate of the HAZ is 2.7 to 3.5 times higher in seawater as compared to air. As the crack propagates and enters into the long crack regime, the ratio decreases to 2.2 times at the transition point of the two-stage crack growth curve and further decreases to 1.5 times when the notch advances towards fracture. The findings indicate that the fatigue crack growth rates documented in standards tend to be on the conservative side. This study significantly enriches the fatigue crack growth data available in literature, which will contribute to a more accurate lifetime assessment offshore wind turbine structures.

Experimental and numerical assessment of fracture toughness variations in steam boiler components after a long period of operation

This study presents an experimental and numerical assessment of the fracture toughness of steam boiler components, which have been in operation for more than 100,000 h. The materials tested are components of two different parts of a steam turbine unit: the pipeline section that connects the headers with turbines, which is made of 13HMF steel, and the valve chamber section, which is made of L17HMF steel grades. Full-size arc-shaped tension specimens (AT) are employed for the experimental evaluation. AT and single-edge notch tension (SENT) specimens are used for numerical assessment. The finite element method (FEM) formulations are developed and examined in Abaqus, which is set up as a shell model with the specimen thickness designated as the mid-surface for the analysis. The notch is modeled using a contour integral definition based on the maximum energy release rate criterion. The fracture toughness is quantified using the stress intensity factor (KI), and the results are compared. The valve chamber is found to have a higher fracture toughness than the pipeline section. The effect of specimen geometry is also noticeable, and the fracture toughness of SENT and AT specimens are compared. The results are useful in assessing the component's durability after long operation.

Comparison of Fracture Behavior in Single-Edge Notched Beams Reinforced with Steel Bars or CFRP Bars.

To explore and compare the failure modes, deformation behaviors, and load-bearing capacities of single-edge notched (SEN) beams strengthened with carbon fiber-reinforced polymer (CFRP) and steel bars, static and dynamic three-point bending tests on both types of concrete beams have been carried out in this study. During the static tests, the electro-hydraulic servo machine served as a loading device to apply pressure to CFRP beams and reinforced concrete (RC) beams. During the impact experiments, different impact velocities were imparted by adjusting the drop hammer's height. Thus, information regarding crack propagation, energy absorption, and deformation was obtained. The results from the static tests showed that the RC beams predominantly experienced shear failure. In contrast, the CFRP beams primarily exhibited bending-shear failure, attributed to the relatively weaker bond strength between the bars and the concrete. Impact tests were conducted at three different velocities in this study. As the impact velocity increased, both types of concrete beams transitioned from bending failure to bending-shear failure. At the lowest velocity, the difference in energy absorption between beams reinforced with different materials was insignificant during the bending process. However, at the highest velocity, CFRP beams absorbed less energy than RC beams. The study of structures' impact failure modes and their mechanical characteristics offers valuable references for the anti-collision design and protection of structures.

Crack development and fracture performance of Recycled Powder Engineered Cementitious Composites (ECC): Experimental investigation

In this paper, recycled concrete powder (RCP) and recycled brick powder (RBP), with particle sizes ranging from 10 to 130 μm, are used to completely replace the fine aggregate quartz sand at a ratio of 1:1. On this basis, recycled glass powder (RGP), with a discussed particle sizes range of 38–300 μm, is used to partially substitute the cementitious materials. Polyethylene (PE) fibers are used as the reinforcement in the Engineered Cementitious Composites (ECC). Using the particle size range and substitution rate of RGP as variables, the crack development, crack mouth opening displacement (CMOD), fracture toughness and fracture energy of Recycled glass powder ECC (RGP-ECC) were discussed through a three-point bending experiment on a single-edge notch beam. A relatively comprehensive evaluation was made on the fracture performance and behavior of recycled ECC, and a suitable mix ratio of recycled powder ECC was given. The results show that the addition of various recycled powders will not affect the multi-slit cracking behavior of ECC, and the cracks can still spread densely, giving the specimen greater deformation capacity and CMOD. The total substitution of fine aggregate by RCP and RBP has a positive impact on the fracture toughness KICini, KICun and fracture energy (GF), but the addition of RGP has some minor negative effects. Among the particle size range and substitution rate of RGP, the influence of substitution rate is more obvious, and the fracture performance will show a downward trend after growth as the substitution rate increases. Choosing the appropriate RGP substitution rate and particle size range can maximize the recycling of construction waste while ensuring fracture performance.

On the suitability of single-edge notch tension (SENT) testing for assessing hydrogen-assisted cracking susceptibility

Combined experiments and computational modelling are used to increase understanding of the suitability of the Single-Edge Notch Tension (SENT) test for assessing hydrogen embrittlement susceptibility. The SENT tests were designed to provide the mode I threshold stress intensity factor (Kth) for hydrogen-assisted cracking of a C110 steel in two corrosive environments. These were accompanied by hydrogen permeation experiments to relate the environments to the absorbed hydrogen concentrations. A coupled phase-field-based deformation–diffusion-fracture model is then employed to simulate the SENT tests, predicting Kth in good agreement with the experimental results and providing insights into the hydrogen absorption–diffusion–cracking interactions. The suitability of SENT testing and its optimal characteristics (e.g., test duration) are discussed in terms of the various simultaneous active time-dependent phenomena, triaxiality dependencies, and regimes of hydrogen embrittlement susceptibility.

High-throughput experimental method for measuring fatigue crack growth rate curve of soft materials

The fatigue properties of soft materials, including fatigue crack initiation and propagation, fatigue life and threshold, have been a significant concern in the past few decades. Measuring the fatigue crack growth rate (FCGR) curve is a standard method to characterize fatigue fracture and has been extensively studied in the literature. Conventional methods employing a pure shear configuration measure only one data point on the FCGR curve per test run. Thus, obtaining the whole FCGR curve with numerous data points becomes excessively time-consuming. This work proposes a high-throughput experimental method for measuring the FCGR curve of soft materials. We prepare ten thermoplastic polyurethane (TPU) specimens in single edge notch tensile configuration with different precut crack lengths, use a mechanical mechanism to apply the same cyclic loads to the ten specimens simultaneously, and identify the crack growth of each specimen by image processing. The proposed high-throughput experimental method enables the measurement of over 90 data points on the FCGR curve in a single test run, facilitating the acquisition of the entire curve with just two runs. The obtained FCGR curve exhibits a linear relationship within its middle region on double logarithmic coordinates. The proposed method is further applied to study the effect of temperature on the FCGR curve of TPU specimens. The current experimental setup can be further optimized to accommodate a wider range of soft materials with diverse mechanical properties and specific loading conditions.

On the validity of the Tada stress intensity factor solution for the single edge notch tension specimen with pinned ends

The validity of the Tada stress intensity factor (KTada) for pinned-ends single edge notch tension [SEN(T)] specimens is assessed via a combined experimental-modeling approach. Analysis of fatigue crack growth rate reductions during constant-ΔKTada loading demonstrates that specific combinations of alloy stiffness, geometry, and loading result in the true K deviating below KTada. Geometrically non-linear, 3-dimensional finite element calculations confirm mild-to-strong influences of these parameters, which are not captured by KTada. Most existing pinned SEN(T) data are found to use parameters where KTada is not significantly reduced, but the present results underscore the need for a more broadly applicable K solution.

A model for modifying the S‐N curve considering the effect of boundary conditions on the fatigue crack growth of welded components

AbstractThe present study proposes a novel model to modify master S‐N curves of components according to their load redistribution capability reflected in different boundary conditions (BCs) based on the fracture mechanics analysis. To that end, a comprehensive numerical study was conducted on a Single Edge Notch Bend (SENB) specimen constrained with different kinematic BCs using discrete fatigue crack growth (FCG) simulation. It was observed that BCs indeed can have a significant effect on the crack growth behavior and consequently on the resulting fatigue life under the same nominal loading conditions. The proposed model was applied to the S‐N curve of a T‐welded joint, and the predicted fatigue life was validated against 3D FCG simulations. Finally, FCG tests were conducted on SENB specimens to experimentally corroborate the effect of BCs on the FCG rate.

Experimental study on crack propagation characteristics of unconventional reservoir rocks

This study investigates the fracture behaviour of three types of specimens (shale, sandstone, and coal), which are commonly found in unconventional reservoirs, based on mode Ⅰ single-edge notch beam (SENB) test. We employ the Digital Image Correlation technique to track the strain evolution and crack propagation paths of the three types of materials and compare their fracture characteristics. The experimental results show that both the surface roughness and crack initiation angle of the coal are larger comparing with that of sandstone and shale, which demonstrates the ductile property of coal and fracturing difficulty in coal seam. Moreover, the average velocity difference of crack propagation is obvious, which is 53.76 × 103, 453, and 0.33 mm/s in shale, sandstone, and coal, respectively. The elastoplastic J-R curves, which reflects the influence of the plastic zone, are presented to analyze the energy dissipation characteristics of three types of specimens during crack propagation. It is found that the J-integral of shale and sandstone does not increase with crack propagation, but the J-integral of coal shows obvious increasing trend with crack propagation due to the effect of plasticity. In addition, microscopic analysis is conducted using a petrographic microscope and scanning electron microscope (SEM) to explain the plastic zone differences at the notch tips of shale, sandstone, and coal. This study is a preliminary and significant step of hydraulic fracturing application in unconventional reservoirs for efficient recovery of the berried natural gas.

Plastic Zone Radius Criteria for Crack Propagation Angle Evaluated with Experimentally Obtained Displacement Fields

The monitoring and maintenance of cracked structures are generally carried out using structural integrity assessments. The plastic zone (PZ) crack path (CP) criteria state that a crack grows in a specific direction when the radius of the plastic zone ahead of the crack tip reaches a minimum value. The PZ can be evaluated using stress intensity factors (SIFs). The SIFs under mixed-mode loading were extracted from the literature from three samples: two single edge notch tension (SENT) samples (E = 2.5 GPa, v = 0.38) made from polycarbonate and one modified compact test (C(T)) sample made from low-carbon steel (E = 200 GPa, v = 0.3). In addition, the CP angle was evaluated for the W and R criteria with experimental data, which included non-linear effects such as fatigue-induced plasticity, crack roughness, and debris. It was found that both can predict the CP for lateral cracks in both tested materials and monotonic and cyclic load when the mode mixity does not change considerably from one crack length to the next or goes beyond 0.2. Moreover, the R criterion exhibited an error as high as 1.7%, whereas the W criterion showed a 6% error on the last crack length for the low-carbon steel sample under cyclic load, which had a 100% increase in mode mixity. Finally, the applicability of LEFM was checked, while the CP was sought by finding the size of the PZ.

Characterization of fracture behavior under impact-like loading conditions of elastomeric grades for high-pressure gas applications

Rapid gas decompression (RGD) is a major concern for elastomeric sealing components in cyclic gas exposure conditions as it generates cracks and can even cause total mechanical damage threatening the sealing integrity. Therefore, to analyze the fracture behavior of elastomeric grades under impact-like loading conditions, a test methodology was developed using a single edge notch in tension (SENT) specimen. To gain more insights into the behavior of differently formulated four nitrile butadiene rubber (NBR) grades (carbon black (CB) or Silica-filled, with different curing characteristics), tests were conducted at a wide range of temperatures, −20, 23, and 80 °C and under a wide range of loading rates (0.2–5 m/s). The single fracture parameters, strain energy release rate (G) at stable crack initiation and G at maximum load, as well as multiple fracture parameters from crack resistance curves (R-curves), G–Δa and crack growth rate vs crack extension curves, da/dt–Δa, were derived. All the grades showed distinct temperature and loading rate sensitivity on fracture behavior, however, the effect was more pronounced in sulfur-cured grades. Higher cross-link density, delivered a lower resistance to fracture and a uniform higher crack growth rate, conversely higher filler loading improved the fracture resistance. Within the tested conditions, the component-level instrumented tests on RGD failure revealed a similar ranking to the tested grades and it emphasizes the importance and relevance of intrinsic fracture properties. The generated information on single fracture parameters and crack growth kinetics is vital in phenomenological modeling attempts of the actual failure of components as well.

Investigating the constraint role in J-CTOD relationship for compact tension (CT) specimen

The Crack Tip Opening Displacement (CTOD or δ) estimation from J-integral (J) defined by ASTM 1820 considers the CTOD dependency on material properties, and the constraint factor (m). The m in Compact Tension (CT) specimen is based on yield to tensile strength ratio (σys/σut) without taking into account of in-plane dimensions as in Single Edge Notch Bending (SENB). Hence, an attempt is made to understand the effect of crack length to specimen width (a/W), specimen thickness to specimen width (B/W) for different σys/σut on CTOD using CT specimen. A new method of estimating the CTOD from FE analysis is demonstrated and validated with 450-intercept method. It has been found that the ASTM 1820 based CTOD (δASTM) assessed values under-estimate the actual CTOD present in the CT specimen. The variation in a/W and B/W doesn’t affect the J-CTOD relationship as stated by ASTM 1820. However, the CTOD measured by FE analysis (δFE) are consistently higher than the δASTM. Therefore, an effort is made to correct the constraint factor, m based on present FE analysis by considering the effect of σys/σut. The proposed corrected constraint factor, mFE, can be employed in fracture applications that generally need both the J-integral and the CTOD.