Fracture Mechanics Investigation Research Articles

Contribution to the Fracture Mechanics Characterization of Repair Welding of Thick-Walled Ductile Cast Iron Components

AbstractFor the spheroidal graphite cast iron materials, also known as ductile cast iron (DCI), EN-GJS-400-18-LT and EN-GJS-450-18, different welding procedures were examined for potential repair welding. A repair weld was performed on thick cast iron plates using the optimum procedure in each case. The weld was evaluated for all areas of the welded joint (weld metal, fusion line/heat-affected zone and base metal). In addition to the metallographic microstructural analysis, the characterization of the repair weld was carried out by means of hardness distribution measurements, static tensile testing and notched bar impact testing as well as fracture mechanics investigations under static and under cyclic loading in a wide load ratio range. On this basis, a generalized description of cyclic crack growth could be made according to the NASGRO® MODEL. NASGRO® is a fracture mechanics and fatigue crack growth software. In accordance with the possible operating conditions of repair-welded, thick-walled components, the static and impact tests were performed in the temperature range down to $$-$$ - 40 $${}^{\circ }\text {C}$$ ∘ C .

Analysis of crack propagation of PLA fabricated by the additive manufacturing technique

Understanding crack propagation is crucial for evaluating the structural integrity and reliability of additive manufacturing components, as cracks can compromise mechanical properties and potentially lead to catastrophic failures. The study of crack propagation in additive manufacturing components is used to develop strategies for mitigating crack initiation and growth, improving material properties, and optimising the design and manufacturing processes. Crack propagation in additive manufacturing components can be influenced by various factors, including material properties, design considerations, manufacturing defects, and loading conditions. Due to the identified issue, the study was carried out to investigate the crack propagation of the Fused Deposition Modeling (FDM) component using compact tension fracture testing. The experimental work started with fabricating the samples using PLA material, followed by a fracture test based on the compact tension specimen test to get a response of the structure and its crack propagation under tensile. Material properties were also collected using the dog bone tensile test. The material properties of the testing were then imported to Finite Element Analysis for further investigation of fracture mechanics. It was found that the maximum force of the sample was 141.7 ± 28 N at 1.70 ± 0.26 mm displacement, and cracks initiated around the tip and propagated upward or downward based on the initial crack location. The deformation patterns of PLA material have shown it to be brittle plastic deformation and low energy absorption before fracture.

Experimental investigation of fracture mechanism in organically modified silica-based coating applied on austenitic stainless steel AISI 904L under monotonic and very high cycle fatigue loading

Surface morphology significantly influences the fatigue life of the metallic materials. Therefore, improving surface performance is important for economic and safety applications. Silica-based coatings are anticipated to protect the metallic materials under synergistic operating conditions, such as fatigue and corrosion. However, pure inorganic silica-based coatings are brittle and fail to protect metallic materials. The properties of the sol–gel coatings can be determined by optimizing synthesis factors, such as synthesis steps, precursors, solvents, and catalysts. Organically modified silica-based sol–gel coatings were synthesized using 3-glycidoxypropyl trimethoxysilane and 3-aminopropyltriethoxysilane as precursors, with ethanol as the solvent. The resulting coating was named EtOH. This coating was applied on the surface of stable austenitic stainless steel AISI 904L with the dip coating method. The fracture behavior of the coating during monotonic loading has been studied by interrupted tensile tests at different load steps and examined closely with scanning electron microscopy. On the other hand, very high cycle fatigue experiments were conducted using an ultrasonic fatigue testing system at load frequency of 20 kHz on coated specimens to investigate the fracture behavior of the coating during cyclic loading. Three different kinds of fracture due to stress relaxation in the coating and at its interface with substrate were observed in monotonic loading. The same kinds of stress relaxation were observed in cyclic loading with the difference being that the range of experienced elongation in cyclic tests was significantly smaller. Therefore, the fracture in cyclic loading were concentrated in the vicinity of macro crack. The application of organically modified coatings could suppress the persistent slip bands and result in increased fatigue life for coated specimens.

Evaluation of fatigue life prediction models and investigation of fracture mechanisms of mild steel

Fatigue is one of the dilemmas of coiled tubing (CT) in oil and gas engineering. This study investigated the fatigue behavior of CT employing mild steel, which is commonly utilized in the CT110. Two fatigue life prediction methods based on cyclic strain-life principles, the Manson–Coffin and Landgraf models, as well as the Smith and Ellyin models based on energy-life principles, were evaluated. The Ellyin model demonstrated superior accuracy in predicting the low-cycle fatigue life of CT110, displaying dispersion within the 1.5 scatter band. This research integrated mechanical property analysis and microscopic deformation assessments for CT110. Fatigue mechanics studies show cyclic softening behavior in the CT110 of mild steel. The results of the microscopic analysis show that the fatigue fracture of CT110 has typical fatigue striations and equiaxial dimples as a plastic fracture. Combined with TEM and EBSD analysis, fatigue deformation increases grain deformation and dislocation density. In addition, the increase in dislocation density results from the combination of cycles and strain amplitude. The results of in situ EBSD further support this view. At 8% strain amplitude, the dislocation density increases to 0.196B when the cycle increases from 0 number to 10 number. Furthermore, the ferrite grain slip becomes accessible, but the 〈111〉 // Z0 direction is still the main texture. These are fundamental reasons for the cyclic softening of the CT110 mild steel, supported by a decrease in stress amplitude.

Investigation of fracture mechanisms in smooth blasting of limestone samples: numerical and experimental approaches

Investigation of fracture mechanisms in smooth blasting of limestone samples: numerical and experimental approaches

Bioinspired soft-hard interfaces fabricated by multi-material additive manufacturing: A fracture mechanics investigation using essential work of fracture

This study employs the Essential Work of Fracture (EWF) concept to evaluate the interfacial fracture toughness of bioinspired interfaces between soft and hard polymer phases. The experimental framework utilizes a model material system with Polylactic Acid (PLA) as the hard phase and Thermoplastic Polyurethane (TPU) as the soft phase. Employing the Fused Filament Fabrication (FFF) technique, bioinspired sutural interfaces are created, characterized by interpenetrating soft and hard protrusions. The modulation of a critical parameter in the FFF process enables the variation of interpenetration length of protrusions at the interface, thereby achieving a spectrum of interfacial strength and toughness. The determination of essential work of fracture from double edge notch tension samples, with varied ligament sizes, allows for the specific EWF measurement. This specific EWF is found to align with the initiation value of plane strain interfacial fracture toughness obtained through the Double Cantilever Beam (DCB) test. Therefore, the proposed approach asserts the elimination of the need for complex interfacial fracture tests, such as DCB. A noteworthy discovery is the establishment of a correlation between specific plastic energy dissipation and protrusion length. This correlation suggests an increased plastic dissipation with longer protrusions within the investigated bioinspired interface. Due to this heightened plastic energy dissipation, the shape of the interfacial nominal stress-displacement curves demonstrates substantial dependence on interface morphology, shifting from a triangular to a trapezoidal shape as protrusion length increases. These findings offer valuable insights into the mechanics of bioinspired interfaces, presenting a more efficient and nuanced approach to characterizing their fracture properties.

Effect of specimen thickness on fatigue crack shape evolution for aluminum alloy

The investigation of fatigue crack propagation and fracture mechanisms in high-thickness metal plates holds pivotal importance in the advancement of a three-dimensional damage tolerance assessment methodology. This research also bears significant implications for ensuring the safe operation of large-scale mechanical equipment. In this study, we examine the influence of specimen thickness on the evolution of fatigue crack shapes in an aluminum alloy. Fatigue crack growth tests were conducted on single-edge notch tension specimens to explore the impact of specimen thickness on the shape of fatigue crack growth. Additionally, we propose a calculation model for crack growth shape based on an energy model in this paper. Both experimental and analytical findings indicate that specimen thickness does indeed exert an influence on fatigue crack shape. Specifically, as the specimen thickness increases, the crack shape transitions from resembling a “fingernail” to a “saddle” shape.

Use of equivalent interface samples during fracture mechanics investigations for the design of adhesively bonded composite reinforcements on steel structures

During an international Joint Industry Project called SBO (Strength Bond Offshore), a design strategy of adhesively bonded composite reinforcements for steel structures was developed and assessed. It relied on the use of fracture mechanics. Yet, an adaptation of the characterization tests was needed to reduce the number of samples while remaining representative of the studied reinforcement. This adaptation was called “equivalent interface samples”.This paper aims at presenting the proposed adaptation of fracture mechanics tests for mode I, mode II and mixed mode, and its use during the design strategy. First, the geometry of the samples will be presented. Then, the obtained results during the fracture mechanics experimental campaign will be introduced. Those investigations relied on the use distributed optical fiber sensor to monitor the crack propagation during the tests. The obtained toughness characterizations allowed us proposing a design strategy that was assessed in regards with experimental tests conducted on large size steel samples reinforced with adhesively bonded composite.

Fracture mechanics investigation for 2D orthotropic materials by using ordinary state-based peridynamics

Fracture behaviors of orthotropic plates are studied by using ordinary state-based peridynamic (OSPD) theory. Based on OSPD, a novel nonlocal formulation of interaction integral is proposed by considering the material orthogonality for the fracture parameter evaluation. By employing peridynamic differential operator, the partial differential terms in the formulation can be transformed into corresponding spatial integral form, which contributes to the calculations of stress intensity factors within the framework of OSPD. It has built up a relationship between classical theory and peridynamic theory. Static and dynamic fracture parameters are carefully evaluated. The crack propagation directions are predicted by prototype microelastic brittle (PMB) criterion in OSPD and maximum circumferential stress (MCS) criterion in classical theory. Several pre-cracked plates with orthotropic material are examined, and results are validated by comparing against the reference solutions. The relationship between fiber orientation and crack inclined angle are also examined. Meanwhile, the crack inclination determined by PMB criterion and MCS criterion are compared and discussed. Accuracy of the peridynamic orthotropic model and proposed nonlocal interaction integral are discussed in detail.

Quantitative investigation of crack propagation and fracture mechanism of fissured granite from the mesoscopic perspective

Natural rock masses comprise numerous fissures, which affect the stability of rock mass engineering. It is crucial for rock engineering to comprehend the crack propagation and fracture mechanisms of fissured rock. In this study, uniaxial compression tests were conducted on fissured granite specimens with various flaw inclinations (0°, 30°, 45°, 60°, and 90°). After specimen failure, the cracks originating from pre-existing fissures were classified (tensile, shear, and tensile-shear cracks). To link the macroscopic failure behavior and mesoscopic fracture mechanism in fissured granite specimens, a quantitative method combining deep learning and scanning electron microscope (QMDL-SEM) was employed for the identification of the mesoscopic fracture mechanism of macroscopic cracks. Identified results suggested that the failure of fissured specimens was attributed to secondary cracks characterized by tensile stress (failure area caused by tensile stress accounting for more than 87%) and far-field cracks dominated by shear stress (failure area caused by shear stress accounting for more than 60%). Additionally, for secondary cracks, as the transition went from tensile crack to tensile-shear crack and then to shear crack in a macroscopic scale, the proportion of failure area caused by shear stress on the failure surface gradually increased. However, tensile stress still dominated the failure area.

Investigation of tensile fracture mechanism of Fe-based coatings deposited on different substrates by arc spraying

The adhesion strength of the coating to the substrate determines the service life and performance of components coated by thermal spraying. FeCrMoCBWNb coatings were deposited via arc spraying process to study the adhesion properties of the coatings on Q235 and 65Mn steel substrates. The adhesion properties and tensile fracture mechanisms of the coatings on different substrates were examined by the tensile bonding strength. The phase intensity and crystallinity of the coatings were studied by X-ray diffractometer (XRD). The microstructure and mechanical properties of the coatings were also characterized. The adhesion properties of the coatings depended on the substrates' mechanical properties. The tensile bonding strengths of the coating on Q235 and on 65Mn were 45.5 ± 5.4 MPa and 40.8 ± 1.4 MPa, respectively. The higher substrate roughness of the Q235 steel enhanced the mechanical interlocking of the coating particles to the substrate. The tensile fracture of the coatings comprised brittle and ductile fracture mechanisms.

Investigation of corrosion behavior, tensile and cyclic fracture mechanism of EH36 steel welded joints

This work systematically revealed the static-dynamic corrosion failure behavior of EH36 joints under air, simulated seawater and cathodic protection in seawater. The result indicated that the seawater environment affected tensile performance slightly but acted a critical role in the fatigue behavior. Specifically, the corrosion pit and cyclic stress would respectively induce high-stress concentration and corrosion current density, significantly decreasing the fatigue strength of joints. The application of cathodic protection on joints in seawater would decrease the tensile behavior, but would continuously provide electrons for the surface and crack tip to prevent corrosion damage, improving fatigue performance of EH36 joints.

Numerical and experimental investigation of impact strength and fracture mechanism of Kevlar and Hemp elastomeric thin biocomposite laminate under high-velocity impact: A comparative study

Thin composite structures reinforced with woven fabrics are widely used in many industrial sectors due to their lightweight, easy access, and high specific performance. Due to safety, the study of fracture mechanisms and energy absorption of thin composite structures is significant. In this study, Kevlar and Hemp elastomeric thin biocomposite laminates with an elastomeric matrix filled with natural lignin filler with a low carbon footprint and with the aim of the sustainability concept of environmental protection have been designed experimentally and numerically for testing under high-velocity impact. The impact resistance of elastomeric composites reinforced with natural fillers is less reported. Good interaction of lignin-filled elastomeric matrix with woven hemp fabric is expected to show impact performance comparable to Kevlar-reinforced composites. For this purpose, Kevlar and hemp thin elastomeric biocomposites are tested under high-velocity impact with almost the same surface density. Also, tensile and dynamic compression tests (Hopkinson test) were performed to check the mechanical properties of the elastomer layer. The proposed laminates' penetration resistance and fracture behavior during an impact according to the Kevlar and hemp fabric constituent material model and the user-defined material model (VUMAT) for the nonlinear behavior of elastomeric materials considering the damage criterion in ABAQUS/Explicit confirmed. The results show, In the thin elastomeric hemp biocomposites, the tensile failure and stress transfer to the surrounding bundles cause more yarns to participate in the load-carrying process. As a result, the low penetration depth and more significant protection margin in fully green hemp composites provide impact performance comparable to Kevlar elastomeric composites.

Experimental investigation of damage evolution and fracture mechanism in rocks with a single flaw under stepwise cyclic compression

Experimental investigation of damage evolution and fracture mechanism in rocks with a single flaw under stepwise cyclic compression

Small-scale fracture mechanical investigations on grain boundary doped ultrafine-grained tungsten

Small-scale fracture mechanical investigations on grain boundary doped ultrafine-grained tungsten

Investigation of Creep Properties and Fracture Mechanism of 430 Ferritic Stainless Steel at Elevated Temperature

Investigation of Creep Properties and Fracture Mechanism of 430 Ferritic Stainless Steel at Elevated Temperature

Increasing the Strain Resistance of Si/SiO2 Interfaces for Flexible Electronics.

Understanding the changes that occur in the micro-mechanical properties of semiconductor materials is of utmost importance for the design of new flexible electronic devices, especially to control the properties of newly designed materials. In this work, we present the design, fabrication, and application of a novel tensile-testing device coupled to FTIR measurements that enables in situ atomic investigations of samples under uniaxial tensile load. The device allows for mechanical studies of rectangular samples with dimensions of 30 mm × 10 mm × 0.5 mm. By recording the alternation in dipole moments, the investigation of fracture mechanisms becomes feasible. Our results show that thermally treated SiO2 on silicon wafers has a higher strain resistance and breaking force than the SiO2 native oxide. The FTIR spectra of the samples during the unloading step indicate that for the native oxide sample, the fracture happened following the propagation of cracks from the surface into the silicon wafer. On the contrary, for the thermally treated samples, the crack growth starts from the deepest region of the oxide and propagates along the interface due to the change in the interface properties and redistribution of the applied stress. Finally, density functional theory calculations of model surfaces were conducted in order to unravel the differences in optic and electronic properties of the interfaces with and without applied stress.

Investigation of Liberation Properties and Mineral Fracture Mechanisms of Iron Ores with Different Mineral Grain Sizes at Different Grinding Degrees

ABSTRACT Understanding the liberation properties of minerals in ores with different grinding degrees is very important for establishing liberation models. However, a detailed study of the effect of grinding degree on the liberation properties of minerals in ores is still highly desirable. In this study, three types of iron ores with different mineral grain sizes were ground to different particle size distributions by locked-cycle grinding and batch grinding. The liberation properties of iron oxide minerals and quartz in each grinding method and the size fraction product were analyzed using Mineral Liberation Analyzer (MLA). Statistical methods were used to quantify the extent of the difference in liberation properties between the two grinding methods. Furthermore, the fracture mechanisms of iron oxide minerals and quartz were investigated. It was found that the liberation degrees of the products were higher in locked-cycle grinding with a lower grinding degree. For fine-grained iron oxide minerals, the iron oxide minerals distribution of 80–100% composition class differed by 22% at −75 + 53 μm size fraction with the two grinding methods, and its corresponding standardized mean differences (SMD) was 20.5, indicating that the difference was statistically significant. The significant change in the percentage of intergranular fracture with different grinding degrees is the reason for the obvious difference in the liberation properties of the products.

Verification of the master curve concept (ASTM E1921) and inhomogeneity analysis of a German RPV weld for various loading rates

Fracture-mechanical investigations were carried out on the weld metal of a German reactor pressure vessel steel 22NiMoCr3-7. The crack tip loading rate and test temperature were varied in the brittle-ductile transition region, while the results were evaluated using the Master Curve methodology (ASTME1921), including inhomogeneity analyses in Appendix X5. The highly inhomogeneous datasets could be uniformly described with a bimodal distribution function with good accuracy compared to the standard procedure. However, this agreement decreases with increasing crack tip loading rate. In contrast, a modified Master Curve with a shape factor of p = 0.03/°C progressively matches the data better with increasing crack tip loading rate. The weld metal has a significantly higher toughness than the base material. SEM analysis suggests that the inhomogeneity is due to the stochastic distribution of sharp microcracks, while the observed inhomogeneity is additionally superimposed by dynamic effects (adiabatic heating and local crack arrest) at increased crack tip loading rates. It is argued that the microstructural inhomogeneity is hereby »neutralized« and could instead be interpreted as a possible artifact of the standard Master Curve under dynamic conditions.

Investigation of Fracture Mechanisms of Self-Reinforced Polypropylene/Polycarbonate Composites Subjected to Fatigue Loading

Self-reinforced polymeric composites have numerous advantages over traditional polymers, including increased mechanical strength, fracture toughness, and other mechanical properties. A special type of such composites can be obtained using non-conventional molding techniques. The investigated self-reinforced composites have layered structure, where layers are created by the same material in numerous molding cycles in the manufacturing process, and have gradient mechanical properties. According to this, it is of high interest to investigate structural behavior of these materials under fatigue loading, including temperature effects and fracture mechanisms occurring during fatigue. The results of the performed studies proved complex fracture mechanisms of self-reinforced polymeric composites manufactured using the applied unconventional injection molding technique due to variability of mechanical properties over the thickness.