Shear Fracture Toughness Research Articles

Comparison of analysis between the classical ESIS method and the ASTM D-7905 standard, for obtaining shear fracture toughness

The present work is part of the characterization of glass fiber composites elaborated with a novel vinylester-urethane resin, with addition of poly for use in reinforcing materials for concrete, as a passive reinforcement or as a reinforcement for structures in use, as these resins have a higher Resistance to chemical agents, basics and vapors along with a lower exotherm facilitating the manufacture of thick laminates.Within Mode II fracture mechanics, until this 2014 there was no standard that would collect a procedure to quantify GIIC. Davies through the European Structural Integrity Society (ESIS) issued a mode II test protocol in 1992 known as ENF (End Notch Flexure) [1]ASTM D7905-14 encompasses a fairly complex procedure in which the basis for the characterization of shear fracture toughness in mode II using the final notch flexure (ENF) test is laid.This paper aims to verify the main differences between this classical procedure and the method of this standard to verify their similarities.For this purpose, a mechanical pre-characterization of the reinforced resin is carried out with high-grade fabric (780 g/cm2) with glass fiber E as well as a thinner carbon fiber composite. This characterization is performed by ENF bending tests, and the procedure of the new ASTM D-7905 standard.   [1] ASTM D5528-01, “Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites”, American Society for Testing and Materials, 2001.

Effects of carbon nanotubes on the interlaminar shear strength and fracture toughness of carbon fiber composite laminates: a review

Effects of carbon nanotubes on the interlaminar shear strength and fracture toughness of carbon fiber composite laminates: a review

A critical review of the mechanical properties of CoCrNi-based medium-entropy alloys

The CoCrFeMnNi alloy is one of the most notable first-generation high-entropy alloys and is also known as a Cantor alloy. This alloy was first proposed in 2004 and shows promising performance at cryogenic temperatures (CTs). Subsequent research has indicated that the equiatomic ternary CoCrNi medium-entropy alloy (MEA), as a subset of the Cantor alloy family, has better mechanical properties than the CoCrFeMnNi alloy. Interestingly, both the strength and ductility of the CoCrNi MEA are higher at CTs than at room temperature. CoCrNi-based alloys have attracted considerable attention in the metallic materials community and it is therefore important to generalize and summarize the latest progress in CoCrNi-based MEA research. The present review initially briefly introduces the discovery of the CoCrNi MEA. Subsequently, its tensile response and deformation mechanisms are summarized. In particular, the effects of parameters, such as critical resolved shear stress, stacking fault energy and short-range ordering, on the deformation behavior are discussed in detail. The methods for strengthening the CoCrNi MEA are then reviewed and divided into two categories, namely, modifying microstructures and adjusting chemical compositions. In addition, the mechanical performance of CoCrNi-based MEAs, including their dynamic shear properties, creep behavior and fracture toughness, is also deliberated. Finally, the development prospects of CoCrNi-based MEAs are proposed.

A model-based investigation of tool-chip friction during precision micro cutting of commercially pure titanium alloy

Understanding interaction between the cutting tool edge radius and the work material is essential to identify the conditions leading to superior surface finish during the micromachining process. The interaction between friction angle and effective rake angle has been investigated based on a slip-line field-based machining model from the literature. Machining forces and cut chip thickness values were obtained from orthogonal cutting tests and employed in the process model. The proposed model also allows for calculating material properties such as shear flow stress and fracture toughness. The proposed model can successfully simulate machining forces during shearing-dominated machining conditions. The results showed the importance of flank and rake face friction in micro-scale machining.

Effects of inter-ply mismatch angle on interlaminar properties and their influence in numerical simulations

In this study, the interlaminar shear strength and mode II fracture toughness of carbon-epoxy laminates with respect to different inter-ply mismatch angles are determined and shown to generally decrease as inter-ply mismatch angle increases. Despite this dependency, most numerical simulations of multi-directional laminates involving delamination adopt interlaminar properties measured from unidirectional laminates only. Using the derived angle-dependent shear strength and fracture toughness, computational simulations of short-beam shear and out-of-plane loading of multi-directional laminate plates were performed and compared with physical experiments. It is shown that delamination tends to form intensively at interfaces with larger mismatch angles where delamination resistance is lower. This characteristic is captured by assigning interlaminar properties corresponding to the inter-ply mismatch angles in the numerical simulation. The practice of applying interlaminar properties obtained from tests on unidirectional samples may lead to erroneous predictions of varying severity for the simulation of multi-directional laminates.

Material Parameters Identification for Modelling of Carbon Rod Structures Delamination

The aim of the present work is to compare the interlaminar shear strength and fracture toughness of glued carbon fiber rods obtained using different experimental approaches and provide the effective way to characterise the interlaminar properties for reliable simulation of the delamination. Five different test methods (tension, single shear test, and double shear test, mode I and mode II delamination tests) were performed. Using the explicit LS-DYNA code the finite element model capable of simulating the damage process of bonded connection was developed. The interlaminar connection and delamination criteria were calibrated using the parameter identification methodology implemented in LS-OPT optimization tool.

On the role of fracture process zone size in specifying fracturing mechanism under dominant mode [formula omitted] loading

The double-edge notched Brazilian disk (DNBD) test is used to investigate the failure mechanism of the polymeric material of polymethyl-methacrylate (PMMA) under dominant shear loading. It has previously been shown that DNBD is a simple and efficient test for measuring shear fracture toughness of rock materials. However, the results in this study show that a tension-based fracturing type prevails in the DNBD samples of PMMA. To predict the type of failure, the previously proposed fracture type (FT) criterion was employed. It is shown that the FT criterion is able to accurately predict the fracturing type, the fracture initiation load and the kink angle of the DNBD specimens. In addition, a comparison made between the results of the DNBD tests of PMMA and rock samples demonstrates the main reason for the different observed fracturing types. Thanks to a large fracture process zone in rocks, DNBD samples of rock materials fail with self-planar extension of cracks due to shear stresses. In contrast, due to small fracture process zone in PMMA, the DNBD samples of PMMA show tension-based failure mechanism with the formations of kinks. As the final discussion, an incremental finite element simulation is used to justify the reason behind the stable tension-based crack growth in the DNBD specimens of PMMA, and to predict the crack propagation trajectory.

Interlaminar shear fracture healing of thermoset CFRP composite using multiphase thermoplastic healing agents

The shear fracture toughness (Mode II) recovery of damaged thermoset carbon fiber reinforced polymer (CFRP) composite by multiphase thermoplastic healing agents is presented here. Bio-mimetic heat-activated intrinsic healing was employed to recover the shear fracture toughness of fractured CFRP specimens. The activation of embedded shape memory polymers (SMP) closes the gap between the delaminated faces. Simultaneously, molten thermoplastic polycaprolactone (PCL) healants repair the micro or macro cracks. End notch flexure (ENF) test specimens were fractured in a quasi-static test to verify the proof-of-concept healing under Mode II shear loading. Rapid melting of PCL healants at ∼80 °C enabled the recovery of the damage site, while subsequent solidification recovered the structural integrity. The healing efficiency was quantified by comparing the shear fracture toughness (GIIc) of a healed specimen to the similar toughness of a virgin specimen. Maximum 88.39±16.41% recovery of virgin toughness was observed during the first heal cycles, and the healing efficiency was always greater than 70% in all subsequent cycles. The fractographic features of the delaminated surfaces were observed and analyzed by scanning electron microscope (SEM) and optical microscope. The fractographic observation elucidated the mechanisms behind the shear toughness recovery in different healing cycles.

Role of dry ozonization of basalt fibers on interfacial properties and fracture toughness of epoxy matrix composites

Abstract The mechanical properties of basalt fiber-reinforced epoxy composites (BFRPs) are significantly dependent on the interfacial adhesion between basalt fibers (BFs) and the epoxy matrix. In this study, we proposed a simple and efficient method for deep and stable penetration of BFs into the epoxy matrix through dry-ozone treatments. To confirm the efficiency of the proposed method, BFRPs were fabricated using two types of composites: untreated BFs and dry-ozonized BFs in varying amounts, and the optimum amount of BFs for all the composites fabricated in this work was 60 wt%. With the addition of this amount of dry-ozonized BFs, the interlaminar shear strength and fracture toughness of the composites were enhanced by 21.2 and 23.2%, respectively, as compared with untreated BFs. The related reinforcing mechanisms were also analyzed, and the enhanced interfacial adhesion was mainly attributed to the mechanical interlocking effect. This approach shows that the dry-ozone treatment of BFs is a simple and efficient method for the preparation of BFRPs with excellent interfacial adhesion, which can be a potential application in the auto parts industry.

Thermal conductivity and mechanical performance of hexagonal boron nitride nanosheets-based epoxy adhesives

Thermosets possess diverse physical and chemical properties and thus they are widely used in various applications such as electronic packaging, construction, and automotive industries. However, their poor thermal conductivity and weak mechanical performance jeopardize their continual spread in modern industry. In this study, boron nitride nanosheets (BNNSs) were employed to promote both mechanical and thermal properties of epoxy nanocomposites. BNNSs and their epoxy nanocomposites were fabricated using in situ solvent ultrasonication and in situ polymerization, respectively. Thermal conductivity was enhanced by 153% increment in epoxy/BNNS nanocomposite at 7 wt% in comparison with neat epoxy. In parallel, Young’s modulus, lap shear strength, fracture toughness (K 1C ) and energy release rate (G 1C ) increased by 69%, 31%, 122% and 118%, respectively at 1 wt% BNNSs. Moreover, fatigue life and strength of lap shear joints were significantly improved upon adding BNNSs. A numerical model of the single lap shear joint was developed to validate the accuracy of the material constants obtained. Epoxy/BNNS nanocomposites exhibited an outstanding mechanical performance as well as high thermal conductivity giving them merits to widen their applications in electronic and automotive industry.

Fracture characterization and modeling of Gyroid filled 3D printed PLA structures

Abstract Polylactic acid (PLA) is a commonly used biodegradable material in medical and increasingly in industrial applications. These materials are often exposed to various flaws and faults due to working and production conditions, and increasing the demand for PLA for various applications requires a full understanding of its fracture behavior. In addition to ABS, PLA is a widely used polymeric material in 3D printing. The gyroid type of filling is advantageous for overcoming the relatively higher brittleness of PLA in comparison with conventional thermoplastic polymers. In this study, the effects of various filling ratios on the fracture toughness of 3D printed PLA samples with gyroid pattern were investigated numerically and experimentally for pure mode I, combined mode I/II, and pure mode II. Two-dimensional finite element modeling was created, and the two-dimensional functions of stress intensity coefficients were extracted in loading mode I, mode I/II, and mode II at varied filling ratios of the gyroid PLA samples. Mixed-mode fracture tests for 3D printed PLA samples with a gyroid pattern at various filling ratios were performed by using a specially developed fracture testing fixture. The results showed that the amount of fracture toughness of the samples under study in tensile mode was much higher than those values in shear mode. Also, as the percentages of the filling ratios in the samples increased, both tensile and shear fracture toughness improved.

Mechanical, Thermal and Thermomechanical Properties of Sponge Iron Slag filled Needle-Punched Nonwoven Jute Epoxy Hybrid Composites

In the present research work, the needle-punched nonwoven jute epoxy composites are fabricated by varying the weight percentages of sponge iron slag in vacuum assisted resin transfer molding technique. This study examines the physical, mechanical and thermomechanical properties (i.e., density, water absorption, tensile, flexural, inter-laminar shear strength, compression, impact, fracture toughness and dynamic mechanical analysis) of unfilled and 8 wt.%, 16 wt.% and 24 wt.% of sponge iron slag filled composites under controlled operating conditions. Moreover, it is shown that all the mechanical properties improved with the increased in sponge iron slag content except for tensile strength which increased only upto 16 wt.%. This study also attempted for the analysis of thermal conductivity of the unfilled and particulate filled composites in experimentally by hot disc method. This work also presents both existing empirical models and numerical simulation analysis to evaluate the thermal conductivity of the developed composites. The numerical simulation values of the thermal conductivity demonstrated good agreement with the experimental values. At the end, Cole-Cole plot is drawn between loss modulus and storage modulus to understand the nature of the proposed composites.

Evaluation of bias voltage-dependent mechanical properties of amorphous TiSi2 thin films on PEEK by nano-characterization techniques

Thin films on PEEK have been designing by magnetron sputtering, using bias voltages ranging from −31 to −157 V. The X-ray diffraction and EDX show how the amorphous films resulting have an elemental composition very close to the stoichiometry TiSi2. The AFM and SEM performed on top and cross-section, respectively, of film reveal a smooth and uniform surface, free of pores and cracks, and a compact microstructure. The evaluation of the resolved shear stress, yield strength, hardness, scratch resistance, and fracture toughness show how these values increase in the TiSi2/PEEK system as the bias voltage increase. The development of these hard and tough thin films has enabled the fracture toughness achieved by the TiSi2/PEEK system increase when a bias voltage equal to or higher than −108 V is used during the deposition process. For these bias conditions, the compressive residual stresses generated are large enough to prevent crack nucleation. The increase of the crack resistance gives as a result that KI reaches values above 32 MPa*m1/2. This value is much greater than those values corresponding to the classic ceramic coatings, such as Al2O3 (4.6 MPa*m1/2) and ZrO2 (7.6 MPa*m1/2).

Stack fabric felting to get PCM GIIc improvement and LVI tolerance

In this work, there was developed a three-axis CNC felting machine with one barbed needle. Felting is a kind of needle-punch technology for joining fabric stack by broken and pulled fibres. The density of needle hits is a valuable factor to make the required shear strength of polymeric composite materials in the specified places. Using the glass-fibre reinforced plastic (GFRP) with plain weave felted fabric, a noticeable decrease in strength with an increase in the needle hits density from 0 to 90 cm−2 is shown. For a laminated GFRP made from a stack of fabrics, it has been shown that the shear strength of the single-lap joint and the interlaminar shear fracture toughness GIIC significantly increase with an increase in the needle hits density. This is important for modern aircraft shells exposed to random low-velocity impacts during ground or flight operations.

Dynamic shear fracture toughness and failure characteristics of Ti–6Al–4V alloy under high loading rates

A novel 2-bar/double-shear impact (2B/2SI) loading technique is used to study the dynamic mode II (shear) fracture characteristics of Ti–6Al–4V. The new specimen design, to be used in combination with a standard split Hopkinson pressure bar, circumvent classical limitations associated with conventional one-point impact methods. This paper presents a combined experimental-numerical approach to determining the mode II fracture toughness of Ti–6Al–4V for a broad range of loading rates between 1.10 × 10−2- 4.98 × 107 (MPa, m1/2s−1). Results showed only a slight initial increase in toughness, which increases abruptly with loading rates beyond 106 (MPa, m1/2s−1). Fractographic examination showed distinctively different mechanisms in operation at the microscale, depending on the rate of loading. Failure is through a brittle-ductile, mixed-mode fracture under quasi-static conditions; by contrast, the fracture surface exhibited fractographic features of adiabatic shear bands (ASB) and material melting/re-solidification under dynamic conditions. High-speed photography showed that both dynamic shear fracture (DSF) and ASB occurred during the same loading process. Interactions between DSF and ASB were observed to influence the dominant failure mechanism of the material at high loading rates.

Parameter identification for phase-field modeling of fracture: a Bayesian approach with sampling-free update

Phase-field modeling of fracture has gained popularity within the last decade due to the flexibility of the related computational framework in simulating three-dimensional arbitrarily complicated fracture processes. However, the numerical predictions are greatly affected by the presence of uncertainties in the mechanical properties of the material originating from unresolved heterogeneities and the use of noisy experimental data. The objective of this work is to apply the Bayesian approach to estimate bulk and shear moduli, tensile strength and fracture toughness of the phase-field model, thus improving accuracy of the simulations with the help of experimental data. Conventional approaches for estimating the Bayesian posterior probability density function adopt sampling schemes, which often require a large amount of model estimations to achieve the desired convergence, thus resulting in a high computational cost. In order to alleviate this problem, we employ a more efficient approach called sampling-free linear Bayesian update, which relies on the evaluation of the conditional expectation of parameters given experimental data. We identify the mechanical properties of cement mortar by conditioning on the experimental data of the three-point bending test (observations) in an online and offline manner. In the online approach the parameter values are sequentially updated on the fly as the new experimental information comes in. In contrast, the offline approach is used only when the whole history of experimental data is provided once the experiment is performed. Both versions of estimation are discussed and compared by validating the phase-field fracture model on an unused set of experimental data.

A comparison of surface preparation techniques for wind turbine field repairs

Wind turbine service lifetimes can exceed 20 years. This extreme operational demand necessitates reliable and consistent field repairs; however, the effects of current wind turbine field repair surface preparation techniques are not well-documented and are thus not well-understood. This leads to reliance on technician experience rather than scientific data for repair procedures, which causes variability in the quality of repairs. Solvent wiping is a common procedure for contaminant removal after damaged material is removed from a wind turbine blade, but it was unknown if solvent wiping reduced the surface energy and consequently the likelihood of a durable repair. The goal of this study was to quantify the effects that various surface preparation techniques have on the overall strength and reliability of wind turbine repairs. Type of reagent-grade solvent, fiber direction, matrix, and adhesive were varied for contact angle, lap shear, Double Cantilever Beam (DCB), and End-Notch Flexure (ENF) testing. The type of solvent used did not affect surface energy, maximum lap shear stress, or modes I and II fracture toughness values. However matrix-adhesive combinations significantly affected maximum lap shear stress and fracture toughness values.

Stress Evolution in Punch-Through Shear Tests: A Numerical Study Based on Discrete Element Method

The discrete element method is adopted to investigate the dynamic failure process in punch-through shear tests. The numerical tests agree well with the laboratory tests on crack propagation sequence and the influence of confining stress on shear fracture toughness. More importantly, the numerical tests indicate that the increase in axial force first causes stress concentrations. When the stresses reaches critical values, cracks initiate and the concentrated stresses dissipate. With further increase in axial displacement, the axial force may fluctuate, accompanying with crack development. In addition, the increase in confining stress promotes the critical values of the concentrated stresses before crack propagation. Thus, the numerical tests verifies the previous statements that the increase in confining stress promotes the shear fracture toughness and restrains the propagation of tensile cracks.

Effect of ozone-treated single-walled carbon nanotubes on interfacial properties and fracture toughness of carbon fiber-reinforced epoxy composites

In this study, superior mechanical properties were obtained for composite materials using single-walled carbon nanotubes (SWCNTs) subjected to a simple surface treatment. The effects of ozone-treatment on the surface properties of the SWCNTs/CFs/epoxy multi-scale composites were investigated in terms of interfacial properties and fracture toughness. The resultant surface free energy after ozone-treated SWCNTs (O-SWCNTs) markedly increased the polar component of the contact angle. In addition, the surface morphology image showed that the surface roughness of O-SWCNTs increased, suggesting that the surface roughness and the number of oxygen functional groups were effectively increased by ozone-treatment. Due to the surface modification, the interlaminar shear strength and fracture toughness of multilayered composites increased by 79.98% and 54.59%, respectively. This shows a direct linear relationship between the morphological changes of O-SWCNTs and the specific polar component of the surface free energy of the material as well as its mechanical properties.

Fabrication and mechanical properties of carbon fiber/epoxy nanocomposites containing high loadings of noncovalently functionalized graphene nanoplatelets

Carbon fiber-reinforced polymer (CFRP) composites containing graphene nanoplatelets (GNPs) were prepared and characterized. Bonding via π−π interactions with poly(4-aminostyrene) (PAS) improved the degree of dispersion of the GNPs in an epoxy matrix. Differences in dispersion and reinforcement between pristine GNPs and PAS-GNPs were established by ultraviolet–visible spectroscopy and scanning electron microscopy. The PAS-GNP/epoxy nanocomposites were characterized by their fracture toughness. The results were discussed in terms of the dispersion quality of the GNPs. Furthermore, the PAS-GNP/epoxy nanocomposites were applied to carbon fiber fabric and the interlaminar shear strength (ILSS) and fracture toughness were measured. The greatest improvements in ILSS (252%) and fracture toughness (142%) were obtained with 4 wt% PAS-GNPs. The superior mechanical properties of the CF/PAS-GNP/epoxy nanocomposites are attributed to better filler dispersion and crack bridging.