Shear Fracture Toughness Research Articles

Suspension mechanical performance and vehicle ride comfort applying a novel jounce bumper based on negative Poisson's ratio structure

Comparing with traditional honeycomb structures, Negative Poisson's Ratio (NPR) structures had better mechanical performances in some certain respects, especially the shear modulus and fracture toughness. However, few publications focused on the cylinder-shape NPR structure, which influence the diversity and possibility of NPR structure applications. In this paper, a cylindrical NPR structure was introduced and applied as a suspension jounce bumper in order to solve the issue that the ideal uniaxial compression load-displacement curve sometimes cannot be realized by traditional Polyurethane (PU) jounce bumper. The load-displacement curve of NPR jounce bumper was proved to be smoother and more ideal than that of traditional jounce bumper. Nevertheless, the influences of NPR jounce bumper on the suspension mechanical performance and vehicle ride comfort were not comprehended yet. In this study, the traditional and NPR jounce bumpers were both assembled into virtual prototypes of Macpherson, double wishbone and multi-link suspensions to conduct single wheel travel virtual tests. The results indicated that NPR jounce bumper can achieve more ideal wheel force vs. jounce height curve without adjusting free travel, which is beneficial to spare precise suspension space. Furthermore, a jounce bumper evaluation method using pulse ride comfort was proposed in this paper. The virtual ride comfort tests of travelling through bump and pothole were conducted using established vehicle virtual prototype. The maximum vertical accelerations and weighted root mean square (RMS) of acceleration of vehicle centroid at most speeds were reduced applying NPR jounce bumper. Thus, the NPR jounce bumper can apparently improve vehicle ride comfort.

Aramid nanofibers for multiscale fiber reinforcement of polymer composites

While aramid fibers have been innovative for ballistic protection because of their high energy absorption, minimal usage has been applied to continuous fiber reinforced polymer (CFRP) composites in structural applications. One of the challenges with aramid fibers results from their processing, which yields smooth and chemically inert surfaces that limit the ability of the fibers to adhere to polymeric matrices. Here, it is shown that aramid nanofibers can adhere to the surface of macroscale aramid reinforcements to improve the strength of the composite interface and reinforce the matrix as well. Aramid nanofibers are formed through the dissolution of aramid fibers followed by isolation and dispersion into an epoxy matrix. When employed in CFRP, aramid nanofibers prove to be effective reinforcement agents through improvement in both matrix properties as well as modifying the interfacial shear strength, which leads to improved interlaminar shear strength and fracture toughness. The interface enhancements are attributed to hydrogen bonding and π-π coordination between the aramid nanofibers and the macro fibers providing improved transfer load from the fiber to the matrix. This work demonstrates that aramid nanofibers may provide the robust mechanical properties that are necessary for structural applications while utilizing a cost-effective and convenient nanoscale building block.

Simultaneous improvement of interfacial strength and toughness between carbon fiber and epoxy by introducing amino functionalized ZrO2 on fiber surface

Amino-functionalized ZrO2 was prepared and incorporated on carbon fiber surface by a simple dip-coating approach to simultaneously strengthen and toughen the interphase of carbon fiber/epoxy composite. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermogravimetry confirmed the successful functionalization of ZrO2 by (3-Aminopropyl) triethoxysilane (APTES-ZrO2). Scanning electron microscopy and atomic force microscope demonstrated that uniform coating with well dispersed particles on fiber surface was achieved at 1.0wt% addition of APTES-ZrO2 particles. Under this condition, the interfacial shear strength (IFSS) and fracture toughness (Gic) revealed respective 41.3% and 257.6% augments, compared with epoxy-only coated fiber composite. The enhanced IFSS is attributed to the improved chemical bonds between fiber and resin. While the increase in Gic can be ascribed to the intermittent distribution of strong/weak bonding zones and the mobility of APTES-ZrO2 particles in the interphase region. This work affords a simple, scalable and cost effective approach to simultaneously increase the interfacial strength and toughness of composites, which has always been sought after for structural materials.

Comparison between the ENF and 4ENF fracture characterization tests to evaluate GIIC of bonded aluminium joints

ABSTRACTAdhesively bonded joints have been increasingly used in structural applications over mechanical joints. Cohesive Zone Modelling (CZM) is the most widespread technique to predict the strength of these joints, and it uses the tensile fracture toughness (GIC) and the shear fracture toughness (GIIC). Different fracture characterization methods are available for shear loadings, among which the End-Notched Flexure (ENF) is undoubtedly the most popular. The 4-Point End-Notched Flexure (4ENF) is also available. This work consists of a detailed comparison between the ENF and 4ENF tests for the experimental estimation of GIIC of bonded aluminium joints. Three adhesives were used: a strong and brittle (Araldite® AV138), a less strong but with intermediate ductility (Araldite® 2015) and a highly ductile (SikaForce®7752). Different data reduction methods were tested, and the comparison included the load-displacement (P-δ) curves, resistance curves (R-curves) and measured GIIC. It was found that the ENF test presents a simpler setup and has a higher availability of reliable data reduction methods, one of these not requiring measuring the crack length (a) during its growth. For the 4ENF test, only one test method proved to be accurate, and the test geometry revealed to be highly affected by friction effects.

Accuracy of cohesive laws with different shape for the shear behaviour prediction of bonded joints

ABSTRACTThe use of adhesive bonding as a joining technique is increasingly being used in many industries because of its convenience and high efficiency. Cohesive Zone Models (CZM) are a powerful tool for the strength prediction of bonded joints, but they require an accurate estimation of the tensile and shear cohesive laws of the adhesive layer. This work evaluated the shear fracture toughness (JIIC) and CZM laws of bonded joints for three adhesives with distinct ductility. The End-Notched Flexure (ENF) test geometry was used. The experimental work consisted of the shear fracture characterization of the bond by the J-integral. Additionally, by this technique, the precise shape of the cohesive law was defined. For the J-integral, digital image correlation was used for the evaluation of the adhesive layer shear displacement at the crack tip during the test, coupled to a Matlab sub-routine for extraction of this parameter automatically. Finite Element Method (FEM) simulations were carried out in Abaqus® to assess the accuracy of triangular, trapezoidal and linear-exponential CZM laws in predicting the experimental behaviour of the ENF tests. As output of this work, fracture data is provided in shear for the selected adhesives, allowing the subsequent strength prediction of bonded joints.

Adhesive thickness influence on the shear fracture toughness measurements of adhesive joints

To increase the confidence in the design of bonded structures, it is important to accurately predict their mechanical strength. In this context, Fracture and Damage Mechanics approaches have significant advantages over Continuum Mechanics. The Fracture Mechanics approach is often applied by an energetic analysis, where the main parameter to predict the evolution of damage and failure is the critical strain energy release rate (GC). This parameter can also be divided into the tensile (GIC) and shear (GIIC) components. Cohesive Zone Models (CZM) are a powerful strength prediction technique within the scope of Fracture Mechanics and take advantage of cohesive laws to induce crack growth in the numerical models. This work aims to estimate GIIC of adhesive joints of composite structures by the End-Notched Flexure (ENF) test, considering different values of adhesive thickness (tA). In this context, different data reduction methods to estimate GIIC are compared. On the other hand, the shear cohesive laws of the adhesive layer are obtained by the Finite Element Method (FEM) and CZM using an inverse technique, enabling to understand the influence of tA on the joints’ shear behaviour. The numerical models accurately captured the observed experimental behaviour of the adhesive. A clear dependency was found regarding both GIIC and the shear cohesive strength (ts0), depending on tA.

Numerical evaluation of the ENF and 4ENF tests for the shear toughness estimation of adhesive joints

Adhesive bonding is widespread in several fields, ranging from construction, packaging, automotive industry, aeronautics and medical applications. As a result, it becomes increasingly necessary to reliably estimate their mechanical behaviour. Modern strength prediction techniques such as Cohesive Zone Modelling (CZM) are based on energetic principles. In the context of CZM, the shear fracture toughness (GIIC) is one of the most influential parameters for the strength prediction. This work aims to make a numerical evaluation of the End Notched Flexure (ENF) and Four-Point End Notched Flexure (4ENF) tests to determine GIIC and to provide shear CZM laws for further application in design. Three adhesives were tested: Araldite® AV138, Araldite® 2015 and SikaForce® 7752. An inverse technique was used to obtain the shear CZM laws of the three adhesives. A parameter sensitivity study was also performed to assess the influence of varying GIIC and the cohesive shear strength (ts0). It was concluded that the GIIC values for each adhesive obtained by the ENF and 4ENF tests are in good agreement. The inverse method enabled obtaining unique shear CZM laws for both ENF and 4ENF tests, which revealed a good agreement between these test configurations.

Mechanical properties and strain monitoring of glass-epoxy composites with graphene-coated fibers

An engineered interphase can improve the mechanical properties of epoxy/glass composites simultaneously inducing a piezoresistive response. To prove this concept, E-glass fibers were coated with graphene oxide (GO) by electrophoretic deposition, while reduced graphene oxide (rGO) coated fibers were obtained by subsequent chemical reduction. The fiber-matrix interfacial shear strength (measured by the single-fiber fragmentation test) increased for both GO and rGO coated fibers. Unidirectional composites with a high content of both uncoated and coated fibers were produced and mechanically tested under various configurations (three-point bending, short beam shear and mode-I fracture toughness, creep). Composites with coated fibers performed similarly or better than composites prepared with uncoated fibers. Finally, composites with rGO coated fibers were tested for their piezoresistive response under both static and dynamic conditions. The electrical resistance changed proportionally to applied strain thus confirming the possibility of using composites with rGO coated fibers as strain sensors in load-bearing components.

Use of Nanoparticles for Enhancing the Interlaminar Properties of Fiber-Reinforced Composites and Adhesively Bonded Joints-A Review.

This review paper aims at reporting some of the notable works carried out concerning the use of nanoparticles (NPs) as a means of improving the resistance of fiber-reinforced polymer composite materials (FRPs) and adhesively bonded joints (ABJs) to delamination initiation and propagation. Applications of various nanoparticles, such as carbon-based, ceramic-based and mineral-based are discussed. The main properties that have been considered for improving the delamination and fatigue resistance of FRPs are the interlaminar shear strength, fracture toughness, and fracture energy. On the other hand, cohesive and interfacial strengths have been the focused parameters in the works that considered enhancement of ABJs. The reported results indicate that inclusion of NPs in polymeric matrices leads to improvement of various material properties, even though some discrepancies in the results have been noted. Notwithstanding, additional research is required to address some of the issues that have not yet been tackled, some of which will be identified throughout this review article.

Assessing local yield stress and fracture toughness of carbon nanotube poly(methyl methacrylate) composite by nanosectioning

A nanosectioning (cutting) method was used to test the local shear yield stress and fracture toughness (specific work of surface formation) of multiwall carbon nanotube (MWCNT) poly(methyl methacrylate) (PMMA) composites, and the effects of MWCNT content on the yield stress and toughness were investigated. The composites were prepared by a solution casting method, with MWCNT content varying from 0.05 to 1.0 wt%. Above 0.1 wt% MWCNT content, the yield stress reduced by the addition of MWCNTs. The fracture toughness of the composite was effectively enhanced by the addition of MWCNTs, ranging from 17 J/m2 for the neat PMMA to 25 J/m2 for the 1.0 wt% composite. The shear yield stresses obtained by nanosectioning were correlated to nanoindentation measurement, and possible contributions from the MWCNTs to the fracture toughness of the composite were analysed.

Experimental and numerical investigation of the punch-through shear test for mode II fracture toughness determination in rock

The punch through shear test is a laboratory test used to determine the shear fracture toughness KIIC in rock. In this paper, the punch-through shear test was simulated using a discrete element particle model, and the dependence of KIIC on loading velocity and confinement ranging from 0.1 to 80MPa was investigated. The numerical results of KIIC vs. confinement match very well with laboratory tests, and the development of complex shear fractures during the test are analyzed in terms of the formation and coalescence of tensile and shear micro-cracking at different confining stresses, and the use of numerical acoustic emission recording.

Mechanical bonding properties and interfacial morphologies of austenitic stainless steel clad plates

The current paper focuses on the correlation between microstructure and mechanical properties in low-carbon steel/austenitic stainless steel clad composite fabricated by hot-roll bonding. For this reason, the morphology and interfacial characteristics, tensile properties shear strength and fracture toughness of the cladded material were evaluated.From the main results, it was found that carbon element diffusion caused the forming of a decarburized ferrite zone (DFZ) of the parent metal and a carburized austenite zone (CAZ) of the clad layer, and between these two area, a thin diffusion layer with rapid element component change are formed in the hot-roll cladding process. Stress-strain curves obtained from tensile testing of parent metal and clad layer can predict the bi-material tensile behavior. The shear test proved that the stainless steel clad plate presents an acceptable shear bond strength at the interface joint. Impact test toughness results confirm that fracture took place only in the parent metal side of cladded specimens; the clad layer was bent but without obvious fracture. Fractography was carried out using scanning electron microscope in the tensile, shear bond test specimens and Charpy impact ones. It reveals the presence of predominantly dimpled fracture. Charpy impact specimens of the interface failed in mixed mode while impact specimens of the base plate failed in ductile mode.

Mixed mode I/II fracture investigation of Perspex based on the averaged strain energy density criterion

In this work, some recent mixed mode I/II fracture toughness results obtained from Perspex (or polymethylmethacrylate (PMMA)) with four simple cracked specimens subjected to the conventional three-point bend loading are reanalysed based on local energy concept. Although all the mentioned samples have been tested under the same and similar mode mixities, different fracture toughness envelopes were obtained for mixed mode I/II fracture of PMMA. The averaged strain energy density (SED) criterion has been applied in the past for different types of notched specimens (including U, V, O and keyhole notches). It is shown that the mixed mode tensile-in plane shear fracture toughness data obtained from the semicircular and triangular crack type specimens are successfully predicted for sharp cracked PMMA samples using the SED criterion.

Shear fracture toughness and cohesive laws of adhesively-bonded joints

Adhesive bonding is a viable technique to reduce weight and complexity in structures. Additionally, this joining technique is also a common repair method for metal and composite structures. However, a generalized lack of confidence in the fatigue and long-term behaviour of bonded joints hinder their wider application. Suitable strength prediction techniques must be available for the application of adhesive bonding, and these can be based on mechanics of materials, conventional fracture mechanics or damage mechanics. These two last methodologies require the knowledge of the fracture toughness (GC) of materials. Being damage mechanics-based, Cohesive Zone Modelling (CZM) analyses coupled with Finite Elements (FE) are under investigation. In this work, CZM laws were estimated in shear for a brittle adhesive (Araldite® AV138) and high-strength aluminium adherends, considering the End-Notched Flexure (ENF) test geometry. The CZM laws were obtained by an inverse methodology based on curve fitting, which made possible the precise estimation of the adhesive joints’ behaviour. It was concluded that a unique set of shear fracture toughness (GIIC) and shear cohesive strength (ts0) exists for each specimen that accurately reproduces the adhesive layer behaviour. With this information, the accurate strength prediction of adhesive joints in shear is made possible by CZM.

Shear cohesive law estimation of adhesive layers by digital image correlation

Modern and competitive structures are sought to be strong, reliable and lightweight, which increased the industrial and research interest in adhesive bonding. With this joining technique, design can be oriented towards lighter structures. The large-scale application of a given joint technique supposes that reliable tools for design and failure prediction are available. Cohesive Zone Models (CZM) are a powerful tool, although the CZM laws of the adhesive bond in tension and shear are required as input in the models. This work evaluated the value of shear fracture toughness (GIIC) and CZM laws of bonded joints. The experimental work consisted on the shear fracture characterization of the bond by a conventional and the J-integral techniques. Additionally, by the J-integral technique, the precise shape of the cohesive law is defined. For the J-integral, a digital image correlation method is used for the evaluation of the adhesive layer shear displacement at the crack tip (δs) during the test, coupled to a Matlab® sub-routine for extraction of this parameter automatically. As output of this work, fracture data is provided in shear for the selected adhesive, allowing the subsequent strength prediction of bonded joints.

Mode II Fracture of GFRP Laminates Bonded Interfaces under 4-ENF Test

This experiment studies the mode II fracture behavior of an adhesively bonded joint composed of GFRP laminates. A new beam model is presented to calculate the mode II ERR for GFRP bonded 4-ENF specimens. In this model, the deformation of 4-ENF specimens caused by the relative deflection angle between the upper and lower layers and by the bending deformation of the upper and lower layers, respectively, is introduced; the effect of the adhesive layer deformation is presented. The closed-form analytical solutions of compliance and energy release rate based on the crack compliance method are obtained. The high accuracy of present analytical solutions are verified by finite element analysis through bonded GFRP 4-ENF specimens and compared to the rigid joint model and the CBT model. The interfacial crack propagation is numerically simulated using shear fracture toughness determined in this experiment, from which the predicted critical load results are in good agreement with the experimental results. The conclusion indicates that the compliance and ERR can accurately be predicted using the new bonded 4-ENF beam model.

Mechanical properties of a carbon fabric-reinforced epoxy composite with carbon nanotubes and a flame retardant

Purpose The purpose of this paper is to present the mechanical and morphological characterization of new multifunctional carbon fibre-reinforced composites (CFRCs) that are able to overcome two of the main drawbacks of aeronautical composite materials: reduced electrical conductivity and poor flame resistance. Multiwall carbon nanotubes and glycidyl POSS (GPOSS) were used to simultaneously enhance electrical conductivity and flame resistance. The effect of these two combined components on the mechanical and morphological properties of the manufactured CFRCs was analysed. Design/methodology/approach This paper describes the mechanical test results obtained for interlaminar shear strength, three-point bending, and tensile and fracture toughness in mode I tests. Carbon fibre-reinforced epoxy resin plates were manufactured in two series with blank resin and CNT+flame retardant GPOSS-enhanced resin. Findings The mechanical properties were decreased by no more than 10 per cent by combined influence of CNTs and GPOSS. Agglomerates of CNTs were observed using scanning electron microscopy. The agglomerates were large enough to be visible to the naked eye as black spots on the delaminated fracture surface. The decrease of the mechanical properties could be caused by these agglomerates or by a changed fibre volume content that was affected by the difficult infusion procedure due to high resin viscosity. Originality/value If we consider the benefit of CNTs as a nanofiller to increase electrical conductivity and the GPOSS as a component to increase the flame resistance of the resin, the decrease of strength seems to be insignificant.

Study of Ligament Length Effect on Mode Mix of a Modified In-Plane Shear Test Specimen

Shear fracture toughness is an important material behavior that needs to be determined and considered in many industrial fields. At the same time, shear testing is one of the complex material testing areas where available methods are few, often need special arrangements, and most of the methods do not strictly satisfy the definition of pure shear. In this study, a modified shear test specimen was proposed to measure the shear fracture toughness by uniaxial loading in a tensile testing machine. High density polyethylene (HDPE) was used as test material for the experiments. The specimen was created in order to suit the most common used tensile test machine. The specimen was then optimized by using finite element analysis (FEA) to find the geometry and the size of the pre-notch to avoid the mixed mode loading and minimize effects of normal stresses. For the specimen in discussion, an upper and lower limit of usable ligament length can be found. A method for determining the fracture toughness was discussed according to the essential work of fracture. Finally, an example of a special application of the proposed specimen was presented where the variation of shear strength of controlled delamination material (CDM) was measured.

Establishment of multi-scale interface in interlayer-toughened CFRP composites by self-assembled PA-MWNTs-EP

A pelote-like hybrid particle was fabricated via self-assembly of PA microparticles and epoxy functionalized carbon nanotubes (MWNTs-EP), and the effects of interlayer films containing self-assembled hybrid particles (PA-MWNTs-EP) on interlaminar shear property and fracture toughness of CFRP composites were studied. The spontaneous self-assembly was ascribed to electrostatic adsorption between MWNTs-EP and PA microparticles. Remarkable improvements in interlaminar shear strength (ILSS) and mode I fracture toughness (GIC) of the CFRP composites were achieved with the incorporation of interlayer films containing PA-MWNTs-EP. The pelote-like microscopic structure of PA-MWNTs-EP was considered to provide extraordinary synergy of one-dimensional MWNTs-EP and three-dimensional PA microparticles through establishment of multi-scale interface based on chemical bonding. A schematic model of stress transfer was proposed to illustrate the effects of multi-scale interface established among PA, MWNTs-EP, epoxy matrix, and carbon fiber in the interlaminar region of interlayer-toughened CFRP composites.

Environmental aging effect on interlaminar properties of graphene nanoplatelets reinforced epoxy/carbon fiber composite laminates

Graphene nanoplatelets are two-dimensional carbon structure materials with single or multilayers graphite plane which possesses attractive characteristics. In this study, the environmental aging effect on interlaminar properties of graphene nanoplatelet containing different proportions (0.25, 0.50, 0.75 wt%) reinforced epoxy/carbon fiber (carbon fiber reinforced plastic) composite laminates including interlaminar shear strength and fracture toughness were investigated. The interlaminar properties of graphene nanoplatelets/carbon fiber reinforced plastic composite laminates were improved over that of neat carbon fiber reinforced plastic composite laminates. Experimental results showed that the composite laminates containing graphene nanoplatelets possesses the appreciable improvement. The mechanisms responsible for the interlaminar enhancement were identified by studying the fracture surfaces using field emission scanning electron microscopy.