Toughness Transfer Research Articles

Investigating the transfer of toughness from rubber modified bulk epoxy polymers to syntactic foams

Syntactic foams are lightweight, high specific strength materials used in the aerospace and naval industries. Their utility is limited by their brittleness. The epoxy polymer matrix in an epoxy/hollow glass microsphere (GMS) syntactic foam was modified using carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber with the aim to increase fracture toughness. The microstructure and fracture properties were investigated, and compared to CTBN modified bulk epoxy polymers. The formation of complex CTBN microstructures was responsible for the increase in fracture energy, from 193 J/m2 for the unmodified syntactic foam, to 296 J/m2 at 12 wt% CTBN modification. However, this increase is much smaller than for the CTBN modification of bulk epoxy polymers, where an increase from 101 J/m2 to 1112 J/m2 was measured for the same CTBN concentration. There is little toughness transfer from the bulk epoxy polymers to the syntactic foams, attributable to small interstitial regions between the GMS, restricting plastic zone size. A statistical approach to the analytical modelling of fracture energy in the bulk epoxy polymers highlights the importance of considering the underlying distribution of rubber particle and void sizes. The increase in fracture energy achieved in this work can increase the overall usefulness of syntactic foams in structural applications.

Effect of polymer nanoparticle morphology on fracture toughness enhancement of carbon fiber reinforced epoxy composites

Poor interlaminar fracture toughness has been a major issue in carbon fiber reinforced thermoset polymer composites . A number of techniques have been studied to improve this property, and resin matrix modification is the most direct and effective one. This study focuses on nanoparticle (NP) toughening of epoxy resins and how well the matrix toughness improvement transfers to its carbon fiber reinforced composites . The NPs used contain the same percentage of soft and hard polymer composition, but have different particle morphology , i.e. homogenous versus core/shell structures. Results show that the core/shell particles, especially when the soft polymer is the core and the rigid polymer is the shell, give 851%, 185%, and 43% enhancement in G IC (epoxy), G IC (composite) and G IIC (composite), respectively, compared to the control specimen. Homogeneous particles are much less effective. Fracture surface morphology analysis suggests that effective toughening of composite requires the combination of well dispersed and strong particle-resin and resin matrix-fiber interfaces. This study provides insightful guidelines in selecting high-efficiency toughening particles, as well as comprehensive data on toughness transfer from neat resin to its corresponding composites.

Effect of rapid manufacturing on the performance of carbon fibre epoxy polymers

The effect of cure rate on the mechanical and fracture properties of both bulk epoxy polymers and carbon fibre-reinforced polymers (CFRP) containing reaction-induced phase-separating liquid rubber was investigated in the current work. The fracture energy of the bulk epoxy polymers increased from 229 to a maximum value of 3434 J/m2. This was achieved by introducing 9 wt% phase separating rubber while keeping a curing isothermal temperature of 45 °C. A reduction in the maximum measured toughness was noted when these polymers were cured at higher isothermal temperatures and hence higher rates. The mode I propagation fracture energy for composite laminates cured using the modified bulk polymers as matrices increased from 633 to only 934 J/m2. The low rise in toughness for the composite laminates is explained due to the inefficient transfer of fracture toughness from bulk polymers to fibre-reinforced polymers for toughened resins. The measured properties of the bulk polymer together with fractography studies demonstrate that this occurs due to the geometrical obstruction of the fibres preventing an unbounded fracture process zone from forming ahead of the crack tip. Increasing the weight fraction of toughening nano-modifiers in CFRP beyond a small amount does not result in further toughness gains. The results of this work underline the importance of considering material processing history when designing composite structures.

On the extent of fracture toughness transfer from 1D/2D nanomodified epoxy matrices to glass fibre composites

In this study, the effects of adding nanofillers to an epoxy resin (EP) used as a matrix in glass fibre-reinforced plastic (GFRP) composites have been investigated. Both 1D and 2D nanofillers were used, specifically (1) carbon nanotubes (CNTs), (2) few-layer graphene nanoplatelets (GNPs), as well as hybrid combinations of (3) CNTs and boron nitride nanosheets, and (4) GNPs and boron nitride nanotubes (BNNTs). Tensile tests have shown improvements in the transverse stiffness normal to the fibre direction of up to about 25% for the GFRPs using the ‘EP + CNT’ and the ‘EP + BNNT + GNP’ matrices, compared to the composites with the unmodified epoxy (‘EP’). Mode I and mode II fracture toughness tests were conducted using double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively. In the quasi-static mode I tests, the values of the initiation interlaminar fracture toughness, G_{text{IC}}^{text{C}} , of the GFRP composites showed that the transfer of matrix toughness to the corresponding GFRP composite is greatest for the GFRP composite with the GNPs in the matrix. Here, a coefficient of toughness transfer (CTT), defined as the ratio of mode I initiation interlaminar toughness for the composite to the bulk polymer matrix toughness, of 0.68 was recorded. The highest absolute values of the mode I interlaminar fracture toughness at crack initiation were achieved for the GFRP composites with the epoxy matrix modified with the hybrid combinations of nanofillers. The highest value of the CTT during steady-state crack propagation was ~ 2 for all the different types of GFRPs. Fractographic analysis of the composite surfaces from the DCB and ENF specimens showed that failure was by a combination of cohesive (through the matrix) and interfacial (along the fibre/matrix interface) modes, depending on the type of nanofillers used.

Investigation of morphologies and tensile impact toughness of immiscible polyphenylene sulfide/polyether sulfone films and carbon fiber composites by quantitative optical methods

Targeting the transfer and improvement of impact toughness in composites deriving from polymer mixtures, we investigated the effectiveness of immiscible polyphenylene sulfide (PPS)/polyether sulfone (PES) blends reinforced by endless carbon fibers. Tensile impact testing of films and composites was performed. Impact strengths of films showed high dependency of the PPS crystallinity and crystal orientation influenced by the present amount of PES. Intermolecular interactions between the immiscible phases and the presence of amorphous areas were assumed to be the driving factor for the load transfer in these films. Optical evaluation of local and global strain at break of films revealed a significant increase in plasticization. Tensile impact tested ±45° carbon composites exhibited high improvements of toughness and deformation ability provided by the PES spheres. The quantification of morphologies evidenced a strong correlation of PES profile's mean diameters and next neighbor distances with impact toughness and deformation ability, qualifying these novel materials as potential alternatives for conventional high‐end composites. POLYM. COMPOS., 40:3725–3736, 2019. © 2019 Society of Plastics Engineers

Matrix toughness transfer and fibre bridging laws in acrylic resin based CF composites

The interlaminar fracture toughness, GIc, of unidirectional carbon fibre composite laminates produced by infusion moulding with two different thermoplastic acrylic matrices, one plain and one rubber toughened, was studied. For the composite with the plain matrix fracture was dominated by failure at the fibre-matrix interface. With the toughened matrix, fracture occurred within the matrix-rich layer, although the matrix toughness could not be fully exploited due to the interaction of the fibres with the development of the process zone. A fibre bridging model based on experimentally derived cohesive laws was developed and it confirmed that fracture occurs within the matrix.

Fracture initiation and propagation in unidirectional CF composites based on thermoplastic acrylic resins

The fracture behaviour of continuous unidirectional carbon fibre composite materials prepared adopting two, one plain and one rubber toughened, thermoplastic acrylic resins as matrices was investigated as a function of temperature and displacement rate. The contributions to fracture toughness of composites given by the matrix and the fibre related mechanisms was analysed by comparing results obtained at crack initiation and during crack propagation stages. It was verified that the transfer of matrix toughness into the composite is only partial when the matrix process zone size is comparable to the interlaminar matrix layer thickness. The effectiveness of fibre bridging mechanism was found to be related to the interfacial strength between the matrix and the fibre.

Nanoscale toughening of carbon fiber reinforced/epoxy polymer composites (CFRPs) using a triblock copolymer

This work explored the incorporation of a triblock copolymer in carbon fiber reinforced epoxy polymer composites (CFRPs) to improve their mode-I fracture toughness, GIc (J/m2). The triblock copolymer poly (styrene)-block-poly (butadiene)-block-poly (methylmethacrylate) (SBM) was used to modify the CFRP matrix at 5, 10 and 15 phr concentrations, respectively. CFRPs were manufactured using an in-house sizing tower system, prepregger, vacuum bag and autoclave method. Mode-I fracture toughness testing revealed a 290% increase in GIc by incorporation of the reactive sizing on the fibers and 10 phr SBM in the matrix. Scanning electron microscopy of the SBM modified CFRP fracture surfaces showed that well distributed, sub 100 nm spherical micelles of SBM underwent cavitation and induced void growth and shear yielding toughening mechanisms to absorb fracture energy. It is noteworthy that longitudinal and transverse composite three point flexural testing showed that the SBM modified matrix did not decrease CFRP strength and stiffness up to 10 phr additive. Further, dynamic mechanical analysis revealed that SBM at 10 phr decreased the glass transition temperature (Tg) of CFRPs by only 2.9 °C; the Tg was then recovered at 15 phr SBM. Finally, the SBM modified CFRP GIc was compared to the neat matrix GIc at 0, 5, 10 and 15 phr SBM to develop a ‘transfer factor’ for SBM modified composites. It was found that only 10% of the increased matrix toughness was transferred from the SBM modified epoxy to the CFRPs. The presence of the rigid carbon fibers constrains plastic deformation of the modified epoxy and limits the toughness transfer in the composite.

Toughened carbon fibre-reinforced polymer composites with nanoparticle-modified epoxy matrices

In the current work, the microstructure and fracture performance of carbon fibre-reinforced polymer (CFRP) composites based upon matrices of an anhydride-cured epoxy resin (formulated with a reactive diluent), and containing silica nanoparticles and/or polysiloxane core–shell rubber (CSR) nanoparticles, were investigated. Double cantilever beam tests were performed in order to determine the interlaminar fracture energy of the CFRP composites, while the single-edge-notched bend specimen was employed to evaluate the fracture energy of the bulk polymers. The fracture energy of the bulk epoxy polymers increased from 173 J/m2 for the unmodified polymer to a maximum of 1237 J/m2 with the addition of 16 wt% of CSR nanoparticles. The toughening mechanisms were identified as (a) localised plastic shear yielding and (b) cavitation of the CSR particles followed by plastic void growth of the matrix. The steady-state propagation value of the interlaminar fracture energy of the CFRP composites increased with increasing nanoparticle concentration, from 1246 J/m2 for the unmodified epoxy matrix to a maximum of 1851 J/m2 with 4 wt% of silica nanoparticles and 8 wt% of CSR nanoparticles. Crack growth in the CFRP composites was dominated by fibre-bridging toughening mechanisms. The efficiency of the transfer of toughness from the bulk polymers to the carbon fibre composites was considered. The measured fracture energy of both bulk and composite materials decreased at a test temperature of −80 °C, compared with room temperature, i.e. 20 °C. Nevertheless, the toughening effects of both the silica and CSR nanoparticles on the bulk epoxy polymers and the CFRP composites, compared with the unmodified epoxy polymers, were still evident even at the lower temperature. Indeed, the toughening effect of the silica nanoparticles was greater at −80 °C than at room temperature.

Comprehensive Study on Using VTBN Reactive Oligomer for Rubber Toughening of Epoxy Resin and Composite

ABSTRACTWhile vinyl-terminated butadiene acrylonitrile is frequently used for the toughening of vinylester and polyester, very limited research has been conducted on modification of epoxy with this oligomer. Herein, the effect of vinyl-terminated butadiene acrylonitrile addition to epoxy in bulk and glass reinforced composite is systematically investigated. Thermo-physical behavior and mechanical characteristics of the samples are determined. To interpret the test results, the void content of reinforced samples is measured and fracture surface of the specimens is investigated. It is found that vinyl-terminated butadiene acrylonitrile improves the toughness with slight negligible effects on other characteristics. Incorporation of 15 phr of vinyl-terminated butadiene acrylonitrile increases the KIC of epoxy from 0.6 MPa to 2.3 MPam0.5. Similarly, addition of 15 phr vinyl-terminated butadiene acrylonitrile leads to 67% enhancement in the interlaminar fracture toughness of composites. The toughening mechanisms and toughness transfer from bulk to composite are discussed.

Toughening performance of glass fibre composites with core–shell rubber and silica nanoparticle modified matrices

The fracture energies of glass fibre composites with an anhydride-cured epoxy matrix modified using core–shell rubber (CSR) particles and silica nanoparticles were investigated. The quasi-isotropic laminates with a central 0°/0° ply interface were produced using resin infusion. Mode I fracture tests were performed, and scanning electron microscopy of the fracture surfaces was used to identify the toughening mechanisms.The composite toughness at initiation increased approximately linearly with increasing particle concentration, from 328J/m2 for the control to 842J/m2 with 15wt% of CSR particles. All of the CSR particles cavitated, giving increased toughness by plastic void growth and shear yielding. However, the toughness of the silica-modified epoxies is lower as the literature shows that only 14% of the silica nanoparticles undergo debonding and void growth. The size of CSR particles had no influence on the composite toughness. The propagation toughness was dominated by the fibre toughening mechanisms, but the composites achieved full toughness transfer from the bulk.

Stress Triaxiality as Fracture Toughness Transferability Parameter for Notched Specimens

The problem of fracture toughness transferability is treated by using the stress triaxiality and by introduction of a new transferability parameter called p. This parameter is a combination of effective critical stress triaxiality (mean value of stress triaxiality over effective distance) and multiplies by a geometrical function. Application of this method has been made on three point bending specimens made in XC 38 steel. Comparison of 2D and 3D approaches is made.

The master failure curve of pipe steels and crack paths in connection with hydrogen embrittlement

This paper provides some critical review of the history and state of two elastic fracture mechanics (K and T) and relationship to crack paths. A particular attention is given in the case of hydrogen embrittlement. A fracture toughness transferability curve (Kρc–Tef) has been established for the X52 pipe steels described by a linear relationship where Tef is the average value of T stress over the characteristic length of the fracture process. A mechanism involving influence of the Tef,c-stress on void growth for ductile failure is proposed; the effects of hydrogen on crack paths from the viewpoint of microstructural aspects are disputed.

A review of fracture toughness transferability with constraint and stress gradient

ABSTRACTIn this review paper, only constraint and stress gradient approaches to transferability of fracture toughness are examined. The different constraint parameters are defined and discussed, and one example is given in each case. Factors that influence the constraint are studied. Special attention is given to the actual trends to use the plastic constraint in the material failure master curve and the material transition temperature master curve. The paper also deals on the influence of T stress on the crack path and out‐of‐plane constraint and on the influence of thickness on fracture toughness. The uses of plasticity with gradient and the relative stress gradient in local fracture approaches are also examined.

The microstructure and fracture performance of styrene–butadiene–methylmethacrylate block copolymer-modified epoxy polymers

The microstructure and fracture performance of an anhydride-cured epoxy polymer modified with two poly(styrene)-b-1,4-poly(butadiene)-b-poly(methyl methacrylate) (SBM) block copolymers were investigated in bulk form, and when used as the matrix material in carbon fibre reinforced composites. The ‘E21’ SBM block copolymer has a higher butadiene content and molecular weight than the ‘E41’. A network of aggregated spherical micelles was observed for the E21 SBM modified epoxy, which became increasingly interconnected as the SBM content was increased. A steady increase in the fracture energy was measured with increasing E21 content, from 96 to 511 J/m2 for 15 wt% of E21. Well-dispersed ‘raspberry’-like SBM particles, with a sphere-on-sphere morphology of a poly(styrene) core covered with poly(butadiene) particles, in an epoxy matrix were obtained for loadings up to 7.5 wt% of E41 SBM. This changed to a partially phase-inverted structure at higher E41 contents, accompanied by a significant jump in the measured fracture energy to 1032 J/m2 at 15 wt% of E41. The glass transition temperatures remained unchanged with the addition of SBM, indicating a complete phase separation. Electron microscopy and cross polarised transmission optical microscopy revealed localised shear band yielding, debonding and void growth as the main toughening mechanisms. Significant improvements in fracture energy were not observed in the fibre composites, indicating poor toughness transfer from the bulk to the composite. The fibre bridging observed for the unmodified epoxy matrix was reduced due to better fibre–matrix adhesion. The size of the crack tip deformation zone in the composites was restricted by the fibres, hence reducing the measured fracture energy compared to the bulk for the toughest matrix materials.

Dynamic Characterization of API 5L X52 Pipeline Steel

The objective of this study is examining the level of degradation caused by the welding process, the influence of defects by third parties and the speed of loading on the integrity of the pipeline. The use of Charpy instrumented pendulium coupled with the the volumetric method analysis allowed us to calculate the dynamic fracture toughness of the API 5L X52 pipeline steel in presence of a real defect characterized by its notch radius but also, to show the need for a second parameter to overcome the problem of fracture toughness transferability.

Gouge assessment for pipes and associated transferability problem

The purpose of this work is to assess a gouge defect in a pipe submitted to internal pressure. To do that a method based on failure assessment diagram and more precisely on a Modified Notch Failure Assessment Diagram (NMFAD) which has been proposed as a mesofracture approach. The safety factor has been determine under conservative conditions i.e. for a X52 pipe steel having a relatively low fracture toughness and a severe gouge defect with high aspect ratio and high constraint. In addition a mesofracture approach of the fracture toughness transferability problem has been proposed. The crack ( K– T) methodology has been modifying to create the ( K ρ – T ef ) two parameters fracture resistance criterion.

Gouge Assessment for Pipes and Associated Transferability Problem

The purpose of this work is to assess a gouge defect in a pipe submitted to internal pressure. To do that a method is used which is based upon a failure assessment diagram and, more precisely, upon a Modified Notch Failure Assessment Diagram (NMFAD) which has been proposed as a mesofracture approach. The safety factor has been determined under conservative conditions; i.e for a X52 pipe steel having a relatively low fracture toughness and a severe gouge defect with high aspect ratio and high constraint. In addition, a mesofracture approach to the fracture toughness transferability problem has been proposed. The crack (K-T) methodology has been modifed to create the ( –Teff) two-parameter fracture resistance criterion.

Weld-metal property optimization from flux ingredients through mixture experiments and mathematical programming approach

This paper presents a new methodology for weld-metal properties optimization from welding flux ingredients. The methodology integrates statistical design of mixture experiment with mathematical programming optimization technique. The mixture experiment is responsible for the modeling of the weld-metal properties as a function of welding flux levels while mathematical programming optimizes the model. Data and confirmed models from the literature were used to perform optimization on the responses. The maximum values possible with the prevailing conditions for acicular ferrite, charpy impact toughness and silicon transfer are 51.2%, 29 J and 0.231% respectively while the minimum oxygen content possible is 249 ppm. The new methodology is able to eliminate the limitations associated with the traditional experimental optimization methodology for flux formulation.

Experimental Study of the Fracture Toughness Transferability to Pressurized Thermal Shock Representative Loading Conditions

Abstract Fracture toughness testing on standard specimens in the ductile to brittle transition regime is well established and was first standardized by ASTM in 1997. However, its applicability to structural components and its potential conservatism, due to the high constraint condition in standard specimens, remain a subject of concern. In this work a study of the transferability of the fracture toughness measurements from laboratory specimens to structural components is performed. The structural component of interest is the Reactor Pressure Vessel subjected to an accidental loading condition called Pressurized Thermal Shock (PTS). As the actual component cannot be reasonably tested, an original experimental set-up is developed to simulate PTS representative loading conditions. A semi-elliptical crack is introduced by fatigue in a disk shape specimen which is biaxial loaded in the ductile to brittle transition regime. The developed disk specimen is called PTS-D specimen. Master Curve tests on 15 PTS-D specimens are performed and compared to standard size specimens having deep and shallow cracks. The reference temperature obtained on PTS-D is equivalent to the one obtained on half-inch compact tension specimens. To support the experimental results, finite element calculations are performed. It turns out that, in the tested range, constraint in PTS-D specimens is high. However, loss of constraint appears earlier than in standard specimens. It is found that the Beremin approach combined with analytical tool allows rationalizing all test results.