mode-II Interlaminar Fracture Toughness Research Articles

Influence of Oxygen/Argon/Nitrogen multi-component plasma modification on interlayer toughening of UHMWPE fiber reinforced composites

The study reveals the influence of glow discharged three gas sources of Oxygen/Argon/Nitrogen plasma modification on interlayer toughening of ultra-high molecular weight polyethylene (UHMWPE) fiber reinforced composite. The failure modes under bending, tensile and interlayer fracture toughness testing were analyzed through acoustic emission (AE) technology. The results showed that Oxygen/Argon/Nitrogen multi-component plasma modified fiber introduced more hydrophilic groups such as hydrogen and hydroxyl groups, and significantly enhanced the binding of fiber and matrix. The GⅠC (Mode-I interlaminar fracture toughness) of multi-component plasma modified composites exhibited increased by 61.67 %, 40.88 % and 18.01 % compared to untreated, Oxygen and Oxygen/Argon plasma, respectively, while GⅡC (Mode-II interlaminar fracture toughness) increased by 38.57 %, 25.57 % and 23.09 %. AE cluster analysis exhibited that multi-component plasma modified composites formed an energy dissipation mechanism dominated by fiber breakage. Additionally, the oxygen, argon, and nitrogen plasma modification have a synergistic effect on interlayer toughening.

An investigation of the impact of functionalized MWCNT reinforcement on interlaminar strength of glass/epoxy composites: a comparative view

ABSTRACT In the present investigation, diverse functionalized states of multi-walled carbon nanotubes (MWCNT) were employed in fabricating glass/epoxy composites, with the MWCNT consistently incorporated at a 0.5 wt.% concentration. These functionalization states included pristine, hydroxyl, amine, and carboxylic conditions, systematically compared against neat glass/epoxy composites. The findings elucidated that all three functionalizations positively influenced the enhancement of interlaminar shear strength, mode-I interlaminar fracture toughness, and mode-II interlaminar fracture toughness properties. Notably, the amine-functionalized MWCNT exhibited superior performance relative to other formulations. The composite reinforced with amine MWCNT demonstrated significant improvements in ILSS, mode-I ILFT, and mode-II ILFT by 41.82%, 37.75%, and 40.93%, respectively. These enhancements are attributed to factors such as the presence of a more functional group, the quantity of individual MWCNT, and the introduction of additional damage mechanisms, including MWCNT pullout and bridging.

Investigation of interlayer toughening of carbon fiber composites using non-woven polyamide veils under different curing pressures

Carbon fiber-reinforced composites have become increasingly favored in the aerospace and automotive sectors for their remarkable performance attributes. Nonetheless, their broader adoption is hindered by the concern of delamination. To address this limitation, our study delved into the influence of incorporating a thermoplastic toughening agent on the interlaminar fracture toughness (ILFT) of woven carbon fiber composites. Our investigation focused on the impact of diverse curing pressures on these composites, analyzing configurations both with and without an integrated non-woven polyamide (PA) veil. The PA veil was meticulously positioned within the central layer of a composite, combining benzoxazine and carbon fiber constituents. Our primary objective entailed optimizing curing conditions to enhance ILFT across both Mode-I and Mode-II fracture modes. Our findings unveiled that elevating curing pressure had a limited effect on ILFT in non-interleaved composites while interleaved samples exhibited substantial ILFT reductions under heightened curing pressure. Raising pressure from 1 atm to 3.5 atm caused approximately a 27% drop in the Mode-I strain energy release rate (GIC) value, and this reduction deepened to 42% at 7 atm. This highlighted the inverse relationship between increased curing pressure and GIC values in interleaved specimens, while non-interleaved counterparts experienced less pronounced changes. A parallel trend manifested in Mode-II ILFT, where escalated curing pressure correlated with diminished Mode-II strain energy release rate (GIIC) values. Interleaved samples suffered a notably more significant decline, attributed to resin content reduction caused by heightened pressure. This, in turn, compromised the bond between the PA veil and the matrix, facilitating the propagation of cracks within the composite. This ultimately culminated in significantly reduced GIC and GIIC values. Our study contributes valuable insights into optimizing the interlaminar fracture toughness of carbon fiber composites, shedding light on the complex interplay of curing conditions and material configurations.

Enhanced interlaminar fracture toughness of unidirectional CFRP laminates with tailored microstructural heterogeneity of toughening layer

This work detailed a novel hierarchical Carbon fiber/CNT/Epoxy composites with tailored microstructural heterogeneity of toughening layer. The heterogeneity in the toughening layer of these laminates was engineered during spray coating process by controlling the length and content of CNT to produce CNT aggregated particles with spatial separation. Each particle contains a number of CNT and is managed to have controlled sizes. The concept of “CNT particle toughening” was firstly proposed and confirmed, opening a new way to fully enable the exploitation of the toughening potential of CNT. The effectiveness and efficiency of this technique are demonstrated by using the extremely low CNT content (0.3 gsm) to achieve remarkable improvements of the Mode-I and Mode-II interlaminar fracture toughness, which are 168 % and 155 %, respectively.

Fracture toughness response of glass/epoxy composite in symmetric and asymmetric design layup

Composites are used in transportation industries for their exceptional strength, reduced weight, and cost. Their performance depends upon many factors, such as stacking sequence, fiber volume fraction (FVF), etc. In the present work, glass/epoxy composites are prepared in symmetric (ɸn/θ2n/ɸn//ɸn/θ2n/ɸn) and asymmetric (θn/ɸ2n/θn//ɸn/θ2n/ɸn) configuration with FVF of 48.13 and 61.34%. The laminate performance was evaluated using interlaminar shear strength, flexural, and mode-II interlaminar fracture toughness tests. The results indicate that symmetric layup has better mechanical strength than asymmetric layup. The interlaminar shear strength, flexural, and fracture toughness of asymmetric layup were lower by 14.72–16.39, 12.54–16.72, and 11.92–17.98%, respectively. Further, low FVF specimens had better mechanical strength, and high FVF specimens showed lower strength by 3.67–5.56, 10.75–15.17, and 23.53–28.79%, respectively,

Design of damage-resistant hybrid lay-up structures for fiber-reinforced composites based on interface properties

In this work, the poor impact damage resistance of carbon fiber-reinforced plastic (CFRP) composites was improved by mixing carbon fibers with ductile fibers (glass fibers/Kevlar fibers). The damage behavior of five interfaces of the prepared hybrid fiber-reinforced plastic (HFRP) composites was investigated through mode-I and mode-II interlaminar fracture toughness tests and micro-droplet debonding tests. On this basis, a numerical model for predicting the impact damage of HFRP composites was established. Through a comprehensive evaluation of the impact damage resistance indexes obtained from the numerical simulations, the hybrid fiber type, hybrid ratio between different fibers, and hybrid lay-up sequence could be optimized. The specimens with optimized lay-ups were prepared by 3D printing, and the effectiveness of the numerical model was verified through low-velocity impact and compression after impact (CAI) tests. The results show that the introduction of glass fiber layers in CFRP composites can significantly improve the damage resistance of the composites. Compared with CFRP composites, when the volume fraction of the glass fibers is 25%, HFRP composites have a smaller delamination damage area (reduced by 74.08%), furthermore, the pit depth is reduced by 43.01%, the CAI is increased by 17.29%, and the strength retention rate is increased by 31.94%.

Post‐curing and fiber hybridization effects on mode‐II interlaminar fracture toughness of glass/carbon/epoxy composites

AbstractFailure by delamination is one of the primary concerns in laminated composites. The development and progression of the delamination results in the degradation of composite stiffness affecting its structural integrity and may ultimately result in composite structure failure. Therefore, knowledge of the evolution of delamination resistance and the factors which affect the delamination in composites is of utmost importance for the selection of materials. This study investigates the effect of fiber hybridization and post‐curing on mode‐II interlaminar fracture toughness (GIIC) of Glass/Carbon/Epoxy (GCE) composites. Carbon/Epoxy (CE), Glass/Epoxy (GE), and GCE composite laminates were molded employing the process of hand layup technique and post‐cured at different temperatures. GE laminates exhibited a superior load for delamination and deformation compared to CE and GCE laminates. The hybrid GCE laminates exhibited higher interlaminar fracture toughness compared to non‐hybrid GE and CE laminates, which can be attributed to the synergetic effect of Carbon and Glass fibers. The laminates post‐cured at 180°C exhibited higher fracture toughness in their corresponding groups. The delaminated surfaces of CE, GE, and GCE laminates were further examined under Scanning electron microscopy (SEM) to understand the effect of post‐curing and fiber hybridization on mode‐II interlaminar fracture toughness.

Interlaminar mechanical performance of latent-cure epoxy joints

Because of uncertainty in bond strength of adhesively bonded composite structures for air flight, redundant load paths, such as mechanical fasteners or other crack arresting features, are required for certification. However, these redundant load paths add extra weight to the structure, induce significantly greater fabrication time and cost, and introduce local stress concentrations (in the case of mechanical fasteners) from which damage can more readily initiate. Structures with co-cured joints are more predictable, require much fewer redundant load paths, and are therefore more desirable. A secondary bonding technique for polymer-matrix composites that aims to produce co-cure-like joints with a continuous interface is briefly presented. A brief progression of mechanical testing results is reported and compared to actual co-cured joint performance. The purpose of the work is to determine the feasibility of this technology to produce a secondary bond equivalent in performance and predictability to that of a co-cured material. Joint performance was measured using six ASTM standard tests to measure interlaminar tensile and shear strength as well as mode-I and mode-II interlaminar fracture toughness at ambient temperature. After 50 process iterations, the final overall average joint performance exceeded 80% of the co-cured performance and in some tests had outperformed the co-cured specimens. The results suggest a high degree of feasibility of the technology to be pursued for future development with the intent to increase performance as well as reliability.

Production and characterization of cellulose nanocrystals of different allomorphs from oil palm empty fruit bunches for enhancing composite interlaminar fracture toughness

The effect of different alkaline treatments during the extraction of α-cellulose from oil palm empty fruit bunches on the properties of subsequently extracted cellulose nanocrystals are reported. Sodium hydroxide in an alkaline process step results in a change of native cellulose from type I to II. Treatment with potassium hydroxide maintains the native allomorph of cellulose I. Compared to commercial cellulose nanocrystals (CNCs), oil palm empty fruit bunch (OPEFB) CNCs type I materials have higher aspect ratios, higher crystallinity index, but lower charge, and thermal stability compared to commercial CNCs. OPEFB CNCs type II have a higher aspect ratio, but lower charge than both types of CNC. Type I CNCs exhibit intermediate charge, but higher thermal stability which might make them more suitable for processing with thermoplastic polymers. A full map of physical properties, comparing the oil palm based CNCs to both commercial and lab-based materials is presented to better understand how they might be exploited in composite applications. It is demonstrated that aligned electrospun cellulose acetate butyrate nanofibrous networks containing these CNCs can enhance the mode-II interlaminar fracture toughness of a carbon fibre-epoxy laminate.

Mode-I and mode-II interlaminar fracture properties of high modulus pitch-based carbon fiber reinforced polymers containing different nanostructures

The mode-I and mode-II interlaminar fracture properties of high modulus pitch-based carbon fiber reinforced polymers (CFRPs) (fiber: K13C; resin: EX-1515 cyanate ester) modified with 20–30 nm β-SiC nanoparticles or multiwalled-carbon nanotubes (MWCNTs) were investigated. Different volume fractions of both the β-SiC nanoparticles (1, 2, 5, and 10 vol%) and MWCNTs (1, 3, 5, and 7 vol%) were tested. The values of the mode-I and mode-II interlaminar fracture toughness of the CFRPs containing the lowest volume fractions of these nanostructures were larger compared with the unfilled composite but decreased with increasing the volume fraction of the inclusions. No differences in mechanical properties were observed among the different nanostructure types.

In situ formation of a carbon nanotube buckypaper for improving the interlaminar properties of carbon fiber composites

To improve the interlaminar properties of carbon fiber reinforced polymer composites (CFRPs), a hybrid fiber mat (i-MBP-PCF) was prepared by in situ deposition of a multiwalled carbon nanotube buckypaper (i-MBP) on the surface of functionalized CF fabric (PCF). The effect of the hybrid fiber mat on the interlaminar and electrical properties was systematically investigated. The results showed that the i-MBP-PCF hybrid fiber mat showed excellent interlaminar and electrical property enhancement functions. The interlaminar shear strength (ILSS), Mode-II interlaminar fracture toughness (GIIC), and electrical conductivity in the out-of-plane direction of i-MBP-PCF/EP increased 68.6%, 44.4%, and 6080%, respectively, compared to those of the CF/EP composite.

Effect of Heating Process on Flexural Strength and Toughness of CFRTP Molded by Multi-layer Press -Investigation on Effect of Residual Stress-

This study investigated the effect of differences in heating process of each mold due to the molding with multi-layer press on changing in mechanical properties of CFRTP. CFRTP specimens molded with 10 pages multi-layer were prepared for 3 point bending tests and DCB tests, ENF tests in order to investigate the different of mechanical properties. The change in the mechanical properties of CFRTP molded by multi-layer press were investigated considering residual stress occurred by differences in heating process of each mold. Average of flexural strength, mode-I interlaminar fracture toughness and mode-II interlaminar fracture toughness of the specimen molded at 5th page were decreased compared with that of 1st page. Assuming the fiber as a rigid and considering the thermal shrinkage of resin, compressive residual stress should be occurred in the fiber since free shrinkage of the resin was constrained. It was considered that the reduction of flexural strength with the failure close to the compression surface was caused due to this additional compressive residual stress. The tensile residual stress also should be occurred in the resin due to constraint of free shrinkage of the resin. The observed reduction of toughness in the resin was also occurred for the procedure of crack propagation. The reduction rate of mode-I interlaminar fracture toughness was much remarkable than that of mode-II interlaminar fracture toughness. Tensile residual stress in the resin was dominant since the mode-I fracture is a crack opening mode.

Mode-II fracture of nanostitched para-aramid/phenolic nanoprepreg composites by end-notched flexure

The mode-II interlaminar fracture toughness considering the end-notched flexure method of nanostitched para-aramid/phenolic composite structures was investigated. The fracture toughness (GIIC) of the nanostitched and stitched composites exhibited a slight increase as compared to the pristine sample. Hence, the nanostitching enhanced the fracture toughness of the para-aramid/phenolic composite structures. Although the type of stitch fiber was not effective, the fabric interlacement frequency, notably prepreg Twaron nanostitched yarn and basket nanoprepreg biaxial interlaced fabric was of critical importance. The principle mechanism for raising the GIIC in the nanostitched composite structure was the interlayer resin fracture particularly as a form of slight shear hackle marks. Cracks grew around the inter- and intrayarn boundaries where the resin was fractured half way around each yarn cross-section. This is called a “zigzag crack path,” and microcracks moved to the through-the-thickness of the composite where nanostitching arrested the crack growth and suppressed delamination in the stitching zone. At the blunt crack tip, carbon nanotubes in the phenolic resin and multiple filament bundles probably diminished the stress clustering via friction/debonding/pull-out/sliding or stick-slip. Thus, nanostitched para-aramid/phenolic composite structures demonstrated better damage tolerance behavior considering the neat structure.

Carbon-Fiber-Reinforced Epoxy Resin with Sustainable Additives from Silk and Rice Husks for Improved Mode-I and Mode-II Interlaminar Fracture Toughness

This paper presents an effective method for enhancing both the mode I (GIC) and mode II (GIIC) interlaminar fracture toughness of carbon fiber reinforced epoxy resin (CFRE). For precursor materials, silk fibroin nanofibers (nSF) and rice husk silica were prepared from sustainable resources. Nanocomposite samples were prepared using various loading ratios of the silica and nSF in epoxy resin (EP). Mechanical stirring and sonication techniques were used to prepare homogenous mixtures of silica and nSF in epoxy resin. Non-isothermal differential scanning calorimetry and the Kissinger equation were used to examine and calculate the cure kinetics and activation energy (Ea) of EP and the composite samples. The CFRE sample with hybrid fillers of nSF and silica at the ratio 0.2/20 (wt%/wt%) exhibited the highest GIC, and improved upon the mode-I and mode-II toughness of the pure-resin sample by 36.08% and 30.06%, respectively. Study of the fracture surfaces indicated that adding nSF and silica as fillers increases the energy required to fracture the CFRE.

Mode-II toughness of nanostitched carbon/epoxy multiwall carbon nanotubes prepreg composites: Experimental investigation by using end notched flexure

The mode-II interlaminar fracture toughness properties following the end notched flexure method of nanostitched carbon/epoxy nanoprepreg composites were studied. The fracture toughness (GIIC) of the nanostitched and stitched composites showed 3.4 fold and 2.7 fold increase compared to the control, respectively. Thus, the nanostitching improved the mode-II toughness of all the carbon/epoxy composites with regard to the nano, and base composites. It was assumed that the type of stitch fiber as well as fabric pattern, in particular prepreg carbon stitching fiber and satin prepreg woven fabric, was effective. The basic mechanism for the enhancement of the GIIC toughness in the nanostitched composite was the interlaminar resin layer failure especially as a form of shear hackle marks where nanostitching arrested the delamination in the stitching zone during crack propagations. Multiwall carbon nanotubes in the matrix and filament also mitigated the stress concentration probably as an outline of debonding/pull-out/stick-slip/friction. Therefore, nanostitched as well as stitched carbon/epoxy woven composites exhibited improved damage tolerance performance with regard to the base composites.

Improvement of mode-II interlaminar fracture toughness of carbon textile composites with modified matrix of thermoplastic and thermoset epoxy-addition of glass fibers

Interlaminar fracture toughness of a material with a layered structure such as CFRP is poor and cracks between the layers easily lead to fatal result. Interlaminar fracture toughness still plays an important role in damage propagation of CFRP. In this study, mode II inter-lamina fracture toughness of CFRP was investigated by ENF test for clarifying effect on weight-average molecular weight of matrix and matrix modification with glass fiber. Micro glass fibers whose diameter was 0.5 μm with 0.3wt% of weight content were added into thermoplastic epoxy resin and thermoset epoxy resin, respectively, for the matrix of the textile carbon fiber composite prepared by our laboratory. State of the fracture surface was also changed accompanying with the change of the weight-average molecular weight of the matrix. In these results, Mode II interlaminar fracture toughness of the composite was improved by preventing the crack propagation with fiber bridging of added glass fibers on the interface regardless of the types of matrix of thermoplastic and thermoset epoxy resin.

Improving the Mode-II interlaminar fracture toughness of polymeric matrix composites through additive manufacturing

Delamination is a common failure mode observed in fiber reinforced polymeric composites subjected to thermo-mechanical loads. Reinforcements at the interlaminar regions or in the through-thickness direction have been previously shown to improve the damage resistance and durability of these composites. In particular additive manufacturing (AM), also called 3D printing, has been shown to increase the interlaminar shear strength when used for custom printing of polymer reinforcements over carbon fiber prepreg prior to the layup process of layered composites. In this paper, printing of interlaminar polymer reinforcements using fused deposition modeling (FDM) technique within AM is proposed for improving the interlaminar fracture toughness of carbon fiber laminates. The rationale behind this improvement in the interlaminar fracture toughness values due to printed polymer reinforcements (PPR) is investigated by elucidating the influence of varying the process parameters, such as print speed, spacing, and dimensions. An in-depth understanding of the bond quality between the prepregs and the printed polymers as well as the interlaminar fracture surfaces formed on the Mode-II fracture toughness values are established by developing an experimentally validated thermo-chemical finite element modeling framework.

Mode-II interlaminar fracture toughness of GFRP/Al laminates improved by surface modified VGCF interleaves

Mode-II interlaminar fracture toughness of GFRP/Al (aluminum) laminates is improved by employing acid treatment to an Al plate and inserting surface modified vapor grown carbon fiber (S-VGCF) between the Al plate and a GFRP layer. End notched flexure (ENF) tests are carried out to investigate the effect of S-VGCF addition on the Mode-II interlaminar fracture toughness. The experimental results demonstrate that the critical load and Mode-II fracture toughness of the specimen with acid treatment of Al plate and 10 g/m2 addition of surface modified VGCF are enhanced by 135.51% and 425.16%, respectively, compared to those of corresponding pristine specimen. Due to the presence of the oxygen containing functional groups, the specimens with addition of S-VGCF possess much higher Mode-II fracture toughness compared to the specimens with addition of pristine VGCF. Laser scanning and scanning electron microscopies are employed to observe the fracture surface of specimens to reveal the improvement mechanisms. Moreover, numerical simulations are performed based on the cohesive zone model to verify the experimental results. The interfacial shear strength between the Al plate and the GFRP layer are also predicted by the numerical simulations.

Interlaminar and Intralaminar Fracture Behavior of Carbon Fiber Reinforced Polymer Composites

Interlaminar and intralaminar fracture behavior of carbon fiber reinforced composites have been experimentally studied. Unidirectional, woven reinforcement and thermoplastic and thermoset polymer matrix laminates have been characterized using double cantilever beam (DCB) and end notch flexure (ENF) specimens for Mode-I and Mode-II fracture toughness, respectively and compact tension (CT) specimens for intralaminar fracture. AS4/PEEK, AS4/8552 and AGP193PW/8552 laminates have been characterized in this study. The fracture toughness determined from the experimental data could be related to the constituents and reinforcements. It has been observed between the two UD laminates, AS4/PEEK exhibit higher fracture resistance under both interlaminar and intralaminar fracture. Woven reinforcement is found to show higher mode-II interlaminar fracture toughness.

The influence of a thermoplastic toughening interlayer and hydrothermal conditioning on the Mode-II interlaminar fracture toughness of Carbon/Benzoxazine composites

Carbon fibre/Benzoxazine laminates with and without non-woven polyamide (PA) fibre veils at the interlaminar regions were manufactured using vacuum assisted resin transfer moulding (VARTM). The effect of the interlaminar thermoplastic veils on the Mode-II critical strain energy release rate (GIIC), under both wet and dry conditions, was determined using two commercially available Benzoxazine resins: a toughened system and an untoughened system. In all samples the toughened system outperformed the untoughened system. The overall resistance to Mode-II crack growth was significantly improved by the inclusion of the interlaminar veils due to an increase in the thickness of the matrix-rich interlaminar region, plastic deformation of the PA fibres and a crack-pinning mechanism. Moisture caused an increase in matrix ductility, which improved the resistance to crack initiation; however, this was counteracted by a reduction in fibre/matrix interfacial strength causing a reduction in resistance to crack growth.