Reduced Notch Sensitivity Research Articles

Damage Identification on Impact and Lightning Damage of Flax Composite Laminates (Linum usitatissimum) Using Long-Pulse Thermography of a Low-Resolution Infrared Camera

The demand for composite fiber material is significantly high due to its excellent mechanical properties and its use in various industries. Recently, with the increasing awareness of environmental issues, researchers are now focusing more on eco-friendly and green materials. A biocomposite offers a good balance of strength and stiffness ratio, bending and membrane mechanical properties, balanced thermal distortion stability, reduced weight and cost, improved fatigue resistance, reduced notch sensitivity, and, comparatively, better performance than synthetic composites. Yet, due to the complex anisotropy of the composite material, the inspection and detection of inner defects become a challenge. Long-pulse thermography is one of the non-destructive evaluations (NDEs) used to detect defects in composite materials. However, very limited research has been carried out on the usage of a low-resolution infrared camera to perform defect or damage inspection on flax composite laminates. In this paper, an experimental setup of a long pulse thermography system using low-resolution infrared camera was performed on flax bio-composite to identify impact and lightning damage. The result highlights that with control parameters, a low-resolution infrared camera has the capability to capture the lightning and impact defects of flax biocomposites using the long-pulse thermography system. An image processing method is then applied to the defect to improve the quality of defect detection and reduce background noise.

Reduced notch sensitivity in pseudo-ductile CFRP thin ply angle-ply laminates with central 0° plies

This paper presents an experimental investigation on the unnotched and open-hole tensile behaviour of pseudo-ductile thin ply angle-ply carbon fibre/epoxy laminates with central 0° plies. Laminates with two different configurations of [±265/0]s and [±252/0]s were designed and tested under unnotched and open-hole tensile loading. Metal-like tensile stress-strain curves with a plateau were observed in both unnotched configurations. The open-hole net-section strength of the [±252/0]s laminate attained 96% of the unnotched “yield” strength. Digital image correlation and X-ray CT-scan images showed that the same damage mechanisms of central 0° ply fragmentation and local dispersed delamination observed in unnotched pseudo-ductile laminates were present in the open-hole specimens of the same configuration. These damage mechanisms caused stress redistribution around the hole and reduced the notch sensitivity in pseudo-ductile laminates under open-hole tensile loading. The main factors governing the open-hole performance are also discussed.

Pseudo-ductility and reduced notch sensitivity in multi-directional all-carbon/epoxy thin-ply hybrid composites

Un-notched and notched tensile response and damage accumulation of quasi-isotropic carbon/epoxy hybrid laminates made of ultra-high modulus and intermediate modulus carbon fibre/epoxy thin-ply prepregs were studied. It was confirmed that the ply fragmentation demonstrated previously in unidirectional hybrids as a successful pseudo-ductility mechanism can be transferred to multi-directional laminates. Furthermore, reduced notch sensitivity was demonstrated in quasi-isotropic specimens for both open holes and sharp notches as a result of local ply fragmentation around the notch.

Transverse Crack Detection in 3D Angle Interlock Glass Fibre Composites Using Acoustic Emission.

In addition to manufacturing cost and production rates, damage resistance has become a major issue for the composites industry. Three-dimensional (3D) woven composites have superior through-thickness properties compared to two-dimensional (2D) laminates, for example, improved impact damage resistance, high interlaminar fracture toughness and reduced notch sensitivity. The performance of 3D woven preforms is dependent on the fabric architecture, which is determined by the binding pattern. For this study, angle interlock (AI) structures with through-thickness binding were manufactured. The AI cracking simulation shows that the transverse component is the one that leads to transverse matrix cracking in the weft yarn under tensile loading. Monitoring of acoustic emission (AE) during mechanical loading is an effective tool in the study of damage processes in glass fiber-reinforced composites. Tests were performed with piezoelectric sensors bonded on a tensile specimen acting as passive receivers of AE signals. An experimental data has been generated which was useful to validate the multi-physics finite element method (MP-FEM), providing insight into the damage behaviour of novel 3D AI glass fibre composites. MP-FEM and experimental data showed that transverse crack generated a predominant flexural mode A0 and also a less energetic extensional mode S0.

Mechanical properties of vinylester resins exposed to marine environments

An experimental investigation was undertaken to determine the effects of moisture on the mechanical properties of vinylester resins (510A and 8084). The resins were exposed to seawater at 40°C and humid air at 50°C until the moisture content equilibrated. Both resins absorbed small amounts of moisture. Mechanical testing of both resins at dry and saturated conditions revealed very brittle behavior under tension and shear loading. The 8084 resin is more ductile than 510A at dry and saturated conditions. Moisture absorption was found to only slightly influence the mechanical response of these resins. Dry shear test specimens failed at quite low shear stresses in a resolved tensile mode and displayed sensitivity to the stress concentration in the Iosipescu shear specimen. Moisture absorption slightly enhanced ductility and reduced notch sensitivity which resulted in increased shear strength. POLYM. ENG. SCI., 53:2413–2421, 2013. ©2013 Society of Plastics Engineers

Tenacidade à fratura de blendas PA 6/ABS avaliada através do método EWF (Trabalho Essencial de Fratura) - parte a: avaliação do efeito do compatibilizante

A poliamida 6 (PA6) é um polímero termoplástico semicristalino que apresenta alta susceptibilidade ao entalhe quando solicitado mecanicamente. Apesar de apresentar um comportamento dúctil sob deformação e sob fratura quando solicitado sem entalhes, torna-se essencialmente frágil e quebradiço quando entalhado. Assim, o comportamento pseudo-dúctil dessas poliamidas requer cuidados especiais na confecção do entalhe e da pré-trinca para análise de seu comportamento mecânico. Devem ser observadas ainda as dimensões ideais dos corpos de prova para garantir a condição de estado de tensão plana no ensaio de tração uniaxial para aplicação do método EWF (Trabalho Essencial de Fratura). Neste trabalho, o método EWF foi utilizado para a avaliação da tenacidade à fratura de blendas poliméricas com uma matriz PA6 modificada com ABS e também a eficiência de um compatibilizante acrílico reativo (MMA-MA) sintetizado em laboratório, na tenacificação da blenda PA6/ABS. Através da aplicação do método observou-se que a simples incorporação do ABS à poliamida aumenta o termo relativo ao trabalho específico de fratura e reduz a sensibilidade ao entalhe da blenda embora com perda na capacidade de absorção de energia plástica. A incorporação do copolímero MMA-MA à blenda refletiu-se nos acréscimos do trabalho específico e recuperação da capacidade de absorção de energia plástica em relação à blenda não-compatibilizada. Os resultados ilustram a aplicabilidade do método EWF para determinação da tenacidade à fratura de blendas poliméricas em relação aos polímeros puros, bem como a avaliação do efeito do compatibilizante reativo na tenacidade da blenda PA6/ABS.

Fiber-reinforced composite foam from expandable PVC microspheres

Abstract The properties of composite foam based on PVC expandable microspheres reinforced with continuous aramid fibers are described. The foam was fabricated by infiltrating low-density non-woven fiber webbing with PVC microspheres. The assembly was subsequently heated to expand the foam. The resulting composite foam consisted of 10 wt% aramid fibers and had a density of 100 kg/m3. Mechanical properties, crack propagation, and microstructure of composite foams were evaluated and compared with properties of similar unreinforced foam and with commercial PVC foam of comparable density. The influence of fiber concentration, fiber architecture and bonding was investigated also. Properties were measured in tension, shear, compression, and flexure using standard ASTM test methods. The composite foam performance equaled or surpassed the performance of most thermoplastic foams commercially available. The tensile strength and modulus of the composite foam increased by factors of 6 and 8, respectively, and the shear strength and modulus increased by factors of 1.8 and 2.4. The composite foam also exhibited improved strain energy density and damage tolerance, and reduced notch sensitivity.

Fracture behaviour and toughness of a plane-woven SiC fibre-reinforced SiC matrix composite

This study investigates the fracture behaviour and fracture resistance of a bi-directional woven SiC fibre-reinforced SiC matrix composite fabricated by chemical vapour infiltration. The tensile stress-strain curve shows nonlinear behaviour above 70 MPa, and the effective Young's modulus decreases above this tensile stress. The composite's effective Young's modulus at fracture was about 40% of the initial value, and all the applied load at fracture was supported by the longitudinal fibre bundle composites (FBCs). The gross fracture stress of a notched specimen showed reduced notch sensitivity in comparison with a monolithic SiC matrix, though it never completely vanished. The origin of the increase in the composite's fracture toughness was explained using an FBC bridging model that employs the strength of an FBC obtained from separated tensile testing. Results demonstrate that the cracking of the transverse FBC, which occurs before longitudinal FBC fracture, plays an important role affecting the bridging condition, and that the tensile fracture strength and toughness of the composite can be modelled by the presented FBC bridging model.

Mechanical properties of the rubber‐toughened polymer blends of polycarbonate (PC) and poly(ethylene terephthalate) (PET)

AbstractThe intrinsically impact‐brittle PC/PET blends can be effectively toughened, in terms of lower ductile brittle transition temperature (DBTT) and reduced notch sensitivity, by incorporating butylacrylate core‐shell rubber. The rubber particles are distributed exclusively in the PC phase. Varying the PC melt flow rate (MFR) is more important than varying the PET I.V. to vary the low temperature toughness of the blends. PC with MFR = 3 is essential to produce the toughest PC/PET/rubber blend. The presence of rubber slightly relieves the strain rate sensitivity on yield stress increase. Lower MFR PC in the blend results in smaller activation volume and, therefore, higher strain rate sensitivity, because a greater number of chain segments are involved in the cooperative movement during yielding. Two separate modes, localized and mass shear yielding, work simultaneously in the rubber toughening mechanism. The plane‐strain localized shear yielding dominates the toughening mechanism at lower temperatures and brittle failure, while the plane‐stress mass shear yielding dominates at higher temperatures and ductile failure. The critical precrack plastic zone volume has been used to interpret the observed phenomenon. © 1994 John Wiley & Sons, Inc.

Rubber-Toughened polymer blends of polycarbonate (PC) and poly (ethylene terephthalate (PET)

The intrinsically impact brittle nature of the PC/PET blends can be effectively toughened by incorporating butylacrylate core-shell rubber. The rubber-modified PC/PET blend possess both excellent low temperature impact properties and reduced notch sensitivity. The ductile-brittle transition temperature of the blend decreases with the increase of rubber content. The presence of rubber in the PC/PET blend does not relieve the strain rate induced yield stress increase. Two separate modes, localized shear yielding and mass hear yielding, work simultaneously in the rubber toughening mechanism. The plane-strain localized shear yielding dominates the toughening mechanism at lower temperature and results in brittle failure. At higher temperature, the planestress mass shear yielding dominates the toughening mechanism and results in ductile failure. The critical plastic zone volume can be used to interpret the observed phenomenon.

The notch sensitivity of polymeric materials

Stress–strain tests were made on about five dozen polymeric materials using unnotched and notched specimens containing six different types of notches. Notches decrease the strength, but they decrease the elongation to break even more drastically in general. Notch sensitivity factors are defined for strength and for energy to fracture in such a manner that the greater the notch sensitivity factor, the greater is the effect of a notch relative to the unnotched material. The notch sensitivity factor for breaking (or yield) strength is not the same as the notch sensitivity factor for energy to fracture as measured by the area under the stress–strain curve. Brittle polymers and composites tend to have greater notch sensitivity factors for strength than ductile polymers. For brittle polymers, the notch sensitivity factor for energy to fracture tends to increase with the elongation to break of the unnotched polymer. Notches generally are more detrimental to ductile polymers than to brittle ones as far as the energy to fracture is concerned. For ductile polymers, the shape of the stress–strain curve is important in determining the sensitivity to notches. The ratio of the upper to lower yield strengths should be small for low notch sensitivity. It is desirable to have the breaking strength greater than the yield strength. Glass fibers and filler in ductile matrices increase the notch sensitivity for strength but decrease the sensitivity for energy to fracture relative to the unfilled polymer. Rubber–filled polymers have a reduced notch sensitivity for strength relative to the unfilled polymer, but the notch sensitivity for energy to fracture may be either increased or decreased, depending upon the system. The energy to fracture for notched specimens correlates better with Izod impact strength than does the energy to fracture for unnotched specimens. It is recommended that notched stress–strain specimens be routinely measured along with unnotched specimens.

Notch sensitivity and fracture toughness of thermally toughened round rolled sections

1. The thermal toughening of carbon steel from rolling heat reduces the rate of the temperaturenotch sensitivity relationship compared with that for steel in the hot-rolled state. 2. With the higher level of resistance to plastic deformation of thermally toughened steel compared with hot-rolled steel, the former is less sensitive to notches (including sharp notches) over a fairly wide range of low temperatures. 3. The thermal toughening of low-carbon steel is the most effective from the point of view of reduced notch sensitivity at low temperatures.