Short Beam Shear Specimens Research Articles

On the cyclic delamination-healing capacity of vitrimer-based composite laminates

This study aims to investigate the self-healing capability of a composite laminate composed of novel bio-based benzoxazine vitrimers when subjected to delamination-healing cycles. The composite laminate is manufactured using compression resin transfer moulding. To quantify the interlaminar shear strength (ILSS) and induced damage, three-point bending tests were conducted on short-beam shear specimens. The healing of interfacial damages was achieved by applying pressure (1 MPa) at 170 °C. Three healing experiments were performed with different thermal cycling durations: 1, 10, and 60 min. The extent of interfacial healing was evaluated through four repetitions of delamination-healing cycles. Despite a gradual decrease in ILSS values with each cycle, the specimens subjected to a 60-minute healing process exhibit remarkable recovery. After three cycles, 80 % of the ILSS is restored, highlighting the highly efficient healing capability of the vitrimer-based composite.

Mechanical properties characterization of fiber reinforced composites by nonlinear constitutive parameter optimization in short beam shear specimens

This work presents a novel approach to quantify the nonlinear shear stress-strain behavior of carbon fiber reinforced polymer composites without finite element stress calculation. Full-field noncontact deformation measurements using Digital Image Correlation are used to measure surface strains in Short Beam Shear IM7/8552 specimens. The presented spline-based optimization technique solves the inverse problem of generating both axial and shear stress-strain curves without ad hoc assumptions on the material model. Accuracy of the analysis method was verified by finite element analysis (FEA). This method improves on available closed-form and iterative FEA based solutions due to flexibility and accuracy without the computational expense.

Microstructure optimizations for improving interlaminar shear strength and self-healing efficiency of spread carbon fiber/epoxy laminates containing microcapsules

The purpose of this study is to improve interlaminar shear strength and self-healing efficiency of spread carbon fiber (SCF)/epoxy (EP) laminates containing microcapsules. Microencapsulated healing agents were embedded within the laminates to impart a self-healing functionality. Self-healing was demonstrated on short beam shear specimens, and the healing efficiency was evaluated by strain energies of virgin and healed specimens. The effects of microcapsule concentration and diameter on apparent interlaminar shear strength and healing efficiency were discussed. Moreover, damaged areas after short beam shear tests were examined by an optical microscope to investigate the relation between the microstructure and the healing efficiency of the laminates. The results showed that the stiffness and the apparent interlaminar shear strength of the laminates increased as the microcapsule concentration and diameter decreased. However, the healing efficiency decreased with decreasing the microcapsule concentration and diameter.

Characterization of Plasma Cured E-Glass-Epoxy composite

Abstract Glass fiber (E-Glass) reinforced polymer Epoxy-resign, composite seats were fabricated in the laboratory by hand-lay-up method with 6,12 and 18 layers of laminates. Short-beam-shear (SBS) specimens were machined out of the fabricated seats. The 6 layer SBS specimens were exposed to low temperature plasma for 5, 10, 15 and 20munits. The specimens developed the maximum ILSS (Inter Laminar Shear Stress), for the 6 layered samples with 15 minute of exposure to the low temperature plasma due to post-cure strengthening. on the basis of the above, 12 and 18 layered SBS specimens were also exposed to the same low temperature plasma for 15 minutes duration. The observed ILSS values for the 12 layered specimens were somewhat higher than the same for the 18 layered specimens while being less than the 6 layered once. The least ILSS values were recorded for the 18 play samples. Thus, the radial migration of low temperature plasma inside the core of the specimens, responsible for post cure strengthening was inferred to bea function of the no of layers in the composite specimens, decreasing with the increase in the no of layers responsible for an increase in the thickness of the specimens. Glass transition temperature (Tg) for these plasma cured samples with 15 minute exposure (optimum) also decreased with the increase in the no of layers in the specimens. The mode of failure, as reviled from SEM fractographs, included fiber-pull-out, fiber-matrix debonding, matrix cracking/ crazing and fiber breaking

Synergetic effects of thin plies and aligned carbon nanotube interlaminar reinforcement in composite laminates

Thin-ply carbon fiber laminates have exhibited superior mechanical properties, including higher initiation and ultimate strength, when compared to standard thickness plies and enable greater flexibility in laminate design. However, the increased ply count in thin-ply laminates also increases the number of ply-ply interfaces, thereby increasing the number of relatively weak and delamination-prone interlaminar regions. In this study, we report the first experimental realization of aligned carbon nanotube interlaminar reinforcement of thin-ply unidirectional prepreg-based carbon fiber laminates, in a hierarchical architecture termed ‘nanostitching’. We synthesize a baseline effective standard thickness laminate using multiple thin-plies of the same orientation to create a ply block, and we find an ∼15% improvement in the interlaminar shear strength via short beam shear (SBS) testing for thin-ply nanostitched samples when compared to the baseline. This demonstrates a synergetic strength effect of nanostitching (∼5% increase) and thin-ply lamination (∼10% increase). Synchrotron-based computed tomography of post mortem SBS specimens suggests a different damage trajectory and mode of damage accumulation in nanostitched thin-ply laminates, notably the complete suppression of delaminations in the nanostitched region. Finite element predictions of damage progression highlight the complementary nature of positive thin-ply and nanostitching effects that are consistent with an ∼15% improvement in Modes I and II interlaminar fracture toughness due to the aligned carbon nanotubes at the thin-ply interfaces.

Effect of Hydrothermal immersion and Hygrothermal Conditioning on Mechanical Properties of GRE Composite

Glass fibre reinforced epoxy (GRE) composite meet several degrading agents like moisture and temperature while its use in real time applications in civil infrastructures. Keeping this in mind, the short beam shear (SBS) specimens of GRE composite were exposed to such laboratory created stringent environment as a combination of moisture and elevated temperature for several periods. The environments are as: immersion in distilled water coupled with 65oC as hydrothermal conditioning and an ambience containing 95% relative humidity at 60oC as hygrothermal conditioning. Moisture treated SBS specimens were subjected to 3-point bend test to reveal inter laminar shear strength (ILSS), stress/strain at rupture and modulus values with periods of exposures. The concerned sample suffered 23% of degradation in ILSS values after 120 days of hydrothermal immersion and 25% after 90 days of hygrothermal conditioning. Samples at some optimum exposing conditions of both the exposures are thermally characterized by adopting differential scanning calorimetry (DSC) test. Glass transition temperature (Tg) of such representing samples were determined from the DSC thermograms. About 8 % reduction in Tg values was observed for the GRE composite sample, expectedly, due to moisture induced matrix plasticization and swelling. The fractographs as obtained through scanning electron microscope (SEM) revealed some causes of failures indicating the prime modes of failure of the treated GRE samples with optimum duration of both the exposures.

グラフェンの超音波分散とグラフェン含有開繊炭素繊維/エポキシ樹脂積層材料の自己修復

Interlaminar damages can cause a reduction in strength and stiffness of laminated composites and is extremely difficult to detect and repair by conventional methods. Thus, self-healing has the potential to mitigate the interlaminar damages. This study examines the interlaminar shear strength and the self-healing of spread carbon fiber/epoxy laminates containing ultrasonicated graphenes. Healing is accomplished by incorporating a microencapsulated healing agent and a catalyst within an epoxy matrix. Healing is triggered by crack propagation through the microcapsules, which then release the healing agent into crack faces. Subsequent exposure of the healing agent to catalyst initiates polymerization and bonding of crack faces. Self-healing is demonstrated on short beam shear specimens and healing efficiency was evaluated by the strain energies of virgin and healed specimens. The damaged area of the specimens was also examined by an optical microscope.

自己修復性を有する開繊炭素繊維／エポキシ樹脂積層材料の強度回復と微視構造最適化

Damage can cause a reduction in the strength and stiffness in FRP and is very difficult to detect and repair. Thus, self-healing has the potential to mitigate damage in FRP. This study examines the effect of microcapsule concentration and diameter on the strength recovery of spread carbon fiber(SCF)/epoxy(EP) laminates encompassing self-healing. Healing is accomplished by incorporating a microencapsulated healing agent and catalyst. Self-healing is demonstrated on short beam shear specimens and the healing efficiency was evaluated by the strain energies of virgin and healed specimens. The damaged area of specimens was also examined by an optical microscope. The results showed that as the microcapsule concentration increased, the apparent interlaminar shear strength decreased and the healing efficiency increased. The trade-off between the strength and the healing efficiency was observed. Furthermore, as the microcapsule diameter increased, the apparent interlaminar shear strength decreased. High healing efficiency was achieved with 50μm microcapsule diameter.

開繊炭素繊維/エポキシ樹脂積層材料の自己修復と強度回復に及ぼす炭素ナノ材料添加の影響

This study examines interlaminar shear strength and self-healing of spread carbon fiber (SCF)/epoxy (EP) laminates. Self-healing is accomplished by incorporating a microencapsulated healing agent and catalyst within the epoxy matrix. Short beam shear specimens of self-healing SCF/EP laminates were prepared and tested. The healing efficiency was evaluated by the strain energies of virgin and healed specimens. The effect of addition of carbon nanomaterials to the matrix on the apparent interlaminar shear strength and the healing efficiency are discussed. The damage area of the healed specimens was also examined by an optical microscope.

Interlaminar Shear and Electrical Resistance Responses of Woven-Carbon-Fiber-Reinforced-Polymer Composite Laminates at Cryogenic Temperatures From Cyclic Short Beam Shear Tests

Abstract This paper studies the interlaminar shear and electrical resistance responses of woven-carbon-fiber-reinforced-polymer (CFRP) composite laminates subjected to fatigue loading at cryogenic temperatures. Fatigue tests were conducted on the composite specimens at room temperature and liquid-hydrogen temperature (20 K) using the short beam shear method, and measurements of the specimen electrical resistance were made during the tests. Also, the tested specimens were examined by microscopy to observe the damage and the failure mode. In addition, a finite-element analysis was performed to examine the stress distributions in the short beam shear specimens. The dependence of the interlaminar shear fatigue performance of the woven-CFRP laminates on the temperature was discussed, and the applicability of the electrical resistance method for assessing the fatigue failure was demonstrated.

Short‐beam Shear Method for Assessment of Stress–Strain Curves for Fibre‐reinforced Polymer Matrix Composite Materials

ABSTRACTThis work presents a thorough description of a short‐beam shear (SBS) test method coupled with digital image correlation (DIC) for the measurement of shear stress–strain curves of polymer matrix composites. The method provides a time‐ and cost‐effective alternative to established methods, and the authors hope this work can serve as a starting point for discussions on the developments of new standards for the characterisation of shear stress–strain curves for composites. The experimental set‐up, including several modifications to the American Society of Testing and Materials current standard configuration, is described. Implementation of the DIC technique to obtain accurate surface strain in the SBS specimens is also presented. Accuracy of beam theory‐based shear stress calculation is assessed using a non‐linear finite element model (FEM). A simple means to improve the accuracy of the closed‐form shear stress approximation is provided. Glass/epoxy and carbon/epoxy composite material systems are used to demonstrate the developed method. Strain surveys are conducted to compare the experimentally generated and FEM‐computed strain fields and verify the accuracy of the shear stress–strain curves. Shear stress–strain response generated from SBS tests is also compared with V‐notched beam test results for further verification.

Short beam interlaminar shear behavior and electrical resistance-based damage self-sensing of woven carbon/epoxy composite laminates in a cryogenic environment

This article investigates the interlaminar shear behavior and damage detection of woven carbon fiber reinforced polymer composite laminates at cryogenic temperatures. Short beam shear tests were performed at room temperature and liquid hydrogen temperature (20 K), and the temperature dependence of the apparent interlaminar shear strength was examined. The electrical resistance of the composite specimens was also monitored during the tests. A detailed observation of the tested specimens was made to assess the damage, and the relationship between the damage and the electrical resistance was discussed. In addition, the stress, strain and current density distributions in the short beam shear specimens were determined by the finite element method. The numerical results were used to better understand and explain the experimental findings.

Characterization of nonlinear shear properties for composite materials using digital image correlation and finite element analysis

Abstract A method for accurate assessment of nonlinear shear stress–strain relations of composite materials is developed in this work using short-beam shear (SBS) tests. The method is based on a combination of a finite element model (FEM) for stress calculation and a digital image correlation (DIC) technique for full-field measurement of deformation. An inverse problem of generating shear stress–strain curves is solved using an iterative procedure which starts with a beam theory-based closed-form approximation of shear stresses, and updates the stresses through a number of finite element simulations. The method is demonstrated on unidirectional carbon/epoxy SBS specimens. Experimentally-generated strain fields are compared with FEM-computed strain fields to verify accuracy of the developed method. Shear stress–strain data are compared with V-notched beam test results for further verification.

Thermal, rheological, and mechanical properties of a polymer composite cured at different isothermal cure temperatures

Thermal, rheological, and mechanical properties of a commercial carbon fiber epoxy prepreg, Cycom 977-2 UD, were obtained for isothermal cure temperatures ranging from 149°C to 182°C. For each cure profile, an encapsulated-sample rheometer (ESR) was used to measure the storage modulus. Each ESR cure profile was followed by the glass transition temperature ( Tg) test. The degree of cure ( α) during cure and the heat of reaction of the prepreg were obtained using a differential scanning calorimeter (DSC). Combined loading compression (CLC) and short-beam shear (SBS) tests were performed to obtain compressive properties and SBS strength, respectively. It was observed that the compressive properties did not vary significantly for the studied isothermal cure temperatures; likewise, the compressive failure mode was the same for all the CLC specimens. However, the SBS strength for the specimens cured at 149°C was approximately 10% less than that of those cured at isothermal cure temperatures ranging from 160°C to 182°C. Further, the failure mode of the SBS specimens cured at 149°C was also different from other specimens. The storage modulus of the ESR sample cured at 149°C also showed a 10% decrease compared to other ESR samples. The SBS strength exhibited a good correlation with the storage modulus and a weak correlation with Tg and α.

Novel Methods for Assessment of Three-Dimensional Constitutive Properties for Composites

Accurate three-dimensional stress-strain constitutive properties are essential for understanding of complex deformation and failure mechanisms for glass-fiber and carbon-fiber reinforced polymer-matrix composites. A large number of different methods and specimen types, which are currently required to generate three-dimensional allowables for structural design, slow down material characterization. Also, some of the material constitutive properties are never measured due to prohibitive cost of the specimens needed. This work shows that simple short-beam shear specimens are well-suited for measurement of 3D constitutive properties for composite systems. In particular, a methodology to measure tensile and compressive material properties, generate shear stress-strain curves and measure the shear strength in a simple short beam shear test will be presented. The methodology is based on the Digital Image Correlation (DIC) full-field deformation measurement. Short-beam and curved-beam tests are accomplished to generate 3D stress-strain response for glass/epoxy and carbon/epoxy tape composite material systems. Accuracy of constitutive properties is also verified using standard methods and data available in the public domain.

Freeze–Thaw Response of Glass–Polyester Composites at Different Loading Rates

The present literature suggests the need for investigating and characterizing the freeze–thaw response of polymer composites under different loading rates and also at ambient and sub-ambient temperatures. The present experiment has been carried out with three different weight fractions (i.e., 0.55, 0.6, and 0.65) of glass fiber-reinforced polyester composites. The short beam shear specimens are suddenly exposed to 80 C for 2 h and then a three-point bend test is conducted instantaneously at this temperature. Another batch of samples is allowed to thaw at ambient temperature for 1 h and tested on flexural mode at ambient temperature. The testing was carried out for a range of 0.5–500 mm/min crosshead speed. The shear strengths at cryogenic temperature and thawing temperature are compared with the room temperature-tested data of the shear value of laminates.

Effects of Thermal and Cryogenic Conditionings on Mechanical Behavior of Thermally Shocked Glass Fiber-Epoxy Composites

A very large thermal expansion mismatch may result in weakening the fiber-matrix interface and/or a possible matrix cracking due to thermal shock stress. The short beam shear specimens of glass-epoxy composite were treated at 40 C for a certain time and then exposed to 40 C for different conditioning times. The treatment was performed in the opposite direction of thermal cycle. The three-point bend test was carried out at room temperature with different crosshead speeds. The debonding effect of different natures of thermal shock (up- and down-cycles) and strengthening phenomena of thermal conditionings (above and subzero temperature conditionings) were assessed in the present study for the different durations of conditioning and different states of thermal conditionings (thermal and cryogenic). The state of the interactions between fiber and polymer matrix by the treatment was reflected in the shear values of the composites.

Thermal Shock and Thermal Fatigue on Delamination of Glass-fiber-reinforced Polymeric Composites

Glass-fiber-reinforced unsaturated polyester and epoxy resin composites were exposed to 75°C temperature gradient thermal shock for ten times in different stages of conditioning times. The other lot of short-beam shear specimens were conditioned with a thermal shock of again 75°C temperature gradient for the different cycles. The three-point bend test was performed on the conditioned specimens to evaluate the value of interlaminar shear strength. These conditionings may induce matrix cracking because of large misfit strain. The present experiments aim to study the fiber-matrix debonding effect of thermal shock-thermal fatigue, post-curing hardening phenomena of thermal conditioning, and also the mechanical keying factor of sub-zero conditioning.

Analytical and Experimental Studies of Short-Beam Interlaminar Shear Strength of G-10CR Glass-Cloth/Epoxy Laminates at Cryogenic Temperatures

Cryogenic interlaminar beam tests in the form of three-point flexure are examined both experimentally and analytically. The use of the short-beam shear test for measuring the interlaminar shear strength of glass-cloth/epoxy laminates at low temperatures is evaluated first. The interlaminar shear tests were carried out with short-beam shear specimens at room temperature, 77 K and 4 K to evaluate the interlaminar shear strength of G-10CR glass-cloth/epoxy laminates. Each specimen was placed on two roller supports that allow lateral motion and a load was applied directly at the center of the specimen. These tests were conducted in accordance with ASTM, 1984, “Standard Test Method for Apparent Interlaminar Shear Strength of Parallel Fiber Composites by Short-Beam Method,” Designation D2344-84. The effects of temperature, specimen width, and span-to-thickness ratio on the apparent interlaminar shear strength are shown graphically. Photomicrographs (scanning electron micrographs, optical micrographs) of actual failure modes were utilized to verify the failure mechanisms. A three-dimensional finite element analysis was also performed to investigate the effects of specimen width and span-to-thickness ratio on the shear stress distribution in the mid-plane. Effective elastic moduli were determined under the assumption of uniform strain inside the representative volume element. The numerical findings are then correlated with the experimental results.

Effects of γ-radiation on mechanical behaviour of carbon/epoxy composite materials

Mainly because of their excellent specific properties, the use of polymeric matrix composite (PMC) materials in space structures and energy applications is growing increasingly. However, when space and nuclear environments are considered, the strength of PMC materials could be adversely affected due to the exposure to different types of radiation and their effects must be assessed in order to evaluate the suitability of the composite material chosen for the application. Radiation is known to affect PMC materials both by breaking polymeric chains and by causing changes in molecular constitution due to the action of free radicals. The combination of these two effects characterizes the mechanical behaviour of the irradiated material. But it is not only the characteristics of the radiation source (type, rate, total dose, etc.) that must be considered, because the presence of other environmental factors, especially the temperature, can be added to the radiation effect, yielding an unforeseen synergistical influence on the results. In this work, the behaviour of short beam shear specimens of a carbon/epoxy material submitted to γ-irradiation up to 10 MGy and tested at temperatures ranging from −60 to 130°C is presented.