Curved Beam Specimen Research Articles

A Rate-Temperature-Dependent Visco-Hyperelastic Constitutive Model for UD CF/PEEK Prepregs During a One-Step Hot Stamping Forming Process

An anisotropic visco-hyperelastic constitutive model for rate-temperature-dependent deformation during one-step hot stamping forming simulation of unidirectional (UD) CF/PEEK prepregs is presented. This constitutive model is based on strain energy decomposition and a multiplicative decomposition of the deformation gradient. Two simple Maxwell models are used to characterize the viscoelastic behavior of the melted PEEK matrix and longitudinal shear deformation, respectively, and a shear invariant of [Formula: see text] is proposed to calculate the shear deformation. Moreover, the fiber stretching deformation is modeled by an anisotropic hyperelastic model. To obtain the model parameters, tensile tests at different strain rates and temperatures above the melt temperature of PEEK are performed on [Formula: see text], [Formula: see text], and [Formula: see text] CF/PEEK prepreg specimens, respectively. In parallel, the [0]8 and [45]8 curved beam specimens are experimentally fabricated to validate the constitutive model. The VUMAT subroutine is developed according to the proposed constitutive model and applied for a [Formula: see text] off-axis tensile simulation and hot-stamping forming simulation of CF/PEEK prepregs. The experimental and simulation results show that the materials flow, distribution of strain and stress, forming defects (wrinkles and overlap) of CF/PEEK curved beam can be captured by the proposed model.

Experiments on the mode II fracture toughness in ENF tests of CFRP curved beams

The purpose of this study is to establish end-notched flexure (ENF) tests of curved beam specimens cut from tanks or cylinders. To provide data reduction methods for experiments, a compliance-based beam method (CBBM) for curved beams in ENF tests is proposed based on the previously established Euler–Bernoulli curved beam theory formulations of ENF tests. ENF tests were conducted using curved beam specimens with three different curvature radii, and the results were in good agreement with the curved beam theory. In addition, the fracture toughness values obtained from the proposed CBBM for curved beams were validated using three-dimensional finite element (FE) analysis. The results revealed that the FE analysis with the measured fracture toughness agreed well with the experimental load–displacement curves, verifying the validity of the obtained fracture toughness values for curved beams.

УЛЬТРАЗВУКОВОЙ КОНТРОЛЬ ОБРАЗЦОВ В ПРОЦЕССЕ РАЗРАБОТКИ И ИСПЫТАНИЙ НОВЫХ МАРОК УГЛЕПЛАСТИКА

Nondestructive testing results of carbon fiber reinforced plastics (CFRP) specimens at the development and testing process are given at that paper. Ultrasonic pulse-echo technique is the most applicable for CFRP monolithic panels and curved beam specimens testing. It is shown that ultrasonic testing allows optimizing the mode in the process of developing molding modes for new CFRP grades. At the test stages it allows to exclude defective specimens from the testing process and estimate the sorts and damage sizes after the tests.

Effect of internal heating on delamination properties of deicing composite curved beams under four-point bending

The deicing composite curved beams can easily experience delamination under internal heating, but fewer studies have been found so far. In this study, the effect of internal heating on the delamination properties of deicing composite curved beams under four-point bending was investigated experimentally and numerically. The composite curved beam specimens were prepared according to the ASTM D6415 standard. A heating circuit in a heating layer was designed to generate heat and tested in compliance with the standard. A three-dimensional (3D) finite element (FE) model was established with cohesive elements for the interfaces of adjacent plies. In the experiments, the load–displacement curves, strain-load curves, and delamination properties of the curved beam specimens in two environments were compared and analyzed. The numerical results agreed well with the experimental data, and the FE model can predict the delamination precisely. The average curved beam failure load of internal heating was about 36% lower than the ones of other environments, and the internal heating can cause the position of delamination to change significantly.

Static and vibration characteristics of thin-walled box beams: An experimental investigation

Static and vibration characteristics of thin-walled straight and curved box beams were investigated experimentally. Three different beam configurations were considered for the tests: one straight and two curved box beams. The load was applied at the centroid of the box section for the straight and one curved beam specimens. However, for the other curved specimen, the load was applied eccentrically to investigate its behavior under the additional torsion induced by the eccentricity. Displacements and strains were obtained using linear variable displacement transducer, one-directional and rosette strain gages. The specimens were excited using an impact at their free ends. The time history of strains was obtained to calculate natural frequencies and damping ratios. The experiment results were compared with those obtained from three-dimensional finite element analysis for all cases. The results obtained from implementing tests on the straight specimen were also used to validate an efficient numerical method recently developed by the authors.

Investigation about the fracture behavior and strength in a curved section of CF/PP composite by a thin-curved beam specimen

A curved beam specimen with a curved section bent at a right angle and straight arms is an efficient experimental method to evaluate the interlaminar tensile strength of a composite laminate. This paper describes a technique to apply such a test method to determining the fracture behavior and strength in a curved section of a thin laminate. The proposed technique is capable of estimating appropriately the compressive or tensile strength as well as the interlaminar tensile strength at failure in a thin-curved section, especially targeting thermoplastic composite materials. The significant point of the proposed technique is a modified calculating method of the stress distribution in a curved section by taking into account the large deformation of the specimen because of thin laminate.

Modelling anomalous moisture uptake, swelling and thermal characteristics of a rubber toughened epoxy adhesive

Experimental studies have been undertaken to characterise the diffusion, swelling and thermal characteristics of a rubber toughened epoxy adhesive. Gravimetric experiments were carried out for three different thicknesses of bulk adhesive. The bulk adhesives were exposed to 81.5%, 95.8% RH and water immersion, all at 50°C. Dual stage moisture uptake profiles were observed for thin specimens at all levels of exposure. It was found that a dual stage uptake model fits the anomalous uptake response excellently. The swelling of the adhesive was characterised using the same exposure environment. Swelling was slow at low levels of moisture uptake and increased gradually before becoming linear with increasing moisture uptake. The initial slow swelling of the adhesive was attributed to the diffusion of water into the existing free volume. A moisture-dependent swelling coefficient was determined using finite element modelling, where the dual stage uptake model and moisture-dependent mechanical properties of the adhesive were taken into account. The thermal expansion of the bulk adhesive was determined using a bi-material curved beam specimen. The curvature of the beam reduced when it was exposed to a wet environment. This behaviour was successfully modelled using coupled moisture diffusion and stress analysis incorporating the derived moisture-dependent swelling behaviour.

Bonded joints with through-thickness adhesive stresses – reinforcing the F/A-18 Y470.5 bulkhead

A bonded composite reinforcement is being developed for the Royal Australian Air Force F/A-18 Y470.5 bulkhead crotch region. Due to the curvature of the surface in this area any bonded reinforcement was predicted to have high through-thickness stresses present in the adhesive under load. A representative specimen, the “curved beam specimen” (CBS) was designed using finite element analysis to identify the stress state in the joint and determine the through-thickness stresses in the adhesive. The specimen was then mechanically tested to failure under static loads and also under spectrum fatigue loading to determine the viability of the repair. The data from the above program was used to determine an estimate for the through-thickness stress design allowable for the structural film adhesive Cytec FM73. It is suggested that the CBS will prove to be a suitable specimen for providing generic data on the through-thickness static and fatigue strength of adhesives.

Edge delamination in curved (04/±456)s glass-fibre/epoxy beams loaded in bending

An approach to predicting delamination due to the free-edge effect in a curved beam specimen loaded in bending is presented. Stress-based criteria do not give satisfactory results because of the stress singularity at the free edge. The conventional fracture-mechanics approach for free-edge delamination cannot be used because the strain-energy release rate does not reach an asymptotic value. An improved approach is developed on the basis of an assumed initial defect. The strain-energy release rate is calculated by using finite elements and this is then used to predict the failure load. Good correlation with the experimental results is obtained.

Interlaminar tensile strength (ILTS) measurement of woven glass/polyester laminates using four-point curved beam specimen

Based on a review of the current methods for the measurement of interlaminar tensile strength (ILTS), a novel specimen of a four-point curved beam is evaluated for this purpose. Detailed finite element analysis is carried out to investigate the appropriateness of the data interpretation formula. A simple design method to choose the appropriate parameters in order to guarantee the delamination failure is provided. This specimen is used to measure the ILTS of woven glass/polyester laminates. The results are found to be comparable with other methods. The most attractive advantage of this test method is its simplicity and that it requires no special techniques in specimen preparation and test set-up.

Interlaminar tension strength of graphite/epoxy composite laminates

An L-shaped curved beam specimen and a tension loading fixture were proposed to measure the interlaminar tension strength of laminated and textile composites. The specimen size was 2 X 2 in. (51 X 51 mm). The use of a standard tension test machine and the introduction of load nearly at the specimen midthickness were the advantages of the proposed specimen. Modified Lekhnitskii and beam theory equations for calculating interlaminar stresses of an L-beam were verified by finite element analysis. The beam theory equation is simple and accurate for mean radius to thickness ratios greater than 1.5. The modified Lekhnitskii equations can be used for detailed stress field calculation. AS4/3501-6 graphite/epoxy unidirectional specimens with thicknesses of 16, 24, and 32 piles were fabricated and tested. The delamination initiation site agreed with the calculated maximum interlaminar tension stress location for all three thicknesses. Average interlaminar tension strengths of 16-, 24-, and 32-ply laminates were 47.6, 40.9, and 23.4 MPa, respectively.

A Curved Beam Test Specimen for Determining the Interlaminar Tensile Strength of a Laminated Composite

A curved beam type of test specimen is evaluated for use in determining the through-the-thickness strength of laminated composites. Two variations of a curved beam specimen configuration (semicircular and elliptical) were tested to failure using static and fatigue loads. The static failure load for the semicircular specimens was found to be highly sensitive to flaw content, with the specimens falling into two distinct groups. This result supports the use of proof testing for structural validation. Static design allowables are derived based on the Weibull distribution. Fatigue data indicates no measured increase in specimen compliance prior to final fracture. All static and fatigue failures at room temperature dry conditions occurred catastrophically. The elliptical specimens demonstrated unusually high failure strengths indicating the presence of phenomena requiring further study. Results are also included for specimens exposed to a wet environment showing a matrix strength degradation due to moisture content. Further testing is underway to evaluate a fatigue methodology for matrix dominated failures based on residual static strength (wearout).

Testing for materials selection : Adv. Mater. Process. June 1990 137, (6), 5–16

A bonded composite reinforcement is being developed for the Royal Australian Air Force F/A-18 Y470.5 bulkhead crotch region. Due to the curvature of the surface in this area any bonded reinforcement was predicted to have high through-thickness stresses present in the adhesive under load. A representative specimen, the “curved beam specimen” (CBS) was designed using finite element analysis to identify the stress state in the joint and determine the through-thickness stresses in the adhesive. The specimen was then mechanically tested to failure under static loads and also under spectrum fatigue loading to determine the viability of the repair. The data from the above program was used to determine an estimate for the through-thickness stress design allowable for the structural film adhesive Cytec FM73. It is suggested that the CBS will prove to be a suitable specimen for providing generic data on the through-thickness static and fatigue strength of adhesives.

A curved beam fracture toughness specimen

A curved beam specimen, with outer-to-inner radius ratio of 1·31, is investigated for fracture toughness testing of material from cylindrical components. Experimental stress-intensity factor calibrations, obtained from fracture tests and compliance measurements, show good agreement with theoretical data from modified mapping-collocation and finite element analyses, and also with the proposed standard C-shape calibration transposed to satisfy three-point bending conditions. The specimen achieves considerable economy of size over a straight specimen, and can have reduced crack length and width without detriment to its performance. Practical aspects of testing this specimen are considered.