Through-thickness Compression Research Articles

Numerical modeling and experimental validation of lamina fracture and progressive delamination in composite dovetail specimens under tensile loading

In this work, a numerical simulation was conducted to investigate the lamina fracture and progressive interlaminar failure behavior of a dovetail specimen of composite fan blades. Through-thickness compression (TTC) will strengthen the interphase properties as the applied pressure increases and degrade the tensile strength combined with the in-plane shear stress, which greatly affects the overall tensile behavior on the composite dovetail specimen. This was realized with the aid of ABAQUS user subroutines VUSDFLD and VUMAT, along with an experimental validation. To demonstrate the performance of the proposed approach, two sets of tensile tests were conducted on specimens with different stacking sequences with a digital image correlation (DIC) device to investigate the deformation configurations, as well as the load–displacement curves of the gauge section. The obtained results exhibited a remarkable agreement with the simulation results, particularly in terms of the first delamination load and lamina fracture load. To further validate these observations, a sensitivity analysis was carried out. This analysis took into consideration the uncertainties associated with parameters such as the longitudinal Young’s modulus, interphase properties, friction coefficient, and the position of the contact pad.

Analysis of interfacial de-bonding failure on stiffened composite panel

orThis article aimed at the interfacial de-bonding between the stringer and skin when the structure is under post-buckling loads. A four-point bending test was set up, and the cohesive zone model (CZM) was employed considering the effects of through-thickness compression on the inter-laminar shear failure of composite laminates. The numerical result is in good agreement with the experimental one. The results indicate that: embedded defect at the flange/skin interface has a slight influence on the load capacity of the structure, but embedded defect at the flange/radius-filler interface has a significant influence on the load capacity of the structure, and delamination always initiated at the flange/radius-filler interface. The presence of inner corner defects in the filled area has a greater impact on the load capacity of the structure, the load capacity is in direct proportion to the width of corner defects.

Fibre reinforcement and stacking sequence influence on the through-thickness compression behaviour of polymer composites

A combined experimental and numerical study was performed to investigate the influence of multi-axial stress states and reinforcement type on the constitutive response of fibre reinforced epoxy polymers. The constitutive behaviour of unidirectional and cross-ply layups of carbon and glass fibre reinforced epoxy resin, IM7/8552 and S2/8552 respectively, was investigated. Prismatic short block specimens at different off-axis angles were tested under quasi-static through-thickness compression. The experiments were simulated with an enhanced non-linear material model based on shear localisation planes and pressure dependency, which represents the elastic and plastic experimental behaviour better than other formulations. The results show that the overall constitutive response is highly dependent on the fibre reinforcement type and multi-axial stress state on the mesoscale. In order to obtain an accurate description of the material behaviour, measurement aspects such as determining the correct orthotropic elastic constants, especially the Poisson’s ratios, are highlighted. Furthermore, particular focus is also drawn on important modelling aspects such as material non-linearity and pressure effects.

A newly proposed damage constitutive model for composite laminates under low-velocity impact by considering through-thickness compression damage

The existing damage constitutive models for a composite lamina under impact loading have mainly considered four failure modes: longitudinal fiber failure in tension and compression, matrix failure in tension and compression along the transverse direction. However, when subjected to an impact load in the out-of-plane direction, through-thickness compression failure may also occur in laminates. Therefore, the present paper has established a damage constitutive model considering through-thickness compression failure and has compared it to those constitutive models without such consideration to see the improvement in the impact response analysis and to discuss the necessity of including through-thickness compression damage in the simulation. To this end, impact testing was conducted on manufactured composite panels, and the results were used to validate the finite element simulation of composite laminates under impact. The results showed that the constitutive model with the compression failure mode in the thickness direction had higher accuracy in predicting the impact force, displacement, damage, and energy absorption of laminates. The predicted results by the developed constitutive model showed that through-thickness compression damage indeed occurred in the laminates during the impact process, and it was mainly concentrated in the layers near the impact side. In the layers near the rear side, the tension damage of longitudinal fiber and matrix mainly initiated and developed in the annular region around the impact center on the laminates.

Comparison of the effectiveness of simplified experimental approaches to studying through-thickness compression effect on interlaminar shear properties of woven CFRP

A correct assessment of the stress-strain state and the strength of composites requires both reliable material deformation models verified by the experimental results and correct failure criteria for the full realization of the material strength potential. It is difficult to assess the material strength due to the complexity of biaxial tests and non-obviousness of the obtained results, especially in the case of biaxial loading in the out-of-plane direction. The aim of this study is to compare simplified experimental methods for determining the interlaminar shear properties of composites under through-thickness compression. The combination of inter laminar shear and through-thickness compression stresses was varied in two ways: by changing the cut angle of cubic specimens and by loading V-notched specimens at different angles in a new device.

Determination of Interlaminar Shear Properties of Fibre-Reinforced Composites under Biaxial Loading: A New Experimental Approach.

The complexity of biaxial tests and analysis of their results makes it difficult to study the interlaminar shear properties of fibre-reinforced composites, particularly under through-thickness compression, which occurs in thick-walled composite elements. The improvements in experimental methods to study the features of the nonlinear behaviour of composites under biaxial loading is now an important and relevant task in the development aircraft structural elements made of carbon fibre-reinforced polymers. This study aimed to develop a new experimental approach for the reliable determination of the interlaminar shear properties of laminates under through-thickness compression using a standard testing machine. An appropriate V-notched specimen was developed based on the configuration of well-known Iosipescu and butterfly-shaped specimens. The approach is demonstrated using woven carbon/epoxy laminates. Both the preliminary assessment of the stress fields under combined compression/shear loading and the analysis of fracture mechanisms were performed with finite-element modelling in a three-dimensional formulation. The digital image correlation (DIC) method was used to obtain experimental, full-field deformations of the specimens and to estimate the uniformity of the strain distribution in the gauge section. The stress–strain curves were obtained under biaxial loading, and the corresponding features of the composite failure behaviour were analysed in detail. It was found that the maximum compression strain on the stress–strain curves, in some cases, corresponded to the discontinuity in the composite structure. In these cases, the disproportionate changes in through-thickness strains in the gauge section of the specimens were recorded at the maximum load. With the increase in through-thickness compression stresses, the difference between the shear strength values, determined by the maximum load and the maximum compressive strain, increased by up to 20%. It was shown that the assessment of the composite strength at maximum load at the design stage significantly increased the risk of premature failure of the composite elements during exploitation.

Experimental investigations on the acoustical performance of open-celled metal foams under grazing flow

Open-celled metal foams offer a possible solution for implementing acoustic liners in non-traditional nacelle locations, such as directly over the turbofan rotor. Previously, we have shown that the normal incidence absorption performance of metal foams can be further improved by compressing them to reduce their effective pore size. In this paper, we focus on the acoustical performance of compressed metal foams under grazing flow conditions. Experimental tests were conducted using a grazing flow impedance tube set up within Spirit AeroSystems' acoustics lab. Metal foams with four different compression ratios were tested under various flow rates and incident sound pressure levels. The sound pressure measurements were then utilized to educe their effective impedance under grazing flow conditions. Our results show that a uniform through-thickness compression can improve the acoustic performance of open-celled metal foams under grazing flow conditions and can aid the further development of acoustic liners for over the rotor applications.

Post-buckling failure analysis of composite stiffened panels considering the mode III fracture

Fully utilizing the post-buckling capacity of stiffened composite panels is important to reduce the weight of aeronautical composite structures. However, owing to the complex post-buckling failure modes and mechanisms, there are no effective analysis methods. This paper explores the impacts of the mode III fracture energy on the damage behavior of the stiffener–skin interface of composite stiffened panels under post-buckling conditions. To characterize the bonding interface damage, an interface damage initiation criterion considering the effects of through-thickness compression on the shear failure was used and combined with the Reeder damage extension criterion, which considers the mode III fracture. The mode III fracture toughness characteristics of the damage evolution of the stiffener–skin interface were obtained using the edge crack torsion (ECT) test method. Moreover, the compressive deformation response, buckling load, buckling modes, failure load, and failure modes of the panel were obtained by a compression test of I-shaped stiffened panels. Based on the aforementioned experimental data and numerical model, the post-buckling behavior of the I-shaped stiffened composite panels with an open cross-section stiffener was studied. It was found that the mode III fracture energy caused by the longitudinal shear stress τ31 played a major role in the stiffener–skin interface damage extension of the stiffened panels. Therefore, the mode III fracture behavior should not be ignored in the post-buckling analysis. The numerical analysis method developed can accurately predict the damage initiation and evolution processes of the composite panel interface. The method can be effectively used for post-buckling analysis of aeronautical composite panels.

High Strain-Rate Through-Thickness Compression Testing of Symmetrical Inter-ply Hybrid Polymer Composites Reinforced with Carbon/Glass and Carbon/Kevlar Fibers

High Strain-Rate Through-Thickness Compression Testing of Symmetrical Inter-ply Hybrid Polymer Composites Reinforced with Carbon/Glass and Carbon/Kevlar Fibers

Predicting interlaminar damage behaviour of fibre-metal laminates containing adhesive joints under bending loads

This study includes experimental and numerical investigations on fibre-metal laminate structures containing adhesive joints under static bending loads. Experimental tests were carried out on Glare® 4B specimens manufactured in-house and containing doubler joint features. Numerical analyses were performed using Abaqus software including damage in the glass fibre reinforced polymer layers, ductile damage in the resin pockets (FM94 epoxy) and plasticity in the metal layers. A new cohesive zone model coupling friction and interfacial shear under through-thickness compressive stress has been developed to simulate delamination initiation and growth at the metal/fibre interfaces with the adhesive joint under flexural loading. This model is implemented through a user-defined VUMAT subroutine in the Abaqus/Explicit software and includes two main approaches, firstly, combining friction and interfacial shear stresses created in the interlaminar layers of the fibre-metal laminate as a result of through-thickness stresses and secondly, considering elastic-plastic damage behaviour using a new cohesive zone model based on the trapezoidal law (which provides more accurate results for the simulation of toughened epoxy matrices than the commonly used bilinear cohesive zone model). Numerical results have been validated against experimental data from 4-point bending tests and a good correlation observed with respect to both crack initiation and evolution. Delamination and shear failure were noted to be the predominant failure modes under bending stresses as expected. This is due to the higher mode-II stresses introduced during bending which cause different damage evolution behaviour to that seen for axial stresses. Finite element results revealed that both friction and shear strength parameters generated from through-thickness compression stresses have a significant effect in predicting damage in fibre-metal laminate structures under this type of loading.

Kinematic and mechanical response of dry woven fabrics in through-thickness compression: Virtual fiber modeling with mesh overlay technique and experimental validation

The through-thickness compressive behavior of fabric reinforcements is crucial in liquid composite molding manufacturing processes. Predictive simulations of the compressive response are thus necessary to enable a virtual processing workflow. These are complex however, as the compressive behavior of the reinforcement fabrics is non-linear. Altough virtual fiber modeling has proven to be a strong kinematical tool, it cannot predict the compressive response due to the lack of bending stiffness in the virtual fibers. Here, we describe a solution that enables predictive compressive simulations through hybrid virtual fibers. It is based on an overlay mesh-element technique, combining both (i) finite elements that determine the in-plane fiber properties as well as (ii) finite elements that determine out-of-plane fiber bending. Using these hybrid virtual fibers, the through-thickness compression of a twill woven fabric ply is simulated and experimentally validated using both μCT-based as compliance-based measurements. Excellent agreement between simulation and experiment is obtained for the right set of input parameters.

A delamination failure criterion considering the effects of through-thickness compression on the interlaminar shear failure of composite laminates

Through-thickness compressive stress of composite laminates can inhibit the interlaminar shear failure which has been proved by experiments. However, common delamination damage initiation criterion cannot well describe this mechanism. For this issue, in this article, based on the insight of the failure mechanism, an interlaminar failure function is constructed at first. Then, a delamination failure criterion considering the effect of through-thickness compression on interlaminar shear failure is established based on logical deductions from a polynomial expansion of the failure function. In the criterion, a constant coefficient with physical meaning is used to describe the effect of through-thickness compression on the interlaminar shear failure, and the estimation method of the coefficient is also given in this study. Here, this failure criterion can be considered as a general form to describe the delamination between laminae, by defining different coefficient, it can be converted to other criteria such as the conventional secondary stress failure criterion or the Christensen failure criterion. Finally, by comparing the numerical calculation and experimental results, the delamination failure criterion proposed in this study exhibited a good prediction ability for the interlaminar damage under different loading conditions.

Assessment in predictability of visco-plastic self-consistent model with a minimum parameter approach: Numerical investigation of plastic deformation behavior of AZ31 magnesium alloy for various loading conditions

By predicting the complex deformation mechanisms, visco-plastic self-consistent (VPSC) model has been found to be a useful tool for investigating plastic deformation behavior of magnesium alloy. The standard version of VPSC model involves a large number of adjustable parameters. Using independent arguments obtained from single tests, VPSC model with a minimum parameter approach immensely reduces the number of adjustable parameters, hence shows huge potential in simulating plastic deformation behavior of thin magnesium alloy sheet. The predictability of this new approach is thoroughly assessed in the present study. Although only the mechanical response of in-plane tension (IPT) in AZ31 magnesium alloy is applied to calibrated the corresponding material parameters, simulated results in terms of texture evolution using VPSC model with a minimum parameter approach show a relatively small difference by comparison with the predicted ones applying the standard version of VPSC model during IPT, in-plane compression (IPC), through-thickness compression (TTC), and plane strain compression (PSC), respectively. Furthermore, the corresponding activated deformation mechanisms during various deformation processes are further analyzed. With the exception of pyramidal <c+a> slip, the predicted activities of remaining deformation mechanisms are generally consistent with each other. This phenomenon is identified to be the root of minor difference in texture evolution. Moreover, the predicted activities in pyramidal <c+a> slip using VPSC model with a minimum parameter approach are relatively higher than the corresponding ones in the case with the standard version of VPSC model. However, these reported results are not beyond the maximum of published literature, and hence are acceptable when simulating various plastic deformation behavior of AZ31 magnesium alloy.

Anisotropic plasticity characterization of 6000- and 7000-series aluminum sheet alloys at various strain rates

This study aims to characterize the high strain rate constitutive behavior and the anisotropic plasticity of three high strength aluminum sheet alloys, AA6013-T6, AA7075-T6 and a developmental AA7xxx-T76 alloy. A comprehensive series of mechanical characterization tests were performed under uniaxial tension, simple shear and through-thickness compression (representing equal-biaxial tension) at room temperature and various strain rates ranging from 10−3 to 103 s−1. It was observed that all three alloys exhibited appreciable plastic anisotropy in terms of the measured r-values along various sheet orientations, although they exhibited minimal anisotropy in the stress response. For all three alloys, the flow stress slightly increases with increasing strain rate, which indicates mild positive strain rate sensitivity. However, the flow stress ratios and r-values do not show any explicit relation with respect to strain rate suggesting a negligible effect of strain rate on in-plane plastic anisotropy. Measurement of the constitutive response to large strains (beyond onset of diffuse necking) was achieved using shear specimens that are not subject to onset of necking. To model the flow stress dependence on equivalent plastic strain and strain rate, an extended Hockett–Sherby constitutive model is proposed and shown to fit accurately the experimental data over the studied range of strain and strain rates. The yield behavior of the sheet alloys was predicted using the Barlat YLD2000-2D plane stress yield function (Barlat et al., 2003) assuming both associative and non-associative (NA) flow rules, as well as for general stress states employing two 3D anisotropic models, Barlat YLD2004 (Aretz and Barlat, 2004) and modified–Yoshida (Lou and Yoon, 2018). An associative flow rule was assumed for both of the higher order 3D yield functions considered herein. The measured flow stress ratios and r-values from the uniaxial tension and equal-biaxial tension experiments, along with the stress ratios from the shear tests, were used to determine the coefficients of the yield functions. The calibrated yield functions exhibited good agreement with the experimental data for most loading conditions considered. The higher order Barlat YLD2004 and modified–Yoshida yield functions provided the better predictive accuracy. The non-associative Barlat YLD2000 yield function also performed reasonably well, while the associative Barlat YLD2000 yield function had the lowest accuracy. The constitutive models were implemented into the FE simulation, in order to evaluate the accuracy of the model parameters calibrated in this work.

Comparative study on yield behavior and non-associated yield criteria of AZ31B and ZK61 M magnesium alloys

The mechanical properties, especially the yield behavior, of AZ31B and ZK61 M magnesium alloys were comparatively investigated in this study. Series of uniaxial tension, uniaxial compression, shear and through-thickness compression (equi-biaxial tension) tests were conducted to obtain yield strengths and Lankford ratios (r values) under different strains and angles to their rolling directions and further the anisotropic characteristics as well as the tension-compression asymmetric features of the materials were analyzed. Furthermore, different yield criteria were adopted to represent yield behaviors of two magnesium alloys and their applicability on such magnesium alloys were compared. Results show that both magnesium alloys show apparent anisotropic and tension-compression asymmetric yield behavior. Compared with AZ31B, ZK61 M performs more remarkably in the anisotropic behavior and more intricately in tension-compression asymmetry and hardening features with changes of angles to the rolling direction and strain. The associated yield criteria of Hill1948, Yld2000-2D and CPB06 cannot concurrently represent anisotropic plastic yield and flow behaviors of AZ31B and ZK61 M magnesium alloys well; while the corresponding yield criteria based on non-associated flow rule can ideally describe these two aspects meantime. Among them, under tension and shear conditions, the non-associated Yld2000-2D criterion provided the most accurate predicting results than the other criteria. Regarding the tension-compression asymmetric characteristics of magnesium alloys, the non-associated model based on the CPB06 was introduced which makes concurrent prediction of tension-compression yield stress and Lankford ratio of these two materials possible. As a result, the comprehensive prediction capability of this model is fairly outstanding among all of three models. This study can provide basis for selection of plastic models, calibration technique and further application in numerical modeling.

Mechanical Deformation of Human Optic Nerve Head and Peripapillary Tissue in Response to Acute IOP Elevation.

PurposeTo measure the deformation of the human optic nerve head (ONH) and peripapillary tissue (PPT) in response to acute intraocular pressure (IOP) elevation.MethodsThe ONH and PPT of 14 human donor globes were imaged with high-frequency ultrasonography during inflation testing from 5 to 30 mm Hg. A correlation-based speckle tracking algorithm was used to compute tissue displacements, and the through-thickness, in-plane, and shear strains were calculated by using least-squares strain estimation methods. The ONH and PPT were segmented along the anterior-posterior direction and the nasal-temporal direction. Regional displacements and strains were analyzed and compared.ResultsThe ONH displaced more posteriorly than the PPT in response to an acute IOP increase. Scleral canal expansion was minimal but correlated with ONH posterior displacement at all IOP levels. Through-thickness compression was concentrated in the anterior of both the ONH and the PPT. Shear was concentrated in the vicinity of the canal with higher shear in the peripheral ONH than the central ONH and higher shear in the PPT near the scleral canal than that further away from the canal.ConclusionsHigh-resolution ultrasound speckle tracking showed a displacement mismatch between the ONH and the PPT, larger compressive strains in the direction of IOP loading in the anterior ONH and PPT, and higher shear strains in the periphery of ONH in response to acute IOP elevation in the human eye. These findings delineate the deformation patterns within and around the ONH and may help understand IOP-associated optic nerve damage.

Measurements on pressboard to understand the effect of solid insulation condition on monitoring of power transformer winding clamping pressure

Pressboard is the main building material of a power transformer winding structure. Through-thickness compression behaviour of pressboard has a direct influence on the transformer winding clamping pressure. Being a cellulose based polymeric material with a porous structure, the through-thickness compression behaviour of pressboard is highly non-linear. Also, the mechanical properties of pressboard are moisture, temperature, and ageing status dependent. However, information on the exact behaviour of the through-thickness compressibility of pressboard under typical operating conditions of a power transformer has not been well documented in the literature. This has posed difficulties in developing reliable techniques to detect loose clamping conditions in power transformer windings. This study presents the results of laboratory measurements on the effect of moisture, temperature and ageing on the through-thickness compression behaviour of oil-impregnated pressboard. From the measured results, it was found that the compressibility of pressboard could increase with increasing temperature, ageing and moisture ingress. It was also found that moisture and ageing tend to exaggerate the thermal softening of pressboard. Subsequent finite element analysis confirmed that transformer clamping pressure has significantly altered with the pressboard condition.

Effect of coining on springback behaviour

Especially due to utilization of high strength steels in automotive parts, accurate prediction and reduction of springback have gained more importance in the recent years. One of the widely used approaches to reduce the springback in deep drawn parts especially in and around the bent regions is coining (restrike operation). In this method, the sheet material in the springback-relevant regions is compressed between two rigid tools having clearences less than the sheet thickness. By this way, compressive stresses and strains are superimposed and therefore the bending effects are reduced. During coining, plastic deformation occurs in the thickness direction. In order to predict this behavior in process planning numerical simulations should be capable of considering the effects of through-thickness compression. In order to analyze this effect, axisymmetrical deep drawing tests were performed. In a further step, the bottom surface or the radius region of the geometry was compressed between the punch and additional tools. To analyze the springback behaviour, drawn geometries were cut along the rolling direction, so that the residual stresses are released. The same procedure was also simulated with shell elements having enhanced formulation to account for through-thickness deformation. Experimental springback results show that the coining process influences the springback and this can be captured by the enhancements in the shell formulation.

Regional Deformation of the Optic Nerve Head and Peripapillary Sclera During IOP Elevation.

PurposeTo measure the deformation of the porcine optic nerve head (ONH) and peripapillary sclera (PPS) in response to intraocular pressure (IOP) elevation.MethodsHigh-frequency ultrasound was used to image the ONH and PPS of 12 porcine eyes during ex vivo inflation testing from 5 to 30 mm Hg. A speckle tracking algorithm was used to compute tissue displacements in the anterior-posterior direction and expansion of the scleral canal. Through-thickness, in-plane, and shear strains were calculated within the ONH. Regional displacements and strains were analyzed and compared.ResultsThe ONH and PPS showed overall posterior displacement in response to IOP elevation. Posterior displacement of the ONH was larger than and strongly correlated with the posterior displacement of the PPS throughout inflation testing. Scleral canal expansion was much smaller and leveled off quicker than ONH posterior displacement as IOP increased. Through-thickness compression was concentrated in the anterior ONH, which also experienced larger in-plane and shear strains than the posterior ONH. Within the anterior ONH, all three strains were significantly higher in the periphery compared with the center, with the shear strain exhibiting the greatest difference between the two regions.ConclusionsHigh-resolution ultrasound speckle tracking revealed the full-thickness mechanical response of the posterior eye to IOP elevation. A mismatch in posterior displacement was found between the ONH and PPS, and regional analyses showed a concentration of strains within the periphery of the anterior porcine ONH. These deformation patterns may help in understanding IOP-associated optic nerve damage and glaucoma susceptibility.

Yield locus prediction using statistical and RVE-based fast Fourier transform crystal plasticity models and validation for drawing steels

A methodology is presented to predict yield loci of drawing steels with virtual mechanical tests using (i) the statistical ALAMEL crystal plasticity model (VEF software of KU Leuven) and (ii) an approach based on representative volume elements (RVE’s) solved with a spectral numerical Fast Fourier Transform (FFT) technique (DAMASK software of MPIE). In the latter case, the RVE’s are defined in the form of a regular mesh of grid points in 3D-space with a crystal orientation assigned to each grid point. The experimental texture is derived from XRD pole figure measurements. Only tensile curves in the rolling direction are used to calibrate the hardening model included in the crystal plasticity models. For an interstitial free steel the predictions are compared to experimental yield loci, as obtained from uniaxial tests, plane strain tests, shear tests and through-thickness compression tests. Analytical FACET yield loci are calibrated to the crystal plasticity (CP) models and these are used in FE simulations of a square cup deep drawing process, validated by experiments.