Open-hole Tensile Test Research Articles

New metal-epoxy-matrix carbon-fibre hybrids to tackle stress concentration

The paper explores the feasibility of improving the performance of composite structures with stress concentrators by local infusion of metals. The resulting material architecture presents Multi-Matrix Continuously-Reinforced Composites (MMCRC) – with shared domains of both the thermoset polymer and metal matrices with fibre-bridged interfaces between the domains. The study explores the manufacturing routes to creating such composites and assesses the resulting performance using the open-hole tensile test. It has been demonstrated that the presence of a rigid multi-matrix patch around the open hole, exhibiting minimal thickness variation, leads to a 15.0% improvement in failure load and a 16.6% improvement in failure strain for quasi-isotropic carbon fibre composite laminates. The strain history analysis of the MMCRC samples indicates the occurrence of plastic yielding within the metal matrix and strain redistribution mechanisms, leading to load sharing over the hybrid matrix area.

Experimental and numerical study of the effects of fiber orientation on stress concentration of CFRE

Carbon fiber reinforced composites are broadly employed, as structural members, in many industrial applications especially in the avionic industry due to their high strength-to-weight ratio. Circular holes are commonly used in these members for fastening, bearing, reducing weight, and others. Therefore, several experimental and analytical studies were introduced to figure out the stress concentration behavior around the vicinity of the holes under different loading conditions. However, no reports are delivered to study the effect of fiber orientation on the stress concentration factor of unidirectional carbon fiber reinforced epoxy (CFRE) composites with a hole under tension. Thus, the current work is targeted to understand the relationship between stress concentration factor (SCF) and fiber orientation in CFRE orthotropic plates. The unnotched and notched composite samples, with four different fiber orientations, were experimentally investigated in a tension test. The SCF of samples was calculated using analytical and computational FE methods. The failure criterion of open-hole tensile (OHT) test samples was also analyzed. The experimental results demonstrated that there is no significant effect of fiber orientation on the normalized strength. Yet based on the stiffness matrix of isotropic materials, a pronounced effect of the fiber orientation on the SCF is detected when using analytical calculations. The present research arises the need for further investigations to find a correlation between experimental and analytical results of SCF in unidirectional CFRE with a central hole at different fiber orientations.

The influence of the hole-generation process on fatigue response of open-hole and assembled titanium samples

The holes required for a bolted connection can be produced in several ways. The defects around the hole and the hole surface finish influence the fatigue life of the produced part. Abrasive waterjet cutting is compared with conventional drilling to assess these influences on titanium alloy parts. Open-hole tensile tests show a preferential fatigue behavior for conventionally drilled samples because of the lower roughness. The originality of this work focuses on linking the surface texture and damage to the life expectancy of the open-hole samples and determining the failure steps/modes of the assembled samples. A model is proposed to track the crack progression that offers adequate life estimations for open-hole samples. Single-shear assembly bearing tests show no difference between the drilling processes, and failure is obtained by secondary bending for both drilling methods.

Comprehensive parametric analyses on the mechanical performance of 3D printed continuous carbon fibre reinforced plastic

This study presents a novel in-house modelling method called “Fibre Path Generator” and comprehensive parametric analyses on the mechanical performance of carbon fibre reinforced plastic (CFRP) with the fibre path along the principal stress direction, using both finite element (FE) simulations and experiments. The reliability of the developed CFRP model is validated through the fabrication of open-hole tensile test specimens using a co-extrusion composite 3D-printer, which demonstrated a highly compatible maximum tensile load with an error less than 3%. A comparative analysis is conducted to assess the mechanical performance of the CFRP specimen with principal stress fibre placement, which result shows that the maximum tensile load achieved is 172.84% higher than a CFRP model with uniaxial fibre placement. Furthermore, comprehensive parametric studies are carried out, varying the carbon fibre location and width-to-depth (w/D) ratio of the open-hole specimen, in order to evaluate the mechanical performance under the tensile loading. This study results provide useful insights to engineers when enhancing the mechanical performance of CFRP composite structures with effective continuous fibre placement.

Enhancing the mechanical performance of notched glass/epoxy composite laminates via hybridisation with thermoplastic fibres

This study examines empirically and theoretically the open-hole tension and compression characteristics of thermoset composites using hybrid glass/polypropylene and glass/innegra yarns. The Abaqus software, finite element analysis (FEA) has been adopted to predict the tensile and compressive failure loads and damage failure modes for un- and notched samples under tensile and compressive strength tests. The commingling technique was used to create hybrid yarns, which were then transformed into non-crimp preforms before the infusion procedure. It was noticed that the inclusion of thermoplastic polypropylene (PP) and innegra fibres increased the ductility of yarns and composites. Densities of hybrid composites were also reduced by up to 20% when compared to pure glass/epoxy composites. Tensile and open-hole tensile (OHT) test results revealed that hybrid composites are less notch sensitive than glass/epoxy composites due to enhanced composite ductility. For glass composites, having holes resulted in a 41% decrease in tensile strength, compared to reductions of only 25% and 26% for PP and innegra fibre composites, respectively. Further, compression and open-hole compression (OHC) tests presented similar results with OHT tests whilst compressive strength reduction was lower for hybrid composites (10% and 11%) compared to glass/epoxy (28%) ones. The results of the numerical simulation for damage failure modes and tensile and compressive failure loads for un-notch and notched composite laminates are consistent with the experimental findings both qualitatively and quantitatively.

DcAFF (Discontinuous Aligned Fibre Filament) – Mechanical properties investigation on multilayer 3D printed parts

DcAFF (Discontinuous Aligned Fibre Filament) is a novel thermoplastic filament developed for 3D printing. This filament is reinforced with highly aligned discontinuous fibres and is based on the High Performance Discontinuous Fibre (HiPerDiF) method which produces thin tapes suitable for a range of different composite manufacturing processes. The HiPerDiF, using fibres longer than the critical length, provides mechanical performance comparable to continuous fibre composites with the high formability typical of short fibre composites. Thanks to the development of the third-generation HiPerDiF machine and the DcAFF filament forming method, circular DcAFF filaments can be produced consistently and at high rates. In this paper, both the physical properties and the internal architecture of the produced filament were investigated. In particular, μCT scanning and image post-processing were used to quantify fibre alignment. The designed filament-forming process ensures that the large fraction of the fibres in the final product are well aligned with the longitudinal axis of the filament. The mechanical properties of the multilayer DcAFF 3D printing part are presented for the first time in this paper with tensile, short beam shear (SBS), and open-hole tensile testing. The comparison with the previous studies and data in the literature shows comparable or indeed superior stiffness of DcAFF over existing methods for 3D printing composite parts; however, to be able to consider this material as a viable candidate for high-performance 3D printing further improvements are required in term of strength, e.g. increasing fibre-matrix adhesion or substituting the proof-of-concept PLA matrix with a high performance one.

Lay-up effect on the open-hole shear strength of composite laminates

The effect of lay-up on the open-hole shear (OHS) strength of quasi-isotropic carbon fibre/epoxy composite laminates was investigated. It was found that laminates with equivalent specific stiffness but different laminate lay-up, i.e. with different arrangements of plies and ply thicknesses, exhibit significant differences in ultimate OHS strength of up to 16.3% and 27.9% for the prepreg material systems IM7/8552 and T800/epoxy, respectively. Using Digital Image Correlation (DIC), X-ray Computed Tomography (CT), and Finite Element Analysis (FEA), it is shown that the lay-up dependent OHS strengths were associated with distinctly different failure modes. These lay-up dependent failure modes are challenging to predict. It is therefore proposed that the OHS test could be used for material screening, and as a new benchmark case for the development and validation of advanced progressive damage modelling frameworks for laminated composites, complementing the commonly used open-hole tensile (OHT) test.

Progressive damage and residual strength of open-hole thin-ply CFRP laminates under tensile fatigue loading

Thin-ply composites are promising structural materials because of their excellent damage resistance and fatigue properties, as well as the advantage of improved design freedom in composite structures. In this study, open-hole tensile fatigue tests were performed on carbon-fiber-reinforced polymer laminates using thin-ply prepregs to investigate the effects of varying ply thickness. Fatigue tests were performed on three types of quasi-isotropic laminates with different ply thicknesses (thin-ply, intermediate-ply, and thick-ply) at several stresses. Fatigue damage growth around a circular hole was examined by evaluating stiffness degradation and internal damage using X-ray radiography. Additionally, the fatigued specimens were subjected to static tensile tests to evaluate the effect of fatigue damage on the residual strength after fatigue tests. The thin-ply laminates can suppress fatigue damage growth and retain the residual strength properties under high-cycle fatigue at high stress level, thus validating their superiority over thick-ply laminates.

Experimental study of notched tensile strength of large open-hole carbon fiber reinforced polymer laminates at low temperature

Carbon fiber reinforced polymer composites used in the aircraft structures often face the problems of open-hole and low temperature. A better understanding of the effects of low temperature and large open-hole on the tensile behaviors of carbon fiber reinforced polymer laminates is highly desirable. In this study the open-hole tensile tests on the carbon fiber/epoxy laminate-T700/SYE20005 with the stacking sequence of [0/90]4S were conducted at both room temperature and low temperature of −60 °C, where the specimens with three different ratios of hole diameter D over the specimen width W were considered. The results showed that the notched strength of the laminates decreases by 54.1% with the increase of the ratio D/W from 0.3 to 0.7 at room temperature. One interesting result is that low temperature greatly improves the strength of laminates with the ratios of D/W = 0.3 and 0.5, producing the increase of 13% in the notched strength, while it has an unobvious effect on the laminate with the ratio of D/W = 0.7. The observations of fracture surface of specimens show that the failure of the laminate with small open hole is mostly dominated by the fiber breakage and axial splits, and the interlaminar delamination and intralaminar matrix cracks are also found for the laminated specimen with large open hole.

Progressive Failure Analysis of Composite/Aluminum Riveted Joints Subjected to Pull-Through Loading

Out-of-plane mechanical properties of the riveted joints restrict the performance of the wing box assembly of airplane. It is necessary to investigate the pull-through performance of the composite/metal riveted joints in order to guide the riveting design and ensure the safety of the wing box assembly. The progressive failure mechanism of composite/aluminum riveted joint subjected to pull-through loading was investigated by experiments and finite element method. A progressive damage model based on the Hashin-type criteria and zero-thickness cohesive zone method was developed by VUMAT subroutine, which was validated by both open-hole tensile test and three-point bending test. Predicted load-displacement response, failure modes and damage propagation were analysed and compared with the results of the pull-through tests. There are 4 obvious characteristic stages on the load-displacement curve of the pull-through test and that of the finite element model: first load take-up stage, damage stage, second load take-up stage and failure stage. Relative error of stiffness, first load peak and second load peak between finite element method and experiments were 8.1%, − 3.3% and 10.6%, respectively. It was found that the specimen was mainly broken by rivet-penetration fracture and delamination of plies of the composite laminate. And the material within the scope of the rivet head is more dangerous with more serious tensile damages than other regions, especially for 90° plies. This study proposes a numerical method for damage prediction and reveals the progressive failure mechanism of the hybrid material riveted joints subjected to the pull-through loading.

Open Hole Tension of 3D Printed Aligned Discontinuous Composites

This paper explores the use of Discontinuous Aligned Fibre Filament (DcAFF), a novel discontinuous fibre reinforced thermoplastic filament for 3D printing, to produce structural complex parts. Compared to conventional composite manufacturing, 3D printing has great potential in steering fibres around small structural features. In this current study, the initial thin carbon fibre (CF)-poly(L-lactic acid) (PLA) tape, produced with the High Performance Discontinuous Fibre (HiPerDiF) technology, is now reshaped into a circular cross-section filament, the DcAFF, using a bespoke machine designed to be scalable to high production rates rather than using a labour-intensive manual moulding method as in previous work. The filaments are then fed to a general-purpose 3D printer. Tensile and open-hole tensile tests were considered in this paper for mechanical and processability of DcAFF. The 3D printed specimens fabricated with the DcAFF show superior tensile properties compared to other PLA-based 3D printed composites, even those containing continuous fibres. Curvilinear open-hole tensile test samples were fabricated to explore the processability and performances of such material in complex shapes. The mechanical performance of the produced specimens was benchmarked against conventionally laid-up specimens with a cut hole. Although the steered specimens produced have lower strength than the fully consolidated samples, the raster generated by the printing path has turned the failure mechanism of the composite from brittle to ductile.

Numerical simulations of combined size effects acting on an open-hole laminated composite plate under tension

Numerical and experimental research programs have been carried out to investigate the effect of scaling on the tensile strength of notched composites. This paper presents a computational study of scaled open-hole tensile tests using the Discrete Ply Model (DPM) method. This finite element model is discrete, and only a small number of parameters are required from experimental characterization tests. Experimental and numerical strength values are compared here, and reveal that DPM simulations tend to slightly overestimate strength values, with an average discrepancy of 9.7%. However, DPM Results show that such modeling simulates both the reduction in strength when specimen size is increased for sublaminate level scaled specimens, where failure is fiber dominated, and the increase in strength when specimen size is increased for ply level scaled specimens, where failure is delamination dominated. In all cases, increasing the total thickness of the specimen leads to a decrease in strength and this effect is dominant over the effect of increasing hole diameter. As well as the variation in strength, three distinct failure mechanisms are observed: fiber failure with extensive matrix damage (pull-out failure), fiber failure with little or no matrix damage (brittle failure) and delamination failure. Comparisons with experiments demonstrate that tensile strengths, damage propagation scenarios and failure patterns are predicted with acceptable accuracy.

Open-hole tensile properties of 3D-printed continuous carbon-fiber-reinforced thermoplastic laminates: Experimental study and multiscale analysis

The mechanical properties of open-hole tensile (OHT) specimens made of 3D-printed continuous carbon-fiber-reinforced thermoplastics (CFRTPs) were investigated. The stacking sequence of the OHT specimens were [0/90]2s, and the as-printed specimens possessed higher porosity (15.19%) than conventional fiber reinforced composites. The OHT tests demonstrated that the tensile modulus and fracture strength of the as-printed specimens exhibited 36.7 ± 0.3 GPa and 226.0 ± 9.0 MPa, respectively. To evaluate the effects of voids on the OHT properties, the 3D-printed CFRTPs were hot-pressed, where fiber orientation and porosity (3.41 ± 0.10%) were improved. Additionally, the tensile modulus was increased to 45.1 ± 0.8 GPa, which is 23% higher than the as-printed specimens, even though the fracture strength were comparable or lower than that of as-printed specimens. To validate such OHT properties, a numerical multiscale model was introduced, with a microscale periodic unit cell (PUC) analysis for determining the effective tensile moduli and mesoscale extended finite element method (XFEM) analysis for OHT properties. In the PUC analysis, we considered a two-scale numerical model including a fiber-resin scale with fiber orientation for effective tensile moduli of a CFRTP filament, and filament-void scale for those of a CFRTP laminate. The porosity and fiber orientation were measured by X-ray computed tomography and digital microscopy observations, and the porosity for the 0° specimen were 14.86%. By substituting the effective tensile moduli of the filament-void scale and those of the fiber-resin scale into the XFEM, respectively, the stress-strain responses of the computational OHT models were found to be in good agreement with those of the experimental results of as-printed and hot-pressed CFRTP, respectively. Both the OHT models showed that the Weibull criterion was satisfied without significant delamination at the failure strain, corresponding to the brittle failure mode due to fiber breakage, which agreed reasonably well with the experimental observations.

In-situ real-time imaging of subsurface damage evolution in carbon fiber composites with shearography

Carbon fiber composites are complex material systems comprising components of different length scales. It is important to understand the interactions of these components and their influence on the overall mechanical properties of the composites for development of new or improved composite materials. However, it is challenging to experimentally image the damage evolution in composites with existing tools. In this paper, we apply shearography technique for real-time in-situ imaging of subsurface damage evolution in carbon fiber composites during open-hole tensile (OHT) tests. Due to its high sensitivity to out-of-plane displacement derivative, shearography can record the initiation and growth of subsurface defects in real-time. Comparison with digital image correlation shows that shearography can provide better defect imaging results. The shearography technique provides a convenient and high-sensitive solution to visualize damage evolution in carbon fiber composites during mechanical tests.

Multi-fidelity progressive damage simulation of notched composite laminates with various ply thicknesses

In this study, a multi-fidelity fracture simulation scheme of composite laminates that accomplishes a seamless transition between medium- and high-fidelity crack models, namely, smeared crack models (SCMs) and discrete crack models (DCMs), is developed. The proposed scheme is based on the adaptive discrete-smeared crack (A-DiSC) method and a theoretical model for saturated crack density. The A-DiSC method first employs a DCM to model the transverse cracks in the laminates. If the crack density of each ply reaches the saturation value calculated using the theoretical model, the DCM is converted to an SCM. At this point, some cracks that are longer than a certain critical length remain explicitly modeled by the DCM, as they can significantly affect the subsequent path to final failure. By combining the two models, this method reduces the computational cost while preserving a sufficient prediction accuracy particularly in the case with numerous noncritical cracks up to failure. A simulation of the open-hole tensile test of composite laminates with various ply thicknesses was performed to verify the prediction accuracy of the developed scheme, and the predicted results were compared with the experimental results.

Application of the Stiffness-Force method to the analysis of a plate in tension with a central hole

In the formulation of the Stiffness-Force method (SFM) are taken into account the equilibrium equations and the compatibility conditions. These are obtained from the principles of virtual work and complementary virtual work. The structure of the governing equation of this method is similar to that of stiffness-method (SM) as the principal unknows are the displacements. This method is equivalent to IFMD formulated by Patnaik and to the hybrid methods formulated by Pian.For the Stiffness-Force method finite element analysis, a user subroutine UEL for a four node quadrilateral element has been developed. This subroutine has been implemented in the commercial program Abaqus. The procedure followed for the analysis of the model has been:1. Analysis of the model using the program Abaqus that employs the SM in its formulation.2. Analysis of the same model using the user subroutine UEL employing the SFM.3. Comparison of the results of both analysis, using varying degrees of discretization, to those obtained by an analytical approach.This procedure has been used in the analysis of the open-hole tensile strength test of a unidirectional composite.

Damage propagation analyses of CFRP laminated OHT test specimens with gap defects

CFRP laminated composite structures manufactured by Automated Fiber Placement (AFP) may have gap and overlap defects and these potentially affect the strength of the structures. This study aims the development of finite element analysis models, which can consider strength reduction due to such defects. This paper proposes the modeling method considering ply-thickness variation induced by gap defects and shows the numerical results obtained by damage propagation analyses for the Open Hole Tensile Test of CFRP laminate with a gap defect.

Quantifying Uncertainty of Damage in Composites Using a Quasi Monte Carlo Technique

Abstract Property variations in a structure strongly impact the macroscopic mechanical performance as regions with lower strength will be prone to damage initiation or acceleration. Consideration of the variability in material property is critical for high-resolution simulations of damage initiation and propagation. While the recent progressive damage analyses consider randomness in property fields, accurately quantifying the uncertainty in damage measures remains computationally expensive. Stochastic damage analyses require extensive sampling of random property fields and numerous replications of the underlying nonlinear deterministic simulations. This paper demonstrates that a Quasi-Monte Carlo (QMC) method, which uses a multidimensional low discrepancy sobol sequence, is a computationally economical way to obtain the mean and standard deviations in cracks evolving in composites. An extended finite element method (XFEM) method with spatially random strength fields simulates the damage initiation and evolution in a model composite. We compared the number of simulations required for Monte Carlo (MC) and QMC techniques to measure the influence of input variability on the mean crack-length in an open-hole angle-ply tensile test. We conclude that the low discrepancy sampling and QMC technique converges substantially faster than traditional MC methods.

Effects of ply thickness and 0°-layer ratio on failure mechanism of open-hole and filled-hole tensile tests of thin-ply composite laminates

Thin-ply technology provides increasing possibilities for composite structures, expanding the design space for ply thickness and laminate layup. This study examines the effects of ply thickness and the 0°-layer ratio on the strength and damage behavior of thin-ply composite laminates. We conducted open-hole tensile (OHT) and filled-hole tensile (FHT) tests on laminates with various ply thicknesses and 0°-layer ratios, observing the internal damage via X-ray radiography. It was confirmed that the damage retardation effect of the thin-ply reduced both the OHT and FHT strengths. Furthermore, the results revealed that laminates with a higher 0°-layer ratio have higher strengths and the failure of laminates with a higher 0°-layer ratio transitioned to splitting-dominated failure. Finite element analysis of the fastener restraint indicated that the fastener restraint alleviated the stress concentration in the 0°-layer around the hole, potentially leading to a difference in the OHT and FHT strengths.

Three notable design strategies for manipulating failure modes in open-hole tensile composite laminates

Different failure mechanisms can lead to very different mechanical responses of notched composite laminates. We are trying to address two scientific problems: how can the occurrence of failure mechanisms be manipulated, and which kinds of failure mechanisms in open-hole tensile (OHT) tests can be manipulated to help improve the ultimate strength of composites? In this paper, three design strategies were proposed to manipulate the failure mechanism of notched composites: decreasing ply thickness at the ply level, hybridizing thin- and thick-ply laminates, and designing helical structures with variable pitch angles and ply thicknesses. Numerical methods are adopted to investigate the influence of the three design strategies on the failure mechanism of notched composites. These three design strategies are not limited to use in an OHT test. The results suggest effective methods for manipulating failure mechanisms to help create laminates with better mechanical properties.