Open-hole Tension Research Articles

Effect of thermal cycling and open-hole size on mechanical properties of polymer matrix composites

This paper investigates the effects of thermal cycling on mechanical degradation of polymer matrix composites (PMCs). Un-notched and open-hole specimens are tested using developed thermal cycling apparatus and tensile test machine. In addition, the hole-size effect of open-hole tension glass/epoxy composite laminates is investigated. The tensile strength, mass loss and surface degradation of the specimens were obtained during 250 cycles. Experimental results showed that the holes diameter is the main parameter to control the thermal cycling effects on open hole structure. Also, it is found that laminates with smaller holes have higher tensile strength variation than those with larger holes. The results showed that increment of the hole diameter and number of cycles decreases the tensile strength.

Comparison of Fracture Characteristics of Open-Hole-Notch Carbon-Fiber-Reinforced Composites Subjected to Tensile and Compressive Loadings

Open-hole tension (OHT) and open-hole compression (OHC) tests were carried out on hot-pressed carbon-fiberreinforced composite samples with a singular open hole. The fracture surfaces of the OHT- and OHC-tested specimens were examined by using scanning electron microscopy (SEM). SEM micrographs showed significant features on the surface of carbon fiber, matrix, and especially in the fiber/matrix interface. Interpretation of these micrographs revealed the possible failure mechanism of composite samples with an open hole under tensile and compressive loadings. Furthermore, a comparative study of these micrographs also pointed to certain specific differences between the fracture characteristics of open-hole composite samples failed under tension and compression. This information is useful in the post-failure analysis of a composite structure.

Investigating the Potential of Using Off-Axis 3D Woven Composites in Composite Joints\u2019 Applications

The effect of circular notch has been evaluated for three different architectures of three-dimensional (3D) carbon fibre woven composites (orthogonal, ORT; layer-to-layer, LTL; angle interlock, AI) through open-hole quasi-static tension and double-lap bearing strength tests in the off-axis (45°) direction. Damage characterisation is monitored using Digital Image correlation (DIC) for open-hole testing and X-ray Computed Tomography (CT) for double-lap bearing strength test. The off-axis notched 3D woven composites exhibits minor reduction (less than 10 %) of the notched strength compared to the un-notched strength. DIC strain contour clearly show stress/strain localisation regions around the hole periphery and stress/strain redistribution away from the whole due to the z-binder existence, especially for ORT architecture. Up to 50 % bearing strain, no significant difference in the bearing stress/bearing strain response is observed. However when ORT architecture was loaded up to failure, it demonstrates higher strain to failure (~140 %) followed by AI (~105 %) and lastly LTL (~85 %). X-ray CT scans reveal the effect of the z-binder architecture on damage evolution and delamination resistance. The study suggests that off-axis loaded 3D woven composites, especially ORT architecture, has a great potential of overcoming the current challenges facing composite laminates when used in composite joints’ applications.

Experimental validation of an empirical nonlinear shear failure model for laminated composite materials

This paper aims to develop a numerical nonlinear progressive damage model for laminated composite materials considering in-plane and out-of-plane shear stresses in combination with cohesive interface elements to predict the structural response and the failure mechanisms of laminated composite materials. For this purpose, the constitutive models for intralaminar and interlaminar damage modes have been developed as a numerical code by a UMAT subroutine and implemented in commercial finite element software. This model, which is based on the continuum damage mechanics approach, enables to predict the gradual degradation of material properties with five distinct damage parameters for different failure modes; three of these damage factors apply the shear damage contribution as a separate damage mode by a separate damage factor into the model and characterize it by shear damage dissipation energy, and two parameters for fiber and matrix in transverse directions. Also, a series of experiments have been performed to characterize and validate the nonlinear behavior of glass/epoxy laminate. This model is used to predict the behavior and the final strength of open-hole tension specimens. A reasonably good agreement was also achieved between numerical predictions and experimental observations in terms of shapes, orientations and sizes of individual intraply damages induced around the notch and also the final strength of open-hole tension specimen.

Mechanical properties of carbon nanotubes based polymer composites

Abstract The objective of this paper was to understand the effect of carbon nanotubes (CNT) additives on the elastic behaviors of textile based composites. The materials consist of three phases namely, carbon fibers fabric, Epoxy matrix and carbon nanotubes. Different volume fractions of CNTs were used (0% as reference, 0.5%, 1%, 2% and 4%). A set of mechanical tests as Open Hole Tension, shear Beam Test and Flatwise Tension tests were performed. A damage initiation and cracks propagation in composite specimens were controlled. The experimental results show an increase the mechanical performance of the composite up to 2% of CNT additives. However, beyond this value, the material strength shows a significant decay.

Progressive failure analysis on scaled open-hole tensile composite laminates

Despite the rapid advances of numerical methods and theoretical models for progressive failure analysis of composites, it’s still a challenge to predict the strength and damage progression of composite laminates under open-hole tension (OHT). One of the main obstacles is to capture the true stress concentration at the hole edge. It has been found that the formation of longitudinal splitting at early loading stage alleviates the extremely high stress concentration. The purpose of this study is to develop an efficient progressive damage model, employing surface-based cohesive contacts for longitudinal splitting and delamination, to predict the thickness size effect of sublaminate scaled and ply-level scaled laminates under OHT. It is found that by using aligned mesh with the fiber direction for each ply, a good correlation with experimental results can be achieved for both strengths and failure modes of the laminates.

In-situ measurements of fracture toughness properties in composite laminates

Prediction of matrix failure in composite laminates can be accomplished using computational methods based on material strength and fracture toughness. Engineering theories and failure criteria, which use such material properties generated by current standard experimental methods, may result in poor accuracy of predicting structural strength and fatigue characteristics due to complexity of deformation and failure mechanisms in composites. In this work the applicability of fracture toughness properties measured using standard test methods is investigated. A customized X-ray Computed Tomography system with a 50kN axial force capacity integrated load frame, enabling full scanning under load, is utilized for accurate 3D in-situ measurements of crack geometry in composite Double Cantilever Beam specimens and Open Hole Tension specimens. These experiments are used to determine Mode I and Mode II fracture toughness based on finite element analysis of models transferred from the tomographic measurements. While measured interlaminar Mode I fracture toughness is in good agreement with standard tests, intralaminar Mode II fracture toughness is shown to depend on crack length.

Laminate damage model for composite structures

A continuum damage model for the prediction of the nonlinear response and the collapse of laminated composite structures is proposed. During laminate hardening the material model considers fiber rotation, plastic strain accumulation and inter-fiber damage to provide accurate ply stress information even at large deformations. In contrast to current continuum damage models for fiber reinforced composites, softening is defined on laminate-level instead of ply-level. Once ultimate failure is predicted, the stiffness of the whole laminate is linearly degraded in consideration of the fracture angle and the energy dissipation. This allows to use the model in combination with layered elements, and thereby its application on structural components. The presented model is validated against open-hole tension tests with different hole-diameters and shows excellent correlation and mesh-size independency. Moreover, sensible extensions of the presented model and further investigations are discussed.

EFFECT OF HYBRIDIZATION ON OPEN-HOLE TENSION PROPERTIES OF WOVEN KEVLAR/GLASS FIBER HYBRID COMPOSITE LAMINATES

Hybrid laminates consisting of woven Kevlar/glass fiber composite plies were studied in terms of their residual tensile strength, stiffness and fracture surface. Residual tensile strength and stiffness were determined from the open hole tension test according to ASTM D5766. The laminates of Kevlar fiber reinforced polymer (KFRP), glass fiber reinforced polymer (GFRP) and hybrid of Kevlar-glass fiber reinforced polymer (KGFRP) were fabricated using a vacuum bagging process. Three different ratios of Kevlar to glass fiber plies were prepared in this study which were 20:80, 50:50, and 80:20. Results showed that hybrid laminate consisting of 80:20 Kevlar to glass fiber plies, produced higher residual tensile strength and stiffness when compared to the other hybrid system. Furthermore, strength and stiffness of hole specimens were reduced within 50-63% when compared to unhole specimens due to existence of the hole. In addition, the effect of adding nanosilica to the hybrid system was also studied. 5 wt% of nanosilica was added to the hybrid composite laminates and results showed that higher tensile strength and stiffness was observed in GFRP and 20:80 KGFRP specimens, while the tensile strength was decreased with an increased number of Kevlar fiber. This research was conducted as there are limited number of studies that have been done on the tensile strength of woven hybrid composite laminates so far, especially on hybridization of Kevlar and glass fiber with consideration on the effect of hole and addition of nanofillers.

Tensile strength of open-hole, pin-loaded and multi-bolted single-lap joints in woven composite plates

The prediction of the tensile strength of multi-bolted joints uses characteristics that are obtained from the open-hole tension (OHT), bolt-filled hole tension (FHT), pin-loaded tension (PLT) and single-bolt single-lap joint (BJ) tests. However, the relative relevance of each of these tests to multi-bolted joints is not clear. This investigation aims to fill the gap by performing these tests on carbon/epoxy and glass/epoxy laminates with quasi-isotropic and cross-ply configurations and on an Al-6065 aluminum alloy. It is found that the highest strength achieved by a multi-bolted joint corresponds to the OHT strength. The number of bolts required to achieve this upper bound depends on the material characteristics. The Al-6065 alloy achieves the OHT strength with two bolts, whereas the composites require up to four bolts. Narrower specimens require fewer bolts to achieve the OHT strength. The stiffness and strength of the BJ and PLT are comparable for Al-6065. However, for the composites, BJ has a lower stiffness but a higher strength than PLT. The pin contact force triggers delamination initiation and propagation in the PLT, whereas the tightened bolt in the BJ suppresses the delamination. In addition, the rotation of the bolt explains the lower stiffness of the BJ.

스마트 폰과 이미지 상관법을 이용한 무선 원격 계측 알고리즘 개발

Article history: A smart phone is a multimedia device that is a necessity for modern people. It includes a wireless networking system to share information and pictures. However, numerous smart phones are discarded every year, since they have a very fast technology development cycle. This paper presents the development of a telemetry algorithm to measure displacement and strain with a discarded smart phone and digital image correlation methods. To implement the measurement algorithm, the LabVIEW 2010 program development platform was used. In order to verify reliability, an open hole tension test was conducted using a smart phone and a universal test machine. In addition, the measurement results from the smart phone were compared with FEM analysis results.

Investigation into the effects of fiber waviness in standard notched composite specimens

This study presents a numerical and experimental evaluation of the standardized material testing specimens (tension and compression) used for determining the strength of composite materials in the presence of defects. The composite specimens contain through the thickness fiber waviness and a circular cutout. The intentional waviness levels have been applied in the out-of-plane direction of the multi and unidirectional laminate during the curing process. The standardized face-stabilized open-hole compression test based on ASTM D6484 and the open-hole tension test according to ASTM D5766 are used for evaluating the interaction of the fiber waviness and the circular cutouts. Temporal evaluations of the load-deformation response in the specimens are coupled with optical microscopy to understand the failure modes and damage progression. Laminates with multidirectional layups show different failure modes and a different damage trajectory when compared with the response in the unidirectional laminates. The damage trajectory is dominated by the notch region and also influenced by notch size and free edge effects.

Deformation gradient tensor decomposition for representing matrix cracks in fiber-reinforced materials

A new method is presented for the representation of matrix cracks in continuum damage mechanics (CDM) models for fiber-reinforced materials. The method is based on the additive decomposition of the deformation gradient tensor into ‘crack’ and ‘bulk material’ components, analogous to the additive strain decomposition of the smeared-crack approach. The potential improvements to the accuracy of CDM models that utilize the presented method are demonstrated for a single element subjected to simple shear deformation and for a unidirectional open-hole tension specimen. The presented method avoids load transfer across matrix cracks and eliminates the prediction of spurious secondary failure modes that occurs when conventional strain-based CDM models are used in geometrically nonlinear finite element analyses involving large shear deformations.

Open-hole tension capacity of pultruded GFRP having staggered hole arrangement

The technique of using staggered bolt holes for connections is common practice in steel design since the method allows for greater design strengths from more compact connections. Steel however is a ductile isotropic material. Pultruded glass fiber reinforced polymer (GFRP), on the other hand, is a relatively brittle, often highly anisotropic material requiring different design formulations to assess its open-hole tension capacity. This study considers the development of open-hole capacity provisions and experimentally investigates the net section rupture limit state of GFRP plates having both a single gage line and staggered-hole arrangements in order to assess the open-hole capacity of pultruded GFRP plates. Tension testing of GFRP plates, having varying thickness, number of holes and hole-spacing arrangements (both staggered and non-staggered) were conducted. A significant strength reduction (beyond the net cross section area) results from the presence of a circular hole in the material, although additional holes do not compound this effect. The test results demonstrate that, unlike in isotropic materials, there is no capacity benefit from using staggered hole arrangements in GFRP plates.

Finite element guidelines for simulation of fibre-tension dominated failures in composite materials validated by case studies

This paper presents a finite element modelling methodology to predict the initiation and damage progression in notched composite laminated plates subjected to increasing in-plane tension load. An important feature of the methodology is it does not rely on customized user-subroutines but solely on the analysis capabilities of the general purpose software Abaqus; thus ensuring that the numerical results can be universally reproduced. The methodology presented copes with intralaminar failure modes and uses the Hashin failure criterion to predict the onset of failure (cracking). To account for damage progression after crack initiation there is a fracture energy calculated for each of four failure modes. Four open-hole laminated plates taken from the literature are used for benchmark examples. The predicted ultimate strength based on the analytically-obtained stress-displacement curve was found to be within 10% of the experimental observations. To study the influence of the interaction of having two or three holes across the mid-plane of a pultruded open-hole tension specimen, a parametric study was carried out. The paper ends giving guidelines for the generalized modelling methodology using Abaqus without user-subroutines.

Stress and structural damage sensing piezospectroscopic coatings validated with digital image correlation

The piezospectroscopic effect, relating a material’s stress state and spectral signature, has recently demonstrated tailorable sensitivity when the photo-luminescent alpha alumina is distributed in nanoparticulate form within a matrix. Here, the stress-sensing behavior of an alumina-epoxy nanoparticle coating, applied to a composite substrate in an open hole tension configuration, is validated with the biaxial strain field concurrently determined through digital image correlation. The coating achieved early detection of composite failure initiation at 77% failure load, and subsequently tracked stress distribution in the immediate vicinity of the crack as it progressed, demonstrating non-invasive stress and damage detection with multi-scale spatial resolution.

Finite Element Guidelines for Simulation of Delamination Dominated Failures in Composite Materials Validated by Case Studies

This paper presents a finite element modelling methodology to predict the initiation and damage progression in notched composite laminated plates subjected to increasing in-plane tension load. An important feature of the methodology is it does not rely on customized user-subroutines but solely on the analysis capabilities of the general purpose software Abaqus; thus ensuring that the numerical results can be universally reproduced. The methodology presented copes with intralaminar failure modes and uses the Hashin failure criterion to predict the onset of failure (cracking). To account for damage progression after crack initiation there is a fracture energy calculated for each of four failure modes. Four open-hole laminated plates taken from the literature are used for benchmark examples. The predicted ultimate strength based on the analytically-obtained stress-displacement curve was found to be within 10% of the experimental observations. To study the influence of the interaction of having two or three holes across the mid-plane of a pultruded open-hole tension specimen, a parametric study was carried out. The paper ends giving guidelines for the generalized modelling methodology using Abaqus without user-subroutines.

Open hole fatigue characteristic and probabilistic model for fatigue life prediction of CCF300/QY8911 and T300/QY8911 composite laminates

This study investigated open-hole tension fatigue characteristic of carbon fiber-reinforced composite laminates subjected to constant amplitude cyclic loading. Two kinds of composite laminates, the CCF300/QY8911 and T300/QY8911, were used for fatigue testing with maximum loading of 70% notched tensile strength. Ultrasonic C-scan method was employed for fatigue damage evolution in composite laminates investigation, and specimens showed that the cracks initiated at the edge of the hole and free edge before final fracture. For most of existing fatigue models for determining material degradation and life of composite laminates have tended to be limited in application to specific materials, laminate stacking sequence and loading conditions; this paper proposed a probabilistic model for fatigue life prediction. This model utilized the equality relationship of failure probability between fatigue life and residual strength for fatigue life connection establishing under different loading stresses, by which the fatigue life or reliability life could be evaluated by the static tensile testing data that was one of the critical ultimate stress point on the S–N curve. In this paper, the tension-dominated fatigue lives of notched CCF300/QY8911 and T300/QY8911 composite laminates were evaluated by this model. To compare with the experimental data, the predicted results showed a good agreement.

A new stress-based multi-scale failure criterion of composites and its validation in open hole tension tests

A new stress-based multi-scale failure criterion is proposed based on a series of off-axis tension tests, and their corresponding fiber failure modes and matrix failure modes are determined at the microscopic level. It is a physical mechanism based, three-dimensional damage analysis criterion which takes into consideration the constituent properties on the macroscopic failure behavior of the composite laminates. A complete set of stress transformation, damage determination and evolution methods are established to realize the application of the multi-scale method in failure analysis. Open-hole tension (OHT) specimens of three material systems (CCF300/5228, CCF300/5428 and T700/5428) are tested according to ASTM standard D5766, and good agreements are found between the experimental results and the numerical predictions. It is found that fiber strength is a key factor influencing the ultimate strength of the laminates, while matrix failure alleviates the stress concentration around the hole. Different matchings of fiber and matrix result in different failure modes as well as ultimate strengths.

Definition of a Constitutive Material Model for Progressive Failure Analysis of Composite Laminates

The aim of this work is to introduce a novel constitutive material model for the progressive failure analysis of composite laminates able to predict the Gradual degradation of laminates up to their complete failure. As a relevant added value with respect to state of the art constitutive models, the presented model uses energy considerations to avoid mesh and time-step dependences. In order to prove its effectiveness and accuracy, the proposed material constitutive model has been implemented in the FE software ANSYS© as a User defined material subroutine (USERMAT) and used to simulate the mechanical behavior of an Open Hole Tension specimen. The obtained numerical results have been compared to experimental literature data and to the numerical outputs of Instantaneous and Standard Gradual Degradation Models.