Fracture Behavior of Heavily Cross-Linked Epoxy: Testing, Fractography, and Numerical Modeling

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
A. Y. Elruby ◽  
Stephen M. Handrigan ◽  
Sam Nakhla
Keyword(s):  
Numerical Modeling ◽  
Fracture Behavior ◽  
Failure Criteria ◽  
Image Correlation ◽  
Unstable Fracture ◽  
Extended Finite Element ◽  
Testing Procedures ◽  
Failure Initiation ◽  
Different Types ◽  
Failure Predictions

Abstract Heavily cross-linked epoxy was characterized under different types of loading. The scope of work involves detailed testing procedures utilizing high-precision digital image correlation (DIC) system for all strain measurements. Fractographic analyses using scanning electron microscopy (SEM) were also provided. Besides, computed tomography (CT) scans were employed to characterize existing manufacturing imperfections, i.e., voids. Numerical modeling using extended finite element method (XFEM) utilizing the actual microstructure is conducted. Testing results and fractographic analyses showed that microvoids led to failure initiation at micro lengths. An unstable fracture behavior dominated the final failure under different types of loading. Global plastic deformation was observed in the case of uniaxial tension, while local plasticity was observed in specimens under three-point loading. It can be concluded that epoxies failure under a combined state of stresses is sophisticated, and straightforward stress/strain-based failure criteria are not well-suited for failure predictions.

Interlaminar/interfiber failure of unidirectional glass fiber reinforced composites used for wind turbine blades

2012 ◽  
Vol 47 (3) ◽  
pp. 353-368 ◽  
Author(s):  
Martin Leong ◽  
Lars CT Overgaard ◽  
Isaac M Daniel ◽  
Erik Lund ◽  
Ole T Thomsen
Keyword(s):  
Glass Fiber ◽  
Turbine Blades ◽  
Axial Loading ◽  
Strain Curve ◽  
Failure Criteria ◽  
Image Correlation ◽  
Fiber Direction ◽  
Stress Strain ◽  
Failure Initiation ◽  
Unidirectional Glass

A unidirectional glass fiber/epoxy composite was characterized under multi-axial loading by testing off-axis specimens under uniaxial tension and compression at various angles relative to the fiber direction. Iosipescu shear tests were performed with both symmetric and asymmetric specimens. Tests were performed on both 1-2 and 1-3 material coordinate planes. Strain gauges and Digital Image Correlation were used to record the stress–strain responses. A new approach was used to define a ‘failure initiation strength’ by analyzing the recorded stress–strain curves. The experimentally determined failure stresses were compared with the predictions of the maximum stress, Tsai-Wu and Northwestern University failure criteria. It was found that using the approach of analyzing the stress–strain curve to define a point of material failure initiation, it was possible to obtain good correlation between the experimental data and predictions by both the Tsai-Wu and the NU failure criteria.

scholarly journals An experimental study on the manufacture and characterization of in-plane fibre-waviness defects in composites

2018 ◽  
Vol 5 (5) ◽  
pp. 180082 ◽  
Author(s):  
W. J. R. Christian ◽  
F. A. DiazDelaO ◽  
K. Atherton ◽  
E. A. Patterson
Keyword(s):  
Residual Strain ◽  
Composite Structures ◽  
Computational Models ◽  
Image Correlation ◽  
Strain Fields ◽  
Strain Measurements ◽  
Ultrasound Measurements ◽  
Residual Strains ◽  
Failure Predictions

A new method has been developed for creating localized in-plane fibre waviness in composite coupons and used to create a large batch of specimens. This method could be used by manufacturers to experimentally explore the effect of fibre waviness on composite structures both directly and indirectly to develop and validate computational models. The specimens were assessed using ultrasound, digital image correlation and a novel inspection technique capable of measuring residual strain fields. To explore how the defect affects the performance of composite structures, the specimens were then loaded to failure. Predictions of remnant strength were made using a simple ultrasound damage metric and a new residual strain-based damage metric. The predictions made using residual strain measurements were found to be substantially more effective at characterizing ultimate strength than ultrasound measurements. This suggests that residual strains have a significant effect on the failure of laminates containing fibre waviness and that these strains could be incorporated into computational models to improve their ability to simulate the defect.

scholarly journals NUMERICAL PREDICTIONS AND MECHANICAL TESTING OF BRAIDED COMPOSITE STRUCTURES UTILISING DIGITAL IMAGE CORRELATION

2016 ◽  
Vol 7 ◽  
pp. 43
Author(s):  
Emil Pitz ◽  
Matei-Constantin Miron ◽  
Imre Kállai ◽  
Zoltán Major
Keyword(s):  
Numerical Model ◽  
Digital Image Correlation ◽  
Digital Image ◽  
Composite Structures ◽  
Experimental Testing ◽  
Single Layer ◽  
Failure Criteria ◽  
Image Correlation ◽  
Braided Composites ◽  
Numerical Predictions

The current paper is describing the implementation of a multiscale numerical model for prediction of stiffness and strength in braided composites. The model is validated by experimental testing of single-layer braided tubes under torsional loading utilising digital image correlation (DIC). For the numerical model the entire braided structure is modelled at yarn detail level, taking into account the yarn behaviour as well as individual yarn-to-yarn interactions by using cohesive contact definitions. By means of Hashin’s failure criteria and cohesive contact damage, failure of the yarns and failure of the yarn-to-yarn interface is being accounted for. Thereby the material failure behaviour can be predicted. For validation of the model, torsion tests of biaxially braided single-layer composite tubes were performed. The strain distribution at the specimen surface was studied using the DIC system ARAMIS in 3D mode.

scholarly journals Moving Assessment Online: Experiences within a School of Pharmacy

2021 ◽  
Vol 25 (1) ◽  
Author(s):  
Kelsey Morgan ◽  
Erin Adams ◽  
Teresa Mary Elsobky ◽  
Marcia Brackbill ◽  
Amber Darr
Keyword(s):  
Clinical Skills ◽  
Internet Access ◽  
Open Book ◽  
Testing Procedures ◽  
Student Stress ◽  
Patient Interviews ◽  
Testing Strategies ◽  
Different Types ◽  
Physical Examinations ◽  
Assessment Procedures

The COVID-19 pandemic required academic institutions to quickly transition to online learning and make changes to assessment procedures. This study examines how a school of pharmacy creatively approached the challenge of online assessment while maintaining the standards necessary to prepare practice-ready student pharmacists. To conduct traditional exams, instructors deployed two different types of methods utilizing testing software: a video conferencing technology approach which mimicked pre-pandemic, on-campus proctored exams; or open-book, internet access-enabled exams that ensured academic integrity and rigor through various testing strategies. To assess students’ clinical skills, faculty used a combination of techniques such as physical examinations, patient interviews, and patient presentations. To understand the student experience with these assessments, students were surveyed using a 12-item questionnaire. Overall, online video proctoring maintained consistency in exam structure and administration, but required extensive instruction for both students and proctors. Students preferred unproctored, open-book, internet access-enabled, standard time exams versus proctored, closed-book, internet-access disabled, extended time exams. Changes to testing procedures, whether with proctored or unproctored methods, appeared to increase student stress.

scholarly journals Open-Hole 3D Braided Composites Strength Prediction and Stress Analysis

2020 ◽  
Vol 38 (4) ◽  
pp. 889-896
Author(s):  
Shuangqiang Liang ◽  
Chenglong Zhang ◽  
Ge Chen ◽  
Qihong Zhou ◽  
Frank Ko
Keyword(s):  
Failure Analysis ◽  
Progressive Failure ◽  
Failure Criteria ◽  
Element Analysis ◽  
Braided Composite ◽  
Failure Initiation ◽  
Geometry Model ◽  
Progressive Failure Analysis ◽  
3D Braided Composites ◽  
Open Hole

The stress concentration caused by notches is a common engineering issue for composite structure application. 3D braided composite possess excellent damage tolerance compared to common laminates. The tensile properties of 3D braided composite with open-hole and un-notched were experimentally examined. The mechanic properties of 3D braided composite in other directions are predicted using FGM (Fabric Geometry Model) and finite element analysis. The stress distributions around the hole and perpendicular to the loading direction are analyzed based on Abaqus software. The simulation results were compared with Lekhnitskii's analytical study. The open-hole strength of 3D braided composite was predicted respectively using Average stress failure criteria, Point stress failure criteria (PSC), and also the progressive failure analysis based on different failure criteria. The predicted strength results were compared to the experimental values. The results show the PSC predicted strength matched the experiment, while the progressive failure analysis can predict the failure initiation, propagation and final failure mode.

scholarly journals Strength characterization of glass/epoxy plain weave composite under different biaxial loading ratios

2020 ◽  
Vol 54 (19) ◽  
pp. 2549-2563
Author(s):  
A Kobeissi ◽  
P Rahme ◽  
L Leotoing ◽  
D Guines
Keyword(s):  
Biaxial Loading ◽  
Central Zone ◽  
Correlation Method ◽  
Axial Loading ◽  
Failure Criteria ◽  
Image Correlation ◽  
Biaxial Test ◽  
Cruciform Specimen ◽  
Plain Weave ◽  
Biaxial Tests

Over the past years, various studies have been investigated in order to characterize the behavior of composite materials under different multi-axial loading conditions. One of the most used biaxial techniques is the in-plane biaxial test on cruciform specimens. To achieve reliable biaxial failure results, the design of the cruciform specimen presents a crucial part. Previous studies show that there is no well-adapted cruciform geometry for the composite biaxial tests. In this paper, an optimal cruciform specimen has been defined numerically for the composite characterization test. The specimen is composed of two aluminum tabs glued on top and bottom side of the plain-weave glass/epoxy composite. Finite element simulations have been carried out in order to study the influence of the aluminum grade and thickness on the stress distribution in the composite. An experimental validation confirms the failure of the specimen in the central zone under three different biaxial tensile ratios. The experimental strains were evaluated using the digital image correlation method. The traction/traction quadrant of the failure envelop was obtained and compared with different failure criteria. The maximum strain criterion shows a good agreement with the experimental results.

scholarly journals Comparison of Different Heat Treatment Processes of Selective Laser Melted 316L Steel Based on Analysis of Mechanical Properties

Materials ◽  
2020 ◽  
Vol 13 (17) ◽  
pp. 3805 ◽  
Author(s):  
Janusz Kluczyński ◽  
Lucjan Śnieżek ◽  
Krzysztof Grzelak ◽  
Artur Oziębło ◽  
Krzysztof Perkowski ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Hot Isostatic Pressing ◽  
Tensile Tests ◽  
Fracture Analysis ◽  
Image Correlation ◽  
Treatment Processes ◽  
Different Types ◽  
316L Steel ◽  
Precipitation Heat

In this study, we analyzed the mechanical properties of selectively laser melted (SLM) steel obtained via different modifications during and after the manufacturing process. The aim was to determine the effects of precipitation heat treatment on the mechanical properties of elements additively manufactured using three different process parameters. Some samples were additionally obtained using hot isostatic pressing (HIP), while some were treated using two different types of heat treatment and a combination of those two processes. From each manufactured sample, a part of the material was taken for structural analysis including residual stress analysis and microstructural investigations. In the second part of the research, the mechanical properties were studied to define the scleronomic hardness of the samples. Finally, tensile tests were conducted using a digital image correlation (DIC) test and fracture analysis. The treated samples were found to be significantly elongated, thus indicating the advantages of using precipitation heat treatment. Additionally, precipitation heat treatment was found to increase the porosity of samples, which was the opposite compared to HIP-treated samples.

scholarly journals Investigation by Digital Image Correlation of Mixed Mode I and II Fracture Behavior of Metallic IASCB Specimens with Additive Manufactured Crack-Like Notch

Metals ◽  
2020 ◽  
Vol 10 (3) ◽  
pp. 400 ◽  
Author(s):  
Ivo Campione ◽  
Tommaso Maria Brugo ◽  
Giangiacomo Minak ◽  
Jelena Janković Tomić ◽  
Nebojša Bogojević ◽  
...  
Keyword(s):  
Digital Image Correlation ◽  
Digital Image ◽  
Mixed Mode ◽  
Fracture Behavior ◽  
Selective Laser Sintering ◽  
Local Stress ◽  
Fatigue Loading ◽  
Image Correlation ◽  
Mode I ◽  
Displacement Fields

This work investigates the fracture behavior of maraging steel specimens manufactured by the selective laser sintering (SLS) technology, in which a crack-like notch (sharp notch) was directly produced during the additive manufacturing (AM) process. For the evaluation of the fracture toughness, the inclined asymmetrical semi-circular specimen subjected to three points loading (IASCB) was used, allowing to cover a wide variety of Mode I and II combinations. The effectiveness of manufacturing crack-like notches via the SLS technique in metals was evaluated by comparing the obtained experimental results with the ones obtained with pre-cracks induced by fatigue loading. The investigation was carried out by using the digital image correlation (DIC) technique, that allowed the evaluation of the full displacement fields around the crack tip. The displacement field was then used to compute the stress intensity factors (SIFs) for various combinations of Mode I and II, via a fitting technique which relies on the Williams’ model for the displacement. The SIFs obtained in this way were compared to the results obtained with the conventional critical load method. The results showed that the discrepancy between the two methods reduces by ranging from Mode I to Mode II loading condition. Finally, the experimental SIFs obtained by the two methods were described by the mixed mode local stress criterium.

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