EFFECTS OF TEMPERATURE ON FIBER TENSION FRACTURE TOUGHNESS OF COMPOSITE LAMINATES AT HIGH LOADING RATE

The understanding of fracture toughness associated with fibre dominated tensile failure is of great important for safety design of composite structrures threatened by extreme loading conditions, such as high/cold temperature and high rate loading. The dynamic fracture toughness of composite laminates in fibre tension is characterized under different temperatures (e.g. -55 °C, 23 °C and 90 °C) with compact tension (CT) sample, at loading rate of 8 m/s using a tension Hopkinson bar intergrated with a experimental chamber. Digital image correlation (DIC) with high-speed imaging is employed for obtaining the full-field strain fields and crack tip location.
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INTERFACE CRACK OR DELAMINATION: WHEN & WHERE TO INITIATE? HOW TO PROPAGATE & HOW BIG AREA TO ATTAIN?

After inserting a matrix, secondary layer in between the two adjacent primary layers of the bimaterial, the overall load causing the weakest secondary layer element to fail can be even bigger than the bimaterial’s strength. The stresses in the element must be untrue. In this paper, two modifying coefficients (MCs) are applied to modify the stresses, which are then substituted into a strength failure equation of the matrix. If it is fulfilled, the failed secondary layer element is deleted, and an interface crack occurs within the two adjacent primary layer elements. Continued in this way step by step, all of the interface failure information is available without iteration. Only the critical displacements at the peak loads of a DCB (double cantilever beam) and an ENF (end notched fracture) tests are required as inputs in addition to the material properties of the two primary layers and the secondary layer.
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EXPERIMENTAL AND NUMERICAL STUDY ON THE DELAMINATION BEHAVIOUR OF INTERLEAVED COMPOSITES WITH AUTOMATED TAPE LAYING

Automation of composite materials manufacturing is an important pre-requisite to upscaling the manufacturing without diminishing quality of the end-products. In this research, interleaved composites have been manufactured by modified skip tow automated tape laying (ATL) process, where each tape can cross many tapes multiple times. This tape architecture has been shown previously to reduce delaminations through the internal/inherent crack arresting features. Here, Single Leg Bending (SLB) experiments were carried out to study the delamination behaviour of the interleaved composites under quasi-static loading. It was found that, following from delamination initiation, the crack plane propagated with a new crack regularly deviating/migrating away from the primary inter-tapes crack plane towards other inter-tapes interfaces due to the gaps, misorientations and interleaves between the tapes. The multiple delaminations fracture toughness was qualified by the compliance calibration (CC) method in which the fracture toughness was theoretically calculated by crack length back-calculated specimen compliance, and was also numerically modelled by accounting for large-scale fibre bridging. Moreover, both optical microscopy and X-ray computed tomography were performed to examine defects in the specimens before testing and damage after testing. It was observed that the generation of multiple cracks was heavily depended on the local lay-up structure. This suggests
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EFFECT OF PROCESS-INDUCED DEFECTS ON MODE I BEHAVIOR OF PMCS: RANDOM DEFECTS VS. CONTROLLED DEFECTS

With advances in composite manufacturing, the need to establish process-microstructure-property relations remains an ongoing challenge. Effective property predictions, including damage tolerance behavior of advanced composites, often requires explicit modeling of defects and investigating the onset and propagation of damage. The high porosity level is a more commonly encountered defect in heterogeneous composite materials. The current work uses the cohesive zone modeling (CZM) approach adapted for explicit defects in the crack path within a finite element (FE) numerical framework. Random and controlled pore distributions have been modeled and numerically compared. Experimental efforts toward creating controlled pores in the crack path are ongoing. The execution of the current approach will enable better material behavior predictions for advanced composite materials.
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SCALE EFFECTS IN THE POST-CRACKING BEHAVIOUR OF CNT-EPOXY COMPOSITES: PREDICTING CRACK JUMPS AND DUCTILE-TO-BRITTLE TRANSITIONS

The scale effects on the global structural response of fibre-reinforced brittle-matrix specimens subjected to bending are discussed in the framework of Fracture Mechanics by means of the Updated Bridged Crack Model (UBCM). This analytical model assumes the composite as a bi-phase material, in which both the brittle matrix and the reinforcing fibres contribute to the global toughness. In particular, the bridging mechanism of the reinforcing layers can be described by an appropriate cohesive softening constitutive law, which takes into account the progressive slippage of the fibre inside the matrix. In addition, the discontinuous phenomena, i.e., crack jumps (snap-back) and snap-through instabilities, which experimentally characterize the post-cracking behaviour of the composite, can be captured in a quantitative way. Furthermore, UBCM predicts different post-cracking regimes depending on two dimensionless numbers: the reinforcement brittleness number, NP, and the pull-out brittleness number, Nw. Finally, UBCM simulations of non-smooth crack evolutions are compared to experimental results reported in the scientific literature, in which carbon nanotube-epoxy specimens are tested in bending.
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THERMAL BEHAVIOR DURING FRACTURE OF HYBRID EPOXY/CNT/GNP COMPOSITES

This work reports the distinct thermal signatures during failure of epoxy-based nanocomposites comprised of multi-walled carbon nanotubes (MW-CNTs) and graphene nanoplatelets (GNPs). These fillers individually alter the material properties, but their synergy dramatically improves mechanical performance and other multifunctionality. CNT/epoxy, GNP/epoxy nanocomposites are fabricated and compared with the mixed GNP/CNT/epoxy hybrid nanocomposites containing the same weight percentage. Temperature profiles during tension tests have been observed using an infrared thermography (IR) camera yielding distinctive temperature profiles.
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DETECTION OF MODE I INTERLAMINAR CRACK IN CNF DOPED GFRP LAMINATES USING ELECTRICAL IMPEDANCE TOMOGRAPHY

Electrical impedance tomography (EIT) is an emerging structural health monitoring tool for self-sensing composites using the piezoresistive effect. This work presented a novel methodology to detect interlaminar crack or delamination developed during mode I loading in carbon nanofillers doped glass fiber reinforced polymer (GFRP) composites using EIT. DCB specimens were fabricated with 1 wt% carbon nanofillers (CNF) doped GFRP laminates to produce interlaminar crack under mode I loading. The boundary voltage datasets, obtained from electrodes mounted on top and bottom surface of the specimen, were used to solve the EIT inverse problem to get the reconstructed conductivity change map. This methodology demonstrated that interlaminar cracks can be successfully detected using EIT.
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NOVEL SERR-CONTROLLED ENVIRONMENTAL FATIGUE TEST METHODOLOGY FOR ADHESIVE-BONDED LAMINATES

In this paper a novel environmental fatigue test methodology is presented, in which the strain energy release rate (SERR) was controlled during the cyclic experiment at an elevated temperature in humid air. Therefore, an electrodynamic testing machine equipped with a custom-built environmental chamber along with a compatible calculation and controlling software package was used. To determine the crack length based on force and displacement data a compliance-based calibration method was implemented. SERR values were deduced for individual load cycles employing an user defined control channel. The control channel allows for definition of a specific starting point (e.g., 10 J/m²) and an increase rate (e.g., +5 J/m² after 50,000 cycles). By holding the SERR constant over many cycles confidence bands were deduced for the measured crack propagation rates accounting for measurement uncertainties. To corroborate the novel methodology, experiments were conducted on double cantilever beam laminates bonded with soft adhesives in both, a SERR and a displacement controlled mode.
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HEALING OF LAMINATED COMPOSITES AFTER STATIC AND FATIGUE DELAMINATION

In this study, static fracture experiments under mode-I and mixed mode loading, and fatigue testing under mode-I loading were carried out on double cantilever beam (DCB) specimens, and subsequent healing of the delamination was investigated. Thermoplastic healants dispersed in a thermoset CFRP composite were used to perform the healing, triggered through brief heating in an oven. It was observed from the test results that delaminations can be healed efficiently and the healing was found to be repeatable. As a result of healing, significant crack closure was observed and fatigue crack growth rate was considerably reduced. These findings can be helpful in extending the service life of laminated composites.
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INFLUENCE OF THE RANDOMNESS OF FIBER DISTRIBUTION ON THE DISPERSION OF FATIGUE RESPONSE IN STEEL FIBER REINFORCED CONCRETE USING MICRO-COMPUTED TOMOGRAPHY

In this work, the dispersion in the flexural fatigue behavior of steel fiber reinforced concrete is studied. For this purpose, the random distribution of the fibers inside the specimens is analyzed by means of micro-computed tomography. The results reveal that fibers are better positioned in some specimens than in others, which partially explains the scatter of the results. In particular, fiber density or average height around the crack plane show a strong correlation with fatigue life.
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