18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS RECENT PROGRESS IN RESISTANCE-BASED DAMAGE SENSING OF CARBON NANOTUBE-FIBER COMPOSITES A.S. Wu, T-W. Chou*, E.T. Thostenson Center for Composite Materials and Department of Mechanical Engineering University of Delaware, Newark, USA * Corresponding author (chou@udel.edu) Keywords : damage sensing, dynamic behavior, composite materials 1 Introduction 0.25% after which microcracks form. Upon failure, the conductive network is severed, resulting in a Sensing the onset of local damage, such as matrix drastic increase in resistance. cracking, delamination, fiber pull-out and breakage in composite materials is challenging. The initiation 2.2 Compression Loading of small-scale damage which leads to ultimate While it is intuitive that damage accrued during failure of a structure is particularly difficult to detect, tensile loading will result in an increase in resistance since the mechanical behavior of a composite part is across a composite, it is interesting and relevant to not noticeably affected by this. In recent years, examine the electrical behavior of composite several different approaches towards non-invasive materials under compression and shear loadings as damage detection, including ultrasonic C-scanning, well. The response of an axially loaded disc-shaped X-ray imaging and acoustic emission measurement, specimen (20 ply, 45º off-axis E-glass/epoxy) with have been utilized. The current work focuses on embedded carbon nanotubes to quasi-static resistance-based damage sensing in electrically compressive loading is plotted in Fig. 2 [4]. conductive composites using carbon nanotubes. In Resistance is measured across the specimen these materials, damage initiation and propagation diameter and is observed to decrease during linear will sever the conductive network, resulting in elastic loading. During damage accumulation, increases in electrical resistance which can be applied load pins cracks closed, causing resistance to measured in situ . increase disproportionally. Only after unloading does resistance increase substantially, accurately reflecting the amount of damage accumulated. 2 Electrical Percolation for in situ Sensing 2.3 Impact Loading Over the past 25 years, the modification of The effect of impact damage on the electrical composites using conductive fibers to achieve response of a composite panel with an electrically- electrical percolation has been extensively studied percolating carbon nanotube network is evaluated [1-2]. In order to sense the onset of local damage, using a drop-weight tower [5]. Electrical resistance the scientific base has progressed from micron-sized measured across the composite panel increases after conductive fibers to carbon nanotubes. Due to the each successive impact, indicating the formation of high aspect ratios of the nanotubes, electrical damage within the composite. percolation can occur in polymer resins at concentrations below 0.1 wt.% [3]. 2.1. Tension Loading 3 Processing of Carbon Nanotube Composites We have demonstrated that resistance-based sensing A major challenge associated with the processing of is highly sensitive to tension-induced damage [3]. In carbon nanotube-based composites is achieving a Fig. 1, significant increases in resistance are high degree of dispersion without sacrificing aspect observed across the composite at strains above ratio. Toward this end, Thostenson and Chou [6]
adopted a calendering method in which a three-roll Efforts including matching specimen and bar cross- mill is used for nanotube dispersion. sectional area, as well as selecting an appropriate specimen thickness and appropriate bar material Recently, an alternative method of processing were made in order to ensure that specimens carbon nanotube composites using a nanotube-based achieved stress equilibrium during Hopkinson bar sizing agent has been developed [7]. This method testing (Fig. 4) [4]. results in composites with localized concentrations of carbon nanotubes at the fiber surface and is 4.2 Dynamic Behavior of Carbon Nanotube- effective in sensing matrix cracking. Furthermore, Based Composites this method has proven to be highly adaptable for Fig. 5. depicts the progression of electrical resistance processing thick-section composites [7]. over time, measured across the diameter of a specimen impacted multiple times via Hopkinson bar loading. Permanent increases in resistance occur 4 New Research Area - Dynamic Behavior after impact loadings #5-7 in Fig. 5; this behavior Due to their complex, rate-dependent mechanical corresponds to a loss of specimen stiffness which is behavior, research into damage sensing in observed in the stress-strain behavior during these composites under dynamic loadings is of great impact cycles [4]. As the striker bar velocity (and interest. Recent efforts at studying the dynamic thus the impact energy) is increased, the specimen compressive behavior of these materials (identical in resistance also increases accordingly (Fig. 6) [4]. morphology to those evaluated under quasi-static Upon failure due to delamination (Fig. 7), resistance compression) using the split Hopkinson pressure bar increases drastically. The sensitivity of resistance- apparatus have been undertaken. based damage sensing is demonstrated through this study and efforts into measuring the high-rate 4.1 Split Hopkinson Pressure Bar Experiment resistance changes occurring during impact are In the split Hopkinson pressure bar experiment, a underway. specimen is sandwiched between two rigid, cylindrical, long (length >> diameter) bars, referred 40 to as the incident and transmission bars, typically assumed to behave in a linear elastic manner [8]. A 150 striker bar (less than one-fourth of the incident bar length) is propelled using a gas gun toward the 30 incident bar free end [9]. The resulting impact causes a compression wave to travel to the bar- σ (MPa) damage initiation 100 ∆ ∆ R (k Ω ∆ ∆ specimen interface. Depending on experimental and 20 material factors (specimen thickness, bar/specimen σ σ σ Ω Ω ) Ω acoustic impedance ratio), a fraction of the compression wave is reflected through the incident 50 10 bar as a tensile wave while the remainder passes through the specimen to the transmission bar [10]. damage propagation The incident ( σ ), reflected ( σ ) and transmitted I R 0 0 ( σ ) pulses (measured via strain gages mounted far 0.0 0.2 0.4 0.6 0.8 1.0 T from the bar-specimen interface) are plotted in Fig. ε (%) ε ε ε 3. Time-shifting these pulses to the bar-specimen Fig.1. Mechanical and electrical response of a interface and applying classic Kolsky data reduction [0/90] s carbon nanotube/E-glass/vinyl ester [11] (which assumes linear elastic behavior of the composite under quasi-static tensile loading [3] bars and stress equilibrium of the specimen) enables the determination of specimen stress and deformation rate during the experiment, based on the incident, reflected and transmitted pulses.
200 200 100 80 σ I 150 150 80 60 σ T 100 100 σ I + σ R 60 40 σ (MPa) σ (MPa) ∆ 50 50 ∆ ∆ ∆ R (k Ω 20 σ σ σ σ σ σ 40 Ω Ω Ω ) 0 0 0 σ R -50 -50 damage propagation 20 -20 -100 -100 0 0 50 100 150 200 0 50 100 150 200 250 300 t (s) t ( µ µ s) µ µ Loading Fig. 4. Demonstration of equilibrium Fig.2. Mechanical and electrical response of a development time [4]. The sum of the time- carbon nanotube/E-glass/SC-15 composite specimen shifted incident σ and reflected σ pulses is I R under quasi-static compression loading [4] equal to the time-shifted transmitted σ pulse T for a carbon nanotube-based composite 80 specimen. incident σ I transmitted 60 50 σ T Gradual damage after 1000 s 40 45 Delamination σ (MPa) 20 40 6 σ σ σ 35 0 Ω ) R (k Ω Ω Ω 5 4 30 3 1 -20 2 σ R 25 7 -40 0 200 400 600 800 1000 20 Contact with support t ( µ µ s) µ µ Fig. 3. Bar stress calculated using strain gage 15 0 500 1000 1500 measurements in real time. Incident, reflected and t (s) transmitted pulses (here, compressive stress is positive) are denoted on the graph. Fig. 5. Resistance increase after multiple impacts for a carbon nanotube/E-glass/SC-15 composite [4] 3
Recommend
More recommend
