УДК 620.179
CHARACTERISATION OF FIBER FAILURE MODE IN T-700 CARBON FIBER REINFORCED EPOXY COMPOSITES BY ACOUSTIC EMISSION TESTING
M. Ramamanohara Babu*, T. V. Bhanu Prakash**
* Advanced Systems Laboratory, Kanchanbagh, Hyderabad — 500 058, A.P., India ** Department of Marine Engineering, A.U. College of Engineering, Visakhapatnam — 530 003, A.P., India
Abstract. Acoustic emissions generated by a structure under stressed condition provide an insight in to the dynamic behaviour of flaws in the structure for characterization of failure modes. Fiber failure mechanism in T-700 carbon epoxy composites is characterized by testing unidirectional specimens in longitudinal mode. Acoustic emission parameters like amplitude, energy, duration, and signal strength have been recorded and studied with respect to the applied load to assess the fiber failure characteristics. The AE data is analyzed with different correlation plots for visual pattern recognition. Significant fiber breakage is observed at above 70 % of the load. Bi-linear trend of the cumulative amplitude distribution curve indicates distinctively matrix and fiber failures. Matrix cracking failure mechanism dominated the entire loading cycle and is represented by AE hits of up to 85 to 90 dB amplitude and the peak amplitude distribution is 58 to 75 dB. The wave forms of matrix cracking hits with less than 90 dB and 100 units of energy are having up to 273 kHz frequency with a peak around 100 kHz. The wave forms of fiber breakage hits with more than 90 dB and 100 units of energy have up to 448 kHz frequency and with a peak from 168 to 437 kHz. From the low amplitude filtering technique the border line for fiber breakage is observed from 89 to 92 dB.
Key words: polymer matrix composites, failure modes, acoustic emission technique.
ОЦЕНКА ВИДА ПОВРЕЖДЕНИЙ ВОЛОКОН В АРМИРОВАННОМ УГЛЕРОДНЫМИ ВОЛОКНАМИ КОМПОЗИЦИОННОМ
МАТЕРИАЛЕ Т-700 МЕТОДОМ АКУСТИЧЕСКОЙ ЭМИССИИ
М. Рамаманохара Бабу*, Т.В. Бхану Пракаш** * Лаборатория прикладных систем, Канчанбаг, Хайдарабад — 500 058, А.П., Индия ** Отдел морской техники, А. У. Технический колледж, Висакхапатнам — 530 003, А.П., Индия
Акустическое излучение, генерируемое композитной структурой под действием напряжений, дает возможность оценки динамического поведения дефектов в структуре с целью определения вида повреждения. Определен механизм повреждения волокон в армированном углеродными волокнами композиционном материале Т-700 при испытании удлиненных образцов. Параметры акустической эмиссии, такие как амплитуда, энергия, длительность и интенсивность сигналов зафиксированы в зависимости от приложенной нагрузки и исследованы, чтобы получить данные о характеристиках повреждения волокон. Данные акустической эмиссии подвергнуты анализу для выявления корреляции путем построения различных графиков. Существенные нарушения волокон наблюдали при приложении нагрузки, превышающей 70 % от максимальной. Зависимости билинейного типа в суммарной кривой распределения амплитуды четко отличают повреждения волокон и матрицы. Механизмы трещинообра-зования доминировал в течение всего цикла нагружения. Он представлен всплесками акустической эмиссии вплоть до 85—90 дБ по амплитуде и от 58 до 75 дБ по распределению амплитуд. Волновые сигналы от трещинообразования матрицы с энергией менее чем 90—100 дБ имеют частоты до 273 кГц с максимумом около 100 кГц. Волновые сигналы от разрыва волокон имеют энергию более 90—100 дБ и частоту до 448 кГц с максимумом от 168 до 437 кГц. С помощью техники низкоамплитудной фильтрации установлено граничное значение от 89 до 92 дБ для разрыва волокон.
Ключевые слова: полимерные матричные композиты, типы разрушения, метод акустической эмиссии.
* Correspondence author e-mail address: babumrm.1963@gmail.com
1. INTRODUCTION
The carbon epoxy composite materials are being widely used in aerospace industry as structural materials due to their high strength-to-weight ratio and corrosion resistance characteristics. One such application is carbon epoxy filament wound rocket motor casings of solid propulsion systems for aerospace and missile structures where the weight saving contributes to greater payload and range capabilities. Quality control of composites must be an on-going process and it needs continuing research in structural integrity assessment. Structural integrity and the performance of composite structures have greatly improved the quality of the components with the help of nondestructive testing. Non destruction evaluation (NDE) of composite structures is complex in terms of testing and interpretation of the data due to anisotropy and non-homogeneity of the material. Acoustic emission testing is a rapidly developing nondestructive tool which can effectively be used for real time structural health monitoring of complex composite structures under stress [1, 2]. The defects/discontinuities in the structure would grow under stressed condition which can be evaluated by acoustic emission testing (AET). This technique has on line structural integrity assessment feature compare with conventional NDT techniques. It enables to evaluate the flaw type, location and damage severity in the structure under the applied stress. AE is defined as the class of phenomena where by transient elastic waves are generated by the rapid release of energy from localized sources within a material under stress [1]. Ae signals, once generated, will be detected by the AE sensors, which are attached to the material, and sent to the AE data acquisition system for recording and processing.
The major failure modes in composites are observed in terms of matrix cracking, fiber breakage and de-laminations [3, 4]. Therefore AE test data interpretation for composite systems is relatively complex due to different failure modes occurring simultaneously. AE data has been generated on coupon level by tensile testing of T-700 carbon fiber based epoxy unidirectional specimens in longitudinal direction to study the characteristics of fiber failure mechanism. The AE data thus obtained is analyzed with different correlation plots by comparing the various AE parameters like amplitudes, energy, signal strength, duration, rise time etc. with respect to applied load. These parameters can be used for correlating the AE characteristics with different types of failure mechanisms [1].
T-700 carbon fiber impregnated with high temperature curing epoxy resin is used to prepare unidirectional composite laminate by filament winding process on a diamond shaped mandrel. The tensile specimens of 200^30^2 mm sizes cut from the laminate in the longitudinal direction to the fiber as per the ASTM standard [5] as shown in Fig. 1. The tensile test specimens are designated as TUDL. The aluminium tabs are bonded at both the ends for the tensile test specimens to ensure better gripping in the grips of universal testing machine (UTM).
2. EXPERIMENTAL PART
2.1. Sample preparation
Fig. 1. Unidirectional longitudinal tensile test specimens.
2.2. Testing Procedure
M/s. Instron make, 100 KN universal testing machine with closed loop screw driven system is used for carrying out tensile testing and load versus displacement or strain curves are obtained independently. M/s. PAC, USA, make acoustic emission system is used for on-line monitoring with suitable software and multichannel computerized AE system that performs AE waveform and signal measurement. M/s PAC make, R15D model resonant piezoelectric transducers and W15D model piezoelectric wide band sensors are used with external preamplifier to pick up the acoustic emissions and wave forms respectively from the specimens. R15D sensor has an operating bandwidth between 100 and 500 kHz with resonant frequency of150 kHz and W15D sensor has an operating band width of10 to 1 MHz. The following AE testing parameters are used during testing of samples.
1) Threshold: 40 dB.
2) Peak definition time [PDT]: 20 ^s.
3) Hit definition time [HDT]: 50 ^s.
4) Hit lock time [HLT]: 300 ^s.
The AE test up is shown in Fig. 2. The specimens are subjected to tensile loading gradually up to failure. The load versus strain and load versus acoustic emissions were measured simultaneously.
Fig. 2. Acoustic emission test setup in UTM.
3. RESULTS AND DISCUSSION 3.1. Mechanical testing results
Six numbers of specimens were tested and the data has been summarized in Table 1. The failure modes in the specimens are shown in Fig. 3. Load versus strain curves for two specimens are shown in Fig. 4. The strain curves are found in linear trend with respect to load. In unidirectional samples the principal failure mode is by fiber breakage and this happens towards the end of the test. However weak fiber failure is observed at the early stage of loading. As stated earlier the sample under mechanical load may exhibit three types of failure mechanisms
[6, 7] occur in sequence as: 1) Matrix micro cracking is excessive during the initial phase of loading and is present during the entire loading phase; 2) Excessive matrix cracking leads to separation of bunch of fibres called delamination; 3) the fiber breakages cause ultimate failure of the specimen [8]. The ultimate failure load depends on the percentage dominance of de-lamination and fiber break-
Table 1
Test data for TUDL specimens
Sl. No Parameter TUDL specimens
1 Failure Load (kN) 42—69.5
2 No. of hits 1207—8922
3 Total energy 26 340—430 650
4 Energy range 2—43 565
5 Total signal strength 1.15E+09—3.24E+09
6 Amplitude range (dB) 47—100
7 Duration range (|s) 28—292 590
8 Rise time range (|s) 1—243
age. The AE data has been analysed by using various correlation plots. The raw acoustic emission signals obtained from six specimens during tensile test have been processed with Matlab program and various acoustic emission parameters are studied for fiber failure. Error in measurements has been minimized by normalization among the acoustic emission parameters like load, energy and signal strength.
Speeimen-2
Fi
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.