N. Jagannathan

Enhanced Fatigue Performance of a Polymer Nanocomposite under Spectrum Loads

A thermosetting epoxy polymer was modified by incorporating 10 wt. % of silica nanoparticles, which were well dispersed in the polymer. Two different glass-fiber-reinforced plastic (GFRP) composite laminates were prepared to give: (1) a GFRP composite with an unmodified epoxy matrix (GFRP neat), and (2) a GFRP composite with a silica-nanoparticle-modified epoxy matrix (GFRP nano). Fatigue tests were undertaken employing a standard wind-turbine spectrum-load sequence, WISPERX. The fatigue life of the GFRP nanocomposite was about four times longer than that of the GFRP neat composite. This was reflected in (1) the development of matrix cracking, and (2) the rate of degradation of the stiffness of the composite, both being more severe in the GFRP neat composite, compared to the GFRP nanocomposite. The underlying mechanisms for the observed improvement in the spectrum fatigue life of the GFRP nanocomposite are discussed. Further, constant amplitude fatigue tests were conducted at various stress ratios. Using the static and fatigue data, constant life diagrams (CLD) were constructed. The spectrum fatigue life was then predicted following a standard procedure using the CLD. Very good correlation was observed between the predicted and experimental fatigue life for both GFRP neat and GFRP nanocomposites.

Variable Amplitude Fatigue Testing to Characterize Mode II Delamination in a Polymer Composite

In this study, the mode II delamination behavior in terms of onset-of-growth and propagation characteristics under a standard variable amplitude (VA) fatigue load sequence was investigated. A special three point bend test fixture was designed and fabricated to perform fatigue tests under VA loads. Standard end notched flexure test specimens of unidirectional IMA/M21 carbon fiber composite were fabricated. A Teflon insert was used to simulate a delamination at the mid plane. The fatigue tests were conducted under a standard mini FALSTAFF VA load sequence. Tests were carried out with different reference values of loads to determine the onset-of-growth, Nonset. Decreasing the reference load was observed to increase the Nonset. Further, the delamination propagation under the mini FALSTAFF load sequence was determined in the same test set-up. It is successfully demonstrated that the test fixture could be used to apply both negative and positive fatigue loads to simulate the service loads.

Micromechanics Modeling and Prediction of Stiffness Degradation Behavior of a Fiber Reinforced Polymer Nanocomposite Under Block Amplitude Fatigue Loads

The micro-mechanisms of fatigue damage initiation and growth in polymer composites lead to observable progressive degradation in global properties such as strength and stiffness. Thus monitoring stiffness degradation behavior of a composite will assist in evaluating the residual strength, stiffness and remaining fatigue life of the material. In the present investigation, the stiffness degradation behavior of a glass-fiber epoxy silica-nano-particle composite (GFRP nanocomposite) under a two step block load sequence was predicted from micro-mechanics based models. The stiffness of nanocomposite was determined from the properties of the constituent materials. To compare the predicted results, experiments were conducted on a GFRP nanocomposite. The stiffness of the specimen was monitored at regular intervals during the fatigue tests. The predicted stiffness degradation behavior of the nanocomposite under variable amplitude fatigue loads was observed to compare quite well with experiments.

Fatigue Life Prediction of a Unidirectional Carbon Fiber Composite Under Off-Axis Spectrum Loads Using 3D Constant Life Diagram

In this study, a novel concept of 3D constant life diagram (CLD) for fatigue life prediction of a unidirectional (UD) polymer composite under spectrum load is proposed. Further, it is constructed and used to predict the fatigue life of a UD carbon fiber composite (CFC) subjected to a standard spectrum load sequence at an arbitrary off-axis angle. First, UD IMA/M21 CFC laminates were fabricated by an autoclave process. Static mechanical tests were conducted to determine the tensile and compressive strength at various off-axis angles ranging from 0° to 90°. Then the constant amplitude (CA) fatigue tests at three different stress ratios, R = σmin/σmax of 0.1 (tension-tension), −1.0 (tension-compression), and 10.0 (compression-compression) and at various off-axis angles between 0° and 90° at each of these stress ratios were performed to determine stress-life curves. Using the static and CA fatigue data generated, 3D CLD for UD CFC was constructed. Further, fatigue life of UD CFC subjected to a standard mini-FALSTAFF spectrum load sequence at an arbitrary off-axis angle of 20° was predicted following an empirical method using 3D CLD. Fatigue tests under the mini-FALSTAFF spectrum load sequence at an off-axis angle of 20° were also conducted at various reference stresses and compared with the predictions. A reasonably good correlation was observed between the predicted and experimental fatigue lives under off-axis spectrum loads.

Effect of Silica Nanoparticles on the Fatigue Life of a Glass Fiber Reinforced Epoxy Composite Under an Aircraft Spectrum Load Sequence

Two types of glass fiber reinforced plastic (GFRP) composites viz., (i) GFRP employing unmodified LY556 epoxy matrix (GFRP-neat), and (ii) GFRP incorporated with 10 wt% of well-dispersed silica nanoparticles in the LY556 epoxy matrix (GFRP-nano), were tested to determine their fatigue life under mini-FALSTAFF, a standard fighter aircraft spectrum load sequence. Spectrum fatigue tests were conducted on standard test specimens in a 50 kN servo-hydraulic test machine with sinusoidal waveform at an average frequency of 3 Hz. Tests were conducted on both types of GFRP composites with various reference stresses to determine the fatigue life expressed as number of blocks required for failure. The fatigue life of GFRP-nano composite was observed to be about four times higher than that of GFRP-neat composite over the entire range of reference stresses investigated. For a given number of applied load cycles, both the matrix crack density and stiffness reduction rates were observed to be lower in GFRP-nano composite when compared to that of GFRP-neat composite. Presence of silica nanoparticles in the epoxy matrix of GFRP appear to reduce matrix cracking and also retard crack growth rate in the composite leading to enhanced fatigue life. Further, using constant fatigue life diagrams of these materials, the spectrum fatigue life under mini-FALSTAFF load sequence was predicted. Good correlation was observed between the predicted and experimental fatigue life for both types of composites.

Prediction of Mode II Delamination Onset Life Under Spectrum Fatigue Loads Using Equivalent Strain Energy Release Rate Concept

End notched flexure (ENF) test specimens of unidirectional IMA/M21 carbon fiber composite (CFC) were fabricated using standard autoclave process. A Teflon insert was used to simulate a delamination at the midplane. Three-point bend setup tests were conducted at an average frequency of 2 Hz using a 25 kN servo-hydraulic test machine in room temperature conditions. Constant amplitude fatigue tests were done at three different stress ratios, viz. R = 0.0, 0.5, and −1.0 to construct the standard G-N onset diagram, similar to S–N curve in its usefulness. N onset was identified as 5% change in initial compliance value. Using an equivalent energy release rate parameter, G eq, all the curves were collapsed into a single curve in the form of Basquin’s equation. The equation was subsequently used in predicting the delamination onset-of-growth life under a standard mini-FALSTAFF spectrum load sequence. A fairly good correlation was found between the predicted and experimental mode II onset-of-growth behavior.

Prediction of mode II delamination propagation life under a standard spectrum loading in a carbon fiber composite

Mode II constant amplitude fatigue delamination propagation tests were conducted at three different stress ratios, viz. R = 0.0, 0.5, and −1.0 using modified three-point bend test fixture and end notched flexure (ENF) test specimens of unidirectional IMA/M21 carbon fiber composite. Delamination length was measured by compliance technique. The delamination propagation rate (da/dN) plots were constructed for various stress ratios. Using an equivalent energy release rate concept, Geq, all the curves were merged into a single curve in the form of Paris equation, da/dN = C(Geq)k. The Paris constants, C and k, determined from the experimental results were subsequently used in predicting the delamination propagation life under a standard mini FALSTAFF spectrum load sequence. Experiments were also conducted under the same spectrum load sequence with various reference loads to determine the propagation lives. A reasonably good correlation was observed between the predicted and experimental mode II delamination propagation life under spectrum loads.

A study on the fatigue performance of a glass fiber-epoxy polymer nanocomposite under random loads

Abstract A glass-fiber reinforced plastic (GFRP) nanocomposite containing 10 wt-% silica nano particles in the epoxy matrix was fatigue tested under a standard helicopter random load sequence, Helix-32. Fatigue life was determined at various reference stresses. The stiffness variation and the matrix crack density in the test specimen were monitored at regular intervals during the fatigue test. The random load fatigue life of the GFRP nanocomposite was about four times higher than that of its neat counterpart over the entire range of reference stress levels investigated. The suppressed matrix cracking and reduced crack/delamination growth rate in nanocomposite were responsible for fatigue life enhancement. Further, the random load fatigue life was predicted by empirical method using constant fatigue life diagrams. Three different damage accumulation models, namely, Palmgren–Miner (PM), Broutman–Sahu (BS) and Hashin–Rotem (HR), were used. All the three models predicted similar results, and a good correlation was observed between experimental and predicted fatigue life.