National Aerospace Laboratories

About: National Aerospace Laboratories is a facility organization based out in Bengaluru, India. It is known for research contribution in the topics: Coating & Corrosion. The organization has 1838 authors who have published 2349 publications receiving 36888 citations.

Fatigue Crack Growth Prediction under Spectrum Load Sequence in an Aluminum Alloy by K*-RMS Approach

Abstract: Constant amplitude fatigue crack growth (FCG) tests were performed at various stress ratios in D16 (2024-T3 equivalent) aluminum alloy using single edge notched tension specimens. Empirical FCG law was derived in terms of crack driving force parameter, K*. The spectrum load sequence of a combat aircraft was approximated as an equivalent constant amplitude (CA) load sequence with maximum and minimum stresses as root-mean-square (RMS) maximum and minimum stresses of the spectrum, respectively. The fatigue crack growth behavior was then predicted under this apparent CA load sequence using the K*-RMS approach. For comparison, the conventional method of FCG prediction using crack closure concept was also performed. In this method, empirical FCG law in terms of effective stress intensity factor range, Keff was obtained from the constant amplitude FCGR data. Assuming a constant Kop level for spectrum sequence, FCG behavior was predicted through cycle-by-cycle method. Experimental FCG behavior under spectrum load sequence was determined and compared with predicted results. It was observed that the K*-RMS approach provided a fairly good correlation with results obtained from experimental as well as those predicted by the crack closure concept. The predicted fatigue life was conservative and the fatigue life ratio, Npred/Nexpt, was about 0.87. The simplicity of the proposed K*-RMS approach and the reasonable accuracy in fatigue life predictions are quite encouraging.

Impactor mass effects in glass-epoxy composites subjected to repeated drop tests

Abstract: Experimental studies were carried out to understand the impactor mass effects on glass-epoxy composites using an instrumented impact machine (DYNATUP 8250) under low-velocity test conditions. Repeated drop tests were carried out at pre-determined energy levels for selective mass height combinations. Number of drops to failure (N1) was chosen as the damage criterion. Analysis of test results clearly showed that, at low incident energies, heavier impactors caused more damage to the laminates as reflected by their lower number of drops to failure. This effect diminished gradually at relatively higher incident energies.

Constitutive modeling of SMA SMP multifunctional high performance smart adaptive shape memory composite

Abstract: Materials design involving the thermomechanical constitutive modeling of shape memory alloy (SMA) and shape memory polymer (SMP) composites is a key topic in the development of smart adaptive shape memory composites (SASMC). In this work, a constitutive model for SASMC is developed. First, a one-dimensional SMA model, which can simulate the pseudoelastic (PE) and shape memory effects (SME) is presented. Subsequently, a one-dimensional SMP model able to reproduce the SME is addressed. Both SMA and SMP models are based on a single internal state variable, namely the martensite fraction and the frozen fraction, which can be expressed as a function of temperature. A consistent form of the analytical solution for the SMP model is obtained using the fourth-order Runge–Kutta method. Finally, the SASMC constitutive model is proposed, following two analytical homogenization approaches. One approach is based on an equivalent inclusion method and the other approach is the rule of mixtures. The SMA and SMP constitutive models are validated independently with experimental results. However, the validation of the composite model is performed using the two homogenization approaches and a close agreement in results is observed. Results regarding the isothermal and thermomechanical stress–strain responses are analyzed as a function of SMA volume fraction. Further, it is concluded that the proposed composite model is able to reproduce consistently the overall composite response by taking into consideration not only the phase transformations, variable modulus and transformation stresses in SMA but also the variable modulus, the evolution of stored strain and thermal strain in the SMP.