PSI - Issue 14

Hemant Chouhan et al. / Procedia Structural Integrity 14 (2019) 830–838 Author name / Structural Integrity Procedia 00 (2018) 000–000

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2.2. Dynamic Testing

The scientific formulations are inferred based on the one-dimensional wave propagation in the elastic bars with particle motion along the longitudinal direction as proposed in the literature by Kolsky (1949). The mathematical formulations of SHPB depend on assumptions, which fill in as an essential for accomplishing the one-dimensional wave theory. The necessity to fulfill one-dimensional loading requires - negligible frictional effects along with homogenous state of stress in the specimen, negligible inertial effects and negligible wave dispersion and perfect flat ends of the specimen in contact with the bars. In composites, stress wave attenuation happens at interfaces between the fiber and matrix which result in the absence of homogeneity of stress state. Henceforth, the state of the homogenous state of stress in the specimen may not really be satisfied while dynamically loading the FRP composite specimen. In any case, this is the main strategy to assess a sensible estimation of dynamic properties of FRP composites. Eq. (1) - (3) are utilized for computing the composite specimen strain rate, strain, and stress. The strain rate, ε̇ s (t) = ( �� � �� ) ε � ( ) (1) The average strain,  s (t)= ±( �� � �� ) ∫ ε � ( ). � � (2) The average stress,  (t) = ± � � � �  t (t) (3) Where, ‘t’ is time duration,  r and  t is reflected and transmitted strain pulse, C 0 is elastic wave velocity of sound in the bars material along the longitudinal axis of bars, Ls is specimen length, A B and A S are the cross-section area of the bar and the specimen and ‘E’ refers to Young’s modulus of elasticity of the bar material. The slope of strain rate-time curve decides the strain rate of homogeneous materials. Due to stress wave attenuation, there could be significant variation in the stress wave and the wave may not be consistent all through the test, as confirmed by Naik et al. (2008). In this situation, a second plan proposed by Nakai et al. (2008) can be utilized for figuring strain rate. According to the new proposed scheme for materials which do not reveal constant reflection pulse, the strain rate may be evaluated by “dividing the area under the strain rate-strain curve, up to maximum strain under loading, by the maximum strain”. In the present investigation, the strain rate is estimated by this new plan to beat the impact of variety because of stress wave attenuation. 3. Results and Discussion The dry UHMWPE-SR specimens kept in potable water had an average increment in the weight 7%, showing significant moisture pick up. The moisture concentration in composites can happen due to multiple phenomena. It might be because of moisture assimilation from the surface, which in the end prompts dissemination through the inside. It may not be essential in the fluid frame, as a composite specimen may collaborate with moisture at an atomic level, by interfacing hydrogen attach to the hygroscopic polymer and fiber surface. Another probability is because of fluid being transported to the insides of the composite specimen by capillary action along the micro cracks and fiber-matrix interfaces and may reside inside. These micro cracks and voids may have been created during composite handling and later during machining for the sample preparation. 3.1. Dynamic compressive testing of dry and wet UHMWPE-SR composite For the dynamic compressive testing, the cylindrical UHMWPE-SR specimen is placed in between the incident and transmission bars. The incident, reflected and transmitted waves are measured by the strain gauges mounted at mid-section of incident and transmission bar and suitably recorded by data acquisition system in terms of incident, reflected and transmitted voltage. The voltage signals are then converted into an equivalent incident, reflected and