Structural integrity of ultrafine grain Al-3%Mg alloy under dynamic loading conditions

Utilization of various materials for constructing dynamic components and equipments has increased ever today. The high speed deformation mechanics was studied in various scale levels, especially in micro and nano scales. Understanding the micromechanics using shock waves led to development of armor plates in military technology. One dimensional elastic stress is applied using Split Hopkinson pressure bar for the ultra-fine grain aluminum samples and microstructural evolution was discussed in detail. The material characterization of equi channel pressing and its effect on stability of material after shock wave testing is provided. The grain size of material is steadily decreased to obtain ultra-fine grain structure during equi channel pressing and by application of shock waves on those pressed samples, the grain size again increases within the material. The recovery, re-crystallization and grain growth was observed in those shock tested samples due to induced temperature during such shock testing. The existing dislocation sub structure in pressed samples devoid after inertia effects. It is proposed further to understand the interaction between precipitate particle and dislocations

Near-tip out of plane displacement fields for dynamic crack propagation in functionally graded materials

Asymptotic expansion for the out of plane displacement field around a crack propagating along the gradient in a functionally graded material is developed. The irregular behavior of one of the terms in the expansion at low crack speeds is further examined and a remedial solution, which is well behaved at low crack speeds, is proposed. The developed out of plane displacement field is used to estimate stress intensity factor from quasi-static finite element solution. The results indicate that inclusion of the proposed nonhomogeneity specific terms gives estimates of stress intensity factor, which are consistent with existing analytical predictions

Dynamic fracture of a functionally gradient material having discrete property variation

A functionally gradient material (FGM) with discrete property variation is prepared, and the dynamic fracture in this material is studied using the technique of photoelasticity combined with high-speed photography. Transparent sheets required for the study are made by casting a polyester resin mixed with varying amounts of plasticizer. The mechanical (quasi-static and dynamic) and optical properties of the material are evaluated as a function of the plasticizer content. Results of material characterization show that the fracture toughness increases with increasing plasticizer content, whereas the Young\u27s modulus decreases. The material fringe constant and the dynamic modulus are observed to be relatively insensitive to plasticizer content. The FGM is then prepared by casting together thin strips having different plasticizer content. The dynamic crack propagation phenomenon is studied for four different property variations along the crack propagation direction, and the effects of these property variations on crack speed, crack jump distance and dynamic stress intensity factor are investigated. Results of this investigation show that increasing the toughness in the direction of crack growth reduces the crack jump distance as compared to an increasing-decreasing toughness variation for the same initial energy. © 1998 Kluwer Academic Publishers

On the use of strain gages in dynamic fracture

Strain gages are the most commonly used type of strain sensors because of the fact that they are easy to use and relatively inexpensive. The application of strain gages to investigate propagating cracks in homogeneous materials, composites and bi-material interfaces is discussed in this chapter. A brief review on strain gage instrumentation and data acquisition is provided at the outset. The importance of using appropriate gage location and gage orientation relative to the crack path in obtaining reliable measurement of the fracture parameters is highlighted. The theory and analysis procedure for obtaining the crack speed and stress intensity factor are discussed for each type of material. Typical results, available from existing literature, are also presented

Experimental Evaluation of the Dynamic Shear Strength of Adhesive-Bonded Lap Joints

An experimental method utilizing a novel specimen geometry was developed to determine dynamic shear strength of adhesive-bonded lap joints using the classical Split Hopkinson Pressure Bar (SHPB) technique in compression. The specimens were loaded dynamically at four different loading rates, and the transmitted load through the joint was obtained from the time-resolved strain history assuming one-dimensional stress wave propagation. The shear strength of the joint was determined from the maximum transmitted load, assuming the load was transferred predominantly as shear load through the adhesive-bonded joint. The shear strength of a lap joint bonded using a general purpose epoxy adhesive was obtained at loading rates varying from quasi-static to 2300 N/μs. The results indicate that as the loading rates are increased to 1000 N/μs the shear strength of the particular adhesive-bonded lap joint increases to three times its static value, after which it stabilizes

Dynamic crack initiation and propagation in nanocomposite materials

A comprehensive series of experiments is conducted to study dynamic crack initiation and propagation in nanocomposite materials. The nanocomposites are fabricated using ultrasonics with an in-situ polymerization technique to produce materials with excellent particle dispersion, as verified by transmission electron microscopy and scanning electron microscopy. Dynamic fracture toughness testing is carried out on three-point bend nanocomposite specimens using a modified split-Hopkinson pressure bar, and the results are compared to those of the matrix materials. Dynamic photoelasticity coupled with high speed photography has also been used to obtain crack tip velocities and dynamic stress fields around the propagating cracks. A relationship between the dynamic stress intensity factor, KD, and the crack tip velocity, á, is established. © 2006 Advanced Study Center Co. Ltd

Study of dynamic underwater implosion mechanics using digital image correlation

The physical processes associated with the implosion of cylindrical tubes in a hydrostatic underwater environment were investigated using high-speed three-dimensional digital image correlation (3D DIC). This study emphasizes visualization and understanding of the real-time deformation of the implodable volume and the associated fluid- structure interaction phenomena. Aluminium 6061-T6 cylindrical tubes were used as the implodable volumes. Dynamic tourmaline pressure transducers were placed at selected locations to capture the pressure history generated during each implosion event. A series of small-scale calibration experiments were first performed to establish the applicability of 3D DIC for measuring the deformation of submerged objects. The results of these experiments indicated that the effects of refraction due to water and the optical windows can be accounted for by evaluation of the camera\u27s intrinsic and extrinsic parameters using a submerged calibration grid when the surface normal of the optical windows is collinear with the camera\u27s optical axis. Each pressure history was synchronized with its respective high-speed DIC measurements. DIC results showed that the highest rate of increase in contact area correlates to the largest pressure spike during the implosion process. The results also indicated that, for a given diameter, longer implodable volumes generated higher pressure spikes

Deformation and failure of alumina under high strain rate compressive loading

For development of structural ceramics e.g., alumina for high strain rate resistant applications, it is extremely important to understand the issues involved in compressive microfracture and the role of compressive microfracture in the global failure process at high strain rates. Thus the present work reports compressive strength of a dense (e.g., rho similar to 97.2% P-th) 5 mu m grain size alumina exposed to high strain rate (e.g., 0.9 x 10(3) s(-1)) loading in SHPB experiments. Concomitant utilization of high-speed videography has been exploited to study in-situ the details of the dynamic fragmentation process. The maximum compressive strength is measured to be similar to 3 GPa. Post-mortem examination of the recovered alumina fragments has been performed by FESEM and TEM. Apart from conventional global brittle fracture, the results show the grain localized microcleavages, intragranular microcracking, plasticity, dislocations as well as formation of subgrain structure to occur in alumina Based on these experimental results the reasons of compressive microfracture and its probable role in the global failure process of alumina ceramic are discussed. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved