Muhammad Kashif Khan

Compressive and lamination strength of honeycomb sandwich panels with strain energy calculation from ASTM standards

Abstract The experiments were designed and performed to find the material properties of honeycomb sandwich panels and to find the difference in properties between aluminium and of glass fibre facing honeycomb sandwich panels, so as to get a path for consideration of panel containing maximum stiffness to weight ratio which is the main idea behind material selection for a spacecraft/satellite structure. Flatwise and edgewise compressive tests were performed on aluminium and glass fibre facing honeycomb sandwich panels of different core thickness to investigate the out-of-plane and in-plane compressive strength. The strain energy and the critical stress were calculated in one of the compression mode. Another set of experiments was performed to find the lamination strength of bond used for joining facing and core of honeycomb sandwich panels. Again the tests were performed on both facing types of sandwich panels to check the difference in strength with the change in core thickness and facing type of the honeycomb sandwich panel. Different ASTM standards for testing honeycomb sandwich panels were used.

The most Influential Articles in Very High Cycle Fatigue

The most influential articles in Very high cycle fatigue (VHCF) during last 20 years have been identified from web of science data. It has been found that VHCF is an important category of fatigue damage and has produced the highest impact in the field. The number of articles and their citations is continuously increasing in the overall published papers in the field of fatigue. In VHCF field, it was found that the participants of VHCF conferences from VHCF 1 to 5 are the main contributors in all the VHCF papers and in its top 100 influential papers. Majority of the articles were experimental studies. The fatigue crack initiation in the VHCF domain is considered the most important area in the field. In VHCF, the most influential papers till date have been identified. This may prove helpful to trainees mastering the most influential literature of the field as well as more established professionals searching for starting points for new investigations

Experimental and Finite Element Based Investigations of In-Plane and Out-of-Plane Properties of Aluminum Honeycomb

Experimental and Finite Element analysis was used for the investigation of the effect of cell size and thickness on the compressive properties of Aluminium honeycomb core. Honeycomb cores were compressed experimentally in in-plane and out of plane directions. The effect of sample size, cell size and thickness on the elastic modulus, yield strength and plateau stress was investigated through FEA. It was found that the mechanical response was independent upon the sample size in in-plane direction. The smallest cell size honeycomb core was deformed at higher yield stress. Similarly, with increase in cell wall thickness, the modulus of the core increased.

Cyclic Bending Response of Single- and Polycrystalline Thin Films: Two Dimensional Discrete Dislocation Dynamics

The bending behavior of single- and polycrystalline thin films is modeled by two-dimensional discrete dislocation dynamics (DDD) to study the cyclic bending response. In the polycrystalline films, grain boundaries (GBs) are simulated with a penetrable dislocation-GB interaction model. Our results reveal that the single- and polycrystalline thin films under pure bending exhibit strong Bauschinger effect but no cyclic hardening or softening. Furthermore, the uploading response of each cycle can be divided into three stages, which are associated with the glide, annihilation and nucleation of dislocations, respectively.

High-Cycle Fatigue Properties and Damage Mechanism of Q345B Structural Steel

The high-cycle fatigue behavior of Q345B structural steel was investigated experimentally. Highfrequency vibration fatigue testing machine and scanning electron microscopy were used to study the high-cycle fatigue S–N curve characteristics and crack initiation mechanism at ambient temperature. The surface temperature of the specimens was monitored. The relation between the fatigue limit and the amount of heat dissipation was also investigated. It was found that the fatigue life changed inversely with the stress amplitude in the high-cycle range. The fatigue limit in high cycle range was obtained from heat dissipation in the specimen and found to have good agreement with the S–N curve. The crack initiation was attributed to the surface defects and the persistent slip bands due to the cycle slip in fatigue loading.

Atomic force microscopy (AFM) for materials characterization

The use of high-resolution microscopic imaging is continuously increasing in engineering, medical, natural science, and other fields. In many applications, the characterization of surfaces requires spatial resolution of nanometers or lower. Atomic force microscopy (AFM), although a relatively newly developed technique, has now become a powerful technology for characterization of the surface of materials down to the atomic scale. AFM can be used to obtain nanoscale chemical, mechanical (modulus, stiffness, viscoelastic, frictional), electrical, and magnetic properties. In comparison with other microscopy techniques, AFM offers low cost, simplicity in operation, and imaging capability to atomic resolution. It is a powerful nondestructive analytical technique which can be used in air, liquid, or vacuum. This chapter discusses the effectiveness of AFM for material characterization.

Micromechanical Modeling of Elastic-Viscoplastic Behavior of Armco-Fe at High Strain Rate

Heterogeneous dynamical stress-strain response of Armco-Fe was investigated at high strain rates through the Split Hopkinson Pressure Bar (SHPB) testing. It was found that the viscoplastic deformation in BCC ferrite grains is affected by the strain rate. Thermal softening and variation in crystal orientations under high-strain-rate loading were used in the elastic-viscoplastic modeling. The micromechanical analysis with self-consistent transition and homogenization was used for estimation of the global impact response of the material. The results from modeling were found in good agreement with the experimental data.

Low cycle fatigue behaviour and life prediction of Q345B steel and its welded joint

Abstract Low cycle fatigue behaviour of Q345B steel and its smooth welded joint were experimentally investigated in fully reversed strain-control at constant strain rate of 0·005 s−1. The cyclic stress response characteristics, strain–life and strain–energy relationships were obtained and analysed in the strain regime 0·3–0·7%. It was found that Q345B showed significant cyclic hardening and softening above and below the 0·4% strain amplitudes, respectively. The welded joints showed cyclic hardening in the entire strain regime because of the mechanical inhomogeneity. The low cycle transition life, fatigue strength and fatigue life of welded joint were found significantly lower with increase in the cyclic hardening. The experimental data were used to extract the Coffin–Manson parameters and the strain–life relationship. In addition, the expressions of plastic strain energy and fatigue life were also obtained using the energy prediction method.

Current understanding of ultra-high cycle fatigue

The fatigue life of numerous aerospace, locomotive, automotive and biomedical structures may go beyond 10 8 cycles. Determination of long life fatigue behavior becomes extremely important for better understanding and design of the components and structures. Initially, before the invention of ultrasonic fatigue testing, most of the engineering materials were supposed to exhibit fatigue life up to 10 7 cycles or less. This paper reviews current understanding of some fundamental aspects on the development of accelerated fatigue testing method and its application in ultra-high cycle fatigue, crack initiation and growth mechanisms of internal fracture, S-N diagram, fatigue limit and life prediction, etc.

Determination of the Residual Stress Field around Scratches Using Synchrotron X-Rays and Nanoindentation

The residual strain field around the scratches of 125µm depth and 5µm root radius have been measured from the Synchrotron X-ray diffraction. Scratches were produced using different tools in fine-grained aluminium alloy AA 5091. Residual stresses up to +1700 micro-strains were measured at the scratch tip for one tool but remained up to only +1000 micro-strains for the other tool scratch. The load-displacement curves obtained from nanoindentation were used to determine the residual stresses around the scratches. It was found that the load-displacement curves are sensitive to any local residual stress field present and behave according to the type of residual stresses. This combination of nanoindentation and synchrotron X-rays has been proved highly effective for the study of small-scale residual stresses around the features such as scratches.