Addis Tessema

The Effect of Particles Size on the Thermal Conductivity of Polymer Nanocomposite

The variation in thermal conductivity of polymer nanocomposite with different particle sizes and volume fractions have been investigated. Particle reinforced nano-composites with two different particle sizes and the volume ratio of each size ranging from 0 to 50 % is considered. The test is conducted using a unidirectional/linear heat transfer device that has six thermocouples to monitor the temperature flow through and across the cross section of the specimen. In addition, based on Lewis-Nielson and modified effective medium approximation, a three phase analytical model is proposed to determine the thermal conductivity of different nanocomposites. It is observed that the thermal conductivity linearly increases as the volume fraction of the particles increases. On the other hand, though the particle size has an effect on the thermal conductivity of the nanocomposites, the effect is minimal compared with the volume fraction. The analytical model has been applied to different batches of specimens, and the results from the experiment and analytical model are compared.

Meso-scale Deformation Mechanisms of Polymer Bonded Energetic Materials Under Dynamic Loading

To understand the plastic deformation mechanism of polymer bonded energetic materials, a meso-scale experiment is conducted under dynamic loading. Energetic simulant material with polymer plasticizer are cold pressed using a mold made of stainless steel. An experimental setup is developed to obtain the local strain field at the meso-scale under dynamic loading conditions. The setup consists of a high speed camera with extension tube and microscope objective lens to obtain magnifications ranging from 1× to 50×. A high intensity halogen light source is used for illumination. The field of view for the experiment is 1700 × 690 μm, with a spatial resolution of 4.427 μm/pixel at 100,000 frames/s. Dynamic loading is performed using a split Hopkinson pressure apparatus to obtain a range of strain rates. The strain fields are obtained using digital image correlation technique. To facilitate for the digital image correlation technique, the specimens are speckled using air brush with average speckle size ranging from 12 to 18 μm. Results are presented for the measured strain fields and the associated deformation mechanisms as a function of loading rate.

Effect of Micro-Cracks on the Thermal Conductivity of Particulate Nanocomposite

The effect of micro-cracks on the thermal conductivity of particle-reinforced nanocomposites is investigated. Two different particles (Carbon nanotube and Silicon dioxide) with different geometries are considered to account for the effect of particle aspect ratio. Three batches of specimens, two with and one without nano-fillers are fabricated. First, the thermal conductivity of the as-fabricated samples were measured using steady state linear heat transfer unit. Afterwards, the samples were subjected to cyclic loading and at the end of every 5000 cycles the samples were taken out and the thermal conductivity was measured. At the same time, the Modulus of Elasticity of the specimens were determined using uniaxial compression test. Based on these results, the effect of micro-cracks on the thermal conductivity of the nanocomposites is presented. In addition, the relation between micro-cracks, stiffness, and thermal conductivity are presented.

On the Mechanical Response of Polymer Fiber Composites Reinforced with Nanoparticles

An experimental study was conducted on the effect of interply nanofiller on the mechanical response of fiber reinforced composite (FRC). Laminate samples were made by hot pressing of woven carbon fiber fabric prepregs. Two batches of samples are prepared, one using five plies of the basic prepreg, the other with silica nanofillers added between the plies during lay-up. Tensile specimen were cut from the laminate under 0, 15, 30, 45, 60, 75 and 90 degrees of fiber orientation are prepared from the laminate. DIC based tensile test is made and the effect of the nano fillers on the mechanical properties are analyzed. Appreciable improvement in strength and Modulus of Elasticity is obtained for fiber orientation of 75° and 60° and reversed response is observed for the fiber angle of 30° and 15°

Experimental Study of Residual Plastic Strain and Damages Development in Carbon Fiber Composite

In this work, the correlation between local damage and residual stiffness in carbon fiber composites is investigated experimentally. The study presents the interaction between local damage mechanisms and the global response of the material, with the aim to capture the gradual local failure phenomenon. High resolution digital image correlation technique at micro length scale is used to measure the local deformation in the specimen made of carbon fiber composite, subjected to tension-tension fatigue loading. During testing, image of a speckled surface inside the gage length of the specimen is captured at specified number of loading cycles. Using digital image correlation, the acquired images are processed and the local deformations and strains are extracted. The growth of local plastic deformation as a function of loading cycle is acquired and different damage modes as a function of loading cycle is explored. The study has able to elucidate the event that shows the gradual growth of matrix cracking into inter-ply debonding. Furthermore the degradation of modulus of elasticity as a function of loading cycle is determined and the corresponding type of damage incorporated within the plastic strain distribution around it is presented.

In-Situ Observation of Damage Evolution in Quasi-Isotropic CFRP Laminates

In this work, a Digital Image Correlation (DIC) impaired with high resolution optical system is applied to capture the damage evolution in composites laminates in-situ. The developed method enables to measure the local (micro scale) deformation across the thickness on the free-edge (8-plies laminate) subjected to a quasi-static tension. Three groups of specimens are prepared by arranging plies at different stacking sequence, and the formation of strain localization, initiation of matrix crack, delamination and other damages are acquired. It is obtained that matrix cracking is the primary and dominant form of damage and it usually occurred in the 90°-plies. However, the orientation and quantity of matrix cracks are highly affected by the stacking arrangement of the plies.

Effect of Crystal Density on Dynamic Deformation Behavior of PBX

Polymer bonded explosives (PBX) are heterogeneous materials that contain solid loading varying from 80 to 95 % and bound together by 5–20 % soft binder. An experimental investigation is performed to study the effect of crystal solid loading on the failure process of PBX subjected to dynamic loading at different strain rates. Model materials, with sugar crystals and binder, are fabricated with solid loading varying from 80 to 95 %. Then dynamic compression experiments are performed on each specimens using split Hopkinson pressure bar. During loading, the deformation is captured using the high-speed camera at 1 million frames/s. Digital image correlation technique is used to obtain the local and full field deformation and strain fields at each strain rate. Based on the local deformation field and the load data, the failure process of each sample are investigated, and the effect of solid loading on the strain localization and failure mode of the PBX is discussed.

Thermo-Mechanically Tunable Elastic Metamaterials with Compliant Porous Structures

Adding programmable function to elastic metamaterials makes them versatile and intelligent. The objective of this study is to design and demonstrate thermomechanically tunable metamaterials with a compliant porous structure (CPS) and to analyze their thermomechanical behaviors. CPS, the unit cell of the metamaterial, is composed of rectangular holes, slits, and bimaterial hinges. By decomposing kinematic rotation of a linked arm and elastic deformation of a bimaterial hinge, a thermomechanical constitutive model of CPS is constructed, and the constitutive model is extended to a threedimensional (3D) polyhedron structure for securing isotropic thermal properties. Temperature-dependent properties of base materials are implemented to the analytical model. The analytical model is verified with finite element (FE) based numerical simulations. A controllable range of temperature and strain is identified that is associated with a thermal deformation of the bimaterial hinge and contact on the slit surfaces of CPS. We also investigate the effect of geometry of CPS on the thermal expansion and effective stiffness of the metamaterial. The metamaterial with CPS has multiple transformation modes in response to temperature while keeping the same mechanical properties at room temperature, such as effective moduli and Poisson’s ratios. This work will pave the road toward the design of programmable metamaterials with both mechanically and thermally tunable capability, providing unique thermomechanical properties with a programmable function. [DOI: 10.1115/1.4038029]

Effect of Nanodiamond (ND) Surface Functionalization on the Properties of ND/PEEK Composites

This paper explores the effect of nanodiamond (ND) surface chemistry on the structure and properties of composites of ND melt-blended with poly(ether-ether-ketone) (PEEK). ND functionalized with phenylphosphonate (PPA) resulted in ND/PEEK composites with improved filler dispersion based on visual observations and surface topography measurements. As-received oxidized ND (OND) reduces PEEK’s lamellar spacing and degree of crystallinity, as does PPA-modified carboxylated ND, which has low PPA graft density. PPA-modified OND does not significantly alter PEEK’s crystalline structure or degree of crystallinity. Sonication during PPA grafting on OND increases composite thermal conductivity by up to 38% relative to pure PEEK. ND/PEEK composites have dielectric permittivity values comparable to and in some cases lower than pure PEEK and electrical conductivity values no greater than pure PEEK. The combination of enhanced thermal conductivity, low dielectric permittivity, and low electrical conductivity makes ND/PEEK composites attractive for high-temperature electronic device applications.

Damage Evolution and Local Strain Redistribution in Composite Laminate with Various Fiber Arrangements

The initiation and gradual development of damage in composites is associated with the degradation of the composite laminate properties. Understanding the characteristics of damage evolution in composite laminates has been one of the major interest in composite studies. There is a lot of progress in this regard, however still there is a lack of clear understanding on how damages are initiated, grown and transformed from one form to another. In this study experiments are conducted to capture the strain localization and cracks formation on the free-edge of composite laminates. Laminates that have a stacking arrangement of (0/−Ɵ/+Ɵ/90)s with plies that have different fiber angles (Ɵ = 15°, 30° and 45°) are manufactured. Coupon samples are made from these laminates and subjected to a uniaxial tension loading until final fracture. Using digital image correlation technique at high magnification, the local deformation field is determined. From the test, it is observed that the strain/stress response of the composite is influenced by the arrangement of the fiber angle of the off-axis plies. From the strain contours obtained on the free-edge, the gradual initiation and growth of matrix cracks is observed to be localized in the 90° plies. In addition, these matrix cracks grow and lead to cause delamination between the 90° plies and neighboring plies. The local strains in each individual ply are seen to fluctuate along with the emergence of cracks at the vicinity of the damage as a result of stress redistribution.