Addis Kidane

On the Failure and Fracture of Polymer Foam Containing Discontinuities

An experimental study is performed to investigate the tensile failure and fracture behavior of polymer foam containing discontinuities. PVC corecell foam, series A800 and A1200 is used for the investigation. Unnotched dog-bone specimen and specimens with center hole and edge cracks are tested under uniaxial tensile loading. Series of experiments are conducted at different defect size to width ratios, and the effect of the defect size on the net-section tensile strength of the foam is investigated. A fracture study is also conducted, and the effect of density and loading rate on the fracture behavior of foam is investigated. A minimal notch-strengthening effect is observed in specimens with center hole, and a notch-weakening effect is observed in specimen with edge notches. Furthermore, the fracture toughness increases with the increase in the foam density and decreases with the increase in loading rate.

Meso-scale study of non-linear tensile response and fiber trellising mechanisms in woven composites:

The non-linear deformation response of plain woven carbon fiber-reinforced composites is experimentally studied at meso-scales. Stereovision digital image correlation is utilized to capture the full-field strain distribution over a 10 × 10 mm2 area of interest located at the center of the specimens. The evolution of local strains on the fiber bundles and matrix-rich regions as a function of loading is extracted. The effect of fiber orientation angle on fiber bundles stretch ratio and their angle of rotation (fiber trellising) and the related underlying failure mechanisms are analyzed using the measured full-field displacement data. The results indicate that the local load-bearing mechanisms are different in on-axis and off-axis loading conditions, whereas the larger global failure strain noticed in off-axis conditions is attributed to the occurrence of fiber rotation. The fiber trellising is also shown to promote high local shear strain and consequently leads to the protrusion of the matrix material on the deformed specimen surface.

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.

Feasibility of Non-Contacting Measurement of Wind-Induced Full-Field Displacements on Asphalt Shingles

Understanding of deformations and the progressive failure mechanisms of asphalt shingles under wind loads is key to develop wind-resistant roofing systems, as well as standard test methods to characterize strength under representative wind loads. In fact, failure of shingles rated as resistant to winds up to 150 mph have been reported at speeds below 115 mph. Damage associated with failure of roof shingles continues to be a major source of insurance claims. Though pressure measurements can be taken at discrete points using pressure taps, no technology has been successfully deployed to measure full-field deformations on roof shingles subjected to wind loads. This paper reports on a feasibility study of three-dimensional digital image correlation (3D-DIC) as a non-contacting technique to measure full-field displacements of roof shingles under high wind loads. Feasibility is assessed based on evidence from load testing of three-tab shingles mounted on a full-scale roof panel specimen that was subjected to straight winds with speed up to 155 mph. Uplift displacements were measured on a target shingle tab. The natural color variations on the shingle exposed surface were used to provide a suitable speckle pattern for 3D-DIC measurements. It is shown that consistent 3D-DIC uplift displacement maps can be obtained up to failure. The evidence gained also highlights the importance of understanding the influence of time-dependent shingle material deformations, together with the progressive physical damage along the sealant strip.

A General Approach to Evaluate the Dynamic Fracture Toughness of Materials

In this paper a combined experimental and numerical approach is proposed to evaluate the dynamic fracture toughness of materials. A circular tube specimen, made of Aluminum alloy, 7050-T7651, having a spiral crack on its outer surface is used to demonstrate the technique. A torsional Hopkinson bar is used to generate a dynamic torsion pulse, which in turn creates predominantly a tensile load along the crack line of the specimen. The torque applied on the sample is obtained from strain gages attached on the bars using one dimensional wave equation. Commercial FE package, ABAQUS, is used to simulate the dynamic fracture parameters. In this case the subspace projection method and standard implicit integration in ABAQUS with time increment are used, assuming the system is linear. The angle of twist associated to the torque measured on the specimen is used as input in the model. The dynamic stress intensity factor is determined using minimum strain energy theory. Digital image correlation is used to measure the deformation field near the crack tip. The measured strain/displacement fields are used to determine the exact time at which the crack initiated. The result show that, all the three classical modes of fracture are existing, but mode I is at least one order magnitude higher than the others. Also the dynamic SIF of the Aluminum alloy 7075-T6571 is higher than the quasi-static SIF (i.e KIc d=1.36KIc), The value obtained in this experiment is in well agreement with the values documented in the literature.

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°

On the Meso-Macro Scale Deformation of Low Carbon Steel

The multiscale deformation response of low carbon steel is investigated. The meso and macro scale displacement and strain fields for specimen subjected to pure tension are measured using in-situ multiscale digital image correlation technique. The specimen is specked with different scale pattern ranging from 5 to 500 μm size. The smallest scale, 5 μm, speckles are used for local meso-scale deformation measurement. In this case an optical microscope is used to record the local information within 1 mm square field of view. On the other hand, the larger size, 500 μm, speckles are used to measure the continuum level deformation. In this case two digital cameras with 5 megapixel resolution are used in 3D arrangement by considering the entire width of the specimen inside the field of view. Both the optical microscope and the digital camera systems are triggered simultaneously to acquire the deformation at the same time scale. The displacement and strain fields are extracted using digital image correlation. The effect of local deformation on the overall displacement and strain of low carbon steel is presented by comparing with the macro scale deformation and strain fields. Furthermore, microstructure images are obtained by optical microscope and used for the analysis of local strain field coupled with the strain field from digital image correlation.

Hybrid Computational and Experimental Approach to Identify the Dynamic Initiation Fracture Toughness at High Loading Rate

In this work, a new hybrid computational and experimental method is proposed which can be used to extract the dynamic fracture parameters in engineering materials at high loading rate. Torsional Hopkinson Bar is used to load an aluminum spiral notched specimen in mode I fracture configuration at different loading rate. A tubular specimen with spiral crack at 45° made of Aluminum 6061-T6, was used for the experiment. Using the experimental measured torque value as input computation analysis is performed to identify the dynamic initiation fracture toughness of the material. For the computational analysis, finite element is implemented in Abaqus. The result show that the dynamic stress intensity factor is loading rate sensitive, giving \( {K}_{Ic}^d\cong \left(1.2\mp 0.2\right){K}_{Ic}. \) The crack initiation time is also influenced by the rate of loading.