P. Raju Mantena

Structural Damage Assessment Using Vibration Modal Analysis

Vibration techniques have been employed for detecting the presence and monitoring the progression of damage in structures. Pinpointing the location of damage is a more complicated and elaborate task. This paper presents modal analysis techniques for locating damage in a wooden wall structure by evaluating damage-sensitive parameters such as resonant pole shifts and mode shapes, residue and stiffness changes. Artificial damage (simulating termite degradation) was created in one of the walls of a specially constructed room. The wall was excited using an impact hammer and its frequency response measured using a laser vibrometer. Resonant poles (plotted in the s-plane) were used for identifying modes that are sensitive to damage, since not all modes are equally affected by the presence of damage. The damaged region was identified by visual comparison of the deformation mode shapes before and after damage. The modal residue and stiffness changes were also quantified for a better representation of the damage location.

Impact and dynamic response of high-density structural foams used as filler inside circular steel tube

Structural foams have good energy absorption properties and are effective in reducing the vulnerability of sandwich structures. This research investigated the impact and dynamic response of three different high-density polymeric structural foams; designated A, B and C for proprietary reasons. Foam-C had the lowest density out of the three; density of foam-B was approximately twice the density of foam-C, while the density of foam-A was about three times the density of foam-C. The cylindrical foam samples were initially impacted at different velocities in a DYNATUP Model 8250 instrumented impact test machine and their energy absorption was characterized from the resulting load–deflection data. Each of the three foams was then modeled as filler inside a circular steel tube of 0.8 mm thickness. Non-linear finite element analysis was performed under displacement controlled quasi-static compressive monotonic loading using PATRAN as pre-processor and ABAQUS Standard commercial software. The area under the load–deflection curve was calculated to obtain the absorbed energy and the crush loads for the three foam fillers were compared. Results indicate that foam-A having the highest density was more effective as filler inside the circular steel tube, with the intermediate density foam-B performing equally well under uni-axial compressive loading. Foam-C, which had the lowest density, was found to be ineffective as filler in this application due to large differences in stiffness between this foam and the enclosed steel tube. A TA Instruments Model 983 DMA (dynamic mechanical analyzer) was used for obtaining the storage and loss modulus along with the damping and glass transition properties of the different density structural foams. Frequency multiplexing was also used in conjunction with the time–temperature superposition principle for characterizing the long-term behavior of these viscoelastic foams.

Experimental and Finite Element Analysis of Pultruded Glass-Graphite/Epoxy Hybrids in Axial and Flexural Modes of Vibration

The effects of hybridization on the extensional and flexural dynamic properties of pultruded composite materials are reported in this paper. The composite materials considered were made up of unidirectional glass, graphite fibers in an epoxy matrix, and hybrids of glass-graphite/epoxy produced by the pultrusion manufacturing process. The dynamic storage modulus and loss factor of the rectangular and circular cross-section samples were first evaluated using the nondestructive impulse-frequency response vibration technique. The long and slender flat specimens were analyzed in both extensional and flexural modes of vibration in a free-free test configuration, whereas the thicker round specimens were analyzed only in the axial mode of vibration. The properties of the monofiber type glass/epoxy and graphite/epoxy composites were used as input to the finite element model for predicting and experimental validation of various pultruded (and hypothetical) glass-graphite/epoxy hybrid combinations. The modal strain energy method was used for computing the structural loss factors of the hybrid specimens based on the element stiffness matrices and estimated mode shapes in the fundamental mode of vibration. Results of this investigation showed excellent agreement between experimental data and numerical predictions for the dynamic extensional properties of both flat and round specimens. The numerical results for dynamic flexural properties of flat specimens were found to have a similar trend as that obtained from the experimental technique. Small variations in flexural properties could be attributed to the irregular shapes (which were not taken into account in the finite element model) of layers in the hybrid combinations formed during the pultrusion manufacturing process. Results also demonstrated that while the extensional properties were independent of the fiber location and fiber packing geometry, the flexural properties were highly dependent upon these two factors. Previously reported data (which was obtained by testing these flat hybrid specimens in the flexural mode of vibration in a clamped-free boundary condition) were re-analyzed using the free-free configuration and modal strain energy method, yielding more reasonable results.

Static and Low-Velocity Impact Response Characteristics of Pultruded Hybrid Glass-Graphite/Epoxy Composite Beams

The influence of hybridization on the crashworthiness and energy-absorption characteristics of pultruded glass-graphite/epoxy composite beams was investigated. Lowvelocity drop weight instrumented impact tests were conducted on these hybrid composites to determine the load-deformation behavior for evaluating the impact performance in terms of the ductility index, damage initiation, propagation, and total absorbed energies. Three-point static flexural tests were also conducted to compare the static load-deformation characteristics with those of the dynamic low-velocity impact tests. The behavior under both static and dynamic loading conditions was simulated using finite element modeling procedures to identify the failure mechanisms for optimizing the performance of pultruded hybrid composites. Experimental results show that the load and strain to failure of all-graphite/epoxy, all-glass/epoxy and other hybrid composites obtained from impact tests are significantly higher as compared to the static test data. The load-bearing capability of composites after damage initiation (which is dictated by the ductility and failure index) has shown marked improvement for the graphite-outside hybrids when compared with the all-graphite, all-glass, and glass-outside hybrids. The high strain to failure glass fibers absorb considerably higher energy before ultimate failure compared to the brittle graphite fibers; as a result, the fiber content and geometric placement of each type of fiber significantly influenced the energy-absorption characteristics of hybrids. Results indicate that the energy-absorption behavior of pultruded hybrids predicted using finite element modeling is in close agreement with the behavior characterized from experimental impact tests. The effectiveness ofhybridization to improve the impact performance of composites was demonstrated.

Energy Dissipation and the High-Strain Rate Dynamic Response of Vertically Aligned Carbon Nanotube Ensembles Grown on Silicon Wafer Substrate

The dynamic mechanical behavior and high-strain rate response characteristics of a functionally graded material (FGM) system consisting of vertically aligned carbon nanotube ensembles grown on silicon wafer substrate (VACNT-Si) are presented. Flexural rigidity (storage modulus) and loss factor (damping) were measured with a dynamic mechanical analyzer in an oscillatory three-point bending mode. It was found that the functionally graded VACNT-Si exhibited significantly higher damping without sacrificing flexural rigidity. A Split-Hopkinson pressure bar (SHPB) was used for determining the system response under high-strain rate compressive loading. Combination of a soft and flexible VACNT forest layer over the hard silicon substrate presented novel challenges for SHPB testing. It was observed that VACNT-Si specimens showed a large increase in the specific energy absorption over a pure Si wafer.

Surface Fractal Analysis for Estimating the Fracture Energy Absorption of Nanoparticle Reinforced Composites

In this study, the fractal dimensions of failure surfaces of vinyl ester based nanocomposites are estimated using two classical methods, Vertical Section Method (VSM) and Slit Island Method (SIM), based on the processing of 3D digital microscopic images. Self-affine fractal geometry has been observed in the experimentally obtained failure surfaces of graphite platelet reinforced nanocomposites subjected to quasi-static uniaxial tensile and low velocity punch-shear loading. Fracture energy and fracture toughness are estimated analytically from the surface fractal dimensionality. Sensitivity studies show an exponential dependency of fracture energy and fracture toughness on the fractal dimensionality. Contribution of fracture energy to the total energy absorption of these nanoparticle reinforced composites is demonstrated. For the graphite platelet reinforced nanocomposites investigated, surface fractal analysis has depicted the probable ductile or brittle fracture propagation mechanism, depending upon the rate of loading.

Effect of Hybridization on the Creep and Stress Relaxation Characteristics of Pultruded Composites

Pultrusion manufacturing process is a well established technique for the cost-effective production of high-modulus and lightweight composite materials having constant cross-sectional profiles. The pultruded composites are widely used as structural members viz., beams, trusses and stiffeners, owing to the presence of high proportion of axial fibers necessary to sustain large tractive forces. These structural members are subjected to a combination of static and dynamic loading conditions at wide temperature ranges and over longer periods. Since polymeric composites exhibit viscoelastic behavior, the effectiveness of these materials as structural members must be thoroughly evaluated to ensure long-term stability. In previous research, the dynamic performance characteristics of pultruded glass-graphite/epoxy hybrids were evaluated at room temperature. The effects of temperature, frequency, post-curing, along with the type and placement of fibers on the dynamic flexural properties of glass/epoxy and hybrid glass-graphite/epoxy were also investigated. This paper focuses on the evaluation of creep and stress relaxation performance characteristics of pultruded glass-graphite/epoxy hybrid composites. Dynamic mechanical analysis technique was adopted for the accelerated creep and stress relaxation testing. Time-temperature superposition principle, which greatly reduced the experimental time, was effectively utilized for predicting the creep and stress relaxation properties of the hybrid composites. Results indicate that the type and amount of fibers as well as their lay-up sequence plays a significant role in determining the flow and load bearing characteristics.

Low-velocity impact response and Dynamic characteristics of glass-resin composites

This paper describes a test methodology used for characterizing different glass-resin composite systems with respect to their low-velocity impact response, dynamic modulus and inherent damping properties. Impact tests were conducted on SMC plaque samples in an instrumented impact test machine. A Dynamic Mechanical Analyzer was used for testing small rectangular samples in the fixed frequency mode of operation. An impulse-frequency response vibration technique was used for obtaining the dynamic modulus and loss factor of cantilever beam specimens at ambient conditions. The dynamic modulus, glass transition and damping peak intensities obtained from the DMA and the vibration techniques are correlated with the impact test data. The potential application for screening glass-resin formulations with respect to their impact performance is discussed.

Ultrasonic and vibration methods for the characterization of pultruded composites

Abstract Characterization of unidirectional fiber reinforced glass/epoxy pultruded composites using ultrasonics (high frequency, 1-5 MHz) and the impulse-frequency response vibration (intermediate frequency, 50-100 Hz) technique, is demonstrated here. This paper compares the response of both of these non-destructive test techniques to the changes in the pultrusion process variables and to the indeed contaminants introduced during manufacturing. The ultrasonic methods use multi-mode techniques of wave velocity and attenuation measurements to measure the viscoelastic constants of the pultruded composite while the vibration technique provides the dynamic flexural modulus and loss factor (damping) measurements. Quasi-destructive assays were also performed using a low frequency (1-50 Hz) Dynamic Mechanical Analyser (DMA) to verify the state of pultruded samples with induced contaminants (simulated porosity and interfacial debonding) and the results compared with the non-destructive measurements. Mathematical models to describe the influence of porosity and debonding agents on the material properties were derived based on statistical regression analysis procedures. Results indicate that the peak damping value of the tan δ curve obtained from the DMA is a sensitive parameter to detect abnormalities in the finished product. The ultrasonic velocity and dynamic flexural modulus measurements provide useful information on the stiffness characteristics while the attention and loss factor can be related to the anomaly-sensitive damping properties of the material.

Axial Loading and Buckling Response Characteristics of Pultruded Hybrid Glass-Graphite/Epoxy Composite Beams

The effects of hybridization on the buckling characteristics of flat pultruded glass-graphite/epoxy composite beams has been investigated. Theoretical buckling loads were obtained based on Euler’s formulation. Finite element models were constructed with different beam dimensions and end constraint conditions to simulate buckling. Experiments were conducted to verify and validate the theoretical and finite element results. The experimental results were found to be consistent with the theoretical and finite element models. The buckling characteristics of composites showed that buckling strengths improve with increase in graphite fiber content. The improvements were more pronounced when glass fibers were replaced with graphite fibers located in the outer layers of the composite. The effectiveness of hybridization for improving the buckling performance of composite beams is highlighted.