Arup K. Maji

Process zone and acoustic-emission measurements in concrete

It is necessary to study the microlevel failure mechanisms of a material in order to improve its quality and to develop a rational constitutive model to describe the material. Nonlinearity and strain-softening behavior of concrete has to be incorporated into any model which can be implemented into efficient design. Acoustic-emission (AE) techniques are useful for obtaining information pertaining to internal cracking and investigating the applicability of a particular material model.The process of localization of cracks and movement of the fracture process zone was studied using acoustic-emission techniques. The rate of acoustic-emission events and sources of acoustic-emission activity were studied for plain-mortar and model-concrete specimens loaded in direct tension.The study shows that acoustic-emission events localize to a region near the notch before peak load is attained. The region of activity progresses through the specimen during further loading and subsequent strain softening. Acoustic-emission events were used to locate the fracture-process zone (FPZ), and to check this location against the location of the effective crack tip as evaluated by a modified linear-elastic fracturemechanics model for concrete as well as by microscopical observations.

Acoustic emissions from reinforced concrete

Acoustic emissions from reinforced-concrete beams, reinforcing bars and plain concrete cylinders were monitored. Acoustic-emission events were used in a study of source locations, frequency characteristics, and other analytical methods that have found use in the past for evaluating acoustic-emission data in other fields of engineering. Tests were done on reinforced-concrete beams under flexural loading, individual reinforcing bars under pure tension, concrete cylinders under compression, and reinforcing bars subject to pullout tests.The experimental data were first analyzed with conventional acoustic-emission methodology. A critical look at many acoustic-emission techniques currently used in other materials (metals, composites, etc.) demonstrated some of the difficulties of applying the same techniques to reinforced concrete. More importantly, it illustrated the limitations of signal processing and parameter estimation of acoustic-emission events as viable nondestructive-evaluation (NDE) techniques for reinforced-concrete structures. Subsequently, on the basis of the experimental results, some of the more promising aspects of developing acoustic emission into a structural monitoring tool are discussed.

Dynamic Response of Retrofitted Masonry Walls for Blast Loading

A full-scale blast test was conducted on eight masonry walls reinforced with two and four layers of carbon fibers and two types of polymer matrices. The walls were then subjected to a 0.45-kg pentolite booster suspended from the ceiling of a test structure. The pressure-time history caused by the blast and the resulting displacement response were measured during the test. This paper presents a summary of the test program and the corresponding results from a nonlinear single degree of freedom analysis. The results provide a basis for determining effective means of retrofitting existing masonry walls and designing new structures to withstand blast loads. The paper also outlines a fiber-reinforced polymer retrofit design procedure for walls subjected to blast loading.

Shape measurement and control of deployable membrane structures

The shape inaccuracies of inflatable antennas and the potential shape control of the surface of those structures are investigated. Surface shape inaccuracies are due to geometric nonlinear deformation. Correcting the shape of these inflatables focused on the integration of piezopolymer actuators on the membranes. The out-of-plane displacements of a membrane structure were assessed with the shadow moiré method. The experimentally measured shape of the structure confirmed the extent of deviation from the ideal optical surface, a paraboloid of revolution. Active control of the shape of the membrane was tested using a piezoelectric material, polyvinylidene fluoride (PVDF). The deformation caused by actuation of the membrane structure was evaluated using electronic speckle pattern interferometry. An analytical solution was developed to verify the extent of shape correction that can be achieved by embedded PVDF actuators. It was confirmed that micron-level shape corrections are possible for future space-based sensors that use inflatable antennae technology.

Fiber print-through mitigation technique for composite mirror replication

The quickest method for generating a lightweight composite optic is to replicate an optical-quality glass tool onto a carbon-fiber- reinforced polymer CFRP. However, fiber print-through creates an un- acceptable sinusoidal surface roughness on replicated CFRP mirrors; chemical and thermal shrinkage during cure are commonly hypothesized to be the dominant causes. In order to mitigate fiber print-through, two methods of generating a polishable resin layer were investigated. The first method employs the application of a resin film to the CFRP surface. The second method, which is a more unconventional approach, generates a cocured resin layer using magnetic fibers. The latter ap- proach is being developed to eliminate the application of additional resin layers to the CFRP surface, since additional layers present structural disadvantages. It was found that the magnetic fiber technique is compa- rable to the conventional approach in mitigating fiber print-through. Due to the presence of a 0.25-mm-thick buffer above the reinforcing phase, a final polishing step was used to attain optical quality features on all of the replicated specimens. CFRP and magnetic fiber samples were polished to within 50-A rms roughness 1-m to 1-mm bandwidth.

Mixed mode crack propagation in quasi-brittle materials

Abstract Crack propagation in rectangular blocks of mortar containing a central notch and subjected to uniaxial compression were studied. Four different notch orientation angles (18°, 36°, 54° and 72°) with respect to the loading direction were used. Holographic Interferometry was used to observe crack initiation and propagation. It was possible to detect crack extensions during the experiments, hence the load vs crack extension curve for each notch orientation angles could be obtained. A separate Holographic Interferometry method was used to measure crack opening and sliding displacements by making four independent observations of holographic fringes from the same holographic plate. Crack initiation theories were employed to study their relative merits for predicting crack initiation angles and loads. A Finite Element Method (FEM) using quarter point singular crack tip element was used to calculate Stress Intensity Factors (SIF) and crack surface displacements for different inclinations and extensions of the propagating cracks. It was found that while crack initiation was predicted well by some of the theories, it was necessary to account for the traction forces on the crack surface before any propagation criterion could be identified. Opening and sliding of the crack faces determined by Holographic Interferometry (HI) and clip gage measurements were used to find the normal and shear traction applied to the propagating crack faces. The SIFs for the traction free cracks were corrected by taking into account normal and shear tractions along the propagating crack. It was concluded that the Maximum Hoop Stress Criterion was reasonable and K I stress intensity factor at the tip of the propagating crack was dominant in the failure mechanism.

Meter-Class Membrane Mirror with Active Boundary Control

Work has been ongoing in the design, fabrication, and testing of a 0.75 meter diameter mirror using thin-film polymer membranes. Advances in polymer film production have resulted in membrane less than 24 microns in thickness with excellent surface roughness and sub wavelength thickness variation. This has allowed the possibility of using a lenticular system consisting of a clear polymer canopy with a reflective polymer mirror integrated and pressurized to a desired focal length. Typical aberrations for such a system consist of spherical aberration as well as those associated with the boundary. The membrane mirror currently being developed accounts for these errors by utilizing a technique to reduce the spherical aberration present in the membrane by coating it with a varied thickness stress coating. This alters the mechanical properties of the membrane so that when pressurized it is shaped in a way matching the design prescription. An active boundary control system is also utilized to help alleviate any errors near the boundary due to any uneven stresses and any mounting errors. The progress to date on this system is presented in this paper.

Identifying source of acoustic emissions in structures using lamb waves and vibration modes

A process for global monitoring of a structure for location of a source of acoustic emissions using the vibration modes of a structure together with the repetition rate of acoustic emission events in said structure thereby allowing determination of source location of acoustic emission events in said structure is provided. Further a process for identifying the arrival of different modes of lamb waves at the same transducer distance, based on the different propagation velocities of these modes is also provided so as to determine the distance from a single transducer to the source of said lamb waves.

Assessment of electronic shearography for structural inspection

Electronic shearography (ES), a laser interferometry technique, has the potential for large-scale structural inspection and for identifying cracks and strain anomalies. A system based on this technology could possibly be used for noninvasive inspection of structures with high insensitivity and robustness. One major problem is the existence of largerigid body motions in typical engineering structures such as bridges and high-rise buildings. These rigid-body motions are large enough to cause a complete decorrelation of the characteristic speckle pattern that is obtained by illuminating the object surface. This nullifies any possibility of interference between speckled images obtained at different stages of loading, as is necessary for interferometry applications. A systematic study was conducted to characterize the speckle pattern obtained from typical civil structures as a function of the illumination and imaging system. Experiments were carried out to quantify speckle decorrelation as a function of object motion and instrumentation characteristics. A fracture mechanics based finite element (FEM) analysis was carried out on an existing fractured bridge to determine the strains and displacements. The results of these studies were subsequently used to define the parameters of a structural inspection system based on ES for field applications.

Validation of the Quadratic Composite Failure Criteria with Out-of-Plane Shear Terms:

The Tsai–Wu Quadratic Failure Criterion is a modified tensor polynomial criterion, that is widely used and the most readily accepted failure criterion for orthotropic materials. While it is an excellent criterion for the majority of composite structures, there is limited experimental validation of this criterion when predicting the failure envelope for “combined shear” failures or, better stated, failures that involve both 2-D (in-plane) and 3-D (out-of-plane) shear stresses in an orthotropic material or laminate. Since several common types of material failure involve complex stress states, the ability to incorporate the effect of 3-D shear stress into failure prediction is attractive. The 3-D Tsai–Wu quadratic failure criterion that incorporates multiple shear stresses is presented in this text and tests were conducted on several laminate specimens in an effort to validate this criterion. Failure load predictions were made to validate the modified 3-D criterion and compare the results with the 2-D Tsai–Wu criterion for specialized cases where multiple shear stresses were present at failure. This testing clearly shows strong agreement between the 3-D criterion’s predictions and experiments for these cases, implying that the standard 2-D Tsai–Wu quadratic failure criterion can be safely extended, through the addition of terms, to 3-D cases.