George C. Kirby

A HYBRID NUMERICAL/EXPERIMENTAL TECHNIQUE FOR DETERMINING THE HEAT DISSIPATED DURING LOW CYCLE FATIGUE

Abstract A hybrid numerical/experimental technique for determining the heat dissipated by a uniformly deforming specimen is presented. The method is applicable under steady as well as non-steady conditions, and the solution is independent of the amount of heat conducted from the grip sections. The technique was applied in a study of the heat dissipated from a specimen under high fatigue loads. For the aluminum alloy considered (6061-T6), it was found that between 85 and 95% of the irreversible work was dissipated as heat. The results also showed that the residue (i.e., the work absorbed by the material) may potentially be used to gauge the fatigue life of a component.

The Relationship of Tensile Specimen Size and Geometry Effects to Unique Constitutive Parameters for Ductile Materials.

Direct observation of solid deformation at all points in a material test specimen is generally not possible. This becomes important when an internal point experiences the maximum effective deformation of the entire specimen. The simple reduction of load-displacement data to material constitutive parameters, as for inelastic tensile specimen necking, is precluded. Although the mechanics of necking has received considerable attention, the complementary issue of constitutive parameter determination from inelastic specimen experimental data has not. This investigation addresses both aspects of the problem for HY-100 steel. An experimental-computational procedure is demonstrated, which generates material constitutive parameters valid at strains beyond those at the onset of necking. Neck deformation, specimen size and specimen geometry effects are used to ensure the uniqueness of the parameters in the sense that a single uniaxial continuum true-stress–true-strain curve predicts the behaviour of laboratory specimens with different geometries. The solution parameters for HY-100 steel are developed, in the context of an incremental elastic–plastic constitutive formulation, by this procedure. These results support the view that tensile specimen necking is the result of the interaction between imperfection-free specimen geometry, material behaviour and applied load for relatively unsophisticated constitutive formulations if the constitutive parameters are accurately determined. In addition, broken symmetry of specimen deformation is computationally predicted and experimentally confirmed for long, slender specimen geometries.

Prediction of classical fracture initiation toughness

Abstract Initiation of crack growth in ductile materials has long been a concern to the manufacturing and design community. The current analysis considers a definition of fracture at the continuum scale to determine the onset of fracture. The strain energy density associated with fracture at the continuum scale is used as the fracture criterion. The critical strain energy density and the scale invariant continuum stress-strain curve were determined from a hybrid computational-experimental analysis of the deformation of a series of tensile tests performed on cylindrical specimens of HY-100 steel. Computational simulation of fracture initiation in a compact specimen of HY-100 steel containing a crack was performed using two-and three-dimensional nonlinear finite element techniques and continuum toughness concepts. Predicted values of classical fracture toughness parameters are within 3.5% of experimentally determined values.

Strain-based shape estimation algorithms for a cantilever beam

Strain-displacement mappings based on linear and quadratic curvature assumptions are derived, compared for a numerical model and applied to a 4.37 m tapered composite boom with a circular cross-section. Displacement estimations are obtained for both the vertical and horizontal directions with displacement estimation errors of less than 0.2 mm in the vertical direction and 1 mm in the horizontal direction. Limitations on strain displacement algorithms for long booms are discussed as well as strain sensor noise effects on estimation accuracy.

Optimal sensor layout for shape estimation from strain sensors

A procedure is developed for mapping strain sensor readings into displacements. Optimal sensor layouts are determined by examining predicted mode shapes from synthesized strain data. The singular values of the transformation matrix bound the error in the inferred displacements. Issues of spatial aliasing as well as sensor spacing are also addressed. The methodology was validated by comparing both static and dynamic shape estimations with experiments.

Ductile alloy constitutive response by correlation of iterative finite element simulation with laboratory video images

Abstract The accurate description of material response consists of a material constitutive formulation and material parameter values. The reduction of material test specimen load versus displacement data to constitutive parameters is often precluded by inelastic material response and deformation inhomogeneity within the specimen. For ductile engineering alloys, these effects are influenced by specimen geometry and must be uncoupled from specimen geometry to characterize the large strain material response. The accuracy of material parameters at such strains should be demonstrated for subsequent applications to design and analysis. Iterative solution for material constitutive parameters is discussed in the investigation. The use of video processing of laboratory tensile test specimens is combined with successive computational simulations of the specimen responses. The solution for HSLA-80 steel constitutive parameters, in the context of incremental plasticity theory, is presented. The material response is treated as the unknown in the computational simulations. It is iteratively modified to achieve correlation with the laboratory experiments. Two different specimen length-to-diameter aspect ratios are utilized to ensure the geometry independence of the material solution and to facilitate efficient solution. The constitutive iteration sequence illustrates the sensitivity of specimen response to material nonlinearity.

Location and estimation of damage in a beam using identification algorithms

We describe an algorithm for locating and estimating damage in a beam used in connection with an identification algorithm. The damage location and estimation is based on a finite element model of the beam. The concept is to start with a model of the beam which is known to accurately model the healthy beam. This model also models the damage that is expected in the beam. Using dynamic response data, an a estimated damage coefficient for each beam element is computed for each identified modeshape and ,modal frequency. Damage in each element is determined by comparing estimated damage coefficients for each element across all modes. The performance of this algorithm is illustrated on a cantilever beam through simulation and experiment.

A Deployable Truss Beam for Long or Lightly Loaded Space Applications

This paper describes development of a deployable graphite truss beam that enables construction of very large space structures such as kilometer-sized solar sails or large sparseaperture interferometers. The development’s objective was to demonstrate that the beam could be fabricated, stowed and deployed as described in the original concept, and this was accomplished. Innovations include roll-up stowage that allows use of highest modulus graphite and unrestricted beam diameter, precise deployment by a powered mechanism and snap-together joints, the ability of the deployment mechanism to walk along the beam after deployment to integrate the beam into a larger system, and a means to increase beam hierarchy. The rigid nodes provide a significant increase in beam strength, compared with pin-jointed beams, for applications with very great L/D ratios.

Strain-displacement mapping for a precision truss using Bragg gratings and identified mode shapes

A procedure for mapping strain measurements to nodal displacements for a 3.74 meter laboratory truss is presented and validated using experimental data. Assumptions of small displacements and a linear displacement-strain relationship were used to develop the strain- displacement mapping. Due to the assumed discrete nature of the space truss structure, the transformation depends only on kinematics and hence only geometric data is required for the mapping. The procedure is therefore valid for quasi-static deformations as well as dynamic deformations. Estimated displacements for several nodes are compared with truth measurements obtained from both a laser interferometer system and accelerometers. It is shown that the accuracy of the predicted displacement for a limited number of sensor depends not only upon the deformation state but also which degree of freedom is being estimated.

Genetic algorithm optimization of feedback control systems

In this paper we are concerned with Smart Materials that contain many actuators and sensors along with digital signal processing electronics that allow for the implementation of a control algorithm. Smart Materials have been proposed for the active control of sound from a vibrating structure. Here we investigate the design of structural control systems for these Smart Structures for noise suppression. First we model the radiated acoustic waves in terms of the velocity of the surface of the structure. Then we formulate an optimal control problem as a linear system that has a transmission zero in the path between the disturbance force and the shape that radiates best. A geometric description of the problem relates control problem to the acoustics. This optimal control problem is solved using a genetic algorithm.