17 research outputs found
A reduced Iwan model that includes pinning for bolted joint mechanics
No full textBolted joints are prevalent in most assembled structures; however, predictive models for their behavior do not exist. Calibrated models, such as the Iwan model, are able to predict the response of a jointed structure over a range of excitations once calibrated at a nominal load. The Iwan model, though, is not widely adopted due to the high computational expense of implementation. To address this, an analytical solution of the Iwan model is derived under the hypothesis that for an arbitrary load reversal, there is a new distribution of dry friction elements, which are now stuck, that approximately resemble a scaled version of the original distribution of dry friction elements. The dry friction elements internal to the Iwan model do not have a uniform set of parameters and are described by a distribution of parameters, i.e., which internal dry friction elements are stuck or slipping at a given load, that ultimately governs the behavior of the joint as it transitions from microslip to macroslip. This hypothesis allows the model to require no information from previous loading cycles. Additionally, the model is extended to include the pinning behavior inherent in a bolted joint. Modifications of the resulting framework are discussed to highlight how the constitutive model for friction can be changed (in the case of an Iwan–Stribeck formulation) or how the distribution of dry friction elements can be changed (as is the case for the Iwan plasticity model). The reduced Iwan plus pinning model is then applied to the Brake–Reuß beam in order to discuss methods to deduce model parameters from experimental data
On Affine Symbolic Regression Trees for the Solution of Functional Problems
No full textSymbolic regression has emerged from the more general method of Genetic Programming (GP) as a means of solving functional problems in physics and engineering, where a functional problem is interpreted here as a search problem in a function space. A good example of a functional problem in structural dynamics would be to find an exact solution of a nonlinear equation of motion. Symbolic regression is usually implemented in terms of a tree representation of the functions of interest; however, this is known to produce search spaces of high dimension and complexity. The aim of this chapter is to introduce a new representation—the affine symbolic regression tree. The search space size for the new representation is derived, and the results are compared to those for a standard regression tree. The results are illustrated by the search for an exact solution to several benchmark problems
An analytical elastic plastic contact model with strain hardening and frictional effects for normal and oblique impacts
No full textOn the Detection and Quantification of Nonlinearity via Statistics of the Gradients of a Black-Box Model
No full textDetection and identification of nonlinearity is a task of high importance for
structural dynamics. Detecting nonlinearity in a structure, which has been
designed to operate in its linear region, might indicate the existence of
damage. Therefore, it is important, even for safety reasons, to detect when a
structure exhibits nonlinear behaviour. In the current work, a method to detect
nonlinearity is proposed, based on the distribution of the gradients of a
data-driven model, which is fitted on data acquired from the structure of
interest. The data-driven model herein is a neural network. The selection of
such a type of model was done in order to not allow the user to decide how
linear or nonlinear the model shall be, but to let the training algorithm of
the neural network shape the level of nonlinearity according to the training
data. The neural network is trained to predict the accelerations of the
structure for a time-instant using as inputs accelerations of previous
time-instants, i.e. one-step-ahead predictions. Afterwards, the gradients of
the output of the neural network with respect to its inputs are calculated.
Given that the structure is linear, the distribution of the aforementioned
gradients should be quite peaked, while in the case of a structure with
nonlinearities, the distribution of the gradients shall be more spread and,
potentially, multimodal. To test the above assumption, data from an
experimental structure are considered. The structure is tested under different
scenarios, some of which are linear and some nonlinear. The statistics of the
distributions of the gradients for the different scenarios can be used to
identify cases where nonlinearity is present. Moreover, via the proposed method
one is able to quantify the nonlinearity by observing higher values of standard
deviation of the distribution of the gradients for "more nonlinear" scenarios
On the origin of computational model sensitivity, error, and uncertainty in threaded fasteners
No full textPredicting the mechanical response of components requires simplifications and idealizations that affect the fidelity of the results and introduce errors. Some errors correspond to the limited knowledge of intrinsic physical attributes while others are introduced by the modeling framework and mathematical approximations. This paper studies the dependence of the force-displacement response of threaded fasteners on modeling attributes such as geometry, material, and friction resistance using finite element simulations. A systematic comparison of View the MathML source1D,2.5D or 3D3D computational models demonstrates the influence of model properties and the limitations of the methodologies. Finally, the paper discusses the sources of model inputs and model form errors for threaded fasteners
The impact of fretting wear on structural dynamics: experiment and simulation
No full textThis paper investigates the effects of fretting wear on frictional contacts. A high frequency friction rig is used to measure the evolution of hysteresis loops, friction coefficient and tangential contact stiffness over time. This evolution of the contact parameters is linked to significant changes in natural frequencies and damping of the rig. Hysteresis loops are replicated by using a Bouc-Wen modified formulation, which includes wear to simulate the evolution of contact parameters and to model the evolving dynamic behaviour of the rig. A comparison of the measured and predicted dynamic behaviour demonstrates the feasibility of the proposed approach and highlights the need to consider wear to accurately capture the dynamic response of a system with frictional joints over its lifetime
Designing energy dissipation properties via thermal spray coatings
No full textThe coefficient of restitution is a measure of energy dissipation in a system across impact events. Often, the dissipative qualities of a pair of impacting components are neglected during the design phase. This research looks at the effect of applying a thin layer of metallic coating, using thermal spray technologies, to significantly alter the dissipative properties of a system. The dissipative properties are studied across multiple impacts in order to assess the effects of work hardening, the change in microstructure, and the change in surface topography. The results of the experiments indicate that any work hardening-like effects are likely attributable to the crushing of asperities, and the permanent changes in the dissipative properties of the system, as measured by the coefficient of restitution, are attributable to the microstructure formed by the thermal spray coating. Further, the microstructure appears to be robust across impact events of moderate energy levels, exhibiting negligible changes across multiple impact events
Observations of variability and repeatability in jointed structures
No full textThe experimental study of joint mechanics has been limited in its effectiveness due to the high uncertainty associated with assemblies of sub-components. In particular, two categories of uncertainty are variability (the uncertainty in measurements of different, nominally identical parts) and repeatability (the uncertainty in measurements of the same set of parts). As a result, the uncertainty measured is often greater than the nonlinear characteristics being studied (such as amplitude dependent frequency and damping), which makes meaningful experimentation challenging. This paper analyzes the contributors to uncertainty in the form of variability and repeatability in order to make recommendations for methods to reduce the uncertainty and to redesign a joint to improve its dynamics. Experiments are summarized that investigate the role of experimental setup, interface roughness, settling versus wear, interface geometry (both meso-scale and macro-scale), and the structure surrounding the joint. From the results of these studies, recommendations for the measurement of nonlinearities in jointed structures are made
A Comprehensive Set of Impact Data for Common Aerospace Metals
No full textThe results of two sets of impact experiments are reported within. To assist with model development using the impact data reported, the materials are mechanically characterized using a series of standard experiments. The first set of impact data comes from a series of coefficient of restitution (COR) experiments, in which a 2 m long pendulum is used to study “in-context” measurements of the coefficient of restitution for eight different materials (6061-T6 aluminum, phosphor bronze alloy 510, Hiperco, nitronic 60A, stainless steel 304, titanium, copper, and annealed copper). The coefficient of restitution is measured via two different techniques: digital image correlation (DIC) and laser Doppler vibrometry (LDV). Due to the strong agreement of the two different methods, only results from the digital image correlation are reported. The coefficient of restitution experiments are in context as the scales of the geometry and impact velocities are representative of common features in the motivating application for this research. Finally, a series of compliance measurements are detailed for the same set of materials. The compliance measurements are conducted using both nano-indentation and micro-indentation machines, providing sub-nm displacement resolution and μN force resolution. Good agreement is seen for load levels spanned by both machines. As the transition from elastic to plastic behavior occurs at contact displacements on the order of 30 nm, this data set provides a unique insight into the transitionary region
