Topologically interlocked material (TIM) systems are load-carrying assemblies of unit elements interacting by contact and friction. TIM assemblies have emerged as a class of architectured materials with mechanical properties not ordinarily found in monolithic solids. These properties include, but are not limited to, high damage tolerance, damage confinement, adaptability, and multifunctionality. The review paper provides an overview of recent research findings on TIM manufacturing and TIM mechanics. We review several manufacturing approaches. Assembly manufacturing processes employ the concept of scaffold as a unifying theme. Scaffolds are understood as auxiliary support structures employed in the manufacturing of TIM systems. It is demonstrated that the scaffold can take multiple forms. Alternatively, processes of segmentation are discussed and demonstrated. The review on mechanical property characteristics links the manufacturing approaches to several relevant material configurations and details recent findings on quasi-static and impact loading, and on multifunctional response.

A nonlinear dynamic methodology is developed for detecting and estimating fatigue damage precursor in isotropic metal structures, prior to fatigue crack initiation, based on measurement of changes in the structure nonlinear response to harmonic base excitation. As an example, a damage precursor feature was extracted for cantilever beams by quantifying the reduction in the nonlinear stiffness because of localized microscopic material softening. The nonlinear dynamic analytical model parametrically tracks changes in the nonlinear structural stiffening as a function of the structural response and the loading cycles. Experimental results are obtained by exciting the base of cantilever beams at various amplitude levels. At high response amplitudes, the beams experience three competing simultaneous nonlinear dynamic mechanisms: (i) stiffening because of high response amplitude at the fundamental mode; (ii) softening because of inertial forces; and (iii) softening because of localized microscopic material damage caused by cyclic micro-plasticity. The third mechanism—a potential precursor to fatigue crack initiation in the structure—resulted in a cumulative softening because of early accumulation of cyclic fatigue damage and is the focus of this study. The loading intensity and the number of cycles influenced the relative contribution of these dynamic mechanisms. Nanoindentation studies near the beam clamped boundary were conducted to confirm the fatigue degradation in the local mechanical properties as a function of loading cycles. The proposed detection method is simple to implement and detects the presence of damage precursors but lacks the resolution to discern spatial distributions of the damage intensity. Based on prior knowledge of the structural dynamic characteristics of the beam and using the proposed methodology, damage level and stiffness reduction can be detected and estimated based on changes in the nonlinear structural response. The nonlinear stiffness term in the equation of motion is found to be very sensitive to the proposed fatigue damage precursor, making it an effective method for monitoring structural degradation prior to crack developments. The proposed methodology provides new insights and constitutes a significant step toward the development of new inspection techniques. Copyright © 2016 John Wiley & Sons, Ltd.

A review of past and recent developments in multiaxial excitation of linear and nonlinear structures is presented. The objective is to review some of the basic approaches used in the analytical and experimental methods for kinematic and dynamic analysis of flexible mechanical systems, and to identify future directions in this research area. In addition, comparison between uniaxial and multiaxial excitations and their impact on a structure’s life-cycles is provided. The importance of understanding failure mechanisms in complex structures has led to the development of a vast range of theoretical, numerical, and experimental techniques to address complex dynamical effects. Therefore, it is imperative to identify the failure mechanisms of structures through experimental and virtual failure assessment based on correctly identified dynamic loads. For that reason, techniques for mapping the dynamic loads to fatigue were provided. Future research areas in structural dynamics due to multiaxial excitation are identified as (i) effect of dynamic couplings, (ii) modal interaction, (iii) modal identification and experimental methods for flexible structures, and (iv) computational models for large deformation in response to multiaxial excitation.

Multiaxial and uniaxial vibration experiments were conducted in order to study the differences in failure modes and fatigue life for the two types of excitation. An electrodynamic (ED) shaker capable of controlled vibration in six degrees of freedom (DOF) was employed for the experiments. The test specimen consisted of six large inductors insertion mounted on a printed wiring board (PWB). Average damage accumulation rate (DAR) in the inductor leads was measured for random excitations in-plane, out-of-plane, and both directions simultaneously. Under simultaneous multiaxial excitation, the average DAR was found to be 2.2 times greater than the sum of the in-plane and out-of-plane DARs. The conclusion was that multiple-step sequential uniaxial testing may significantly overestimate the durability of large/heavy structures with high center of mass in a multiaxial dynamic environment. Additionally, a test method utilizing uniaxial vibration along a direction other than the principal directions of the structure was examined. This method was found to have significant limitations, but showed better agreement with simultaneous multiaxial vibration experiments.