Challenges in Mechanics of Biological Systems and Materials, Thermomechanics and Infrared Imaging, Time Dependent Materials and Residual Stress, Volume 2

Within the context of this study, it is intended to develop new biocompatible polymer-based composite materials. For this purpose, PEEK (polyether ether ketone) was used as the matrix of the composites, and carbon nanotube (CNT) was used as the reinforcement. The PEEK matrix and 1 wt. % CNTs were melt mixed in a twin-screw extruder to obtain the compound in the form of pellets. Then, using these pellets, composites were manufactured using a pellet extruder type 3D printer. After manufacturing, microstructure of the specimens was observed using optical microscopy, and mechanical characterization was performed through three-point bending tests and Charpy impact tests. The most noteworthy result of the microscopy was the absence of any discontinuity between the layers of the specimen for pure PEEK specimens. Furthermore, the mechanical improvement was not apparent by the CNT incorporation for the tested composites. In conclusion, pellet extrusion is thought as a promising tool for the manufacturing of biocompatible materials for biomedical applications.

Parkinson’s disease is a neurodegenerative disease affecting over eight million people globally. Among its symptoms is an involuntary rhythmic tremor in the body that manifests in the hands. These hand tremors affect one’s ability to perform basic motor functions, often leading to frustration, social anxiety, and isolation. Approaches to mitigating these tremors generally involve surgery or medications. Those approaches that are non-invasive are often still expensive or conspicuous. The goal of this research is to make a wearable, non-invasive, mechanical device that can significantly diminish hand tremors. The device is targeted to be accessible to large populations through low cost, ease of use and care, and adjustability for individual needs. The research builds from an ongoing study where a simplified proof of concept was established that reduced tremors in some instances by over 70%. In this study, novel kinematic mechanisms have been introduced to optimize the range of effectiveness while reducing the size and weight of the device by more than 20%. Prototyping and testing have been performed on a mechanical test hand, and initial testing shows quantifiable improvements in size, weight, and effectiveness.

Glaucoma is a leading cause of irreversible blindness that affects over 60 million people worldwide. Glaucomatous eyes are associated with risk factors such as elevated intraocular pressure (IOP) and low corneal hysteresis. Reliable non-invasive measurement of IOP remains a formidable challenge that limits the accurate diagnosis of glaucoma and associated intervention therapies. This work investigates the propagation of shear-dominated elastic waves in hydrostatically inflated corneal tissue phantoms based on the optical coherence elastography (OCE) technique. Unlike previous approaches reported in the literature, we analyze the dispersion relation of guided elastic waves in the phantoms by accounting for both small amplitude viscoelastic wave propagation and finite static deformations. The analytical approach we adopted will enable the determination of the storage and loss shear moduli dependence on finite strains in the cornea that results from hydrostatic pressures. This work provides a modeling and experimental framework for accurately characterizing viscoelastic properties and the IOP of corneal tissues.

The contour method for assessing residual stress is a widely accepted technique and has been widely applied to validate and verify modelling predictions of welding processes since its inception 20 years ago. Briefly, it comprises the stress-free cutting of a body containing residual stress. Then, using the resulting averaged distortion measured from both cut faces, the calculation of the elastic loading to “force” the cut face “flat” is possible. However, a wider application as compared to other residual stress assessment techniques has been somewhat limited due to the absence of a common computational framework to process experimental data. This is at odds with the relatively accessible equipment requirements to generate this data: a wire electrodischarge machining (EDM) facility for cutting and a coordinate measurement machine or surface profilometer to capture the resulting deformation of the cut surfaces. In the present work, an open-source platform is presented that accommodates the full analysis work flow, from registration of surface profiles from each side of the cut, alignment and averaging of these profiles, numerical fitting of this surface, and finite element pre- and post-processing to recover a contour plot of the residual stress. Programmed in Python, this has been accomplished via a graphical user interface for initial preprocessing steps, followed by close integration of commercial finite element code (Abaqus) and open-source equivalents (Gmsh, CalculiX). Called pyCM (Python Contour Method), the overall analysis sequence is described employing a thick-section weld in pressure vessel steel (SA533), as well as showing that the results stemming from this tool employing either Abaqus or CalculiX are identical. Further advantages of pyCM will be discussed including diminished cost of accessing the technique and the ease of sharing of results and analysis strategies among practitioners. Factors, which necessitate these decisions, are inherent to the technique, and the present contribution lends itself to making the results obtained by the contour method more transparent.

Milled thin-walled monolithic aluminum structural parts are widely used in the aerospace industry due to their appropriate properties such as a high overall strength-to-weight ratio. The semifinished products made of aluminum alloy 7050 undergo a heat treatment to gain this increased strength and hardness: Typically, three steps including solution heat treatment, quenching, and age hardening are carried out. This also leads to high initial bulk residual stresses (IBRS) within the part in the range of ±200 MPa. In rolled aluminum plate, plastic stretch (T7451 heat treatment designation) provides very effective relief of residual stress, decreasing IBRS to a level of ±20 MPa. In large parts with low bending stiffness, even that low level of IBRS can cause appreciable distortion. Besides the IBRS, the machining-induced residual stresses (MIRS) contribute to the distortion. This study investigates how IBRS in different stress relieved 7050-T7451 semifinished products vary and how this variation affects the distortion of milled thin-walled monolithic structural parts. A linear elastic finite element distortion prediction model, which considers the IBRS as well as the MIRS as input, was used to analyze the effect of varying IBRS on the distortion for different part sizes and geometries. The model was validated by machining of those parts and measuring their distortion. IBRS were measured via slitting technique and MIRS via incremental hole-drilling.

The slitting method is a well-known technique for residual stress measurement. It consists of cutting a slot in the specimen while monitoring the relieved strains with one or more strain gauges. The stress distribution is then computed by solving a reverse problem.

The use of Digital Image Correlation to replace the standard strain-gauge-based approach has been proven successful in previous work. In this new contribution, the authors want to explore the integrated Digital Image Correlation (iDIC) potential as a direct replacement for the entire procedure. Instead of using DIC as an optical extensometer—i.e., using a large DIC subset to extract single-point data—in the iDIC formulation, the shape functions describing the surface displacement field satisfy the equilibrium conditions. Thus, the minimization parameters (both in space and in time) are the Legendre polynomial weights directly connected to the residual stress values.

To verify the new approach, an aluminum beam is loaded in bending above the yield stress and then unloaded using a four-point configuration; then, the slot is performed using a milling machine while imaging the back face. The use of strain gauges allows comparing the results of the new approach with the standard one.

The use of thermal indexes assessed by monitoring rapid fatigue tests with infrared detectors as reliable damage parameters to reconstruct the S/N curve is currently a topic still presenting open points. First of all, the selection of the proper thermal index representing damage is a topic to be explored, and also the relationship between the damage from rapid stepwise tests and constant amplitude tests is another point of discussion.

In the present work, we deal with such an issue by investigating the first amplitude harmonic (FAH) of thermal signal related to thermoelastic phenomena and dissipative effects too. It has been demonstrated that FAH is related to stiffness degradation and stress-induced effects. Moreover, it provides a local analysis of specific effect related to the material fatigue damage without artefacts.

The results show that due to the relationship between stiffness degradation and FAH and specific material properties, it is possible to reconstruct the S/N curve by carrying out just one constant amplitude test and a stepwise rapid test. Moreover, the capability of temperature FAH to study fatigue behaviour and detect damage during any loading procedure is also presented.

A direct correlation exists between the microstructure of a composite (defect distribution, residual stresses, fiber geometry, and orientation) and the global composite mechanical behavior. Manufacturing processes govern the composite microstructure characteristics; thus, rapid process–microstructure–performance relationships must be established for emergent materials and manufacturing processes. In this chapter, a preform was manufactured via a modified Additive Manufacturing followed by Compression Molding (AM-CM) process with carbon fiber (CF) filled acrylonitrile butadiene styrene (ABS). The AM-CM plates were prepared with varying process parameters yielding distinct composite microstructures with varying porosity and fiber orientations. Micro-computed tomography has been used to characterize the microstructure of these composites. Preliminary mechanical testing (static) was conducted to ascertain these composites’ static strength and stiffness properties. The current chapter strives to correlate the microstructure of these composites with their fatigue performance using rapid infrared thermography (IRT). A typical “staircase” loading has been adopted for IRT-based fatigue testing via self-heating. The stabilized temperature versus applied maximum stress profile plots yields a bi-linear curve indicating a pseudo-fatigue limit of each composite configuration. This bi-linear curve, coupled with stiffness degradation plots, can map composite microstructure with its fatigue performance. The approach outlined will provide a basis for rapidly characterizing and inserting emergent materials and manufacturing processes in fatigue-critical applications.

The cost and size of instrumentation for thermoelastic stress analysis (TSA) have often been an inhibiting factor for its use in industrial applications. This has been alleviated to some extent by the development of packaged infrared (IR) bolometers which have become popular for non-destructive evaluation of structures. Recent work has demonstrated that an original equipment manufacturer (OEM) microbolometer, combined with a single circuit board with dimensions equivalent to a credit card, can be used to detect cracks of the order of 1 mm long and to monitor their propagation. The monitoring system costs about one tenth the price of a packaged bolometer and can provide results in quasi real time without the need for calibration. The system uses the principles of TSA to acquire thermal images and evaluate the amplitude of the thermal signal over the field of view, i.e., a map of thermal emission amplitude. Feature vectors are extracted from the time-varying maps of thermal emission amplitude and used to identify changes in them that occur when a crack initiates or propagates in the field of view. Since the technique does not generate TSA data but uses uncalibrated thermal emission data, it has been named Condition Assessment by Thermal Emission (CATE). The CATE system’s crack detection capability has been evaluated in laboratory conditions and compared against a state-of-the-art IR photovoltaic effect detector. It was demonstrated that the CATE system is capable of detecting cracks as small as 1 mm at loading frequencies as low as 0.3 Hz. Evaluations in industrial conditions on large-scale structures are being concluded and imply that there will be little loss of capability in the more demanding applications.

Thermoelastic stress analysis (TSA) is an experimental technique that provides maps of stress variation in loaded solids related to the variation of temperature, under linear and adiabatic conditions. Traditionally, TSA has been employed in mechanical components under monotonic harmonic excitation measuring with infrared thermography. Hence, the amplitude of the temperature signal in a pixel can be related to the stress amplitude, after proper calibration of the thermoelastic properties. The main purpose so far has been the analysis of stress maps for fatigue and fracture mechanics by applying a load at a low frequency, about 10 Hz, but high enough to avoid heat conduction. However, recent studies have exploited harmonic excitation for a new use, which is the evaluation of stress maps associated with mode shapes. These studies have shown the capability to provide the shape information at much higher frequencies, of hundreds and even thousands of Hertz. In this work, the authors evaluate the capabilities of this methodology, based on harmonic excitation of mechanical components, to identify the effect of discontinuities on the stress maps associated with mode shapes. It is based on the fact that each mode shape is unique, changing the stress distribution from one to another. For this purpose, different kinds of geometrical discontinuities on flat components were evaluated. First, their natural frequencies were identified, and, afterwards, a harmonic excitation was applied at those frequencies. The response of the specimen was recorded with an infrared thermocamera. The analysis of the thermal signals, related to stress, reveals that how much the maps are altered depends on the location of the discontinuity regarding the modal shape of the sound specimen. In this way, with a set of some mode shapes, it is possible to identify, at least, one that highlights the effect of discontinuities.

In recent years, induction thermography has been proposed to detect and characterize shallow and open cracks on steel components. The specimen is inspected by an infrared camera that records the heating and cooling behavior after a short induced electrical current pulse. The presence of a crack changes the distribution of the induced eddy currents with a consequent more significant local heating around the region of interest than the related sound one. This work investigates the influence of the testing parameters with an experimental approach, considering the analysis of a ferromagnetic master specimen with imposed simulated open cracks of different depths. The influence of the relative position between the coil and simulated crack has been evaluated, considering both numerical and experimental results. Two different coils have been adopted to optimize the experimental tests in terms of inspection time and improve the signal-to-noise ratio. It has been demonstrated that the post-processing of the raw thermal data is necessary to have useful information about defect detection for a pulse duration of 10 ms with only 20 A. A linear relationship exists between the phase contrast and the nominal defect depth.

Fatigue is an irreversible process accompanied by the heat dissipation which is significant when the transition from anelastic to inelastic strains happens. In view of this, in the last years, the heat dissipation has been accepted as an appropriate damage indicator of the material.

The estimation of the heat dissipation can be obtained by detecting the surface thermal footprint of the specimen by using thermography-based techniques. However, the energy dissipation as heat is highly sensitive to the environmental and test conditions and the microstructure status. Therefore, the experimental measurement is always associated with some inaccuracies and only provides an estimation of the heat dissipated during fatigue.

This paper is mainly focused on the numerical modeling of the heat dissipation performed by COMSOL Multiphysics software in order to investigate the factors that can affect the estimation of the heat source by means of thermography. The obtained results have been compared with an analytical solution derived from the one-dimensional heat equation. This study can provide valuable insights about the shape of the heat sources produced during the cyclic loading and differences associated with thermographic measurements and actual values, which are the main goals of this work.

Part of the sealing function in the pneumatic system of the French high-speed train (TGV) is provided by elastomeric components, such as nitrile butadiene rubber (NBR) O-rings. Like most elastomers, NBRs are sensitive to thermo-oxidative aging. In the current application, aging results in a strong hardening of the O-rings, which alters the sealing function and impacts the lifetime of the mechanical pneumatic system. Therefore, understanding the origin of this aging, studying its effects on the mechanical properties, and modeling it are of a paramount importance in optimizing time in maintenance operations of the TGV.

In a previous study, accelerated aging tests have been carried out with different NBR formulations at different times and temperatures. These tests enabled us to reproduce ex situ the hardening observed on the O-rings. Aging mechanisms have been identified and related to the aging conditions of the NBR formulations through different characterization techniques: infrared spectroscopy, swelling tests, X-ray fluorescence spectrometry, differential scanning calorimetry thermogravimetric analysis, and micro-hardness.

To consider the effects of the mechanical loading on the aging process and the mechanical behavior of tested NBRs, mechanical spectroscopy (DMA) analyses have been carried out. The results obtained highlight how mechanical loadings impact the aging of these NBRs and quantify the effects of aging on their viscoelasticity. From the collected data, a hyperelastic model has finally been identified to simulate the behavior of O-rings by the finite element method. These first results lay the foundation of a selection methodology for O-rings used in the pneumatic system of the TGV.

We present an extension of our previously published work in which we utilized an in situ/embedded Digital Image Correlation (DIC) technique to evaluate the effect of aging on the interface debonding strength in an idealized particulate composite. The idealized composite consists of a single ~650 μm glass bead inclusion at the center of a Sylgard 184 dog bone tensile sample along with an embedded DIC speckle pattern at the mid-plane. The speckle pattern enables the measurement of the strain field in the region surrounding the embedded glass bead while the sample undergoes a tensile load to failure. The measured strain field can then be used in conjunction with the global load measurement to characterize the local stress required to induce a failure at the interface. In previous works we have demonstrated how this approach can be used to characterize interface debonding. Construction of these tensile samples is done in a stepwise fashion with partial cure cycles in order to achieve a reliable in situ/embedded DIC speckle pattern. This stepwise construction introduces two uncertainties into the experiment, namely, cold joints between the casting layers and functional grading of the stiffness properties due to each layer experiencing different curing histories. In this work we present an evaluation of the effect of the cold joints and functional grading by performing tension tests to measure the global stiffness of dog bones of various sample configurations spanning the number of cold joints and the cure cycle history. By comparing the measured stiffness of each sample configuration, the effect of the cold joints and functional grading are determined.