Difference In Strain Energy Research Articles

The sensitivity of the stiffness and thickness of a titanium inlay in a cementless PEEK femoral component to the micromotions and bone strain energy density

Polyetheretherketone (PEEK) has been proposed as alternative material for total knee arthroplasty implants due to its low stiffness, which may reduce stress-shielding. In cementless fixation, a proper primary fixation is required for long-term fixation. Previous research showed that the lower stiffness of a cementless PEEK femoral component results in larger micromotions at the implant-bone interface compared to a cobalt-chrome femoral component. A titanium inlay on the PEEK implant surface may improve the primary fixation while maintaining the favourable stiffness properties. Therefore, the effect of thickness and stiffness of a titanium inlay on the primary fixation and stress-shielding was investigated. A finite element model of the femur and femoral component was created with five titanium inlay variants. The micromotions and strain energy density (SED) were quantified as outcome measures. The distal thin – proximal thick variant showed the largest resulting micromotions (51.2 µm). Relative to the all-PEEK femoral component, the addition of a titanium inlay reduced the micromotions with 30 % to 40 % without considerably affecting the stress-shielding capacity (strain energy difference of 6 % to 10 %). Differences in micromotions (43.0–51.2 µm) and SED between the variants were relatively small. In conclusion, the addition of a titanium inlay could lead to a reduction of the micromotions without substantially affecting the SED distribution.

FEA Based Design and Stability Study of Electroless Ni-P Coating Plated over a Stepped Shaft under Thermal Load

ABSTRACT Modelling of Ni-P coating and substrate assembly is done using the finite element analysis framework. Coating having a thickness of 50 µm is applied over a stepped shaft made of stainless steel and analysed in Ansys Workbench at an elevated temperature of 200°C. The thermal effect on the system is studied and results are discussed in detail with respective figures. Elastic modulus of Ni-P coating required for the analysis is evaluated from the nanoindentation test under 10 mN load. The analysis focuses on the evaluation of the critical design parameters such as equivalent stress, total deformation, strain energy and thermal strain over the entire body. Besides, four separate paths are created on the model to study the difference of induced stress and strain energy between the contacting surfaces of the shaft and the coating. It is found that there is a sharp discontinuity in these values and this discontinuity is further interpreted as a measure of the stability of coating over the substrate. It is found that the higher the difference of stress and strain energy, lower is the stability of the assembly which is considered to be realistic and in accordance with the elasticity principles. .

A strain energy-based elastic parameter calibration method for lattice/bonded particle modelling of solid materials

A generic strain energy-based parameter calibration method is proposed to calibrate the elastic parameters of lattice or bonded-particle modelling of solid materials. Unlike surrogate models or general optimisation-based calibration methods in the discrete element method, this approach does not require running any physical simulations to obtain tentative solutions and thus ensures high calibration efficiency. The novelty of this method is achieved in five aspects: (1) the calibrated parameters are physically decoupled as elastic and strength parameters; (2) the calibration of elastic parameters is converted as an optimisation problem to minimise the difference of strain energy between the discrete and continuum systems; (3) the strain energy is analytically determined in both systems; (4) explicit expressions of gradients of the loss function facilitate obtaining the optimal particle-scale parameters; and (5) when subjected to two independence strain fields, a closed-form solution is derived for a pair of uniform normal and tangential stiffnesses. The proposed calibration framework is validated with four different examples. All the results confirm the reliability of the proposed calibration method in lattice or bonded-particle modelling of solid continua. • A generic strain energy-based parameter calibration method is proposed. • Parameter calibration is physically decoupled as elastic and strength parts. • The method does not require running simulations to obtain tentative solutions. • Elastic parameters can be accurately calibrated.

Atomistic insight into ideal strength, fracture toughness, deformation, and failure mechanisms of magnetocaloric Fe2AlB2

AbstractThe thermal and magnetic cycling of a magnetocaloric material degrades its mechanical properties and device performance. We used ab initio tensile and shear simulations to investigate the mechanical properties such as ideal strength, fracture toughness and deformation and failure mechanisms of Fe2AlB2 at finite strain. The weakest direction of Fe2AlB2 is [010], and the weakest slip system is (010)[100]. The ideal tensile strength (σm = 12.51 GPa) of Fe2AlB2 is less than its ideal shear strength (τm = 13.32 GPa). The strain energy difference (ΔE = −13 eV/f.u.) of Fe2AlB2 confirms cleavage fracture as its most plausible failure mode. The concomitant changes in the c‐lattice parameter and Al–Al bond along the c‐axis determine the ideal tensile strength of Fe2AlB2. Likewise, the subtle changes in the a‐lattice parameter and Al–Al bond along the a‐axis specify its ideal shear strength. The tensile strain induces a magnetic to nonmagnetic transition in Fe2AlB2 at the critical tensile strain (εc = 0.08). A similar transition occurs at the critical fracture strain (εcf = 0.48) due to shear deformation. The brittle nature of Fe2AlB2 is predicted by its anisotropic Poisson's ratios, strength ratio, and failure mode. The fracture toughness of Fe2AlB2 for mode I fracture is (KIc = 2.17 MPa m1/2), mode II fracture is (KIIc = 1.33 MPa m1/2), and mode III fracture is (KIIIc = 1.16 MPa m1/2). The failure mechanism of Fe2AlB2 due to the tensile deformation is marked by the sharp and appreciable changes in the lattice parameters, bonding characteristics, and magnetic moment of Fe at the critical fracture strain (εcf = 0.44). This study provides a fundamental understanding of the mechanical behavior of Fe2AlB2 at the finite strain relevant to the cycling stability of the magnetocaloric Fe2AlB2.

A new damage detection method of single-layer latticed shells based on combined modal strain energy index

The existing damage detection methods are challenging to be directly applied to single-layer latticed shells due to the complex mechanical mechanism and modal characteristics of such structures. In this regard, an effective damage detection method based on the combined modal strain energy index is proposed in this paper to detect the location of the damaged members in such structures. Based on the vibration modal features before and after damage of structures, the modal matching and combination method is presented to generate the combined mode, which include the modal matching procedure based on the modal assurance criterion and the modal combination procedure using the Kriging interpolation. Compared with the damaged mode, the specially constructed combined mode can significantly reduce the mode shape discrepancy in the undamaged region on the basis of retaining the characteristic differences in the damaged members. On this basis, the combined modal strain energy change index is proposed according to the modal strain energy difference between the original mode and the combined mode. Since the proposed index is established according to the difference between the combined vibration mode and the original vibration mode, the values of damage index of the undamaged members are greatly reduced, so as to effectively avoid the occurrence of misjudgement. Several numerical examples with different damage scenarios are studied in order to evaluate the proposed approach. The results clearly indicate that the proposed method features high damage localization accuracy compared with traditional methods and good measurement noise robustness.

Optimal Design and Biomechanical Analysis of a Biomimetic Lightweight Design Plate for Distal Tibial Fractures: A Finite Element Analysis

The treatment of fractures of the distal tibia can be problematic due to the insubstantial soft-tissue covering this part of the anatomy. This study investigates a novel strategy for minimally invasive plate osteosynthesis of distal tibia fractures called bionic lightweight design plating. Following the structure of the animal trabecular bone, we utilized topological mathematical methods to redesign the material layout of the internal fixation device to fulfill the desired lightweight design within given boundary conditions. The results showed that this method can maintain the same stability of the construct as the original plate after a reduction in the original volume by 30%, and the differences in strain energy of plates and maximum node displacement of constructs between the constructs [RP construct vs. LP construct] were not statistically significant (p > 0.05). In the safety assessment of the constructs, the peak stress of plates between constructs was found to not be statistically significantly different under a doubled physiological load (p > 0.05). The average stress of the plates’ elements exceeding the allowable stress was analyzed, and no statistically significant differences were found between the two constructs under axial compression stress conditions (p > 0.05). The average stress of the plates’ elements in the redesigned plating construct under torsional stress conditions was 3.08% less than that of the locked plating construct (p < 0.05). Under the double physiological load condition, 89% of the elements of the plate in the redesigned plating construct and 85% of the elements of the plate in the locked plating construct were lower than the maximum safe stress of the plate, which was 410 MPa (secondary allowable stresses). That reminds us the topology optimization offer a possible way to improve the capacity of soft tissue protection while ensuring the safety of the RP construct by reducing the volume of the implants.

Introduction of low strain energy GdAlO3 grain boundaries into directionally solidified Al2O3/GdAlO3 eutectics

In order to tune the mechanical properties of the Al2O3/GdAlO3 (GAP) eutectic ceramics, low energy GAP grain boundaries (GBs) were introduced into Al2O3/GAP eutectic ceramics. We prepared single-crystal Al2O3/bi-crystal GAP eutectic ceramics with the directional solidification technique, in which the Al2O3 was single-crystal, but the GAP was bi-crystal. The crystallographic orientation relationships between Al2O3 and GAP was determined as [101¯0] Al2O3 || [001] GAP-I || [11¯0]GAP-II, (112¯0) Al2O3 || (200) GAP-I, (112¯0)Al2O3 || (112) GAP-II, and (110) GAP-I || (110) GAP-II. Such GAP GB had ultra-low strain energy. The phase boundaries (PBs) strain energies are higher than that of the GAP GB. However, the difference in the interfacial strain energy between the two PBs was quite small. The reason for the successful preparation of single-crystal Al2O3/bi-crystal GAP eutectic ceramics was attributed to almost the same driving force requirements for the two PBs during solidification. We envisaged that the concept of interfacial structure design could open new pathways for high-performance materials design.

MODERN CONSTRUCTION SOLUTIONS FOR PRESTRESSED CABLE DOMES AND WAYS TO IMPROVE THEM

To create fundamentally new innovative large-span structures of buildings and structures coverings, modern design solutions of prestressed cable domes of the Tensegrity type are considered. The service life of the first built Tensigrity domes is only 35 years. These are fairly new, effective structures that require careful study and use of modern scientific approaches for their design using software systems, since their work under load and the construction process are quite complex. The design analysis and erection of self-stressed structures is based on the invention of an equilibrium structure, the so-called tensegrity form. The search for the shape is multidimensional and consists of the stage of computational analysis of a self-stressed dome for the equilibrium position of elements and their nodes, selection of the most stable and rigid structure, as well as taking into account possible unfavorable loads during operation and the initial load in the elements from the application of prestressing. To determine the shape of cable domes, a nonlinear programming problem with given axial forces is formulated, which can be considered as the problem of minimizing the difference in the total strain energy between the elements of the cables and struts under constraints on the compatibility conditions. The first step in calculating the prestressing of a cable dome is to assess the feasibility of its geometry. The possibility of forming a cable dome of negative Gaussian curvature is considered and a method for calculating the prestressing for this new shape is investigated. The proposed method is effective and accurate in determining the allowable prestressing for a cable dome with negative Gaussian curvature and can be used for other types of prestressed structures. The new directions for the development of effective constructive solutions for large-span coatings are presented, including a suspended-dome structure, which combines the advantages of a mesh shell and a cable dome. Special attention should be paid to experimental studies on models of tensegrity domes, the results of which demonstrate the positive and negative aspects of the behavior of structures under load, the process of their erection, as well as the possibility of control and restoration during operation.

Indirect structure damage identification with the information of the vertical and rotational mode shapes

In this paper, a robust method is proposed to detect damage extent and location of structural elements focusing on data type and acquisition method and aiming to the promotion of model updating tools. The novelty of this method is rotational mode shape acquisition, which provides valuable information on the damage. In this method, the damaged elements are indirectly identified by detecting the healthy elements and eliminating them from searching space. This method requires to minimize the modal strain energy difference between the damaged model and numerical model via an optimization algorithm, then an improved genetic algorithm (IGA) is used. In this study, four numerical and two experimental damage scenarios are applied on a simply supported beam to examine the performance of the proposed method. Modal data acquisitions have been made by vision-based method and accelerometer sensors. The results demonstrate that this method could accurately figure out the location and severity of damage using just the first mode shape since rotational mode shapes are more sensitive than vertical mode shapes in damage detection.

Constitutive Modeling of Structural Steels: Nonlinear Isotropic/Kinematic Hardening Material Model and Its Calibration

Numerical models of structural components that deteriorate primarily due to geometric instabilities under multiaxis cyclic loading are sensitive to both the assumed geometric imperfections and the nonlinear material model assumptions. Therefore, the accuracy of the constitutive model is a desirable feature in finite-element simulations. However, the classic Voce-Chaboche metal plasticity model, ubiquitous among commercial finite-element software, is found to underestimate the initial yield stress in structural steels by about 10%–30% when calibrated to minimize the overall difference in strain energy between the model and test data of load protocols representative of earthquake loading. This paper proposes a refined version of the Voce-Chaboche material model. When compared with the original model, the updated one improves the prediction of the initial yield stress, can simulate initial yield plateau behavior, and better estimates experimental cyclic stress-strain data. Constraints on the model parameters are established, a calibration procedure is developed, and model parameters are proposed for nine structural steels used worldwide. Source code for the material model is also made publicly available. A case study demonstrates that steel component behavior is sensitive to subtle differences in the material response that arise between the Voce-Chaboche and the proposed material models.

On the Origin of Superiority in Wear Response of Superelastic NiTi Compared to Conventional Ti6Al4V

The difference in wear behavior of superelastic NiTi and conventional Ti6Al4V alloys with similar elastic limits was investigated. Reciprocating dry sliding wear tests were performed with a ball-on-plate tribometer at different normal loads and sliding times. Mechanical properties were also examined using different microindentation tests, and wear track morphologies and chemical compositions in the scar area were characterized by scanning electron microscopy (SEM) and energy-dispersive spectrometry. Superelasticity of the NiTi alloy was significantly beneficial to its tribological properties, and the wear resistance of the NiTi alloy was up to 40 times higher than that of the Ti6Al4V alloy. Such a significant difference cannot be attributed only to the relatively small difference in hardness (H) or H/E ratios (E = elastic modulus) of the materials but, rather, to a difference in strain energy, which dissipates in the material and results from normal load of the sliding ball. To estimate this energy, spherical microindentation tests in the wear scar were carried out. Understanding the difference between the tribological behavior of the NiTi shape memory alloy and conventional Ti6Al4V material provides insight into wear progression, and an investigation of its mechanisms can effectively prevent the destruction of components, prolonging their safe service life.

Damage Detection in Glass/Epoxy Laminated Composite Plates Using Modal Curvature for Structural Health Monitoring Applications

This paper deals with detection of macro-level crack type damage in rectangular E-Glass fiber/Epoxy resin (LY556) laminated composite plates using modal analysis. Composite plate-like structures are widely found in aerospace and automotive structural applications which are susceptible to damages. The formation of cracks in a structure that undergoes vibration may lead to catastrophic events such as structural failure, thus detection of such occurrences is considered necessary. In this research, a novel technique called as node-releasing technique in Finite Element Analysis (FEA), which was not attempted by the earlier researchers, is used to model the perpendicular cracks (the type of damage mostly considered in all the pioneering research works) and also slant cracks (a new type of damage considered in the present work) of various depths and lengths for Unidirectional Laminate (UDL) ([0]S and [45]S) composite layered configurations using commercial FE code Ansys, thus simulating the actual damage scenario. Another novelty of the present work is that the crack is modeled with partial depth along the thickness of the plate, instead of the through the thickness crack which has been of major focus in the literature so far, in order to include the possibility of existence of the crack up to certain layers in the laminated composite structures. The experimental modal analysis is carried out to validate the numerical model. Using central difference approximation method, the modal curvature is determined from the displacement mode shapes which are obtained via finite element analysis. The damage indicators investigated in this paper are Normalized Curvature Damage Factor (NCDF) and modal strain energy-based methods such as Strain Energy Difference (SED) and Damage Index (DI). It is concluded that, all the three damage detection algorithms detect the transverse crack clearly. In addition, the damage indicator NCDF seems to be more effective than the other two, particularly when the detection is for damage inclined to the longitudinal axis of the plate. The proposed method will provide the base data for implementing online structural health monitoring of structures using technologies such as Machine Learning, Artificial Intelligence, etc.

Selection and mechanical evaluation of γ/γ boundary in γ-TiAl alloy

Simulation of lamellar boundary (γ/γ) selection during the transformation of α2’→α2+γ in γ-TiAl alloy has been investigated using an improved 3D Phase Field (PF) model, and the mechanical properties of γ/γ boundaries are evaluated by Molecular Dynamics (MD) simulation. The effects of elastic strain energy and interfacial energy difference among different types of γ/γ interfaces on the occurrence frequencies of the three types of γ/γ boundaries (i.e., true twin (TT), pseudo-twin (PT) and ordered domain (OD)) have been studied. It is found that the boundary fraction of TT increases with increasing elastic strain energy or increasing interfacial energy difference among γ/γ interfaces. Besides TT, the elastic interaction also favors OD while PT is suppressed. The relative fractions of PT and OD could be effectively controlled by elastic strain energy. Furthermore, the frequency of PT or OD can also be adjusted by their interface energy ratio. The simulation results well explain the phenomenon that boundary frequencies via experimental statistics deviate from those under random distribution without interface selection. It can also be found that one-twin-dominant zones (OTDZ) can be induced by higher elastic strain energy and larger interface energy difference among different γ/γ boundaries through the merging of adjacent small clusters with the same γ/γ boundary type under the premise of correlated and sympathetic nucleation. Through MD simulation, it is found that the strengths of the configurations with true twin boundary (TTB) and ordered domain boundary (ODB) are almost the same, while the strength of that with pseudo-twin boundary (PTB) is relatively low. The order of plasticity of the configurations with three types of γ/γ interfaces is ODB > TTB > PTB.

Preferential grain growth and textural evolution of AZ31B Mg alloy during annealing after isothermal compression at 400 °C

Optimizing post-deformation cooling conditions can to a great extent control grain size and texture in the hot forming components, especially larger-size ones. Here, we investigated the effects of pre-strains and 400 °C isothermal annealing on the microstructural evolution of AZ31B magnesium alloy after isothermal compression. For the pre-strain exceeding ~6%, grains coarsen immediately without the need of an incubation period. The rapid coarsening mainly happens for a group of strain-free grains with grain orientation spread value lower than 0.5°. Two kinds of strain-free grains in the pre-strained microstructure are mainly responsible for the rapid increase in grain size. One is the undeformed grains at the pre-strain of 6%. The other is the dynamic recrystallized grains when the pre-strain is higher than 10%. Driving force for the preferential grain growth is ascribed to the difference in stored strain energy between deformed and strain-free grains. The final grain size after 900 s annealing decreases with the increase in pre-strain level. As both c-axes of undeformed grains and dynamic recrystallized grains are nearly parallel to the compression direction, the preferential growth of these grains results in an enhanced basal texture after annealing. These results demonstrate that larger deformation extent and instant cooling can effectively refine grains and weaken the texture of magnesium alloy during hot forming.

Distributed Strain Damage Identification Technique for Long-Span Bridges Under Ambient Excitation

Distributed strain measurement, such as long-gauge fiber bragger grating (FBG) techniques, has developed rapidly in the field of structural health monitoring. However, strategies of corresponding damage identification still need to be enhanced. The damage identification technique based on distributed strain measurement is proposed identifying the structural damage under ambient excitation. Damage indices, like the distributed strain energy difference (DSED) and the relative distributed strain energy (RDSE), are derived from the power spectral density of the frequency response function of distributed strain response and further employed by detecting damages in the structure. A numerical analysis is performed on a long-span cable-stayed bridge with several assumed damage scenarios at various degrees in the girder. Damage localization capability and robustness of the proposed damage indices are discussed. In addition, damage quantification utilizing the proposed indices is conducted. The results indicate that the proposed technique can accurately identify the locations and extent of the damage under ambient vibration excitation.

Hip Implant Design With Three-Dimensional Porous Architecture of Optimized Graded Density

Even in a well-functioning total hip replacement, significant peri-implant bone resorption can occur secondary to stress shielding. Stress shielding is caused by an undesired mismatch of elastic modulus between the stiffer implant and the adjacent bone tissue. To address this problem, we present here a microarchitected hip implant that consists of a three-dimensional (3D) graded lattice material with properties that are mechanically biocompatible with those of the femoral bone. Asymptotic homogenization (AH) is used to numerically determine the mechanical and fatigue properties of the implant, and a gradient-free scheme of topology optimization is used to find the optimized relative density distribution of the porous implant under multiple constraints dictated by implant micromotion, pore size, porosity, and minimum manufacturable thickness of the cell elements. Obtained for a 38-year-old patient femur, bone resorption is assessed by the difference in strain energy between the implanted bone and the intact bone in the postoperative conditions. The numerical results suggest that bone loss for the optimized porous implant is only 42% of that of a fully solid implant, here taken as benchmark, and 79% of that of a porous implant with uniform density. The architected hip implant presented in this work shows clinical promise in reducing bone loss while preventing implant micromotion, thereby contributing to reduce the risk of periprosthetic fracture and the probability of revision surgery.

Elucidating the Trends in Reactivity of Aza‐1,3‐Dipolar Cycloadditions

This report describes a density functional theory investigation into the reactivities of a series of aza‐1,3‐dipoles with ethylene at the BP86/TZ2P level. A benchmark study was carried out using QMflows, a newly developed program for automated workflows of quantum chemical calculations. In total, 24 1,3‐dipolar cycloaddition (1,3‐DCA) reactions were benchmarked using the highly accurate G3B3 method as a reference. We screened a number of exchange and correlation functionals, including PBE, OLYP, BP86, BLYP, both with and without explicit dispersion corrections, to assess their accuracies and to determine which of these computationally efficient functionals performed the best for calculating the energetics for cycloaddition reactions. The BP86/TZ2P method produced the smallest errors for the activation and reaction enthalpies. Then, to understand the factors controlling the reactivity in these reactions, seven archetypal aza‐1,3‐dipolar cycloadditions were investigated using the activation strain model and energy decomposition analysis. Our investigations highlight the fact that differences in activation barrier for these 1,3‐DCA reactions do not arise from differences in strain energy of the dipole, as previously proposed. Instead, relative reactivities originate from differences in interaction energy. Analysis of the 1,3‐dipole–dipolarophile interactions reveals the reactivity trends primarily result from differences in the extent of the primary orbital interactions.

Comparative Study of Pore Formation Energy by High Intensity, Nanosecond Electrical Pulse.

Nanosecond, high intensity electric pulses create nanopores in the cell membrane. Pore formation energy is probed by taking account of the strain energy based on the continuum model. Maxwell stress acting on the cell membrane is included in the 3D model calculation as well as the effect of membrane curvature. In addition, comparison between cylindrical and toroidal pores were made to explore the difference of strain energy and force over the pores at a range of radii. Through the analyses the transmembrane potential were kept constant in order to obtain a transient response in that the electric pulse has a ultrashort duration and pore-evolving process is rapid as well. Our results demonstrate that under the same circumstances toroidal pores have higher strain energy than cylindrical pores due to the surface area and volume of the pore shape.

Mechanistic investigations on asymmetric N-H insertion of amines catalyzed by palladium-chiral guanidine complex

The mechanism and stereoselectivity of the asymmetric N-H insertion reactions between α-diazocarbonyl compounds and amines mediated by palladium-chiral guanidine complexes were investigated at the BP86-D3(BJ)/def2TZVP (SMD, CH2Cl2)//BP86-D3(BJ)/def2SVP (SMD, CH2Cl2) level at 303 K. The non-catalytic reaction occurred through a stepwise mechanism, with a high activation barrier of 56.4 kcal mol−1. Good linear correlations between the global nucleophilicity index (N) of amine, Hammett substituent constants (σP), and the activation energy barriers (ΔG≠) were found. The Pd(0)-guanidine-catalyzed reaction consisted of three continuous steps, including: (i) generation of Pd-carbene intermediate by dinitrogen loss from α-diazoesters substrate, (ii) formation of CN bond, and (iii) 1,2-H transfer by metal-associated ylide, accompanying with the regeneration of catalyst. A water molecule accelerated the final H-transfer by constructing hydrogen bonding network. The cyclohexyl group in ligand provided sufficient steric shielding around Pd-carbene intermediate from the re-face attack by amines. The combination of the hydrogen bonding orientation of amide moiety of guanidine ligand, as well as the steric repulsion between the ester group of α-diazoester substrate and bulky CH(Ph)2 group in ligand played an important role in controlling the stereoselectivity, affording the predominant S-configuration product observed in experiment. Introducing one aromatic ring to chiral backbone of the guanidine ligand enhanced the enantiodifferentiation of products by increasing the difference of strain energy (ΔΔEstrain) of Pd-carbene moiety along two competing pathways. Different from Pd(0)-catalyst, the Pd(II)-chiral guanidine complex accelerated N-H insertion reaction via Lewis acid catalysis. In this process, the formation of free ylide in the reaction led to low ee. These results were in good agreement with experimental observations.

The controversy about nonclassical ions: Finished too early?

AbstractThe debate on nonclassical ions centered almost exclusively on the behavior of 2‐norbornyl esters. The hallmark of a nonclassical charge distribution in solvolysis reactions was always seen in the seemingly exceptional fast reaction of the 2‐exo‐derivatives, with slow rates of the endo‐esters. While the nonclassical bridged structure of the free norbornyl ion has been unequivocally established, possible steric contributions to the rate differences in solution remained a matter of debate. Many formerly used solvents were less suited to exhibit uniform reaction mechanisms; the most appropriate solvent hexafluorisopropanol was introduced rather late. Solvolysis rates of many cyclic esters including medium rings can be predicted by force field calculated strain energy differences ΔSIsp3/sp2 between sp3 and sp2 states. Large deviations from the correlation are observed for 2‐tosyloxy norbornane‐ and for 17‐tosyloxy steroids; they indicate that both nonclassical charge delocalization and steric hindrance play a major role for the reactivity differences. These reach an unprecedented rate constant ratio of &gt;30.000 between, eg, the 17‐steroid epimers, much larger than the ratio of 1.550 in 2‐norbornyl reactions. The presence of alkyl groups or hydrogen antiperiplanar to the leaving group can lead to extreme reactivities by 1,2‐shifts or Wagner‐Meerwein rearrangement if asymmetric bridged transition states can occur which approach the stability of tertiary cations. The long controversy about nonclassical ions illustrates how progress in science can deviate significantly from the ideal pathways lined out by Karl Popper.