Initial Force Research Articles

Robust Control Method Design for Underactuated Double-Pendulum Overhead Cranes

Overhead travelling cranes are infrastructure for building construction and their main purpose is to ensure accurate and rapid transport of goods to a given location in the shortest possible time without any residual oscillation. The overhead crane is also a typical underdrive method, which has fewer inputs than the amount being controlled. In some practical applications, uneven weight and f-loads on the hook can lead to load and hook oscillations and the bridge crane can develop the so-called double-pendulum properties. This leads to a high degree of coupling and a high degree of non-linearity between the various state variables of the crane system. The study that follows suggests a new energy-coupled control strategy for an underactuated double-pendulum overhead crane with initial control force constraints. Strong robustness and superior control performance to parameter changes and external disturbances are the proposed controller’s noticeable characteristics. To ensure the smooth start of the trolley, a robust control adds the hyperbolic tangent function to the control procedure. In addition, the stability analysis and stability verification of the control system is given by using the LaSalle’s invariance principle and Lyapunov techniques. The simulation results show that the new energy coupling control method has stability and strong robustness under various conditions such as different rope lengths, load quality, target position and external disturbance when the initial control force decreases.

An In-vitro Evaluation of Retention Force of All PEEK, All Zirconia and Zirconia-PEEK Telescopic Attachment for Mandibular Overdentures.

The objective of this study is to compare retention capacity of mandibular implant-supported overdenture with all zirconia, zirconia-PEEK, and all PEEK telescopic attachments. Sixty acrylic resin models of mandible were used. Telescopic attachment system was made up of all zirconia, all PEEK, and zirconia-PEEK. Evaluation of retention force was done using universal testing machine. Obtained data were statistically evaluated. Highest initial force value was obtained with zirconia-PEEK group. All PEEK and zirconia-PEEK groups showed significantly decreased final retention values in comparison to their initial values. Zirconia can be regarded as the material of choice for telescopic crown, whereas polyetheretherketone and zirconia-PEEK telescopic crowns showed retention loss over a period of time.

Online ride-hailing regulation: a simulation study based on evolutionary game theory

ABSTRACT Game theory contributes to the quantitative study of online ride-hailing regulations; however, prior game models of the online ride-hailing market fail to comprehensively consider government regulation strategies as well as multiple stakeholders in various regulation contexts. This study constructs two system dynamic models of evolutionary games among online ride-hailing platforms, drivers, and passengers. One is the basic model not subject to government regulations, while the other considers government regulations systematically regarding penalty policy, incentive policy, policy adaptability, and public participation. By solving and simulating the model, we study evolutionary stable strategies to control fluctuations in the game process. The results show that an unregulated online ride-hailing system is volatile, and government regulations help stabilize the system. The effect of government regulations can be optimized by adopting a dynamic penalty with a greater initial force, considering platforms as agents in incentive policy, improving policy adaptability, and rewarding public participation.

Axial crushing of novel hierarchical multi-cell square tubes

Inspired by the concept of hierarchy, two types of hierarchical multi-cell square tubes (HMSA and HMSB) are proposed. The crashworthiness of HMSA and HMSB tubes with various wall thicknesses and hierarchical orders is numerically studied. It has been found that the peak crushing force of HMSA and HMSB tubes is smaller than that of single square tubes, which is preferred to minimize the possible injury or damage caused by the initial peak force. Moreover, the mean crushing force, energy absorption, crushing force efficiency and specific energy absorption of HMSA and HMSB tubes increase with the hierarchical order and wall thickness. Compared with single square tubes, the specific energy absorption of the 3rd order HMSA and HMSB tubes increases by more than 1.5 and 1.7 times, respectively. Moreover, the 3rd order HMSB tubes show slightly greater specific energy absorption than that of the 3rd order HMSA tubes. In addition, theoretical models are developed which can well predict the mean crushing force of two new types of tubes.

Initial impact force analysis and simulation of planetary helical gear transmission

The planetary gear transmission is widely used because of its advantages such as large transmission ratio and high transmission efficiency. But at the moment of starting, there is a speed difference between driving gear and driven wheel due to the backlash between the teeth, this will easily cause the initial collision and impact force, which seriously affect the life of gear transmission, even lead to tooth surface pitting and root fracture failure, therefore, it is necessary to conduct a quantitative analysis of the initial collision. Based on the gear meshing principle and the system dynamics theory, the analytical calculation formula of initial impact force for the internal and external meshing gear pair was derived. The impact simulation analysis model of planetary helical gear transmission was established based on the finite element LS-DYNA, and the simulation analysis results were compared with the analytical calculation results, which verified the correctness of the analytical calculation model. This paper reveals the impact collision mechanism and studies the influence law of the speed and temperature on the impact force, thus laying a good theoretical foundation for the dynamic modification design of the planetary gear transmission and preventing the failure of the gear teeth from impact collision.

Feasibility study of self-centring beam-to-column joints with posttensioned SMA strands

The hysteretic behaviour of self-centring beam-to-column joints with posttensioned shaped memory alloy (SMA) strands under cyclic loading was studied in this paper. The material test of the dog bone-shaped SMA coupon was carried out and used to verify the applicability of the SMA constitutive in finite element (FE) models. The FE models of self-centring joints were established by ABAQUS, and the accuracy of the FE models was verified by the relevant test results available from the existing literature. With the verified FE modelling technique, the FE model of a novel self-centring joint was generated, and the numerical results were compared with the hysteretic performance of regular joints. Afterwards, the influences of the steel angle thickness, initial posttensioning force, vertical distance of two SMA strands on one side of the beam, and SMA strand number on the hysteretic performance of the novel joint were discussed in detail. The novel joint exhibited excellent seismic performance. The initial posttensioning force, numbers and vertical distance of SMA strands have significant effects on the stiffness, bearing capacity and self-centring performance of the proposed joints, but have little effects on the energy dissipation. The bearing capacity, stiffness, energy dissipation as well as the residual deformation of the proposed joints improve with increasing steel angle thickness.

Experimental Investigation on Reversible Swelling Mechanisms of Lithium-Ion Batteries under a Varying Preload Force

The safety of lithium-ion batteries has to be guaranteed over the complete lifetime considering geometry changes caused by reversible and irreversible swellings and degradation mechanisms. An understanding of the pressure distribution and gradients is necessary to optimize battery modules and avoid local degradation bearing the risk of safety-relevant battery changes. In this study, the pressure distribution of two fresh lithium-ion pouch cells was measured with an initial preload force of 300 or 4000 N. Four identical cells were electrochemically aged with a 300 or 4000 N preload force. The irreversible thickness change was measured during aging. After aging, the reversible swelling behavior was investigated to draw conclusions on how the pressure distribution affected the aging behavior. A novel test setup was developed to measure the local cell thickness without contact and with high precision. The results suggested that the applied preload force affected the pressure distribution and pressure gradients on the cell surface. The pressure gradients were found to affect the locality of the irreversible swelling. Positions suffering from large pressure variations and gradients increased strongly in thickness and were affected in terms of their reversible swelling behavior. In particular, the edges of the investigated cells showed a strong thickness increase caused by pressure peaks.

Investigation on Dynamic Impact Mechanism of Single Crystal Sapphire 3D Model Based on Cohesive Zone Law

AbstractBased on mesh reconstruction technology, a 3D solid model of single crystal sapphire (SCS) with global embedded cohesive element is established in this study. Using exponential cohesive zone law (CZL), the anisotropy of SCS can be effectively characterized by assigning the mechanical properties of matrix element and cohesive element. Taking the C‐plane SCS model as the research object, the whole process of dynamic evolution of cracks in the model under high‐speed surface impact and high concentrated stress impact is reproduced by finite element simulation technology. The simulation is compared with the experimental results from the aspects of crack initiation morphology, crack propagation path, sample fragmentation form, and mechanical response curve. The dynamic compressive strength reaches 1630.3 MPa with the impact velocity of 30 m s–1 in surface impact type. The initial maximum loading force reaches 320 N when the impact velocity is 8.35 m s–1 in the indentation impact type. In addition, by combining the simulation results, this study clarifies the mechanism of strain rate effect exhibited by SCS from the perspective of stress–strain data and crack evolution characteristics. Therefore, the model established in this study can effectively reflect the mechanical behavior of SCS under impact load.

Unintentional force drifts in the lower extremities.

We explored the phenomenon of unintentional force drift seen in the absence of visual feedback during knee extension contractions in isometric conditions. Based on the importance of knee extensors for the anti-gravity function, we hypothesized that such force drifts would be slower and smaller compared to those reported for the upper extremities. We also explored possible effects of foot dominance and gender on the force drifts. Young healthy persons produced isometric knee extension contractions to different levels, ranging from 15 to 25% of maximal voluntary contraction force, with the help of visual feedback, and then, the visual feedback was turned off. Force change over the time interval without visual feedback was quantified. In the absence of visual feedback, force drifted to smaller magnitudes. The drift magnitude expressed in percent of the initial force magnitude was smaller for smaller initial force levels, ranging between 8 and 15% of the initial force for the initial force magnitude of 15% and 25% of maximal voluntary contraction force. The time exponent of the force drift was independent of the initial force magnitude and was, on average, 6.45s. There were no significant effects of foot dominance or gender, although the male subjects tended to show stronger scaling of the drift magnitude with the initial force level compared to the female subjects. The results show that unintentional force drift is a common phenomenon across limbs and muscle groups. This conclusion fits the theory of control with spatial referent coordinates and the general tendency of all natural systems to drift to states with lower potential energy.

A self-centering hybrid braced precast wall with replaceable U-shaped flexural plates: Experimental and theoretical analysis

A self-centering hybrid-braced precast wall with replaceable U-shaped flexural plates (UFPs), which is composed of a hybrid braced wall, post-tensioned (PT) strands, and steel wall toes, is proposed. The UFPs bolted inside the steel wall toes, which yield at the same thickness outside the plane of the steel plate when wall rocking, providing plastic bending energy dissipation. The steel wall toe made of H-shaped steel provided increased compressive strength and space for the installation of the UFPs. Four wall specimens with varying initial PT forces, UFPs of different thicknesses, and different dampers were investigated using low-cyclic loading tests. After replacing the UFPs, one wall was retested to assess its performance. The precast walls sustained significantly low damage and showed large self-centering capabilities. The maximum drift was up to 4.5%, which far exceeded the specified requirements, and the lateral load still maintained an increasing trend. When the initial PT force was increased, the bearing capacity, initial stiffness, and self-centering capacity of the precast wall increased. When the thicknesses of the UFPs were increased, the energy-dissipation capacity and self-centering capacity of the wall increased and decreased, respectively. The self-centering and bearing capacities of the repaired wall met the seismic requirements but were slightly lower than those of the original wall. The wall with UFPs performed better bearing capacity and stiffness than the wall with variable friction dampers (VFDs) before 1.5% drift. Additionally, the opening load of the wall, the load at the yield of UFPs, and the load at different compression sections were calculated and compared against the test results. The analysis method was able to predict the behavior of the walls reasonably well.

Influence of density gradient and hybrid effect on quasi-static axial crushing behavior of lattice cylindrical structures

Uniform/gradient lattice cylindrical structures (ULCS/GLCS) and hybrid uniform/gradient lattice cylindrical structures (HULCS/HGLCS) are innovatively designed and fabricated. The axial crushing behaviors of these four-type cylindrical structures are investigated by the quasi-static axial compression experiments and numerical simulations. The influence of the density gradient and the hybrid effect (metal skin+ABS core) on the deformation mode, loading capacity, and energy absorption of the lattice cylindrical structures are elaborated systematically. The results show that the gradient design realizes the function of regulation and controllability of the structural deformation. The softer layers (with smaller relative density) in GLCS begin to deform first and then gradually extend to the harder layers (with larger relative density). And it effectively reduces the initial peak compression force (PCF), and makes the load more stable on the premise of energy absorption. However, unstably catastrophic failures are observed before the final densification in both ULCS and GLCS during the compression due to the brittleness of the ABS material and the compression–expansion​ deformation of structure. For HULCS/HGLCS, it presents progressive crushing mode and overcome the unstably catastrophic failure due to the ductility and constraint of the metal skins. The effect of “1 ＋ 1 > 2” on energy absorption from hybrid effect is achieved in HULCS/HGLCS, in which the EA of HULCS and HGLCS increases by 74.62% and 100.78% comparing the sum of all components individually compressed. Moreover, the progressive peak with increasing trend of HGLCS demonstrated that the hybrid design enhances the advantages of gradient design in sandwich structures.

Large-scale igneous intrusion emplacement as a trigger for fluid seepage on the northern South China Sea margin

Widespread mud eruptions have been identified along the northern margin of the South China Sea. However, the ultimate driving force for initiation of the eruptions remains to be discussed. Here, we investigated the discovered hydrothermal vents and the presence of gas/fluid occurring above deep-seated igneous intrusions and volcanic edifices and consider them as a likely trigger. Using a one-dimensional (1D) numerical model, SILLi 1.0, we studied the thermal effect of a 100-m-thick dolerite sill complex emplaced at 1100 °C and 3.8 km depth within the organic-rich Triassic series (∼3.5 wt% total organic carbon). The modeled scenarios also account for variations in sill count and thickness and the effects of erosion. The calculated CH4 produced from contact metamorphism reaches a peak of ∼700 kg/m2/yr shortly after emplacement of the first sill in the complex. When scaled to a sill size of 100 m by 25 km2, i.e., a sill volume of 2500 km3, ∼42−550 Mt of CH4, depending on individual sill thickness, have been produced within 10,000 yr of sill emplacement from sedimentary organic matter in the aureole, with peak generation of ∼17.5 Mt CH4/yr just after emplacement. The newly discovered vents are suggested to have formed as a direct result of sill emplacement and by overpressure generated in the sediments around the sill by hydrothermal fluids and gases liberated during organic matter degradation. Combining data from seismic images with the likely production of carbon gas released from contact metamorphism with igneous intrusions, we suggest that volcanically induced gas release is common along the South China Sea margin. This study supports the important role of magmatism in driving regional-scale gas/fluid seepage. Our findings also suggest that the hydrothermal vents and intrusive magmatic bodies provide postemplacement fluid focusing pathways to shallower strata or the seafloor over long time scales.

Motion Synchronization Control for a Large Civil Aircraft’s Hybrid Actuation System Using Fuzzy Logic-Based Control Techniques

The motion synchronization of the hybrid actuation system (composed of a servo-hydraulic actuator and an electro-mechanical actuator) is very important for all applications, especially for civil aircraft. The current research presents a nested-loop control design technique to synchronize motion between two different actuators, such as a servo-hydraulic actuator (SHA) and an electro-mechanical actuator (EMA). The proposed strategy consists of a trajectory, an intelligent position controller (fuzzy logic-based controller), a feed-forward controller, and an intelligent force controller (fuzzy logic-based controller). Position, speed, and acceleration signals are produced by trajectory at a frequency that both SHA and EMA can follow. The SHA/EMA system’s position tracking performance is enhanced by the feed-forward controller and intelligent position controller working together, while the intelligent force tracking controller lowers the issue of force fighting by focusing on the rigid coupling effect. To verify the effectiveness of the proposed strategy, simulations are performed in the Matlab/Simulink environment. The result shows that the proposed intelligent control strategy not only reduces initial force fighting, but also improves load-rejection performance and output-trajectory tracking performance.

A machine learning based optimisation method to evaluate the crushing behaviours of square tubes with rectangular-hole-type initiators

Rectangular-hole-type crush initiators have attracted much attention due to their exceptional advantages in improving crashworthiness performance. Unfortunately, undesired deformation buckling modes, i.e. bifurcation buckling, can occur and challenge optimisation processes. This paper aims remedies to this limitation and proposes a machine learning based optimisation method that removes these undesired deformation modes to effectively evaluate the effect of initiators’ parameters in improving crush response. First, finite element models of square tubes with different numbers and positions of initiators were built and computed. Second, the complex proportional assessment (COPRAS) method was adopted to rank the crashworthiness characteristics. The results demonstrated that square tubes with two sets of four corner holes along the axial direction were the best choice. Finally, the optimisation method based on machine learning was employed to evaluate the influence of the initiator’s size on the crushing behaviours for minimizing the initial peak force (IPF), maximizing the energy absorption (EA), and crushing force efficiency (CFE). The results illustrated that the IPF of the optimised design Pareto front upper limit is about 21.94% lower, with 0.08% and 36.67% higher EA and CFE, respectively, than those of the best choice observed from COPRAS. This study highlights the ability of the proposed machine learning based optimisation method to improve crashworthiness performance.

Feasibility study and material selection for powder-bed fusion process in printing of denture clasps

Background and objectiveThe retention of selective laser melting (SLM)-built denture clasps is inferior to that of cast cobalt-chromium (Co–Cr) clasps engaging 0.01-in undercuts, which are commonly used in clinical practice. Either the clasps engage in excessively deep undercuts or inappropriate printing process parameters are applied. With appropriate undercut engagement and levels of process parameters, the retention of SLM-built clasps (including Co–Cr, commercially pure titanium [CP Ti], and Ti alloy [Ti–6Al–4V] ones) may be comparable to that of cast Co–Cr clasps. Therefore, this feasibility study aimed to evaluate their retention to guide dentists during material selection for the powder-bed fusion process during the printing of denture clasps. MethodsWe engaged the clasp arm at an appropriate undercut depth (0.01 or 0.02 in), built clasps at the orientation of their longitudinal axes approximately parallel to the build platform, generated square prism support structures at a critical overhang angle of 30°, applied optimized laser parameters (laser power, scan speed, and hatch space), and adopted annealing treatment for Co–Cr, CP Ti, and Ti–6Al–4V clasps. After postprocessing and accuracy measurement, an insertion/removal test of the clasps for 15,000 cycles was performed to simulate 10 years of clinical use, and the retentive force was recorded every 1500 cycles. Permanent deformation of the retentive arms of the clasps was measured. Cast Co–Cr clasps engaging 0.01-in undercuts were designated the control group. ResultsThe initial retentive forces of the SLM-built Co–Cr clasps engaging 0.01-in undercuts and CP Ti and Ti–6Al–4V clasps engaging 0.02-in undercuts were comparable to those of the control group. SLM-built Co–Cr clasps engaging 0.01-in undercuts and Ti–6Al–4V clasps engaging 0.02-in undercuts had similar final retentive force and less permanent deformation compared with those of the control group; SLM-built CP Ti clasps engaging 0.02-in undercuts had lower final retentive force and greater permanent deformation. ConclusionsConsidering the long-term retention and permanent deformation of the retentive arms, Co–Cr and Ti–6Al–4V alloys, except CP Ti, are recommended for printing denture clasps. SLM-built Co–Cr clasps should engage 0.01-in undercuts, and Ti–6Al–4V clasps should engage 0.02-in undercuts.

Design and Development of Flapping Wing UAV

Flapping wing air vehicles are unmanned miniature aircraft’s that draw inspiration from birds, bats and insects. They utilize continuous flapping wing motion to generate aerodynamic forces and moments. The fluid structure interaction analysis of biological and artificial flapping flyers is extremely complicated thus an efficient unsteady aerodynamic model is required in such time consuming problems as optimal flapping wing simulation. This project deals with designing a flapping wing air vehicle which includes applying inertial impulsive force to UAV in such a way that the flapping wing air vehicle reaches almost constant flight speed after removing the initial impulsive forces. The flow dynamics are different of the flapping wing air vehicles with that of the fixed wing UAV and this project deals with the utilization of the flapping wing in the UAV to overcome the challenges faced by a fixed wing UAV like maneuverability in remote areas, sound produced and identification during mission. Structural and aerodynamic characteristics of a bird in flight offer benefits over typical propeller or rotor driven unmanned air vehicle (UAV). Hence this study deals with the designing of an flapping wing UAV to overcome these drawbacks of fixed wing UAV.

Lower-limb resistance training reduces exertional dyspnea and intrinsic neuromuscular fatigability in individuals with chronic obstructive pulmonary disease.

Skeletal muscle atrophy, dysfunction, and fatigue are important complications of chronic obstructive pulmonary disease (COPD). Greater reliance on glycolytic metabolism and increased type III/IV muscle afferent activity increase ventilatory drive, promote ventilatory constraint, amplify exertional dyspnea, and limit exercise tolerance. To investigate whether muscular adaptation with resistance training (RT) could improve exertional dyspnea, exercise tolerance, and intrinsic neuromuscular fatigability in individuals with COPD (n = 14, FEV1 = 62 ± 21% predicted), we performed a proof-of-concept single-arm efficacy study utilizing 4 wk of individualized lower-limb RT (3 times/wk). At baseline, dyspnea (Borg scale), ventilatory parameters, lung volumes (inspiratory capacity maneuvers), and exercise time were measured during a constant-load test (CLT) at 75% maximal workload to symptom limitation. On a separate day, fatigability was assessed using 3 min of intermittent stimulation of the quadriceps (initial output of ∼25% maximal voluntary force). Following RT, the CLT and fatigue protocols were repeated. Compared with baseline, isotime dyspnea was reduced (5.9 ± 2.4 vs. 4.5 ± 2.4 Borg units, P = 0.02) and exercise time increased (437 ± 405 s vs. 606 ± 447 s, P < 0.01) following RT. Isotime tidal volume increased (P = 0.01), whereas end-expiratory lung volumes (P = 0.02) and heart rate (P = 0.03) decreased. Quadriceps force, relative to initial force, was higher at the end of the stimulation protocol posttraining (53.2 ± 9.1 vs. 46.8 ± 11.9%, P = 0.04). This study provides evidence that 4 wk of RT attenuates exertional dyspnea and improves exercise tolerance in individuals with COPD, which in part, is likely due to delayed ventilatory constraint and reduced intrinsic fatigability. A pulmonary rehabilitation program beginning with individualized lower-limb RT may help mitigate dyspnea before performing aerobic training in individuals with COPD.NEW & NOTEWORTHY This study presents the novel finding that 4-wk resistance training (RT) focused specifically on the lower limbs can reduce exertional dyspnea during constant-load cycling, improve exercise tolerance, and reduce intrinsic fatigability of the quadriceps in individuals with COPD.

In vitro measurement of the retention force of two stud attachment systems during cyclic load.

To investigate the retentive behavior of the Locator legacy and Novaloc attachment systems with different retention inserts both within and across systems under cyclic load. Three retention inserts of each system (green, yellow, and white for Novaloc; green, orange, and red inserts for extended range for legacy Locator) were tested on abutments of both systems with a sample number of 10 per force and 10,000 cycles of insertion and removal. The loading was applied in the axial direction of the abutments, which were placed in artificial saliva. The retention force was measured in each cycle. The results were compared with the manufacturer's specifications and evaluated for a simulated period of use of 10 years. Characteristic time constants were determined, and subsequently, the two systems were compared regarding their wear behavior. The manufacturer's specifications could only be confirmed for the green Novaloc retention insert on a Novaloc abutment (t-test: p=0.50); for all other inserts, the baseline exceeded the manufacturer's specifications by 30%-75% (Novaloc; t-test: p<0.001) and 75%-550% (Locator; t-test: p<0.001). After 10,000 cycles performed, themanufacturer's specifications were confirmed on a Novaloc abutment for the white Novaloc retention insert (t-test: p=0.86) and on a Locator abutment for the green Novaloc retention insert (t-test: p=0.32). Both systems lost retention force during the experiment. Overall, Novaloc inserts on both abutments showed less wear (decrease to 56%-85% of initial force) and a slower decrease in retention force compared to Locator inserts (decrease to 6%-31% of initial force). In both systems, wear leads to a varying loss of retention; therefore, regular checks with possible replacement of the inserts are necessary in clinical use. Novaloc attachments seem to be more resistant to the loss of retention than Locator attachments. A cross-combination may be clinically useful in some cases.

Cyclic load tests and finite element modeling of self-centering hollow-core FRP-concrete-steel bridge columns

This paper presents experimental and numerical investigations of the seismic performance of a novel self-centering (SC) hollow-core (HC) fiber-reinforced polymer (FRP)-concrete-steel (SC-HC-FCS) bridge column. This new structure is fabricated by mounting external energy dissipators (ED) and applying unbonded post-tensioned (PT) basalt FRP (BFRP) tendon to the conventional HC-FCS column that consists of an outer FRP tube and an inner steel tube, with the space between filled with concrete. The SC-HC-FCS column combines the advantages of accelerated bridge construction and self-centering. The effects of the initial prestress force values and the configuration of the energy-dissipated aluminum bar on the column performance are studied. Based on the experimental and numerical results, it is found that the proposed SC-HC-FCS column shows adequate self-centering and energy dissipation capacities. However, it is required to properly select the configuration and material properties of the aluminum bar as energy dissipators to further refine the seismic resistance of the SC-HC-FCS column.

Prediction of ring crack initiation in ceramics and glasses using a stress–energy fracture criterion

AbstractCrack initiation in brittle materials upon spherical indentation is associated with the tensile radial stresses during loading. However, location of crack onset often differs (offset) from the site of maximal stress. In addition, experiments reveal a strong dependency of crack initiation forces on geometrical parameters as well as the surface condition of the sample. In this work, a coupled stress–energy fracture criterion is introduced to describe the initiation of ring cracks in brittle materials, which takes into account the geometry of the contact and the inherent strength and fracture toughness of the material. Several experiments reported in literature are evaluated and compared. The criterion can explain the location offset of the ring crack upon loading, as observed in various ceramics and glasses. It also predicts the ring crack initiation force upon contact loading, provided that surface compressive stresses, introduced during grinding or polishing processes, are taken into account. Furthermore, the stress–energy criterion may be employed to estimate the surface residual stress of ceramic parts, based on simple contact damage experiments.