Tip Loading Research Articles

Crack path predictions and experiments in plane structures considering anisotropic properties and material interfaces

In many engineering applications special requirements are directed to a material's fracture behavior and the prediction of crack paths. Especially if the material exhibits anisotropic elastic properties or fracture toughnesses, e.g. in textured or composite materials, the simulation of crack paths is challenging. Here, the application of path independent interaction integrals (I-integrals), J-, L- and M-integrals is beneficial for an accurate crack tip loading analysis. Numerical tools for the calculation of loading quantities using these path-invariant integrals are implemented into the commercial finite element (FE)-code ABAQUS. Global approaches of the integrals are convenient considering crack tips approaching other crack faces, internal boundaries or material interfaces. Curved crack faces require special treatment with respect to integration contours. Numerical crack paths are predicted based on FE calculations of the boundary value problem in connection with an intelligent adaptive re-meshing algorithm. Considering fracture toughness anisotropy and accounting for inelastic effects due to small plastic zones in the crack tip region, the numerically predicted crack paths of different types of specimens with material interfaces and internal boundaries are compared to subcritically grown paths obtained from experiments.

Effect of Cyclic Loading Frequency on Flexural Modulus of GFRP Laminates with Resin Rich Intermediate Layers

The flexural modulus of composite laminates subjected to cyclic loads of varying cycle periods are experimentally investigated in this work. GFRP composite laminate specimen of configuration 0/R/0/0/0, 0/R/30/60/0, 0/R/90/90/0 and 0/R/45/-45/0 are prepared by hand lay-up technique with a uniquely processed resin-rich intermediate layer for the testing. Specimens are exposed to constant amplitude cyclic loading of frequencies 4.6 Hz and 8.6 Hz. A cantilever configuration of specimen with cyclic tip loading is considered to simulate the conditions of a leaf of a laminated automobile leaf spring. The flexural response of the laminate is measured using three point bend test as per ASTM D 790 and damping by hysteresis loop. The observed reduction in flexural modulusand the increase in damping factor after loading are compared with virgin. Minimised flexural modulus deviations between the loading frequencies are favoured by the cushioning of resin rich intermediate layer in the lay-up.

Reconsiderations of fatigue crack growth at negative stress ratios: Finite element analyses

Two important aspects of fatigue crack growth at negative stress ratios are reconsidered within this study. First, the controlling crack tip loading parameters are discussed. As already shown in former studies K (Kmax or ΔK) and R are not suitable to describe properly the crack tip loading for the negative part. It is proven that an alternative stress based parameter correlates with the crack tip loading for negative loads. Second, the deformation mechanisms at negative stress ratios are intensively discussed. Therefore a new method is proposed in order to visualize the plastic deformations before and behind the crack tip, even at the negative loading part. Phenomena of constant and variable amplitude loading are investigated.

Brittle interfacial cracking between two dissimilar elastic layers: Part 2—Numerical verification

A thorough program of 2D finite element method (FEM) simulations is carried out parametrically on a bimaterial double cantilever beam (DCB) model in MSC/NASTRAN. The Young’s modulus ratio, the Poisson’s ratio, the thickness ratio, and the DCB tip loads are varied over their entire practically useful domains for different values of the crack extension size. Extensive comparisons are made between the results of the analytical theory that was developed in Part 1 by Harvey et al. (2015) and FEM results. This paper reports the outcome of these comparisons. The present analytical theory and the supporting mathematical techniques are thoroughly verified. Overall, excellent agreement is observed between the present analytical theory and the FEM results for the crack extension size-dependent energy release rate (ERR) components and the stress intensity factors (SIFs).

A molecular dynamics study of nanofracture in monolayer boron nitride

In this paper, we use molecular dynamics (MD) modeling to study the fracture properties of monolayer hexagonal boron nitride (h-BN) under mixed mode I and II loading. We investigate the impact of crack edge chirality, crack tip configuration and loading phase angle on the crack propagation path and critical stress intensity factors. The MD results predict that under all the loading phase angles cracks prefer to propagate along a zigzag direction and the critical stress intensity factors of zigzag cracks are higher than those of armchair cracks. Under mixed mode loading, the h-BN sheets can undergo out-of-plane deformations due to the buckling induced by compressive stresses. The out-of-plane deformations can be significant when mode II loading is dominant. An excessive amount of out-of-plane deformation can induce buckling cracks. Depending on the loading phase angle and crack configurations, buckling cracks can nucleate before or after the propagation of the original cracks.

Behaviour of single pile in consolidating soil

Piles located in consolidating soil are subjected to indirect loading resulted from the settlement of adjacent soil. Both pile and soil undergo downward movements caused by the axial pile load and surcharge loading on surrounding area. Both dragload and downdrag imposed on the pile are time dependent. The purpose of this research was to analyse and quantify the dragload and down drag imposed on single pile. Effect of pile head load was considered. Shielded and unshielded piles are analysed. Three dimensional nonlinear analyses using ABAQUS 6.12 have been utilised. The research investigates the size of the surcharge loaded area, location of the pile tip and pile head load on the dragload and down drag imposed on the pile. Based on numerical results the study revealed that sacrificing piles, which are unloaded piles, “hang up” the soil between the piles in the group and, thus, the vertical effective stress around the shielded pile is reduced. Numerical results proved that an average value of the shear stress mobilisation factor β mobilised along unloaded shielded central pile varies between zero at neutral plane (NP) and 0.27. The long term dragload of the shielded floating pile is about 67% of the long term dragload of unshielded floating single pile.

Crack path prediction in rolled aluminum plates with fracture toughness orthotropy and experimental validation

The paper presents a numerical procedure for the accurate calculation of crack paths in plane structures. The J-integral is used to determine crack tip loading quantities such as stress intensity factors or the energy release rate. Crack paths are predicted applying a modified maximum energy release rate approach accounting for the fracture toughness orthotropy in rolled plates of aluminum alloy Al-7075-T651 where the anisotropy of elastic constants is negligible. Experiments with CT-specimens are carried out to determine the fracture toughnesses in and perpendicular to the rolling direction. The numerically predicted crack paths for differently shaped specimens are in good agreement with those obtained from experiments.

Atomistic simulation of the effect of roughness on nanoscale wear

At the macroscale, it is expected that surface roughness decreases adhesive wear and increases abrasive wear, and both increase with load. Here we evaluate whether these trends are also applicable to nanoscale contacts. Molecular dynamics simulation is used to model dry sliding of Si atomic force microscope tips with varying roughness on a Cu substrate at a range of normal loads. Surface wear is quantified as the number of substrate atoms displaced during steady-state sliding and characterized as a function of tip roughness and load. The interaction energy between the tip and substrate and a geometric roughness factor are used to investigate the contributions of adhesive and abrasive wear. By predicting how roughness affects nanoscale wear, and isolating the adhesive and abrasive wear modes, it is possible to describe the relationship between roughness, load and wear and, further, to determine an optimum tip roughness that will minimize wear at a given load.

Design of an Experimental Test System to Investigate Parameters Affecting Distal Tip Loads of Pacemaker and Defibrillator Leads

The purpose of this study was to design and evaluate a system to test the mechanical behavior of pacemaker and defibrillator leads. Over 300,000 pacemaker and implantable cardioverter defibrillator (ICD) procedures are performed every year in the U.S. for the treatment of cardiac arrhythmias, ventricular dysrhythmias, and congestive heart failure. These procedures require implanting transvenous leads into the interior wall of the heart. A serious and sometimes fatal complication that may occur during or after lead implantation is perforation of the lead tip through the heart wall. The factors that lead to perforation are not fully understood. This illustrates that the mechanical interactions between the lead tip and the cardiac tissue need to be further investigated to improve the outcome for pacemaker and ICD patients. To improve the performance of lead tips, the testing protocols must reproduce physiological and clinically relevant tip-tissue interactions. As a first step toward this goal, testing parameters that influence those interactions must be identified. We investigated the effect of test system parameters, which reproduce potentially critical physiological constraints, on the load experienced at the distal tip of thirteen pacemaker and defibrillator active-fixation leads. We evaluated the use of a constraint to simulate the effect of the right ventricle (RV constraint) in vivo, how and where the lead was fixed in the test configuration, location of the load cell in the test system, rotation and frequency of the test protocol, and the effect of stylets. Results showed the RV constraint and load cell placement had the largest impact on lead tip load, while rotation of the test setup and test frequency had a minimal impact. Recommendations are made for a test system and protocol for in vitro testing of leads that take into consideration in vivo conditions. Better approximations of the in vivo environment may lead to improved product development. The potential of this system to more effectively evaluate new pacemaker and defibrillator lead designs will require further study.

Infrapopliteal arterial recanalization: A true advance for limb salvage in diabetics.

The world is facing an epidemic of diabetes; consequently in the next years, critical limb ischemia (CLI) due to diabetic arterial disease, characterized by multiple and long occlusions of below-the-knee (BTK) vessels, will become a major issue for vascular operators. Revascularization is a key therapy in these patients as restoring adequate blood supply to the wound is essential for healing, thus avoiding major amputations. Endoluminal therapy for BTK arteries is now a key part of the vascular specialist armamentarium. Tibial artery endovascular approaches have been shown to achieve high limb salvage rates with low morbidity and mortality and endovascular interventions one should now consider to be the first line treatment in the majority of CLI patients, especially in those with associated medical comorbidities. To do so, the vascular specialist requires detailed knowledge of the BTK endovascular techniques and devices. The first step decision in tibial endovascular therapy is access. In this context, the anterograde ipsilateral approach is generally preferred. The next critical decision is the choice of the vessel(s) to be approached in order to achieve successful limb salvage. Obtaining pulsatile flow to the correct portion of the foot is the paramount for ulcer healing. As such, a good understanding of the current angiosome model should enhance clinical results. The devices used should be carefully selected and optimal choice of guide wire is also extremely important and should be based on the characteristics of the lesion (location, length, and stenosis/occlusion) together with the characteristics of the guide wire itself (tip load, stiffness, hydrophilic/hydrophobic coating, flexibility, torque transmission, trackability, and pushability). Passing through chronic total occlusions can be quite challenging. The vascular interventional radiologist needs therefore to master the techniques that have been recently described: anterograde approaches, including the drilling technique, the penetrating technique, the subintimal technique and the parallel technique; subintimal arterial flossing with anterograde-retrograde procedures (Safari); the pedal-plantar loop technique and revascularization through collateral fibular artery vessels.

Spring back of infinite honeycomb sheets beyond plastic deformation

Cellular structures are promising for applications where high stiffness and strength are required with the minimal use of material. They are often used in applications where the plastic deformation plays an important role, such as those involving crashworthiness, energy absorption, and stents. The elastic analysis of a honeycomb sheet has been carried out in the past [1]. The present analysis extends this classical work in the elasto-plastic regime. Recoil analysis due to elastic recovery is absent from the published literature. This work aims to develop an analytical model to calculate the spring back for a simplified case, that of an infinite honeycomb sheet. An elastic-perfectly plastic material model is assumed. The recoil for a clamped beam with a load and moment applied at the free edge is analytically calculated first. This is carried out by relating the stress distribution of the cross section to the final deformed shape. The part corresponding to the elastic contribution is subsequently subtracted in order to obtain the final configuration after the external load is removed. This simple elasto-plastic analysis is then incorporated into the analysis of an infinite sheet made of uniform hexagonal cells. The translational symmetry of the lattice is exploited along with the analysis of a beam under tip loading through to plastic stage and recoil. The final shape of the struts upon the removal of the remote stress is completely determined by the plastic deformation which cannot be recovered. The expression for the beam thus obtained is then used to build an analytical model for an infinite honeycomb sheet loaded in both directions.

Friction properties of amorphous carbon ultrathin films deposited by filtered cathodic vacuum arc and radio-frequency sputtering

The friction properties of ultrathin films of amorphous carbon (a-C) deposited on Si(100) substrates by filtered cathodic vacuum arc and radio-frequency sputtering were investigated by surface force microscopy. Deposition parameters yielding a-C films with high sp3 content were used to deposit films of thickness between 5 and 35nm. The coefficient of friction of both types of a-C films was measured with a 1-μm-radius conical diamond tip and normal loads in the range of 20–640μN. The results show a strong dependence of the friction properties on the surface roughness, thickness, and structure of the a-C films, which are influenced by the intricacies of the deposition method. The dependence of the coefficient of friction on normal load and the dominance of adhesion and plowing friction mechanisms are interpreted in terms of the through-thickness variation of carbon atom hybridization of the a-C films.

Lifetime comparison of Y-TZP/porcelain crowns under different loading conditions

ObjectivesTo compare the lifetime of Y-TZP/porcelain crowns under three different load conditions using step-stress accelerated lifetime testing. MethodsThe Y-TZP frameworks were milled using CAD/CAM, veneered with a porcelain and cemented onto dentine analogue dies. Specimens were divided according to the occlusal load condition (n=20): central fossa load (CFL), cusp tip load (CTL) and sliding contact (SC). For CFL and CTL, the cyclic load was applied parallel to the long axis of the preparation using a ceramic piston. For SC, the axial load was associated to 1mm lateral displacement at the disto-lingual cusp. Different stress profiles were used. Failures were detected with an acoustic system. A Weibull distribution (95% confidence boundary) was used to analyse the data, and fractographic principles were used to evaluate fractured specimens. ResultsThe acoustic monitor was able to detect the initial crack. The probability of failure (at 300N load and 200,000 cycles) was statistically greater for CTL (0.63; 0.44–0.81) compared to CFL (0.23; 0.12–0.43). The Weibull modulus of CFL (2.1; 1.5–3.6) was greater than for SC (0.7; 0.5–1.2), with no difference in the lifetime. All specimens failed by chipping, which originated predominantly at the contact (66.7%) on CTL, and in the bulk of the porcelain on CFL (100%) and SC (80%). ConclusionsContact at the cusp tip is more harmful than at the central fossa. Data from sliding contact are less consistent than from axial contacts, but more clinically relevant. Clinical significanceThe loading condition of Y-TZP/porcelain crowns can influence on the probability of failure and failure mode. The contact at the cusp tip is more harmful than at the central fossa, where the stress is better distributed. Sliding contact is a better simulator of the chewing cycle compared to axial contacts.

Rates of decay to non homogeneous Timoshenko model with tip body

We consider the uniform stabilization of a hybrid elastic model consisting of a Timoshenko beam and a tip load at the free end of the beam. Our main result proves that the semigroup eAt associated to this model is not exponentially stable. Moreover, we prove that the semigroup decays polynomially to zero as t−1/2. When the damping mechanism is effective only on the boundary of the rotational angle, the solution also decays polynomially as t−1/2 provided the wave speeds are equal. Otherwise it decays as t−1/4 for any initial data taken in D(A).

Discussion of “Instrumented Static Load Test on Rock-Socketed Micropile” by Hoyoung Seo, Monica Prezzi, and Rodrigo Salgado

Fig. 9 of Seo et al. (2013) shows load reaching at the tip of the micropile, but from the text, it appears that loads were calculated using readings of strain gauge (SG1) located 177 mm above the tip. The 177-mm-long segment of the pile above the tip also develops side frictional resistance; if that is deducted from the measured load at SG1, then it can be shown that the tip load is almost zero at all load increments. At the final load increment, the test load is about 3,600 kN, which is equal to 78% of the pile structural capacity of 4,600 kN. The easiest way of determining load at the tip is to extend the load transfer lines between strain gauges SG2 and SG1 to the tip of the pile. For brevity, the strain gauge located at Location 1 in Table 5 of the original article has been termed as SG1 and Location 2 as SG2, and so on. For example, Osterberg Cell is placed on a 150–300-mm-thick concrete bearing pad, which rests on the base of the hole. The side friction developed around the bearing pad is deducted from O-cell readings to determine tip load (Schmertmann et al. 2001). Nearly zero tip loads for all the load increments when accompanied with a significant amount of tip settlement shall indicate that the tip resistance is negligible because of the presence of soft sediments between the tip and the base of the hole.

Wing–vortex interaction: unraveling the flowfield of a hovering rotor

This paper focuses on one of the most prominent flow features of the hovering rotor wake, the close interaction of the tip vortex with a following blade. Such vortex interactions are fundamental determinants of rotor performance, loads, and noise. Yet, they are not completely understood, largely due to the lack of sufficiently comprehensive experimental data. The present study aims to perform such comprehensive measurements, not on hovering helicopter rotors (which hugely magnifies test complexity) but using fixed-wing models in controlled wind tunnel tests. The experiments were designed to measure, in considerable detail, the aerodynamic loading resulting from a vortex interacting with a semi-span wing, as well as the wake resulting from that interaction. The goal of the present study is to answer fundamental questions such as (a) the influence of a vortex passing below a wing on the lift, drag, tip vortex, and the wake of that wing and (b) the strength of the forming tip vortex and its relation to the wing loading and/or the tip loading. This paper presents detailed wing surface pressure measurements that result from the interaction of the wing with an interacting vortex trailing from an upstream wing. The data show large lift distribution changes for a range of wing–vortex interactions including the effects of close encounter with the vortex core. Significant asymmetry in the vortex-induced lift loading was observed, with the increase in wing sectional lift outboard of the interacting vortex (closer to the tip) being much smaller than the corresponding decrease inboard of the vortex.

Numerical crack path prediction considering orthotropic effects and experimental verification

AbstractFor a reliable prediction of crack paths, on the one hand the accurate calculation of crack tip loading quantities is inevitable, on the other hand orthotropic features of the fracture toughness need to be taken into account. The interplay of crack tip loading and material response due to fracture is still unclear and seems to have a crucial effect on crack path predictions. Numerical tools for the accurate calculation of crack tip loading quantities using path‐invariant J‐integrals and interaction integrals (I‐integral) are presented. Here, global approaches are beneficial when considering crack tips approaching other crack faces or internal boundaries. Curved crack faces have to be taken into account and special treatment regarding crack face integrals is necessary. Experimental investigations are carried out at standard CT‐specimens of rolled aluminum alloy Al‐7075 exhibiting a directional orthotropy of the fracture toughness. Considering that property, the numerically predicted crack paths based on FE calculations show very good agreement with subcritically grown paths obtained from experiments. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A gel electrophoresis loading system to prevent laboratory contamination by amplification products.

Pipet tip loading of polymerase chain reaction (PCR) and other amplification products into an electrophoresis gel represents a potential source of laboratory contamination. We have developed a prototype of the gel contamination control system (GelCCS) that enables gel loading by bottom puncture of PCR tubes. Puncture occurs within a sealed gel casing, preventing contamination of the surrounding environment. The system was designed for inexpensive manufacture so that after the results are visualized, the gel casing and PCR tubes are discarded intact with the amplification products sealed inside. We demonstrate that gel loading is reliable and that the resulting bands are equivalent in appearance to manually loaded gels.

Apparent Fracture Toughness Versus Micro-Scale Fracture Toughness of Interfaces—The Challenge of Critical Values

AbstractThis study aims to shed light on the conflict between macroscopic fracture toughness and small-scale measures of interface strength when the interface is simulated with an increasing resolution. In the design of practical structures, crack initiation is the critical, limiting phase of material behavior and needs to be defined by lucid means. In this study, we model an example structure, namely a fracture test specimen, including a bi-material interface having micro-roughness and thermal residual stresses. The initiation of secondary cracks, which is one of the microscopic mechanisms leading to crack coalescence, is studied. The work focuses on the effects of the bi-material's Young's modulus, residual stresses, and flaws. The simulation of crack propagation shows that the overall crack tip loading mode mixity distribution is not essentially affected when the Young's modulus of the bi-material's coating is doubled. The analysis of the secondary crack initiation due to interfacial flaws in turn showed that increasing the Young's modulus of either the bi-materials substrate or coating resulted in immediate interface collapse via an elongated ‘damage region’ along the interface.

Dynamic Response of a Metal and a CMC Turbine Blade During a Controlled Rub Event Using a Segmented Shroud

Ceramic matrix composites (CMCs) provide several benefits over metal blades including weight and increased temperature capability, and have the potential for increased engine performance by reduction of the cooling flow bled from the compressor and by allowing engines to run at higher turbine inlet temperatures. These CMC blades must be capable of surviving fatigue (high cycle and low cycle), creep, impact, and any tip rub events due to the engine missions or maneuvers that temporarily close blade tip/shroud clearances. As part of a cooperative research program between GE Aviation and the Ohio State University Gas Turbine Laboratory, OSU GTL, the response of a CMC stage 1 low-pressure turbine blade has been compared with the response of an equivalent metal turbine blade using the OSU GTL large spin-pit facility (LSPF) as the test vehicle. Load cells mounted on the casing wall, strain gages mounted on the airfoils, and other instrumentation are used to assess blade tip rub interactions with a 120-deg sector of a representative turbine stationary casing. The intent of this paper is to present the dynamic response of both the CMC and the metal blades with the turbine disk operating at design speed and with representative incursion rates and depths. Casing wear and blade tip wear are both characterized for several types of rub conditions including a light, medium, and heavy rub at room temperature. For each condition, the rub primary dynamic modes have been evaluated, and the corresponding blade tip loads have been calculated. The preliminary results suggest that a CMC blade has similar abilities to a metal blade during a rub event.