Stop Hole Research Articles

Peridynamic analysis of thermomechanical effects in fracture toughening mechanisms of stop holes

In this study, a bond-based fully coupled thermomechanical peridynamic framework is implemented to study the fracture toughening of stop-holes in brittle materials subjected to thermomechanical loads. The developed modelling approach aims to provide comprehensive guidance about thermal effects on the fracture toughening mechanism of inclusions under realistic engineering conditions. First, the model is verified using simulation of several benchmark case studies and predicts the mechanical, thermal and damage parameters with high accuracy. Then, a fully coupled thermomechanical analysis of a 2D structure is performed to study the effect of the location of a single stop-hole on fracture toughening. A thorough investigation of the arresting and accelerating effect of the stop-hole is conducted. Additionally, the accuracy of various degrees of thermomechanical coupling including one-way weak coupling and no coupling is compared with full coupling. The study of crack propagation dynamics reveals that ignoring thermal effects underpredicts the toughness enhancement of stop-holes. Furthermore, although the weak thermomechanical coupling offers a reasonable prediction of crack propagation dynamics in most cases of our study, it is not accurate in certain cases. In the final section, some benchmark cases of complex arrangements of multiple stop-holes in literature are revisited. Our results indicate a different toughness trend for some cases when considering the full thermomechanical coupling. This suggests the proposed framework provides a more accurate tool for designing toughened structures using inclusions.

Gradiently Foaming Ultrasoft Hydrogel with Stop Holes for Highly Deformable, Crack-Resistant and Sensitive Conformal Human-Machine Interfaces.

Hydrogels are considered as promising materials for human-machine interfaces (HMIs) owing to their merits of tailorable mechanical and electrical properties; nevertheless, it remains challenging to simultaneously achieve ultrasoftness, good mechanical robustness and high sensitivity, which are the pre-requisite requirements for wearable sensing applications. Herein, for the first time, this work proposes a universal phase-transition-induced bubbling strategy to fabricate ultrasoft gradient foam-shaped hydrogels (FSHs) with stop holes for high deformability, crack-resistance and sensitive conformal HMIs. As a typical system, the FSH based on polyacrylamide/sodium alginate system shows an ultralow Young's modulus (1.68kPa), increased sustainable strain (1411%), enhanced fracture toughness (915.6 Jm-2 ), improved tensile sensitivity (21.77), and compressive sensitivity (65.23 kPa-1 ). The FSHs are used for precisely acquiring and identifying gesture commands of the operator to remotely control a surgical robot for endoscopy and an electric ship in a first-person perspective for cruising, feeding crabs and monitoring the environmental change in real-time.

Thickness-dependent selection of adhesive joints

ABSTRACT The fracture resistance of adhesive joints depends on the bondline thickness, especially in the toughest systems. While increasing the adhesive thickness to operate at peak fracture toughness is anticipated to be attractive, it leads to a mass increase and to a decrease of the effective shear stiffness and strength, calling for a trade-off. Here, we follow a rational materials selection approach for mechanical design to address different sets of objectives and constraints representative of different classes of applications. The approach is applied to select the best conventional adhesive based on a novel database. Mechanical analysis with two different degrees of complexity is considered. The elementary level only accounts for the influence of thickness on toughness, while more advanced models introduce the dependence of shear strength on thickness as well as the stiffness mismatch with the adherends. The best adhesive choice strongly depends on the objectives, constraints, and loading configuration. However, the ranking is not drastically affected by the adherends’ stiffness. Gaps found for stiff/strong and tough adhesives motivate the development of architected joints. Inserting stop holes improves damage tolerance and density but reduces the shear strength and stiffness, requiring an optimum design.

Investigation of Crack Repairing Technique to Delay Fracture Initiation of Steel Members Subjected to Low Cycle Fatigue

Stress concentrations have become a common phenomenon in steel elements when arresting a fracture by implementing the crack stop hole (CSH) technique. Embedding the CSH with Carbon Fibre-Reinforced Polymer (CFRP) enhances the fatigue life by delaying fractures while achieving stiffness recovery due to the superior mechanical characteristics of the CFRP material. Hence, the low cyclic fatigue (LCF) behaviour of 90 strengthened and non-strengthened CSH specimens was examined in this context. These specimens were subjected to a range of 0 to 10,000 fatigue load cycles at a frequency of 5 Hz. At the end of fatigue exposure, the average tensile strength was measured in each case. The application of a CFRP patch on the CSH effectively recovered the strength losses while enhancing the strength in the range of 32% to 45% with respect to the non-strengthened specimens. The developed numerical model based on the cyclic J-integral technique agrees with the test results. This study introduced geometry-related design guidelines for this novel CSH hybrid technique.

Research on heavy-haul adaptive technology and strengthening measures for existing railway steel bridge

PurposeThis research addresses the diverse characteristics of existing railway steel bridges in China, including variations in construction age, design standards, structural types, manufacturing processes, materials and service conditions. It also focuses on prominent defects and challenges related to heavy transportation conditions, particularly low live haul reserves and severe fatigue problems.Design/methodology/approachThe study encompasses three key aspects: (1) Adaptability assessment: It begins with assessing the suitability of existing railway steel bridges for heavy-haul operations through comprehensive analyses, experiments and engineering applications. (2) Strengthening: To combat frequent crack defects in the vertical stiffener end structure of girder webs, fatigue performance tests and reinforcement scheme experiments were conducted. These experiments included the development of a hot-spot stress S-N curve for this structure, validating the effectiveness of methods like crack stop holes, ultrasonic hammering and flange angle steel. (3) Service life extension: Research on the cruciform welded joint structure (non-fusion transfer type) focused on fatigue performance over the long life cycle. This led to the establishment of a fatigue S-N curve, enhancing Chinese design codes.FindingsThe research achieved several significant outcomes: (1) Successful implementation of strengthening and retrofitting measures on a 64-m single-span double-track railway steel truss girder on an existing heavy-duty line. (2) Post-reinforcement, a substantial 26% to 32% reduction in live haul stress on bridge members was achieved. (3) The strengthening and retrofitting efforts met design expectations, enabling the bridge to accommodate vehicles with a 30-ton axle haul on the railway line.Originality/valueThis research systematically tackles challenges and defects associated with Chinese existing railway steel bridges, providing valuable insights into adaptability assessment, strengthening techniques and service life extension methods. Furthermore, the development of fatigue S-N curves and the successful implementation of bridge enhancements have practical implications for improving the resilience and operational capacity of railway steel bridges in China.

An improved peridynamic approach for fatigue analysis of two dimensional functionally graded materials

This study investigates the fatigue behavior of functionally graded materials (FGMs) within an enhanced peridynamic (PD) framework that eliminates the need for surface and volume correction factors. FGMs offer mechanical advantages by varying material compositions and properties in one or more directions. The distribution of mechanical properties in FGMs is achieved by employing a power law. Comparative analysis is conducted between the PD predictions for reaction forces and outcomes obtained from the reference solutions for a quasi-static problem. Minimal divergence is observed between the PD and reference solutions, affirming the accuracy of the present PD formulation. Furthermore, the PD fatigue results for a Mode I Compact Tension (CT) specimen problem match the experimental findings. An experimental stop-hole problem is used to evaluate the accuracy of the PD fatigue code in mixed-mode circumstances. The load cycle determination is achieved via ABAQUS Computational Finite Element Method (CFEM) for the mixed mode problem. There is a good correlation between CFEM and the present PD outcomes. Furthermore, the effect of stop-hole existence on fatigue life in mixed mode is examined, revealing a significant retardation effect. Notably, this study adds a novel perspective by using the PD approach to investigate the fatigue analysis of FGMs and examine the impact of the stop hole on fatigue life- a component previously unexplored in the existing literature.

Fatigue improvement of rib‐to‐deck welded joints using adhesively bonded steel patches ‐ LEFM‐based parametric study

AbstractRib‐to‐deck welded joints in orthotropic steel decks of bridges are prone to develop fatigue cracks. Repairing such cracks allows for prolonging the life of the infrastructure, provided that structural safety is guaranteed. Among others, the repair technique of adhesively bonded steel patches is relatively simple and inexpensive. Moreover, it has the advantage that the original structure is not altered, e.g. as in the case of crack stop holes. This technique has been used in other details like the diaphragm of steel bridges. However, limited information is available when applied to rib‐to‐deck joints, which are the most common joint type in orthotropic steel decks. This study proposes two repair schemes using adhesively bonded steel patches for the rib‐to‐deck joints. A 3‐D finite element model which takes account of the crack size is established. Based on linear elastic fracture mechanics (LEFM), a parametric study is performed to investigate the effect of patch geometry and adhesive thickness on the performance of repair schemes.

Effect of plasma nitriding of a stop hole along the fatigue crack path on crack propagation

The effect of nitriding of the fastener hole along the Fatigue Crack Growth (FCG) path on FCG was investigated. A standard Compact Tension (CT) specimen made of AISI 4140 steel with a 5 mm diameter hole drilled on the FCG path was used. It was subjected to a plasma nitriding process. Crack growth was monitored using the digital imaging method. The results were compared with the paired sample t-test. As compared to the un-nitrided sample without a hole (control group), FCG lifetime was increased 1.5 times un-nitrided without a hole and nitrided with a hole samples. And 2.5 times the total FCG lifetime was observed in drilled un-nitrided samples. It was determined that nitriding treatment of the drilled samples eliminated the positive effect of the hole on FCG life.

Failure characteristics and the law of the energy evolution of granite with different pre-crack inclination angles under uniaxial compression loading

Cracks affect the strength of rock masses and eventually threaten their stability in engineering. In order to study the fracture characteristics and mechanical properties of cracked rocks, uniaxial compression tests of pre-cracked granite samples with a central circular through hole were carried out by using MTS816 rock mechanics testing system. The inclination angles of different pre-cracks are 0°, 15°, 30°, 45°, 60°, 75°, and 90° respectively, and the influence of the crack stop hole near the crack tip on the failure behavior of pre-cracked samples is also considered. The results indicate that, compared with the intact sample, the peak strength of pre-cracked samples decreases significantly and is related to the pre-crack inclination angle. The failure mode of the sample varies with the pre-crack inclination angle, and the crack stop hole near the crack tip also has a certain influence on the crack growth to a certain extent. However, in terms of failure mode and its transformation law, the influence of central circular through hole and crack stop hole can be ignored. Generally speaking, the larger the inclination angle of the pre-crack, the more the total energy required for failure of the sample, and the more the stored elastic strain energy. Before the peak strength, the elastic strain energy of the sample is greater than the dissipated energy, after the peak strength, the dissipated energy gradually exceeds the elastic strain energy due to energy conversion. It is found that the pre-crack reduces the energy storage capacity of the sample, and the total energy is ultimately dominated by sample integrity. The dissipated energy rate increases first, then decreases, and finally increases again, the inflection points are the end of micro-crack closure and the peak strength, respectively. The crack stop hole changes the law of energy evolution to a certain extent, which can improve the ability of rocks to accumulate energy when designed at an appropriate position, so as to improve its load-bearing capacity in a certain range. The results display the mechanical properties of pre-cracked granite samples under uniaxial compression and are conducive to its application in engineering.

Drill Hole Optimization for Augmenting Structural Integrity

Structures develop cracks during the course of there life resulting in catastrophic failures. This can be reduced by drilling holes near the crack tip. Here the aspect of optimal position and size of the drill hole has been analysed. Additionally the effect of supplementary hole has also been made. Finite element code ANSYS was used for the analysis. To verify the fracture capabilities of ANSYS the results were corroborated using photoelastic techniques. It was observed that reduction in stress intensity factor to a great extent depends upon the position of the stop hole. Also it was observed that as the radius of the crack was increased the stress intensity factor increased. Creating supplementary hole further reduced the stress intensity factor. So it can be concluded that for employing the concept of drill hole its position and size should be accurately determined.

Numerical analysis of crack propagation in fretting fatigue specimen repaired by stop hole method

Fretting fatigue is a common structural failure phenomenon. It can greatly reduce the fatigue life of a structure, and because of the complexity of its load condition, it is a challenging research topic. This paper applies, for the first time, the widely used stop hole repair technology to a fretting fatigue specimen in crack propagation stage in order to extend its lifetime. LEFM approach is adopted for the crack propagation phase using Paris’ law to predict the fatigue life. The critical plane parameter and variable length critical distance method are used together to predict the second crack initiation lifetime. This prediction method is validated by the central crack tensile fatigue experiment, which considers the stop hole. It is shown that the stop hole changes the propagation path and leads to a secondary crack initiation from the lower edge of hole. The increase of crack propagation life mainly comes from the secondary crack initiation. It is also found that the larger the radius of the stop hole, the greater the fatigue life is.

An investigation on the fatigue performance of cracked steel plates reinforced with FRP and stop hole

In this paper, we implement the combined method of drilling stop hole and FRP reinforcement to repair cracked steel plates subjected to cyclic tension. The crack initiation and growth are experimentally investigated. The stress distribution at the stop hole and the residual fatigue life are evaluated by the FEM. The effects on prolonging residual fatigue life are analyzed. The results show that the effectiveness of only using the stop hole is limited, while the combined method has dramatically prolonged the residual fatigue life. The effect mainly owes to the increasing of the crack initiation life from the stop hole.

Ultimate strength of a cracked stiffened panel repaired by CFRP and stop holes

An analysis of the ultimate strength of a cracked stiffened panel repaired by carbon fibre reinforced polymer (CFRP) under axial compression is presented. The analysis method adopts the nonlinear mechanics of adhesives in the interaction of CFRP with the steel structure. The numerical model is validated by tests of cracked plates repaired by CFRP. The collapse of the cracked stiffened panel repaired by CFRP revealed that the debonding of CFRP happens after buckling. The CFRP repair can recover the ultimate strength of the cracked structure to the one of the intact panel. A series of analysis is performed with the validated numerical model to investigate the effects of geometry, configuration of CFRP and crack length on the ultimate strength. Crack treatment with stop holes is also studied, and the results show that the repair of the CFRP can recover the ultimate strength of the cracked stiffened panel with the occurrence of the stop holes, at the same time as it may reduce the stress levels significantly.

Improvement of Stop-Hole Method on Fatigue-Cracked Steel Plates by Using High-Strength Bolts and CFRP Strips

Steel bridges are extremely damaged by fatigue subjected to cycling load. Therefore, it is often necessary to put forward effective reinforcement to strengthen steel structures during the daily maintenance. In this study, two repairing methods of high-strength bolts and high-modulus CFRP strips on the basis of stop-hole repair method were introduced, respectively, to investigate fatigue improvement of cracked steel plates. First of all, numerical analysis was conducted to predict the repair efficiency and investigate the optimal parameters of each method. Variables studied were stop-hole diameter, pretightening force of bolt, and size of CFRP patch. Subsequently, a total of 12 specimens were tested to study the repairing efficiency of cracked steel plates with various strengthening methods through cyclic loading. At the same time, the failure mode and fatigue life were analyzed to present the improvement of fatigue performance. In addition, the experimental results were compared against the S-N curves of this strengthened fatigue detail. The outcomes of this study revealed that an improvement in the influence of fatigue-crack repair with the adoption of these two strengthening methods was evident. Numerical results showed that the addition of these materials could significantly diminish stress concentration factor around hole edge and improve their fatigue performance in comparison with only stop-hole method. Fatigue test results indicated that the crack initiation life of specimens repaired by stop-hole method was more than 20 times that of the unrepaired specimens. The high-strength bolt reinforced stop hole and CFRP patched stop hole can extend the crack initiation life by 9 and 8 times, respectively, in contrast to control specimens with sole stop-hole method. Finally, it was demonstrated that repairing damaged steel plates with stop-hole method alone was not enough to satisfy the fatigue strength requirements of various countries. But the fatigue strength category of damaged steel plates after further repairing with high-strength bolts and high-elastic-modulus CFRP, respectively, was higher than category A of AASHTO.

Reduction of Stress Concentration at Stop-Hole by Bolting a Crack

A stop-hole, which is drilling at the crack tip, is one of the famous tentative repairs of fatigue cracks in steel structures. When the crack becomes longer, the stress concentration of the stop-hole also increases. Therefore, during the tentative repair of long fatigue cracks, the chances for re-initiation of the fatigue cracks from the stop holes are high. The bolting stop-hole method is a useful technique against fatigue cracks. In this technique, bolting is applied only for the stop-holes at the crack tips. Therefore, as the crack length increases, the effect of the bolting stop-hole method decreases. In this paper, the authors propose a stress concentration reduction technique for a stop-hole by bolting the fatigue crack between stop-holes. The effect of the proposed technique was verified by tensile and bending tests of specimens as well as finite element analysis. From the tensile and bending tests, it was clarified that the stress concentration of the stop-hole was drastically reduced by bolting a crack under the condition before the slippage of bolts. Additionally, multi-bolting cracks have the advantage of improving the slippage load of the bolts in a crack. Furthermore, finite element analysis verified the reduction in stress concentration of the stop-hole by bolting a crack, and the effectiveness of bolting cracks and stop-holes was also shown.

New Crack Arresting Methodology Using Structural Optimizing Technique. (Dept. M. ( Production ) )

The stop-hole method is a simple and economic repair technique widely used to retard or even to stop the propagation of a fatigue crack in structural components that cannot be replaced immediately after the detection of the crack. Its principle is to drill a circular hole at or close to the crack tip to transform the crack into a notch, reducing in this way its stress concentration effect. In the present study, the stop-hole method was investigated with creating a non circular hole. The aim of the present work is to obtain an optimum stop hole shape that gives maximum fatigue crack initiation life. Fatigue crack initiation life depends at least on the crack size and on the hole diameter. An optimization technique had been used and a finite element program had been built to find this optimum shape of stop hole. It was found that the fatigue life obtained by using the optimum hole shape ranges from 2 to 9 times the fatigue life obtained by using the circular holes depending on the initial hole size and initial crack length. The effect of this optimum hole shape on crack reinitiating life for different specimen geometries has been studied. It was found that the optimum hole shape increased the initial fatigue life for all specimens used whatever its geometry was. A global optimum hole shape (which is practically suitable for all geometries ) is defined. Global optimum hole shape helps engineers to use this global hole shape directly in practice without carrying out any calculation much like the use of the stop hole size found in the literature. Opposite to the traditional stress concentration factor minimization problem where the nominal area remains constant during optimization, here it is allowed to vary nominal area using design variables resulting in decreasing of the nominal stress in addition to decreasing of the stress concentration factor. This leads to higher fatigue life compared to previous studies.

Effects of stop hole on crack turning, residual fatigue life and crack tip stress field

The aim of this paper is to research the effects of stop hole on crack turning, residual fatigue life and crack tip stress field, in which the location, size and shape of stop hole are considered. The numerical model is established according to M-integral technology, and crack turning angle is determined by maximum hoop stress criterion. The calculation results show that stop hole location has a significant effect on crack turning angle and residual fatigue life, and it should be drilled with a reasonable distance in vertical and horizontal direction around crack tip to achieve a better residual fatigue life retardation effect. The larger the stop hole size is, the wider effect range of stop hole on crack turning and residual fatigue life will be. The effects of elliptical stop hole on crack turning angle and residual fatigue life retardation are greater than that of round hole when the distance in vertical direction increases. Crack tip stress distribution is changed by increasing σy and decreasing σx, and the most dramatical changes occur at the circumferential direction of 90° and 270°.

Fatigue Performance Evaluation and Cold Reinforcement for Old Steel Bridges

Fatigue and corrosion damage in existing old steel bridges are accumulated continuously in service stage, probably leading to structural failure. One old welded steel bridge and one old riveted steel bridge are taken as examples for structural performance and safety evaluation. A large number of out plan deformation-induced long fatigue cracks were found at the connection plates connecting the bottom flange of end cross-frames and main girders of the two old steel bridges, and serious corrosion damage was found at such details of the old riveted steel bridge. Cold reinforcement techniques of drilling stop holes and bonding and bolting steel plates were adopted to strengthen the old steel bridges. Fracture mechanical models using the short-term monitoring data were established to assess the fatigue lives before and after the reinforcement, considering the coupling effect of fatigue and corrosion, and the evaluation results indicate that corrosion and fatigue coupling is very significant for old steel bridges. Monitoring and evaluation results show that bonding and bolting steel plates leads to about 90% reduction in the maximum tensile stress and about 8 times fatigue lives extension, while drilling stop holes can only temporarily arrest the crack growth. Additionally, the future maintenance strategies recommendations are provided.

Effect of stop holes on structural integrity of offshore structures: a numerical model

As offshore structures are reaching their original design life, effective repair and life-extension techniques are required, in order to ensure continued safe operation. A stop hole may be drilled at the end of a crack, in order to delay further fatigue crack propagation. To compare the stop hole technique with other relevant repair techniques, its effect needs to be modelled. Here, a procedure for modelling the stop hole-induced fatigue crack growth delay for a crack propagating under mixed-mode I + II conditions is presented. The procedure combines the S–N curve for fatigue crack initiation and the Paris law for fatigue crack propagation, with models for mixed-mode crack propagation and finite-element analysis of the cracked geometry. The failure criterion inherent in the S–N curves is used to define the transition between initiation and propagation. The procedure is implemented on an example, and the importance of considering the mixed-mode conditions is indicated.

Retrofit against fatigue cracking at web penetration details with a slot in steel girder

Fatigue cracking in steel girder web penetration details is so dangerous that it can break steel girders. The purpose of this study is to clarify fatigue behavior of web penetration details with cross beam flange welded to upper or lower surface of a slot by fillet welding, and verify the effect of retrofitting methods against fatigue cracking in web penetration details. As a result, fatigue cracks were initiated at the web-side toe of the fillet weld along cross beam lower flange edges both in upper and lower welded specimen. In the upper welded specimen, fatigue cracks were propagated into the web plate diagonally upward almost perpendicularly to the maximum principal stress direction. Whereas in the lower welded specimen, fatigue cracks were propagated into the web plate vertically downward faster than in the case of the upper welded specimen, and broke the lower flange. The fatigue life of the lower welded specimen was shorter than that of the upper welded specimen. As a preventive measure, weld toe grinding can extend fatigue life a little, but cannot prevent crack initiation perfectly. As a post cracking measure, a stop hole can stop crack propagation of long cracks, whereas crack removal and shaping up method by grinding can prevent crack propagation of short cracks.