Reduction Of Stress Intensity Factor Research Articles

Investigation of Stress Intensity Factor Reduction using Steel and GFRP Patches for Repairing Edge Cracks in Steel Plates

The ultimate objective of this study was to compare the performance of repaired edge cracks in steel plates before and after repair with patches made of steel patch and glass fiber-reinforced polymer composite patches (GFRP) in different shapes: circular, rectangular, and trapezoidal, under two conditions: unsymmetric patch (one patch) and symmetric patch (two patches). A three-dimensional finite element model of the one-sided and two-sided repaired examples is used to study how the steel and composite patch affect the stress intensity factor (SIF). Under uniaxial tensile loads, the use of steel patches and GFRP composite patches to repair cracks was studied. The results showed that the steel patch performs better than the GFRP patch because it significantly lowers the stress intensity factor (SIF). The symmetric patch arrangement (two patches) is better than the un-symmetric patch arrangement (one patch) because it significantly reduces the stress intensity factor (SIF).

Enhancing Stress Intensity Factor Reduction in Cracks Originating from a Circular Hole in a Rectangular Plate under Uniaxial Stress through Piezoelectric Actuation

Enhancing Stress Intensity Factor Reduction in Cracks Originating from a Circular Hole in a Rectangular Plate under Uniaxial Stress through Piezoelectric Actuation

Numerical Analysis of Fracture Behaviour for Cracked Joints in Corrugated Plate Girders Repaired by Stop-Holes.

The efficient crack eliminated stop-hole measure was proposed to repair and reduce the stress concentration associated fracture risk of the corrugated plate girders by setting it at the critical joint of flange plate with tightened bolts and gaskets under preloading. To investigate the fracture behaviour of these repaired girders, parametric finite element analysis was conducted, focusing on the mechanical feature and stress intensity factor of crack stop-hole in this paper. The numerical model was verified against experimental results first, and then the stress characteristics due to the presence of crack open-hole were analysed. It was found that the moderate-sized open-hole was more effective than the over-sized open-hole in the reduction of stress concentration. For the model with prestressed crack stop-hole through bolt preloading, the stress concentration was nearly 50% with the prestress around open-hole increased to 46 MPa, but such a reduction is inconspicuous for even higher prestress. Relatively high circumferential stress gradients and the crack open angle of oversized crack stop-holes were decreased owing to additional prestress effects from the gasket. Finally, the shift from the original tensile area around the edge of the crack open-hole that was prone to fatigue cracking to a compression-oriented area is beneficial for the reduction of stress intensity factor of the prestressed crack stop-holes. It was also demonstrated that the enlargement of crack open-hole has limited influence on the reduction of stress intensity factor and crack propagation. In contrast, higher bolt prestress was more beneficial in consistently reducing the stress intensity factor of the model with the crack open-hole, even containing long crack.

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.

Finite element analysis on properties evolution of slab track FLC under the coupling action of rainfall and fatigue load

The filling layer concrete (FLC) is the crucial component materials in the CRTS III slab track structure. In order to investigate the properties evolution of FLC under the coupling action of environment and fatigue load, a three-dimensional finite element model of the CRTS III slab track structure system was established, and extended finite element method (XFEM) was used to calculate the stress intensity factor (SIF) of the crack tip under different conditions through the domain interaction. Results show the fatigue load significantly decreases stiffness of FLC. Coupling actions of water-saturated and fatigue load, acid-rain and fatigue load accelerate the deterioration of FLC. The decrease of stiffness leads to stress redistribution in FLC. With the increase of fatigue load, the stress of FLC decreases, but the stress of steel in the FLC increases. Positions of fatigue load on FLC take on significant influence on the evolution of properties. The deterioration rate of FLC at the end of the slab is greater than that of the middle FLC. Under the coupling of fatigue load and environment, FLC at the end of the slab is more likely to crack and propagation, and the cracks are most likely to appear from the bottom of the FLC. Due to the influence of the stress field and displacement field at the crack tip, with the fatigue load increasing, the SIF at the external crack of FLC has an exponential decrease. The coupling action of environment and fatigue load accelerates the deterioration of FLC, and also leads to the rapid reduction of SIF. In this study, the coupling of acid-rain and fatigue reduces the properties of FLC most obviously.

Estimation of stress intensity factor reduction for a center-cracked plate integrated with piezoelectric actuator and composite patch

Active repair of a damaged structure using piezoelectric (PZT) actuators has played a significant role during the last two decades in reducing crack damage propagation in structures due to its electro-mechanical effect. Similarly, passive repair of damaged structures using composite materials has been widely used in recent years, and it has been divided into many types. In this paper, the hybrid repair of center-cracked plate using PZT actuators at the front of a plate and a composite patch at the back of the plate is analytically and numerically investigated. First, the solution is obtained from Rose's equations for the center-cracked plate integrated with a composite patch and a passive PZT actuator under uniform uniaxial load. Second, the stress intensity factor (SIF) for a center-cracked plate due to stress produced by a PZT actuator is analytically modeled using the weight functions method. The superposition principle is then used to superimpose the aforementioned solutions in order to yield the PZT actuator and composite patch's hybrid stress intensity factor for the center-cracked plate. Finally, the proposed analytical method was verified using the finite element method. The results demonstrate low relative errors between the analytical and the FE of below 5% in all the cases studied in this paper.

Performance assessment and optimization of hybrid composite patch repair of aircraft structure

PurposeIn this article, a novel hybrid composite patch consisting of unidirectional carbon fiber and glass fiber is considered for repair of the aircraft structure. The purpose of this paper is to assess the performance of hybrid composite patch repair of cracked structure and propose an optimized solution to a designer for selection of the appropriate level of a parameter to ensure effective repair solution.Design/methodology/approachElastic properties of the hybrid composites are estimated by micromechanical modeling. Performance of hybrid composite patch repair is evaluated by numerical analysis of stress intensity factor (SIF), shear stress, and peel stress. Design of experiment is used to determine responses for a different combination of design parameters. The second-order mathematical model is suggested for SIF and peel stress. Adequacy of the model is checked by ANOVA and used as a fitness function. Multiobjective optimization is carried out with a genetic algorithm to arrive at the optimal solution.FindingsThe hybrid composite patch has maintained equilibrium between the SIF reduction and rise of the peel stress. The repair efficiency and repair durability can be ensured by selection of an optimum value of volume fraction of glass fiber, applied stress, and adhesive thickness.Originality/valueThe composite patch with varying stiffness is realized by hybridization with different volume fraction of fibers. Analysis and identification of optimum parameter to reduce the SIF and peel stress for hybrid composite patch repair are presented in this article.

Optimization of composite patch repair for center-cracked rectangular plate using design of experiments method

The repair of aircraft structures using composite material patches are well known in aerospace industries. In the present work composite patch bonded with a superglue over a cracked rectangular plate under uniform uniaxial tensile stress is considered. A three-dimensional finite element method was used to define the stress intensity factor for a repaired plate at mode-I crack propagation. Later, the design of experiments method was used to investigate the parametric effect on repair structure in order to achieve the optimum solution. The size and mechanical properties of the adhesive bond and composite patch that affect the repair quality are considered as the parameters for reduction in stress intensity factor. The outcome of this work will serve as a guideline for designer to improve the repair quality and durability.

Effect of constraints on stress intensity factor for dissimilar metal plate with centre crack under uniform tension

Dissimilar metal joints (DMJs) are more common in the coolant systems of most of the engineering applications and are subjected to variety of loadings. To understand the constraint effect of the dissimilar metal welded joints (DMJs), fracture studies have been carried out on dissimilar metal plate (DMP) with center crack made up of ferritic steel (SA 508 Gr.3 Cl.1), austenitic steel (SA312 Type 304LN) and Inconel – 182 alloy (ENiCrFe-3). 2-D and 3-D finite element analysis (FEA) is carried out on the DMP with centre crack (length = 250 mm, width = 150 mm) for various crack lengths (15, 20, 30 and 50 mm) and various dissimilar metal welded joint widths (DMJWs) (5, 10, 20, 30, 40 and 50 mm) to evaluate stress intensity factor (SIF). SIF has been evaluated by employing domain integral method. It is observed from the studies that, (i) SIF increases with increase of crack length and DMJW, (ii) 3-D finite element analysis yields realistic SIF (iii) SIF of DMP reduces compared to homogenous plate, (iv) constraint effect is found to be significant if DMJW is less than the initial crack length, resulted in reduction of SIF (iv) constraint effect is found to be insignificant if DMJW is more than 20 mm and the ratio of crack length to DMJW is greater than 0.75. The study will be useful for damage tolerant analysis of structures/components which in turn will be helpful for repair, retrofitting of components and in-service inspection.

Experimental and numerical analysis of piezoelectric active repair of edge-cracked plate

This article proposes an alternative model for active repair for an edge-cracked plate with adhesively bonded piezoelectric actuator. It computes the Mode-I stress intensity factor (SIF) produced by the piezoelectric actuators using appropriate derived geometrical weight functions. Furthermore, this article presents an experimental study to verify the proposed analytical model and the finite element analysis using ANSYS software. Therefore, the analytical, finite element and experimental results for Mode-I opening of the crack conditions are demonstrated. Parametric analysis to understand the influence and to study the efficiency of the piezoelectric actuator on mitigation of the Mode-I SIF was conducted. The obtained analytical solution is applicable in the calculation of Mode-I SIF with reasonable accuracy. The result indicated that the maximum reduction of SIF is achieved with the application of high external voltage and thin thickness actuator. The relative errors of the analytical model and the experimental results are less than 10% in all the cases studied in this article.

On the static strength of aluminium and carbon fibre aircraft lap joint repairs

The behaviour of various aircraft lap joint repair configurations is investigated experimentally and numerically under static loading. The lap joints consist of aluminium alloy (AA) 2024-T3 substrates repaired with twin single-sided AA 2024-T3 or Carbon Fibre Reinforced Epoxy (CFRE) doublers. Pure riveted, pure bonded and hybrid (riveted and bonded) joints of metal–metal and metal–composite configurations are investigated. From experimental results, joints with adhesive bond showed nearly 5 times higher average strength than pure riveted joints, while hybrid joints performed better than riveted and bonded joints because of higher stiffness. On the other hand, hybrid metal–metal joint has 70% higher average strength compared to hybrid metal–composite joint. Rivet-shear has caused failure of riveted joints, and adhesive failure is observed in pure bonded joints. Hybrid joints with metal doublers have failed initially due to adhesive failure and later rivet shear. Interestingly, net-section failure is observed in composite doublers with breakage of doublers due to the presence of holes in the doublers. Experimental results are complimented with numerical analysis using commercial finite element code ABAQUS. Load–displacement curves obtained from the numerical results are in good agreement with experiments with a marginal error of 2%. In addition to load–displacement curves, a detailed stress analysis is performed numerically on metal–metal and metal-composite joints under riveted, bonded and hybrid configurations to study stress distribution on substrate and doublers. Numerical analysis showed hybrid and bonded joints have lower stresses in substrate and doublers compared to the riveted joints. Bonded joints have smoother load transfer due to the adhesive spread over a larger area. And finally, Stress Intensity Factors (SIFs) are performed numerically for unreinforced and reinforced metal substrate with crack length of 1, 5 and 10 mm with metal and composite doublers under riveted and bonded configuration. For crack of 10 mm, 35% reduction in SIFs is observed for reinforced substrate with bonded metal or composite doublers compared to unreinforced cracked substrate.

Evaluating the Reduction of Stress Intensity Factor in Center-Cracked Plates Using Piezoelectric Actuators

Active repairs using smart materials such as piezoelectric actuators can play a significant role in reducing the crack damage propagation in engineering structures. This study analytically and numerically investigated the active repair of center-cracked plates using piezoelectric actuators. First, the stress intensity factor (SIF) for a center-cracked plate due to stress produced by a piezoelectric actuator is analytically modeled. This analytical model is obtained by applying the method of weight functions. In the second step, the solution is found for the center-cracked plate due to external loading from known linear elastic fracture mechanics. These solutions are then superimposed, taking into account the superposition principle to yield the total stress intensity factor for the integrated piezoelectric actuator to the center-cracked plate. Finally, the proposed theoretical model is verified by finite element simulation. The results indicated that the relative errors of the analytical model and the FEA results are less than 5% in all the cases studied in this paper.

Modeling approach to evaluating reduction in stress intensity factor in center-cracked plate with piezoelectric actuator patches

Active repairs using piezoelectric actuators can play a significant role in reducing the crack damage propagation in thin plate structures. Mode-I crack opening displacement is the most predominant one in tension, and it is responsible for the failure which in turn affects the load carrying capability of the cracked structure. In addition, there are limited studies that investigated the effect of the piezoelectric actuator over mode-I active repair. In this study, the mode-I stress intensity factor for a plate with a center crack, and a bonded piezoelectric actuator was modeled using the linear elastic fracture mechanics. For this, an analytical closed-form solution is developed using the virtual crack closure technique taking into account mode-I as the only effective mode, coupling effects of the piezoelectric patch, and the singular stress at the crack tip. In addition, the total stress intensity factor was obtained by the superposition of the stress intensity factor obtained from the stresses produced by the piezoelectric actuators on the crack surfaces as the only external loads on the cracked plate and the stress intensity factor due to the far-field tension load. The proposed analytical model for mode-I stress intensity factor was verified by a finite element–based approach using ANSYS finite element software. The results demonstrated a good agreement between the analytical and finite element models with a relative error of less than 4% in all the cases studied. The results illustrated that the piezoelectric patch is efficient in reducing stress intensity factor when an extension mode of the actuator is applied. However, applying a contraction mode of the piezoelectric actuators produced negative strain which increased the stress intensity factor and thus the severity of the cracked structure and could lead to damage propagation.

An innovative tooth root profile for spur gears and its effect on service life

An innovative approach to the design of the gear-tooth root-profile, and its effects on the service life is reported in this paper. In comparison with the widely used trochoidal and the recently proposed circular-filleted root profiles, the optimum profile proposed here is a $$G^2$$ -continuous curve that blends smoothly with both the involute of the tooth profile and the dedendum circle. Following the AGMA and ISO standards for fatigue loading, the von Mises stress at the critical section and stress distribution along the gear tooth root are studied. The process leading to gear-tooth failure is composed of the crack initiation phase, in number of cycles $$N_i$$ , and the crack propagation phase, in $$N_p$$ cycles. The strain-life ( $$\epsilon$$ -N) method is employed to determine $$N_i$$ , where the crack is assumed to initiate at the critical section. Based on the ANSYS crack-analysis module, the effects of $$G^2$$ -continuous blending on the stress intensity factor (SIF) are investigated for different crack sizes. Paris’ law, within the framework of Linear Elastic Fracture Mechanics, is used to correlate the SIF with crack size and, further, to determine $$N_p$$ . The optimum profile provides a significant reduction in SIF and improvement in both $$N_i$$ and $$N_p$$ . Spur gears are made of high-strength steel alloy 42CrMo4, the effects of its properties and surface treatment on service life improvement not being included in this study.

Investigation of composite coating effectiveness on stress intensity factors of cracked composite pressure vessels

The use of composite coating is among favorite reinforcement methods for repairing cracked pressure vessels. In the present study, the effects of mechanical properties and geometry of composite coatings on Stress intensity factors (SIF) is investigated. In this regard, after verification of modeling and analytical procedure, some 3D longitudinal and transverse semi-elliptical cracks in inner and outer layer of pressure vessels are modeled in FEM software. In addition, mechanical properties and thickness are adjustable. Then SIFs in throughout of the pressurized crack face were computed. Results indicate that critical crack occurred whenever inner longitudinal crack present in composite pressure vessel. Also, via 2 mm thickness of graphite/epoxy and glass/epoxy composite coatings, 55 and 43 percent reduction in Stress intensity factors (SIF) was observed, respectively. It is shown that increasing the thickness of the composite layers reduces stress intensity factor and hence can be considered as a suitable solution for reinforcement of the cracked pressure vessel in practical situations.

Extended finite element fracture analysis of a cracked isotropic shell repaired by composite patch

AbstractAn extended finite element method (XFEM) is developed to study fracture parameters of cracked metal plates and tubes that are repaired on top of the crack with a composite patch. A MATLAB® stand‐alone code is prepared to model such structures with eight‐noded doubly curved shell elements in the XFEM framework. Crack trajectory studies are performed for a diagonally cracked panel under fatigue loading. Verification studies are investigated on different shell type structures such as a cracked spherical shell and cracked cylindrical pipe with different crack orientations. The effects of using patch repairs with different fibre orientations on the reduction of stress intensity factors (SIFs) is also studied which can be useful for design purposes. XFEM is selected as any crack geometry can be embedded in the finite element mesh configuration with no need to coincide the crack geometry with meshed elements and so re‐meshing with fine mesh generation is not needed in the current method.

Pressurized hole as a Stress Reducing feature

In many structures, cracks may appear during manufacturing process or in service. Stress intensity factor is used in fracture mechanics to more accurately predict the stress state (stress intensity) near the tip of the crack caused by remote load or residual stresses. In this work a rectangular plate with central hole & two cracks emanating from his hole is analyzed for remote tensile loading. Stress Intensity Factor is determined for this configuration using finite element based software ANSYS. This value is compared with the one obtained by analytical method. Then hole as a stress reducing feature is studied. The effect of hole location, hole size on the stress intensity factor is studied. Feasibility of pressurized hole as a stress reducing feature is studied for the best hole location & size obtained earlier. Further variation in the pressure was done to study effect of pressurized hole. Fracture Mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the material resistance to fracture. Stress intensity factor, K, is used in fracture mechanics to more accurately predict the stress state (stress intensity) near the tip of the crack caused by remote load or residual stresses. It is theoretical construct applicable to a homogeneous elastic material and is useful for providing failure criteria for brittle materials. The stress intensity factor remains finite and provides a basis for determining the critical load. If the value of SIF is less than the toughness of the material, the structure is safe. If SIF value is more than the toughness of material, the structure fails. Typical Stress Intensity Factor Reduction Techniques used by earlier researchers are: Strip or patch repair method, Artificial crack closure by crack filling, Welding repair, Hole drilling. In this work, finite element analysis of a cracked plate is done. Initially a hole is used as a stress reducing feature. Then the effect of pressure applied in this hole is studied. II. LITERATURE REVIEW Miloud Souiyah, Abdulnaser Alshoaibi. et al (1) considered a rectangular plate with crack starting from a circular hole and double edge notched plate. Both geometries were in tensile loading and under mode-I conditions. In this paper a displacement extrapolation technique was employed particularly to predict the crack propagations direction and to calculate SIFs and validated with the relevant numerical and analytical results obtained by other researcher. Xiangqiao Yan (2) studied the stress intensity factors (SIFs) of cracks emanating from a rhombus hole in a rectangular plate subjected to internal pressure by means of the displacement discontinuity method with crack-tip elements (a boundary element method) proposed by the author. Moreover, an empirical formula of the SIFs of the crack problem was presented and examined. It was found that the empirical formula is very accurate for evaluating the SIFs of the crack problem. Junping Pu (3) studied a Boundary value problem of a plate with crack and defect such as the circular and/or elliptical holes. This kind of problem was suitable for solving by boundary element method with its higher precision. The sub-region method was used in this work. A center cracked plate subjected to remote tensile and shear loading was studied numerically. Levend Parnas and Omer G. Bilir (4) calculated Stress Intensity Factor by experimental procedures using strain gages. In this method, cracks were opened at the tip of crack starter slot on the standard compact tension test specimens by using Electrical Discharge Machining (EDM) and Wiring Discharge Machining (WDM). Strain gage data from the crack tip region were used to calculate stress intensity factors. But the values obtained were on lower side. This was partly due to unavoidable deformations in the adhesives used to fasten the strain gages to the specimen. This might be also due to the use of the relatively long strain gages, local yielding and three-dimensional effects and limited regions for strain gauge.

Ni基超合金の薄板疲労試験における振動リード法の適用についての研究

Vibrating reed machine to thin plate fatigue test is a kind of resonance type, invented by Hirata 1995 as the method of a speaker unit was used as the vibrator. In the case of the fatigue evaluation of thin plate plane bending stress as typified by bellows are needed, it was expected as the effectual method. There are little research papers that made adequacy evaluations about absolute level of fatigue data. Furthermore, even though the test method is high-speed and simple, it doesn't become universal. And so, this paper is appeared to make adequacy evaluations of thin plate vibrating reed fatigue test with testing thin test pieces actually. In this time, adding the determined points of the displacement, and using a vibration exciter, accuracy is increased. Vibrating reed fatigue tests were carried out for Inconel 718 in ambient air using 5 mil and 8 mil notched specimens with 64 Hz and 110 Hz. Fatigue results appeared that the fatigue strength in vibration reed method was higher than that in rotating bending fatigue tests with 55 Hz reported by Kawagoishi et al. 2006. Then pointing out the stress field of crack tip in the difference of thickness, the calculations of each stress intensity factor conduced to the effect of thickness. The increase in fatigue strength of thin plate was caused by the reduction of stress intensity factor in the crack tip with early crack growth.

Multiscale thermodynamic analysis on hydrogen-induced intergranular cracking in an alloy steel with segregated solutes

Abstract A multiscale analysis has been conducted on hydrogen-induced intergranular cracking at ambient temperature in medium strength (840 MPa) Ni-Cr steel with antimony, tin, and phosphorous segregation. Combining first-principles calculations and fracture mechanics experiments, a multiscale relationship between threshold stress intensity factor (K th) and cohesive energy of grain boundary (the ideal work of interfacial separation, 2γ int) was revealed. The K th was found to decrease rapidly under a certain threshold of 2γ int, where the 2γ int decreases mainly by mobile hydrogen segregation on fracture surfaces. This segregation is considered to arise during formation of the fracture surfaces under thermodynamic equilibrium in slow crack growth. The resulting strong decohesion probably makes it difficult to emit dislocations at the microcrack tip region, leading to a large reduction in stress intensity factor. Our analysis based on this mobile hydrogen decohesion demonstrates that the K th decreases dramatically within a low and narrow range of hydrogen content in iron lattice in high-strength steels.

Evaluation of Stress Intensity Factor for CFRP Bonded Steel Plates

Recent studies on the application of carbon fibre reinforced polymer (CFRP) materials to defected steel structures have demonstrated the potential for significant reduction of stress intensity factor (SIF) values at crack tips leading to extended fatigue lives. However, most of the previous research relied on experimental and numerical methods, which were either expensive or time-consuming. In this paper, the SIF values at crack tips of steel plates strengthened with bonded composite materials were evaluated using linear elastic fracture mechanics. The analysis was based on the classical solution of SIF values of plain steel plates, considering load share effect and geometry correction factor change resulted from the overlay patch. The effect of different parameters were demonstrated and compared with experimental results, including initial damage degrees of specimens, geometric and mechanical properties of retrofitting materials and bond locations. Good agreement with the experimental data indicated that this approach could conservatively predict the SIF values with reasonable accuracy. A parametric study on variables including the CFRP modulus, the bond width and bond length was conducted based on this method to further investigate their effect on the SIF values.