FEM Stress Analysis Research Articles

Fatigue-induced fracture assessment for duplex stainless steel protruding tube-to-tubesheet welded joints in multiple-tube test specimen

A novel fatigue testing result and its numerical discussions are presented for protruding tube-to-tubesheet welded joints. The welded joint specimen is made up of an innovative stainless steel grade for pressure vessels of urea production plant, i.e., duplex stainless steel (DSS). Multiple-tube test specimen in which a bundle of tubes fabricated to the tubesheet by welding, is employed. During the fatigue test, several initial cracking positions were observed from the central tube and surrounding tubes. The crack propagation (CP) phenomena, i.e., CP from the central tube to surrounding tube and from the surrounding tube to central tube, were characterized for the multiple-tube problem. Two types of fracture pattern were occurred. These distinctive cracking behaviors are studied using numerical analyses, i.e., FEM stress analysis and X-FEM CP simulation. The possible crack initiating positions as well as the fracture patterns are identified based on the stress distributions. The CP behaviors between the tubes are carefully examined using stress intensity factor (SIF). Additionally, fatigue life cycles and fatigue resistance of the specimen are discussed. The fatigue-induced fracture of the DSS specimen is rigorously investigated and presented using the experimental and numerical methods.

Effect of Residual Stresses on Fatigue Strength on the Smooth and Splined Parts of Spring Torsion Bars

The article discusses the effect of residual stresses on the fatigue strength of torsion bars by using the Dang Van fatigue criterion, at the stress concentration at the root of the tooth and at the smooth part of the torsion bar. Residual compressive stresses were induced on the surface during the technological processes by cold surface rolling and torsional pre-strain (pre-setting) to the plastic range, to increase the elastic linear range and to reduce the stress state during torsional loading. Normally, the stress reduction in the torsion bar at the mounting point is provided by the increased diameter of the splined part, compared to the smooth part of the torsion bar. In the case of a small difference between the diameter of the splined and the smooth parts of the torsion bar, a fatigue crack can be initiated in the notch, which causes the torsion bar to break during operation after a few hundred loading cycles. FEM stress analysis of a torsion bar was performed in the article. The actual residual stresses were measured by the non-destructive X-ray method at the place of stress concentration on the smooth and splined parts with a notch of the torsion bar, and Dang Van’s failure criterion was used to determine the fatigue behavior during torsional loading. Therefore, in the paper, the three methods were combined to determine the optimum lifetime of a torsion bar exposed to the required cyclic torque. The results of the analysis showed that the residual compressive stresses are useful up to a certain limit where the retarded fatigue crack initiation occurred, but a further increase of compressive stress caused an increase of the maximum shear stresses, which, in turn, caused the initiation of fatigue and failure at the splined part of the torsion bar before achieving the required number of cycles.

On the morphology and failure of worn human molar cusps

ObjectivesEnamel wear is a common occurrence that may lead to tooth failure. Beyond reducing enamel thickness, wear exposes different regions of enamel microstructure to various types of stresses. This work was aimed at elucidating the effect of enamel wear on enamel morphology and tooth resilience in human molar teeth undergoing large-scale contact. MethodsIntact/polished molar cusps were indented with a hard disk/ball. The unloaded specimens were sectioned longitudinally or transversely, and the damage examined by optical and scanning electron microscopy. The onset of cracks at the dentin horn apex was determined by a FEM stress analysis that modeled a cusp as truncated, conical enamel shell supported by dentin. ResultsThe damage consisted of radial and cylindrical cracks growing under the contact, sparsely distributed radial cracks in the enamel shell region, and cracking from tufts at the dentin horn apex (TA cracks). The damage under the contact circle exhibited shear deformation zones. These zones helped relieve contact stresses, absorb energy, contain damage, and solicit cylindrical cracks in order to avoid growth of wear-sensitive cone cracks. Enamel tufts provided stress shielding while Hunter-Schreger Bands helped maintain this benefit by enforcing a collaborative cracking. The TA cracks were deemed a primary cause for tooth failure. The FEA predicted well the onset of these cracks. Making use of in vivo enamel wear data, the analysis showed that cusp failure might routinely occur at old age. SignificanceThe results provided new information on the response of the interior part of enamel to contact loading. This included the resistant effects of the waviness of enamel rods, interrods and tufts. The unique microstructure of enamel gave rise to shear bands under the contact circle that helped relieve contact stresses, absorb energy and contain damage. Enamel interrods solicited cylindrical cracking in order to avoid the wear-sensitive cone cracks. The onset of cracks at the dentin horn apex (TA cracks) was deemed critical to a tooth survival. The FEA showed that this cracking mode might routinely occur at old age due to natural enamel wear. Finally, the occlusal force needed to initiate the TA cracks in intact cusps can help explain the natural design logic involving the maximum bite force of hominid species.

Improvement of Fatigue Life of Arc Welded T-joints by Local Shape Change Near the Joint Line

ABSTRACT: The objective of this study is to improve fatigue life of T-shape arc welded joint structures which are often used in automobiles. In conventional T-shape joint, ST-joint (Standard T-joint), a vertical part is welded to a flat surface in the horizontal part. In this paper, a new structure, CCT-joint (Concave shape T-joint), is proposed which can reduce stresses around a joint line in car driving. In a CCT-joint, a vertical part is welded to a concave surface in the horizontal part. Fatigue life of a CCT-joint was significantly longer than that of a conventional ST-joint. Results of this study suggest that the drastic improvement in fatigue life was due to stress reduction by the concave shape around the joint line. FEM stress analysis revealed that the maximum stress near the weld line of CCT-joint was reduced to less than half that of the ST-joint. Furthermore, it was found that the locations of the maximum stress were different between the CCT-joint and the ST-joint. This suggests that the crack origin positions of both joints are different, which was also confirmed experimentally. The geometry, hardness, and residual stresses after welding were investigated around the weld toe. The effects of geometry and hardness were small on the fatigue life. The residual stresses were decreased by CCT-joint. The evaluation of the advantage of residual stress reduction by CCT-joint compared with that of stress reduction in car driving will be investigated and discussed in the next step of this study.

Accelerated fatigue testing on special tracks as new part of methodology for bus/trolleybus development

Over the last twenty years, the Research and Testing Institute in Pilsen (former SKODA Research Company) has been developing a methodology of computational and experimental investigation of strength and fatigue life of bodies of road vehicles for mass passenger transport. The methodology includes multibody dynamic simulations, FEM stress analysis, test bench testing and stress measurement during the service of a prototype vehicle aiming the evaluation of data about its fatigue life. The first presented case study demonstrates an example of using the methodology in the design stage of a trolleybus. There is a bus manufacturer that plans to include accelerated fatigue testing on special test tracks as a final test to demonstrate a satisfactory operational fatigue life of the vehicle in a shorter time. In real urban traffic and on the test circuit, extensive stress measurements on a number of structural details and components have been carried out and analysed. In collaboration with the Regional Technological Institute, which is the research centre of the Faculty of Mechanical Engineering of the University of West Bohemia, fatigue life was calculated for critical structural details and components of various parts of the bus. Based on these calculations, the development potential was assessed and planning of accelerated tests on proving grounds was considered.

Fem stress analysis of a barrel reamer

Abstract Horizontal Directional Drilling (HDD) is a method of installing underground pipelines, cables and service conduit through trenchless methods. The tools and techniques used in the HDD process are an outgrowth of the oil well drilling industry, too. Installation of a pipeline by HDD is a three stages process. The first stage consists of directionally drilling a small diameter pilot hole along a designed directional path. Drilling fluid is pumped through the drill pipe to the drill bit where high pressure jets and the bit will grind the soils ahead of the drill stem. The second stage involves enlarging this pilot hole to a diameter suitable for installation of the pipeline. A reamer is pulled back and rotated while pumping drilling fluid to cut and remove solids to enlarge the hole. A kind of reamer is the subject of this paper. The final stage consists of pulling the pipeline back into the pre-reamed hole. The most important part of the mechanical analysis is to define the boundary conditions for operating situations when the tool is under maximum load. In this paper we present a helpful description for giving boundary conditions to perform the numerical simulation of a barrel reamer.

Calibration of blade tip-timing sensor for shrouded 40″ last stage blade

Abstract Today’s power generation market requests steam turbines of high efficiency, reliability and wide operation range. The last stage moving blades are designed with integral shroud, mid-span, and fir-tree dovetail. The blades are continuously coupled by the blade untwist due to the centrifugal force, and thus the vibration control and increased structural damping are provided. Unfortunately this shroud complicates the blade vibration measurement using contactless methods. This paper describes in detail the Blade Tip-Timing measurement and calibration provided in test rig and in steam power station where the new eddy-current and optical sensors were used together to measure blade vibration in various conditions. Detail calibration procedure for Blade Tip-Timing sensors will be presented as well as the results of the measurements and comparison to FEM Maxwell calculation and to FEM stress analysis.

FEM thermal and stress analysis of bonded GaN-on-diamond substrate

A three-dimensional thermal and stress analysis of bonded GaN on diamond substrate is investigated using finite element method. The transition layer thickness, thermal conductivity of transition layer, diamond substrate thickness and the area ratio of diamond and GaN are considered and treated appropriately in the numerical simulation. The maximum channel temperature of GaN is set as a constant value and its corresponding heat power densities under different conditions are calculated to evaluate the influences that the diamond substrate and transition layer have on GaN. The results indicate the existence of transition layer will result in a decrease in the heat power density and the thickness and area of diamond substrate have certain impact on the magnitude of channel temperature and stress distribution. Channel temperature reduces with increasing diamond thickness but with a decreasing trend. The stress is reduced by increasing diamond thickness and the area ratio of diamond and GaN. The study of mechanical and thermal properties of bonded GaN on diamond substrate is useful for optimal designs of efficient heat spreader for GaN HEMT.

Options for a high heat flux enabled helium cooled first wall for DEMO

Helium is considered as coolant in the plasma facing first wall of several blanket concepts for DEMO fusion reactors, due to the favorable properties of flexible temperature range, chemical inertness, no activation, comparatively low effort to remove tritium from the gas and no chemical corrosion. Existing blanket designs have shown the ability to use helium cooled first walls with heat flux densities of 0.5MW/m2. Average steady state heat loads coming from the plasma for current EU DEMO concepts are expected in the range of 0.3MW/m2. The definition of peak values is still ongoing and depends on the chosen first wall shape, magnetic configuration and assumptions on the fraction of radiated power and power fall off lengths in the scrape off layer of the plasma. Peak steady state values could reach and excess 1MW/m2. Higher short-term transient loads are expected.Design optimization approaches including heat transfer enhancement, local heat transfer tuning and shape optimization of the channel cross section are discussed. Design points to enable a helium cooled first wall capable to sustain heat flux densities of 1MW/m2 at an average shell temperature lower than 500°C are developed based on experimentally validated heat transfer coefficients of structured channel surfaces. The required pumping power is in the range of 3–5% of the collected thermal power. The FEM stress analyses show code-acceptable stress intensities. Several manufacturing methods enabling the application of the suggested heat transfer enhanced first wall channels are explored. An alternative cooling technology is suggested, extending the tolerable heat loads up to 3MW/m2.

Biomechanical outcomes of tooth-implant-supported fixed partial prostheses (FPPs) in periodontally healthy patients using root shape dental implants

Background: Connecting an osseointegrated implant and a natural tooth is a treatment alternative for partially edentulous patients in some clinical situations. The main issue of a connected tooth-implant system is derived from the dissimilar mobility patterns of the osseointegrated fixtures and natural abutments causing potential biomechanical problems within the entire system. Purpose: The aim of this review was to multilaterally analyze and discuss the main biomechanical factors that may question the reliability of splinted tooth-implant system and the long-term success of fixed partial prostheses (FPPs) supported by both teeth and implants with an emphasis on the disparity of mobility of these two different abutments. Material and methods: An electronic MEDLINE (PubMed) search supplemented by manual searching was performed to retrieve relevant articles. An assessment of the identified studies was performed, the most valuable articles were selected and biomechanical outcomes of tooth-implant splinting system were analyzed. Results: 3D FEM stress analyses and photoelastic studies show uneven load distribution between the tooth and the implant and stress concentration in the crestal bone around the implant neck when connected to a natural tooth by FPPs. However, clinical studies demonstrate good results for both the implants and FPPs supported by splinted implant-tooth abutments. Conclusion: Connecting implants to natural teeth is not a preferable treatment option because of possible inherent biomechanical complications. Whenever possible, this treatment option should be avoided.

Tests and Examination of Drive Sheave in Mine Skip Hoist

Abstract This article describes a certain stage of work aimed at strength analysis of the structure of drive sheaves in mine shaft hoists. The work was initiated in connection with the emergence of cracks in those sheaves as a result of operation, and is an attempt to develop a new improved design of the drive sheave. The first stage involved several series of strain gauge measurements of stress in the drive sheaves of machines exposed to their rated load. Measurement technology and results are described in detail in (Rokita & Wójcik, 2013). The results of those measurements were the basis for FEM stress analysis and discussion of the changes in the structure of those sheaves. This article focuses on a description of the developed computational models for the currently used sheaves, and presents the results of strength calculations. As a result of those actions, a new drive sheave design was developed, for which a computational model was also prepared and FEM calculations were performed. Comparison of the results of calculations for both sheave models is, therefore, also an assessment of the impact of the proposed changes to the design of the drive sheave on its strength and durability. It was decided to adopt the results of strength analysis of the sheave assembly currently in service as the baseline for this assessment. This required not only the performance of a strength analysis on the basis of a specially developed drive sheave calculation model, but also identification and assessment of the allowable stress values for the analysed structure.

157 中空円筒締結体の三次元有限要素法応力解析と応力特性

157 中空円筒締結体の三次元有限要素法応力解析と応力特性

158 引張り荷重を受ける円形フランジボルト締結体のFEM応力解析と設計について

158 引張り荷重を受ける円形フランジボルト締結体のFEM応力解析と設計について

内圧作用下における大口径管フランジ締結体のFEM応力解析及び密封性の評価

内圧作用下における大口径管フランジ締結体のFEM応力解析及び密封性の評価

FEM stress analysis of the cooling hole of an HPDC die

Cracking at a cooling hole is a typical die failure mode in a high-pressure die-casting (HPDC) die. We simulated the thermal distortion of a die considering the HPDC machine deflection and revealed a stress concentration at the cooling hole. The stress concentration at the cooling hole changes after injection or after spraying and blowing air. The cooling hole top remains in a compression stress state 5, 10, and 20 mm deep from the die surface, but the stress amplitudes are higher when the depths are shallower. It was suggested that cracking takes place due to the high compressive stress and that the shear stress assists the propagation of the initiated crack. On the other hand, the stress condition at the R portion of the cooling hole is always a tensile state, but the mean stress and stress amplitude values were not found to be in the range that causes fatigue fracture. It was demonstrated that the developed analysis is valuable in designing the cooling hole of an HPDC die.

曲げモーメントを受ける同種被着体材料による中実円柱および中空円筒段付重ね合わせ接着継手の有限要素法応力解析と強度について

三次元有限要素法（FEM）を用いて曲げモーメントを受ける同種材料中実円柱および中空円筒段付き重ね合わせ接着継手の応力分布を解析し，接着層の縦弾性係数，接着層厚さ，スカーフ角，段数が接着界面での最大主応力分布に及ぼす影響を調べた。同種材料中実円柱および中空円筒段付き重ね合わせ接着継手のどちらの場合も突合わせ部接着界面の外径端部で最大主応力が特異性を示し，荷重は重ね合わせ部より大方突合わせ部で負担されることが分かった。両継手の場合とも最大主応力の値は接着層の縦弾性係数が小さくなるほど，接着層の厚さが小さくなるほど，また段数とスカーフ角が大きくなるほど小さくなることが示され，角状段付き接着継手の特性との比較検討も行った。FEM 計算による継手強度推定値は実験結果とかなりよく一致し，本研究で扱った寸法では同種材料中実円柱継手の方が中空円筒継手よりも，やや強度が大きくなることが示された。

内圧を受ける大口径 (20B) リングジョイントガスケット付き管フランジ締結体の有限要素法応力解析と密封性能評価

内圧を受ける大口径 (20B) リングジョイントガスケット付き管フランジ締結体の有限要素法応力解析と密封性能評価

Three-dimensional FEM stress analysis and strength prediction of scarf adhesive joints with similar adherends subjected to static tensile loadings

The interface stress distributions in scarf adhesive joints with similar adherends under static tensile loadings were analyzed using two-dimensional and three-dimensional finite element calculations for the two cases where adhesive length and width of the adherends were held constant. The effects of adhesive Young׳s modulus, the scarf angle and the adhesive thickness on the interface stress distributions were examined. In addition, the joint strength was predicted using the interface stress distributions based on the maximum principal stress theory and von Mises׳ stress criterion. It was found that when the scarf angle was around 60°, the singular stress at the edges of the interfaces was minimal in the 3-dimensional FEM calculations while the singular stresses vanished at 52° in the 2-dimensional FEM calculations. The value of the normalized maximum principal stress at the edges of the interfaces obtained from the 3-D FEM was larger than that from the 2-D FEM. It was found that the maximum principal stress increased at the edge of the interfaces as the scarf angle decreased. In addition, the effect of the adherends where the width was held constant was demonstrated. Experiments to measure strain and joint strengths were carried out. The measured strains were in a fairly good agreement with those obtained from the 3-D FEM calculations. Also, the measured joint strengths were fairly consistent with the predicted results based on the maximum principal stress theory. As a result, the maximum joint strength was observed when the scarf angle was around 60°. The difference in the interface stress distributions was demonstrated between the two cases.

Transverse strength of unidirectional non-crimp fabric composites: Multiscale modelling

A multiscale approach is used to predict transverse tensile and transverse compressive strength of unidirectional non-crimp fabric (NCF) composites. Numerical analysis on fibre/matrix scale is performed to obtain the transverse strength of the fibre bundle to be further used in an analytical mesoscale model to predict the strength of the unidirectional NCF composite. Design of unidirectional layer composites with the same fibres, interface, matrix and volume fractions as in the bundle is suggested as an alternative method for bundle strength determination. Good agreement of both methods for bundle transverse strength determination is demonstrated. The simple analytical model used on mesoscale gives accurate predictions of the tensile transverse strength whereas the compressive strength is underestimated. The necessity of including bundle waviness in models when bidirectional NCF composites are analysed is demonstrated by FEM stress analysis and by experimental data showing differences in transverse cracking pattern due to bundle waviness.

引張り荷重を受ける異種中実円柱および中空円筒段付重ね合せ接着継手の有限要素法応力解析と強度について

引張り荷重を受ける異種材料中実円柱段付き重ね合せ，および中空円筒段付き重ね合せ接着継手の応力分布を軸対称有限要素法を用いて解析し，被着体の縦弾性係数比，接着層の縦弾性係数，接着層厚さ，スカーフ角，段数などが最大主応力分布に及ぼす影響を調べた。その結果，異種材料中実円柱段付き重ね合せ継手の場合，接着界面の外径端部で最大主応力が特異性を示した。また中空円筒段付き重ね合せ接着継手の場合は，外径端および内径端で最大主応力が特異性を示し，内径端の方が大きくなった。両継手の場合とも最大主応力の値は被着体の縦弾性係数比が小さくなるほど大きく，さらに接着層の厚さが小さくなるほど，また接着層の弾性係数，スカーフ角が大きくなるほど、小さくなる。段数は内径端と外径端で逆の影響を示す。FEM計算による継手強度は実験結果とよく一致し，異種材料中実円柱継手の方が中空円筒継手よりも，やや強度が大きくなることが示された。