Showing papers on "Fatigue limit published in 1992"

Characterization of eutectic Sn-Bi solder joints

Abstract: This report presents experimental results on 58Bi-42Sn solder joints, optical and SEM microstructures of their matrix and of their interface with copper, solidification behavior studied by differential scanning calorimetry, wettability to copper, creep, and low cycle fatigue. These results are discussed in comparison with 60Sn-40Pb solder, and with three low temperature solders, 52In-48Sn, 43Sn-43Pb-14Bi, and 40In-40Sn-20Pb. The 58Bi-42Sn solder paste with RMA flux wets Cu matrix with a wetting angle of 35° and had a 15° C undercooling during solidification. The constitutive equation of the steady state shear strain rate, and the Coffin-Manson relation constants for the low cycle shear fatigue life at 65° C have been determined. The test results show that this solder has the best creep resistance but the poorest fatigue strength compared with the other four solders.

Cyclic Plasticity and Low Cycle Fatigue Life of Metals

Abstract: 1 Introduction 2 Stress and Strain in Cyclic Loading Monotonic stress-strain curve Stress-strain relationship in cyclic loading Hysteresis loop Cyclic hardening/softening curves Cyclic stress-strain curve 3 Cyclic Plasticity and Microstructure Metals and simple alloys with fcc structure Metals and single phase alloys with bcc structure Other metals and single phase alloys Multiphase materials 4 Dislocation Mechanisms in Cyclic Plastic Straining Athermal mechanisms in fcc metals Thermally activated cyclic straining Dislocation mechanisms in particle strengthened metals 5 Statistical Description of Cyclic Stress-Strain Response Internal and effective stress in an elementary volume Statistical approach 6 Experimental Investigation of the Dynamics of Cyclic Plastic Straining Stress-dip method Stress and strain relaxation Strain rate changes Analysis of hysteresis loop shape Evaluation of results using individual methods 7 Cyclic Creep Relevant experimental investigations Dislocation arrangements Mechanisms and models 8 Fatigue Crack Initiation Observation of surface relief evolution Models of surface relief evolution Mechanisms of crack initiation Role of grain boundaries Role of inclusions 9 Growth of Fatigue Cracks Fracture mechanics approach to fatigue crack growth Crack growth under small scale yielding General yield fatigue crack growth Short crack growth 10 Fatigue Life of Smooth Bodies Strain controlled cycling Plastic strain controlled cycling Stress controlled cycling Energy criterion Evaluation of fatigue life of a smooth body using the fatigue process model 11 Fatigue Life of Notched Bodies Stress and strain concentration in a notched body Fatigue life evaluation 12 Variable Amplitude Loading Phenomenological description Analysis of load history Sudden changes of strain amplitude Cyclic plasticity in repeated block loading Hypothesis of cumulative damage Fatigue life prediction 13 Effect of Depressed Temperature Cyclic plasticity Fatigue life 14 High Temperature Low Cycle Fatigue Cyclic plasticity at elevated temperatures Fatigue life and its evaluation Damage mechanisms Fatigue life prediction 15 Thermal and Thermomechanical Fatigue The effect of temperature changes under constraint Reversed plasticity and thermal cracking Thermal ratchetting 16 Multiaxial Loading Multiaxial stress and strain Cyclic stress-strain response Fatigue life 17 Computer Controlled Fatigue Testing Role of the digital computer Low cycle fatigue test Crack growth test Variable amplitude test Other tests 18 Characterisation of Low Cycle Fatigue Resistance of Metallic Materials Basic characteristics Review of materials properties References Subject Index

Standardization of fretting fatigue test methods and equipment

Abstract: Papers contained in this book are grouped under the topics of the fundamental aspects of fretting fatigue testing (conceptual framework and mechanics of contact), methods and equipment for fretting fatigue testing, environmental and surface conditions, and nonconventional materials and test methods. Papers are presented on the problems of fretting fatigue testing, a critical appraisal of testing methods in fretting fatigue, the determination and control of contact pressure distribution in fretting fatigue, and fretting fatigue analysis of strength improvement models with grooving or knurling on a contact surface. Other papers include a critical review of fretting fatigue investigations at the Royal Aerospace Establishment, techniques for the characterization of fretting fatigue damage, improving fretting fatigue strength at elevated temperatures by shot peening in steam turbine steel, the fretting fatigue properties of a blade steel in air and vapor environments, and fretting fatigue of carbon-fiber-reinforced epoxy laminates.

A fractographic investigation of PMMA bone cement focusing on the relationship between porosity reduction and increased fatigue life.

Abstract: Fracture surfaces of both monotonic and fatigue loaded bone cement samples were examined to investigate the fractographic characteristics of PMMA. Classic cleavage step river patterns were observed on all monotonically loaded samples, running downstream in the direction of crack propagation. All fatigue cracks initiated at internal pores and the direction of crack propagation of many cracks was discernible. Porosity, pore size, and pore size distribution were found to affect the crack initiation and fatigue behavior of bone cement. Statistical analysis revealed a strong negative correlation between two-dimensional porosity present on the fracture surfaces and the cycles to failure. The fractographic observations of these fatigue samples elucidate one reason why porosity reduction by centrifugation or vacuum mixing increases the fatigue life of PMMA bone cement.

Fatigue of aluminium—lithium alloys

Abstract: Aluminium-lithium alloys are a class of low density, high strength, high stiffness monolithic metallic materials that have been identified as prime candidates for replacing 2000 and 7000 series aluminium alloys currently used in commercial and military aircraft. In this review, the cyclic fatigue strength and fatigue crack propagation characteristics of aluminium-lithium alloys are reviewed in detail with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage tolerant design and lifetime computations. Compared with traditional aerospace aluminium alloys, results on the fatigue of binary Al-Li, experimental Al-Li-Cu, and near commercial Al-Li-Cu-Zr and Al-Li-Cu-Mg-Zr systems indicate that alloying with Li degrades the lowcycle fatigue resistance, though high-cycle fatigue behaviour remains comparable. The alloys, however, display superior (long crack) fatigue crack growth properties, resulting from a prominent role of crack tip shielding, principa...

A study of progressive failure of rock under cyclic loading by ultrasonic and AE monitoring techniques

Abstract: New experiments were conducted to expand the existing information on AE and pulse velocity changes In this study, a few Hyderabad granites were subjected to uniaxial compressive cyclic loading conditions at room temperature The laboratory tests were carried out by loading the rock to a constant stress maximum ranging between 40% and 80% of failure stress in the initial cycles, and then by progressively increasing the stress maximum, and also increasing the time gap between the cycles Compressional wave velocity and amplitude changes were monitored in directions perpendicular to the applied stress, and acoustic emission event count data were recorded continuously until the fatigue failure of the rock

An analysis of stress concentrations caused by shot peening and its application in predicting fatigue strength

Abstract: — In order to calculate the stress concentration caused by shot-peening dents, three simplified models, namely a one-pit model, a seven-pit model and a pit-loop model, are proposed in this paper. By using a finite element program for automatic dynamic incremental non-linear analysis, the stress concentration coefficients based on these models are calculated for different pit parameters and different stress fields. When considering the stress concentration and residual stress field produced by shot peening, a modified Goodman formula is proposed and used for forecasting the fatigue strength of peened specimens having the fatigue sources at the surface. The forecasted results were verified by peening followed by fatigue tests on 40Cr steel.

Short Fatigue Cracks

Abstract: For more than a century, metal fatigue research has been associated with producing S-N endurance curves from experiments on plain and notched specimens. Recently a detailed interpretation of S-N curves generated from smooth specimen tests has become possible because of our increased understanding of the behaviour of very small cracks ie cracks growing to about 500 pm in depth, which can involve a high proportion of the fatigue lifetime at high endurances. This recent knowledge has been due to the development of elastic-plastic fracture mechanics of crack growth, new techniques to monitor the growth of small surface cracks, and not least, from commercial pressures to use advanced materials.

The fatigue behaviour of T- and X-joints made of square hollow sections

Abstract: This work presents the results of experimental and numerical research on the fatigue behaviour of T- and X-joints between square hollow sections of which the brace is welded to the face of the chord, without any additional stiffeners. The work has been carried out in the framework of the CIDECT programme 7K "Fatigue behaviour ofuniplanar joints", and an earlier ECSC programme "Fatigue strength of welded unstiffened RHS joints in latticed structures and Vierendeel girders" (CECA Convention nr. 721 O-SA/111). Furthermore, experimental results of the CIDECT programme 7H "The low cycle fatigue behaviour of axially loaded T -joints between rectangular hollow sections" have been used in this work. The aim of the research programmes is to establish a better design method for the fatigue strength of joints in square hollow sections, based on the hot spot stress method. The results are to be proposed for inclusion in Eurocode 3.

Advanced aerospace materials

Abstract: 1. Situation and Perspectives.- 2. Metallic Materials and Metal Matrix Composites.- 2.1 Metallic Materials.- 2.1.1 Aluminium Alloys.- 2.1.1.1 High-Strength Aluminium Alloys.- 2.1.1.2 Aluminium-Lithium Alloys.- 2.1.1.3 Powder Metallurgy of Aluminium Alloys.- 2.1.2 Titanium Alloys and Aluminides.- 2.1.2.1 Titanium Alloys.- 2.1.2.2 Titanium Aluminides.- 2.1.3 Superalloys and Coatings.- 2.1.3.1 Superalloys.- 2.1.3.2 High Temperature Corrosion.- 2.1.3.3 Coatings.- 2.2 Metal-Matrix Composites.- 2.2.1 Metal-Matrix Composites with Aluminium Matrix.- 2.2.2 Fibre Reinforced Aluminium Laminates.- 2.2.3 Titanium Matrix Composites.- 2.2.4 Interfaces in Metal Matrix Composites.- 3. Ceramic Materials and Ceramic Matrix Composites.- 3.1 Non-Oxide Materials (Silicon Nitride).- 3.1.1 Fabrication and Microstructural Development of Non-Oxide Ceramics (Silicon Nitride).- 3.1.2 Silicon Nitride Matrix Composites.- 3.1.3 Fracture and Fatigue of Non-Oxide Ceramics.- 3.2 Oxide Materials (Mullite).- 3.3 Imaging Microstructures of Monolithic Carbons and Carbon/Carbon Composites in the TEM.- 4. Polymer Matrix Composites.- 4.1 Quasi-Static Strength of Polymer Matrix Composites.- 4.2 Fatigue Strength of Polymer Matrix Composites.- 5. Materials Characterization and Life Prediction.- 5.1 Microstructural and Microanalytic Methods.- 5.2 Fatigue and Fracture of Metallic Materials.- 5.2.1 Random Load Fatigue and Life Prediction.- 5.2.2 Physical Reasons for the Existence of ?Keff.- 5.2.3 Crack Growth Life Prediction.- 5.3 Special Testing Techniques.- 5.3.1 Stress Corrosion Testing.- 5.3.2 Biaxial Testing.- 5.3.3 Chevron Notched Specimen Testing.- Abbreviations.- Laudatio.

Simplified Contact Fatigue Life Prediction Model—Part II: New Model

Abstract: This paper presents the new model that uses the applied alternating shear stress field to define the critical stress as a function of depth, and computes life as crack propagation time through this field.

Fatigue Behavior of Direct Post-and-core-restored Premolars

Abstract: Evaluation of long-term mechanical behavior of new types of restorations in clinical trials is time-consuming. A partial alternative can be found in experimental fatigue-testing, which simulates accelerated mechanical deterioration. The aim of this study was to determine the feasibility of using fatigue-testing of a complex dental restoration and to evaluate the mechanical fatigue behavior of premolar teeth restored with a titanium alloy post and an amalgam or composite core. Eighty-seven human upper premolar teeth were decoronated, embedded, and restored with a prefabricated post of 1 mm diameter. The teeth were randomly assigned to one of two groups corresponding with a core build-up of amalgam or chemically-cured core composite, respectively. Five to 21 days after restoration, the specimens were subjected to cyclic loading (frequency, 5 Hz), at an angle of 45 degrees to the long axis of the tooth. The boundary technique was used for determination of the mean fatigue strengths of the restorations at 10(4), 10(5), and 10(6) cycles, simulating up to 1-3 years of clinical functioning. Mean fatigue strength was expressed in percentage of initial strength: For 10(4), 10(5), and 10(6) cycles, the results were 66%, 58%, and 52%, respectively, for the amalgam and 62%, 62%, and 53% for the composite group. It is concluded that fatigue-testing of more complex systems is possible, if a suitable testing method is selected. The restorations showed a comparable strength reduction after 10(6) cycles of about 50% of their initial strength. The composite core build-up showed a behavior less predictable than that of the amalgam, which might be attributed to handling parameters.

Structural Efficiency of Internally Ring-Stiffened Steel Tubular Joints

Abstract: Welded tubular structures are widely used for offshore platforms. In India, there are more than 120 offshore platforms in the Bombay High region, all of which are jacket‐type structures consisting of welded tubular members. The tubular joints of these structures are subjected to stress concentration, which because of the increased hot‐spot stresses reduces the fatigue life of the structure. Also, the ultimate strength of the tubular joints is very much reduced due to the local buckling effect of the tubular members. Based on the analytical and experimental studies conducted at the Structural Engineering Research Centre, at Madras, it is observed that these deficiencies of tubular joints can be reduced to a great extent by providing internal ring stiffeners in the chord members. Internal ring stiffeners are found to reduce stress concentration, increase fatigue life under cyclic loads, and also improve the ultimate strength of the joints. The results of investigations on internally ring‐stiffened steel tub...

Extended Charles–Hillig Theory for Stress Corrosion Cracking of Glass

Abstract: The work originally performed by Charles and Hillig (C&H) on the chemical stress corrosion cracking of glass is based on the chemical reaction rate theory and restricts the analysis to only the kinetic change at the exact location of the crack tip. As a result, crack sharpening/blunting is predicted when the applied stress lies above/below the static fatigue limit. The present paper extends the investigation within the same physical framework to the geometric change of the entire cavity surface, particularly in the vicinity of the cavity apex region. It has been found that a physical-property-dependent parameter (m) exists which exerts a strong influence on the crack-tip morphology. In the case of m=mth, where mth is a threshold m which assumes a value of ∼ 45 for an elliptical cavity having a minor to major axes ratio of 0.01, the current predictions reduce to the C&H results. In general, however, m≠mth, and the single-valued fatigue limit degenerates into a range of applied stresses under which either enhanced blunting (m > mth) or necking (m 45, suggesting that enhanced blunting takes place at the crack tip when external loads are applied at a moderate level for a typical crack having an initial major to minor axes ratio > 100 in a soda–lime glass specimen.

Detail design of welded ship structures based on hot-spot stresses

Abstract: Details of ship structures are characterised by a relatively complex geometry, showing a large variety in shape and plate thickness. These particulars make the fatigue strength assessment rather difficult, especially if the common nominal stress approach based on standard notch cases is applied. Local approaches can help overcome these difficulties. The hot-spot or structural stress approach is particularly well suited in view of the large number of different details in ships and the short time available for structural design. With regard to practical application, several problems have been investigated concerning the numerical evaluation of hot-spot stresses and their assessment with respect to fatigue strength, being described and discussed in the paper. Different examples of structural details illustrate the problems and solutions found. The investigations result in proposals for a general application of the hot-spot stress approach in ship structural design, being advantageous particularly in those cases, where no experience is existing or where structural details are optimised with respect to fatigue strength.

Fatigue properties and fatigue fracture mechanisms of SiC whiskers or SiC particulate-reinforced aluminium composites

Abstract: Fatigue properties and fracture mechanisms were examined for three commercially fabricated aluminium matrix composites containing SiC whiskers (SiCw) and SiC particles (SiCp) using a rotating bending test. The fatigue strengths were over 60% higher for SiCw/A2024 composites than that for the unreinforced rolled material, while for the SiCp/A357 composites, fatigue strengths were also higher than that for the unreinforced reference material. For the SiCp/A356 composites at a volume fraction of 20%, the fatigue strength was slightly higher than that of the unreinforced material. Fractography revealed that the Mode I fatigue crack was initiated by the Stage I mechanism for the SiCw/A2024 and SiCp/A357 composites, while for the SiCp/A356 composite, the fatigue crack initiated at the voids situated beneath the specimen surfaces. On the other hand, the fatigue crack propagated to the whisker/matrix interface following the formation of dimple patterns or the formation of striation patterns for SiCw/A2024 composites, while for the SiCp/A356 and SiCp/A357 composites the fatigue crack propagated in the matrix near the crack origin and striation patterns were found. Near final failure, dimple patterns, initiated at silicon carbide particles, were frequently observed. Mode I fatigue crack initiation and propagation models were proposed for discontinuous fibre-reinforced aluminium composites. It is suggested that the silicon carbide whiskers or particles would have a very significant effect on the fatigue crack initiation and crack propagation near the fatigue limit.

High-temperature low-cycle fatigue behavior of a NIMONIC PE-16 superalloy—Correlation with deformation and fracture

Abstract: Low-cycle fatigue (LCF) responses of NIMONIC PE-16 for various prior microstructures and strain amplitudes have been evaluated and the fatigue behavior has been explained in terms of the operative deformation mechanisms. Total strain-controlled LCF tests were performed at 923 K on samples possessing three different prior microstructures: alloy A in solution-annealed condition (free of γ′ and carbides), alloy B with double aging treatment (spherical γ′ of 18-nm diameter and M23C6), and alloy C with another double aging treatment (γ′ of size 35 nm, MC and M23C6). All three microstructures exhibited an intial cyclic hardening followed by a period of gradual softening at 923 K. Coffin-Manson plots describing the plastic strain amplitudevs number of reversals to failure showed that alloy A had maximum fatigue life while C showed the least. Alloy B exhibited a two-slope behavior in the Coffin-Manson plot over the strain amplitudes investigated. This has been ascribed to the change in the degree of homogeneity of deformation at high and low strain amplitudes. Transmission electron microscopic studies were carried out to characterize the various deformation mechanisms and precipitation reactions occurring during fatigue testign. Fresh precipitation of fine γ′ was confirmed by the development of “mottled contrast” in alloy C. Evidence for the shearing of the ordered γ′ precipitates was revealed by the presence of superdislocations in alloy C. Repeated shearing during cyclic loading led to the reduction in the size of the γ′ and consequent softening. Coarser γ′ precipitates were associated with Orowan loops. The observed fatigue behavior has been rationalized based on the micromechanisms stated above and on the degree of homogenization of slip assessed by slipband spacing measurements on tested samples.

The effect of a thin coating of polymethylmethacrylate on the torsional fatigue strength of the cement-metal interface.

Abstract: Recent studies have established that a mechanism of initiation of failure of fixation of cemented femoral components is debonding of the cement-metal interface. Other studies have shown that the torsional forces induced by stair climbing and rising from a chair are very high. Thus, the interface between the femoral prosthesis and the bone cement in total hip arthroplasty (THA) is required to transmit high torsional loads from the metal to the cement in a cyclic shear mode many times per year. These torsional loads likely contribute to the debonding. This study evaluated the efficacy of a thin layer of polymethylmethacrylate (PMMA) precoating in increasing the torsional fatigue strength of the cement-metal interface. Fatigue studies were performed on 15 specimens. Each specimen was tested with and without PMMA precoating. The PMMA precoat significantly and substantially increased the torsional fatigue strength of the cement-metal interface. Thus, PMMA precoating is likely to be a clinical advantage in maintaining the long-term integrity of the cement-prosthesis interface.

Effect of Nonlinear Elasticity on Measured Fatigue Data and Lifetime Estimations of Optical Fibers

Abstract: The compressive stress–strain relation in optical fiber, which is important for a fiber during bending, is shown experimentally to be an asymmetric continuation of the already known nonlinear relation in the tensile region. Measured strengths of optical fibers in fatigue experiments can be 50% higher because of nonlinear corrections. Consequently decreases in measured corrosion susceptibilities, up to 10%, result in estimated fiber lifetimes, which decrease by an order of magnitude. The elastic line of a fiber inserted in a two-point-bending testing apparatus, however, is hardly influenced by nonlinear elastic effects.

A model for the fatigue limit and short crack behaviour related to surface strain redistribution

Abstract: A model based on surface strain redistribution and the reduced closure stress of short cracks is shown to successfully predict the fatigue limit and short crack growth behaviour for aluminium alloy 2024-T351. Using this approach, the length of non-propagating cracks can be anticipated. The local stress intensity range may be resolved into two components (first the linear elastic fracture mechanics component and the second is due to surface strain concentration). Consequently, the local stress intensity range of aluminium alloy 2024-T351 is a maximum at a depth of approximately half a grain diameter and a minimum at a depth slightly in excess of three grain diameters. The reduced closure stress for short cracks coupled with the increased applied stress intensity caused by surface strain redistribution accounts for the variation of the effective stress intensity parameter as a function of crack depth. This parameter is a maximum for the smallest possible crack (3 μm) and decreases as crack length increases.