In-service Loading Research Articles

Modeling of Residual Stress in Grinding of Nodular Cast Iron

In grinding operations, the high specific energy generates high temperatures in the grinding zone, and therefore causes various types of thermal damage on the workpiece surface such as burn or high tensile residual stresses. High tensile residual stresses attract significant attention because they may initiate cracks on the surface, either immediately after grinding or under in-service loading. Cracking will significantly reduce component life. Thus, avoidance of surface damage in general, and residual stresses in particular, dominates any discussion of quality/productivity trade-offs in grinding. By increasing the material removal rate (MRR) productivity is enhanced but the temperature and temperature gradient in the grinding zone are increased as is the likelihood and severity of surface damage. Currently there is no analytic or numerical tool for predicting residual stresses in ground parts. Thus developing a robust grinding process while minimizing residual stress is a lengthy trial and error process. This report proposes an analytic model, based on the temperature profile in the workpiece, for predicting the severity of the residual stress under various grinding cycles. Further, the model also comprehends the cumulative effects of multiple grinding passes (which are routinely employed in any production grinding environment) and predicts the final residual stress after the complete process cycle has been completed. In addition to achieving excellent correlation with measured residual stresses, the validity of the model assumptions was evaluated and independently verified.

Ferritic Steel Welds – A Neutron Diffraction Standard

The Versailles Project on Advanced Materials and Standards 'VAMAS' initiated in 1996 a Technical Working Area 'TWA20' entitled 'Measurement of residual stress'. The project was designed to facilitate the basic research necessary for, and to draft, the first international standard for the measurement of residual stresses using neutrons. An ISO/VAMAS TTA was delivered in 2001 as a precursor to a full standard. A substantial part of the project was a series of four 'Round Robin' inter-laboratory comparative measurements on single and multiphase materials containing residual stresses which were introduced by a variety of fabrication techniques and in-service loads. One of the 'Round Robin' studies involved the measurement of the residual stress field in a ferritic steel weld. This paper describes the investigation and outlines the results of the measurements and its implications for the neutron diffraction standard.

Effect of Reinforcement Corrosion on Reliability of Bridge Girders

Aging reinforced concrete structures have been not only subjected to in-service loading but also exposed to an aggressive environment which will cause corrosion of reinforcing steels embedded in concrete. An approach that is akin to the nested reliability method is proposed for the time-dependent reliability analysis of bridge girders under corrosion attack. The approach can be used to investigate directly the effects of the uncertainty in surface chloride concentration and critical chloride concentration for corrosion initiation, and the possible nonlinear corrosion growth on the reliability. The consideration of the nonlinear corrosion growth is due to the fact that the corrosion current density may not be a constant. Analyses of the reliability of bridge girders to the parameters governing the corrosion are performed using the proposed approach and typical statistics of load effects and material strength parameters. Results suggest that consideration of the nonlinear corrosion growth is very important in determining the remaining service life of existing concrete structures, and that the uncertainty in the variables controlling the corrosion initiation has only small effect on the estimated time-dependent reliability.

Investigation of mechanical behavior of transverse stitched T-joints with PR520 resin in flexure and tension

An investigation of the mechanical behavior of transverse stitched T-joints using a fiber insertion process and PR520 toughened epoxy resin was undertaken. Experimental and numerical analysis was performed under flexure loading and preliminary experiments were conducted under tensile loading. These conditions were selected as representative of the in-service loads found in the application of this type of joint in industry. Experiments were conducted to determine the modes of failure and ultimate failure strength for each load condition. Flexure specimens were instrumented with strain gages to measure far field strains. Initial and ultimate damage moment is investigated and failure mechanisms are discussed. The results indicate that the flexural specimens fail in part from unsymmetrical loading of the fiber insertions and in part from high stress concentration in the “resin-rich” fillet region. Tensile specimens have symmetric loading of both sets of fiber insertions and initially fail due to matrix cracking at the web-to-flange interface. Both flexure and tension specimens are shown to exhibit additional load-carrying capability beyond initial failure indicating a significant damage tolerance. A linear elastic finite element model was developed for flexural loading and results are compared to experimental data.

Advances in probabilistic design: Manufacturing knowledge and applications

Product lifetime prediction, cost and weight optimization have enormous implications for the business of engineering manufacture. Deterministic design fails to provide the necessary understanding of the nature of manufacture, material properties, in-service loading and their variation. Probabilistic approaches offer much potential in this connection, but have yet to be taken up widely by manufacturing industry. This paper reports on advances in probabilistic design made possible through new progress in manufacturing engineering. Methods for predicting process capability indices for given design geometry, material and processing route, and for estimating material property and loading stress variation, are developed to augment probabilistic design formulations. The techniques are used in conjunction with failure mode and effects analysis (FMEA) to facilitate the setting of reliability targets for design schemes. A case study is included to illustrate the application of the methods and the benefits that can accrue from their usage.

A new measurement technique for the estimation of core shear strain in closed sandwich structures

Sandwich structures have been widely used for many years in applications such as aircraft panels, marine-craft hulls, racing car bodies and spacecraft solar arrays. Most sandwich panel designs include a lightweight core such as paper honeycomb or closed cell foam encased between two face plates, and in the case of aircraft panels constructed from carbon fibre reinforced plastics, the core is bevelled and edge pan plies are included to totally enclose the core. This type of design restricts access to the core making it almost impossible for the engineer to measure the shear strain developed in the core during in-service static or dynamic loading. This paper introduces a new measurement technique whereby the shear strain in the core can be estimated from face plate measurements using a linear finite difference approximation. The estimation method is presented and supported by calculations on a statically loaded sandwich beam. Static and dynamic experiments were conducted in order to validate the technique using a honeycomb sandwich beam instrumented with strain gauges on the core and face plates. The results showed excellent agreement between measured and estimated core shear strain for sandwich configurations with thin face plates, such as those encountered in aircraft and marine-craft constructions.

A Study on Failure Analysis of Low Pressure Trubine Blade Using AFM and FEM

Mechanical component has striation with constant width and SEM can estimate fracture type and loading condition. SEM has benefit to fatigue fracture analysis but striation can be observed according to the kind of material and range of crack growth rate and can't. In this case, it needs AFM that can measure 3-dimensional surface profile with resolution of atomic size. In this study. to find fracture reason of torsion-mounted blade in nuclear plant, we estimate the relation between stress intensity factor range and root mean square roughness in 12% Cr steel by AFM and predict in-service loading condition of turbine blade. failure analysis is performed by finite element method and Goodman diagram on torsion-mounted blade.

Advanced textile composite ring for Ilizarov external fixator system.

The use of a radiolucent composite material permits easier and more accurate radiographic evaluation of the bone healing process, and results in a much lighter system. The finite element method (FEM) is employed to determine the worst possible in-service loading condition and the ring dimensions are modified accordingly. Half-ring prototypes are produced using two types of composite materials: knitted aramid fibre fabric reinforced epoxy and random short carbon (RSC) fibre reinforced epoxy. The in-plane compressive strength and axial stiffness of the complete frame are tested according to ASTM specifications. The performance is evaluated, and compared with an existing system in simulated in-service conditions.

Assessment of Reliability of Aging Reinforced Concrete Structures

Aggressive environment and in-service loading can cause the resistance degradation of existing reinforced concrete structures. They must be considered for the safety evaluation and service-life prediction of deteriorating structures. This paper presents an integrated approach for time-dependent system reliability analysis considering the stochastic nature of load processes, uncertainties in strength, degradation initiation time, and strength degradation mode. The approach takes into account the partial correlation among the failures of structural elements, which arises from the fact that elements of a structural system are subjected to the same load processes and/or depend on some common basic random variables. The approach is efficient, since it does not require simulation. Analysis results obtained by using the proposed approach indicate that the reliability of series systems is relatively insensitive to the correlation between the failure of structural elements. However, the reliability of parallel system is highly sensitive to the correlation. The use of the time-dependent reliability obtained by neglecting possible correlation as a safety measure, therefore, should be avoided.

Case studies in probabilistic design

Deterministic design fails to provide the necessary understanding of the variability associated with the properties of materials, manufacturing tolerances and inservice loading. Probabilistic design offers much potential in this area, providing an effective way of determining product life-time estimates, reducing failure costs and aiding the process of weight optimization in product design, but has yet to be taken up widely by manufacturing industry. In this paper, case studies are used to explore some important probabilistic design concepts and introduce methods that allow reliability performance measures to be assessed for product designs early in the product development process. The methods described are used in conjunction with Failure Mode and Effects Analysis to facilitate the setting of reliability targets for design proposals.

Behavior and Modeling of Wood-Pegged Timber Frames

Traditional timber connections with wood pegs are encountered in the renovation and rehabilitation of historic wood structures and in the construction of new structures where rustic appearance and traditional methods are desired. This study examined four types of traditional connections: mortise and tenon, mortise and tenon with a shoulder, mortise and tenon with a shoulder and a knee brace, and fork and tongue. A total of 60 specimens, primarily white oak and Douglas fir, were tested. About a quarter of the specimens were assembled and allowed to season under simulated in-service load. All specimens were tested under simulated gravity load up to failure or to about five times typical design load. Some knee-braced specimens were retested to simulate lateral loads. Test results showed that joint behavior is primarily linear. A tightly fitting joint carries load with less peg damage than a loose joint allows. A mortise and tenon with a shoulder performs better than a mortise and tenon or a fork and tongue. ...

Fatigue crack growth and threshold measurements at very high frequencies

Fatigue testing at ultrasonic frequencies makes it possible to investigate the fatigue properties of materials in an effective and time saving manner. The mechanical and electrical components of ultrasound equipment are described in detail. Different testing and evaluation procedures are reviewed. Recommendations on how to perform high frequency fracture mechanics studies and to evaluate experimental results are described. Recent developments for multiaxial loading, for performing in-service loading fatigue experiments, and for realising high frequency torsional vibrations are described. A survey of fatigue investigations of metallic materials, composites, metallic glass, and ceramics, determined with the ultrasound method, is presented. Intrinsic frequency effects are reviewed, emphasising the influences on dislocation structures, crack initiation, and fatigue crack growth for pure metals and for technical materials. The advantages and disadvantages of the ultrasound method are compared with those of conventional fatigue testing equipment.

Fatigue crack growth and threshold measurements at very high frequencies

Fatigue testing at ultrasonic frequencies makes it possible to investigate the fatigue properties of materials in an effective and time saving manner. The mechanical and electrical components of ultrasound equipment are described in detail. Different testing and evaluation procedures are reviewed. Recommendations on how to perform high frequency fracture mechanics studies and to evaluate experimental results are described. Recent developments for multiaxial loading, for performing in-service loading fatigue experiments, and for realising high frequency torsional vibrations are described. A survey of fatigue investigations of metallic materials, composites, metallic glass, and ceramics, determined with the ultrasound method, is presented. Intrinsic frequency effects are reviewed, emphasising the influences on dislocation structures, crack initiation, and fatigue crack growth for pure metals and for technical materials. The advantages and disadvantages of the ultrasound method are compared with those of conventional fatigue testing equipment.

Structural Design Codes: Strain-Life Method and Fatigue Damage Estimation for ITER

A preferred route is suggested for implementing the design rules and requirements of the design codes for the International Thermonuclear Experimental Reactor (ITER), such as ASME and RCC-MR, and for preliminarily assessing which of the in-service loading conditions inflicts the greatest damage on the structure. Some of the relevant design code rules and constraints are presented, and lifetime and fatigue damage, with some data on fatigue life for Type 316 stainless steel, are predicted. A design curve for strain range versus the number of cycles to failure is presented, including the effect of neutron damage on the material. An example calculation is performed on a first-wall section, and preliminary estimation of the fatigue usage factor is presented. One must observe caution when assessing the results because of the assumptions made in performing the calculations. The results, however, indicate that parts of the component are in the low-cycle fatigue region of operation, which thus supports the use of strain-life methods. The load-controlled stress limit approach of the existing codes leads to difficulties with in-service loading and component categorization, whereas the strain-deformation limit approach may lead to difficulties in calculations. The conclusion is that the load-controlled approach shifts the emphasis to themore » regulator and the licensing body, whereas the strain-deformation approach shifts the emphasis to the designer and the structural analyst. 11 refs., 7 figs., 2 tabs.« less

Design of multilayer polymeric coatings for indentation resistance

Indentation is often a mode of in-service loading for a thin coating deposited on a substrate. Under such a loading, the strong adherence of the coating to the substrate is a basic necessity for successful performance of the coating. In this study, we investigate the indentation behavior of thin single and multilayered polymeric coatings using the finite element method. The deformation patterns and the stress fields that are generated during indentation are obtained by employing constitutive models which accurately represent the elastic-viscoplastic and hyperelastic behavior of the glassy and rubbery states of the polymeric layers under investigation. Three types of loading conditions are considered: indentation to (1) a fixed depth; (2) a fixed work; and (3) a fixed force. For these loading conditions, we then investigate the mechanical performance of various composite coatings subjected to an overall thickness design constraint. The composite structure is altered via variation in individual layer material composition, layer thickness and layer arrangement. It is shown how the placement of different material layers in a multilayer coating can alter the flow pattern and hence the distribution of stress state and resulting failure. It is also shown that a soft rubber elastic layer acts to greatly minimize the interfacial shear stress at the substrate, thereby reducing the risk of delamination of the coating, but the presence of the rubber can also produce detrimental tensile stresses on the surface. We then demonstrate that the tensile stress state can be eliminated through manipulation of the rubber layer thickness, without increasing the interface shear stress. Through these examples, a framework for evaluation and design of multilayer coatings for indentation resistance is provided.

The morphology, chain structure and fracture behaviour of high-density polyethylene

High-density polyethylene (HDPE) is being used more and more in critical long-term applications. For this reason it is important to have a strong understanding of those parameters which control the fracture behaviour of HDPE. In Part I of this work, fracture results were presented for eleven HDPE samples tested using a tensile testing machine. Such short-term tests do not accurately reflect the in-service loads on HDPE components, which tend to be low and static. It is, therefore, important to perform fracture tests under long-term static loads. The results of such testing are presented in this paper. The resistance to static fatigue was found to be most strongly dependent on molecular weight. Short branch concentration and short branch length were also found to exert an influence on the resistance to static fatigue. This result is similar to the findings presented in Part I of this work. However, there is some evidence that molecular weight influences fracture behaviour to a greater extent in the long-term tests. Notwithstanding, the similarity between the short-term and long-term results is important. It means that an early indication of the long-term performance of HDPE resins can be obtained from rapid comparative tests conducted using a tensile testing machine.

Fatigue of Drill String Connections

Abstract This paper reports full-scale tests on threaded connections used in drill strings. A concise background is given concerning the in-service environment and loading conditions on the connections. This details some of the reasons particular steels are used in preference to others. Crack growth data is given for ten full-scale axial and rotating bend tests. This is compared with predictions from a dedicated weight function fracture mechanics solution designed for threaded connections. Crack aspect ratio is considered with a view to development of an appropriate empirical model.

Fatigue and fatigue crack propagation in AlSi7Mg cast alloys under in-service loading conditions

Abstract In this study, the influence of microstructure and microporosity on fatigue crack initiation and propagation in AlSi7Mg cast alloy has been investigated at very high numbers of cycles (10 8 –10 9 ). Two alloys with similar chemical composition were used; one had finer microstructure, somewhat high microporosity and better static strength properties. The experiments were performed both at constant cyclic loads and with load sequences that simulated random in-service loading conditions of automobiles. The results show that casting pores predominantly influence the number of cycles to crack initiation. For in-service loading conditions, the detrimental effect of microporosity on crack formation cannot be adequately predicted on the basis of linear damage calculation, because of premature crack initiation during high loads of the random sequence. Comparison of the number of cycles to fracture as determined with smooth cylindrical specimens and with pre-cracked flat specimens shows the relatively high portion of the crack propagation time. The good fatigue crack growth properties of both AlSi7Mg alloys in the threshold regime (which are even better for the coarser alloy) are explained by roughness-induced crack closure.

In-service loading of AlSill aluminium cast alloy in the very high cycle regime

The fatigue properties of two AlSi11 cast alloysas used for car wheels have been tested in the regime of 10 8−10 9 cycles. An AlSi11 aluminium alloy with 0.12–0.20% Mn and one with 0.05% Mn and increased H 2 content during the pouring procedure are compared. It is shown that mainly cast voids determine the fatigue properties. These material inhomogeneities reduce the fatigue strength and lead to early fracturing. The number of cycles to failure decreases with increasing void cross-section. With a statistical analysis of the fatigue data, it is shown that the mean number of cycles to failure is higher for the alloy with fewer large cast voids and a higher Mn content. This alloy is less susceptible to cast voids, and the threshold stress intensity value is approximately 10% higher.

Bolted Joints Under Tensile Service Loads: The Effect of Torsion in the Bolt on In-Service Clamping Loads

In order to determine how bolted joints behave under conditions of tensile external loading, tests were conducted on bolts which had been tightened into joints by means of (a) elastic range, (b) yield, and (c) over-yielding tightening. The loading tests showed that the maximum clamp load which the bolt would exert on the joint without permanent extension was the uniaxial tensile yield load of the bolt. This was observed even when torsional stress was present in the as-tightened bolt, and for all three tightening methods.