This tech brief provides a succinct review of the structural parameters required to assess the risk associated with possible net section failure. The ultimate capacity of a load path is often evaluated employing the assumption of a perfect elastic/plastic alloy and the load redistribution based on the plastic section modulus. 

Three parameters are reviewed. First, the difference between the elastic and plastic section modulus of a net section. Secondly, the implications that actual alloy properties have on the true stress/strain behavior compared to an idealized perfect elastic/plastic material. Thirdly, how displacement versus force controlled loading influences the ultimate capacity of a limiting net section.  The review provides several reasons for considering the use of a plastic analysis when managing risk in features having low margins against full plastic moments as predicted with the plastic section modulus and a perfect elastic/plastic material model.

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A well-designed vibratory Accelerated Life Test (ALT) will ensure the acquired data can be used to characterize the dynamic parameters of a product as well as screen its endurance capacity.   Dynamic characterization enables decision makers, using correlated structural models, to assess the product’s fitness for service, significantly compressing the time to market compared to employing testing alone.  

In optimizing a design; avoiding both an overdesigned product or one which has too high of a risk exposure; good dynamic characterization is essential.   The focus of this white paper are the considerations of good test fixture design which will facilitate recovering response data to dynamically characterization as well as ensuring the screening requirement of the ALT has been met.

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Migrating existing designs to new applications can create unexpected risk when their performance sensitivity to system inputs have not been characterized. Typically, designs are assessed solely on margins at a single set point within the design space. Transferring a previously successful design to a new application without having characterized its performance to changes in input variables, however, can result in significant risk exposure and false economies.

This tech brief provides an example of using Design of Experiments (DOEs) strategies in the process of Analysis Leading Design (ALD) to reduce the risk of experiencing the costs of a false economy. An annotated EXCEL spreadsheet is available upon request containing the numerical methods employed to characterize the example design space.

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Weld fatigue characterization as formulated by AWS, AISC, TWI, and many other organizations takes an approach of classifying stress life curves based on joint categories without reference to specific alloys or mean stress effects. This Tech Brief discusses the warrants grounding this methodology but also reviews how design practices and post weld operations can substantially improve fatigue performance beyond these characterizations.

Both strain life and fracture mechanic methods are used to illustrate the mechanisms supporting the basis of the fatigue characterizations, simultaneously revealing the options at the disposal of a designer to improve performance.

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Successfully managing design projects requires a vast array of skills, one of them being able to assess the actual information content in technical data and employing it in the design decision-making process.  Honing this skill results in delivering conforming products within cost, on schedule, and which reliably meet performance targets.  This tech brief looks at the role of information in the decision-making process and how analytical simulations, employing Analysis Leading Design (ALD), can increase the information content used at project decision points.

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Integrated Systems Research provides custom structural fatigue seminars for clients. Two day on-site seminars are available as well as one hour webinars. Course content is provided in below.

Please contact us for seminar rates.

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Blunting of stress concentrations in fatigue behavior is a universally recognized phenomenon.  The actual causal mechanism, however, is not fully understood.  Although several detailed explanations have been posited as to causation, it is generally accepted that the stress gradient associated with the concentration feature is the primary variable controlling the blunting characterization. 

The most common methodology for characterizing fatigue blunting is employing a notch sensitivity factor used to estimate an effective stress concentration which is then used in the fatigue calculation. This tech brief provides a means of using the stress intensity field associated with a notch to predict fatigue behavior.  The method provides a coherent approach that accounts for both the geometric and material effects governing the phenomenon. The approach can be used to provide fatigue predictions and to cross check notch sensitivity estimations obtained from other methods.

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The concept of a maximum Kf is a very useful tool in evaluating the relative merits of joint designs that have non-controlled notch features. Welded joints are typically prime examples of such notch geometry. Two different Kf material models are presented in this paper and compared with ANSYS finite element results. The results underscore the care that should be exercised in using the maximum Kf concept in the design process.

Neuber\\'s rule is present using a modified Kf. The modified Kf uses Neuber\\'s Kf model for the stress concentration but employs the elastic Kf for the strain concentration. A comparison of strain range predictions is provided for a butt weld joint using the finite element approach and Neuber\\'s rule for both the modified and unmodified Kf.

The brief also illustrates the importance of identifying the characteristic joint dimensions. These dimensions, along with the materials local ductility govern the stress strain behavior at the notch. A subsequent paper, in December, will discuss the stress gradient behavior around the butt neld notch and will present a means of estimating the process zone size. The size of the process zone will be used to present a rational basis for cross checking infinite life predictions of a joint using LEFM concepts.

To obtain a hard copy of this brief, contact us.

This bulletin provides an illustrated example of employing the local strain approach and finite element results in estimating life to crack initiation.

The tech brief expands on information obtained in Chapter 4 of the Practical Design Technology for Fatigue seminar. An annotated Office 97 Excel spreadsheet is available, which contains the calculations presented in this paper. It also contains an additional problem set, which allows the reader to go through the process by him or herself in order to reinforce the concepts.

To obtain a hard copy of this brief, contact us.

This is the first in a series of three bulletins dealing with structural damping. In this tech brief, two different damping models are discussed. The subsequent papers will cover the effects of damping in forced and transient vibrations, and design strategies for utilizing its potential benefits. Proportional damping is discussed in this paper and in particular how __ damping is used in the two models. The relationship between Q factors and the generalized mass of the system is also addressed.

An annotated Excel spreadsheet is available that contains the damping models discussed in this bulletin. This spreadsheet can be obtained upon request.

To obtain a hard copy of this brief, contact us.

This tech brief provides an example of estimating strains in a local notch root when net section plasticity is present. The example employs both a modified Neuber and a Strain Energy Density model to predict the local strain in the notch. The SED model is based on work done by Ellyin and Kujawski.

The brief illustrates the practical importance of considering the level of net section stresses as well as the concentrated stress in estimating local strain behavior. The plasticity in ductile material below the 0.2% yield value can have a significant influence on the local notch strain and predicted life.

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Suddenly applied loads are characterized by rapid changes in magnitude, which can result in structural responses significantly greater than those that are gradually applied. Whether or not a change in load magnitude is rapid enough to be classified as suddenly applied is governed by the mass-elastic properties of the system and the rise time of the load. The resulting system response to a time varying load, therefore, is both a function of how rapidly the magnitude of the load changes and parameters that the designer typically has more control over such as geometry and material selection.

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This tech brief outlines a method of employing fracture mechanics as a means of cross checking high cycle fatigue (HCF) predictions. The technique enables an engineer to evaluate the robustness of a design against accumulated HCF fatigue damage in a notch using a damage tolerance approach. The intent is to help increase the designer\\\'s understanding of the mechanistic processes involved in crack initiation and provide a tool in assessing the likelihood of a crack being arrested from a stress concentration feature as it transitions from a micro to a macro crack .

The limitations of linear elastic fatigue mechanics (LEFM) in the near threshold crack growth regime are discussed and how it relates to fatigue initiation. The J-integral is employed in evaluating loading scenarios where local and net section plasticity is present. The CINT macro in ANSYS version 11 is employed to perform the integration.

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Bolted joint separation is greatly facilitated when the loading on the connection is eccentric to the fasteners. Such joint behavior increases the potential for bolt fatigue, primarily due to the bending moment carried by the fasteners as lift off occurs. Most conventional flange designs create some amount of eccentric loading on the fasteners. 

This tech brief addresses the behavior of eccentrically loaded bolted connections by evaluating the relative benefits that various joint features have on increasing the resistance to separation. The results of a response surface analysis of a conventional cylindrical flange design is presented. The results are coalesced into a methodology which an engineer can employ to efficiently explore options in the preliminary design phase. As with any critical bolted joint, the preliminary design should be finalized with a finite element evaluation.

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The RR4500 Auxiliary Turbine Generator (ATG) incorporates an isolation system addressing four main design requirement environments. These environments include high impact shock, structureborne vibration, sea state motion, and installation/integration into the machinery space. Multiple design iterations were performed, beginning with a simplified system representation and expanding to full system finite element models. Specific resilient isolation mounts were selected to satisfy the competing criteria from the different requirement sets. Design resolutions passed specific requirements down to the component level and were addressed during detail design. Structures, system components, and flexible ship connections were adapted to meet the requirements needed by the isolation system. Testing of the system indicates good correlation between system predictions and actual performance.

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When structural fatigue issues occur they are typically associated with regions of load path transitions. Transitions can take on many different forms, but the feature common to them is the local concentration of stress created by an interruption of the stress field. This concentration can facilitate the accumulation of fatigue damage under cyclical loading and eventually crack initiation. 

This tech brief outlines a design technique for optimizing local transition features by minimizing the disruption of the stress field. The approach uses crack fronts as probes into the stress field. The resulting stress intensity factors are used to assess the behavior of the transition stress field from which local transition features can be optimally located and sized. The optimization is based on the alignment of the transition feature with respect to the maximum first principle stress. With the latest ANSYS 14.5 release, probing a stress field in complex geometry has become extremely efficient due to the ease at which elliptical cracks can be inserted into the base geometry. 

This technique, however, can also be used with geometry in a primitive state to guide the detail design towards an optimal solution. This white paper provides several simple illustrative examples of how this design technique can be used. The approach can be extrapolated to more complex design challenges.

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The primary value of a Failure Mode and Effect Analysis (FMEA) is being able to identify the key decision points which drive the success of a design. The FMEA, by defining the negative space, focuses the creativity and analytical effort where it is most needed. This tech brief addresses the inherent weakness of the multiplicative approach in determining the Risk Priority Number (RPN) in a FMEA and the potential pitfalls to consider when mitigating risk through redundancy.

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This Tech Brief discusses the use of Response Surface Methodologies (RSM) in the process of optimizing design performance. Identifying design points where performance is maximized and variation in system response minimized is central to implementing successful designs. This paper discusses a method for evaluating the significance of parameter estimations in generating response surface models and how to use them in the decision making process.

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This Tech Brief discusses potential pitfalls to be aware of when employing shake table data and the half power method in estimating the damping associated with structural modes. Data provided to the engineer is oftentimes incomplete. Being cognizant of the limitations in the data, how to assess what has been provided, and extract damping values at resonances are topics covered in this paper.

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This Tech Brief discusses an approach for estimating cutting tool forces for use in finite element evaluations of work piece deflections. Cutting tool forces vary considerably based on the alloy, cutting tool geometry, material removal rate and cutting speed. 

An approach for using an energy conservation method in estimating cutting forces is outlined in this paper. Additionally, a means of cross checking the estimates is discussed and illustrated. The cross check employs a Single Shear Plane (SSP) model and is used in a Design of Experiments to identify corner points of the tool force design space. The corner points can then be used in the finite element evaluation of the work piece deflection to assess the limits of the fixture design

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This tech brief outlines an approach for estimating stress concentrations without employing high mesh density finite element models. The method estimates stress concentrations by evaluating the behavior of the stress intensity or K field in the vicinity of a geometric discontinuity. 

In components with complex geometry, ensuring that the elastic stress concentration of a geometric discontinuity has actually been captured typically requires several runs with mesh density refinements. The Minimal Mesh Density (MMD) method outlined in this brief is relatively insensitive to mesh density and local geometry features and therefore can be used as a means of quickly cross checking the reasonableness of a single pass stress concentration evaluation. 

This Tech Brief discusses the basis and process of computing the relative kinetic and potential energy in free torsional vibration patterns. The utility of these computations is that they facilitate identifying where, in a system, stiffness and inertia changes will provide the greatest leverage for influencing the system’s torsional natural frequencies and inertia participation.

Two types of hydrogen embrittlement can occur in high strength alloys. The first type is referred to as hydrogen environment assisted cracking (HEAC) and the second is internal hydrogen assisted cracking (IHAC). High strength bolts under significant preload can exhibit both types of failures.

This tech brief identifies the difference between the two mechanisms and how they can be linked to the process zone of the stress concentration in the component. The relationship between the crack front K field and the stress concentration process zone, where HEAC and IHAC initiate, is discussed.

Since high strength bolts can be susceptible to these types of failures, a brief discussion on an overall design strategy of bolted joints is also undertaken.

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