No description available.

The main purpose of this research project was to determine the fatigue-life behaviour of AISI 1008 sheet steel which has been mechanically formed to a radius of curvature of 120mm and then to correlate the fatigue-life behaviour to that of the parent or “as manufactured” material. During the forming process it was felt important to induce plastic strain through stretch-bending by clamping the sides of a plate sample’s (width) edges in the bending fixture before being bent by a single acting mechanical press. It was determined through actual testing that there was a decrease in fatigue-life when the mechanical formed data was compared to fatigue data of the parent material. Standard fatigue mathematical models were used to relate the actual fatigue data. Due to the material being cold formed to a radius of curvature of 120mm, residual stresses induced during the forming process played an essential role during the fatigue-life prediction calculations. The maximum relieved stress in the parent material was compressive in nature having a magnitude of 11percent of the “as manufactured” yield strength (265 MPa). For the mechanical formed material compressive residual stresses were measured on the outer surface while tensile stresses were measured on the inner surface. The difference between actual number of cycles to failure to that calculated using the standard mathematical models for the parent material, ranged between 48 percent and 18 percent and for the mechanical formed samples between 35 percent and 1percent, depending on the strain amplitude used. An important aspect of this study was to determine the criteria required for mathematical modelling of the parent material as testing occurred between the limit of proportionality and yield point. Although this aspect requires further investigation the mathematical results obtained during this study were considered to be acceptable.

In recent years there has been much engineering effort channeled toward obtaining higher strength, lower weight and cost product designs. In pursuit of this goal some carefully designed and tested parts have catastrophically failed after a long history of repeated successful service, even though the load at the time of failure was no higher than the part had repeatedly carried previously. These mechanical failures are commonly classified as fatigue failures.

Ph. D.

The couple stress theory as presented by Mindlin, Koiter, Stokes and others is reviewed with particular attention being given to stress concentration and size effects. That theory is compared on a qualitative and quantitative basis with Neuber's theory for stress concentration for very small or sharp notches and with observed notch sensitivity effects in aluminum and steel. It is established that, in order to explain observed elastic size effects in these materials, the new material constant ℓ which is characteristic of the couple stress theory must be on the order of magnitude of .02". In order to determine order of magnitude of the constant, a series of experiments was designed based on a thin plate analysis developed here. The plate theory uses the basic assumptions of classical plate theory as well as several, as explained in the text, necessary when couple stress is included. The formulation resulted in a set of equations for plate deflection including classical and couple stress terms in such a way that when the constant ℓ was allowed to be zero, the classical equations resulted. Experimentally, it was found that the couple stress effect in steel and aluminum is at least one order of magnitude too small to account for observed size effects as was suggested by Mindlin. In addition, the magnitude of ℓ was found to be such that its effect would undoubtedly be clouded by inhomogeneity and anisotropy of metals at the microstructural level. The results represent the only known experimental data at the time of publication dealing with couple stress in metals and would seem to be the next obvious step in the development of the theory which has proceeded on a purely mathematical plane. An alternate approach to account for the effects of microstructure in notch sensitivity is discussed. / Ph. D.

Mechanical cyclic variations in mean stress and strain amplitude is a well-known occurrence for metals even at room temperature. Many fatigue analysis procedures ignore these variations. Fatigue analysis which included both time and cycle dependent mechanical material behavior for metals at room temperature had not been previously studied except for the case of creep. An investigation studying transient mechanical effects on Ti-6Al-4V titanium and 7475-T651 Al alloys was done to determine how great an effect transients at room temperature would have on fatigue life under cyclic conditions. The mechanical material response was modeled using viscoplasticity constitutive laws and Neuber’s rule eliminating the need for finite element modeling of uniaxially loaded notched members. However, the Nenber’s modeling may be used with any material constitutive law. The procedures for fatigue damage used cycle counting to compute strain amplitude and mean stress. Since a large amount of fatigue data is reported as strain-life curves, the fatigue analysis was developed using this fatigue data although it did not include transients. If favorable results are obtained, development of modeling and testing to include transients in strain-life fatigue data could be avoided, and the existing fatigue data base utilized. Experimental work was undertaken and nonlinear optimization techniques used to compute model constants for the two alloys. However, small amounts of rate dependence was found for cyclic strain control testing. The viscoplasticity models became stiff when rate dependence was low causing numerical problems, and model constants for the viscoplastic constitutive law could not be determined since convergence was not achieved. Also, only small amounts of transient static stress relaxation was observed for extended hold periods. Finally, experimental verification was done for the local surface stresses in a notched member under load using advanced x-ray stress equipment. Measurements during brief pauses were made over a cycle. From the x-ray results, an anomalous surface behavior was observed. The surface yielded before the bulk material with the lower surface yielding seeming to be time dependent in nature. Since rolled plates of the alloys were used, texture was measured and studied in the form of pole figures, and extreme texture was found for both alloys. However. successful x-ray measurements were made for the alloys studied even though assuming linear <i>d</i>-spacing versus <i>sin²ψ</i>. Finally. x-ray measurements for a cycled notched member, exhibited relaxation of mean stress and not relaxation of residual stress. / Ph. D.

Abstract not available

This work presents a novel methodology for evaluation of in service loads applied to the output shafts of automatic transmissions. It also presents a novel methodology of data reduction for shaft load signals as an alternative to the cycle counting methods. Current durability testing of automatic transmission output shafts uses 50 000 stall torque cycles from zero to wide open throttle. In the majority of cases, these requirements lead to an over design that can result in an unnecessarily bulky transmission system. As a solution to this problem a novel methodology for evaluation of loads applied to the output shafts of automatic transmissions was developed. The methodology is based on real world loading conditions and therefore leads to a more realistic estimation of the fatigue life of shafts. The methodology can be used as a tool for shaft optimisation in different drive conditions. Using the developed methodology the effects of different road conditions on the fatigue life of a transmission output shaft were compared. Four routes having differing driving conditions were investigated and of those routes, the route with most stop-start events resulted in the greatest reduction in fatigue life. A novel methodology of data reduction for shaft load signals was also developed. The methodology is based on knowledge of the bandwidth and dynamic range of the expected in-service load signal. This novel methodology allows significant reduction of the volume of data to be acquired. It preserves the time sequence of peaks and valleys of the signal, which is vital in the case of fatigue analysis. This is in contrast to current methods based on cycle counting. Cycle counting methods achieve high data reduction but do not preserve the time sequence of the signal. The developed novel methodology has been validated on the newly developed data acquisition system capable of real time data acquisition and compression of shaft torque signal. The performed tests show that the proposed one-channel low cost system equipped with 1 GB compact flash card can store well over 10 000 hrs of load history.

The aircraft engine industry is constantly striving to increase the operating temperature and stresses in hot section engine components, a goal that can only be achieved by accurately modeling and predicting damage mechanisms of potential engine materials. The objective of this work is to develop physically-based models that are able to accurately predict the high temperature crack initiation behavior of Rene 88DT, a commonly used aircraft engine disk material, under low cycle fatigue (LCF) conditions. Two different microstructural conditions were produced by subjecting the material to two separate heat treatments; the heat treatments were selected so that grain size remains the same but the size distribution of the strengthening gamma prime precipitate is different between the two conditions. LCF experiments were performed on specimens from each condition at 650C and R = -1 under strain ranges of 0.66%, 0.75%, and 1.5%. A third microstructural condition with a similar grain size but different gamma prime size distribution was tested by another source at 650C and R = 0 under strain ranges of 0.66%, 0.79%, 0.94%, and 1.14%. The results indicate that there are two competing crack initiation mechanisms: initiation from a microstructural defect such as an inclusion and initiation from slip band cracking. A physically based model, in the form of a modified Fatemie-Socie parameter, is utilized to predict the crack initiation mechanism and approximate cycles to failure based on the microstructure of the material and applied strain. Long crack growth models are also developed to model crack growth from subsurface inclusions and surface semi-elliptical cracks. These models predict that long crack growth is a small portion of the total fatigue life in these conditions, which suggests that the majority of the fatigue life is spent initiating a dominant fatigue crack.

A36 steel is a commonly used material in civil engineering structures where fatigue damage can lead to catastrophic failure. In this research, nonlinear Rayleigh surface waves are used to characterize damage in A36 steel specimens caused by monotonic tension and low cycle fatigue. Fatigue damage produces the increased acoustic nonlinearity that leads to the generation of measurable higher harmonics in an initially monochromatic Rayleigh wave signal. One specimen is subjected to static tension and four specimens are used for low cycle fatigue tests in the tension-tension mode with a constant stress amplitude. The fatigue tests are interrupted at different numbers of cycles for the nonlinear ultrasonic measurements. Tone burst Rayleigh wave signals are generated and detected using a pair of oil coupled wedge transducers. The amplitudes of the first and second harmonic are measured at varying propagation distances to obtain the nonlinearity parameter for a given damage state. The experimental results show an increase of acoustic nonlinearity in the early stages of fatigue life. Furthermore, a close relationship between plastic deformation and the acoustic nonlinearity is found, which indicates that the acoustic nonlinearity is indeed a measure of microplasticity in this material.