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.

An investigation of the effect of R-ratio on the mode II fatigue delamination of AS4/3501-6 carbon/epoxy composites has been undertaken. Experiments have been performed on end notched cantilever beam specimens over a wide range of R-ratios (-l ≤R ≤0.50). The measured delamination growth rate data have been correlated with the mode II values of strain energy release rate range ∆G[formula omitted]), maximum strain energy release rate (G[formula omitted]) and stress intensity factor range (∆K[formula omitted]). The growth rate is dependent on the R-ratio over the range tested. For a constant level of ∆G[formula omitted], the crack growth rate decreases with increasing R-ratio. A similar trend is observed when the data is plotted as a function of G[formula omitted]. The effect of plotting the growth rate as a function of ∆K[formula omitted] is to produce an R-ratio dependence opposite to that obtained by either the ∆G[formula omitted] or G[formula omitted] approach. For a constant level of ∆K[formula omitted], the crack growth rate increases with increasing R-ratio. Master equations which completely characterize the fatigue behaviour as a function of ∆G[formula omitted] and ∆K[formula omitted] have been derived, based on the observation that the growth rate law exponent, n and constant, A are unique functions of R-ratio. Values for n are surprisingly large and increase with increasing R-ratio whereas values for A decrease with increasing R-ratio. The effect of time-at-load has been considered in an attempt to explain the existence of the R-ratio dependence of the growth rate. The correct trend can be established for the exponent, n but not for the constant, A. Friction between the crack faces, particularly at higher R-ratios, is proposed as a possible explanation for the observed anomaly. Further evidence of a frictional mechanism operating at higher R-ratios has been discovered through a postmortem fracture surface examination. Additional fractographic observations are presented over the entire range of R-ratios tested. In regions subjected to negative R-ratio cycling, there is no evidence of the characteristic mode II hackle features. Instead, loose rounded particles of matrix material are found. An extensive amount of hackling is observed in regions subjected to low positive R-ratio cycles. The extent of hackle damage visibly decreases in areas where higher levels of R-ratio are imposed. A correlation between the general fracture surface morphology and the fatigue data provides support for the hypothesis that energy for delamination is always available in sufficient quantity, and that growth is dependent on the stresses ahead of the crack tip being sufficiently high. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Submitted in fulfilment of the requirements for the degree of Master of Engineering: Mechanical Engineering, Department of Mechanical Engineering, Faculty of Engineering and the Built Environment, Durban University of Technology, Durban, South Africa. 2017. / In this study, the fatigue life of fiber reinforced composite (FRC) materials system was investigated. A nano-filler was used to increase the service life of the composite structures under cyclical loading since such structures require improved structural integrity and longer service life. Behaviour of glass fiber reinforced composite (GFRC) enhanced with various weight percentages (1 to 5 wt. %) of Cloisite 30B montmorillonite (MMT) clay was studied under static and fatigue loading. Epoxy clay nanocomposite (ECN) and hybrid nanoclay/GFRC laminates were characterised using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The mechanical properties of neat GFRC and hybrid nanoclay/GFRC laminates were evaluated. Fatigue study of the composite laminates was conducted and presented using the following parameter; matrix crack initiation and propagation, interfacial debonding, delamination and S–N relationship. Residual strength of the materials was evaluated using DMA to determine the reliability of the hybrid nanoclay/GFRC laminates. The results showed that ECN and hybrid nanoclay/GFRC laminates exhibited substantial improvement in most tests when compared to composite without nanoclay. The toughening mechanism of the nanoclay in the GFRC up to 3 wt. % gave 17%, 24% and 56% improvement in tensile, flexural and impact properties respectively. In the fatigue performance, less crack propagations was found in the hybrid nanoclay/GFRC laminates. Fatigue life of hybrid nanoclay/GFRC laminate was increased by 625% at the nanoclay addition up to 3 wt. % when compared to neat GFRC laminate. The residual strength of the composite materials revealed that hybrid nanoclay/GFRC showed less storage modulus reduction after fatigue. Likewise, a positive shift toward the right was found in the tan delta glass transition temperature (Tg) of 3 wt. % nanoclay/GFRC laminate after fatigue. It was concluded that the application of nanoclay in the GFRC improved the performance of the material. The hybrid nanoclay/GFRC material can therefore be recommended mechanically and thermally for longer usage in structural application. / M

Ph. D.

No description available.

Several experimental methods are examined for stress concentration factor determination. Tests are performed on a chosen 2-D specimen using strain gages, brittle coating, brittle specimen failure, photoelasticity, and Moire interferometry. Results are compared to each other and to finite element analysis performed on the same geometry. Strain gaging and photoelasticity were chosen as the best methods for stress concentration factor determination. / Master of Science

Past efforts at implementing Load Spectrum Generation and Neuber Control have centered around minicomputers and analog circuits. The use of a personal computer to implement the tasks is presented. On implementation of the load Spectrum Generation software, the response of the Materials Testing System was investigated for distortion and attenuation. In particular, the effect of the resolution of the waveform on the test system response was noted. There was negligible attenuation for full scale frequencies of up to 20 Hz. Greater waveform resolution was required at lower frequencies than at higher frequencies. On implementation of the Neuber Control program, the accuracy obtained at the Neuber hyperbolas was noted. Better accuracy was obtained at ramp frequencies below 0.1 Hz. Based on the results obtained after implementing the Load Spectrum Generator program and the Neuber Control program, the performance of the personal computer in controlling fatigue tests is evaluated. Cost effectiveness and versatility favor the use of a personal computer for the control of fatigue tests. / Master of Science

The response of a flake mat to the mechanical stress applied during consolidation is a function of mat structure, raw material properties and the environmental conditions created during pressing. This research project was aimed at improving the understanding of raw material behavior during the hot-pressing of wood based composites by examining the response of a wood flake mat to the compression encountered during press closure in the flakeboard manufacturing process. The structure of a flakeboard mat can be considered to be that of a cellular material, the properties of which are governed by the cellular geometry, or arrangement of cells, and the properties of the solid cell wall material. A method for quantifying the cellular structure of a wood flake mat was developed and implemented. The structure of thin mat sections and small flake mats was quantified using image analysis techniques. The applicability of theories developed for modelling the compressive behavior of cellular materials to the consolidation of wood flake mats was investigated using thin mat sections. Narrow mat sections and small laboratory mats has similar void sizes but significantly different void shapes. Void size was not significantly affected by flake orientation, but void shape was. / Master of Science

Temperature is a parameter which is important because of its influence on other material properties. Many temperature measurement techniques are available but few of them permit a direct visualization of surface temperature Variation. The Liquid crystal method is one of the rare methods that permits a complete color mapping of surface temperature and that is also fast enough to respond to surface temperature Variation on plates heated by cyclic bending. A mathematical model for irreversible mechanical heating of plates is developed to support the experimental investigation. The results, which include comparison of the theory and the experiment, show that liquid crystals allow good qualitative measurements and can lead, with certain precautions, to quantitative results. / Master of Science

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.