William F. Deans

Novel dynamic fatigue-testing device: design and measurements

The influence of dynamics on a propagating fatigue crack has not been studied experimentally yet mainly due to quasi-static loading from traditional fatigue-testing machines. To overcome this serious drawback, a novel base-excited fatigue-testing device was designed and built to allow measurement of the dynamic responses of a single-edge-notch beam (SENB) under a growing fatigue crack. In this paper, the details of the novel test rig including initial development, modification and instrumentation are given. The experimental time histories obtained for harmonic and chaotic excitations have shown that the fatigue rig is capable of generating a wide range of loading patterns. Moreover, the experimental crack growth curves and features of the fracture surface have confirmed that the rig is capable of inducing proper fatigue cracks.

Chaos caused by fatigue crack growth

Abstract The nonlinear dynamic responses including chaotic oscillations caused by a fatigue crack growth are presented. Fatigue tests have been conducted on a novel fatigue-testing rig, where the loading is generated from inertial forces. The nonlinearity is in the form of discontinuous stiffness caused by the opening and closing of a growing crack. Nonlinear dynamic tools such as Poincare maps and bifurcation diagrams are used to unveil the global dynamics of the system. The results obtained indicate that fatigue crack growth strongly influences the dynamic response of the system leading to chaos.

An Experimental Rig to Investigate Fatigue Crack Growth Under Dynamic Loading

A novel experimental rig capable of generating a versatile dynamic loading has been designed and tested to overcome the shortcomings of conventional fatigue testing machines such as the difficulty in providing zero crossing aperiodic loading. The main principle of this new design is based on two, single degree of freedom based excited oscillators, where inertial forces act on a specially designed specimen. By changing the natural frequency of the oscillator, the extent of the preloads and pattern of the excitation signal on the shaker, the rig provides a new and robust means of fatigue testing, particularly for aperiodic loading.

Influence of microstructure changes on corrosion resistance of super duplex stainless steel

Effect of microstructure on pitting corrosion of super duplex was investigated in 3.5% NaCl solution at 90°C. The microstructures were controlled by applying two different cooling rates of water quench and air from the heat treatment temperatures of 1,000°C and 1,300°C. The amount of ferrite and austenite and other precipitates were measured using optical and image analyser. The results revealed that the ferrite percentage increased as the heating temperature increased to 1300°C. Metallographic results showed the presence of intermetallic phases. Back scattering analysis revealed presence of sigma (σ) and chi (χ) phase. The volume fraction of ferrite to austenite as well as the precipitation of harmful intermetallic phase during cooling process affected the corrosion resistance. Reformation of austenite during slow cooling from 1,300°C enhanced corrosion resistance while intermetallic precipitates promoted pitting damage and decreased pitting potential to more active values.

Hydrogen Embrittlement Enhanced by Plastic Deformation of Super Duplex Stainless Steel

The effect of variations in plastic deformation percentage on hydrogen embrittlement of super duplex stainless steel alloy was investigated. Samples were strained to 4%, 8%, 12%, and 16% of plastic strain prior to hydrogen charging. Sufficient hydrogen for embrittlement was achieved by cathodic charging in 0.1 M H2SO4 for 48 h at a current density of 30 mA/cm2. Hydrogen embrittlement susceptibility was highly dependent on the amount of plastic deformation. Experimental results showed that prestraining of super duplex stainless steel and hydrogen charging affected the elongation and the values of the strain required to failure. The total elongation for the samples with no prestraining deformation and tested in air was 29%. This elongation reduced to 25% when the same sample condition (no prestraining) charged with hydrogen. Further reduction in elongation and strain to failure was observed when the prestraining samples were charged with hydrogen prior to tensile testing. Load—displacement results showed that as the percentage of the plastic deformation increased, the elongation and strain to failure decreased. Comparison between the prestrained samples, charged and uncharged with hydrogen, showed a noticeable difference in strain at failure in the hydrogen charged specimens.

The Effect of Surface Roughness on the Erosion of Mild Steel

This paper investigates the relationship between the surface roughness parameter, Ra , and the erosion of the material surface. Jet impingement tests were carried out on the surface of mild steel plates at an impact angle of 30 degrees to the material surface. The erosion test rig utilised sand as the erodent at a concentration of 2%wt, with average particle size of 270 μm. The average surface roughness Ra , of the test specimens ranged from 0.07 μm to 1.14 μm. The erosion damage was assessed by a unique application of the surface profilometry technique. In addition, by using Fast Fourier Transforms to convert the surface profile scans into wave-number spectra, the changes in the material surface after erosion exposure were easily monitored. The investigation showed that the surface roughness parameter plays a significant role in the erosion process, particularly in the initial stages. The operating conditions of the impinging jet were associated with an equilibrium condition, both for the surface roughness and the eroded depth. As a result, materials having an initial surface roughness value greater than the equilibrium Rae value showed a decrease in surface roughness, whereas those having an initial value less than the equilibrium value showed an increase in surface roughness. This trend was confirmed by the changes in the amplitudes of the wave-number spectra for each plate, over the erosion period. The eroded depth was maximum for the plate having the smallest difference between the initial Ra value and the equilibrium value Rae . This subsequently decreased as the deviation from the equilibrium value was increased.Copyright