A. Plumtree

Cyclic stress-strain response and substructure

Abstract A series of fully reversed cyclic strain tests have been conducted on a range of ferrous and non-ferrous metals. For metals with finely dispersed particles and single phase low stacking fault metals the stabilized hysteresis loops coincided with the cyclic stress–strain curve magnified by two (Masing behaviour). In this case the Bauschinger strain increased linearly with cyclic plastic strain. For high stacking fault energy metals, where the cyclic deformation was matrix controlled, the cyclic stress–strain response was non-Masing. However, Masing behaviour was observed below a threshold strain level. Above this threshold, where non-Masing behaviour occurred, a dislocation cellular microstructure formed and the Bauschinger strain increased at a progressively slower rate with increasing cyclic strain.

A fatigue damage accumulation model based on continuum damage mechanics and ductility exhaustion

Using continuum damage mechanics, a fatigue damage accumulation model related to ductility exhaustion has been developed. This model was applied to study fatigue damage evolution in a pressure vessel steel and two-level cyclic tests have been carried out to verify the damage accumulation model. The experimental results showed that a damage variable, D, based on ductility exhaustion can be used to measure cumulative damage and provide a good estimate of the fatigue damage process, as well as to predict the fatigue life.

Modelling compressive cyclic stress-strain behaviour of structural foam

Abstract Polyurethane foam, similar to that used for protection in automotive interiors has been subjected to continuous compressive cycling. The loading and unloading stress–strain relationships for any given cycle can be fully described by applying Ogdens modified model for compressible rubber. The accumulated dissipated energy that indicates the vibration damping capacity of the material may be expressed in the form of a simple mathematical function. The concept of loading path dissipated energy (LDE) using the Weibull function has been applied to express the manner in which cyclic damage accumulates.

Stress corrosion cracking of X-60 line pipe steel in a carbonate-bicarbonate solution

Abstract An experimental system was developed to reproduce stress corrosion cracking (SCC) of API X-60 line pipe steels in highly alkaline (pH =10) carbonate-bicarbonate (1 N sodium carbonate [Na2C...

Transgranular Stress Corrosion Cracking of X-60 Pipeline Steel in Simulated Ground Water

Abstract An investigation was carried out on API X-60 steel specimens with multiple edge cracks to determine transgranular stress corrosion crack (TGSCC) growth rates in simulated ground water (NS-...

Fatigue damage evolution in off-axis unidirectional CFRP

Damage mechanisms have been evaluated for a unidirectional carbon fibre reinforced epoxy matrix composite under off-axis cyclic loading. The variation in fatigue life for a given cyclic strain was found to be dependent upon the microstructure and the ease with which coalescence of matrix microcracks took place. When matrix cracks coalesced quickly, the fatigue life was relatively short. On the other hand, when the microcracks were well distributed, coalescence was delayed and the fatigue life was much longer.

A fatigue damage parameter for off-axis unidirectional fibre-reinforced composites

Abstract A fatigue damage parameter based on that of Smith Watson Topper is developed and applied to predict the fatigue-life of off-axis unidirectional fibre reinforced composite materials. A microstress analysis is used to quantify the parameter that takes into account the maximum shear and normal stresses, and shear and normal alternating strains on the fracture plane parallel to the fibres. The major advantage is that this parameter takes into account the effect of fibre orientation and mean stress. By applying this parameter it is possible to develop a set of fatigue lives for various fibre orientation angles and stress ratios using experimental data obtained from different fibre/load angles and stress ratios. The fatigue parameter has been applied to off-axis unidirectional glass/epoxy composite fatigue data. The predicted and experimental results are in good agreement for different fibre/load angles and stress ratios.

Cyclic Crack Growth Rates of X-60 Pipeline Steel in a Neutral Dilute Solution

Abstract An experimental investigation was carried out to measure transgranular stress corrosion crack growth rates (CGR) in precracked X-60 steel specimens exposed to neutral (pH 7) dilute simulated ground water (designated NS4) while being cyclically loaded in cantilever bending. Testing was carried out over short-term (generally < 40 days) and long-term periods (∼ 1 y). Loading and environmental conditions in the long-term tests were similar to those for buried natural gas pipelines, with the R ratio (minimum/maximum load) varying from 0.82 to 0.98 and frequencies ranging from 1 to 400 cycles/day. Cyclic loading conditions for specimens in the short-term tests were more severe, with R ratios ranging from 0.5 to 0.9 and frequencies from 40 cycles/day to 5,000 cycles/day. Post-mortem metallographic examination by optical (OM) and scanning electron microscopy (SEM) was used to evaluate crack growth. Average CGR from 4.50 × 10−8 mm/cycle to 1.25 × 10−4 mm/cycle (equivalent to 1.4 × 10−9 mm/s to 7 × 10−7 mm...

Weight function for the surface point of semi-elliptical surface crack in a finite thickness plate

Abstract The weight function for the surface point of a semi-elliptical crack in a plate of finite thickness has been derived and tested against numerical data available in the literature. Derivation was accomplished by using the universal weight function form and two reference stress intensity factors. It was found that a four term expression was sufficient to approximate weight functions for a variety of crack shapes and depths. Using this approach it was possible to calculate stress intensity factors for several nonlinear stress fields to an accuracy of better than 3% when compared with the finite element data.

Stress-substructure relationships in cyclically and monotonically deformed wavy slip mode metals

Polycrystalline aluminum was cyclically deformed under strain control at room temperature in the as-extruded form and different annealed conditions after forging. The dislocation substructure developed on saturation was examined by transmission electron microscopy. An inverse relationship was found between the saturation stress and dislocation cell size. Comparison of these results with those for aluminum, copper, and iron obtained under both monotonie and cyclic conditions at various temperatures indicated that the same form of equation with a different constant was characteristic of each of the three wavy slip mode metals. The relationship appears to be dependent upon the stacking fault energy of the particular material and independent of the type of deformation(i.e., monotonie or cyclic).