Prestressed concrete beams with stirrups subjected to torsion, bending and shear

Reported in this thesis are test results of 37 eccentrically prestressed beams with stirrups. Single variable parameters were investigated including the prestressing force, the prestressing steel area, the concrete strength, the aspect ratio h/b and the stirrups size and spacing. Interaction of bending, torsion and shear was also investigated by testing a series of beams subjected to varying bending/torsional moment ratios. For the torsional strength an empirical expression of linear format is proposed and can be rearranged in a non-dimensional interaction form: T/To+V/Vo+M/Mo+Ps/Po+Fs/Fo=Pc2/Fsp. This formula which is based on an average experimental steel stress lower than the yield point is compared with 243 prestressed beams containing ' stirrups, including the author's test beams, and good agreement is obtained. For the theoretical analysis of the problem of torsion combined with bending and shear in concrete beams with stirrups, the method of torque-friction is proposed and developed using an average steel stress. A general linear interaction equation for combined torsion with bending and/or shear is proposed in the following format: (fi) T/Tu=1 where (fi) is a combined loading factor to modify the pure ultimate strength for differing cases of torsion with bending and/or shear. From the analysis of 282 reinforced and prestressed concrete beams containing stirrups, including the present investigation, good agreement is obtained between the method and the test results. It is concluded that the proposed method provides a rational and simple basis for predicting the ultimate torisional strength and may also be developed for design purposes.

Fatigue crack initiation and propagation from defects in aluminium welds

Fatigue crack initiation and propagation in aluminium butt welds has been investigated. It is shown that the initiation of cracks from both buried defects and. from the weld reinforcement may be quantified by predictive laws based on either linear elastic fracture mechanics, or on Neuber's rule of stress and strain ooncentrations. The former is preferable on the grounds of theoretical models of crack tip plasticity, although either may be used as the basis of an effeotive design criteria against crack initiation. Fatigue lives fol1owing initiation were found to follow predictions based on the integration of a Paris type power law. The effect of residual stresses from the welding operation on both initiation and propagation was accounted for by a Forman type equation. This incorporated the notional stress ratio produced by the residual stresses after various heat treatments. A fracture mechanics analysis was found to be useful in describing the fatigue behaviour of the weldments at increased temperatures up to 300°C. It is pointed out, however, that the complex interaction of residual stresses, frequency, and changes in fracture mode necessitate great caution in the application of any general design criteria against crack initiation and growth at elevated. temperatures.

The mechanics of creep crack growth in a Magnox alloy

Magnox AL80 has been used for a study of creep crack propagation. A number of variables have been considered such as specimen geometry,notch root radius, material thickness, creep prestrain and stress level.The work has covered the material behaving under two values of the creep exponent, n=3.5 and n=7, according to the stress level. As well as observing initiation times and crack growth rates, scribed grids have been used to examine the near crack tip strain levels and distributions. It was shown that estimations of COD from notch flank opening can give misleading indications of material behaviour and that a more informative method was to monitor displacements in the material surrounding the crack tip. Strong evidence was found for crack advance being displacement controlled, however it was shown that the COD approach should be considered geometry dependant. The summation of ∈xx and ∈yy provided the most successful description of crack advance as it produced a single value that described propagation in all the cases concidered. The strain distributions indicates that σyy was related to distance from a point ahead of the crack tip by the exponent - (l/n+l) and that σxx is proportional to σyy. The constraint stresses arising in the DEN and CN specimens were evaluated. Initiation time was found to be principally affected by the stress level but was modified by the constraints arising from specimen geometry. Crack growth was found not to obey either the empirical K or σpett relationships but was reviewed in context of the observed strain behaviour.

Creep crack growth in cast steam turbine casing steels

The creep rupture properties of cast ½Cr½Mo¼V and 1Cr1Mo¼V alloy steel used in the manufacture of power station steam generating plant. have been investigated. The effects of constraint and geometry on the creep rupture properties are also considered. The validity of various criteria controlling macroscopic creep crack growth in cast CrMoV alloys has been examined. It is found that neither the stress intensity factor nor reference stress correlate satisfactorily the creep crack growth rates at the test temperature of 550°C. Certain minimum displacements must be achieved for crack initiation and propagation. It is found that this displacement as measured by crack opening displacement or crack aspect ratio, is the same in both compact tension and centre-cracked panel geometries, is invariant with crack length and decreases with increasing constraint. The effect of constraint on creep crack growth rate in the two geometries is less conclusive. A new model describing creep crack growth in cast CrMoV alloy steels has been developed. The model is based on the results from a numerical finite element creep analysis of the relaxation and redistribution of stress ahead of an incubating creep crack . It is found that macroscopic creep crack growth in a material undergoing either plane stress or plane strain deformation can be described by a fracture stress which is based on the Von Mises equivalent stress. It has been shown that this model is capable of rationalising all of the experimental crack velocity data from the cast CrMoV alloys. The resultant degree of data correlation is far superior to that obtained when using the stress intensity factor or reference stress. A cumulative damage creep fracture model based upon the results from the numerical analysis has been developed. It is found that the model is capable of predicting the behaviour of propagating creep cracks in cast CrMoV alloys from smooth bar creep rupture data.

Ductile fracture mechanisms in a structural steel

Microstructural fracture processes in a BS4360 Grade 50D structural steel with lower sulphur content were studied in smooth tensile specimen tests and Charpy-size bend bar tests. Based on the experimental analysis, an experimental void growth relation with the plastic strain and stress triaxiality and multiplying factor on void growth were determined. Experimental results show that the void growth relation can be reasonably used to estimate the constraint in the specimens containing the notch or crack, also they can be used to evaluate the variations of the stress triaxiality in front of the notch and crack tip under general yielding condition. Side-grooves obviously increase the constraint of the CVN specimens. Strain hardening leads to increasing the stress triaxiality, and decelerating the net void growth. This is especially true for the values of stress triaxiality more than about one. Additionally, the effect of the stress triaxiality on the critical void growth corresponding to the onset of ductile tearing was preliminarily investigated. In this work, a large number of smaller specimens were tested to investigate the ductile-brittle transition behaviour of the structural steel. A void growth rate explanation was suggested for evaluating the temperature transition behaviour. The elastic-plastic fracture tough-ness values based on small specimen tests, such as pre-cracked side-grooved bending specimen and short bar tensile specimen, may give large overestimates of the plane strain fracture toughness.

Crack-growth of medical-grade silicone using pure shear tests

Silicone elastomers are commonly used in the manufacture of single-piece joint replacement implants for the finger joints. However, the survivorship of these implants can be poor, with failure typically occurring from fracture of the stems. The aim of this paper was to investigate the crack growth of medical-grade silicone using pure shear tests. Two medical-grade silicones (C6-180 and Med82-5010-80) were tested. Each sample had a 20 mm crack introduced and was subjected to a sinusoidally varying tensile strain, with a minimum of 0 per cent and a maximum in the range 10 to 77 per cent. Testing was undertaken at a frequency of 10 Hz. At various times during testing, the testing machine was stopped, the number of cycles completed was noted, and the crack length measured. Graphs of crack length against number of cycles were plotted, as well as the crack growth rate against tearing energy. The results show that Med82-5010-80 is more crack resistant than C6-180. Graphs of crack growth rate against tearing energy can be used to predict the failure of these medical-grade elastomers.

Arms, bending discounting functions:how motor actions affect intertemporal decision-making

We demonstrate that task-irrelevant somatic activity influences intertemporal decision making: Arm movements associated with approach (arm flexion), rather than avoidance (arm extension), instigate present-biased preferences. The effect is moderated by the sensitivity of the general reward system and, owing to learning principles, restricted to arm positions of the dominant hand.

Vectorial bending characteristics of long-period grating written in D-shaped fibre

The curvature- or bend-sensing response of long-period gratings (LPG) UV-inscribed in D-shaped fiber has been investigated experimentally. Strong fiber orientation dependence of the spectral response when such LPGs are subjected to dynamic bending has been observed and is shown to form the basis for new vector sensors.

Bending characteristics of fiber long-period gratings with cladding index modified by femtosecond laser

A femtosecond laser has been used to asymmetrically modify the cladding of fiber containing long-period gratings. Following modification, devices in single-mode fiber are shown to be capable of sensing the magnitude and direction of bending in one plane by producing blue and red wavelength shifts depending upon the orientation of the bend. The resulting curvature sensitivities were -1.62 and +3.82 nm·m. Devices have also been produced using an elliptical core fiber to study the effects of the cladding modification on the two polarization eigenstates. A cladding modification applied on the fast axis of the fiber is shown to affect the light in the fast axis much more significantly than the light in the orthogonal state; this behavior may ultimately lead to a sensor capable of detecting the direction of bending in two dimensions for applications in shape sensing. © 2006 IEEE.

H-induced platelet and crack formation in hydrogenated epitaxial Si/Si0.98 B0.02 /Si structures

An approach to transfer a high-quality Si layer for the fabrication of silicon-on-insulator wafers has been proposed based on the investigation of platelet and crack formation in hydrogenated epitaxialSi/Si0.98B0.02/Si structures grown by molecular-beam epitaxy. H-related defect formation during hydrogenation was found to be very sensitive to the thickness of the buried Si0.98B0.02 layer. For hydrogenated Si containing a 130nm thick Si0.98B0.02 layer, no platelets or cracking were observed in the B-doped region. Upon reducing the thickness of the buried Si0.98B0.02 layer to 3nm, localized continuous cracking was observed along the interface between the Si and the B-doped layers. In the latter case, the strains at the interface are believed to facilitate the (100)-oriented platelet formation and (100)-oriented crack propagation.

Fatigue crack propagation in some PM steels

Mechanisms of fatigue crack growth have been studied for a range of PM steels at relative densities of 0.90 and 1.0, for which strength, fracture toughness, and microstructural information was also available. It is shown that the Paris exponents for steady state crack growth are between 8 and 18 when ρr is approximately 0.9 but when ρr is approximately 1.0 the exponents are between 2.6 and 4.0, i.e in the range typical of wrought steels (2-4). At both densities, threshold stress intensities are between 5.5 and 10.8 MPa m1/2 when R = 0.1. Combinations of these thresholds and yield strengths are comparable with those for wrought steels. When R = 0.8, reductions in threshold to between 2.7 and 5 MPa m1/2 are attributed to crack closure effects. At ρr = 0.90, Fe-0.5C fails by progressive rupture of sinter necks. Astaloy A, with 0.2%C and 0.6%C, and Distaloy AB-0.6C have smaller plastic zone sizes and the cracks follow more difficult paths through particles as well as necks. When ρr is approximately 1.0, fracture is partially by true fatigue modes and partly by cleavage, the bursts of cleavage being more noticeable when Kmax is high.

The effect of frequency and microstructure on corrosion fatigue crack propagation in high strength aluminium alloys

Fatigue crack growth in high strength aluminium alloy 7150 commercial plate material has been studied in both laboratory air and acidified aqueous salt solution. The aggressive aqueous environment enhanced fatigue crack growth rates by up to an order in magnitude compared to laboratory air. The enhancement in fatigue crack growth rate was accompanied by evidence of embrittlement in the crack path, involving both brittle intergranular and transgranular failure modes. Both the enhancement of fatigue crack growth rates and the extent of intergranular growth modes are dependent on cyclic frequency which, along with the absence of a similar frequency effect in a spray-formed version of the material with a significantly different grain structure, supports a mechanism of grain boundary hydrogen diffusion for intergranular corrosion fatigue crack growth. The convergence of corrosion fatigue crack growth rates at high ΔK in both spray-formed and conventional plate materials coincides with the operation of identical transgranular corrosion fatigue modes dependent on strain-controlled hydrogen diffusion ahead of the crack tip. © 1997 Acta Metallurgica Inc.