Initial Stage Of Crack Growth Research Articles

Fatigue crack propagation in steels with different yield strengths

1. Fatigue crack propagation in low-alloy and low-carbon steels can be described by the relationship d2a/dN=C(ΔK)n. Two sections — lower and upper — are observed on the curves of crack growth, differing from each other in their slope to the ΔK axis. 2. The length of the fatigue crack corresponding to the change from one section to the other increases with increasing yield strength of the steels. 3. On the lower section, with a steeper slope to the ΔK axis, corresponding to the initial stage of crack growth, the average rate of crack propagation is considerably smaller than in the upper section. 4. In the upper section the relationship between exponent n and coefficient C and the yield strength and ultimate strength of the steels is almost linear. 5. The average rate of fatigue crack propagation in low-alloy and low-carbon steels decreases with increasing yield strength, as the result of which the life of the steel increases. The largest values of vav were obtained for steel 15kp and the smallest for steel 10G2S1.

Crack-growth monitoring: Optimisation of the electrical potential technique using an analogue method

1) Electrical potential field distributions have been obtained for SEN and CTS fracture testpieces and used to derive the optimum conditions for current input and probe positions. 2) The most suitable position for the potential measurement probes is on the top face of the specimen as close to the open end of the notch as possible, since attachment near the crack tip leads to much larger errors from any slight variations in positioning the probes. 3) For SEN specimens, the greatest sensitivity is obtained by introducing the current at points close to the notch. However, the uniform current configuration (input of current at specimen ends) is prefered since it is not liable to errors from slight variations in current input position. 4) Introduction of the current by area contact on the top surface in CTS specimens is preferred because very low sensitivity is achieved for the initial stages of crack growth when the current is applied from the side flanks. 5) The mode of current lead attachment on CTS testpieces is dependent on the testpiece materials. In steels, reproducible area contact mayy be achieved by the use of brazed or soldered copper terminals. Howwever, in aluminium and its alloys, the samde degree of reproducibility cannot be achieved for any practical application by area contact. Point application of current through a single screw is therefore advocated.

Photoelastic study of initial stages of brittle fracture in compression

Initial stages of crack growth within various arrays of elongate open flaws were studied in photoelastic models subjected to uniaxial compression. The growth of inclined flaws into stable configuration with respect to stress for crack growth, with certain exceptions, is similar to that described previously (Bombolakis, 1964). Critical arrays for initial crack growth, however, include certain sets of closely spaced parallel flaws inclined 45° to the stress axis, in contrast to the 30° orientation predicted by Griffith theory. Despite the fact that flaws of various orientation and en-échelon configuration were located very close to one another in each array before crack growth began, coalescence of flaws did not occur during the first stage of crack growth within any array. Coalescence occurred during the second stage of crack growth, but this was in only one of the arrays less favorable for initial crack growth. It resulted in a partial fracture surface inclined approximately 36° to the stress axis. Characteristic features of the first and second stages of crack growth are the occurrences of small “shocks” and minor drops in the applied stress that causes crack growth. Such features were not observed in the several arrays in which subsequent stages of crack growth were obtained; crack growth was gradual instead of spasmodic, and it could be controlled by the loading. These results suggest a possible mechanism for some of the seismic events associated with shallow-focus earthquakes. Tests on several duplicate arrays indicate that the initial stages of crack growth depend in a remarkable way on the past history of deformation. Inclined flaws in those arrays either grew more or less parallel to the axial direction or perpendicular to it during the second stage of crack growth, depending on whether the stress for first-stage crack growth was increased directly or cycled to the stress for second-stage crack growth. This kind of dependency on history of deformation is of potential importance in interpretations of rock-deformation experiments and of fracture patterns in the field.

Initial stages of damage in high stress fatigue in some pure metals

Abstract The results to date are reported of an investigation into the initiation and propagation of fatigue cracks at high and low stresses in aluminium, nickel and copper. It is shown that crack propagation involving the formation of ripples on the fracture surface occupies, at high stresses, a large portion of the fatigue life and the whole of the fracture surface area, whereas at lower stresses a considerable part of the fracture surface may be covered with ripples if the specimen has been previously annealed, but only a few per cent of the total number of cycles are consumed in this type of fracture. Observations and measurements on the slow initial stages of crack growth at high and low stresses have been made, and it is shown that the variables of metallurgical condition of the specimen, environment and frequency of cycling affect this stage more significantly than propagation involving ripples. In discussion it is suggested that the environment assists the fatigue process by a corrosive action rather than by preventing welding of the crack tip during the compression part of the cycle.