Crack Growth In Tension Research Articles

Effects of deformation twinning on the mechanical properties of biodegradable Zn-Mg alloys

To satisfy the property requirements for biodegradable medical implants, Zn alloyed with low levels of Mg (≤0.8 wt%) has attracted increased research interest. In the present study, deformation twinning was observed in tensile tests and twinning appears to have an adverse impact on ductility. The profuse twinning in the as-cast Zn-Mg alloys accelerated crack growth in tension due to twinning impingement which caused local stress concentrations and initiates cracking. As-rolled Zn-Mg alloys have better ductility than their as-cast counterparts due to the inhibition of twinning by the refined Mg2Zn11 intermetallic phase and the finer grain size.

Interface debond crack growth in tension–tension cyclic loading of single fiber polymer composites

Fiber/matrix interface debond crack growth from a fiber break is defined as one of the key mechanisms of fatigue damage in unidirectional composites. Considering debond as an interface crack its growth in cyclic loading is analyzed utilizing a power law, where the debond growth rate is a power function of the change of the strain energy release rate in the cycle. To obtain values of two parameters in the power law cyclic loading of fragmented single fiber specimen is suggested. Measurements of the debond length increase with the number of load cycles in tension–tension fatigue are performed for glass fiber/epoxy single fiber composites. Analytical method in the steady-state growth region and FEM for short debonds are combined for calculating the strain energy release rate of the growing debond crack. Interface failure parameters in fatigue are determined by fitting the modeling and experimental results. The determined parameters for interface fatigue are validated at different stress levels.

Positive and negative feedback in the earthquake cycIe: the role of pore fluids on states of criticality in the crust

Fluids exert a strong physical and chemical control on local processes of rock fracture and friction. For example they may accelerate fracture by stress corrosion reactions or the development of overpressure (a form of positive feedback), or retard fracture by time-dependent stress relaxation or dilatant hardening (negative feed-back), thereby introducing a variable degree of local force conservation into the process. In particular the valve action of dynamic faulting may be important in tuning the Earth to a metastable state of incipient failure on all scales over several cycles, similar to current models of Self-Organised Criticality (SOC) as a paradigm for eartiquakes However laboratory results suggest that ordered fluctuations about this state may occur in a single cycle due to non conservative processes involving fluids which have the potential to be recognised, at least in the short term, in the scaling properties of earthquake statistics. Here we describe a 2-D cellular automaton which uses local rules of positive and negative feedback to model the effect of fluids on failure in a heterogeneous medium in a single earthquake cycle. The model successfully predicts the observed fractal distribution of fractures, with a negative correlation between the predicted seismic b-value and the local crack extension force G. Such a negative correlation is found in laboratory tests involving (a) fluid-assisted crack growth in tension (b) water-saturated compressional deformation, and (c) in field results on an intermediate scale from hydraulic mining-induced seismicity all cases where G can be determined independently, and where the physical and chemical action of pore fluids is to varying degrees a controlled variable. For a finite local hardening mechanism (negative feedback), the model exhibits a systematic increase followed by a decrease in the seismic b-value as macroscopic failure is approached, similar to that found in water-saturated laboratory tests under controlled «undrained» conditions, and where dilatancy hardening is independently known to be a local mechanism of negative feedback. A similar pattern is suggested from selected field observations from natural seismicity, albeit with a lesser degree of statistical significance.

Hydrogen-Assisted ductile fracture in spheroidized 1520 Steel: Part I. axisymmetric tension

The effects of hydrogen on ductile fracture were investigated in a spheroidized steel similar to AISI 1520, which contained negligible amounts of P and S. Cylindrical tensile data indicated that hydrogen does not significantly affect void initiation and the early stage of void growth. Instead the effects of hydrogen were observed in the final fracture stages. It was inferred that hydrogen could assist the later stage of void growth by an internal pressure and affect the center crack growth in tension by the enhanced strain localization caused by hydrogen. The fracto-graphic response to hydrogen in spheroidized steels was rationalized by the combination of these two effects, that is, internal pressurization and enhanced strain localization. The critical role of manganese sulfide was manifested by comparison with previous work.

Ductile fracture in spheroidized 1520 steel

Ductile fracture was investigated in a spheroidized steel, similar to AISI 1520, which contained negligible amounts of P and S. The detailed substeps of the ductile fracture process were characterized by careful sectioning studies. It was found that 1. (i) void initiation appeared to be controlled by a criterion involving stress, 2. (ii) void growth was predominantly affected by plastic strain, 3. (iii) additional strain was needed to arrive at final fracture even after the void coalescence stages, comprising microvoid coalescence and macrovoid coalescence, had occurred and 4. (iv) the central crack growth in tension mainly depended on the strain concentration at the crack tip. The effect of localized deformation on fracture could not play a role in axisymmetric tension, due to a high critical strain for shear band formation.

A Microcrack Model of Dilatancy in Brittle Materials

For a solid containing preexisting flaws, overall nonlinear constitutive relations are developed on the basis of a model which endows a preexisting flaw with frictional and cohesive resistance, and which includes nucleation and growth of tension cracks at the preexisting flaw, as it deforms under the action of an overall compressive load. The preexisting flaws may be randomly distributed or may have an initial preferential distribution. They may be of varying sizes and orientations. Even when the flaws are randomly distributed, their preferential activation, and the nucleation and growth of tension cracks at preferential flaws, render the overall response of the solid highly anisotropic. As a first step toward a more complete constitutive micromechanical modeling, a dilute distribution of preexisting flaws is assumed, rate constitutive relations are developed for loading and unloading, which include hysteresis, dilatancy, and other characteristics observed experimentally in rocks, ceramics, concrete, and similar brittle materials. A number of illustrative examples are worked out, and the results are compared to relevant experimental observations.

Dynamic analysis of wedge-assisted surface crack growth in tension

The dynamic analysis of constant-speed tension crack growth in an elastic half-plane assisted by a rigid wedge traveling also at a constant speed is performed. By modeling the crack as an array of dislocations distributed w.r.t. speed, the analysis reduces to dual integral equations. These equations can, in turn, be replaced by one singular integral equation of the Cauchy type subject to constraint equations. A standard numerical solution approach then yields not only the solution but information on the crack growth in terms of various dimensionless input parameters.

Causes of mixed fatigue-tensilecrack grovvth and significance of microscopic crack behaviour

Abstract Alternating fatigue- and tension-crack growth is often observed in both laboratory and service failures. This phenomenon is described for some aluminium-alloy plate fatigue specimens where it has been found that for conditions where σ max remains substantially constant each tensionjump occurs at a constant value of √a/1, where a is maximum depth of crack and 1 is crack-front length. The √a/1 value for the ‘pop in’ crack front also remains constant, but at a lower value. Clearly plate thickness, σ max , and K IC will affect the value of √a/1, and the general equation can be written:√a/1=KIC/Cloσmax√πwhere lo+plate thickness=shortest possible crack front, and C is the appropriate factor, excluding any correction for crack shape. The fatigue period causes a reduction of curvature of the ‘pop in’ crack front, thereby shortening it until the critical jump value for √a/1 is reached. Straightening of the crack front also occurs on a microscopic scale, starting from the fragmented condition of the tensil...