Localized Strain Zones Research Articles

Thermal defect characterization and strain distribution of CFRP laminate with open hole following fiber laser cutting process

High power fiber laser cutting of CFRP composites has been proven to be feasible with high efficiency, while there are still several challenges due to different carbon fiber-epoxy resin properties and sensitivity to thermal heat. This paper studied the influence of processing parameters on machined hole quality and quasi-static tensile behavior of unidirectional CFRP laminate with open hole (6.0 mm in diameter) following fiber laser cutting process. Hole surface morphology was analyzed using optical microscope and scanning electron microscopy. The area of heat affected zone and defect factor were quantified to understand the effect of machining parameters on thermal defect. It was found that both cutting speed and laser power were statistically significant with respect to HAZ recorded at hole entry, while cutting speed was the only significant factor for HAZ observed at hole exit. The full-field strain distribution during tensile loading was characterized using 3D digital image correlation (DIC) technique. Surprisingly, laser processing parameters showed limited effect on surface strain distribution (εxx, εxy, εyy) during tensile loading. Some highly localized strain zones were observed on the tangent of transverse edge of the hole, corresponding to final failure mode of splitting due to inevitably slight shear force during tensile loading and weak interface strength between carbon fiber and epoxy resin. Numerical simulation results of strain distribution were in good agreement with the DIC analysis.

In-situ microscale visualization experiments on microcracking and microdeformation behaviour around a pre-crack tip in a three-point bending sandstone

A new microscale visualization experimental system is proposed for in-situ measurement of microcracking and microdeformation behaviour of macroscopic specimens during real-time loading. A series of three-point bending experiments were conducted on sandstone beam specimens containing a Mode I pre-crack. The newly developed microscopic experimental system and two other traditional experimental techniques, including acoustic emission (AE) monitoring and macroscopic charge coupled device (CCD) monitoring, were applied together to capture and measure the cracking process and deformation characteristics around the pre-crack tip in real-time. The results show that the crack initiation determined by the proposed microscopic monitoring system was earlier than that determined by AE monitoring and macroscopic CCD monitoring. The microscopic monitoring system observed that the cracks initiated at multiple locations at the pre-crack tip, and they continuously propagated, opened, and coalesced with each other to eventually form a main crack in a zigzag and irregular way, which is significantly different from the macroscopic CCD monitoring observations that suggested that the crack only initiated from the pre-crack tip and rapidly extended upwards along the pre-crack length direction in a relatively simple way. Furthermore, the full-field deformation characteristics at the pre-crack tip were quantitatively obtained by the macrograph-based digital image correlation (DIC) and the micrograph-based DIC calculations. The results indicate that the calculated localized strain zones from the micrograph-based DIC were narrower than those calculated from the macrograph-based DIC. The former had a very similar morphology to realistic microscopic cracks, whereas the latter was wider and longer than the induced macroscopic cracks. The average critical crack opening displacement (COD) calculated using the micrograph-based DIC was approximately 1.3 times larger than that calculated using the macrograph-based DIC.

Influence of hydrogen on the localization of plastic strain in low-carbon steel

The localization of plastic strain in the extension of 08ps low-carbon steel is compared after hot rolling and after electrolytic saturation with hydrogen by means of a three-electrode electrochemical cell. Analysis of the strain curves shows that they include a plasticity region, a stage of linear strain hardening, and a stage of parabolic strain hardening (Taylor hardening). Double-exposure speckle photography is used to visualize the localized strain zones. On that basis, the quantitative deformation characteristics may be obtained. In other words, the displacement-vector field in a plane sample under extension may be determined, and then the components of the plastic distortion tensor (the local elongation exx, shear exy, and rotation ωz) may be calculated. The basic types of plastic flow localization are identified at different stages of strain hardening, and their parameters are determined.

Localization of plastic deformation in alloyed γ-iron single crystals electrolytically saturated with hydrogen

On chromonickel γ-iron single crystals with \(\left[ {\bar 111} \right]\) orientation and low packing-defect density, the plastic-flow localization during electrolytic saturation with hydrogen within a three-electrode electrochemical cell is investigated, with constant controllable cathode potential. On the plastic-flow curve for the extension of single crystals in the initial state (without hydrogen), beyond the transition from elasticity to developed plastic flow, linear strain hardening and then parabolic (Taylor) strain hardening may be observed. The plastic-flow curve for single crystals of austenitic steel saturated with hydrogen includes a small projection and a flow trough, stages of linear strain hardening and parabolic strain hardening, and a prefailure stage. Saturation of \(\left[ {\bar 111} \right]\) single crystals with hydrogen reduces the yield point, increases the plasticity to failure by a factor of 1.3, and suppresses necking in crystals oriented for multiple slip. By double-exposure speckle photography, the basic types of plastic flow location at different stages of strain hardening may be identified, in the presence and absence of hydrogen, and the corresponding parameters may be determined. Hydrogenation of chromonickel γ-iron single crystals intensifies the localization of deformation and leads to considerable changes in the characteristic distances between the plastic-shear bands and the localized-strain zones.

On the localization of plastic strain under compression of LiF crystals

The plastic flow localization patterns for alkali halide LiF crystals under compression have been investigated. The main spatiotemporal regularities of the strain localization at different stages of deformation hardening in the single crystals have been established. The relation has been traced between the orientation of localized strain zones and the crystallography of slip systems of the test specimens studied simultaneously by the double-exposure speckle photography and photoelasticity methods.

Orientation Mapping Study on the Inhomogeneous Microstructure Evolution during Annealing of 6013 Aluminum Alloy

Orientation mapping in transmission electron microscope was successfully applied to study microstructural changes at the initial stage of recrystallization in the aluminum alloy with a bimodal second-phase particle distribution. The alloy samples were reversibly cold rolled resulting in the formation of laminar structure with zones of localized strain around large second-phase particles. Orientation mapping and in-situ investigations carry information about the processes which are active in the deformation zones during annealing.

Lower crustal earthquakes near the Ethiopian rift induced by magmatic processes

Lower crustal earthquakes are commonly observed in continental rifts at depths where temperatures should be too high for brittle failure to occur. Here we present accurately located earthquakes in central Ethiopia, covering an incipient oceanic plate boundary in the Main Ethiopian Rift. Seismicity is evaluated using the combination of exceptionally well resolved seismic structure of the crust and upper mantle, electromagnetic properties of the crust, rock geochemistry, and geological data. The combined data sets provide evidence that lower crustal earthquakes are focused in mafic lower crust containing pockets of the largest fraction of partial melt. The pattern of seismicity and distribution of crustal melt also correlates closely with presence of partial melt in the upper mantle, suggesting lower crustal earthquakes are induced by ongoing crustal modification through magma emplacement that is driven by partial melting of the mantle. Our results show that magmatic processes control not only the distribution of shallow seismicity and volcanic activity along the axis of the rift valley but also anomalous earthquakes in the lower crust away from these zones of localized strain.

On the localization of plastic flow under compression of NaCl and KCl crystals

The plastic flow localization patterns for alkali halide crystals under compression are investigated. The main spatiotemporal regularities of the strain localization at the stages of deformation hardening in these single crystals are established. The relation is traced between the orientation of localized strain zones and the crystallography of slip systems of the test specimens at the initial stages of plastic deformation. The velocity of motion of localized strain zones under compression is determined.

Gradient Deformation Models at Nano, Micro, and Macro Scales

Various deformation models incorporating higher-order gradients are discussed and their implications are considered in a variety of problems ranging from the determination of the size of dislocation cores or elastic dislocation interaction to the determination of wavelengths of dislocation patterns or heterogeneous dislocation distributions and the determination of the structure of solid interfaces and of localized strain zones during adiabatic shear deformation. Different scales are involved in each one of these problems: the nanoscale for single dislocations, the microscale for dislocation patterning, and the macroscale for adiabatic shear banding. Accordingly, different gradient models apply for each case, different types of gradient terms are involved and different expressions of the gradient coefficients are assumed.

Movement and structural features observed in ice masses, Framnes Mountains, Mac.Robertson Land, East Antarctica

Ice-movement surveys in the Framnes Mountains from austral summers 1993–94 and 1994–95 are presented with complementary surface elevations and structural interpretations. Surface velocities of 21 m a−1 have been recorded in Ice Stream B and 31 ma−1 in Ice Stream C. The flow rate varied between 11.6 and 21.0 m a−1 over a lateral range of 2.1 km west of Rumdoodle Peak and between 17.1 and 31.5 m a−1 over 2 km west of Mount Parsons. Two ice streams of relative high flow between the central Masson and David Ranges and several zones of incremental increase in flow rate have been identified within Ice Stream B, using shear-sense criteria based as ice-fracture geometry and subsequent deformation of crevasse traces. These zones of localized strain correlate with ice-flow survey measurements.

Movement and structural features observed in ice masses, Framnes Mountains, Mac.Robertson Land, East Antarctica

Ice-movement surveys in the Framnes Mountains from austral summers 1993–94 and 1994–95 are presented with complementary surface elevations and structural interpretations. Surface velocities of 21 m a−1 have been recorded in Ice Stream B and 31 ma−1 in Ice Stream C. The flow rate varied between 11.6 and 21.0 m a−1 over a lateral range of 2.1 km west of Rumdoodle Peak and between 17.1 and 31.5 m a−1 over 2 km west of Mount Parsons. Two ice streams of relative high flow between the central Masson and David Ranges and several zones of incremental increase in flow rate have been identified within Ice Stream B, using shear-sense criteria based as ice-fracture geometry and subsequent deformation of crevasse traces. These zones of localized strain correlate with ice-flow survey measurements.