Observation Of Failure Mechanisms Research Articles

Cohesive zone model for crack propagation in crystalline silicon nanowires

A fracture of the active particle such as silicon anode results in a potential degradation in lithium-ion batteries with charge/discharge cycle. The massive volume expansion during the lithiation generates large stress which induces the cracking on its surface. We propose the finite element model to have a deep insight into the diffusion-induced stress with fracture process for crystalline silicon nanowires. This model is constructed based on the large deformation theory and cohesive zone model that can describe the failure of Si nanowires. At the beginning of lithiation, high compressive hoop stress is generated on its surface. However, further lithiation, the hoop stress near the surface become less compressive and tensile, which is associated with the “push out effect” as a result of the volume expansion of lithiated material near the phase boundary. When the diffusion induced tensile stress exceeds the fracture threshold, a crack is initiated on the surface and propagates continuously to the center of the silicon nanowire. This crack follows the phase boundary rapidly, but the crack growth speed decreases due to the neighboring compressive hoop stress generated near the phase boundary. From the observation of failure mechanism in Si, we characterize a critical size of Si nanowire, below which fracture can be averted, and we suggest a safe state of charge (safe SOC) depending on its radial size during lithiation.

Performance of single vertical helical anchor embedded in dry sand

An investigation into the uplift behavior of single vertical helical anchor embedded in dry sand presented. A series of laboratory tests have been conducted to determine the effect of the embedment ratio, shaft diameter ratio and sand density against the uplift capacity of helical anchor. The laboratory tests were conducted in a small scale model in loose and dense sand. A sand placement technique was utilized over the testing program to achieve the predetermined depth. In this testing program, the uplift load and uplift displacement have measured. The observation of failure mechanism and the measurement of pullout load and the vertical displacement analyzed and discussed. A number of graphs will be plot between the uplift capacity and the factors to obtain their relationships. From the analysis, the uplift capacity is increase with the increase of embedment ratio and sand density however the shaft diameter ratio is not significantly influent to the uplift capacity. In the observation of failure mechanism, the failure surface proposed for the helical anchor embedded in loose and dense sand. For loose sand package, local failure surface observed however for dense sand package, a truncated cone shape failure surface was observed. From the analysis, an empirical model was proposed. The proposed empirical model compared with the experiment result and existing theories to verify the validity of the empirical model.

トンネル切羽の3次元崩壊メカニズムの可視化

A number of experimental studies have been done regarding the mechanism of tunnel face failure but most of the studies are based on the observation of failure mechanisms using experimental models in two-dimensional and pseudo-three-dimensional model tests. And three-dimensional behavior of the face failure is not well studied yet. The objective of this paper is to investigate failure mechanism behind a tunnel face quantitatively by using X-ray computed tomography (CT) and to visualize the face failure on a three-dimensional basis through a tunnel pull-out model test. As a result of this study, the development of the failure zone at the time of face failure could be described elaborately. And the quantitative findings include that arching effect occurs when the overburden ratio is more than 2D (two times the outer diameter of the pull-out core) and that the volume of the failure zone behind the face is independent of the overburden ratio.

The effects of matrix and interface on damage in GRP cross-ply laminates

A study has been made of the transverse cracking behavior of a series of cross-ply laminates with different matrices, fiber coatings (sizes) and fiber volume fractions. On the basis of unpublished results, a correlation was assumed between material effects on leakage pressure in pressure vessels of given stacking sequence and transverse cracking behavior in cross-ply laminates. Mechanisms for crack initiation and growth were studied by optical microscopy. The strain at the onset of transverse cracking, ε TOS, increased and the slope, K, of the curve of crack density as a function of strain decreased as the G IC of the matrix increased. Improved fiber/matrix adhesion and lower fiber content had similar effects. The properties of polyester-based composites were usually inferior to those of vinylester composites as a consequence of pre-existing debonds and subcritical cracks resulting from microlevel curing stresses. From observations of failure mechanisms one may infer that proof testing of pipes or pressure vessels will increase the extent of subcritical damage in the material.

Failure mechanisms of 3D woven composites in tension, compression, and bending

Observations of failure mechanisms in monotonic loading are reported for graphite/epoxy composites containing three-dimensional (3D) interlock weave reinforcement. The key phenomena are delamination and kink band formation in compression, tow rupture and pullout in tension, and combinations of these in bending. The materials exhibit great potential for damage tolerance and notch insensitivity. This is partly due to the presence of geometrical flaws that are broadly distributed in strength and space; and partly to the coarseness of the reinforcing tows, which leads to extensive debonding and reduced stress intensification around sites of failure. Rules of mixture corrected for the effects of tow irregularity suffice to estimate elastic moduli. Rough estimates of the stress at which the first failure events occur in compression or tension can be made from existing micromechanical models. Ultimate tensile failure might be modeled by regarding failed tows that are being pulled out of the composite as a cohesive zone. The characteristic length estimated for this zone, which is a direct measure of damage tolerance and notch insensitivity, has very large values of order of magnitude 0.1–0.5 m.

Centrifuge Model Tests on Screening Effect of Piles Behind A Vertical Wall

ABSTRACT In the present paper the authors attempted to evaluate quantitatively the screening effect of reinforcement piles in sand backfills by centrifuge model tests. A thin steel plate and a perspex plate with a hinge at the bottom were used as model walls. The reinforcement consisted of thin steel bars tied at the top and bottom. The ratio of the diameter of the pile to the spacing of piles was varied. The model tests were carried out under 30 g of centrifugal acceleration with the measurement of total force on the wall and the observation of failure mechanism in the backfill. From the tests with no reinforcement in the backfill, it was found that the failure mechanism and the total force on the wall compared well with the prediction by the Rankine theory on the basis of the angle of shearing resistance deduced from the plane strain shear tests. The screening effect observed for the present tests was considerably smaller than that reported by other workers, and it was larger for the smaller diameter of the piles. The difference of flexural regidity of one order did not give rise to any significant difference in the screening effect.