Failure Process Of Specimens Research Articles

Effect of specimen size on energy dissipation characteristics of red sandstone under high strain rate

In this experiment, red sandstone specimens, having slenderness ratios of 0.5, 0.7, 0.9 and 1.1 respectively, were subjected to blow tests using a Split Hopkinson Pressure Bar (SHPB) system at a pressure of 0.4 atmospheres. In this paper, we have analyzed the effect of slenderness ratio on the mechanical properties and energy dissipation characteristics of red sandstone under high strain rates. The processes of compaction, elastic deformation and stress softening deformation of specimens contract with an increase in slenderness ratio, whilst the nonlinear deformation process extends correspondingly. In addition, degrees of damage of specimens reduced gradually and the type of destruction showed a transformation trend from stretching failure towards shear failure when the slenderness ratio increased. A model of dynamic damage evolution in red sandstone was established and the parameters of the constitutive model at different ratios of length to diameter were determined. By comparison with the experimental curve, the accuracy of the model, which could reflect the stress–strain dynamic characteristics of red sandstone, was verified. From the view of energy dissipation, an increase in slenderness ratio of a specimen decreased the proportion of energy dissipation and caused a gradual fall in the capability of energy dissipation during the specimen failure process. To some extent, the study indicated the effects of slenderness ratios on the mechanical properties and energy dissipation characteristics of red sandstone under the high strain rate, which provides valuable references to related engineering designs and academic researches.

Experimental Study on Properties of Different Angles of FRP Shear Reinforced Timber Beams

Study of fiber reinforced plastics (i.e. CFRP, BFRP and AFRP) for rectangular beams reinforcement effect, using the FRP were 90o vertically and 45o slantingly parceled 10 rectangular timber beams shear performance, recorded specimen failure process. This paper analyzes the different FRP reinforcement plies, different reinforcement on beam properties of specimen, failure mode, bearing capacity, load-deflection relationship and strain distribution. The results showed that: FRP changed beams brittle failure mode, improve the beam bending, shear performance, ultimate bearing capacity increased by 28.2%~55.1%, the stiffness of the specimen and also improved ductility. In addition, its more effective to paste the FRP sheets vertically than slantingly with the angle of 45°. Using FRP strengthening beams the performance of the method is effective, the method can significantly improve the performance of wood. To improve the utilization efficiency of wood, expand the application range of timber provides useful reference.

Stress evolution and failure process of Brazilian disc under impact

To reveal stress distribution and crack propagation of Brazilian discs under impact loads, dynamic tests were conducted with SHPB (split Hopkinson pressure bar) device. Stress states of specimens were monitored with strain gauges on specimen surface and SHPB bars. The failure process of specimen was recorded by ultra speed camera FASTCAM SA1.1 (675 000 fps). Stress histories from strain gauges offer comprehensive information to evaluate the stress equilibrium of specimen in time and space. When a slowly rising load (with loading rates less than 1 200 N/s for d 50 mm bar) is applied, there is usually good stress equilibrium in specimen. The stress distribution after equilibrium is similar to its static counterpart. And the first crack initiates at the disc center and propagates along the load direction. But with the front of incident wave becoming steep, it is hard for specimens to get to stress equilibrium. The first crack may appear anywhere on the specimen together with multiple randomly distributed secondary cracks. For a valid dynamic Brazil test with stress equilibrium, the specimen will break into two halves neatly. While for tests with stress disequilibrium, missing strap may be found when broken halves of specimens are put together. For those specimens broken up neatly at center but having missing wedges at the loading areas, it is usually subjected to local buckling from SHPB bars.

NUMERICAL STUDY OF FAILURE OF CEMENTITIOUS COMPOSITE SPECIMENS IN MODIFIED COMPACT TENSION FRACTURE TEST

The paper introduces a numerical analysis of failure process in specimens loaded in modified Compact Tension (CT) test configuration which are intended to be used for estimation of fracture parameters of quasi-brittle cement-based materials, including the extent of the fracture process zone. Specimen set (consisting of three sizes), designated to future testing in lab by X-ray imaging, was modelled in ATENA 2D finite element method software. Several variants of the tensile load eccentricity, the way of the load application and the initial crack length were considered in order to optimize the range of demanding experiments.

Experimental Investigation of Damage Characteristics in Steel Honeycomb Sandwich Beams

Honeycomb sandwich structures are generally designed to carry flexural loads. However, mechanical properties can be influenced by accidental impacts, or service conditions. Thus a nondestructive detection testing is desired for them. In contrast to many conventional nondestructive evaluation (NDE) techniques, acoustic emission (AE) technique permits continuous damage inspection, classification and identification of failure modes in real time. In this work the fracture process of steel honeycomb sandwich beams has been investigated by in-situ AE technique. Pre-cracks were made both for L-direction and W-direction specimens subjected to three-points bending loads. Damage initiation sites were observed in the vicinity of the crack tip. A series of curves among the AE hits, AE amplitude, AE energy and loading time were obtained. Damage characteristics were discussed based on the above parameters. The results indicate AE characteristic parameters can reflect the damage and failure process of specimens. A good agreement was found between the experimental and analytical results.

Experimental Investigation of Damage Characteristics in Steel Honeycomb Sandwich Beams

In this work the fracture process of steel honeycomb sandwich beams has been investigate by situ acoustic emission(AE) technique. Pre-cracks were made both for L-direction and W-direction specimens subjected to out-plane three-points bending loads. Damage initiation sites were observed in the vicinity of the crack tip. A series of curves among the AE counts, AE hits, AE amplitude, AE energy and loading time were obtained. Damage characteristics were discussed based on the above parameters. The results indicate AE characteristic parameters can reflect the damage and failure process of specimens. A good agreement was found between the experimental and analytical results.

Numerical simulation of the rock fragmentation process induced by two drill bits subjected to static and dynamic (impact) loading

Rock fragmentation processes induced by double drill bits subjected to static and dynamic loading are examined by a numerical method. Micro-heterogeneities of the rock are taken into account in this numerical model. For the static case, the simulated results reproduce the progressive process of brittle rock fragmentation during indentation. For the dynamic case, numerical simulations represent radial cracks, incipient chips, pulverized zones, and shell cracks. Comparing the static and dynamic cases, the dynamic loading can lead to rock fragmentation more efficiently. In addition, numerical results indicate that the dynamic pressure (P max) plays an important role in the failure process of specimens with two indenters. Furthermore, the heterogeneity of the rock can also affect the failure modes of the rock when two indenters are used. Finally, the numerical results demonstrate the effect of the spacing between the indenters on the rock.

Dynamic response and failure behavior of rock under static-dynamic loading

Dynamic response and failure behavior of rock under static-dynamic loading were studied. The effects of initial static load on the total energy dissipated during the failure process of specimen were analyzed. To simulate the engineering situation that in-situ rock experienced and obtain the dynamic loading with an intermediate strain rate, a low cycle fatigue load with the frequency from 0.5 to 5 Hz was adopted by servo-controlled Instron material testing system. The results show that the obtained strain rate increase with the increase of load frequency. The initial static load has great influence on both the energy and dynamic response of rock. Both the energy and the maximum failure load P f decreases with the increase of initial static load. P f under the static-dynamic loading is larger than that under only the static loading but less than that under only the dynamic loading. The load-displacement curves become nonlinear as the pre-added static load reaches the transition point which is about one third of static strength. With the increase of initial static load, Young’s modulus decreases and poisson ratio increases. It shows that rock has a lower strength and a tendency to soften under a higher initial static load. Rock may be broken more easily static-dynamic loading than under only the dynamic loading. The proposed method is useful in the investigation of constitutive relationship and failure behavior of rock under quasi-dynamic loading.