Stress-wave loading

Up to this point we have addressed primarily the flaw structure and energy concepts in stress-wave loaded solids governing the creation of new fracture surface area (or the mean fragment size) in catastrophic fragmentation events. It remains to consider a concept which is frequently the end concern in impulsive fracture applications, namely, the distribution in sizes of the particles produced in the dynamic fragmentation event. 

L. B. Freund, Crack Propagation in an Elastic Solid Subjected to General Loading IV-Stress Wave Loading, Journal of Mechanics and Physics of Solids, 21, 47-61 (1973). 

The objective in these gauges is to measure the time-resolved material (particle) velocity in a specimen subjected to shock loading. In many cases, especially at lower impact pressures, the impact shock is unstable and breaks up into two or more shocks, or partially or wholly degrades into a longer risetime stress wave as opposed to a single shock wave. Time-resolved particle velocity gauges are one means by which the actual profile of the propagating wave front can be accurately measured. 

The motion of disloeations under eonditions of shoek-wave eompression takes plaee at sueh high veloeities (approaehing the elastie sound speed) that many vaeaneies and interstitials are left behind. However, these point defeets ean anneal out at room temperature and are thus diflieult to study by shoek-reeovery teehniques. The presenee of point defeets has little effeet on the material eompressibility and other properties related to equation of state. While they also have little direet influenee on the relief of shear stresses, point defeets do influenee the mobility and multiplieation of disloeations. This, in turn, affeets most of what happens under shoek-wave loading eonditions. 

On the loaded side of a slab subjected to an intense reflected blast wave, a region of the slab will fail if the intensity of the compressive wave transmitted into the slab exceeds the dynamic compressive strength of the material. For an intense wave striking a thin concrete slab, the failure region can extend through the slab, and a sizeable area turned to rubble which can fall or be ejected from the slab. For a thicker slab or localized loaded area, spherical divergence of the stress wave can cause it to decay in amplitude within the slab so that only part of the loaded face side is crushed by direct compression. 

Acoustic emissions are the stress waves resulting from sudden movements in the stressed sample material. The most obvious example is the tin cry due to stress in a wooden structure due to the load. It is very important that the measured acoustic emission signal be corrected for the background noise signal. Acoustic emission energy is due to the elastic stress field in the sample material. Acoustic emission is monitored by applying a controlled variable load. The principle of the method is illustrated below (Figure 2.14). 

Measuring loads in high speed tests is difficult because of the short times involved and the presence of transient effects from stress waves. The more simple method which has been developed in both the Izod and Qiarpy tests is to break the spedmoi with a pendulum and measure the energy absorbed. Some form of scaling parameter from the specimen dimmsions is then applied and an impact number derived. The practical utility of these numbers is beyond question but they have many, weU known, drawbacks. In particular, the nund)ers are geometry dependent and do not agree, for example, between the Izod and Charpy tests. 

Mulholland, S.E., Lee, S., McAuliffe, D.J. and Doukas, A.G. (1999) Cell loading with laser generated stress waves the role of stress gradient . Pharm. Res. 16, 514-518. 

Stress variables mean stress, maximum stress, minimum stress, constant load/constant strain, strain rate, plane stress/plane strain, biaxial, cyclic frequency wave shape. 

With the rapid development of the national economy and the increased mining depth coal and rock dynamic disaster is increasing ay by day. In order to ensure life safety, safe, reasonable and efficient mining method are very necessary. Then the research on dynamic mechanical properties of underground rock is also very importanti l Almost all of the dynamic disaster, such as broken rock, rock burst, coal and gas outburst and other dynamic disasters had certain correlation with the rock failure under impact load and stress wave s propagation in the rocU f Because research on the dynamic mechanical properties of rock is minimal in our country, many law needs to refer to foreign datal - l, which leads to the lack of theoretical basis in China. Rock burst and other dynamic disaster tend to occur in coal seam with limestone roof Therefore, in order to effectively control the mine dynamic disasters, we must reveal its dynamic mechanical properties, thus it is very important to do the comparative study on dynamic mechanical properties of coal and limestone. 

The propagation of water waves over a permeable seabed exerts a time varying pressure at the sediment/water interface. The time varying pressure will cause cyclic variations in pore pressure and stresses within the bed. The effective stress varies in response to wave loading. Since soil strength is directly related to effective stress, any change in the effective stress state within the bed will affect bed strength and stability. Many coastal structures such as pipelines, platforms, anchors, and breakwaters that interact with the seabed will be affected by both cyclic effective stresses and the erosion potential of the bottom sediments. 

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