Impact rapid load

The term shatter fracture applies to a situation where the applied energy is well in excess of that necessary for fracture. Many regions of the particle are overloaded under these conditions, yielding a comparatively large number of particles with a wide distribution of sizes. This type of fracture occurs under conditions of rapid loading such as those obtained in a high-velocity impact. 

The commonest way of measuring the toughness of plastic and composites is by means of impact test. The specimen is supported at each and struck at the centre so that it is rapidly loaded in three point bend (Charpy impact test). In This experiment the specimen was unnotched(fiat), and was broken by flow from a stricking pendulum. Figure 3 shown schematically the experimental arrangement. The IS of the specimen for this test can be calculated by equation ... 

Now consider the effects of strain rate. Ejqieriments show that Kic increases with increasing crack speed, as illustrated in Figure 5.18, but the resulting shift in fracture stress with increasing strain rate is usualfy smaller than the increase in yield stress. Consequent, a deoreases. As a result, impact and other forms of rapid loading tend to cause brittle failures. 

Impact failures of engineering components are not uncommon, but they are probably oumumbered by those from other brittle modes fatigue and slow crack growth. The fracture surfaces which witness all three modes are superficially similar, but profoundly different rate and temperature dependencies emphasise that different mechanisms are at work. Slow crack grow failures are favoured by long periods under static load at higher temperatures, whilst impact failures are favoured by rapid loading at lower temperatures. 

Impact and Erosion. Impact involves the rapid appHcation of a substantial load to a relatively small area. Most of the kinetic energy from the impacting object is transformed into strain energy for crack propagation. Impact can produce immediate failure if there is complete penetration of the impacted body or if the impact induces a macrostress in the piece, causing it to deflect and then crack catastrophically. Failure can also occur if erosion reduces the cross section and load-bearing capacity of the component, causes a loss of dimensional tolerance, or causes the loss of a protective coating. Detailed information on impact and erosion is available (49). 

Shock-compression processes are encountered when material bodies are subjected to rapid impulsive loading, whose time of load application is short compared to the time for the body to respond inertially. The inertial responses are stress pulses propagating through the body to communicate the presence of loads to interior points. In our everyday experience, such loadings are the result of impact or explosion. To the untrained observer, such events evoke an image of utter chaos and confusion. Nevertheless, what is experienced by the human senses are the rigid-body effects the time and pressure resolution are not sufficient to sense the wave phenomena. 

Fig. 5.2. Current-versus-time records for x-cut quartz impact loaded to stresses of 2.5, 3.9, 4.5, 5.9, 6.5, and 9.0 GPa are shown, illustrating the drastic changes occurring with mechanical yielding and conduction. Time increases from right to left. The current pulses are in the center of each record and are characterized by a brief horizontal trace (zero current before impact) followed by a rapid jump to a current value (after Graham [74G01]).

For the most part, many of the behavioral characteristics discussed are valid for a wide range of loading rates. There may be significant shifts in behavior, however, at load or strain durations that are much shorter than those discussed, usually take about a second or less to perform (Figs. 2-47 and 2-48). This section deals with loading rates significantly faster than those covered so far, namely rapid and impact loading. 

Whenever a product is loaded rapidly, it can be said to be subjected to impact loading. Any product that is moving has kinetic energy. When this motion is somehow stopped... 

In conclusion, it may be mentioned that the characterization of the mechanical behaviour of materials has many facets. Different methods of testing pertain to different aspects and conditions. The tensile properties, as determined by the tensile test, correspond to slowly applied single load applications. Rapidly applied and cyclic load applications respectively provide the impact and the fatigue properties. Hardness is an analog of the tensile strength which a tensile test measures. The creep test pertains to mechanical behaviour under long term loading at elevated temperatures. 

PP bead foams of a range of densities were compressed using impact and creep loading in an Instron test machine. The stress-strain curves were analysed to determine the effective cell gas pressure as a function of time under load. Creep was controlled by the polymer linear viscoelastic response if the applied stress was low but, at stresses above the foam yield stress, the creep was more rapid until compressed cell gas took the majority of the load. Air was lost from the cells by diffusion through the cell faces, this creep mechanism being more rapid than in extruded foams, because of the small bead size and the open channels at the bead bonndaries. The foam permeability to air conld be related to the PP permeability and the foam density. 15 refs. 

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