Rebound heights

Mohs hardness values may vary Shore hardness number is derived from rebound height of standard steel ball when dropped on material from standard height. 

The coefficient of restitution is defined as the ratio of the vertical components of the impact and rebound velocities resulting when a bad is dropped or thrown onto a playing surface. The velocities or related rebound heights may be measured photographically. Criteria such as bad inflation pressure, air temperature, and other detads must be specified. 

The surface of the material to be tested should be free of imperfections since they could affect the rebound height. 

Indentation hardness determinations were performed in dynamic mode ( 1500 mm/sec impact speed) using a pendulum impact device and in quasistatic mode ( 0.008 mm/sec impact speed) with a custom-built indentation tester. The spherical indenters were of 2.54 cm diameter and 65.6 g mass, and the pendulum length was 92.3 cm with a release angle of 30°. Quasistatic indentation forces were selected to produce indentations of a similar size to the dynamic indentation test (1.5 to 2.0 mm radius). The compact indentations were measured using a white light interferometer (Zygo Corporation, Middlefield, Connecticut, U.S.A.) and the dent depth, dent diameter, apparent radius of curvature, and pendulum initial and rebound heights were used to calculate the indentation hardness of the compacts. 

Several methods have been developed for hardness testing. Those most often used are Brinell, Rockwell, Vickers, Tukon, Sclerscope, and the files test. The first four are based on indentation tests and the fifth on the rebound height of a diamond-tipped metallic hammer. The file test establishes the characteristics of how well a file takes a bite on the material. 

The energy absorbed when an object strikes a surface is related to the hardness of the surface the harder the surface, the less the energy absorbed, and the greater the rebound height of the object after impact. Several methods have been developed to measure hardness in this way. The most common method uses a Shore scleroscope, in which the hardness is determined fi om the rebound height after the impact of a diamond cone dropped onto the surface of the test piece. Typical values of Scleroscope hardness together with the Rockwell M values (in parentheses) for some common plastics are as follows PMMA 99 (M 102), LDPE 45 (M 25), polystyrene 70 (M 83), and PVC 75 (M 60). 

Barquins and Charmet [50] applied the JKR theory to the bouncing of a steel hall on natural rubber, by measuring rebound heights. This permitted the evaluation of the work of adhesion for short contact times. 

This characteristic makes it difficult to interpret hardness measurements except with the ball-identation method which can be considered reasonably accurate. The resulting contact hardness is defined as the average pressure required to indent the material to a depth equal to 2/100th. of the radius of the ball. Other hardness-meeisurement methods such as the Scleroscope (which is the measure of the rebound height of a falling diamond-tipped hammer) are convenient but, as they are not based on the same principle, cannot readily be correlated with the ball hardness. 

OLo=h /h, where h (respectively, h ) the release altitude (respectively rebound height), characterizes the losses in the bulk of the viscoelastic material, without intervention of molecular attraction forces. Due to the shortness of the collision time 19) the penetration of the ball into the rubber surface occurs according to the classical theory of Hertz, even for an adhesive surface, so that maximum values of radius Omax of the contact area and the elastic penetration depth Smax are closely related to the release altitude h by and Smax-A through known prefactors depending upon mass M... 

For h>hc the rebound height is obviously provided by A =(l motor drag)/A(g Using the above expression of he it can be expressed in the reduced form ... 

Rebound heights. Curves I on Figure 6, show rebound heights A as a function of release heights h for six balls made of stainless steel surface effects are superimposed on bulk properties. Experimental data (symbols) verify computed predictions (heavy lines) established from the classical theory of Hertz and the general equation of the adherence previously indicated, with for a natural rubber... 

For comparison, curve II shows rebound heights h versus release altitudes h when the surface was dusted with talcum powder in order to prevent adhesion of the balls. These results have been used to determine the energy restitution coefficient oo. It was calculated as ao=0 625, which is a value commonly found on a soft natural rubber sample when molecular attraction forces do not act. 

Fig. 4.13 Variation of rebound height with time for a 200 pm refinish paint film on glass.

Figure 14 shows the experimental setup for the free-fall experiments. Before it is dropped, a pellet is held at a height hi above the target (flat plane) with the aid of vacuum tweezers. The pellet falls freely onto a target and reaches a rebound height hin after the impact. The movement of the pellet near the contact point before and after the impact is recorded by a highspeed video camera with a frequency of 8.000 frames per second and a resolution of 1200 X 1200 pixels. 

Vickers microhardness is most widely used for microhardness fests of thin coatings. The Shore scleroscope hardness test is a dynamic test that measures rebound height/energy as an indicator of surface hardness by dropping a test hammer onto the surface. The rebound height/energy is heavily dependent on the material elasticity therefore, the Shore hardness should only be... 

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