Impact testing

When a material is loaded quickly or shocked or impacted, its behavior is, in many cases, different from the behavior exhibited in the tensile test, where the load is applied slowly. Whether shocks occur as part of 

I A Before After 1 3efore 1 After Before / After 1 

FIGURE 15.28 Left, the swinging pendulum. Right, the pendulum after striking an object. 

FIGURE 15.30 Left, Charpy test. Right, Izod test. 

FIGURE 15.31 The dimensions and specifications of a metal impact specimen. 

As opposed to window glass, in a plastic the energy associated with the deformation can often be dissipated in ways that do not lead to breaking of chemical bonds or cause similar interatomic disruption. As a consequence, there can be a greater resistance to failure and this gives certain plastics high impact strengths. 

Samples can also be notched, the size and shape of the notch is also standardised. This ensures that the samples fractnre. 

3 Tensile and Impact Testing of Recycled Expanded Polystyrene 

To show the kind of effects that are observed due to degradation, the following example shows the results of a study using Method lA on recycled expanded polystyrene. This method is typically used for foam cups and packaging. 

Cogswell, Polymer Melt Rheology , A Guide for Industrial Practice, Woodhead Publishing, Cambridge, UK, 1994. 

ASTM D638-03, Standard Test Method for Tensile Properties of Plastics, 2003. 

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Component Composition, wt % Heat of reaction, kj/g" Gas volume, cmVg Relative brisance, % TNT Ignition temp, °C Impact test, % TNT... 

ASTM E23-93, "Notched Bar Impact Testing of MetaUic Materials," Annual Book of ASTM Standards, ASTM Puhhcations, Philadelphia, 1993. ASTM D256-92, "Impact Resistance of Plastics and Electrical Materials," Annual Book of ASTM Standards, ASTM Puhhcations, Philadelphia, 1993. A. N. Atiuri and T. Nishioka, Int. / fract. 27, 245 (1985). 

Subsequent to processing, an inspection is made for incomplete bonding, inside dirt, and glass quaUty. In the case of windshields, rigid optical standards must be met, and these must be evaluated for the completed windshield. Extensive test requirements are described in the appropriate codes (11,12,15,18—24), and they include light stabiUty, resistance to optical distortion, humidity, boil test, abrasion resistance, and assorted impact tests. 

Testing of Painted Products. The enhancement of paint adhesion is one of the principal functions of conversion coating (20—22). A group of tests based on product deformation is used to test the painted product. The appHance and cod-coating industries use the mandrel bend, the cross-hatch adhesion test, and the direct and reverse impact tests. Adhesion after a water soak is judged using a cross-hatch test performed on the exposed surface. 

Four modes of characterization are of interest chemical analyses, ie, quaUtative and quantitative analyses of all components mechanical characterization, ie, tensile and impact testing morphology of the mbber phase and rheology at a range of shear rates. Other properties measured are stress crack resistance, heat distortion temperatures, flammabiUty, creep, etc, depending on the particular appHcation (239). 

Impact resistance is determined usiag flyer plate impact tests, long rod impact tests, Hopkinson bar tests (50), and the Hquid jet technique (51). Impact damage resistance is often quantified by measuring the postimpact strength of the ceramic. 

Certain forms of this material, as stated in Table 10-57, must be impact-tested to qualify for service below -29 C (-20 F). Alternatively, if provisions for impact testing are included in the material specification as supplementary requirements and are invoked, the material may be used down to the temperature at which die test was conducted in accordance with the specification. 

The minimum temperahire shown is tliat design minimum temperature for which the material is normally suitable without impact testing otlier tlian that required by the material specification. However, the use of a material at a design minimum temperature below -29 C (-20 F) is established by rides elsewhere in the code, including any necessary impact-test requirements. 

This material when used below -29 C (-20 F) requires impact testing if the carbon content is above 0.10 percent. 

AU other carbon steel, low-intermediate, and high-aUoy steels, ferritic steels Base metal Deposited weld metal and heat-affected zone (See Note 1) 2. Except when conchtions conform to Note 2, the material shall be heat-treated to control its microstructure by a method appropriate to the material as outlined in the specification applicable to the product form and then impact-tested. (See Note 1.) Deposited weld metal and heat-affected zone shall be impact-tested. 

Austenitic stainless steel 3(3. If (1) the carbon content by analysis is greater than 0.10 percent or (2) the material is not in the solution-heat-treated conchtion, then impact testing is required for design temperatures below-29 C (-20 F). See Note 2. ib. When materials are fabricated or assembled by wel(hng, the deposited weld metal shall be impact-tested for design temperature below —29 C (—20 F) unless cou(htious conform to Note 2. 3. The material shall be impact-tested. See Note 2. 

Austenitic ductile iron, ASTM A571 4(3. No ad(htional requirements. 4b. Wel(hng not permitted. 4. The material shall be impact-tested. This material shall not be used at design minimum temperatures lower than —196 C (—320 F). Wel(hug is not permitted. 

Impact testing is not required if the design temperature is helow —29 C (—20 F) hut at or above —46 C (—50 F) and the maximum operating pressure of the fabricated or assembled components will not exceed 25 percent of the maximum allowable design pressure at ambient temperature and the combined longitudinal stress due to pressure, deadweight, and displacement strain (see Par. 319.2.1) does not exceed 41 MPa (6000 Ibfiin ). 

Impact Testing Materials conforming to ASTM specifications listed in the code may generally be used at temperatures down to the lowest temperature listed for that material in the stress table without additional testing. When welding or other operations are performed on these materi s, additional low-temperature toughness tests may be reqiiired. The code requirements are listed in Table 10-57. 

Fabrication tolerances are covered in this subsection. The tolerances permitted for shells for external pressure are much closer than those for internal pressure because the stability of the structure is dependent on the symmetry. Other paragraphs cover repair of defects during fabrication, material identification, heat treatment, and impact testing. 

Low-temperature service is defined as being below—29°C (—20°F), and impact testing of many materials is required. The code is restrictive in the type or welding permitted. 

Figure 9.20. Experimental setup for symmetric plate impact test.

Figure 9.21. Comparison of calculational and experimental results for plate impact test.

Figure 9.20 shows the setup for a symmetric plate impact test. The projectile here has a facing plate of ceramic and is backed with a low-density foam, for support of the ceramic during launch. The facing plate of the target is also made of ceramic. The lithium fluoride slab, which backs the target sample, serves as a window for the laser velocity interferometer (VISAR) that measures the time-resolved particle velocity at the sample/window interface. 

Temper 650/1100°F Very low impact test Heat treated alloy Heat treated alloy Slow cooling Add about. 50%... 

It is probably most useful to consider toughness as a property of a plastics part under some specified conditions of service. Whilst it is possible to devise impact tests and to rank a series of plastics materials according to the results obtained in such tests it remains almost impossible to use such tests to try to predict whether or not an article made from a specific material will or will not be satisfactory in service. 

Impact tests are, however, used to try to compare the impact strength of different materials. Of these tests four require specific mention. These are the Izod test, the Charpy test, the falling weight tests and the tensile impact test. 

Other impact tests widely used are the falling weight tests where a weight is allowed to fall onto a supported flat or domed surface and tensile impact tests in which a sample is subjected to a sudden shock in tension. Whilst the... 

VINCENT, p. I., Impact Tests and Service Performance of Plastics, Plastics Institute, London (1971)... 

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