Impact bending test

Loading of the material in a triaxial stress state that keeps Mohr s circle small and shifts it to the right in the direction of the cleavage strength. Such a stress state can be found in the notched bar impact bending test [42]. In components, changes in cross section and notches cause such... 

Loading of the material at high strain rates, for example in the notched bar impact bending test, or at low temperature. This is due to the fact that the yield strength always depends on these two parameters (see section 6.3.2), whereas the cleavage strength is almost constant. This is particularly the... 

In view of the technical relevance the impact resistance of polymers has been intensively investigated. The cited references [88—103] may serve as an introduction into the large body of literature. Vincent [88] and Bucknall et al. [89] give a general survey of the impact testing of polymers. Other references concern the molecular aspects [88-96], instrumentation [97—100], and particle impact [101—103]. The extensive literature on the fracture mechanical analysis of impact bending tests will be referenced in Chapter 9. 

Tensile and flexural properties were studied with an Instron 4204 testing machine. Tensile tests were performed on the drawn strands at a test speed of 3 mm/ min, while three-point-bending tests (ISO 178) at a speed of 5 mm/min were applied to the injection molded specimens. Charpy impact strength was measured of the unnotched samples with a Zwick 5102 pendulum-type testing machine using a span of 70 mm. The specimens (4 X 10 X 112 mm) used for three-point-bending tests were also used for the impact tests. It should be noted that neither the tensile tests for the strands nor the impact tests were standard tests. The samples were conditioned for 88 h at 23°C (50% r.h,) before testing. 

Fig. 6.3. A theoretical plot of fracture toughness, R, with variation of frictional shear stress, Tf, compared with experimental fracture toughness values (A) Charpy impact and (A) slow bend tests for carbon-epoxy composites ( ) Charpy impact and (O) slow bend tests for carbon polyester composites. After...

Block polymer B differs substantially in its failure characteristics from BP A polycarbonate. For the block polymer a mixed failure mode predominates in three-point bend tests of notched specimens from —100°-90°C. In the mixed mode craze breakdown and plane strain fracture occur first inside the specimen subsequently shear failure occurs in the surface regions of the specimen. Shear lips (11) are formed as a result. Shear lips are also found on the notched Izod impact fracture surfaces of block polymer B, implying that the same mixed mode of failure occurs under high speed loading conditions. 

Two types of mechanical tests have been used the low rate of deformation tensile, compressive or bending tests and the high speed impact tests. Immiscibility of polymers is reflected in both. For example, in tensile tests the maximum strain at break (or the maximum elongation), and the yield stress (or the maximum strength) can be... 

The simplest way to approach detection of brittleness point is quickly to bend the material while holding it at the low temperature, and a considerable number of tests of this type have been devised and some standardized. The degree of strain is often not precisely defined, and these tests tend to be operator-dependent, so that reproducibility can be poor. A more satisfactory approach is to impact a test piece at low temperatures. Adaptations of falling weight impact tests are tests are commonly used for rigid plastics, but for flexible materials a procedure in which a strip test piece held as a simple cantilever and impacted by a striker is very widely standardized. 

SENB specimens offer further flexibility in that they are. suitable for static as w ell as impact three-point bending tests, where the loading span 5 can be adjusted. 

The 5 ms failure time in a Charpy impact test is 2000 times smaller than the 10 s failure time in a slow bend test. This increases the value of the yield stress, without necessarily increasing the stress for crazing. For some polymer/temperature combinations there may be a changeover from yielding to crazing and plane strain fracture. 

Somewhat more sophisticated instruments are frequently used for so-called instrumented impact tests. These instruments carry devices for measuring quantities such as the force applied to the sample and its displacement as a function of time and they allow the impact energy to be obtained more directly. Bending tests are, however, frequently carried out in a less sophisticated way using specimens without notches. The energy of fracture divided by the area of fracture is then called the impact strength. This quantity cannot be simply related to the quantities such as the critical strain-energy-release rate defined in fracture mechanics. 

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