Impact fracture testing

Flexural experiments were carried out in three-point loading geometry with inner span being 20 mm, and crosshead speed 1 mm/min. The impact fracture tests were conducted in a Zwick impact tester using Charpy specimens 4x10x50 mm. They contained razor-sharp initial cracks of 1,2,3,4 and 5 mm in the depth direction. The impact fracture toughness (G ) was obtained using the equation of Plati and Williams [7]. 

Name and describe the two impact fracture testing techniques. 

Fig. 7.5. (a) Transverse impact fracture toughness and (b) fiber pull-out length versus testing temperature for carbon fiber-epoxy matrix composites with and without PVAL coatings on fibers. After Kim and... 

Although the processing variables appear to have correlations with morphology, results of impact tests fail to produce a definitive description of the morphological response to impact. However, this problem is shown to be substantially eliminated through the use of instrumented impact testing (17). Such tests measure the time dependence of the impact load borne by the sample and indicate that impact fracture is a... 

L. R. Deobald and A. S. Kobayashi A Bar Impact Tester for Dynamic Fracture Testing of Ceramics and Ceramic Composites, Experimental Mechanics, 32, 109-116 (1992). 

Failure Morphologies. Ductile failure of notched polycarbonate specimens has long been recognized to occur with shear yielding from the notch tip (6). This occurs for the block polymers for all rates of test. Hull and Owen (5) recently reported from micrographic studies of impact fracture surfaces that the brittle failure of polycarbonate involves the formation and breakdown of a craze at the notch tip. The ductile-... 

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. 

Numerous studies have been made of the mechanical properties of fibrous composites these include recently published papers on impact properties by Izod (1,2, 3,4) and Charpy (5,6) and drop weight (7) tests. We reported the Charpy impact fracture behavior of various glass-polyester composites regarding the effects of temperature (8,9,10), specimen size (8), and fiber orientation (10). This paper describes the effects of the tough-brittle transition in the impact behavior of glass-polyester composites which occurs with a variation of temperature and specimen size. 

It has been shown that fracture is a very complex process and the fracture performance depends on both the initiation and the propagation of a defect [6-10] in the material. Under impact, most polymers break in very distinct manners. Several types of fracture have been identified depending on the amount of plastic deformation at the crack tip and the stability of crack propagation. For each type, an appropriate analysis has been developed to determine the impact fracture energy of the material. These methods have also been verified in various plastics [11,12]. The different fracture behaviors in most polymers are illustrated in Figure 27.1, which shows a schematic drawing of the load-deflection diagram of Charpy tests on HIPS [13] under an impact velocity of 2 m/s at various temperatures. 

The fracture test on three-point-bend samples revealed that the fracture behavior of ABS remains ductile for temperatures above -80 °C at a crosshead speed of 5mm/min. The maximum fracture energy at crack initiation is also observed around -80 °C. Figure 27.20 shows that above this temperature, G, decreases continuously with increasing temperature. When the test speed is increased to lOOmm/min, the temperature at brittle ductile transition is shifted to about -40 °C as shown in Figure 27.21. At an impact velocity of 2.5 m/s, the brittle-ductile transition occurs at around -20 °C as shown in Figure 27.22. The results also confirm that the fracture energy at crack initiation is maximum at the brittle-ductile transition. 

The variation of the loading curve shape with fibre orientation (angle a) is less regular in Im/s fracture tests and the identification of the point of fracture initiation from the load diagram was often problematic. Load-point displacements at fracture initiation in 1 m/s tests appear to be larger than in low rate tests. This apparent result is not unexpected, in view of the damping technique used in the impact tests which increases the compliance of the test system initially. 

Figure 5.15. Impact fracture surface of C/SiC composites [24] (a) after Charpy impact test and (b) after the nails penetrated test...

Most of the authors cited observed a strong dependence of the toughening mechanisms on the testing rate. However, the loading rates used for fracture testing in these studies were generally one static rate and one impact rate. 

The studies of impact fracture in PP/EPR blends have been carried out by Tam et al. (11), Hayashi et al. (12) and Yang et al. (13). In particular, Hayashi et al. (12) examined the microdeformation of EPR domains in the PP matrix after the impact test by using TEM. They reported that certain part of the sample showed whitening, where the corresponding TEM images are illustrated in Fig. 8.8. 

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