Notching machine

Three modes are clearly defined for crack propagation from a very thin (radius of the order of 10 gm) notch-machined in the specimen (Fig. 12.3). This notch induces a stress concentration effect, higher than those produced by all the other defects already present in the specimen, which governs the fracture initiation. For isotropic materials, mode I (the most severe) is generally used and gives the lowest value of toughness. In the case of adhesives and laminates, modes II and III are also performed. 

The test can be performed using either unnotched or notched test pieces. Three types of notches arc standardized (Fig. 13). the preferred one having a radius at the noteh base of 0.25 mm (the type A notch). A blunt 1.0 mm (type B notch) and a very sharp 0.1 mm (type C) notch are also covered. Notches of different base radius are useful for more extensive characterization of plastics than a simple quality lest or data sheet entry, in that they enable an estimate of the notch sensitivity of the plastic to be investigated. The flatwise test can also be performed notched or unnotehed. except that here the notched test has two notches machined across the 4 mm direction and directly opposite... 

Test specimens (400 mm wide) of the three types shown in Fig. 6a, with notches machined in the region of maximum residual tensile stress were tested at... 

The ceramic specimens mentioned above, tested by dynamic loading, namely impact testing, are notched as is usual during the performance of such tests. During conventional impact testing, the radius at the notch-root is significant, since it affects the outcomes. For metals, the dimensions of the specimens and the notches are standard. A standard CIT specimen consists of a bar of metal or other material (ceramics included), 55 x 10 x 10 mm having a notch machined across one of... 

Tensile tests of notched specimens from plain and welded plate have been made to measure the notch sensitivity of aluminum-magnesium alloys at low temperatures. For the plain plate, specimens with four different notches, as shown in Fig. 2b were tested at room temperature and at -320 F. For the welded plate, Type I specimens from across the weld with the deep sharp notch machined in the center of the weld itself were tested at the same two temperatures. The results were compared with those obtained on standard 1/2 in. diameter tensile specimens (ASTM E8-57T). 

The four-point bending method requires a simple bend bar of the coated substrate with a notch machined in the coated layer (Fig. 6.1). The four-point bend delamination... 

Figure 2-39. Notching machine for impact test bars. (Courtesy of Testing Machines, Inc.)...

Nelson, D.V., Ricklefs, R.V., Evans, W.P. The role of residual stresses in increasing long life fatigue strength of notched machine members. Achievements of High Fatigue Resistance in Metals and Alloys. ASTM STP 467, 228-253 (1970)... 

A photograph of the DNS test fixtures and the test specimen is shown in Figure 2. Failure of the specimen is expected to occur by shear in the joint layer between the two offset opposing notches machined halfway through the thickness. The DNS specimens were machined with dimensions of 20 x 6 x 7.6 mm from SiC-MAX-SiC joined panel. The distance between the notches is 3 mm, the notch width is 0.5 mm, and the notch raius of curvature is 0.2 mm. The notch roots sit inside the joint section. The apparent shear strength is calculated by dividing fracture force by the joint area between the notches. The test frame crosshead displacement was 0.6 mm per minute. 

EWF tests were carried out with DENT specimens based on the ESIS TC4 protocol [4], prepared with these thin-wall injection moulded plastics, as shown in Figure 1. Notches were induced by a notching machine (MARUTO Testing Machine Company) which slides in a razor blade 1mm ahead of a machined notch, to make N=5 specimens each for five nominal ligament length of L, = 5.00 mm, L = 6.62 mm, L, = 8.24 mm, L = 9.86 mm, L = 11.48 mm (< M3). 

Shear Horizontal (SH) waves generated by Electromagnetic Acoustic Transducer (EMAT) have been used for sizing fatigue cracks and machined notches in steels by Time-of-Flight Diffraction (TOED) method. The used EMATs have been Phased Array-Probes and have been operated by State-of-the-art PC based phased array systems. Test and system parameters have been optimised to maximise defect detection and signal processing methods have been applied to improve accuracy in the transit time measurements. 

Due to the limitation posed by the initial electrical leakage signal and by the chosen angle of incidence of 52 deg. diffracted signals from 8 mm deep machined notch were hidden. Defects with depth exceeding 12 mm could be detected and sized. The same difficulty was observed when the thickness of the sample was less than 30 mm. 

The second parameter, tear strength, describes the film resistance to tear propagation. It is measured with a special apparatus, the Ehnendorf Tear Tester (ASTM D1922), and defined as the weight of a loaded pendulum capable of tearing a notched piece of film. Two values are usually measured for each film sample. One determines tear propagation in the machine direction of the film, the other in the transverse direction. 

Depth-of-Gut Notching. Depth-of-cut notching (DOCN) is a localized wear process common when machining materials such as austenitic stainless steels or high temperature alloys. Notching is attributed to the chemical reaction of the tool material and the atmosphere, or to abrasion by the hard, sawtooth outer edge of the chip. DOCN may lead to tool fracture. 

The data presented in Figure 19.7 were obtained on a Sonntag-Universal machine which flexes a beam in tension and compression. Whereas the acetal resin was subjected to stresses at 1800 cycles per minute at 75°F and at 100% RH, the nylons were cycled at only 1200 cycles per minute and had a moisture content of 2.5%. The polyethylene sample was also flexed at 1200 cycles per minute. Whilst the moisture content has not been found to be a significant factor it has been observed that the geometry of the test piece and, in particular, the presence of notches has a profound effect on the fatigue endurance limit. 

The common heater block shown in Fig. 19.23 can itself be subject to corrosion leading to different heat transfer conditions for different tubes, and in some later versions of the apparatus individual short heating jackets are used for each tube, which are heated with oil from either a steam-heated or electrically heated heat exchanger. This modification not only avoids corrosion problems but also obviates the necessity to machine a length of the outside of each tube to fit the semi-circular notches in the single heater block. The oil flow is adjusted to give an oil temperature of 95°C at each outlet. 

The popular Izod impact tester can use different size specimens depending on the type of plastic and their method of fabrication. The specimen is usually 1/8 in. x l/2 in. x 2 in. other sizes are also used. Specimens can be notched or unnotched. A notch is cut in a specified manner on the narrow face of the specimen. The sample is clamped in the base of a pendulum testing machine so that it is cantilevered upward with the notch facing the direction of impact. The pendulum is released, and the force expended in breaking the sample is calculated from the height the pendulum reaches on the follow-through. The speed of the pendulum at impact is controlled. 

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