Three-point flexure

The Imass Dynastat (283) is a mechanical spectrometer noted for its rapid response, stable electronics, and exact control over long periods of time. It is capable of making both transient experiments (creep and stress relaxation) and dynamic frequency sweeps with specimen geometries that include tension-compression, three-point flexure, and sandwich shear. The frequency range is 0.01—100 H2 (0.1—200 H2 optional), the temperature range is —150 to 250°C (extendable to 380°C), and the modulus range is 10" —10 Pa. 

As with tensile properties, both compressive strength and modulus depend on the fiber content and hber orientation (see Table 5.8). The interlaminar shear strength reported in Table 5.8 is a measure of the shear strength in the thickness direction of the SMC sheet. It is determined by three-point flexural testing of beams with short span-to-depth ratios and is considered to be a quality-control test for molded composites. 

Small samples were stressed in three-point flexure while being viewed with scanning electron or optical microscopes. The fracture behavior was recorded by video and correlated with acoustic emission measurements. The samples were oriented in the flexure-test fixture with the plane of the cloth plies perpendicular to the force direction therefore, the tensile and compressive stresses are primarily in the plane of the cloth plies where the fibers resist deformation. 

Static mechanical properties of aerogels have been measured mainly by the three-point flexure test [54] or by direct longitudinal compression [55]. The... 

The samples were exposed to dry, flowing air at 500°C, 600°C, and 700"C for periods out to 4000 hours. Five samples from each set were retained for as-prepared mechanical evaluation and at least five samples of each set were removed from the furnaces at intervals of 24, 100, 500, 1000, 2000, and 4000 hours. All samples were tested in three-point flexure at room temperature, using an outer loading span of 25.4 mm and a loading rate of 25 MPa/s. 

Another composite structure we have investigated recently is a particulate-filled epoxy. The epoxy is EPON 828 (Z-hardener) filled with irregularly shaped alumina particles with an average diameter of approximately 10/xm. This material is quite strong and brittle so we fractured most of the samples in a three-point flexure mode. The cross section of the sample was 2 mm x 6 mm. A typical EE curve plotted on a log scale is shown in Fig. 9, where t = 0 corresponds to the instant of failure. The material for this emission curve is filled at an Al203/epoxy ratio a of... 

The elastic modulus, measured using the pulse echo method, was about 180 GPa, and the fracture strength determined by the three-point flexure test was about 800 MPa in the grain-alignment direction. This strength, which is relatively high for the large porosity of 25%, is almost equivalent to that of commercially available... 

This measurement is performed by a three-point flexure method on short beams, with the span-to-width ratio chosen to produce interlaminar shear failure [1,2]. The geometry of this test is described in Figure 1. Fibres are oriented in the direction perpendicular to the axes of the supports. 

Three-point flexural tests provide values for the modulus of elasticity in bending, flexural stress, flexural strain, and the flexural stress-strain response of the tested material [63, 64], i.e., the determination of strain recovery and stress recovery ratio. 

Fig. 12 Scheme of a three-point flexural test, (a) Scheme of the shape-memory effect (SME) in polymers as defined by four critical temperatures. The value of 7 is a material property. Tiow is always less than Tg, whereas Tiugh may be above or below Tg, depending on the desired recovery response. (b) Schematic of the three-point flexure thermomechanical test setup. Taken from lef. [63], Copyright 2005. Reprinted with permission of John Wiley Sons, Inc. 

Besides the variation of the thermomechanical test set-up (e.g., cyclic, thermomechanical tests or three-point flexural bending tests) and the application of different types of programming and recovery modules as described in Table 2, the resulting... 

A similar behavior was observed in three-point flexural tests for epoxy based polymer networks (Tg = 90°C), where Tgw, Tg max, and (Tmax increased when ]8h was raised from 2.5 to 10Kmin [64]. 

Test method Three-point flexure Four-point flexure (a/W as 0.2... 0.3) ... 

This appeared as Eq. (1.12a) in Chap. 1). Next, the fracture toughness, Kj, was measured on single-edge crack-Uke specimens with notches of one-half the specimen thickness. The specimen was tested in three-point flexure over a 6-in. span on a commercial testing machine at a crosshead speed of 0.05 in./min. The Kic s were calculated from Brown and Strawley s [14] equation ... 

Woignier et al. (1989) used the three-point flexural technique to determine the of an aerogel prepared via sol gel technique concluding that the process parameters such as concentration of silicon compounds, catalytic conditions, and the preparation conditions affect the Young s modulus. They concluded that the Fa obeys the power law dependence relative to its density with the exponent of 3.7. 

---