Three-point bending tests

Instron Three-Point Bending Failure. Three-point bending tests have also been conducted on notched Izod specimens at crosshead rates of 0.02-20 inches/min (Figure 5). Plots of work to break obtained from the areas under the Instron force-displacement traces show abrupt ductile-brittle transitions these are displaced to lower temperatures as the test rate is decreased. However the temperature interval between ductile-brittle transitions of the two materials remains about the same. The force-displacement trace for each specimen shows a yield point with a ductile failure but a sharp termination when the sample breaks in a brittle fashion. 

Instrumented impact tests, three-point bending, bending force, analogical model, inverse problem. 

Flexural tests may be carried out in tensile or compression test machines. In standard tests, three-point bending test is preferred, although it develops maximum stress localized opposite the center point (support). If the material in this region is not representative of the whole, this may lead to some errors. Four-point test, offers equal stress distribution over the whole of the span between the inner two supports (points) and gives more realistic results for polymer blends (Figure 12.3). Expressions for the calculation of flexural strength and modulus for differently shaped specimens are given in Table 12.4. 

Two methods are used for bend testing—three- and four-point bending tests. Here, the specimens are rectangular, without notches. It is obvious from Fig. 1.48 that the applied force (downward arrows) is compressive by nature, resisted by the tensional force (upward arrows). Thus, the longitudinal stresses at the lower surfaces (convex) in the specimens are tensile and compressive at their upper surfaces (concave). As a consequence, a calculable bending moment develops. The modulus of rupture is the stress of the specimen at its failure and represents the flexural strength of the specimen. 

Tensile tests CSR Tensile test fatigue 1 Tensile test fatigue 2 Tensile creep Three-point bending CSR Three-point bending fatigue 1 Three-point bending creep... 

FIGUREl.l Schematic of flexural strength test fw ceramics (a)Three-point bend, (ft) three-point loading, three-point bend, and (c) quarter-point loading, four-point bend. 

As a further step towards the optimization of the coating process, the influence of the operational variables on the final composite mechanical properties was studied. Four different parameters (polymer powder size, fiber pull speed, and furnace and PCT die temperatures) were independently varied. Then towpregs were produced under preset conditions, and composite plates were obtained by compression molding and subjected to mechanical testing (three-point bending and tensile tests). The overall results are summarized in Table 3. 

Fig. 6. Fracture toughness test specimens (a) single-edge notch (b) center notch (c) compact tension and (d) three-point bend. Terms are defined in text.

ROR = ring-on-ring bending FP = four-point bending TT = tensile test and TP = three-point bending. 

FIG. 20-75 Fluid-bed erosion or wear rate as a function of granule material properties. Kq is fracture toughness and H is hardness as measured hy three-point bend tests. [Ennis [Pg.1888]

In the deflection temperature under load test (heat distortion temperature test) the temperature is noted at which a bar of material subjected to a three-point bending stress is deformed a specified amount. The load (F) applied to the sample will vary with the thickness (t) and width (tv) of the samples and is determined by the maximum stress specified at the mid-point of the beam (P) which may be either 0.45 MPa (661bf/in ) or 1.82 MPa (264Ibf/in ). 

Fig. 18. Yield strengths in three-point bend tests of highly filled composites of polyfvinyl butyral) and silica particles treated with methylsilane and octylsilane coupling agents to varying degrees of surface coverage vs. work of adhesion measured independently using IGC. Redrawn from ref. [90].

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. 

Flexural modulus is the force required to deform a material in the elastic bending region. It is essentially a way to characterize stiffness. Urethane elastomers and rigid foams are usually tested in flexural mode via three-point bending and tite flexural (or flex ) modulus is obtained from the initial, linear portion of the resultant stress-strain curve. 

Flexural strength is determined using beam-shaped specimens that are supported longways between two rollers. The load is then applied by either one or two rollers. These variants are called the three-point bend test and the four-point bend test, respectively. The stresses set up in the beam are complex and include compressive, shear and tensile forces. However, at the convex surface of the beam, where maximum tension exists, the material is in a state of pure tension (Berenbaum Brodie, 1959). The disadvantage of the method appears to be one of sensitivity to the condition of the surface, which is not surprising since the maximum tensile forces occur in the convex surface layer. 

We perform flexural testing on polymer rods or beams in the same basic apparatus that we use for tensile or compressive testing. Figure 8.6 illustrates two of the most common flexural testing configurations. In two-point bending, shown in Fig. 8,6 a), we clamp the sample by one end and apply a flexural load to the other. In three-point bending, shown in Fig. 8.6 b), we place the sample across two parallel supports and apply a flexural load to its center. 

We use a variant of flexural testing to measure a sample s heat distortion temperature. In this test, we place the sample in a three point bending fixture, as shown in Fig. 8.6 b), and apply a load sufficient to generate a standard stress within it. We then ramp the temperature of the sample at a fixed rate and note the temperature at which the beam deflects by a specified amount. This test is very useful when selecting polymers for engineering applications that are used under severe conditions, such as under the hoods of automobiles or as gears in many small appliances or inside power tools where heat tends to accumulate. 

The short beam shear test designated in ASTM D 2344 (1989) involves loading a beam fabricated from unidirectional laminate composites in three-point bending as... 

Fig. 3.18. (a) Shear stress eontours and (b) shear stress distributions aeross the thickness of a three-point bending specimen in a short beam shear test. After Cui and Wisnom (1992). Reproduced by permission of... 

Boukhili, B., Hubert, P. and Gauvin, R. (1991). Loading rate effect as a function of the span-to-depth ratio in three-point bend testing of unidirectional pultruded composites. Composites 22, 39-45. 

ASTM D 2023-95a, Standard Test Method for Measuring the Dynamic Mechanical Properties of Plastics Using Three Point Bending, ASTM, West Conshohocken, PA, 1997. 

Tensile moduli were measured from standard dog-bone samples (2.0 mm thickness, 4.7 mm width, and 22.0 mm gauge length) in a Model 1122 Instron. Flexural modulus was determined using a testing apparatus which consists of two aluminium/steel pieces attached to the Instron which is fitted with a tensile load cell. This device effectively performs an inverted three-point bend the two side bars remain stationary above the sample as the central bar below the sample moves upward. Flexural samples measured ca. 52.0 x 1.7 x 13.1 mm and were tested using a 25.4 mm span (distance between the two side bars). Crosshead speed (CHS) for both flexural and tensile testing was 1.0 mm/min. 

Pig. 1 a and b. Fracture toughness test specimens a three-point bend and b compact tension. Pre-crack length denoted by a. Movement of crosshead is vertical for both... 

This is discussed in standard tests on elasticity, and the anisotropic case in Lekhnitskii171. The simplest assumption is that of the Bernoulli-Euler theory in which plane sections remain plane and Poisson contraction is ignored. Young s modulus measured by the deflection of a simply supported beam under three point bending is given by... 

---