Crosshead displacement

The double torsion test specimen has many advantages over other fracture toughness specimen geometries. Since it is a linear compliance test piece, the crack length is not required in the calculation. The crack propagates at constant velocity which is determined by the crosshead displacement rate. Several readings of the critical load required for crack propagation can be made on each specimen. 

Stress-Strain Data. Tensile tests were made with an Instron tester at some seven crosshead speeds from 0.02 to 20 inches per minute at five or six temperatures from 30° to —46°C. The tests were made on rings cut with a special rotary cutter from the circular sheets of the elastomers. The dimensions of each ring were determined from the weights of the ring and the disc from its center, the thickness of the ring, accurately measured, and the density of the rubber. Typically, the outside and inside diameters were 1.45 and 1.25 inches, respectively, and the thickness was about 0.085 inch. The test procedure used is described elsewhere (11), and the cubic equation, eq 4 in ref. j 2, was used to compute the average strain in a ring from the crosshead displacement. 

Studies of the tensile properties concerned true stress-true strain measurements taken by means of an INSTRON Tensile Testing Machine and video system. The course of the deformation process running at 20%/min. (2 mm/min.) rate was recorded by means of a video camera. Observations were performed on the volume elements of each sample (see Fig. 2). Changes of sample thickness were measured with a properly modified micrometer screw. Elasticity moduli E, elasticity limit eg and elongation at yield ey were determined on the basis of registered load - crosshead displacement curves. 

Quasi-static and dynamic tests were conducted at the DLR Institute of Stmctores and Design, Stuttgart. The quasi-static test was conducted in a Zwick 1494 servo-hydraulic uniaxial loading frame (max. loads 500 kN, max. crosshead displacement 850 mm) with the stmcture test setup as shown in Figure 10.14(a). Vertical compression loads were applied by the test machine crosshead via a load cell to the test stmcture through the I-beam and measured at the load cell and on the load platform. The test was performed initially at a crosshead velocity of 5 mm/min for the first 20 mm of crosshead displacement, then increased and maintained at 20 mm/min until final collapse at 62 mm crosshead displacement. From the measured load-deflection curve the absorbed energy at failure can be calculated, which was measured to be 6.3 kJ at 62 mm displacement. This energy represents... 

These tests were performed using an Instron Model 8561 (single screw) machine in air and the furnace was adapted to perform four-point bend tests. The rates indicated in Fig. 2.3 relate to crosshead displacement. Figure 2.4 shows the resolved shear stress at yield for the specimens tested ate = 4.2 x 10 s above Tc at the indicated orientations. The mechanism for slip is dislocation glide, which explains the orientation dependence of yield, as seen in Fig. 2.4. Thus, the BDT temperature, Tc, of the sapphire (AI2O3) varies not only with the strain rate, but also with the crystallographic orientation of the fracture plane. 

Recall that A1 is the displacement (change in dimensions of the specimen). Denoting the rate of the crosshead displacement, I, and if it is assumed to be constant during the test, then the initial strain rate is sq, which is given by ... 

Reduced (dimensionless) volume element Energy per unit volume Work of adhesion Crosshead displacement Crosshead speed... 

Simple beam equations are used to determine the stresses on specimens at different levels of crosshead displacement. Using traditional beam equations and section properties in Fig. 2.5, the following relationships can be derived where Y is the deflection at the load point ... 

Fig. 7.12 Detected electric charge during cyclic three-point-bending in dependency of crosshead displacement.

Fig. 1 Typical load-crosshead displacement curves of (a) C- and (b) BN-SiCf SiC composites after three-point bending test at room temperature.

Figure 7.24 Effect of crosshead displacement rate on the measured fracture energy, Gic, and type of crack growth [108]. (a) DGEBA-TEPA adhesive (b) DGEBA-TETA adhesive. O stable brittle crack growth (Type C) A initiation and A arrest of unstable crack growth (Type B). (Test temperature = 22 °C TDCB joints aluminium alloy substrates.)...

The stress-strain behavior of a material provides important information relevant to its range of applicability. Load bearing applications may require certain stiffness or strength properties, the latter of which has been addressed in (see Chap. 19). For strain-controlled loading modes experienced by sealants, the modulus must be sufflciently low and the strain capabilities sufficiently high to provide adequate flexibility to meet the mechanical or thermally driven deformations. Due in part to the popularity of screw-driven test frames, most stress-strain characterization experiments have traditionally been carried out at a constant crosshead displacement rate, effectively straining the specimen at the desired rate. Results obtained are often quite rate and temperature dependent, so care is needed in reporting these details. 

The test speed recommended by the standard ISO 11003-2 is 0.5 mm/min. However, a constant crosshead displacement rate will induce an accelerating strain rate in the adhesive once it starts yielding, influencing the yielding properties. This is a common feature of all the joint tests, contrarily to the bulk tests where the strain rate is much more constant for a given crosshead rate. To have a constant adhesive strain rate, the crosshead speed can be controlled using the adhesive displacement measurements. 

Peel force versus crosshead displacement obtained during fixed-arm peeling at 10 mm/min at an angle of 90°. The adhesive used was Bondmaster ESP110 and the peel arm was 0.1-mm thick aluminum... 

Figure 17 Stress-crosshead displacement behavior of ZrBj single crystal deformed at 2125°C. (From Ref. 66.)...

As stated earlier, adhesion is a major concern in electronic applications involving thin polyimide films either coated on hard substrates or laminated with metal ribbons. In these cases, neither lap-shear nor die-shear techniques allow the determination of the adhesion strength this can be done by using either the 90° peel test or the island blister test whose principles are sketched in Figs 12.26 and 12.27. The 90° peel test provides reliable data for the measurement of practical adhesion, especially useful for comparing the effect of surface treatments on the interfacial adhesion. The standard peel test procedure has been modified to determine the adhesive strength of thin polyimide films coated onto 10-cm silicon wafers. The equipment illustrated in Fig. 12.26 maintains a 90° peel effort during the test conducted at room temperature with a constant rate of crosshead displacement of 2 mm min . ... 

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. 

In Figure 8, the torsion test showed extremely large test frame crosshead displacement of 11 mm because of slack in the complex fixturing system and also because of deformation of the soft aluminum coupler relative to the hard CVD SiC. The latter effect is discussed in the next section. The stress-displacement curve indicated britde failure of the joined torsion specimen in... 

Failure of the substrate material (CVD SiC) was observed for the torsion specimens, as shown in Figure 11. Deformation of the aluminum coupler and hence the relative rotation between the coupler and the joint specimen can be seen, which contributes partly to the large crosshead displacement observed in the torsion test. 

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