To investigate structural modification during deformation X-ray scattering experiments were performed on samples mounted in a miniaturized tensile rig placed in the synchrotron X-ray beam. The synchrotron radiation is necessary to have sufficient intensity to get high time resolution. A general description of the equipment was given by Davies et al.14
The actually used experimental arrangement and the specimen geometry are illustrated in Figure 1.
Figure 1. Sketch of the experimental arrangement for SAXS and WAXS during deformation (left) and waisted specimen (mini-dumbbell) used for simultaneous structure and mechanical investigation (right), the dimensions of the specimen can be scaled.
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To investigate local strain-dependent properties, small waisted specimens were used to concentrate the stress in the center of the specimen. By using a relatively large radius of curvature (12 mm) compared with the specimen width (3 mm), the stress state is in good approximation maintained constant in the middle of the specimen. The strain was determined optically by observing the deformation of a grid pattern applied on the specimen surface. The grid pattern was applied using a self-made flexible ink and a mesh size of 0.35 mm. The middle of the specimen, where the beam crosses, was left blank. Alternatively, also a classical image correlation analysis for strain estimation can be used. A comparison of stress-strain curves of the waisted specimens with standardized dog-bone specimens show a good consistency: The curve progression from the beginning to the yield point as well as during strain hardening, that is, where the parallel region of the dog-bone specimen is deformed in a uniform way, are identical. For the range of neck formation and propagation in the dog-bone specimen, an estimation of true stress and strain via global strain measurement are not possible. Only a local strain measurement solves the problem. The local measurement of the waisted specimens provides true stress and strain data in a good approximation.
In this context, the specimens are assumed as incompressible. This allows calculating the true stress σt from the measured force F, initial cross section A, and tensile strain εt as σt = F × (1+εt)/A.
To characterize the structure at certain strains mostly step-loading experiments were performed, stretching the sample to a certain strain and then recording the patterns at this constant strain. This is also the reason for the presented slightly fluttering stress-strain curves. In some cases also continuous stretching was performed.
To keep the beam in a fixed position relative to the gauge length of the waisted sample throughout the measurement, both grips were moved simultaneously in opposite direction.
To get simultaneous SAXS and WAXS patterns, the WAXS could be monitored only in a limited range. By using a horizontal tensile direction, the vertically arranged WAXS detector monitors mainly the equatorial scattering of the sample.
For the validity check, the sample was rotated around the tensile direction and the patterns were compared. To detect oriented crystallites, the sample can be rotated around an axis perpendicular to the beam.
To perform temperature-dependent tensile and scattering experiments, a small heating device locally blows preheated air at the sample.
Besides the global scanning of samples, the experimental setup also permits spatially resolved pattern recording, for example, around a crack tip, if it is used in a microfocus beamline. The described arrangement was successfully used investigating semicrystalline polymers during deformation (Schneider et al.16) and fracture (Schneider17, 18).
WAXS and SAXS measurements were performed at the synchrotron beamline BW4 at HASYLAB in Hamburg, Germany.13 The wavelength of the X-ray beam was 1.3808 Å, the beam diameter was about 400 μm. The SAXS images were collected by a two-dimensional MarCCD-detector (2048 × 2048 pixels of 79.1 × 79.1 μm2). The sample-to-detector distance was set to 4080 mm. The WAXS images were collected by a two-dimensional PILATUS 100K-detector (487 × 195 pixels of 172 × 172 μm2). By a special procedure, the position of the tilted WAXS-detector was determined to have a tilt angle of 22.7°. The distance between the sample and the point of normal incidence was 247 mm.
Exposure times were chosen in the range of 5–60 s per pattern. The frame rate, determined by exposure and data storage, was 15–70 s per pattern.
For the discussion, all SAXS and WAXS 2d-patterns are shown with vertical tensile direction. For specimens with fiber symmetry, this means that the fiber axis and so the scattering vector s3 also are vertical, the scattering vector s12 always are horizontal.
For quantitative comparison of crystallinity, some DSC measurements were performed on drawn and undrawn samples using a DSC Q 1000 from TA-instruments. The specimens were measured between −80 °C and 230 °C with heating and cooling rates of 20 K/min. Before each scan, the sample was equilibrated at constant temperature for 300 s.
As qualitative check of the discussed structures, some SEM images of the drawn samples were taken on a Zeiss Gemini Ultra plus SEM.