Stress evaluation of crystalline polymers using X-ray diffractometry

Highlights

• Demonstrates that X-ray diffraction can quantitatively measure stress in crystalline polymers, opening a practical path for evaluating residual stress in engineering plastics.
• Shows that the polyacetal (100) diffraction plane is highly sensitive to applied stress, while the (105) plane is largely insensitive because of differences in crystal bonding directions.
• Establishes that high-quality optics, parallel-beam geometry, and experimentally determined X-ray elastic constants are essential for accurate polymer stress measurements.

Summary

The increasing use of engineering plastics as lightweight structural materials has created a need for reliable methods to quantify residual stress introduced during molding. While X-ray stress measurement is well established for metals, applying the technique to crystalline polymers presents unique challenges because polymers typically produce diffraction peaks only at relatively low diffraction angles, exhibit viscoelastic behavior, and allow deeper X-ray penetration, all of which reduce strain sensitivity and measurement precision.

This work evaluates the feasibility of X-ray stress measurement in injection-molded polyacetal (POM) using the sin²ψ method. Tensile specimens, both untreated and annealed, were characterized by conventional XRD and two-dimensional WAXS before tensile loading. Annealing produced slightly narrower diffraction peaks and marginally higher crystallinity, indicating improved crystal quality while leaving the overall crystallinity and molecular orientation largely unchanged. WAXS confirmed the expected molecular orientation generated during injection molding.

Stress measurements focused on the (100) and (105) diffraction planes during controlled tensile loading. The (100) reflection exhibited clear, measurable shifts in the 2θ-sin²ψ relationship as applied stress changed, enabling accurate determination of the X-ray stress constant. Annealed specimens produced substantially improved linearity and reduced scatter because of their sharper diffraction peaks. In contrast, the (105) reflection showed little response to applied stress despite its higher diffraction angle.

The difference between the two reflections is explained by polyacetal's crystal structure. The (100) plane corresponds primarily to intermolecular bonding along the a-axis, where weaker intermolecular forces allow measurable lattice deformation. The (105) plane is oriented along the c-axis, where strong covalent bonds within the polymer chains greatly reduce elastic deformation. This demonstrates that selecting diffraction peaks based solely on diffraction angle is insufficient; the crystal structure and deformation mechanism of the polymer must also be considered.

The results demonstrate that X-ray diffraction can provide accurate stress evaluation for crystalline polymers when appropriate diffraction peaks, instrument optics, and experimentally determined X-ray elastic constants are used. The study also establishes a practical methodology for qualifying measurement conditions using tensile specimens before applying X-ray residual stress analysis to molded components, while identifying future opportunities for extending the technique to reinforced polymers, additional engineering plastics, and amorphous materials.