Abstract
The mechanical durability of paper is a key consideration for applications ranging from shipping boxes to disposable medical substrates. Paper commonly experiences fatigue loading in such applications, but a high-cycle fatigue mechanism has not been identified. This research details paper’s high-cycle fatigue degradation mechanism. Paper specimens were loaded with monotonically increasing, constant, and sinusoidally varying cyclic stresses, and the resulting tensile, creep, and fatigue damage accumulation rates were compared. The difficulty in defining the size and growth of cracks in paper’s cellulosic fiber network were overcome with optically measured strain fields. We found that fatigue damage can accumulate via a fiber fracture mechanism, while ratchetting, creep, and tensile overload damage accumulation occurs due to failure of inter-fiber bonds. We also discovered the synergistic interaction between creep and high-cycle fatigue damage accumulation mechanisms, which is critical for extending the high-cycle fatigue life of paper.
Highlights
The mechanical durability of paper is a key consideration for applications ranging from shipping boxes to disposable medical substrates
As the web moves through the mill along this “machine direction (MD),” water is removed and the fibers bond to each other
Commercial copy paper has a strongly bonded fiber network, so its stress–strain behavior is qualitatively similar to structural metals—linear, elastic deformation is followed by permanent, nonlinear deformation that ends with catastrophic failure (Fig. 1)
Summary
The mechanisms that control the deformation, degradation, and failure of wood fiber pulp-based paper and packaging materials depend on how the stresses are applied (Fig. 2). A constant load can cause time-dependent damage via processes known as creep, again causing fracture surfaces that are hairy in appearance (Fig. 3e) because of a time-dependent interfiber bond failure and fiber pull-out fracture mechanism. A longer fatigue damage region (fractured fibers) was observed in the crack profile of long-life specimens (Fig. 3d) (Nf = 98,808 cycles at stress range of 5.4 MPa). Time-dependent, and cyclic damage accumulate in most organic and inorganic materials in ambient laboratory air environments These three types of damage accumulation mechanisms enable subcritical crack growth that can cause delayed failures in specimens, components, and structures. Distinguishing pure cyclic fatigue from static and time-dependent crack growth is challenging because all three mechanisms can synergistically and simultaneously cause crack growth during cyclic loading
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