A Viscodamage Model to Simulate Both Fatigue and Static Failures in Soft Fibrous Tissue

Musculoskeletal soft tissues in joints, including ligament, tendon, and meniscus, are frequently torn, leading to pain, joint instability, and increased risk of osteoarthritis. Tissue tears are most commonly associated with single high-magnitude loading events, known as static failures; but tears may also develop from repeated exposure to low-magnitude loads, known as fatigue failures. The risk of fatigue failures in conventional materials (e.g. polymers, composites) has been greatly reduced by the development of failure models that accurately predict fatigue damage, yet comparable fatigue models currently do not exist for soft fibrous tissues.

 

Fatigue damage can be mathematically described and predicted through constitutive models. Constitutive frameworks that have had success in simulating the damage processes of soft biological tissue include continuum damage mechanics (CDM), pseudoelasticity, and elasto-viscoplasticity where most of these models have been validated with experiments that use displacement control loading. A loading condition that is more physiological to fatigue injury is repeated application of a targeted stress (force-control), which results in three characteristic stages of cyclic creep (stages I, II, III). However, to our knowledge, no study has modeled force-control fatigue data in any soft tissue. Moreover, no study has used the same constitutive model to simulate both fatigue and static failures in soft tissues.

 

A novel and elegant solution for modeling cyclic creep behavior in soft tissue may be possible using discontinuous CDM with viscoelasticity. Previous work has determined that discontinuous CDM with strain energy-based failure criteria can model static failure in soft tissue but has intrinsic limitations in modeling steady state creep since damage evolution is restricted under a constant load. Conversely, viscoelastic models can represent steady state creep, but not damage propagation to rupture. We hypothesize that by pairing a discontinuous CDM model with a viscoelastic model, damage will evolve in response to viscoelastic creep during fatigue loading. The objective of this study is to determine the feasibility of using a viscodamage model to simulate high-cycle tensile loading in human meniscus. A validation study was performed to determine if the viscodamage model is also predictive of static failure.