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Stop, wait and restart: the three-step protocol looks simple, yet stress, orientation, stretch and network constraints can forget on different clocks. The restart transient is a functional test of what the rest interval did not erase.
Open the chapter contents · Polymer Science root · Gold Standard of Education · How X Works
Chapter contents
Choose the route closest to the question, or read from Chapter 1 for the complete argument. The chapters remain open on the page; these grouped routes are an index.
Route 1 · Define interrupted flow
Route 2 · Work the restart ledger
Route 3 · Test direction and cycles
Route 4 · Check limits and evidence
Route 5 · Ask and continue
CHAPTER 1 OF 12 · DEFINE INTERRUPTED FLOW
1. An interrupted flow has three histories
An interrupted-shear test has a first shear interval, a rest interval and a resumed shear interval. The response after restart depends on what relaxed during rest: stress, chain orientation, stretch, entanglement constraints, reversible associations or larger-scale structure.
“The sample rested for ten seconds” is not enough. Record the first shear rate and duration, strain at interruption, rest boundary, restart direction, temperature and the full restart signal.
CHAPTER 2 OF 12 · DEFINE INTERRUPTED FLOW
2. Separate fading memory from full reset
If the restart response matches a fresh start, the measured variables may have largely reset under that protocol. If the restart peak is smaller, shifted or direction-dependent, some memory remains. That memory need not be stored in one scalar structural parameter.
Parallel restart probes persistence along the original deformation direction. Orthogonal restart can expose tensorial orientation that a parallel test misses.
| Restart protocol | Question | Caution |
|---|---|---|
| same direction | how much of the original flow memory remains | different mechanisms can yield the same peak |
| reverse direction | whether stored orientation changes the sign response | reversal can create its own transient |
| orthogonal direction | whether memory is directional | geometry and normal stresses matter |
CHAPTER 3 OF 12 · WORK THE RESTART LEDGER
3. Work one restart ledger
Imagine a polymer melt whose fresh startup peak is 120 Pa at strain 2.4. After first shearing past its peak, it rests for 20 seconds and is restarted in the same direction. The new peak is 84 Pa at strain 1.6.
A simple peak-recovery index is 84/120 = 0.70. This says the restart peak is 70% of the fresh value. It does not say that the material is “70% recovered”, because peak height mixes orientation, stretch, entanglements and the imposed protocol.
| State | Peak stress | Strain at peak | Indexed peak |
|---|---|---|---|
| fresh startup | 120 Pa | 2.4 | 1.00 |
| restart after 20 s | 84 Pa | 1.6 | 0.70 |
| missing evidence | structure and orientation | direction-dependent | not inferred |
CHAPTER 4 OF 12 · WORK THE RESTART LEDGER
4. Keep the rest boundary explicit
During rest, a fixed tool position, zero commanded shear rate and zero measured stress are different boundaries. Elastic recoil can occur if the boundary permits motion. Instrument compliance can also release strain while the command reads zero.
The shear-cessation guide separates stress relaxation, recoil and structural recovery. Interrupted flow adds a functional test: what happens when deformation resumes?
CHAPTER 5 OF 12 · WORK THE RESTART LEDGER
5. Read peak recovery across rest time
Use several rest times spanning the relevant relaxation spectrum. A short rest may recover fast chain stretch while preserving orientation; a longer rest may allow orientation and constraints to relax. One exponential fit is a summary unless a constitutive model justifies it.
Plot both peak stress and strain to peak. A restored height with a shifted peak position can still show incomplete reset.
CHAPTER 6 OF 12 · TEST DIRECTION AND CYCLES
6. Changed context: turn the restart ninety degrees
Keep the 20-second rest but restart orthogonally. Suppose the peak becomes 102 Pa rather than 84 Pa. The scalar rest time is unchanged, yet the response differs because residual orientation relative to the new flow has changed.
This transfer check rejects the idea that one “degree of recovery” fully describes the state. It also motivates measuring normal stresses, chain orientation or scattering when the molecular explanation matters.
CHAPTER 7 OF 12 · TEST DIRECTION AND CYCLES
7. Distinguish rejuvenation from resumed deformation
In thixotropic and ageing materials, strong shear may reduce structure and reset part of the age distribution; this is often called shear rejuvenation. In polymer melts, interrupted-flow memory may instead be discussed through orientation, stretch and entanglement dynamics.
Do not use “rejuvenation” as a universal synonym for lower viscosity. Define the structural or rheological quantity that was reset and how a fully rejuvenated reference was prepared.
CHAPTER 8 OF 12 · TEST DIRECTION AND CYCLES
8. Design cycles without erasing the evidence
Repeated shear–rest cycles can reveal convergence, training, damage or drift. Use identical first intervals and rest times before changing one factor. Randomise long and short rests where ageing or temperature drift could otherwise mimic memory.
The cyclic-loading guide provides return-map logic for distinguishing repeated coordinates from reversible local structure.
CHAPTER 9 OF 12 · CHECK LIMITS AND EVIDENCE
9. Limits and misconceptions
The worked peak index is not a constitutive parameter. Wall slip, edge fracture, shear banding, torque limits and temperature can change the restart transient. Unentangled melts, entangled melts, associating networks and particle-filled gels need different mechanisms.
A lower restart overshoot is not direct proof of lost entanglements. Orientation and segment stretch can dominate under some conditions. A recovered steady stress does not prove that transient memory has disappeared.
CHAPTER 10 OF 12 · CHECK LIMITS AND EVIDENCE
10. Build an evidence check
Store torque, angular position, normal force, gap and temperature continuously through stop and restart. Repeat fresh startup controls between cycles. Test at least two gaps or roughened fixtures when slip is plausible.
Report restart peak, strain to peak, stress just before interruption, rest-time relaxation and steady state. Add structural evidence where the claim requires it. Use Stress Overshoot, Start-Up Shear and Transient Network Breakdown for the loading-side controls.
CHAPTER 11 OF 12 · ASK AND CONTINUE
11. Questions readers should ask
Is the restart peak a memory meter?
It is a protocol-dependent memory descriptor. It is not a unique measurement of one molecular variable.
Why vary restart direction?
Directional tests can reveal tensorial memory hidden by a same-direction restart.
When is rejuvenation an appropriate word?
When the article defines the age or structural variable being reset and provides a reproducible reference state.
CHAPTER 12 OF 12 · ASK AND CONTINUE
12. Continue through verified routes
Begin with the Polymer Science root, Gold Standard of Education and How X Works. Continue through shear cessation, startup overshoot and cyclic loading above, then Amplitude Sweeps, Yield Onset and Cycle-to-Cycle Training.
Primary sources include Venerus and Kahvand’s interrupted shear flow of unentangled polystyrene melts, Masubuchi, Doi and Uneyama’s primitive-chain simulations of interrupted shear, and Cunha, Olmsted and Robbins’ orthogonal interrupted-shear simulations. The restart ledger is an original teaching example.
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