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Two terminal pull tests run at the same crosshead speed can apply different force rates to the joint. Part of the machine movement stretches the fixture and grip, while another part deforms the specimen. Changing that stiffness distribution changes how quickly the joint is loaded. A fair comparison therefore records the actual loading history and distinguishes a controlled crosshead speed from a controlled force or local deformation rate.
Measurement purpose
Determine whether terminal pull comparisons have equivalent actual loading rates despite differences in test-system stiffness or control mode.
Specimens and conditions
- Mechanical boundary
- Identified joint, terminal, support and grip geometry with specimen conditioning
- Rate definition
- Approved controlled variable and the pre-failure interval relevant to the comparison
Equipment and records required
- Mechanical test system: Suitable force and position acquisition under the authorized control and safety procedure
- Local observation: Defined local displacement evidence where needed to distinguish specimen movement from system compliance
Method sequence
- Method definition
Name the controlled and reported rate quantities
Record: Control mode and comparison requirement
- Rate verification
Evaluate actual force and displacement histories in a justified interval
Record: Measured slopes and model limits
- Comparison
Separate rate differences from construction effects
Record: Supported comparison or required matched-rate follow-up
Decision and uncertainty
Do not assume equal crosshead speed establishes equal force or joint-deformation rate; use the actual history and approved comparison requirement.
Changing stiffness, seating, slip, rate sensitivity and dynamic response can invalidate a constant series-stiffness interpretation.
The mechanical-test authority approves control mode and safe operation; the product owner defines the assembly comparison and acceptance requirement.
Traceable outputs
| Record | Required contents |
|---|---|
| Loading-history record | Control settings, actual force/time/position and evaluated rate interval |
| Comparison conclusion | Fixture stiffness changes, rate equivalence evidence, failure classes and unresolved influences |
Method review decisions
- Name the rate quantity required by the test method.
- Check how fixture and specimen stiffness distribute crosshead movement.
- Use measured pre-failure histories to verify rate comparability without inventing a universal correction.
Separate movement rate, force rate and local strain rate
Crosshead speed has units of length per time. Force rate has units of force per time. Local joint opening rate is another length-per-time quantity, measured between specific physical locations. Strain rate requires an additional defined gauge length and is not simply the displayed crosshead speed renamed.
Determine which quantity the approved test method controls and which it only records. An assembly-level harness pull may intentionally specify actuator movement, while a material comparison may require a different rate definition. Do not replace the contractual method with another control mode solely because it appears more sophisticated.
Trace where actuator movement is absorbed before failure
The terminal, joint, ceramic support, grips and machine can all contribute displacement. In a stable approximately linear loading interval, compliant elements in series carry the same force and share the total movement. A softer fixture absorbs more movement for each increment of force, reducing the force-rise rate at a fixed crosshead speed.
This rate consequence is different from subtracting a fixed fixture deflection from a displacement result. Even when the final displacement could be corrected, the specimen has already experienced the actual time-dependent loading history. A later correction cannot undo a rate-sensitive physical response.
Use a series-stiffness model only over a supported interval
For a simplified joint stiffness kj and surrounding test-system stiffness kf, equivalent stiffness is the reciprocal of the sum of their reciprocal stiffnesses. If both remain constant and the crosshead moves at speed v, the force rate is equivalent stiffness times v. The joint opening rate is that force rate divided by kj.
The equations exclude grip slip, seating changes, plastic deformation and dynamic inertia. They describe a limited pre-failure interval, not the full fracture process. Additional series elements can be included where their meaning and state are known, while a changing load path requires a more appropriate analysis.
keq = (1/kj + 1/kf)⁻¹; dF/dt = keq v; dδj/dt = (keq/kj) v
- kj is joint stiffness and kf is the remaining series-system stiffness in N/mm.
- v is actual crosshead speed in mm/s.
- δj is the defined local joint opening; dF/dt is force rate in N/s.
Quasi-static linear series response, constant stiffnesses, stable contact and no slip in the evaluated interval. This is not a prediction of terminal failure force.
Compare equal speeds with different fixture stiffnesses
Assume a joint stiffness of one hundred newtons per millimetre. With a remaining-system stiffness of one hundred newtons per millimetre, equivalent stiffness is fifty newtons per millimetre. At an illustrative crosshead speed of 0.1 millimetre per second, force rises at five newtons per second and joint opening at 0.05 millimetre per second.
If remaining-system stiffness increases to four hundred newtons per millimetre, equivalent stiffness becomes eighty newtons per millimetre. The same crosshead speed now produces eight newtons per second and 0.08 millimetre per second joint opening. The assumed sixty-percent rate increase is not a claim about any production fixture; it demonstrates why speed labels alone do not establish matched loading.
| Remaining-system stiffness | Equivalent stiffness | Force rate at v = 0.1 mm/s | Joint opening rate |
|---|---|---|---|
| 100 N/mm | 50 N/mm | 5 N/s | 0.05 mm/s |
| 400 N/mm | 80 N/mm | 8 N/s | 0.08 mm/s |
Measure the rate in the interval that matters to the comparison
Retain force and time together with actual actuator position and any suitable local displacement observation. Define the evaluation window relative to preload, seating and the intended failure event. A slope fitted through initial slack and a slope fitted through the later loaded interval describe different conditions.
Estimate rates over a justified window rather than differentiating noisy points without a method. Keep the fitted interval and residuals available. If the force history changes slope or the local response is nonlinear, report that behaviour instead of compressing the entire test into one apparently constant loading rate.
Do not attribute a rate-induced difference entirely to the joint process
A joining material or terminal construction may respond differently when loading occurs faster, but the magnitude and direction require appropriate evidence for that construction and state. Do not transfer a metal tensile-test percentage or an adhesive supplier example into a universal ceramic-terminal correction. Temperature and conditioning can also change the relevant response.
Where fixture stiffness changes between groups, compare actual histories before interpreting a peak-force difference as a joining improvement. A matched-rate follow-up may be necessary if the test objective depends on rate comparability. Preserve the original tests and the reason for the follow-up rather than silently replacing them.
Treat a control-mode change as a method change
Force control, crosshead-speed control and local deformation control can produce different trajectories, especially as the specimen stiffness changes near failure. A controller that maintains one variable may need to change another rapidly. The selected machine, sensor and control method must be suitable for the test and approved by the responsible laboratory.
Do not prescribe controller gains or attempt to maintain a simple linear rate formula through unstable fracture. The equation is a diagnostic comparison tool, not an instruction to force a machine through a changing specimen. Include safe stopping, fragment containment and specimen disposition in the authorized test procedure.
Release a rate-comparable result with its physical boundary
The record should identify the commanded control mode, actual time histories, evaluated loading interval, fixture revision and any local measurement. State whether the required rate was achieved and which rate quantities were only inferred under a model. Keep failure location and grip anomalies attached to each specimen result.
ChipSimple can review the terminal geometry and loading conditions needed for a project-specific comparison. A useful report distinguishes equal machine settings from demonstrated equal specimen conditions. It supports a bounded assembly decision without treating one peak, one speed or one compliance correction as an intrinsic joint-strength specification.
Define the loading rate for a terminal comparison
Provide the actual mechanical boundary and the rate quantity the result must represent.
- Terminal and support geometry with fixture revisions
- Approved force, movement or local deformation rate requirement
- Actual force-time and position-time records
- Preload, seating and comparison interval
- Conditioning, failure classification and local displacement evidence
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