When vulcanized rubber, elastomeric gaskets, or thermoplastic buttons are squished to a fixed height stop, they do not maintain constant push over time. Internal polymer chain realignments cause the material to relax, gradually reducing counter-force, a physical behavior defined as compressive stress relaxation. The Qualitest Stress Relaxation Test Rig, from our Stress Relaxation Tester – Creep Tester family, maintains a locked compression height while continuously logging force decay, isolating true elastomeric relaxation without structural frame flex.
Apparatus Design and System Mechanics
To capture compressive force decay accurately without equipment interference, a test rig must function as a rigid, unyielding structure. If the testing frame or pressure plates flex even a tiny fraction of a millimeter under load, that structural deflection feeds displacement back into the rubber specimen, distorting force decay readings.
┌─────────────────────────────────────────────────────────────┐
│ Manual Wheel Actuator Assembly │
│ (Smooth manual wheel drive for precision height stops) │
└──────────────────────────────┬──────────────────────────────┘
│
▼
┌─────────────────────────────────────────────────────────────┐
│ Ultra-Rigid Steel Support Frame │
│ (Built like a brick wall to stop all frame flex) │
└──────────────────────────────┬──────────────────────────────┘
│
▼
┌─────────────────────────────────────────────────────────────┐
│ LOAD CELL Force Sensor + U20 Monitor │
│ (1/20,000 resolution, ±1% load accuracy, multi-unit) │
└──────────────────────────────┬──────────────────────────────┘
│
▼
┌─────────────────────────────────────────────────────────────┐
│ 42 mm Compression Platens & Gauge │
│ (0.01 mm thickness gauge precision, 25 mm compression stroke)│
└──────────────────────────────┴──────────────────────────────┘
Qualitest System Assemblies for Rubber Testing
| System Assembly | Engineering Configuration | Practical Function in ISO 3384 Testing |
|---|
| Manual Compression Drive | Manual handwheel actuator with 25 mm compression stroke | Compresses rubber buttons to an exact height stop without needing complex automated motors |
| Force Transducer | High-precision LOAD CELL sensor (50 kgf capacity) with ±1% load accuracy | Logs continuous force decay across selectable units (Kgf, gf, Ton, lb.f, N, kN) |
| U20 Digital Indicator | Digital display unit with 1/20,000 decomposition resolution | Tracks real-time counter-force, percentage force loss, and compression metrics |
| Compression Platens | Dual 42 mm diameter pressure plates with 0.01 mm thickness gauge precision | Compresses rubber buttons evenly without surface binding or sliding friction |
| Compact Rig Frame | Rigid benchtop layout operating on single-phase 220V 3A (110V optional) | Prevents frame flexing during long-term testing while saving valuable laboratory bench space |
Why Mechanical Rigidity Determines Data Quality
In rubber compression testing, the apparatus is modeled as a rigid frame connected in series with a viscoelastic polymer sample. If the support frame flexes during initial clamping, it will slowly spring back as the rubber relaxes, introducing unexpected movement into the sample. The Qualitest Stress Relaxation Test Rig eliminates frame compliance, keeping specimen height frozen at the target compression ratio.
Control Modes and Compression Protocols
Evaluating seals, O-rings, and rubber buttons requires standardized fixed-height loading protocols:
Fixed Height Compression Protocol
Using the ergonomic handwheel drive, the technician compresses a cylindrical rubber specimen between the 42 mm pressure plates to a set height, typically 25% compression per ISO 3384. The U20 indicator locks the initial force value before logging continuous force decay over specified time intervals.
Single-Step vs. Multi-Step Testing
- Single-Step Compression: Compresses the rubber sample once to the target height, locks the position, and tracks force decay. This provides the cleanest, most unconfounded relaxation curve for seal lifespan prediction.
- Multi-Step Compression: Applies successive compression increments to the same sample. Because prior strain history remains trapped in the polymer matrix, standard step-loading superposition rules cannot be applied since force continuously bleeds away.
Viscoelastic Models for Rubber and Elastomers
To project how a rubber seal or gasket will perform years into service based on short-term laboratory tests, engineers fit compression decay data to viscoelastic mathematical formulations:
| Model Classification | Mathematical / Physical Mechanics | Application to Polymers & Elastomers |
|---|
| Maxwell Model | Connects an elastic spring in series with a viscous fluid dashpot. | Captures rapid initial force decay in vulcanized rubber buttons |
| Burgers Model | Combines Maxwell and Kelvin-Voigt elements in series. | Models both instantaneous elastic recovery and long-term viscous flow in sealing compounds |
| Standard Linear Solid (SLS) | Features two parallel branches: an elastic spring branch and a viscous dashpot branch. | Simulates equilibrium stress levels in structural thermoplastics and pipe gaskets |
| KWW Stretched Exponential | A flexible stretched exponential curve fitting multi-scale polymer chain relaxation. | Projects long-term stress retention in glassy thermoplastics and composite seals |
Force Decay Stages and Environmental Stability
Under constant compression, rubber samples exhibit three distinct phases of force reduction:
- Phase 1 (Initial Rapid Decay): Immediately after reaching target compression height, force drops sharply, accounting for 55% to 95% of total stress reduction as polymer chains realign rapidly.
- Phase 2 (Steady Linear Decay): The decay rate transitions into a stable, gradual decline over extended hold times.
- Phase 3 (Equilibrium Plateau): The decay curve flattens out, reaching long-term residual sealing force equilibrium.
Sealing Force
│
│ |\
│ │ \ Phase 1: Steep Initial Drop (55% to 95% force loss!)
│ │ \
│ │ └──────┐
│ │ \ Phase 2: Steady Gradual Decline
│ │ └─────────────────────────── Phase 3: Residual Sealing Plateau
│ │
└───────────────────────────────────────────────────────────── Time Elapsed
Environmental Thermal Controls
Thermal expansion in rubber or steel platens directly distorts fixed compression heights, making strict environmental controls mandatory:
- Temperature Stability: Ambient test chamber temperatures must remain locked within ±0.7 °C to prevent thermal expansion artifacts during long-term runs.
- Humidity Control: Relative humidity should be held within ±3.5% for moisture-sensitive elastomers and synthetic compounds.
Industry Standard Compliance
The Qualitest Stress Relaxation Test Rig is engineered specifically to meet global compression relaxation testing standards for elastomeric materials:
- ISO 3384-1: Rubber, vulcanized or thermoplastic, Determination of stress relaxation in compression, Testing at constant temperature.
- ISO 3384-2: Rubber, vulcanized or thermoplastic, Determination of stress relaxation in compression, Testing with temperature cycling.
- ASTM D6147: Standard Test Method for Vulcanized Rubber and Thermoplastic Elastomer, Determination of Force Decay in Compression.
- ASTM E328: Standard Test Methods for Stress Relaxation Tests for Materials and Structures (Compressive Testing Section).
