IRHD Micro Hardness Tester

Fundamental Principle of Micro IRHD Indentation

The IRHD Micro Hardness Tester determines elastomer stiffness by measuring the penetration depth of a spherical indenter into a test piece under controlled static forces. Governing international standards, including ISO 48-2 (Method M), ASTM D1415, ASTM D1414, and FIAT 50408, define a differential, two-stage loading sequence to isolate actual specimen displacement from surface irregularities:

1. Initial Minor Contact (L₀): A gem-grade synthetic ruby sphere (0.395 mm diameter) presses into the sample with a contact force of 0.0083 N for 5 seconds inside an annular seating foot. This establishes an exact zero-displacement baseline.
2. Major Indentation Load (L₁): An additional indenting force is applied to bring the total load to 0.1533 N for a dwell period of 30 seconds, driving the ruby sphere deeper into the elastomer matrix.
3. Penetration Depth Conversion: The system measures the net displacement (d) between the minor and major loads, converting penetration depth into an International Rubber Hardness Degree from 0 to 100 at a display resolution of 0.01 IRHD points.

For isotropic elastic materials, penetration depth maps directly to Young’s modulus (E) using non-linear Hertzian contact equations:

F = (4/3) × E* × R1/2 × d3/2

An IRHD value of 0 represents a material with zero elastic modulus, while 100 represents an infinitely rigid body.

Method M (Micro) vs. Method N (Macro) Theory and Parameters

Standard macro testing (Method N) requires thick specimens to prevent hard support anvils from distorting the displacement calculation. Micro testing (Method M) scales down indenter geometry and applied forces proportionally, enabling non-destructive evaluations on thin sheets (≥ 1.0 mm), O-ring cross-sections, and small molded seals.

Because diffusion rates scale with sample volume, applying Method M on thin cross-sections allows direct monitoring of local crosslinking gradients, fluid swell, or thermal degradation across the thickness profile of a part.

Force Control and Laser-Guided Centering Methodologies

Conventional micro-testers use hanging weights or spring levers that introduce mechanical friction drag. Our IRHD Micro Hardness Tester incorporates modern force application and alignment methods:

• Load Cell Force Application Method: Direct dynamic load sensors replace mechanical weights, eliminating drag resistance and ensuring precise force application during both L₀ and L₁ loading phases.
• Ruby Sphere Contact Mechanics: Synthetic ruby provides low surface friction and extreme resistance to deformation, maintaining a constant sphere radius (0.395 mm) over thousands of test cycles.
• Laser-Guided Apex Alignment Method: Testing curved O-rings requires placing the indenter on the highest point of the cross-section. The unit pairs a Class 2 laser finder (0.002 mm sensor resolution) with a motorized sample stage (0.005 mm positioning accuracy) to scan the part profile, locate the geometric apex automatically, and position the sample beneath the indenter without manual adjustment.
• Automated Sample Handling Methods: Offers a sliding holder method for single-part positioning or a rotating disc holder method for continuous automated serial testing across multiple O-rings in a single sequence.

Viscoelastic Calculation Methods and Profile Scanning

Rubbers exhibit time-dependent viscoelastic responses during indentation. Our HardnessCheck analysis software executes specialized calculation methods during and after the test run:

• Stress Relaxation Angle Method: Measures force decay over time while holding constant indenter displacement, calculating the slope angle of the stress relaxation curve.
• Hysteresis Loss Calculation: Tracks mechanical energy dissipation by evaluating indenter recovery depth during load retraction.
• Thickness Correction Algorithm: When testing thin sheets (1.0 mm to 2.0 mm), proximity to the rigid support anvil causes artificially elevated hardness readings. The system applies algorithmic thickness correction formulas to eliminate anvil boundary effects.
• Part Scan Method: For complex, non-symmetrical profiles, the system scans specimen geometry, establishes multi-point measurement coordinates, and saves those spatial coordinates to specific part numbers for batch automation.

Precision Metrics and Calibration Verification Methods

Automating indenter placement with an IRHD Micro Hardness Tester reduces operator variance, securing a verified measurement repeatability of 0.22 IRHD. When evaluating low-modulus silicones or soft foam seals, applying the IRHD Supersoft (IRHD-ss) testing method provides far clearer material differentiation than Shore A scales.

System calibration follows a three-part verification method prescribed by ISO 48-2:

1. Geometric Verification: Optical quantification of ruby indenter sphere diameter and annular foot dimensions.
2. Force Sensor Verification: Direct load cell verification against traceable force standards at 0.0083 N and 0.1533 N loading thresholds.
3. Displacement Calibration: Optical measurement of vertical indenter displacement across defined loading intervals.