Elastomeric isolator Destructive Testing: A Complete Guide to GB/T 20688.1 Ultimate Property Tests

1. Introduction

Elastomeric isolator - also known as elastomeric seismic isolators - are critical components in the seismic protection of buildings and bridges. They support heavy vertical loads while allowing controlled horizontal movement, isolating structures from the damaging effects of earthquakes. But how do engineers prove that a bearing will survive extreme loading without catastrophic failure?

The answer lies in destructive testing - the process of deliberately loading a bearing until it breaks, buckles, or rolls over. In China, this is governed by the national standard GB/T 20688.1-2007, Elastomeric isolator - Part 1: Seismic-Protection Isolators Test Methods, a modified adoption of ISO 22762-1. In Europe, equivalent requirements are set by EN 15129:2018, Anti-seismic Devices. This article explains what destructive testing of elastomeric isolator involves, why it matters, and how GB/T 20688.1 structures these procedures - and compares them with EN 15129.

Elastomeric Isolator

2. What Is a Rubber Bearing?

A rubber bearing is a laminated device made of alternating layers of rubber and thin steel plates, bonded together during vulcanization. The steel plates confine the rubber laterally, giving the bearing high vertical stiffness while keeping horizontal stiffness low. Common types covered by the standard include:

LNR (Linear Natural Rubber Bearing) - provides elasticity without significant damping.

LRB (Lead Rubber Bearing) - contains a vertical lead core that dissipates seismic energy.

HDR (High-Damping Rubber Bearing) - uses specially compounded rubber to absorb energy.

These bearings are used for both building and bridge seismic isolation.

3. What Counts as "Destructive" Testing?

Under GB/T 20688.1, a bearing reaches its ultimate properties when it experiences one of three failure states:

Breaking - fracture of the bearing caused by combined compression (or tension) and shear loading.

Buckling - loss of stability under combined compression and shear.

Roll-out - overturning or rolling instability that can occur in bearings connected with dowels or grooves during horizontal displacement.

The ultimate properties diagram (UPD) records the relationship between shear force and shear displacement at the point of failure - the core output of destructive testing. EN 15129 applies the same conceptual framework of failure under compression-shear loading, expressed as ultimate behaviour at breaking or buckling.

4. Rubber Material-Level Destructive Tests

Before testing whole bearings, the standard requires destructive tests on the rubber material itself, following referenced Chinese standards:

Tensile properties (GB/T 528): measures tensile strength, elongation at break, and modulus at 100% strain.

Fracture / shear failure (GB/T 12830): a four-plate shear method that determines failure shear stress and strain of the rubber compound.

Bond strength (GB/T 7760): evaluates the type of failure at the rubber–metal interface, verifying the vulcanized bond that holds the bearing together.

These tests confirm the material's basic strength and its adhesion to the steel plates - prerequisites for any full-size destructive test. EN 15129 similarly specifies characterization of the elastomeric compound, including hardness, tensile, tear, compression set, ozone ageing and bond strength to the substrates.

5. Isolator-Level Ultimate Tests

5.1 Ultimate Shear Property Test

The most important destructive test is the ultimate shear property test. The bearing is subjected to a constant design compressive stress while horizontal shear displacement is increased until failure occurs. According to GB/T 20688.1 section 6.5:

Testing is performed under the maximum design compressive stress.

For bearings connected by dowels or grooves, or those that may experience uplift, the test is also conducted under the minimum design compressive stress.

The ultimate shear displacement state is defined as the point of breaking, buckling, or roll-out.

If no obvious damage appears and the force–displacement curve rises monotonically once the specified ultimate displacement is reached, the test may be stopped, and the ultimate performance is determined from the maximum shear force and displacement recorded.

5.2 Tensile Property Test

Bearing uplift during strong earthquakes can place isolators in tension. GB/T 20688.1 section 6.6 therefore requires a tensile test under a constant shear displacement, with tension applied slowly until the specimen yields or breaks. The standard defines:

Yield tensile force - determined by intersecting the test curve with a line offset by 1% of the total internal rubber thickness from a reference line based on the shear modulus.

Breaking tensile force and the corresponding shear strain.

The bearing's performance in combined shear and tension is critical for structures where overturning moments are expected.

5.3 Compression Property Test

While primarily a serviceability test, compression forms the baseline for destructive evaluation. Loading is applied in 0–Pmax–0 cycles repeated three times (or an equivalent multi-step pattern), and the vertical compression stiffness (Kv) is computed from the third cycle. Compression stiffness must be established before shear and ultimate tests, because the bearing's stability depends on the confining effect of vertical load.

6. Test Conditions and Specimen Preparation

Reliable destructive testing requires strict control of test conditions:

Conditioning: specimens are stored at standard temperature (23 °C) for a defined period before testing.

Loading cycles: shear tests use 3 or 11 loading cycles; performance is taken from the third cycle or averaged over cycles 2–11.

Shear strain levels: tests cover a wide range, from ±5% up to ±400%, allowing engineers to map behavior all the way to failure.

Frequency and temperature: loading frequency and temperature (from −20 °C to 40 °C) are selected to represent service conditions, since rubber properties are rate- and temperature-dependent.

Displacement sensors: at least two sensors are arranged symmetrically, with measured values averaged.

7. Comparison with EN 15129:2018 (European Standard for Anti-Seismic Devices)

7.1 Overview of EN 15129

EN 15129:2018, published by CEN, is the harmonized European standard for anti-seismic devices. Unlike GB/T 20688.1 - which is primarily a test-method standard - EN 15129 is a comprehensive product and conformity standard that integrates functional requirements, general design rules for the seismic situation, material characteristics, manufacturing and testing requirements, evaluation of conformity (CE marking), installation and maintenance for displacement-dependent devices such as elastomeric bearings.

7.2 Key Similarities

Despite their different roles, the two frameworks share several common features:

Both define ultimate or failure states under compression-shear loading (breaking and buckling; roll-out is addressed in the Chinese framework).

Both require compression stiffness tests, combined compression-shear tests, and shear or ramp loading to characterize isolator behavior.

Both cover LNR, LRB and HDR-type elastomeric isolators.

Both prescribe rubber material tests (tensile, hardness, bonding, ageing) as prerequisites for whole-bearing qualification.

7.3 Key Differences

The principal differences reflect the purpose and structure of each standard:

Scope and role. GB/T 20688.1 is a test-method standard based on ISO 22762-1; EN 15129 is a full design, conformity and CE-marking standard.

Loading protocol. GB/T 20688.1 conducts shear tests under a constant compressive stress. EN 15129, especially for the highest importance classes, requires coupled vertical–horizontal loading to reproduce the P-Δ effect.

Design limits. EN 15129 limits the maximum total design shear strain (typically εq,max ≤ 2.5, i.e. 250%) under the design seismic displacement. GB/T 20688.1 tests shear strains up to ±400% and permits stopping when no damage appears and the force–displacement curve rises monotonically.

Importance classes. EN 15129 classifies structures into importance classes and imposes stricter prototype testing for critical buildings (for example hospitals and emergency centres). GB/T 20688.1 applies a uniform type-testing approach without importance grading.

Conformity. EN 15129 ties testing to conformity assessment procedures and factory production control for CE marking; GB/T 20688.1 supports type inspection under the Chinese product certification regime.

Damping frequency. EN 15129 specifies damping tests at a reference frequency such as 0.5 Hz; GB/T 20688.1 lists frequency groups ranging from 0.05 Hz to 2.0 Hz.

7.4 Summary Comparison Table

Aspect	GB/T 20688.1-2007	EN 15129:2018
Standard role	Test-method standard (MOD ISO 22762-1:2005)	Product + conformity standard (CE marking)
Scope	Building and bridge seismic isolators	Anti-seismic devices, including elastomeric bearings
Failure states	Breaking, buckling, roll-out	Ultimate behaviour at breaking / buckling
Shear test loading	Constant compressive stress	Constant, plus coupled vertical–horizontal loading (P-Δ) for high importance classes
Shear strain range	Tested up to ±400%	Design limit εq,max ≤ 2.5 (250%)
Damping test frequency	Groups 0.05–2.0 Hz	Reference frequency, e.g. 0.5 Hz
Importance classification	Uniform type testing	Importance classes; stricter testing for critical buildings
Conformity	Supports Chinese type inspection	CE marking + factory production control

Table 1 - Comparison of GB/T 20688.1 and EN 15129 for elastomeric isolator testing

8. Why Destructive Testing Matters

Destructive testing serves a simple but essential purpose: it proves that a bearing will not fail before the structure it protects. Seismic isolation works only if the bearing survives the largest expected earthquake. By deliberately pushing bearings to breaking, buckling, and roll-out, manufacturers and engineers:

Verify that design assumptions about ultimate capacity are correct.

Produce ultimate properties diagrams (UPD) used directly in seismic design.

Identify weak points in materials, bonding, or manufacturing.

Establish quality benchmarks for type testing and routine inspection.

Because these tests destroy the specimen, they are typically performed on prototype or type-test bearings rather than every production unit. Both GB/T 20688.1 and EN 15129 rely on such ultimate tests to validate the safety margin of isolators under extreme compression-shear loading.

9. Conclusion

Destructive testing of elastomeric isolator, as defined by GB/T 20688.1-2007, is the final line of defense in seismic design. From tensile and fracture tests on the rubber compound to full-size ultimate shear, tensile, and compression tests, the standard provides a comprehensive framework for verifying that elastomeric isolators can carry extreme loads and still perform their life-saving function.

Compared with EN 15129:2018, the two frameworks pursue the same conceptual goal - proving that a bearing survives extreme compression-shear loading - but differ in emphasis. GB/T 20688.1 offers a broad, purely method-based test framework that reaches very high shear strains (up to ±400%) and covers breaking, buckling and roll-out, while EN 15129 embeds testing within a harmonized design and conformity system, imposes a 250% design shear-strain limit, grades requirements by importance class, and requires coupled vertical–horizontal loading for critical structures. For engineers working globally, understanding both standards is essential: a bearing qualified under GB/T 20688.1 can be validated against EN 15129 requirements with appropriate adjustments to loading protocol and limits.

Whether you are a structural engineer, a manufacturer, or a researcher working with LNR, LRB, or HDR isolators, mastering these destructive test procedures is essential to ensuring the safety and reliability of seismic isolation systems in any jurisdiction.