Brake Pad Shear Strength Testing to the Chinese National Standard (GB/T 22309): Procedures, Equipmen

Brake pad shear strength is one of the most critical safety indicators for evaluating the bond between friction material and the steel backing plate or brake shoe. In China, this property is verified through the brake pad shear strength test conducted in accordance with the national standard GB/T 22309, which is technically aligned with ISO 6312 and complemented by the acceptance requirements of GB 5763 for automotive brake linings. This article explains the test principle, sample preparation, equipment, step-by-step procedure, calculation methods, typical acceptance criteria, and practical applications for manufacturers, suppliers, and accredited third-party laboratories.

Technical Overview

The shear strength test measures the maximum force required to detach the friction material from its backing plate (in disc brake pad assemblies) or from the shoe table (in drum brake shoe assemblies), in a direction parallel to the backing surface, divided by the sheared area. The result is expressed in megapascals (MPa) or newtons per square millimeter (N/mm²). A weak bond can lead to partial or complete lining separation during braking, causing loss of braking force, pedal vibration, abnormal noise, and in extreme cases catastrophic brake failure.

In the Chinese national standard system, the governing test procedure is GB/T 22309-2008, "Road vehicles — Brake linings — Shear strength test procedure for disc brake pad assemblies and drum brake shoe assemblies," which corresponds to the international standard ISO 6312. The product-oriented mandatory standard GB 5763 for automotive brake linings defines the applicable acceptance framework, while individual vehicle manufacturers and Tier-1 suppliers typically impose their own, often stricter, limit values for cold shear, hot shear, and post-corrosion shear performance.

The test evaluates the weakest link in the composite structure, which may be the adhesive bond line, the cohesive strength of the friction material itself, the mechanical interlock created by the backing plate geometry, or a combination of these mechanisms. For this reason, fracture surface examination and failure-mode classification form an integral part of the test report.

Testing Samples

Disc Brake Pad Assemblies

Complete, unused disc brake pad assemblies are tested in their as-delivered condition, including the backing plate. Accessories such as acoustic shims, wear indicator clips, or sensors are removed when they interfere with fixture engagement. The shear plane is located at the interface between the friction material and the backing plate. A minimum of five test pieces per batch is common practice, although the exact sampling plan is defined by the product standard or the customer specification.

Drum Brake Shoe Assemblies

Bonded drum brake shoe assemblies are tested as complete units, with the lining sheared from the shoe table. The sheared area is determined from the arc length of the lining segment multiplied by its width. Riveted linings are generally assessed with separate retention or pull-out procedures rather than the bonded-interface shear method.

Conditioning and Preparation

• Room-temperature samples are conditioned at 23 ± 5 °C for a sufficient period to reach thermal equilibrium before testing.
• Hot shear samples are pre-heated in an oven at a specified elevated temperature, frequently in the range of 250 °C to 350 °C for approximately 30 minutes, and tested within a short defined window after removal so that the interface remains at the target temperature.
• Corrosion-conditioned samples are exposed to neutral salt spray in accordance with GB/T 10125 / ISO 9227 for a defined duration before shear testing to simulate service aging.
• Before testing, the length, width, and thickness of each lining are measured, and the effective sheared area is calculated after deducting grooves, holes, and slots.

Testing Items

Depending on the validation or quality-control program, the following items are typically assessed:

• Room-temperature (cold) shear strength of disc brake pad assemblies, in MPa.
• Elevated-temperature (hot) shear strength after thermal conditioning.
• Residual shear strength after corrosion (salt spray) conditioning.
• Maximum shear force in kilonewtons (kN) for each individual test piece.
• Failure-mode classification: adhesive failure at the lining/backing interface, cohesive failure within the friction material, mixed failure, or backing plate fracture.
• Statistical evaluation of the sample set, including minimum value, mean value, and scatter.

Testing Methods

Equipment and Fixture

The test is performed on a universal testing machine or a dedicated shear strength tester equipped with a purpose-built fixture conforming to GB/T 22309. The fixture consists of a rigid anvil that supports the backing plate or shoe and a hardened shear blade or pushing tool that engages the friction material parallel to the backing surface. Load measurement accuracy is typically within ±1% of the indicated value. The shear blade is set at a small, controlled distance from the backing plate so that the shear plane coincides closely with the bonding interface.

Test Procedure

• Measure the lining dimensions and compute the effective sheared area in square millimeters.
• Condition the sample according to the selected test condition (ambient, hot, or post-corrosion).
• Mount the assembly in the fixture with the backing plate fully supported and the shear tool aligned parallel to the backing surface.
• Apply the shearing load at a controlled rate, commonly around 10 mm/min of tool travel, unless otherwise specified.
• Continue loading until complete separation of the friction material occurs, and record the maximum force (Fmax).
• Examine the fracture surfaces and classify the failure mode.

Calculation and Evaluation

The shear strength is calculated as the maximum failure force divided by the sheared area: shear strength (MPa) = Fmax (N) / A (mm²). For disc pads, the area is the plan area of the friction material, with holes and slots deducted; for drum linings, the area is the arc length multiplied by the lining width. Acceptance thresholds in the range of approximately 2.5 MPa to 4 MPa at room temperature are widely applied in the automotive sector, with lower limits permitted after high-temperature or corrosion conditioning; the exact values must be taken from the applicable product standard or customer drawing. The test report documents sample identification, dimensions, area, test temperature, loading rate, individual and mean results, and failure-mode descriptions.

Applications

Shear strength testing to the Chinese national standard supports the entire life cycle of friction products:

• New product development and design validation for original equipment manufacturers, including adhesive formulation and curing-window optimization.
• Routine production quality control and batch release at friction material plants.
• Compliance demonstration against GB 5763 for the domestic automotive aftermarket.
• Verification testing by ISO/IEC 17025-accredited third-party laboratories for export shipments and international type-approval regimes such as UN Regulation No. 90.
• Failure analysis and root-cause investigation of delamination, edge lifting, or noise complaints in the field.
• Benchmarking of competitive products and incoming inspection of purchased pad assemblies.
• Extension to adjacent sectors such as motorcycle, commercial vehicle, and railway friction components that reference comparable shear test principles.

FAQ

Which Chinese national standard governs brake pad shear strength testing?

GB/T 22309-2008 specifies the shear strength test procedure for disc brake pad assemblies and drum brake shoe assemblies and corresponds to ISO 6312 at the international level. Acceptance limits for automotive applications are drawn from GB 5763, OEM drawings, or customer specifications.

What causes low shear strength results?

Common root causes include inadequate adhesive mixing or curing, contamination of the backing plate surface, insufficient surface preparation or mechanical keying, thermal degradation from overheating in service, corrosion of the steel plate, or low cohesive strength of the friction formulation itself.

What is the difference between cold and hot shear testing?

Cold shear is performed at ambient temperature and reflects the as-manufactured bond quality. Hot shear is performed after heating the sample to an elevated temperature to assess how the adhesive system and friction material retain their integrity under the thermal loads experienced during heavy or repeated braking.

How many samples should be tested?

A minimum of five test pieces per batch is typical, with larger sampling plans applied for statistical process control or formal type testing. The sampling scheme should always follow the relevant product standard or the agreed customer protocol.

Can used or worn brake pads be tested?

Yes, residual shear strength testing of service-exposed pads is a recognized diagnostic tool in failure investigations, although results must be interpreted with regard to thermal history, wear level, and corrosion state, and they are not directly comparable to new-part acceptance criteria.

Where can this test be performed?

Testing can be conducted in manufacturer laboratories or by accredited third-party laboratories holding ISO/IEC 17025 accreditation, which ensures traceable calibration, validated fixtures, and technically competent reporting for regulatory or commercial purposes.