ASTM D7248 Bearing-Bypass Interaction Testing for Composite Laminates
In ASTM D7248M – 12 test method, a two-fastener specimen is loaded in either a double-shear tensile or single-shear tensile or compressive manner to measure the uniaxial bearing/bypass interaction response of multi-directional polymer matrix composite laminates reinforced by high-modulus fibres.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
Overview
ASTM D7248M-12 describes a test method for measuring the bearing/bypass load interaction response of polymer matrix composite laminates using two-fastener specimens. In bolted composite joints, loads are transferred through a combination of bearing (direct pin contact) and bypass (net section tension/compression), and their interaction determines the actual failure load of the joint.
This test provides essential data for the design and certification of bolted composite structures in aerospace, automotive, and defense applications.
Scope, Applications, and Benefits
Scope
ASTM D7248M-12 covers continuous fiber-reinforced PMC laminates and determines:
- Combined bearing and bypass failure load and mode
- Bearing/bypass interaction envelope for specific laminate configurations
- Effect of fastener torque, clearance, and laminate layup on interaction response
- Comparison of joint efficiency across different composite systems
Applications
- Aerospace bolted joint structural design allowables
- Fighter aircraft and commercial aircraft composite structure certification
- Automotive carbon fiber chassis bolted connection design
- Bolted wind turbine blade root connection characterization
- Structural composite repair design with fastened overlaps
Benefits
- Provides direct bearing/bypass interaction data unavailable from single-parameter tests
- Enables accurate bolted joint design without conservative knockdowns
- Supports FAA and EASA composite certification requirements
- Applicable to a range of fiber architectures and matrix systems
- Reduces reliance on expensive full-scale structural tests
Test Process
Specimen Fabrication
Two-fastener specimens are fabricated with defined geometry, fastener hole diameter, edge distance, and laminate layup.
1Fixture Setup
Specimens are installed in a specially designed two-fastener test fixture that controls the bearing/bypass load ratio.
2Loading
Tensile load is applied at a controlled rate; load, displacement, and failure mode are recorded.
3Data Reduction
Bearing and bypass stresses at failure are calculated and plotted to form the interaction diagram.
4Technical Specifications
| Parameter | Details |
|---|---|
| Standard | ASTM D7248M-12 |
| Test Principle | Two-fastener bearing/bypass interaction loading |
| Applicable Materials | Continuous fiber PMC laminates |
| Fastener | Pin or bolt with defined torque level |
| Measured Outputs | Bearing stress, bypass stress, failure mode |
| Loading Rate | Typically 1–2 mm/min |
Instrumentation Used for Testing
- Universal testing machine with appropriate load cell
- Two-fastener interaction test fixture
- Extensometers or DIC (digital image correlation) for strain measurement
- Torque wrench for fastener installation
- Optical microscopy for failure mode examination
Results and Deliverables
- Bearing stress and bypass stress at failure for each load ratio tested
- Bearing/bypass interaction diagram (failure envelope)
- Failure mode descriptions (bearing, net section, shear-out)
- B-basis or A-basis allowables (with sufficient replicates)
- Test report for use in composite structural design analysis
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Frequently Asked Questions
Bearing stress is the compressive stress from pin-hole contact at the loaded fastener; bypass stress is the tension or compression carried across the joint at the net section. Most real joints experience both simultaneously.
A two-fastener configuration allows independent control of the bearing and bypass load fractions by distributing total load between two holes with different load transfer ratios, enabling different points on the interaction curve.
Bearing/bypass interaction governs how load is shared between fasteners and surrounding laminate. Improper interaction can lead to premature failure, while optimized interaction improves joint efficiency, strength, and durability under complex structural loading conditions.
Fiber orientation determines load transfer paths and stiffness distribution, significantly influencing strength, failure mode, and interaction behavior.
The interaction diagram plots bearing stress against bypass stress, defining failure envelopes. It helps engineers understand combined loading limits, identify safe design regions, and predict failure under varying load combinations in composite joints.
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