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Glossary of Terms (Based on BSI and ASTM definitions)
Loading Modes
Failure Modes
Safety Factors
Design Data Requirements
Variable Amplitude Spectrum Loading
Palmgren-Miner Cumulative Damage Rule
General Approaches to Fatigue Design
Stress-Life Approach
Fatigue Strength of Typical Details
Strain-Based Approach
Engineering structures are frequently expected to sustain static (creep) or cyclic fatigue loads for considerable periods of time, often in hostile environments, without any adverse effect on the load-bearing capacity of the structure (rail road bridges and automotive parts are expected to last at least 120 and 10 years, respectively). Adhesively bonded joints have excellent fatigue properties, and thus particularly attractive for use in fatigue critical applications. Adhesives are used extensively for secondary (not main load-bearing frame) aircraft and automotive parts, and in some cases primary parts of aircraft. A major concern is that under dynamic fluctuating loads, joints will often fail at stress levels much lower than the static strength of the joint. It is estimated that 90% of all structural failures that occur in service are caused by fatigue. The tendency is to “overdesign” structures. Safety authorities will often require that adhesively bonded structures, particularly those employed in primary load-bearing applications, include mechanical fasteners (e.g. bolts) as an additional safety precaution. Conservative design and engineering practices result in heavier and more costly components.
This module addresses the various aspects that need to be considered in the design and testing of adhesive joints to ensure long-term durability (i.e. resistance to creep, cyclic fatigue and environmental degradation) with emphasis on structural applications where adhesives are used to join primary load-bearing multi-component assemblies. It includes advice on design approaches (including fracture mechanics), data requirements and testing procedures required to generate quality assurance and design data. A list of recommended test methods for determining input data for the design and analysis of bonded joints is provided in Test Methods and Standards. A number of joint geometries, including single-lap, T-peel and T-joints are used to demonstrate various aspects associated with design and testing of joined systems. The module is primarily concerned with metals and fibre-reinforced polymer composites bonded with structural adhesives (i.e. rubber-toughened epoxies).
The intention of the module is to provide designers and users with sufficient information which, when coupled with their own expertise, can be used to reliably assess the key parameters that affect the performance of adhesive joints. If the intention is to generate design data, then the information should be used in conjunction with the appropriate structural design codes. The module assumes some basic knowledge of the materials and mechanical engineering, and is not intended as a textbook or as a design protocol. The reader is advised to check other modules within the Adhesive Design Toolkit for specific issues:
There are also a number of NPL Measurement Good Practice Guides " Preparation and Testing of Bulk Specimens of Adhesives" , " Thermal Analysis Techniques for Composites and Adhesives" , " Fibre Reinforced Plastic Composites - Machining of Composites and Specimen Preparation" , " Characterisation of Flexible Adhesives for Design" , " Preparation and Testing of Adhesive Joints" and " The Use of Finite Element Methods for Design with Adhesives" , that provide advice on issues relating to the preparation and testing of bulk adhesive and adhesive joint specimens, acquisition of design data from bulk specimens, finite element modelling of adhesives, flexible adhesives and durability testing. The intention of the module is to complement the work covered elsewhere within the Adhesive Design Toolkit and within the above mentioned published works.
It is recommended that specialist advice be sought from adhesive manufacturers on adhesive selection, use of associated technologies and health and safety requirements.
Expert advice should be obtained from the adhesive manufacturer on selection and use of surface treatments and that the detail requirements specified by the manufacturer are completely satisfied. Where tests are performed to characterise the adhesive material then it is recommended that the surface preparation is as good as possible to minimise premature adhesion failure. Where tests are performed to evaluate a bonding system then the surface preparation procedures for test specimens will need to mirror those for the final bonded component.