Energizing ASTM lap joint fracture standards.

Area of Science:

• Materials Science
• Fracture Mechanics
• Composite Materials

Background:

• Current ASTM standards for welded and glued joints neglect the Griffith energy criterion.
• ASTM's stress-based failure definition, originating from Galileo, is inadequate for fracture analysis.
• Standardized testing of carbon fibre reinforced polymer (CFRP) lap joints reveals significant issues with existing methods.

Purpose of the Study:

• To demonstrate the inadequacy of current ASTM 'shear strength' standards for welded lap joints.
• To propose replacing stress-based criteria with Griffith's energy conservation principle for evaluating joint performance.
• To highlight the importance of fracture surface energy as the primary parameter resisting failure in lap joints.

Main Methods:

• Analysis of ASTM D5868 test limitations for CFRP lap joints.
• Application of Griffith's energy conservation argument to lap joint fracture.
• Investigation of tensile delamination in lap joints, focusing on stored elastic energy changes.

Main Results:

• ASTM standards fail to account for fracture energy, leading to inaccurate assessments of joint toughness.
• Fracture force in lap joints is not proportional to area, invalidating the concept of size-independent strength.
• The study identifies a size effect where larger joints exhibit lower strength, contrary to Galileo's observations on bent beams.

Conclusions:

• ASTM standards for welded joints require revision to incorporate fracture mechanics principles.
• Griffith's energy criterion offers a more robust framework for assessing lap joint performance than stress-based methods.
• Tensile delamination, driven by stored elastic energy, is a more accurate description of lap joint failure than 'shear strength'.