Residual stress and its effects on thermoplastic bonding to metals

Wenping Zhao, Purdue University

Abstract

The objective of this research is to study the residual stress and its effects on thermoplastic bonding to metals. Thermoplastic adhesives have the potential to become important in bonding various materials. Residual stresses can occur in a thermoplastic bonded system due to the deferential material properties between adherend and adhesive such as thermal expansion and viscoelastic property. In bonding a thermoplastic to metal, residual stresses in the thermoplastic adhesive are generated during cooling down the thermoplastic-metal joint. An initial slow cooling (∼4.5°C/minute) to ∼210°C followed by a fast cooling (∼30°C/second) of PEEK, a semi-crystalline polymer, resulted in a ∼52% increase in residual stress compared to a cycle with only fast cooling. For PEI, an amorphous polymer, fast cooling during glass transition produced ∼15% higher residual stress than slow cooling. A mechanics based curvature model, combined with the elasticity and viscoelasticity of thermoplastics, predicted the residual stress development. The residual stresses were further studied with a thermoplastic-metal cylindrical configuration. Polymethylmethacrylate (PMMA) was injection-molded around a metal rod and a joint assembly was created during the cooling. The in-process residual stresses were visualized using plane polariscope. Slitting and strain gage techniques were used along with a FEM model to measure and interpret the circumferential residual stress at ∼5 MPa in the PMMA ring. Two methods, water soaking and slitting, reduced the residual stresses. Environmental stress cracking of the thermoplastic adhesive was revealed in this study. Combination of experimental observation and a FEM model demonstrated that the surface roughness of the adherend can induce stress concentration at the interface. The stress concentration was shown to be a significant cause of the adhesive environmental cracking. For two new thermoplastic and thermosetting adhesives, Bemis 6343 and FM 300-2, preliminary fracture studies were carried out. As the loading speed of the Bemis 6343 DCB increased from 2.5 mm/minute to 381.0 mm/minute, the fracture toughness increased from ∼2000 J/m2 to ∼4000 J/m2. Fiber additives stabilized the fracture propagation of the FM 300-2 DCBs.

Degree

Ph.D.

Advisors

Ramani, Purdue University.

Subject Area

Mechanical engineering|Materials science

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