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Research Papers

Classification of Failure Modes in Friction Stir Blind Riveted Lap-Shear Joints With Dissimilar Materials

[+] Author and Article Information
Wei-Ming Wang

Department of Mechanical Engineering,
University of Hawaii at Manoa,
Honolulu, HI 96822
e-mail: wmwang@hawaii.edu

Haris Ali Khan

Department of Industrial and Manufacturing Engineering,
Pennsylvania State University,
State College, PA 16801
e-mail: harisali@hawaii.edu

Jingjing Li

Mem. ASME
Associate Professor
Department of Industrial and Manufacturing Engineering,
Pennsylvania State University,
State College, PA 16801
e-mail: jul572@psu.edu

Scott F. Miller

Associate Professor
Department of Mechanical Engineering,
University of Hawaii at Manoa,
Honolulu, HI 96822
e-mail: scott20@hawaii.edu

A Zachary Trimble

Assistant Professor
Department of Mechanical Engineering,
University of Hawaii at Manoa,
Honolulu, HI 96822
e-mail: atrimble@hawaii.ed

1Corresponding author.

Manuscript received February 8, 2016; final manuscript received July 8, 2016; published online September 6, 2016. Assoc. Editor: Yannis Korkolis.

J. Manuf. Sci. Eng 139(2), 021005 (Sep 06, 2016) (10 pages) Paper No: MANU-16-1100; doi: 10.1115/1.4034280 History: Received February 08, 2016; Revised July 08, 2016

In transportation sector, there is an increasing need for joining dissimilar materials for lightweight structures; however, substantial barriers to the joining of dissimilar materials have led to an investigation and development of new joining techniques. Friction stir blind riveting (FSBR), a newly invented method, has shown great promise in joining complex structures with dissimilar materials. The process can be utilized more effectively if knowledge regarding the failure mechanisms of the FSBR joints becomes available. This research focuses on investigating the different mechanisms that lead to a failure in FSBR joints under lap-shear tensile tests. An in situ, nondestructive, acoustic emission (AE) testing method was applied during quasi-static tensile tests to monitor the initiation and evolution of damage in FSBR joints with different combinations of dissimilar materials (including aluminum, magnesium, and a carbon-fiber reinforced polymeric composite). In addition, a fractographic analysis was conducted to characterize the failure modes. Finally, based on the analysis, the distinct failure modes and damage accumulation processes for the joints were identified. An AE accumulative hit history curve was found to be efficient to discriminate the deformation characteristics, such as the deformation zone and failure mode, which cannot be observed through a traditional extensometer measurement method. In addition, the AE accumulative hit history curve can be applied to predict the failure extension or moment of FSBR joints through an identification of the changes in curve slope. Such slope changes usually occur around the middle of Zone II, which is defined in this study.

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References

Figures

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Fig. 1

Illustration of the FSBR process: (a) Stage 1, a rivet approaches the work materials, (b) Stage 2, the rotating rivet is driven through the softened work materials (by frictional heat) with reduced force (a process called FSR), (c) Stage 3, the mandrel is pulled upward (the BR process), and (d) Stage 4, the mandrel is broken at a notch to complete the FSBR joint [3]

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Fig. 2

The transducer on (a) the Al workpiece for FSBR Mg/Al and CFRP/Al joints and (b) the CFRP workpiece for CFRP/Mg joints

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Fig. 3

Failure modes of as-fabricated Mg/CFRP joints: (a) tension failure (CFRP fractured), (b) pullout failure, (c) mixed failure with tension and shearing (Mg fractured), where the circled region indicates the compression on the Mg workpiece introduced by the edge of the rivet head, and (d) cross section of fractured Mg in mixed failure with tension and shearing

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Fig. 4

SEM of Mg fractured surface from a mixed failure mode, where the inset picture highlights the investigated area. The rough surfaces and circled region indicate the tensile and shearing fracture characteristics, respectively.

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Fig. 5

Failure modes of as-fabricated CFRP/Al joints: (a) tension failure and (b) cleavage failure. The circled regions were further analyzed using SEM, the results of which are shown in Figs. 7 (a) and 7 (b).

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Fig. 6

Mechanical locking in CFRP/Al joint due to friction stir process: (a) location of the mechanical locking; and (b) a detailed view of mechanical locking under SEM

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Fig. 7

Microstructures of fractured surfaces of CFRP for as-fabricated joints: (a) a cross section view of CFRP failed in tension failure; (b) SEM image of tension failure sample where circled region shows a fiber pullout failure; (c) cleavage failure, where the arrow indicates the direction of the fractured fibers; and (d) crushed fibers in CFRP

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Fig. 8

Different failure modes in as-fabricated FSBR Mg/Al joints: (a) tension failure, (b) shearing failure, and (c) bearing followed by cleavage failure

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Fig. 9

Mechanical locking at the interface of Mg and Al sheets

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Fig. 10

SEM of fractured surfaces: the circled regions highlight the characteristic dimples caused by a ductile failure

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Fig. 11

Load-extension and accumulative hit curves of as-fabricated Mg/CFRP joints, where an AE sensor was placed on the CFRP: (a) tension failure (CFRP fractured), (b) rivet pullout failure, and (c) mixed failure of tension and shearing (Mg fractured). The circles show the threshold extension.

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Fig. 12

Load-extension and accumulative hit curves of as-fabricated CFRP/Al FSBR joints, where an AE sensor was placed on the Al: (a) tension failure, and (b) cleavage failure, where the circled region shows the threshold extension

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Fig. 13

Load-extension and accumulative hit curves of as-fabricated FSBR Mg/Al joints, where an AE sensor was placed on the Al: (a) tension failure, (b) shearing failure, and (c) bearing followed by cleavage failure

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Fig. 14

Secondary bending of Mg/CFRP FSBR joint: (a) regular secondary, and (b) nonregular secondary bending (when the rivet body rotates in the CFRP workpiece)

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Fig. 15

Load and AE amplitude curves of as-fabricated FSBR Mg/Al joints: (a) tension failure, (b) shearing failure, and (c) bearing failure followed by cleavage failure

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