Evaluation of friction welded bi-material joint strength subjected to

impact loads

Qasim H. Shah, Kassim A. Abdullah

Department of Mechanical Engineering, Faculty of Engineering, International Islamic University

Malaysia, Jalan Gombak, 50728 Kuala Lumpur, Malaysia.

Keywords: Bi-material, bond strength, rotary friction welding, polymers, impact.

Abstract

In the present work the bond strength of two friction welded polymers was investigated when the

bi-material specimens were subjected to projectile impact at 86 ms-1

. Two transparent polymer rods

of Polycarbonate(PC) and Polymethylmethacrylate Acrylic(PMMA) were joined together using

rotary friction welding. Specimens were cut into two different batches. In one batch the central

portion of the rod specimens were removed and in the other batch the complete specimens along

with their central portion were retained. When the bi-material specimens were subjected to

projectile impact the cracks initiated in the comparatively brittle PMMA specimens and were able

to propagate across the interface and subsequently into the PC specimen for the first batch of the

specimens while the cracks were either arrested at the bi-material interface or the cracks propagated

along the interface in the second batch of specimens. From the experimental work it was deduced

that the crack propagation along the interface or across the interface could be a good measure of the

bond strength difference of a bi-material joined using rotary friction welding process. Weak point of

rotary friction welding has also been identified.

Introduction

Bi-materials possess a significant importance due to their extensive applications in manufacturing

industry where ductile materials maybe joined to comparatively brittle materials or where

physically mismatching materials have to be joined to obtain technical or economical objectives.

The bond strength at the bi-materials interface plays a greater role in assuring the joint reliability. A

few examples of bi-material applications are narrated in the following references.

Composite tubes can be made by joining stainless steels and low-alloy steels[1]. These tubes are

used in applications where it is difficult to meet the demands for both mechanical strength and

corrosion resistance with a component made of a single material. Due to wide differences in the

chemical composition between the two steels in a component, considerable transport of elements

occurs over the interface. Bi-material layered systems composed of concrete and asphalt were

investigated in [2] for the interface failure under static and dynamic loading. Fracture toughness

values were experimentally found for the interface crack. Inclined loading to interface was also

carried out and comparisons between static and dynamic fracture toughness were made.

Comparison between new and old concrete and asphalt interface was also made showing that old

interface strengths were twice as high compared to the newly built bi-material system. The

transonic interfacial fracture phenomena in bi-material specimens of PMMA and aluminum alloy

were experimentally recorded in [3] where it was observed that the crack along the interface of non-

homogeneous materials propagates exceeding transonic of the compliant material. The model of a

bi-material notch is suitable to simulate a number of construction points from which a failure is

initiated at the junction of two materials at the free surface. They are layered composites, some

configurations of fibres at the free surface, edges of protective layers etc. In the locations where the

layers or fibres touch the surface of the composite body the singular stress concentrations occur.

Such places are often responsible for crack initiation and consequently for the final failure of the

whole body [4].

Advanced Materials Research Vols. 264-265 (2011) pp 719-725Online available since 2011/Jun/30 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.264-265.719

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Alumina and Aluminum bi-material system was investigated [5] to know the crack propagation

trajectories depending upon the loading angle. Loading angle dependence on whether the crack

remains in the interface or deviate into either of the two materials was investigated based on loading

angle. It is possible that the crack may be shielded due to ductile material presence ahead of the

crack tip.

It has been shown that the difference between the cracks propagating into polymers possess

different characteristics compared to that of metals plasticity and therefore the influence of a

polymer and rigid material interface behave distinctly. Constitutive model of a polymer softening at

and beyond yielding was established [6]. The study of impact loading of polymer-metal strips

showed that Mode II, cracks running along a weak interface achieved steady state crack speeds

greater than the shear wave velocity and approaching the dilation wave speed. Such failures have

been termed “inter-sonic” or “supershear” and generated much interest. The work has also been of

considerable interest in geophysics since the experiments can be used to model earthquakes if an

unbonded interface is loaded laterally to generate friction forces and a disturbance propagated. In

such experiments supershear failures have also been observed and indeed there is evidence of their

occurrence in real earthquakes [7]. Finite element analysis for a crack propagating perpendicular to

alternate layers of steel/Alumina have been conducted in [8] where a brittle failure of alumina and

subsequent ductile deformation of steel layer is considered. Ductile fracture initiation has been

found to occur before plasticity spreads in multiple ductile layers. Research challenges include

improving our understanding on how cracks in brittle/ductile layered systems behave under a

general state of applied loading.

In case of metals it has been claimed that the bond strength of the rotary friction weld is higher at

the middle of the joined shaft[9] but there is no report available regarding the polymer rotary

friction welding in the literature. The joining of similar material like Ti-6Al-4V alloy using rotary

friction welding has been stated in [10]. It was found that a certain rotational speed was the most

suitable condition to achieve the best quality joint with highest tensile strength. The final fracture of

the specimen was always at a location away from the joint interface.

Four failure criteria for a crack along the bi-material interface were investigated [11] and compared

for their validity. Energy release rate criteria was declared to be the most suitable failure criteria for

a bi-material interface failure.

Experimental Work

Circular cross-section solid rods of transparent polymers PMMA and PC were joined using rotary

friction welding. The specimens were machined to rectangular cross-section as shown in Fig.1. Two

batches of specimens were prepared whereby in the first batch the central portion of the specimen

was removed. The central portion of PC and PVC joint is elaborated in Fig.2. In the other batch the

central portion was retained, the specimen dimensions were kept same. A steel sphere of 9 mm

diameter was forced into the specimens at various locations statically. When the forced dent was

within the PC material, only a ductile dent was the deformation observed. As the dent engaged the

interface a crack along the interface was observed in few cases that separated the specimen into two

halves but most of the specimens underwent only the dent deformation.

Fig.1. Bi-material specimen (PMMA-PC) joined by rotary friction welding.

720 Advances in Materials and Processing Technologies II

Fig.2. Central portion of bi-material interface after separation between PVC and PC material, the central portion joint

is weaker than the periphery of the rotary friction welding.

Impact Test

To investigate the damage and failure of a bi-material a 9 mm diameter steel sphere was impacted at

the impact points shown in Fig.1 at and in the vicinity of the interface at 86 ms-1

resulting in a

variety of crack patterns. For the impact locations sufficiently farther than the interface no cracks

were observed in the specimen. A shallow dent was the only damage observed in the PC material. It

is to be noted that for any such impact in the PMMA, large cracks were observed therefore in the

current study attention was focused only on the impacts in the PC.

At the given projectile velocity as the impact dent engaged the bi-material interface the cracks were

observed in the PMMA material. As the distance of impact from the interface narrowed down the

severity of crack initiation and propagation in PMMA was enhanced significantly.

Fig.3. Impact crater slightly engages the bi-material interface. The cracks originally initiated in PMMA cross into PC

across the interface.

Advanced Materials Research Vols. 264-265 721

Fig.4. Crack length dependence in PC upon the impact distance from the bi-material interface.

Fig.5. (a) Heat affected zone at center of interface, (b) Heat affected zone at the outer periphery.

As the impact distance decreases from the bi-material interface the cracks initiated in PMMA cross

the interface into PC to a limited distance as shown in Fig.3. The dependence of the crack length

upon the impact distance from the bi-material interface is shown in Fig.4. Maximum crack size in

PC is obtained for an impact exactly at the interface.

Discussion

Unlike metal joints obtained using rotary friction welding [9] the joint strength distribution at the

polymer bi-material interface is low at the center of the weld because the central portions of two

polymer rods do not join properly. A convex and a concave surface can be observed near the center

of the weld on the rod ends after the material was separated at the interface due the impact at the

interface as shown in Fig.2. At the center of the weld a glassy surface can be observed where the

joint strength is negligibly small while the peripheral region shows a rough ductile failure where the

joint strength is inferred to be higher. The failure of bi-material interface with central portion of

weld included is shown in Fig.6a while in Fig. 6b the bi-material specimen without central portion

is shown after an impact at the interface. It can be observed that when the central portion is

removed there is a great resistance for a crack to travel along the bi-material interface. Due to high

joint strength the bi-material behaves like a single integral part therefore crack rather traverses

across the bi-material interface into PC material but is arrested shortly due to higher resistance to

crack propagation in a comparatively tougher material like PC.

722 Advances in Materials and Processing Technologies II

Heat affected zone at the bi-material interface was observed under the optical microscope and the

extent of heat affected zone (HAZ) due to friction welding is shown in Fig.5. HAZ at the center of

weld varies between 86~132 µm while near the periphery of the weld the HAZ size varies between

226~380 µm which is more than 4 times larger than the one at the center. As the HAZ is smaller at

the center of weld the material joint is inferred to be comparatively weaker at the center.

Fig.6. (a) Crack at the bi-material interface (PMMA & PC) for the specimen that included the central portion of the

rotary friction weld, (b) Crack pattern for the specimen with central portion removed.

In the rotary friction welding of the polymers the melting of the polymer rod ends is not uniform at

the welding surface. The outer periphery melts faster than the center of rod which leaves crests and

troughs in the vicinity of the center of the rod ends. The HAZ size is also not uniform at the center

and at the outer periphery which results in defective or weak joint. When such joint is subjected to

impact loading the crack ensues at the center and propagates towards the periphery. Due to this

reason the bi-material specimen shows trough the thickness separation when impacted by a

projectile at the interface line.

When the central portion was removed from the specimens, their strength was enhanced

significantly so that the cracks were either observed only in the brittle material (PMMA) or the

crack traversed across the interface into PC material depending upon the severity of the impact i.e.,

the distance of impacted point from the bi-material interface. Fig.7 shows the PC and PMMA

broken joint where the molten material was observed to have shifted to one side leaving a wake

behind where the joining process of the two materials remained incomplete. On impact the

disjoined material caused the crack to open from the center which then propagated along the bi-

material interface separating the two halves of the specimen. This is to be noted here that Fig.7 is

the broken interface taken from Fig. 6(a).

The bond strength of rotary friction welded circular cross-section polymer rods can be considered to

be flawed due to incomplete joining at the center of the weld and should therefore be avoided.

Instead of solid rods the rotary friction welding of hollow rods can be suggested as an effective

application in joining process of polymers. Solid polymer parts can be joined using linear friction

welding process [12] whereby the joint strength is uniform.

Advanced Materials Research Vols. 264-265 723

Fig.7. Bi-material (PC & PMMA) specimen broken interface taken from Fig.6(a).

Conclusions

Polycarbonate (PC) and Polymethylemethacrylate Acrylic (PMMA) circular cross-section rods

were joined using rotary friction welding. Two batches of specimens including the central welded

portion and the other without were prepared. The specimens were subjected to static and dynamic

(impact) loads by a spherical steel projectile. Following conclusions were made.

1. When the impact distance is sufficiently greater and far(1/3 of projectile diameter) from the

bi-material interface only a ductile crater was the result of an impact in PC.

2. Crack length in the ductile material (PC in this case) depends on the distance of the impact

point from the bi-material interface with a linearly varying relationship.

3. Rotary friction welded joints of solid circular rods are of inferior strength when subjected to

impact loads at or near the bi-material interface due to a strength gradient at the welded

interface.

4. When the central portion was removed from the specimens the joint strength was enhanced

drastically.

5. Rotary friction welding is suitable for hollow circular shafts if higher strength joints are

desired.

Acknowledgement

The authors are grateful to our undergraduate student M. Mokhtar for his assistance in the

experimental work to complete this research.

724 Advances in Materials and Processing Technologies II

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Advances in Materials and Processing Technologies II doi:10.4028/www.scientific.net/AMR.264-265 Evaluation of Friction Welded Bi-Material Joint Strength Subjected to Impact Loads doi:10.4028/www.scientific.net/AMR.264-265.719