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Infrared thermography for inspecting the adhesion integrity of plastic

welded joints

M. Omara, M. Hassana, K. Donohueb , K. Saitoa and R. Allooc

(a) Mechanical Engineering Department, University of Kentucky

(b) Electrical Engineering Department, University of Kentucky

Lexington KY 40506

(c)Toyota Motor Manufacturing North America Inc.

Erlanger KY 41018

Abstract

This work aim at developing a non-destructive tool for the evaluation of bonded plastic

joints. The paper examines infrared thermographic transmission and reflection mode

imaging and validates the feasibility of the thermal NDT approach for this application.

Results demonstrate good estimation performance for adhesion integrity, uniformity and

bond strength using a transmission mode application of infrared thermography. In

addition, results from a pulsed infrared thermographic application using a modified

dynamic infrared tomography scheme show good performance for estimating adhesion

layer thickness mapping and detecting delaminations.

Key words: Plastic kissing bonds, Adhesion uniformity, Bond strength, Pull force,

Dynamic tomography, Delaminations detection.

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Introduction

Plastic bonded structures have found increased acceptance in new products and

applications due to its light weight and strength in preserving structural integrity. Infrared

thermography has been reported to be successfully applied in the evaluation of wide

range of materials including wood (wood poles inspection and lumber industry) [1,2] ,

brick and reinforced concrete (in buildings and smokestacks [3-5]) and finally a variety of

composites and polymers such as, Carbon Fiber Reinforced Plastic CFRP and graphite

epoxy composites [6,7] which motivated the use of the infrared thermography for this

application.

Motivation

The non-destructive investigation of the kissing bond region in plastic welded joints

constitutes an important challenge in the plastic molding industry because, of the wide

variety of defects that exist in these joints such as, delaminations, the adhesion layer

thickness and uniformity. Those defects may lead to the breakage of the bonds or,

misalignments in the final product structure. Since these joints are manufactured

through mass production lines, the application of a classical non-destructive testing

procedure may hinder the production cycle times of these products due to its contact

nature, or slow operation time. The infrared thermography doesn’t pose these limitations,

but has the potential to provide the flexibility of detecting wide variety of defective

behaviors by relating those defects to a specific thermal signature that could be detected

remotely and in real time. Thermal NDT techniques have been applied for the inspection

of some typical encountered defects in similar applications. These applications included

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the composite crack detection [19,20], the evaluation of impact damage in graphite

epoxy [8] and the delaminations detection [9]. In more related applications Turler [10]

reported the use of steady state thermographic technique in predicting the geometry and

location of defects in adhesive and spot welded steel lap joints. Aglan et al [11] used the

infrared thermography for the detection of cumulative fatigue disbond of adhesive steel

joints. None of the currently applied techniques including classical and thermal NDT

provides a quantative value for the bond strength under inspection, which is a potential

that may result from this work. The use of infrared thermography for the inspection of

plastic bonded joints is considered to be a novel approach.

Problem Description

The plastic joints under study are made through an extrusion molding process. The

material involved is a composite of two layers of a polymer (High Density Polyethylene

HDPE) with an adhesive interface. This composition is shown in figure (1). Carbon

pigments have been added to one of HDPE layers for darkening to provide better

thermal absorption and emission properties for insulation purposes. The geometry of the

joints under inspection along with a cross section are shown in figure (2), this geometry

is represented in two cups welded together at the neck region. The bond interface is

located between the white HDPE layer and the rest of the geometry.

The main defectives encountered in this particular product could be categorized into

structural and adhesion. Structural defects include misalignment in the neck region and

the variations in the diameter of contact, which may affect the breakage mode of those

joints as shown in figure (3). Adhesion related defects include lack of adhesion,

adhesion thickness, uniformity and delaminations. All of these defectives ultimately

affect the strength of joint.

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In the following the transmission mode of infrared thermography will be introduced and

applied for the validation of the technique feasibility through studying the effect of bond

integrity on the thermal wave conduction through the bond region. The pulsed reflected

mode also will be presented for the thickness mapping and delaminations detection.

The transmission mode

The transmission mode of infrared thermography is where the stimulation and the

infrared detector are located at opposite sides, to allow for monitoring of the transmitted

heat through the part. This mode is used as a first qualitative step due to its simplicity

[18] and the fact that it doesn't require complicated setup.

The experimental setup for this mode is shown in figure (4). Boiling water at 100oC is

(HDPE tolerates temperature of 120 Co ) used as a contact stimulant; the choice of

stimulant is based on the fact that water at phase change will preserve a constant

temperature. An infrared bolometer (commercial name ThermaCam SC 2000 of Flir, MA)

is used to monitor the temperature rise. The transient temperature evolution curve is

recorded for a number of samples. The evolution curves exhibited a linear behavior with

different slopes for some samples; this behavior is related to the strength of contact

since a well-contacted joint will conduct heat faster than a joint with air gaps presenting a

larger thermal resistance for the heat flow leading to heat entrapment over the defect.

To quantify this observation a criterion to describe the strength of the joint is needed.

This criterion is chosen to be the pull force needed to break the bond between the two

cups using a tensile machine to pull the two cups apart. Figure (5) shows the results of

this application, where the heat evolution curves are shown for joints corresponding to

different pull force values. In this application there are two sets of plastic joints that differ

in size, and therefore the heat evolution curves are recorded separately in figures (5 a,b).

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It's clear that some of the samples with low slope values exhibit unique breakage modes

due to the weak bond such as samples number (1) and (2). The no-fusion sample refers

to the case of contact without bonding, even though this technique is not mainly aimed to

detect such cases. Transmitted mode infrared thermography provides a quantitative

measure of this plastic bond strength by applying a simple and inexpensive setup that

emphasizes its potential utility. The transmission tests results were consistent upon

repeatable measurements.

The transmission mode is found to be useful in predicting the uniformity of the joint weld.

Figure (6,a) shows a thermogram image captured simultaneously for two samples, and

the corresponding pulled samples are shown in figure (6,b). Seeking a qualitative

description for testing uniformity of weld; the time needed for each pixel within the bond

region for some samples to exhibit a certain temperature change is recorded and

rendered into 3 dimensional plots to produce figure (7) which shows the results for

a CT o0.3=∆ . From figure (7) it's clear that the regions of disbond require larger time

periods to conduct heat.

The reflection mode

For the reflection mode of infrared thermography, the stimulant and detector are situated

at the same side and monitor the reflected thermal wave effect on the surface

temperature. The main aim of using this technique is to investigate some of the

subsurface features such as delaminations using a non contact mode. The transmission

mode isn't suitable for non-contact rapid scanning applications due to the fact that the

thermal wave must conduct through the whole bond making the average observation

time period in the order of 100 seconds per scan for this mode.

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An analytical model was devised in this study to help predict the behavior of this material

under the pulsed stimulation. This analytical model is shown in figure (8) and is intended

to simulate the bond region. This model utilizes an implicit finite difference approach to

represent the general homogenous heat conduction equation in cylindrical coordinates

equation (1).

tT

zTT

rrT

rrT

∂∂

=∂∂

+∂∂

+∂∂

+∂∂

αθ111

2

2

2

2

22

2

(1)

The symmetry simplifies the analysis to where only a pie slice is studied. The back and

side walls are assumed to be thermally adiabatic. The boundary conditions are applied

according to a radiation excitation source but could be adjusted to fit the transmission

mode with contact source.

Tossell [12] discussed the boundary conditions and the effects of the method of

stimulation delivery on the thermography numerical modeling.

The convection and radiation surface heat losses are included in this model as in

equations (2) & (3).

))(( 44ambsurfrad TtTq −⋅⋅= εσ (2)

))(( ambsurfconvfreeconv TtThq −⋅= − (3)

where 28 .1067.5 mW−×=σ , ε is the emissivity, assumed to be 1. surfT is the

temperature of a surface node. ambT is ambient temperature and convfreeh − is the free

convection heat transfer coefficient assumed to be 10.

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The delaminations in the analytical study are modeled as an air layer [21] that will

constitute an interface to reflect the thermal waves due to the thermal mismatch factor

existed between the polymer and the air, the thermal mismatch factor is computed

through equation (4)

airHDPE

airHDPE

eeee

+−

=Γ (4)

where HDPEair ee , is the effusivity of air and HDPE respectively computed as:

cke ⋅⋅= ρ (5)

where k is the thermal conductivity KmW ./5.0= , ρ is the density 3/950 mkg= and

c kgKJ ./1900= is the specific heat.

The reflection of thermal waves from subsurface features affects the facial temperature

value at that point causing a deviation from its surroundings. This deviation is monitored

and recoded as the thermal contrast. Figure (9) shows the sample-cooling curve after

depositing a rectangular pulse of 2/100 mkWQ = over a sound (non-defective) spot

and another one that is located over an air gap. Figure (10) shows the thermal contrast

for different delaminations at different depths, such figure is computed from similar

figures as in (9). The contrast peak decays as the depth increases. which is expected

since the thermal front will weakens due to the diffusion effect. Figures (9) and (10)

provide useful information to guide the pulse infrared thermographic procedure in

deciding on the best observation time window and sensitivity limits.

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The pulse reflected infrared thermographic setup is shown in figure (11) where a pulse

unit with radiation power of 6.4 kJ (through two banks of capacitors) and duration of 15

ms is used (commercial name BALCAR Source, France). The geometry of the joints

posed the challenge of scattering the radiation power off the sides of the cups, so the

use of a Bi-tube pulse head is warranted to provide a focused pulse. The pulsed

infrared thermography results could be interpreted and processed in different schemes

such as, Pulse Phase Thermography Maldague et al [13], Synthetic Infrared

Thermography Shepard et al [14] and Infrared Dynamic Tomography Vavilov et al [15].

In this study the Dynamic Infrared Tomography will be utilized to process the data

resulted from the pulse application with some modifications applied to this scheme.

Infrared Tomography

This technique is intended in this case study to map the thickness of the adhesion layer

of the kissing bond. The application of this technique is based on the procedures

proposed by Vavilov et al in 1986 (Vavilov et al 1990 [15]) the details of the computation

involved in this technique could be found in [16]. This technique is based on

establishing a maximum contrast matrix and a time-gram matrix. The maximum contrast

matrix presents the maximum deviation values exhibited through the transient response

to the pulse and the time of occurrence for these values is reported by the time-gram

matrix. The Full Width Half Maximum (FWHM) contrast could be used in other

application [22]. In this application a self-referencing procedure (Shepard et al [17]) will

be utilized in computing the thermal absolute contrast needed for the maximum contrast

matrix. The self-referencing procedure is based on computing the deviation between

each pixel in the thermogram image and a small local neighborhood (kernel of pixels)

surrounding it. This procedure eliminates the need for a known sound area within the

thermogram for thermal deviation calculations and guarantees the consistency since

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each thermogram is referenced to itself. A computer code using MATLAB is prepared for

the modified infrared dynamic tomographic calculations. A further modification is applied

for the representation of the tomographic results to enable it for thickness mapping. The

traditional representation of this technique is based on slicing the material under

inspection into depth slices that correspond to the distribution of the thermal properties

at those depths. In this application the tomographic output will be represented in the

form of a thickness map in order to include all the information in a single image rather

than multiple slices. The thickness mapping scheme is applied using Balageas et al [7]

approach in relating the minimum effusivity curves to the depth through equation (6)

95.0min ][

oo e

etZ ⋅⋅= α (6)

Where Z is the depth of the defect, mint is the occurrence time of the minimum of the

normalized effusivity oee curve. Another form of this equation in terms of the thermal

contrast could be found in [18] as equation (7).

bjiCjitajiZ ),(),(),( maxmax ⋅⋅= (7)

where ),( jiZ is the depth at location ),( ji , 334.0,432.0 −== ba are constants

determined experimentally (dependant on the material under inspection), ),(max jit is the

time at position ),( ji taken from the time-gram matrix, ),(max jiC is the contrast value at

),( ji from the contrast matrix.

The result of using this approach is shown in figure (12) that shows a drop in thickness

map in the middle of the kissing bond indicating delaminations. To validate this results

an ultrasound C-scan was obtained with an Ultrascan 5 (US Ultratek, Martinez, CA). For

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this scan the transducer frequency was 5 MHz, the sampling rate was 100 MHz, the X-Y

scan increment was .012 inches, and the sonic velocity was .0741 inches/microsecond.

The velocity of sound was estimated with a contact transducer applied over parts of

samples including regrind and virgin layers that could be measured directly with calipers.

The X-Y increment was chosen to be about ¼ the width of the beam filed of the

transducer (.04 inches). The C-scan result is shown in figure (13) which verifies the

existence of delaminations in this sample. The average error between the depths

reported by the thermal technique and those of ultrasound measurements is about 12 %.

Conclusion

The infrared thermographic applications in transmission and reflection modes have been

successfully used for the evaluation of adhesion integrity in welded polymer plastic joints.

This evaluation included the inspection of adhesion uniformity and thickness mapping

was achieved using a modified dynamic infrared tomographic scheme. The infrared

thermography provided a quantative non-destructive tool for evaluating the strength of

the plastic polymer welded joint by correlating it with the thermal wave travel within the

joint under inspection. The infrared thermography could be utilized for the inspection of

different defective behaviors in plastic kissing bond applications making it a flexible and

an effective inspection tool for this application.

Acknowledgement

This study was sponsored by Toyota Motor Manufacturing North America Inc. Kentucky.

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3

2

1

Legend: 1 Virgin High Density Polyethylene HDPE (black colored due to carbon addition) 2 Adhesive 3 Virgin High Density Polyethylene HDPE

Figure 1. HDPE material composition

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Figure 2. Joint geometry, cross section A-A

AA

~ 1 cm

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Figure 3. Diameter of contact effect on the breakage mode of the joints

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Figure 4. Transmission setup

IR detector

Heating Element

Sample Under inspection

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Figure 5. Left: Heat evolution curves A,B Legend: shows breakage pull force values in Newtons Right: Sample 1 and 2 breakage mode

1

2

1

2

A

B

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Figure 6. Adhesion uniformity effect on heat conduction A: Thermograms for the two samples, B: Corresponding breakage mode

A

B

Pull force = 3055 N Pull force = 7355 N

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Figure 7. Time plots needed for temperature change of 3 oC. Z axis: time in seconds. X,Y axis : bond spatial coordinates

Time in seconds

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Figure 8. The Analytical model

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Figure 9. thermal response to the pulse stimulation

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Figure 10. The thermal contrast vs. the depth of delaminations

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Figure 11. Pulse reflection setup

Pulse Head

IR Detector

Sample

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Figure 12. Thermal thickness map showing delaminations

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Figure 13. Ultrasound c-san thickness map