EXPERIMENTAL SETUP OF INFRARED TESTING

Experimental Setup

Concrete test pieces, 80x400x100mm with known maximum crack widths of 0.5 and 1 mm, are mounted vertically and heated by a uniform output heating element. The concrete mix complies to British Standard BS8110 specification and is designed to an approximate cube strength of 40 MPa. This is a generic mix which is meant to represent many of the concrete structures in the UK and EU, hence adding realism to the experiments. The concrete mix can be procured from most contractors for industrial use. The surface temperature distribution of the specimen is registered by the IR camera which has a resolution of 240 x 360 pixels, and a sensitivity of 0.15 °C. The IR camera is capable of capturing consecutive images using a Frame Grabber through an image acquisition expansion card for PC. Appropriate software is used to control the grabber, and many frames of recorded images may be displayed simultaneously in pseudo-colours. A schematic diagram of the experimental set-up is shown in Fig. 1.

Fig 1: The Schematic of the Experimental Set-up

Experimental Results

The experimental results are analysed using a bespoke software package. Surface temperature distribution is presented in the form of the images. Results for cracks with a width 1 mm are shown in Fig. 2 (images 1-8).

Fig 2: A Set of Infrared Images Which Show the Extent of Heat Flow Blocking due to Surface Discontinuity Caused by A Mechanical Crack. Image 7 is at Steady-State Condition after 394 Minutes of Test Starting

The heat 'front' is allowed to propagate at various time intervals and its progress recorded as isotherms by the Frame-Grabber software. The temperature field variation graph indicates the sharp change in temperature between the leading and the lagging crack-tip, due to interference with the natural heat conduction of the continuum. Images 1-6 are taken at given intervals in Table 1 for a total time lapse of 354minutes. This constitutes a transient heat flow condition. Image 7 and 8 are at steady-state after 394 minutes. Table 1 includes the complete time and image recording details for this experiment at which the images were taken.


Image Number	1	2	3	4	5	6	7	8
Lapse time ts in minutes(t start = t 0 = 0 )	0	26	90	210	266	354	394	¥
Transient (Tn)Steady-state (Ss)	Tn	Tn	Tn	Tn	Tn	Tn	Ss	Ss
Table 1: Image and Time Lapse Specified

Numerical Modelling

Fig 3: Typical FE Crack Geometry Mod

Fig 4: Temperature Distribution for a T dimensional Model

A commercially available finite element (FE) software is used to model the geometry of the test specimens as well as determine the temperature variation field in the material due to a specified heat source (ANSYS, Release 5.4, 1997). There are standard text books discussing the use of FE method for heat transfer, for example see Bonet and Henwood[9], Zienkiewicz[10].
All present analytical models are formulated in two-dimensions, see Fig. 3. The 2-D model which includes the crack, is generated automatically using 2480 triangular FE's from which the temperature distributions are obtained. Material parameters such as the thermal conductivity k, specific heat C, and the heat transfer coefficient a are used based on the referenced literature such as Carslaw[11], Chapman[12], DeWitt and Incropera[13], Jaluria and Torrance[14], and Minkowycz et al [15]. The density of the specimen is obtained accurately from the standard laboratory measurements.
The characteristic of the temperature distribution is identical to the images gained from the experiments. Figure 4 is an example representing the shape of the predicted temperature field. As the heat is continuously delivered by the source, the temperature distribution along the concrete surface, at steady-state conditions, can be plotted by a single curve. A group of such curves are shown in Fig. 5. This shows surface temperature distributions due to a concentrated heat source (generating heat) as multiples of the ambient temperature (T). Similar results are shown in Fig. 6, where each curve represents the surface temperature variation due to input of heat, in this instance assumed to be uniformly distributed as a plateau, also as multiples of the ambient temperature (T).


Fig 5: Surface Temperature Distribution due to Different Temperatures of an Applied Point Heat Source (curves relate to various source temperatures, from T to 5T), S/d = 0.5

Fig 6: Surface Temperature Distribution due to Different Temperatures of the Distributed Heat Source Applied (S is the distance of the leading edge of heat source to crack centre, refer to Fig. 1), S/d = 0.5

A series of calculations are carried out with both heat source models in order to study the effect of varying geometric parameters of the crack on heat flow characteristics. As an example, the temperature distributions along the specimen for different widths of the cracks are presented in Fig. 7, where the source temperature is 3T due to a plateau-type heat source.


Fig 7: Surface Temperature Variation for Different Crack Width (S = leading edge heat source to crack centre, refer to Fig1)

It is surmised that if there were no cracks present, the temperature distribution on the concrete surface would be expected to be symmetrical and continuous. In the section of the surface with at least 1 crack, there would be a sharp local change in the temperature distribution. The temperature value is seen to reduce with the increase of distance from the heat source. To summarise, the theoretical study indicates that:

1. As expected, the temperature distribution on the surface is symmetrical relative to the source centre-line, in the absence of surface discontinuities (assuming that the edge effects are negligible).
2. Even very small cracks have significant effects which interfere with the heat flow.
3. The difference of crack geometry would have a varying impact on the temperature difference between the leading and trailing edges across the crack width.
4. Geometrical parameters of the crack influence surface temperatures only in close vicinity of damage.
5. It can be seen in Fig 6 that on the left side of heating element temperature variation has a fixed variation irrespective of different crack widths.
6. Increasing the source temperature (within limits) exaggerates the temperature difference across the crack width, for the same damage geometry.

Comparison contd................