Ultrasonic Technique and Equipment for Residual Stresses Measurement Part 1

ABSTRACT

Different methods exist to measure the residual stresses (RS). One of the advantages of ultrasonic techniques for RS measurement is that they are non-destructive. Using such techniques, one can measure the RS in the same points many times, studying, for instance, the changes of RS under the action of service loading or effectiveness of stress-relieving techniques. An Ultrasonic Computerized Complex (UCC) for non-destructive measurement of residual and applied stresses was developed recently. The UCC includes a measurement unit with transducers and basic supporting software and an advanced database and an Expert System, housed in a laptop, for analysis of the influence of RS on the fatigue life of welded elements. In general, the ultrasonic method allows one to measure the RS in both cases: averaged through thickness or in surface layers. The advanced ultrasonic method, the equipment and some examples of RS measurement in welded elements and structures are discussed in this paper.

INTRODUCTION

Residual stresses (RS) can significantly affect engineering properties of materials and structural components, notably fatigue life, distortion, dimensional stability, corrosion resistance, brittle fracture [1]. Such effects usually lead to considerable expenditures in repairs and restoration of parts, equipment and structures. For that reason, the RS analysis is a compulsory stage in the design of parts and structural elements and in the estimation of their reliability under real service conditions.


Systematic studies had shown that, for instance, welding RS might lead to a drastic reduction in fatigue strength of welded elements [2]. In multi-cycle fatigue (N>106 cycles of loading), the effect of RS can be compared with the effect of stress concentration. Figure 1 illustrates one of the results of these studies. The butt joints in low-carbon steel were tested at symmetric cycle of loading (stress ratio R=-1). There were three types of welded specimens. The relatively small specimens (420x80x10 mm) were cut from a large welded plate. Measurements of RS revealed that in this case the specimens after cutting had a minimum level of RS. Additional longitudinal weld beads on both sides in specimens of second type created at the central part of these specimens tensile RS close to the yield strength of material. These beads did not change the stress concentration of the considered butt weld in the direction of loading. In the specimens of third type longitudinal beads were deposited and then the specimens were bisected and welded again. Due to the small length of this butt weld the RS in these specimens were very small and approximately the same as those within the specimens of first type [2].

Tests showed that the fatigue strength of specimens of first and third types (without RS) is practically the same with the limit stress range 240 MPa at N=2-106 cycles of loading. The limit stress range of specimens with high tensile RS (second type) was only 150 MPa. In all specimens the fatigue cracks originated near the transverse butt joint. The reduction of the fatigue strength in this case can be explained only by the effect of welding RS. These experimental studies showed also that at the level of maximum cyclic stresses close to the yield strength of base material the fatigue life of specimens with and without high tensile RS was practically identical. With the decrease of the stress range there is corresponding increase of the influence of the welding RS on the fatigue life of welded joint.

Fatigue curves of butt welded joint in low-carbon steel: 1 - without residual stresses; 2, 3 - with high tensile residual stresses (fatigue testing and computation)

Figure 1. Fatigue curves of butt welded joint in low-carbon steel: 1 – without residual stresses; 2, 3 – with high tensile residual stresses (fatigue testing and computation)

The effect of RS on the fatigue life of welded elements is more significant in the case of relieving of harmful tensile RS and introducing of beneficial compressive RS in the weld toe zones. The beneficial compressive RS with the level close to the yield strength of material are introduced at the weld toe zones by, for instance, the ultrasonic peening (UIT/UP) [1, 3]. The results of fatigue testing of welded specimens in as-welded condition and after application of UIT/UP are presented in Figure 2. The fatigue curve of welded element in as-welded condition (with high tensile RS) was used also as initial fatigue data for computation of the effect of the UP. In case of non-load caring fillet welded joint in high strength steel (ay = 864 MPa, ctu = 897 MPa), the redistribution of RS resulted in approximately two times increase in limit stress range and over 10 times increase in the fatigue life of the welded elements.

Fatigue curves of non-load caring fillet welded joint in high strength steel: 1 - in as-welded condition; 2, 3 - after application of ultrasonic peening (fatigue testing and computation)

Figure 2. Fatigue curves of non-load caring fillet welded joint in high strength steel: 1 – in as-welded condition; 2, 3 – after application of ultrasonic peening (fatigue testing and computation)

The RS, therefore, are one of the main factors determining the engineering properties of materials, parts and welded elements and this factor should be taken into account during the design and manufacturing of different products. The efficient approach to the problem of RS includes, at least, stages of determination, analysis and beneficial redistribution of residual stresses. The combined consideration of the above-mentioned stages of the RS analysis and modification gives rise to so called Residual Stress Management (RSM) concept approach [4, 5]. The RSM concept includes the following main stages:

Stage 1. Residual Stress Determination:

- Measurement: Destructive, Non-destructive,

- Computation.

Stage 2. Analysis of the Residual Stress Effects:

- Experimental Studies,

- Computation.

Stage 3. Residual Stress Modification (if required):

- Changes in Technology of Manufacturing and Assembly,

- Application of Stress-Relieving Techniques.

Over the last few decades, various quantitative and qualitative methods of RS analysis have been developed [4]. In general, a distinction is usually made between destructive and non-destructive techniques for RS measurement. The first series of methods is based on destruction of the state of equilibrium of the RS after sectioning of the specimen, machining, layer removal or hole drilling. The most common destructive methods are:

- the hole drilling method,

- the ring core technique,

- the bending deflection method,

- the sectioning method, etc.

The application of the destructive or so-called partially-destructive techniques is limited mostly to laboratory samples.

The second series of methods of RS measurement is based on the relationship between the physical and the crystallographic parameters and the RS and does not require the destruction of the part or structural elements and could be used for field measurements. The most developed non-destructive methods are:

- the X-ray and neutron diffraction methods,

- the ultrasonic techniques,

- the magnetic methods etc.

Although certain progress has been achieved in the development of different experimental techniques, a considerable effort is still required to develop efficient and cost-effective methods of residual stress analysis [4, 6]. The application of an ultrasonic non-destructive method for residual stress measurements had shown that, in many cases, this technique is very efficient and allows measuring the residual stresses both in laboratory conditions and in real structures for a wide range of materials [4-11].

ULTRASONIC MEASUREMENT OF RESIDUAL STRESSES

Ultrasonic Method of Residual Stress Measurement

One of the promising directions in development of non-destructive techniques for residual stress measurement is application of ultrasound. Ultrasonic stress measurement techniques are based on the acoustic-elasticity effect, according to which the velocity of elastic wave propagation in solids is dependent on the mechanical stress. The relationships between the changes of the velocities of longitudinal ultrasonic waves and shear waves of orthogonal polarization under the action of tensile and compressive external loads in steel and aluminum alloys are presented in Figure 3. As can be seen from Fig. 3, the intensity and character of such changes could be different, depending on material properties.

Different configurations of ultrasonic equipment can be used for residual stress measurements. In each case, waves are launched by a transmitting transducer, propagate through a region of the material and are detected by a receiving transducer as shown in Figure 4 [6]. The technique when the same transducer is used for excitation and receiving of ultrasonic waves is often called pulse-echo method (Figure 4a). This method is effective for analysis of residual stresses in the interior of material. In this case the through-thickness average of residual stresses is measured. In the configuration shown in Figure 4c, the residual stress in a surface/subsurface layer is determined.

Change of ultrasonic longitudinal wave velocity (C L) and shear waves velocities of orthogonal polarization (C SX3 ; C SX2 ) depending on the mechanical stress a in steel A (1), steel B (2) and aluminum alloy (3) [10]: • - C SX3 ; 0 - C SX2 ; x - C l

Figure 3. Change of ultrasonic longitudinal wave velocity (C L) and shear waves velocities of orthogonal polarization (C SX3 ; C SX2 ) depending on the mechanical stress a in steel A (1), steel B (2) and aluminum alloy (3) [10]: • – C SX3 ; 0 – C SX2 ; x – C l

Schematic view of ultrasonic measurement configurations: (a) through-thickness pulse-echo, (b) through-thickness pitch-catch and (c) surface pitch-catch

Figure 4. Schematic view of ultrasonic measurement configurations: (a) through-thickness pulse-echo, (b) through-thickness pitch-catch and (c) surface pitch-catch

The depth of this layer is related to the ultrasonic wavelength, often exceeding a few millimeters, and hence is much greater than that obtained by X-ray method. Other advantages of the ultrasonic technique are the facts that the instrumentation is convenient to use, quickly to set up, portable, inexpensive and free of radiation hazards.

In the proposed in [7, 9, 10] technique, the velocities of longitudinal ultrasonic wave and shear waves of orthogonal polarization are measured at a considered point to determine the uni- and biaxial residual stresses. The bulk waves in this approach are used to determine the stresses averaged over the thickness of the investigated elements. Surface waves are used to determine the uni- and biaxial stresses at the surface of the material. The mechanical properties of the material are represented by the proportionality coefficients, which can be calculated or determined experimentally under an external loading of a sample of considered material.

In general, the change in the ultrasonic wave velocity in structural materials under mechanical stress amounts only to tenths of a percentage point. Therefore the equipment for practical application of ultrasonic technique for residual stress measurement should be of high resolution, reliable and fully computerized.

Ultrasonic Equipment and Software for Residual Stress Measurement

The Ultrasonic Computerized Complex (UCC) for residual stress analysis was developed recently based on an improved ultrasonic methodology [7-10]. The UCC includes a measurement unit with supporting software and a laptop with an advanced database and an Expert System (ES) for analysis of the influence of residual stresses on the fatigue life of welded components. The developed device with gages/transducers for ultrasonic residual stress measurement is presented in Figure 5. The UCC allows determining uni- and biaxial applied and residual stresses for a wide range of materials and structures. In addition, the developed ES can be used for calculation of the effect of measured residual stresses on the fatigue life of welded elements, depending on the mechanical properties of the materials, type of welded element, parameters of cyclic loading and other factors.

The Ultrasonic Computerized Complex for measurement of residual and applied stresses

Figure 5. The Ultrasonic Computerized Complex for measurement of residual and applied stresses

The developed equipment allows one to determine the magnitudes and signs of uni- and biaxial residual and applied stresses for a wide range of materials as well as stress, strain and force in various size fasteners. The sensors, using quartz plates measuring from 3×3 mm to 10×10 mm as ultrasonic transducers, are attached to the object of investigation by special clamping straps (see Figure 5) and/or electromagnets.

The main technical characteristics of the measurement unit:

- stress can be measured in materials with thickness 2 – 150 mm;

- error of stress determination (from external load): 5 – 10 MPa;

- error of residual stress determination: 0.1 ay (yield strength) MPa;

- stress, strain and force measurement in fasteners (pins) 25-1000 mm long;

- independent power supply (accumulator battery 12 V);

- overall dimensions of measurement device: 300x200x150 mm;

- weight of unit with sensors: 7 kg.

The supporting software allows controlling the measurement process, storing the measured and other data and calculating and plotting the distribution of residual stresses. The software also allows an easy connection with standard PC’s.

An example of presentation of the residual stress measurement data, using the developed software, is shown in Figure 6. The software allows comparing different data on residual stress measurement and transferring selected data for further fatigue analysis. In Figure 6, the left side of the screen displays information on the measured ultrasonic wave velocities as well as other technical information on the sample. The right side of the screen displays the distribution of calculated residual stresses.

In the example of residual stress measurement presented in Figure 6, a plate made from low carbon steel, with yield strength of 296 MPa, was heated locally, with the focal point of heating located approximately 50 mm from the left side of the plate. The distribution of both components of residual stresses in the specimen, as a result of this local heating are shown in the right side of Figure 6. As can be seen, in the heating zone, both residual stress components are tensile and reach the yield strength of the considered material. In the compression zone, located between the edge of the plate and the centre of the heating zone, the longitudinal component of residual stresses reaches minus 140 MPa.

Distribution of residual stresses in a low carbon steel plate after local heating [10]. 2.3. Examples of Residual Stress Measurements Using the Ultrasonic Method

Figure 6. Distribution of residual stresses in a low carbon steel plate after local heating [10]. 2.3. Examples of Residual Stress Measurements Using the Ultrasonic Method

One of the main advantages of the developed technique and equipment is the possibility to measure the residual and applied stresses in samples and real structure elements. Such measurements were performed for a wide range of materials, parts and structures. A few examples of the practical application of the developed technique and equipment for residual stress measurement based on using of the ultrasonic technique are presented below.

Specimen for Fatigue Testing

The residual stresses were measured in a 500x160x3 mm specimen made of an aluminum alloy (ay = 256 MPa, au = 471 MPa) with a fatigue crack. The residual stresses were induced by local heating at a distance of 30 mm from the centre of the specimen. As can be seen from Figure 7, in the heating zone, both components of the residual stress are tensile. In the compression zones, the longitudinal component of residual stresses reaches minus 130 MPa.

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