Lecture 4 Weld Residual Stresses, Distortion and Fatigue

1. Lecture 4 Weld Residual Stresses, Distortion and Fatigue 助理教授：王惠森 huei@mail.isu.edu.tw 您可以在以下的網址 Down Load 本檔案 http://140.127.180.27/www/index.htm

2. Overviews-I • Residual stresses are those that would exist in a body if all external loads were removed. They are sometimes called internal stresses. Residual stresses that exist in a body that has previously been subjected to nonuniform temperature changes, such as those during welding, are often called thermal stresses σ

3. Residual Stress • Depending on the scale at which the matter is analyzed, different kinds of stresses are classically defined. Metallurgists and engineers mainly use optical methods to study the material (optical or electron microscope, etc.). • Three kinds of residual stresses are therefore usually defined : the macro stresses (or stresses of first kind) over a few grains, the stresses of second kind over one particular grain and the stresses of third kind across sub-microscopic areas, say several atomic distances within a grain. The stresses of second and third kind are also called micro stresses.

4. Changes in temperature and stress during welding-1 • Since section A-A is not affected by the heat input, the thermally induced stress σxis zero. • Along section B-B, σxis close to zero in the region under-neath the heat source, since the weld pool does not support loads. In the regions somewhat away from the heat source, stresses are compressive (i.e., σx is negative), because the expansion of these areas is restrained by the surrounding metal of lower temperatures. σx reaches the yield strength of the base metal at corresponding temperatures; σxin the areas farther away from the weld are tensile and balance with compressive stresses in areas near the weld. • Section A-A is ahead of the heat source and is not yet significantly affected by the heat input • section B-B, which crosses the heat source, the temperature distribution is rather steep • Distribution becomes less steep at some distance behind the heat source (i.e., along section C-C), • uniform very far away from the heat source, along section D-D

5. Changes in temperature and stress during welding-2 • Along section C-C, the weld metal and the adjacent base metal have cooled and hence have a tendency to shrink, thus producing tensile stresses;that is, σx is positive, As the distance from the weld increases, σx first changes to compressive and then becomes tensile. • Finally, along section D-D, high tensile stresses are produced in regions near the weld, and compressive stresses are produced in regions away from the weld. Since section D-D is well behind the heat source, the stress distribution does not change significantly beyond it, and this stress distribution is therefore the residual stress distribution.

6. Typical Distributions of Residual Stresses • Figure shows typical distributions of residual stresses in a butt weld. According to Masubuchi and Martin (3), the distribution of the longitudinal residual stress σx can be approximated by the following equation: • where σm is the maximum residual stress, which is usually as high as the yield stress of the weld metal. The parameter b is the width of the tension zone of σx

7. Distribution of the Transverse Residual Stress • The distribution of the transverse residual stress σy along the length of the weld is shown in Figure. • As shown, tensile stresses of relatively low magnitude are produced in the middle part of the weld, and compressive stresses are produced at the ends of the weld. • If the lateral contraction of the joint is restrained by an external constraint, approximately uniform tensile stresses are added along the weld as the reaction stress (1). This external constraint, however, bas little effect on σx

8. 鑽孔法量測殘留應力 • 根據彈性力學基本觀念，在垂直於任何自由表面的應力必自動歸零。由此觀點，欲量測試件表面的殘留應力，我們可經由鑽孔法獲得。 • 理由是在試件表面鑽一圓柱形小孔，如果在試件表面具有應力場，那麼在沿著孔壁表面的應力勢必會釋放出來，由此種應力從新分配而產生的應變，吾人可藉由預先黏貼的應變規量得，再由線性彈性力學推導得到試件表面殘留應力。

9. X-Ray position sensitive detector • Goniometer Ffor the measurement of the stress tensor. • X-Ray P.S.D. that permits a quick acquisition of diffraction peaks. Many type of Anti-cathodes are available (chromium, manganese, cobalt, copper and iron). • Electronic Displacement Pick-up, allowing precise centering of goniometer on material to be measured. • X-Ray collimator • Y goniometer for stress measurements. • Sample to examine

10. Basic principle of X-Ray diffraction method • A Y goniometer takes several measures of the q diffraction angle as function of the incidence angle Y; then one traces on a diagram a straight line passing through the measures points . • The slope of the straight line linked to the elastic constants of the material, determines the stress.

11. Magnitude Of Stresses- A Simple Analogy

14. Distortion - Prevention

15. Method-1 Tack welding procedures used for tack welding to prevent transverse shrinkage • tack weld straight through to end of joint • tack weld one end, then use back-step technique for tacking the rest of the joint • tack weld the centre, then complete the tack welding by the back-step technique

16. Method-2 Back-to-back assembly Back-to-back assembly to control distortion when welding two identical components a) assemblies tacked together before weldingb) use of wedges for components that distort on separation after welding

17. Method-3 Stiffening • Longitudinal shrinkage in butt welded seams often results in bowing, especially when fabricating thin plate structures. Longitudinal stiffeners in the form of flats or angles, welded along each side of the seam (Fig ) are effective in preventing longitudinal bowing. Stiffener location is important: they must be placed at a sufficient distance from the joint so they do not interfere with welding, unless located on the reverse side of a joint welded from one side.

18. Method-4 Welding process • General rules for selecting a welding process to prevent angular distortion are: • deposit the weld metal as quickly as possible use the least number of runs to fill the joint • Unfortunately, selecting a suitable welding process based on these rules may increase longitudinal shrinkage resulting in bowing and buckling.

19. Method-5 Welding technique • keep the weld (fillet) to the minimum specified size • use balanced welding about the neutral axis • keep the time between runs to a minimum

20. FATIGUE Adapted from Fig. 8.16, Callister 6e. (Fig. 8.16 is from Materials Science in Engineering, 4/E by Carl. A. Keyser, Pearson Education, Inc., Upper Saddle River, NJ.) • Fatigue = failure under cyclic stress. • Stress varies with time. --key parameters are S and sm • Key points: Fatigue... --can cause part failure, even though smax < sc. --causes ~ 90% of mechanical engineering failures. 17

21. FATIGUE DESIGN PARAMETERS • Fatigue limit, Sfat: --no fatigue if S < Sfat Adapted from Fig. 8.17(a), Callister 6e. • Sometimes, the fatigue limit is zero! Adapted from Fig. 8.17(b), Callister 6e. 18

22. FATIGUE MECHANISM • Crack grows incrementally typ. 1 to 6 increase in crack length per loading cycle crack origin • Failed rotating shaft --crack grew even though Kmax < Kc --crack grows faster if • Ds increases • crack gets longer • loading freq. increases. Adapted from Fig. 8.19, Callister 6e. (Fig. 8.19 is from D.J. Wulpi, Understanding How Components Fail, American Society for Metals, Materials Park, OH, 1985.) 19

23. IMPROVING FATIGUE LIFE 1. Impose a compressive surface stress (to suppress surface cracks from growing) Adapted from Fig. 8.22, Callister 6e. --Method 1: shot peening --Method 2: carburizing 2. Remove stress concentrators. Adapted from Fig. 8.23, Callister 6e. 20

24. THERMAL FATIGUE OF SOLDER • Fatigue failures in the solder joints may occur even though the cyclic thermal stresses and strains generated may be well below the yield limits. For this reason, the failures are immature and unexpected.