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is subjected in the welding operation. The joint may crack through the dam- aged zones during fabrication and this phenomenon may be carried over into the service behavior of the structure if the damage is not corrected. Second of the factors controlling the production of homogeneous welded joints is the design of the joint it- self. It is the rule rather than the exception that stress concentrations of a very severe nature may exist in the joint. Fig. 4 shows the hetero- geneous stress condition which exists in an improperly designed welded joint. This photoelastic study repre- sents a loaded butt joint between two pieces of metal of equal thickness. Stress Variations Too High The two welds, deposited from each side, did not meet at the center. The unfused portion forms an_ internal boundary which produces an extreme variation in stress over the joint. At the end of this crack, the stresses are extremely high. In addition, inter- mediate high stresses exist at the ends of the reinforcements which are usu- ally, and vainly, applied in order to strengthen this type of joint. Such reinforcements are effective only un- der static load; under impact or fatigue loads, the changes in contour at the ends of the reinforcements in- troduce additional stress maximums. Third of the factors influencing the production of homogeneous welded joints is the element of residual stress introduced in the structure through contraction of cooling weld metal. Lit- tle is known of the direction or mag- nitude of these thermal stresses. There is direct evidence, however, that the residual stresses may in many cases exceed the yield point of low-carbon steel. When a known service load is super- imposed on a structure which is al- ready loaded by residual stresses in an indeterminate manner, the elastic Fig. 7 (Left and center)—Stress distribution usually found in welds made from one side of a plate. from one side. Penetration of weld metal cannot be completely controlled Fig. 8 (Right)—The overlap, in which weld metal runs onto the plate surface and solidifies without fusing, is another common defect conditions which exist from point to point are highly chaotic. The structure may be so highly preloaded that its permissible load in service is only a small fraction of the designed load. Structures, the natural shape of which is such that the stresses are self-lock- ing, often fail through the partially completed welds during fabrication, due to some shop handling condition —-such as hammering or dropping— which applies even a small shock load to the structure. This behavior is not due to “brittle welds,” as has been commonly supposed. Warping Often Occurs In addition to the elastic chaos that residual strains may produce in a welded structure, severe warping will occur when the strains are relieved, Fig. 6—Contrasting conditions around a fillet weld with concave contour and another with triangular cross section. The triangular throat weld has a low average stress. but a high maximum stress because of abrupt change in contour. The concave throat weld has high average stress but low maximum stress and better distribution MARINE REview—January, 1934 either in the machining operation or by a gradual creep over a period of time. A battery of machines, in which proper functioning depended upon maintained accurate alignment, was rendered progressively ineffective as the machine housings gradually crept out of shape. In two years, the machines were useless. Fig. 5 represents graphically the abortion which can result from the improper execution of a welded joint. This joint is far from homogeneous. Physical properties are not uniform from point to point, since there eare zones of badly damaged metal. The stress values throughout the joint when loaded vary tremendously from point to point, because, in addition to the stress concentrations which exist at internal and external bound- ary discontinuities, the unknown re- sidual stresses add to the concentrated load stresses, and thus produce utter chaos. Parent Metal Damaged Note the points of extreme stress concentration at the ends of the in- ternal boundary, the points of lesser stress concentration at the ends of the reinforcement and around the under- cut. Zones of badly overheated and quenched parent metal surround the weld metal. The situation is aggra- vated further by the co-existence of damaged material and high stress at the same place. The undercut local- izes a high stress at a point where the parent material is in its poorest condition. Whenever a welded joint, whatever its type, functions in a nonhomogene- ous manner, one or more of three de- structive factors are operating—sharp changes in contour, damaged mate- rial, and preloading. For example, 17