DESIGN DETAILS 



3-23 



stiff doubled portion of the bottom of the hull will deflect as a unit, and modify the deflec- 

 tion curve which would exist if the hull were single skin throughout. The junction of the 

 two sections then becomes a hard spot liable to failure. The solution to the problem is to 

 run the inner bottom to the side as shown in Fig. 3-52b, even at the cost of introducing a 

 slight slope to the outboard portion of the inner skin. 



-CENTERl I NE 



DEFLECTION CURVE 

 UNDER BOTTOM LOAD 



WEAK SPOT DUE TO 

 ABRUPT CHANGE IN 

 RIGIDITY 



EDGE OF INNER BOTTOM 



CARRIED OUTBOARD TO 



NEARLY VERT I CAL SI DE 



SHELL AND LANDED ON 



STIFFENER OR REINFORCED 



AREA TO EASE TRANSITION 



b. IMPROVED 



Fig. 3-52. Double Bottom Arrangement 



Stress Concentrations 



A major problem in all structural design is stress concentration. The usual design 

 formulas give stresses based on the assumption that the member being considered is of 

 uniform, or at worst gradually changing shape. When an abrupt change in shape occurs, 

 such as a hole, sharp bend, or a lap joint in a tension member, stresses much higher than 

 simple standard calculations indicate can occur. As explained in Chapter 5, this is 

 particularly important in fiberglass design because of the material's lack of ductility. 



The nature of a reinforced laminate is such that discontinuities causing stress concen- 

 trations are often introduced by the laminating process. For instance, a change in the 

 number of plies of reinforcement, small laps between adjacent pieces of reinforcement, 

 thick and thin spots in mat, voids and resin rich or poor areas are all examples of discon- 

 tinuities in the material which should be avoided or compensated for. In the case of the 

 change in the number of plies, the over-all thickness of the laminate determines the degree 

 of severity of the discontinuity. A change from 8 to 9 plies is obviously less serious than 

 a change from 8 to 4 plies. Abrupt changes in the number of plies such as 8 to 4 will 

 create a high stress concentration which can be easily avoided by gradually reducing the 

 plies with a generous distance between endings. 



There exists a considerable amount of theoretical and experimental data and experience 

 on stress concentrations in elastic, isotropic materials, to assist designers in determining 

 their effect and to establish design rules. Unfortunately, similar technical information does 

 not exist for fiberglass laminates. Some experimental work (4) has been done with laminates 

 reinforced with 181 glass cloth only, and the number of samples tested for each configura- 

 tion was limited. Therefore the effects of differences in reinforcements as well as varia- 

 tions in fabrication could not be evaluated. These effects must be considered when lami- 

 nates reinforced with different types of reinforcement are used. 



The major causes of stress concentrations in any material are holes, notches, and 

 abrupt changes in the geometric property, such as, area or section modulus, which con- 

 trols the stress in the cross section being considered. If holes must be cut, they should 

 be kept as small as possible in relation to the size of the member. If a transition must be 

 made in the depth of a shell longitudinal, it should be made gradually, preferably, at a 4 

 to 1 slope. 



