CHAPTER 16 EXPERIMENTAL STRESS ANALYSIS Introduction We live today in a complex world of manmade structures and machines.We work in buildings which may be many storeys high and travel in cars and ships,trains and planes;we build huge bridges and concrete dams and send mammoth rockets into space.Such is our confidence in the modern engineer that we take these manmade structures for granted.We assume that the bridge will not collapse under the weight of the car and that the wings will not fall away from the aircraft.We are confident that the engineer has assessed the stresses within these structures and has built in sufficient strength to meet all eventualities. This attitude of mind is a tribute to the competence and reliability of the modern engineer. However,the commonly held belief that the engineer has been able to calculate mathemati- cally the stresses within the complex structures is generally ill-founded.When he is dealing with familiar design problems and following conventional practice,the engineer draws on past experience in assessing the strength that must be built into a structure.A competent civil engineer,for example,has little difficulty in selecting the size of steel girder that he needs to support a wall.When he departs from conventional practice,however,and is called upon to design unfamiliar structures or to use new materials or techniques,the engineer can no longer depend upon past experience.The mathematical analysis of the stresses in complex components may not,in some cases,be a practical proposition owing to the high cost of computer time involved.If the engineer has no other way of assessing stresses except by recourse to the nearest standard shape and hence analytical solution available,he builds in greater strength than he judges to be necessary (i.e.he incorporates a factor of safety)in the hope of ensuring that the component will not fail in practice.Inevitably,this means unnecessary weight,size and cost,not only in the component itself but also in the other members of the structure which are associated with it. To overcome this situation the modern engineer makes use of experimental techniques of stress measurement and analysis.Some of these consist of"reassurance"testing of completed structures which have been designed and built on the basis of existing analytical knowledge and past experience:others make use of scale models to predict the stresses,often before final designs have been completed. Over the past few years these experimental stress analysis or strain measurement techniques have served an increasingly important role in aiding designers to produce not only efficient but economic designs.In some cases substantial reductions in weight and easier manufactur- ing processes have been achieved. A large number of problems where experimental stress analysis techniques have been of particular value are those involving fatigue loading.Under such conditions failure usually starts when a fatigue crack develops at some position of high localised stress and propagates 430CHAPTER 16 EXPERIMENTAL STRESS ANALYSIS Introduction We live today in a complex world of manmade structures and machines. We work in buildings which may be many storeys high and travel in cars and ships, trains and planes; we build huge bridges and concrete dams and send mammoth rockets into space. Such is our confidence in the modern engineer that we take these manmade structures for granted. We assume that the bridge will not collapse under the weight of the car and that the wings will not fall away from the aircraft. We are confident that the engineer has assessed the stresses within these structures and has built in sufficient strength to meet all eventualities. This attitude of mind is a tribute to the competence and reliability of the modern engineer. However, the commonly held belief that the engineer has been able to calculate mathemati￾cally the stresses within the complex structures is generally ill-founded. When he is dealing with familiar design problems and following conventional practice, the engineer draws on past experience in assessing the strength that must be built into a structure. A competent civil engineer, for example, has little difficulty in selecting the size of steel girder that he needs to support a wall. When he departs from conventional practice, however, and is called upon to design unfamiliar structures or to use new materials or techniques, the engineer can no longer depend upon past experience. The mathematical analysis of the stresses in complex components may not, in some cases, be a practical proposition owing to the high cost of computer time involved. If the engineer has no other way of assessing stresses except by recourse to the nearest standard shape and hence analytical solution available, he builds in greater strength than he judges to be necessary (i.e. he incorporates a factor of safety) in the hope of ensuring that the component will not fail in practice. Inevitably, this means unnecessary weight, size and cost, not only in the component itself but also in the other members of the structure which are associated with it. To overcome this situation the modern engineer makes use of experimental techniques of stress measurement and analysis. Some of these consist of “reassurance” testing of completed structures which have been designed and built on the basis of existing analytical knowledge and past experience: others make use of scale models to predict the stresses, often before final designs have been completed. Over the past few years these experimental stress analysis or strain measurement techniques have served an increasingly important role in aiding designers to produce not only efficient but economic designs. In some cases substantial reductions in weight and easier manufactur￾ing processes have been achieved. A large number of problems where experimental stress analysis techniques have been of particular value are those involving fatigue loading. Under such conditions failure usually starts when a fatigue crack develops at some position of high localised stress and propagates 430