This problem was observed repeatedly in expensive one thousand hours thermal fatigue test which is mandatory for this component approval. After elimination of various reasons in course of failure analysis and careful review of all the processes during manufacturing, it was suspected that residual stresses developed either in the casting process itself or the subsequent machining were leading to cracks at the vulnerable location of valve bridge. So it was decided to measure residual stresses at the valve bridge after different stages of manufacturing. Hence it was essential that the measurement technique to be nondestructive. The method of X-ray diffraction was not feasible in this case due to non-accessibility at this location. Hence it was decided to try Barkhausen noise-magneto elastic method to measure residual stresses which is a well established method for ferrous materials.

STRESSCAN 500C, equipment by American Stress Technologies was used which works on Barkhausen noise principle. To convert this noise into stress units, uniaxial calibration in tension and compression was carried out with a specimen machined from the casting. Various experiments conducted to evaluate the residual stress concluded that the source of tensile residual stresses was the casting process and the stress relieving cycle used was incapable of relieving the stresses fully. Hence the stress relieving cycle was modified and was evaluated by the same method for the effectiveness in relieving stresses. The cylinder heads stress relieved by the modified process passed the thermal fatigue test without cracking. This has helped in indigenizing the critical component and has resulted in considerable saving of foreign exchange.