Strength versus Toughness
Although counter intuitive increasing yield strength may not prevent failure.
In situations where there have been previous failures, designers often feel tempted to specify a ‘higher strength material’. This is all very well if the failure mechanism has been one of plastic overload, but in our experience, such plastic overload failures are remarkably rare.
Certainly, the component needs to be strong enough to sustain the applied loading, but once above that limit added strength is often detrimental as it is frequently achieved at the cost of ductility or toughness. It is very rare that materials are both strong and tough, and once a materials design is such that the material is strong enough, specifying additional strength capability is often self-defeating as such strength gain comes at a cost of reduced fracture toughness.
As an example, the casing of a rocket motor was specified with a grade 250 maraging steel, with a yield stress in excess of 1520MPa (but fracture toughness less than 65MPa√m) and a consequent critical flaw size under proof test conditions of only about 3mm! The whole structure failed under such proof testing.
Had the designers chosen a Grade 200 alloy which had a suitably high 1100MPa yield strength (but critically a fracture toughness in excess of 150MPa√m) the critical flaw size under the same proof testing stress would have been 18mm and have led to “leak before break” and a non catastrophic result. The additional strength specification was not needed. Once the component is “strong enough” strength additions are often detrimental (as toughness generally decreases with strength) and a suitably strong, tough material is a better choice.
This is also often the case with bolts. Although grade 12.9 bolts have a greater yield and tensile strength (and hence fatigue resistance), they are inherently less tough than lower grade components (e.g. Grade 8.8 or 10.9). As such, the higher strength variants are less crack tolerant and it is unwise to simply increase the grade where lower strength grades would suffice.
Published in Technical Tips by Origen Engineering Solutions on 1 October 2014