Introduction to forensic engineering
5 Containment problem: car radiator reservoir
The polymer tips and feet of the ladder considered in Section 4 are the most vulnerable to wear and tear because of their contact with the wall and ground. They are the safety-critical parts of the structure, not just for extension ladders but also for step ladders, loft ladders and other domestic and industrial ladders. So are the seals briefly mentioned in Section 2 of this unit. Such components are cheap and easy to replace, but the costs of either not doing so or using the wrong replacement can be serious. Like seals, containment materials must be resistant to the fluids they contain especially where the fluids can be hazardous. The two applications considered in this section and the next are a car radiator reservoir and a bulk storage tank.
The problem of the quality of materials and manufacturing processes used to shape those materials, apply to all systems in a car cooling system. A car radiator reservoir must resist the high temperature of the water,high pressure, and the high temperature in the engine compartment. New materials, such as glass-reinforced nylon, have become widespread for applications under the bonnet, and have proved themselves in applications such as inlet manifolds sitting directly on the engine. However, manufacturing standards must be high to eliminate defective mouldings.
Problems can be created by faulty design of storage tanks, where polymeric materials have good resistance to corrosion from powerful chemical liquids. The dangers of tank leakage, however, mean the design must meet minimum standards.
The storage tank case study raises the problem of standards in general, where national institutions often cannot supply appropriate guidance. It occurs because of the often inevitable delays in forming a committee to draft standards, so standards do not become available until several years after design and manufacture have begun. In the case study, a German standard was available to the engineers concerned, but was not apparently applied to the tanks in question.
In Section 2, you saw that high-temperature seals in central heating systems must at least resist hydrolytic degradation, and no less is true of new radiator materials. Radiator reservoirs must resist water temperatures of 70–80° C, high internal pressures and high external temperatures.
New materials such as glass-reinforced nylon, have become widespread for under-the-bonnet applications, and have proved ideal for inlet manifolds for example (Figure 63). The reasons for their adoption are multifold: they can be moulded into complex shapes so that air flow into the engine can be controlled effectively. This is a requirement of advanced fuel management systems.
Interestingly enough, inlet manifolds are made using metal cores that must be melted out after each injection moulding cycle, a new process route known as lost-metal moulding. This overcomes the so-called re-entrant angle problem faced by conventional injection moulding, where the core must be capable of being withdrawn at the end of the cycle to release the moulded product.
Such manifolds are lighter than cast metal equivalents, so that total engine weight is kept down and makes the car more efficient. A subsidiary bonus comes from greater ease of installation during assembly in the car factory. On the deficit side of the costs equation comes the greater capital cost of tool and machine investment than conventional materials, and the problems of developing new materials.
Such developments have encouraged use of, glass-filled nylon (GF nylon 6,6) for other car engine parts, and in the case examined here, car radiator reservoirs. The case study is presented in detail in Paper 3 which is attached as a pdf and should be printed out (if possible) to gain the maximum benefit from the discussion of this case study. The paper describes in a scientific format, the way the failure investigation developed, what initial inspection revealed and what tests were used to explore the failure. The results are discussed below by extracting key information needed for analysis, but you will need to read the paper in detail to gain a full understanding of the problem. As in succeeding case studies in this block, information not given in the original paper is also provided here. It will give you a wider perspective of the problem.
Click on the 'View document' link below to read Paper 3 (keep the paper open on your desktop for reference throughout Section 5).