Total life Durability is a major factor that must be considered in the manufacture of transportation vehicles. Stresses and torsion have to be considered due to vehicle being regularly subjected to potholes, kerbs, reduced height accessibility mechanisms in its daily service life.
When a vehicle is designed, it must be suitable for any situation that my occur, these include:
1 - Potholes
2 - Speed bumps
3 - Towing
4 - Hoisting
5 - Jacking
6 – Full life durability
There are various testing methods that can determine if the vehicle is able to withstand these;
Distortion testing – the objective of this test is to observe how a vehicle will react when subjected to speed bumps and potholes. The vehicle is held in a longitudinal twist and the wheels are lifted or lowered depending on the test type (lifted to simulate speed bumps, lowered to simulate pot holes).
Towing – there are two types of tow testing; static and dynamic. A static tow test is used to assess the strength of the towing fixtures during static loading conditions of the vehicle. The test entails a static tension load 12 times the vehicle's curb weight applied to the front and rear of the vehicle, with pulls in multiple directions.
The dynamic tow is used to test the integrity of the vehicle's towing fixtures and the feasibility of towing a bus using heavy duty wreckers. A vehicle is towed at curb weight for five miles at 20mph for each towing configuration.
Hoisting – The objective of this test is to determine any damage that might be caused by the jack stands or the jacking pads. The vehicle is raised to a sufficient height and the jack stands are placed under the jacking pads or axles and are checked for damage to the pads or bulkheads.
Jacking – this test is used to determine the damage caused by a deflated tyre and to determine if the vehicle can be jacked using a portable hydraulic jack to replace a deflated tyre. The test simulation involves deflating each tyre on the vehicle (corner by corner) and jacking the vehicle up with three inches of clearance to replace the tyre.
Full life durability – as well as simulating situational testing, it is also important that the durability of the vehicle is tested to ensure it can withstand any additional stresses it may see throughout its service life. This test simulates 25% of the service life of the vehicle, by adding several different stresses to the vehicle and inducing elements that subject the vehicle to the types of events that are expected to be encountered during transit.
Durability is an important factor that must be considered when designing a vehicle; it must not need extensive repair procedures and must withstand or fulfil the life intended on the road. The vehicle must be flexible to allow for vehicle movement on uneven road surfaces, rapid turning / lane change (heavy load shift in weight) and hitting foreign objects which may be in the road. It must be able to sustain impact without dismantling and it must be safe for both the driver and the passengers on board.
Durability and joint design go hand in hand when it comes to the sustainability of an adhesive joint. Joint strength is ultimately dependent on the adherend material, adherend thickness and bond-line thickness. Small variations in bond-line thickness can result in significant changes to the bond strength. To ensure that joining adhesive remains intact and in service, the bond-line thickness must be controlled using a uniform layer across the entire joint; ensuring that the ideal volume of adhesive is used for the joint area. Using a thicker bond-line on a joint can give rise to high level of voids leading to debonding. Conversely, a bond-line which is too thin can lead to adhesive starvation and ultimately de-bonding. Bond-line thickness can be controlled by a physical fixture of separation; spacers, wires and glass balls.
The adhesive type used to join materials together plays a large part in vehicle movement and torsion. An adhesive’s flexibility and structural integrity varies vastly depending on the chemistry type and the adhesive properties. It is imperative that the correct adhesive is chosen for the right application and the right questions are asked before joining materials together with adhesives;
- Does it adhere to the substrates to be bonded?
- Does it withstand chemicals?
- Does it provide a water tight bond?
- How flexible is the adhesive?
- Does the viscosity and cure time suit the application and processing?
- Is the adhesive conductive or insulative?
Recently, Forgeway conducted testing on a plywood floor joint using two different structural adhesives. The reason for this testing was; a customer was experiencing witnessing through vinyl flooring on a subfloor and a ‘tacking’ noise was evident from the distribution of passenger loads. To tackle this problem Forgeway tested the previous adhesive used by the manufacturer (external, 2-component structural product) and another that was manufactured by Forgeway; Aerok KF500. To test the structural integrity of the products, the following tests were undertaken:
- Compression testing (ASTM D3410)
- Tensile testing to plywood flooring (ASTM D412-16)
- Overlap shear strength (ASTM D1002)
- Substrate specific adhesion testing to plywood flooring (FGY009)
Results showed that the external product had lower compressive and tensile extension loads compared to that of Aerok KF500. Aerok KF500 also showed excellent elastic recovery, whereby the material did not crack and achieved a recovery of 57%. Whereas the external product cracked and only recovered by 16%. In the following graphs, it is possible to see the difference in both flexibility and strength of the two products tested.
Overlap shear strength
Tensile testing on plywood
The outcome of the testing found that the witnessing through the flooring was related to the flexibility and elastic recovery of the adhesive and once replaced with Aerok KF 500, the witness and ‘tacking’ noise was eradicated and enabled movement throughout the plywood flooring joints.
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