Pseudo dynamic loads energy dissipation in mechanical shock absorber seismic damper

Earthquake tremor from neighbouring country had caused several cases of structural and non-structural damages toward the buildings in Malaysia. This issue had raised public attention about the safeness of the low-rise, mid-rise and high-rise building in the country. Buildings without proper seismic resistance design will collapse during the earth earthquake and people who could not evacuated from the building will be buried alive. Viscous Damper is one of the passive structural control systems in attenuating the seismic response on building. The damper utilized principle of fluid flow through orifices to create friction that turns the movement into heat energy to be release into atmosphere. This study is to investigate the effectiveness and behaviour of the mechanical shock absorber as damper in resisting seismic force. Three different type of dampers are being tested under 6 different loading displacement amplitude to measure the hysteresis loops and energy dissipation capability. The results showed that the extra features of the coil over in EX5 Kriss Wave damper with 17 kNmm had significantly increase the energy dissipation capability up to 5.6 time better than the lowest 3 kNmm APIDO type damper.


Introduction
Earthquake has been recorded nearly 4000 years and earthquake is one of the deadliest natural disasters in human history [1]. For instance, one of the most devastating earthquakes with the 8.0 magnitude happened in Sichuan China province in the year 2008 had took away over 70,000 lives; Algeria 2,700 people were killed in the earthquake with magnitude 6.8 in year 2003; Marmara earthquake in Turkey had also killed 17,000 people in year 1999 [2]. Apart from the human death tolls, the earthquake also adversely affects the economy of the country especially Marmara earthquake had caused over 5 billion USD dollar economic impact towards the country [3]. Malaysia is a country being unique and strategic in its geological location, being located away from the Ring of Fire. However, the risk of an earthquake strike to Malaysia is underrated by considering the recent geological trend manifested in neighbouring countries [4]. Being near to the Sumatran Island, Peninsula Malaysia poses risk from far-field earthquakes especially from Sumatran Fault and the subduction zone as Sumatran Island. These zones are known to be in the 'hot area' to earthquake due to the geological locations in the Ring of Fire [5]. On 5th June 2015, an earthquake with a magnitude of 5.9Mw hits Sabah with epicentre 16 km away from Ranau caused 19 people lose their lives in such event [6]. Due to aftermath of an earthquake such as casualties, epidemics, economic impacts, and cost of repair, Malaysia has started to practice seismic design in reinforced concrete structure especially in the high-risk seismic regions in Sabah [7].
To maintain the structural integrity, passive damping systems have been developed and used in the recent two decades in seismic active countries such as Japan, Italy, and New Zealand. These passive damping systems are including with state of art viscous damper, tuned mass damper, seismic isolation systems, mechanical energy dissipator, and friction dampers [8]. Recent application of the fluid viscous dampers to reduce the seismic damage caused towards the buildings is also located in Japan's Saitama Citizen Medical Center, a new 6 story hospital equipped with twelve fluid viscous damper in addition to a base isolation system to shield the building from any seismic caused ground motions and vibrations [9].
The experiment done by Constantinou (1995) with three-storey height of scale down model was tested with and without fluid viscous damper (FVD) which six dampers were installed in pair at each storey. Based on the tested experimental results, a building with FVDs can provide a 50% reduction in structural response than those without FVDs [10]. A shock absorber or otherwise known as the damper as shown in Figure 1, is a mechanical or hydraulic device designed to absorb damp and shock impulse. In a vehicle, shock absorbers are used to stabilize the car when traveling across rough and bumpy ground. The mechanism behind the mechanical shock absorber is absorbing dampens or impact due to load and turn out to become vibration or spring oscillations [11]. These vibration and oscillation eventually turn into heat energy stored in the viscous fluid due to fluid friction and exhausted to the atmosphere by thermal conductivity [12]. The aim of the present study is to investigate the compatibility of using the mechanical shock absorber [13] to replace the role of energy dissipation devices or control system in structures to reduce excessive movement and vibration during seismic events. In this study, the main objective is to investigate the behaviour and energy dissipation of gas pressure shock absorber damper [14] in resisting seismic force to close the research gap in utilizing automotive mechanical damping system on to the structure.

Methodology
In this study, three types of mechanical shock absorber from different manufacturers were employed namely EX5 Kriss Wave Dash, APIDO and SKK Racing. All shock absorbers chosen for this study are adjustable gas pressured mechanical shock absorber, adjustable in length and filled with methyl silicone oil. The three types of the absorbers have same piston diameter 10mm and maximum length of 340 mm.

Dynamic universal tensile machine testing setup
Servo-Hydraulic Dynamic Universal Tensile Machine was used to carried out the cyclic loading test in this study. The Servo-Hydraulic Dynamic Universal Tensile Machine utilized the E-type loading frame from the Shidmazu Servopulser Series [15]. The capacity of the actuator is 50 kN and the available apply stroke is ±25mm for the selected testing machine. Figure 2 showed the actual installation of the damper on to the universal testing machine. The damper (Mechanical Shock Absorber) was mounted on the universal testing machine. The load cell mounted on the hydraulic crosshead and stroke detector are responsible to record the loads and displacement data and store in the computer with controller.

Test parameter
A preliminary test was carried out for each condition to pinpoint the errors, precautions, and improvements that are required before the actual test on the mechanical shock absorber was carried out [16]. The tests were carried out at the room temperature. There are 6 sets of tests were carried out for each type of mechanical shock absorber with maximum spring length of 18 cm where the applied stroke from universal testing machine varies between the range of 1mm, 3mm, 5mm, 10mm, 15mm and 20mm. The input frequency is 1.0 Hz with total of 10 steady cycles obtained from a total of 20 repeated cycle to ensure the data obtained eliminated the extremes end of both the initial and the end of the testing process. The data collected from the tests are then used to generate a hysteresis curve along together with force-displacement chart.

Theoretical energy dissipation capacity of mechanical shock absorber
The experimental results are in terms of damping force versus displacement at different applied amplitude will be generated through the cyclic loading test. The force-displacement relation under each condition described previously were obtained. In this study, the model is  (1). This equation can be applied in the spring-damper system to obtain desired results [17]. Where the p is the output force from " " mass multiply with acceleration plus " " damping coefficient multiply with velocity plus "ku" stiffness multiply with displacement and all of them are associated with time (t). (1) where, The spring in the damper system provides compressive resistance and serve as the selfrestoration ability in the structural system back to the original position after the seismic motion. Spring is very good in absorbing sudden tension and compression loads to lower down the shock wave. However, spring potential energy and kinetic energy are zero due to elasticity in nature and thus area enclosed in hysteresis curve by the force, fs = ku is minimal. Figure 3 shows the force-displacement graph or commonly known as hysteresis curve or energy dissipation curve built up by the force fs and fd, where spring force in relation with stiffness and displacement, fs = ku and damper force in relation with damping and velocity, fd = . As such, the area of the hysteretic loop per cycle can be used to denotes the energy dissipation capability. By comparing the hysteresis loops, performances of the varied type of dampers under varied amplitude or displacement can be very different [17].  Figure 4 shows the changing process of the hysteresis curve with different dampers. By observing the changing process of the hysteresis curve in Figure 4(a); the damper's hysteresis curve begins with wide rounded shape. When the applied displacement of the damper increased to 3 mm and 5mm the hysteresis curves started to become narrow. When the displacement increased further to 10 mm and 15 mm a rounded irregular shape of hysteresis has formed. This phenomenon could be due to the irregular movement of the fluid within them damper. As the damper approach to 20mm displacement the compressive zone had spring stiffness influences to make the hysteresis curve formed a tail spike for SKK brand damper. These behaviour shares the same pattern for EX5 Kriss Wave brand damper as well. Figure 4(b) show the APIDO brand with perfect parallelogram under consistent incrementation of displacement from 1mm and 15mm. This damper shows the stability of load and unloading sequences unlike other dampers have instability in dissipating seismic energy which may cause unforeseen structural damages if applied on the structure. However, with increase of the loading displacement, the hysteresis curve started to distort in shape in a similar manner for all three types of shock absorber as shown in Figure 4 (a) -(c) especially at 20mm displacement which may be presume that the elastic properties of these damper started to dominate by spring effect. This is because spring also contributes compressive stiffness in term of force when the displacement increases tremendously.

Maximum damping force versus loading displacement of dampers
The maximum damping force versus loading displacement for various brands have been summarized in Figure 5. Maximum damping force at 5mm, 10mm, 15mm and 20mm loading displacement shows increment as the displacement increases. Only the starting loads for 1mm, 3mm and 5mm displacement for those 3 brands of the damper are similar. This is because in general most of the absorber damper shall perform similarly regular performance for all the end users. Significant load is required for EX5 Kriss Wave brand absorber damper with 0.94kN to achieve 10mm of displacement indicates this damper is stiffer than the other two brands which at the 5 to 10mm displacement region. This increase in stiffness contributes an additional energy dissipation capacity to the viscous damper by the increase in energy dissipation capacity observed in the graph. A damper with a softer stiffness is much preferable for a smooth energy dissipation to prevent sudden surge of energy to be experienced by the suspended object. SKK Racing brand also shows the same characteristic when 2.28kN of loads are required to achieve 20mm of damper's displacement indication it's intended design to keep as little as possible compression of the damper at 15-20mm displacement. Such loads showed in SKK Racing and EX5 Kriss Wave brands reflected the high spring stiffness which may not be ductile enough to absorb the dynamic movement of the structure as compared by soft 1.21kN at 20mm APIDO brand absorber damper. However, spring with low stiffness such as APIDO brand absorber may also post a threat where damper could not dissipate seismic energy through spring movement. Thus, the spring stiffness must be balanced together with the liquid damper to achieve greater seismic energy dissipation.

Energy dissipation of dampers
Under identical test conditions, the comparison of energy dissipation capacity between the 3 selected type of mechanical shock absorber was made as shown in Figure 6. Among 3 types of mechanical shock absorbers, EX5 Kriss Wave showed a higher energy dissipation capacity of 17.3172 kNmm. However, referring to the backbone curve on Figure 5, EX5 Kriss Wave has the highest activation force of 0.1804 kN compared to the other two types of mechanical shock absorbers. In other words, more force is required before EX5 Kriss Wave shock absorbers can play its role in damping the seismic force of the structure. SKK Racing however has a relatively lower activation force of 0.1226 kN and a high energy dissipation capacity of 13.7297 kNmm. APIDO mechanical shock absorber has the lowest activation force 0.0435 kN yet the energy dissipation capacity is also the lowest 2.611 kNmm only among the three types of shock absorbers due to the poor and narrow hysteresis loops. In short, EX5 Kriss Wave damper energy dissipation has 5.6 times better than the APIDO damper. Table 1 shows the comparison between the 3-shock absorber and its categorization for critical damping.

Conclusion
After conducting the cyclic loading test on three types of mechanical shock absorber, the behaviour and energy dissipation mechanism has successfully captured. In conclusion, the extra feature of the coil over spring for EX5 brand can increase the energy dissipation capacity considerably and proven from the positive relationship between the hysteresis loop that generated during the testing. These had led to positive energy dissipation capacity for all three types of mechanical shock absorber. Results proven as the energy dissipation capacity relates to the loading displacement for all types of mechanical shock absorbers.