NONLINEAR STATIC AND MULTI-AXIAL FATIGUE ANALYSIS OF AUTOMOTIVE LOWER CONTROL ARM USING NEiNASTRAN
NONLINEAR STATIC AND MULTI-AXIAL FATIGUE ANALYSIS OF AUTOMOTIVE LOWER CONTROL ARM USING NEiNASTRAN 
Dr. J.M. Mahishi, Director Engineering
MS&M Engineering Inc, Farmington Hills, MI, USA
SUMMARY
The Lower Control arm is the most vital component in a suspension system. It is usually a steel bracket that pivots on rubber bushings mounted to the chassis. The other end supports the lower ball joint. Significant amount of loads are transmitted through the control arm while it serves to maintain the contact between the wheel and the road and thus providing precise control of the vehicle. The finite element analysis of the control arm using NEiNASTRAN is presented. Inertia relief analysis was carried out as the measured road loads were in self equilibrium. The zone of maximum stress in most of the load cases studied is close to the left strut bush and around the lower ball joint bush. Since the stresses exceed the material yield strength, a nonlinear static analysis of the control arm was also carried out using NEiNASTRAN and the results are compared well with MSC.NASTRAN and ABAQUS analysis. Multi-axial Fatigue analysis using NEiNASTRAN and WINLIFE is discussed.
1 Background 
All the loads acting on the control arm are dynamic in nature. The vehicle dynamics and desired ride and handling specifications of the vehicle require that the control arm has certain stiffness. The design of control arms involves optimising for strength, stiffness and weight. Designing for some less frequent severe loads (pot holes, curb impact etc.) will lead to heavier sections. Based on years of experience, designing practice allows for occasional overloads. As a result the control arms have a limited life. A reliable fatigue analysis is required to ensure that the control arms at least survive the expected life span of the vehicle.
The approach in this study is to subject the control arms with bushings to peak loads of varies operating conditions individually and perform static linear Inertia relief analysis (Ref. 1-3). For the load cases in which the stresses exceed yield strength of the material perform static non-linear analysis. Assuming that
NONLINEAR STATIC AND MULTI-AXIAL FATIGUE ANALYSIS OF AUTOMOTIVE LOWER CONTROL ARM USING NEiNASTRAN
all load cases are in sync and proportional, the fatigue life is estimated from individual load conditions using the Miner’s cumulative theory. The assumption that all loads act in sync provides a safe conservative estimate. Whereas if more accurate estimate is required, the multi-axial alternating fatigue stress state can be analysed using multi-axial module in WINLIFE.
2 Lower Control Arm FEA
The lower control arm is subjected to different magnitude of forces depending on the event (Table 1). In real life these events occur in varying sequence in and in varying combination.
Table 1: Typical road loads.
In order to develop an optimum weight, strength and stiffness of the control arm, one has to study the response of the control arm during all operational loading conditions.
The concept cast steel control arm was modeled using second order 19,000 tetra elements (Figure 1). The bushings were represented by spring elements with spring rates in 3 directions.
NONLINEAR STATIC AND MULTI-AXIAL FATIGUE ANALYSIS OF AUTOMOTIVE LOWER CONTROL ARM USING NEiNASTRAN
3 Inertia relief analysis
The forces acting on the control arm are dynamic in nature. The measured road loads are in equilibrium with the inertial forces from the sprung control arm. To solve such systems NEiNASTRAN (also MSC.NASTRAN / ABAQUS) provide a method in which the inertia forces are computed and subtracted from applied loads. In applying static inertia relief method to dynamic loading, it is assumed that the natural frequency of the system is at least twice that of the highest loading frequency. (2). The results of static inertia relief analysis are shown in Table 2.
NONLINEAR STATIC AND MULTI-AXIAL FATIGUE ANALYSIS OF AUTOMOTIVE LOWER CONTROL ARM USING NEiNASTRAN Von Misses Stress | Maj. Principal Stress |
NEi | MSC | NEi | MSC |
1 | 1g Vert | 287 | 299 | 329 | 321 |
2 | 3g Vert | 891 | 887 | 990 | 986 |
3 | Curb Push off Left Leading | 349 | 341 | 386 | 379 |
4 | Curb Push off Left Trailing | 390 | 382 | 318 | 311 |
5 | Max Aft Acc | 444 | 438 | 498 | 490 |
6 | Max Aft Brake | 238 | 246 | 216 | 159 |
7 | Max Corner Left Turn | 119 | 132 | 123 | 127 |
8 | Max Corner Right Turn | 720 | 720 | 430 | 429 |
9 | Max Fore Acc | 183 | 180 | 199 | 189 |
10 | Max Fore Brake | 495 | 488 | 557 | 550 |
11 | Max Roll Left In Jounce | 683 | 678 | 735 | 731 |
12 | Max Roll Right In Jounce | 125 | 127 | 127 | 131 |