Thought for the day “Engineering is the art of modelling materials we do not wholly understand into shapes we cannot precisely analyse to resist loads we cannot properly assess in such a way that the community has no reason to suspect the extent of our ignorance” President, Scottish Branch - Institute of Engineering UK, 1946

Hydraulic System Overview - Norman Thornton, Engineering Consultant A brief description of the basic system elements will be followed by the effect of performance when specifying amplitude, frequency combined with equipment.

Servo Hydraulics in the field of mechanical testing for performance A fundamental need in all testing is repeatability which is enhanced by equipment operating in a stable and reliable condition. Changes should be to the specimen not the equipment doing the test. Do we have enough power? Why do we need this power? What can we do with the power we have. How can we reduce power? What is limiting my predicted performance. What do we provide to improve the predictive modelling in Design. Do we have a complete understanding of the test specimen in terms of material, manufacture and residual stress state.

BASIC HYDRAULIC CIRCUIT lcu pump Controller pressure return drain Distribution manifold

Road Load Data (Force & Deflection)

Pump Flow Capacity Check

Test Rig Hydraulic Flow

Accumulator Flow

Actuator Flow Calcs (Sine Wave) Pv (Peak Velocity for a Sine Wave) Pv = 2 π x frequency x ½ amplitude Pf (Peak Flow for a Sine Wave) Pf = 2 π x frequency x ½ x area

Compressibility of Oil As a general rule oil compresses at 0.5% per 1000psi (70 Bar) Example:- This represents an equal area actuator operating at 210 bar At null position, the pressure P on both sides of the piston is equal (105 bar) The pressure drop across the servo-valve is 70 bar (∆p), therefore the maximum dynamic pressure available is 210 – 70 = 140 bar At this pressure the compression volume = 140 / 70 = 2 and as a % = 2 x 0.5 = 1 % And the dynamic compression flow Pcf = 2 π x frequency x ½ ampl. x (1% of stroke) P P

Actuator flow calculation

Actuator Performance Calculator

Considering The Actual Test A number of factors can affect the reliability and repeatability of the test:- INERTIA REACTION OF FIXTURES (loads & deflections) VELOCITY BANDWIDTH ATTEMPTS TO ACCELERATE A TEST

Considerations for Accelerating a Test The definition of a “load cycle” and the decision whether it can be omitted or not is only possible if all simultaneous acting forces and the local stresses are taken into account. Every change (shortening) of one individual channel requires an appropriate change of all other channels, due to existing correlations (phase Relations) The shortened load-time histories should have qualities that allow us to exploit the capacities of the test rig as far as possible during the whole test without exceeding the given limits.

Vauxhall Vectra Suspension Components

Summary of parameters for Simultaneous Procedures Load cycles of the resulting local stresses in a component being below a certain limit can be omitted e.g 15% of max stress. The significant turning points of the local curve must be maintained. The maximum deviations and frequencies of the load-time histories shortened should not exceed the given limits. RESTRICTIONS Inertia Forces – Application of Simultaneous Procedures is restricted to tests with test rigs and components that can be considered as STIFF. Damping Elements – Forces depend on speed and frequency which are changed by Simultaneous Procedures. Rotating Components – Not currently applicable.

Actual Component Effects on Test Performance Material Metallurgy Residual Stress State Design Stress Concentration Process Machining Welding Heat Treatment

Process Control - Example Three components: •Steel from same supplier and factory •One prototype assembly •One assembly from production at Factory 1 (–50% of reference life) •One assembly from production in Factory 2 (+50% of reference life) Rear suspension arm

Process Control – Residual Stress State Compression Prototype Tension Factory 2 (+50%) life) Factory 1 (–50% life)

SUMMARY ANY COMPONENT TEST WILL BE IRELLAVANT IF YOU DON’T KNOW THE STARTING POINT:- The specimen:- Material, manufacture, stress state The test specification:- Quality and interpretation of the data The test system:- Fixtures, reaction points Limitations of the servo hydraulic system Repeatability of the test